University of Kassel Faculty of Ecological Agricultural Sciences Department of Agricultural Engineering in the Tropics and Subtropics Kassel/Germany In cooperation with Federal Agricultural Research Centre (FAL) Institute of Technology and Biosystems Engineering Braunschweig/Germany

Biogas production from forage and sugar beets Process Control and Optimization – Ecology and Economy

Dissertation for THE DOCTOR DEGREE of Engineering (Dr.-Ing.)

By

Elhussein Abdien Hassan

Kassel / Witzenhausen 2003

Die vorliegende Arbeit wurde vom Fachgebiet Agrartechnik im Fachbereich Landwirtschaft, Internationale Agrarentwicklung und Ökologische Umweltsicherung der Universität Gesamthochschule Kassel als “Dissertation zur Erlangung des Grades eines Doktors der Ingenieurwissenschaften (Dr.-Ing.) angenommen.

Tag der mündlichen Prüfung: 04. Juli 2003 Prüfungskommission: 1. Gutachter 2. Gutachter 3. Gutachter Prüfer Prüfer

Prof. Dr.-Ing. Rüdiger Krause Prof. Dr. Konrad Scheffer Prof. Dr.-Ing. Peter Weiland Prof. Dr. Ralf Bokermann Prof. Dr. Ezzat S. Tawfik

Ó im Selbstverlag Bezugsquelle

Universität Kassel Fachgebiet Agrartechnik Nordbahnhofstraße 1a D-37213 Witzenhausen

To my parents

Acknowledgments The Institute of Technology and Biosystems Engineering of the Federal Agricultural Research Centre (FAL), Braunschweig-Germany is a leading research Institute in production of biogas in agriculture, and I was drawn to this study by the influence of Professor Dr.-Ing. Rüdiger Krause, my immediate supervisor. I would like to thank him for the opportunity, and express my delight in being able to work under his supervision. He has been generous and gracious to me at all times. I am deeply indebted to Professor Dr.-Ing. Peter Weiland, my co-supervisor, for his insight, unlimited help and valuable advice during this study. I am thankful for his willingness to spend time discussing my research questions and for his generosity in involving me in one of his projects. I also wish to express my thanks to Prof. Scheffer for taking on the role of a co-referee. Recognition is due to my colleagues, Christa Rieger, Alexander Schattauer and Thorsten Ahrens for their co-operation, interesting discussions and for being such a wonderful team. Thanks to our laboratory personnel, specially, Anna Friedrich and Hans Berg for all help with analytical equipment and creating a great working environment. Part of this research was supported by 'Bundesanstalt für Landwirtschaft und Ernaehrung (BLE), Bundesministerium für Verbraucherschutz, Ernährung und Landwirtschaft (BMVEL) and the University of Kassel, for which I am deeply grateful. Outside the research area, my deepest gratitude to my parents in the Sudan for their encouragement; their belief in me gives me the strength to persevere. Finally, I wish to give special recognition to my wife for her patience and understanding during the execution of this task.

Preface Human power and energy very soon comes to its limitations. But people very early learnt to use support by draught animal power, wind- and water energy and later by energy carriers of high energetic density like coal, petroleum or natural gas for heating, driving machines (like steam engines) or internal combustion engines, especially for locomotion. At least since the oil crisis’s during the seventies of last century everybody knows the vital role of energy and I am afraid mankind will suffer more wars for access to energy wells. While fossil energy resources are becoming scarce and expensive, research is focusing on renewable energy. Organic matter is one of the traditional but again promising feed stuffs, biomethanization one of the possible technologies to produce gas for energetic use. In this thesis the author presents his thoroughly elaborated, valuable results, investigating the gas production potential of fodder – and sugar beets, also mixed with cow dung. Systematically he investigates the effect of composition of feedstock, frequence and rate of load, process temperature, batch and nearly continuous flow, the quantity and the quality of produced gas. He also looks for the impact of beet-denaturalizing-chemicals. Important is the attempt to compare ecological consequences of the full production line of beets compared with fossil fuel which biogas is able to substitute. Concerning economy of beet-based biogas production realistic calculations demonstrate that only under optimal conditions a very limited profit is possible. This is especially now important when many farmers, attracted by not very serious calculations based on less thoroughly basic data are in the phase of thinking over high investment into biogas technology. Working on such an important aspect as renewable energy I am convinced that Dr.-Ing. Elhussein Abdien Hassan can contribute to future energy safety, here in Germany or in his home country, the Sudan.

Rüdiger Krause, Witzenhausen im Juli 2003

Abstract For the energetic use of renewable resources the anaerobic digestion of forage and sugar beets silage as well as their mixtures with cow manure were examined in batch and semi-continuous reactors under different process conditions in order to efficiently compare biogas production from beets to conventional electricity production with respect to energy balance, ecological balance and economy. Generally, the results show that forage and sugar beets silage are suitable for mono- and co-fermentation. Mixing of beets with cow manure reduces the degradation efficiency and hence the methane production but on the other hand increases the process stability. In the semi-continuous experiments, the maximum organic loading rate that could be reached for forage and sugar beets under stable conditions was 4 g organic total solid per liter per day [oTS/l*d]. This increased to 4.5 g oTS/l*d as a result of the addition of manure to the beets. Through the anaerobic digestion the NH4-N content increased to more than 50 %, which increased the efficiency of using the digested substrates as fertilizer. Regarding energetic aspects, it was found that producing electricity from forage and sugar beets can have 4 times higher Output/Input factor compared to conventional energy which can save fossil energy. The use of biogas from 1 hectare forage (sugar) beets for electricity could avoid the release of 21 (20) tons CO2-equivalent when using fossil energy. From today’s perspective, the biogas production from beets can not be economically recommended. Biogasproduktion von Futter- und Zuckerrüben, Prozesssteuerung und Optimierung Ökologie und Ökonomie Für die energetische Nutzung von Nachwachsende Rohstoffen wurden in diskontinuierlichen und quasi-kontinuierlichen Reaktoren die anaerobe Vergärung von Futterrübensilage und Zuckerrübensilage sowie deren Mischungen mit Rindergülle unter verschiedenen Parameter genau untersucht, um eine effiziente Energiebilanz, Ökobilanz und die Wirtschaftlichkeit des gesamtes Prozesses beim Wechsel von fossilen Energieträgern zur Energieproduktion auf Basis von Biogas zu ermitteln. Die Ergebnisse zeigen, dass Futter- und Zuckerrübensilage grundsätzlich für Mono- und Kofermentation geeignet sind. Durch Beimischung von Rindergülle sinkt der Abbaugrad und infolgedessen auch die Methanausbeute, jedoch wird gleichzeitig die Prozeßstabilität erhöht. In den quasi-kontinuierlichen Versuchen betrug die maximal erzielbare Raumbelastung unter stabilen Prozessbedingungen für Futter- und Zuckerrüben 4 g oTS/l*d. Durch die Beimischung von Gülle stieg dieser Wert auf 4.5 g oTS/l*d. Durch die Vergärung nahm der NH4-N-Gehalt um bis zu 55 % zu, wodurch die Effizienz bei der Verwendung der Gärrückstände als Wirtschaftdünger verbessert wird. Von der energetischen Seite her betrachtet wurde festgestellt, dass bei der Betrachtung der elektrischen Energieproduktion aus Futterrüben (Zuckerrüben) ein 4 (4) mal größerer Output/Input–Faktor gegenüber einem dem der fossilen Energieträgern erzielt werden kann, der zu Einsparungen von fossile Energie führt. Durch die Produktion von Biogas aus Futterbzw. Zuckerrüben kann im Vergleich zur Verwendung von Fossilenergie pro Hektar die Freisetzung von 21 bzw. 20 Tonnen CO2-Äquivalenten vermieden werden. Aufgrund der ökonomischen Analyse ist die Biogasproduktion aus Futter- bzw. Zuckerrüben derzeit wirtschaftlich noch nicht empfehlenswert.

Table of Contents

List of Abbreviations and Symbols ____________________________________________ I List of Tables ____________________________________________________________ III List of Figures ____________________________________________________________IV 1 Introduction ____________________________________________________________ 15 2 Literature review ________________________________________________________ 18 2.1 Energy _____________________________________________________________ 18 2.1.1 Energy resources ________________________________________________ 18 2.1.2 Energy situation in Germany_______________________________________ 19 2.1.2.1 Electrical energy__________________________________________ 19 2.1.2.2 Renewable energy ________________________________________ 19 2.1.2.3 Energy policy ____________________________________________ 20 2.1.3 Energy from biomass_____________________________________________ 21 2.1.3.1 Energy crops_____________________________________________ 22 2.1.3.2 Biogas production ________________________________________ 23 2.2 Anaerobic fermentation ________________________________________________ 24 2.2.1 Principles of anaerobic fermentation_________________________________ 24 2.2.1.1 Hydrolysis ______________________________________________ 25 2.2.1.2 Acidogenesis ____________________________________________ 26 2.2.1.3 Acetogenesis_____________________________________________ 26 2.2.1.4 Methanogenesis __________________________________________ 26 2.2.2 Factors affecting the anaerobic fermentation __________________________ 27 2.2.2.1 Type of substrate _________________________________________ 27 2.2.2.2 pH and buffer capacity _____________________________________ 28 2.2.2.3 Temperature _____________________________________________ 29 2.2.2.4 Toxicity effects___________________________________________ 29 30 2.2.2.5 H2S 2.2.2.6 Loading rate and retention time ______________________________ 31 2.2.2.7 Mixing 31 2.3 Co-fermentation ______________________________________________________ 32 2.4 Biogas plants ________________________________________________________ 33 2.4.1 Requirements of biogas plants______________________________________ 33 2.4.2 Types of biogas reactors __________________________________________ 36 2.4.2.1 Batch reactor ____________________________________________ 36 2.4.2.2 Continuously stirred tank reactor _____________________________ 37 2.4.2.3 Two-phase reactor ________________________________________ 37 2.4.3 Purification of biogas ____________________________________________ 38 2.4.4 Storage of biogas ________________________________________________ 39 2.5 Biogas as energy source________________________________________________ 40 2.5.1 Gas composition and quality _______________________________________ 40 2.5.2 Biogas application _______________________________________________ 41 2.6 Principles of system evaluation __________________________________________ 42

2.6.1 Description and methods of analysis systems __________________________ 42 2.6.2 Energy balance _________________________________________________ 43 2.6.3 Ecological balance_______________________________________________ 49 3 Materials and Methods ___________________________________________________ 57 3.1 Laboratory experiments ________________________________________________ 57 3.1.1 Type of reactors used_____________________________________________ 57 3.1.1.1 Batch reactors ____________________________________________ 57 3.1.1.2 Semi-continuous reactors ___________________________________ 58 3.1.2 Necessary analysis_______________________________________________ 59 3.1.2.1 Gas production and composition _____________________________ 60 3.1.2.2 pH 61 3.1.2.3 Dry matter (TS) and organic dry matter (oTS)___________________ 61 3.1.2.4 Chemical oxygen demand (COD) ____________________________ 62 3.1.2.5 Ammonium-nitrogen (NH4-N) _______________________________ 63 3.1.2.6 Total nitrogen (Total-N)____________________________________ 64 3.1.2.7 Phosphate-phosphorus (PO4-P) ______________________________ 64 3.1.2.8 FOS/TAC _______________________________________________ 65 3.1.2.9 Volatile fatty acids (VFA) __________________________________ 66 3.1.3 Controlling and measuring parameters _______________________________ 67 3.1.3.1 Loading rate and hydraulic retention time ______________________ 67 3.1.3.2 Methane yield ____________________________________________ 68 3.1.3.3 Methane productivity ______________________________________ 69 3.1.3.4 Degradation efficiency _____________________________________ 69 3.1.4 Substrate properties ______________________________________________ 71 3.1.4.1 Experiment with forage beets silage (EXP1) ____________________ 71 3.1.4.2 Experiment with sugar beets silage (EXP2)_____________________ 73 3.1.5 Description of the experiments _____________________________________ 74 3.1.5.1 Experiments with forage beets silage (EXP1) ___________________ 75 3.1.5.1.1 Batch experiments ________________________________ 75 3.1.5.1.2 Semi-continuous experiments _______________________ 76 3.1.5.2 Experiments with sugar beets silage (EXP2) ____________________ 80 3.1.5.2.1 Batch experiments ________________________________ 80 3.1.5.2.2 Semi-continuous experiments _______________________ 80 3.2 Systems evaluation____________________________________________________ 82 3.2.1 Method of energy balance _________________________________________ 84 3.2.2 Method of ecological balance ______________________________________ 85 4 Results and discussion____________________________________________________ 86 4.1 Experimental results___________________________________________________ 86 4.1.1 Mesophilic experiments___________________________________________ 86 4.1.1.1 Batch experiments ________________________________________ 86 4.1.1.1.1 Inoculum________________________________________ 86 4.1.1.1.2 Forage beets silage ________________________________ 88 4.1.1.1.3 Sugar beets silage _________________________________ 90 4.1.1.1.4 Sugar beets silage with manure ______________________ 91 4.1.1.2 Semi-continuous experiments _______________________________ 93 4.1.1.2.1 Methane productivity ______________________________ 93 4.1.1.2.1.1 Forage beets silage ______________________ 93

4.1.1.2.1.2 Forage beets silage with manure ____________ 95 4.1.1.2.1.3 Sugar beets silage _______________________ 96 4.1.1.2.1.4 Sugar beets silage with manure _____________ 97 4.1.1.2.2 Biogas composition _______________________________ 99 4.1.1.2.3 Degradation efficiency _____________________________ 99 4.1.1.2.4 Laboratory analyses ______________________________ 101 4.1.1.2.4.1 Chemical oxygen demand ________________ 101 4.1.1.2.4.2 Total and ammonium-nitrogen ____________ 102 4.1.1.2.4.3 Phosphate-phosphorus___________________ 104 4.1.1.2.4.4 FOS/TAC and volatile fatty acids __________ 105 4.1.1.2.4.5 pH __________________________________ 108 4.1.1.2.5 Experiments of FBS containing plant denaturation agents 109 4.1.2 Thermophilic experiments________________________________________ 112 4.1.2.1 Batch experiments _______________________________________ 112 4.1.2.1.1 Inoculum_______________________________________ 112 4.1.2.1.2 Forage beets silage _______________________________ 113 4.1.2.1.3 Forage beets silage with manure ____________________ 114 4.1.2.2 Semi-continuous experiments ______________________________ 116 4.1.2.2.1 Methane productivity _____________________________ 116 4.1.2.2.1.1 Forage beets silage _____________________ 116 4.1.2.2.1.2 Forage beets silage with manure ___________ 118 4.1.2.2.2 Biogas composition ______________________________ 119 4.1.2.2.3 Degradation efficiency ____________________________ 119 4.1.2.2.4 Laboratory analyses ______________________________ 120 4.1.2.2.4.1 Chemical oxygen demand ________________ 120 4.1.2.2.4.2 Total and ammonium-nitrogen ____________ 121 4.1.2.2.4.3 Phosphate-phosphorus___________________ 122 4.1.2.2.4.4 FOS/TAC and Volatile fatty acids _________ 123 4.1.2.2.4.5 pH __________________________________ 126 4.1.3 Comparison between forage beets and sugar beets _____________________ 127 4.2 Energy balance______________________________________________________ 129 4.3 Ecological balance ___________________________________________________ 134 4.4 Economical evaluation________________________________________________ 141 5 Error discussion________________________________________________________ 146 6 Conclusions ___________________________________________________________ 147 7 Summary _____________________________________________________________ 149 8 Zusammenfassung ______________________________________________________ 152 9 References ____________________________________________________________ 155 10 Appendix _____________________________________________________________ 164

List of Abbreviations and Symbols hCOD h(fluid) h(gas) r C CH4 CHP CM C:N CO CO2 COD CSTR EXP1 EXP2 FBS FM FOS Total-N GWP IPCC ha H2 H2S H2SO3 H2SO4 HPLC HRT HS IN kWh l LCA m m(feed) m(effl.) N2 N2O NH3 NH4+ NH4-N NMHC oTS OLR PDA PCH4 PO4-P I

COD-degradation degree Degradation efficiency throughout fluid phase Degradation efficiency throughout gas phase Density Methane concentration Methane Combined heat and power engine (plant) Cow manure Carbon nitrogen ratio Carbon monoxide Carbon dioxide Chemical oxygen demand Continuously stirred tank reactor First experiment Second experiment Forage beets silage Fresh mass (wet mass) Volatile organic acids Total nitrogen Global warming potentials Intergovernmental panel on climatic change Hectare Hydrogen Hydrogen sulfide Sulfurous acid Sulfuric acid High performance liquid chromatography Hydraulic retention time Reactor head space Inoculum Kilo watt hour Liter Life cycle assessment Mass Daily organic matter added to the reactor Daily organic matter removed from the reactor Nitrogen Dinitrogen oxide Ammonia Ammonium Ammonium-nitrogen Non-methane hydrocarbon Organic total solid (organic dry matter) Organic loading rate Plant denaturation agent Methane productivity Phosphate-phosphorus

PPA ppm PVC REA RI SBS t TAC Total-N TS UV VFA VR Vbiogas VCH4 & feed V YCH4

Plant protection agent Parts per million Polyvinyl chloride Renewable energy act Refractive index detector Sugar beets silage Time Total anorganic carbon Total nitrogen Total solid (dry matter) Ultra-Violet Volatile fatty acids Reactor volume Biogas production rate Methane production rate Daily substrate volume fed to reactor Methane yield

II

List of Tables Table 1: Some benefits and risks of the application of co-fermentation________________ 33 Table 2: Technical options in biogas plants _____________________________________ 35 Table 3: Overview of techniques used for biogas treatment _________________________ 39 Table 4: Characteristics of biogas components___________________________________ 41 Table 5: Mean calorific value of fossil energy sources_____________________________ 45 Table 6: Mean calorific value of some bio-energy sources _________________________ 45 Table 7: Diesel consumption of different tractors and applications ___________________ 46 Table 8: Energy input in fertilizer production____________________________________ 47 Table 9: Physical calorific values of main biomass constituents _____________________ 49 Table 10: Global warming potentials (GWP) given in kg CO2-eq./kg gas ______________ 53 Table 11: Acidification potentials (AP) for acidifying substances ____________________ 54 Table 12: Characteristics of the substrates and inoculum used in EXP1 _______________ 72 Table 13: Concentration of organic acids in substrates (EXP1) ______________________ 72 Table 14: Characteristics of the substrates and inoculum used in EXP2 _______________ 73 Table 15: Concentration of organic acids in substrates (EXP2) ______________________ 73 Table 16: Concentration of alcohol and sugar in substrates (EXP2) __________________ 74 Table 17: Weights of substrates mixture used in batch reactors (EXP1) _______________ 75 Table 18: Hydraulic retention times and loading rates of EXP1______________________ 77 Table 19: Operational plan of semi-continuous reactors with PDA ___________________ 78 Table 20: Weights of substrates mixture used in batch reactors (EXP2) _______________ 80 Table 21: Hydraulic retention times and loading rates of EXP2______________________ 81 Table 22: Mean results of the batch experiments (mesophile) _______________________ 93 Table 23: Biogas composition (mesophilic experiments) ___________________________ 99 Table 24: Mean results of batch experiments (FBS) under different temperatures ______ 116 Table 25: Biogas composition (thermophilic experiments) ________________________ 119 Table 26: Energy input for forage beets silage per hectare_________________________ 131 Table 27 :Energy input for sugar beets silage per hectare _________________________ 132 Table 28: Energy balance of biogas (FBS) and fossil cycle ________________________ 133 Table 29: Energy balance of biogas (SBS) and fossil cycle ________________________ 133 Table 30: Emission of toxic substances from biogas and fossil cycles________________ 140 Table 31: Forage (and sugar) beets production and biogas running costs _____________ 143 Table 32: Investment cost and annual income of biogas plant ______________________ 144

III

List of Figures Figure 1: Energy life cycle from biomass _______________________________________ 21 Figure 2: Stages of anaerobic fermentation______________________________________ 25 Figure 3: Principle layout of biogas plant _______________________________________ 34 Figure 4: Energy flux scheme ________________________________________________ 44 Figure 5: Energy output scheme ______________________________________________ 48 Figure 6: Phases of life cycle analysis (LCA)____________________________________ 50 Figure 7: Batch reactor _____________________________________________________ 58 Figure 8: Semi-continuous reactor ____________________________________________ 59 Figure 9: Hydraulic retention time and loading rate for mesophilic experiments (EXP1) __ 79 Figure 10: Hydraulic retention time and loading rate for thermophilic experiments (EXP1)_________________________________________________________ 79 Figure 11: Hydraulic retention time and loading rate for EXP2 ______________________ 82 Figure 12: Energy production cycles of biogas and fossil fuel _______________________ 83 Figure 13: Methane accumulative curve of inoculum used for mesophilic FBS batch experiments _____________________________________________________ 87 Figure 14: Methane accumulative curve of inoculum used for mesophilic SBS batch experiments _____________________________________________________ 87 Figure 15: Accumulative methane of mesophilic FBS batch experiments ______________ 88 Figure 16: Daily methane production of mesophilic FBS batch experiments ___________ 89 Figure 17: Accumulative methane of mesophilic SBS batch experiments ______________ 90 Figure 18: Daily methane production of mesophilic SBS batch experiments ___________ 91 Figure 19: Accumulative methane of mesophilic SBS:CM batch experiments __________ 92 Figure 20: Daily methane production of mesophilic SBS:CM batch experiments ________ 92 Figure 21: Methane productivity of FBS in semi-continuous reactors _________________ 94 Figure 22: Methane productivity of FBS:CM in semi-continuous reactors _____________ 95 Figure 23: Methane productivity of FBS and FBS:CM under different OLR ___________ 96 Figure 24: Methane productivity of SBS in semi-continuous reactors _________________ 97 Figure 25: Methane productivity of SBS:CM in semi-continuous reactors _____________ 98 Figure 26: Methane productivity of SBS and SBS:CM under different OLR ___________ 98 Figure 27: Degradation efficiency of substrates with different loading rates according to gas phase ______________________________________________________ 100 Figure 28: Chemical oxygen demand of effluent at different OLR. __________________ 101 Figure 29: Total-N and NH4-N concentration of FBS and FBS:CM effluent. __________ 102 Figure 30: Total-N and NH4-N concentration of SBS and SBS:CM effluent. __________ 103 Figure 31: PO4-P concentration of effluent for mesophilic EXP1 and EXP2___________ 104 Figure 32: FOS/TAC-value with different OLR throughout the mesophilic experiments _ 105 Figure 33: VFA concentration of FBS throughout mesophilic experiments ___________ 106 Figure 34: VFA concentration of FBS:CM throughout mesophilic experiments ________ 106 IV

Figure 35: VFA concentration of SBS throughout mesophilic experiments ___________ Figure 36: VFA concentration of SBS:CM throughout mesophilic experiments ________ Figure 37: pH-value of reactors for mesophilic EXP1 and EXP2 ___________________ Figure 38: Methane productivity of FBS mixed with PDA in semi-continuous reactors __ Figure 39: VFA concentration of FBS mixed with Tieröl _________________________ Figure 40: VFA concentration of FBS mixed with Arcotal ________________________ Figure 41: VFA concentration of FBS mixed with Arbin__________________________ Figure 42: Methane accumulative curve of inoculum used for thermophilic FBS batch experiments ____________________________________________________ Figure 43: Accumulative methane of thermophilic FBS batch experiments ___________ Figure 44: Daily methane production of thermophilic FBS batch experiments _________ Figure 45: Accumulative methane of thermophilic FBS:CM batch experiments _______ Figure 46: Daily methane production of thermophilic FBS:CM batch experiments _____ Figure 47: Methane productivity of FBS in semi-continuous reactors ________________ Figure 48: Methane productivity of FBS at different OLR_________________________ Figure 49: Methane productivity of FBS:CM in semi-continuous reactors ____________ Figure 50: Degradation efficiency of substrates with different loading rates ___________ Figure 51: COD of FBS and FBS:CM throughout the experiment___________________ Figure 52: Total-N and NH4-N concentration for effluent of FBS and FBS:CM ________ Figure 53: PO4-P concentration of effluent for FBS and FBS:CM___________________ Figure 54: FOS/TAC-value of FBS throughout the thermophilic experiments _________ Figure 55: FOS/TAC-value of FBS after feeding break throughout the thermophilic experiments ____________________________________________________ Figure 56: VFA concentration of FBS after feeding break throughout thermophilic experiments ____________________________________________________ Figure 57: FOS/TAC-value of FBS:CM throughout the thermophilic experiments______ Figure 58: pH-value of FBS and FBS:CM under thermophilic digestion______________ Figure 59: Methane production from FBS and SBS ______________________________ Figure 60: NH4-N concentration in influent and effluent of FBS and SBS ____________ Figure 61: Methane productivity of FBS and SBS with different OLR._______________ Figure 62: CO2-equivalent emission from biogas production and fossil life cycle ______ Figure 63: N2O-emission from biogas and fossil life cycle ________________________ Figure 64: SO2-equivalent emission from biogas and fossil life cycle ________________ Figure 65: NOx-emission from biogas and fossil life cycle ________________________ Figure 66: NH3-emission from biogas and fossil life cycle ________________________ Figure 67: NMHC-emission from biogas and fossil life cycle ______________________ Figure 68: Reactor specific investment________________________________________

V

107 107 108 109 110 111 111 112 113 114 115 115 116 117 118 120 121 122 123 124 124 125 125 126 127 128 129 135 136 137 138 138 139 143

1

Introduction

The world energy demand is continually increasing due to the increase in the world’s population, economic growth, and energy usage (Heinloth, 1997). At today’s rate of increase the sources of fossil energy, which meet the majority of the current world energy demand, will not be sufficient in the centuries to come. Moreover, CO2 emissions, which are the main cause of the greenhouse effect, and other atmospheric pollutants from energy generation using fossil fuels, cause environmental pollution. In 1997, over 180 nations, including Germany, met in Kyoto, Japan, to finalize negotiations on a legally binding international treaty aimed at lowering greenhouse gas emissions. The use of renewable energy sources can contribute to solving present and future energy problems. Replacing fossil fuels as an energy source with energy derived from agricultural crops is one of Germany’s policies for reducing carbon dioxide in the atmosphere. The neutral CO2-emission from renewable resources is only realized when the emission from fossil energy used in the production of renewable resources is less than the emission from production and use of equivalent fossil energy. Anaerobic digestion, as a source of biogas, has been used in the past mainly for degradation of waste materials or toxic compounds. Recently, there has been great interest in producing biogas from energy crops. Since the Renewable Energy Sources Act (REA) became effective in Germany in 2000, the interest in anaerobic fermentation of energy crops as a source of electrical energy has increased. In 2002, the REA supports the production of electricity from biogas through a refund of approx. 0.1023 Euro per kWh. Forage and sugar beets are considered to be important energy crops for biogas production because of their high organic matter yield per hectare. Additionally, they contain a high fraction of light degradable components. Moreover, forage and sugar beets have optimal conditions for easily ensiling so that they can be stored and used for the whole year. Another advantage of forage beets is the low dry matter which makes forage beets pumpable and hence the possibility of fully automated biogas plants.

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Generally, the anaerobic degradation of organic matter to produce methane relies on the complex interaction of several different groups of bacteria. Effective anaerobic biodegradation of organic material can be reached by achieving the optimal condition for the bacteria. On the other hand, the adaptation of the bacteria to the substrate used in the digestion is also an important factor in the evaluation of the process. Therefore, the adaptation of new substrates must be examined. Until now there are no data available about the anaerobic degradation of sugar beets. The data found for forage beets showed large variations. A detailed examination of the anaerobic processes of forage and sugar beets is therefore essential to obtain a thorough evaluation of the process. Defining the optimal conditions of anaerobic processes is not enough to identify or to evaluate the whole system. Additional energetic, ecological, and economical analysis must be achieved to completely identify the system. For energetic and ecological evaluation the entire life cycle of the system must be considered.

Objectives of the thesis The general purpose of this study is to report the status and the prospects of the production and utilization of biogas from forage and sugar beets silage as a source of alternative energy. The primary objective of this study is to obtain and examine under laboratory conditions the parameters within the anaerobic digestion system that contribute the largest impacts. These include: ·

Characteristic of forage and sugar beets silage ingredients which can affect the degradation.

·

Determination of the maximum degradation degree conversion and associated methane yields under different process temperatures.

·

Determination of the maximum methane content of the gas.

·

Determination of the optimal operating range of the process under continuous conditions.

·

Examination of the suitability of forage and sugar beets for co-fermentation with cow manure.

·

Determination of the efficiency of effluent as a secondary fertilizer.

·

Determination of the effect of plant denaturation agent on the process of forage beets.

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The objectives also include energetic, ecological and economical evaluation of the total process. The aim is to compare the whole life cycle of biogas production from energy crops with the fossil life cycle from an energetic and ecological point of view, so that the advantages and disadvantages of the substitution of fossil energy can be determined. Another objective it is to determine the economic efficiency of the production of electricity from energy crops.

Hypothesis Forage and sugar beets are carbohydrate crops which have high organic dry matter yield per hectare. The current research was carried out to test the following hypotheses. ·

Forage and sugar beets have high biogas production with 50–65 % methane content.

·

Ensiling forage and sugar beets, so that they are available throughout the whole year, do not affect the anaerobic digestion process.

·

Production of biogas from beets will have positive energy balance due to the high biogas production per hectare.

·

The total CO2-emission from biogas have a lower ecological impact than from fossil energy because of the neutral CO2 emission of biogas burning.

·

The selling the biogas electricity covers the input costs.

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2

Literature review

2.1 Energy We are aware of energy in our life in many forms. Energy, for example, transports us, fuels machines, cooks our food. In short, energy maintains our entire economic system and supplies us with comfortable lives. Energy also allows plants to grow through the process of photosynthesis from the sunlight. Moreover, the sun supplies us with the bulk of our fuels, primarily the fossil fuels (coal, oil and natural gas), which are the stored energy of the sun resulting from plant growth millions of years ago. Nevertheless, the amount of energy that fossil fuels can provide is ultimately limited. This means that the energy supply of the future needs solutions at the present. In order for sufficient energy to be available in future centuries, it is essential to further develop the use of renewable energy sources.

2.1.1 Energy resources Energy resources can be divided into renewable and non-renewable resources. Non-renewable resources can be divided into a) fossil fuels which are divided into coal, crude oil and natural gas and b) uranium ores – nuclear power. Renewable resources can be divided into geothermal, hydro-electric, solar, wave, tidal, wind, biomass, etc. Fossil fuels are the most widely used energy resources. Renewable energy resources can be defined as energy resources that are replaced rapidly by natural processes. Renewable energy is beginning to grow out of its fledgling status and has experienced exponential growth in usage over recent years. There can be no doubt that it will play a major role in the global, regional and local energy supply systems of the 21st century and beyond. Non-renewable energy resources are energy resources that are not replaced or are replaced only very slowly by natural processes; i.e., they are being used up at rates much greater than rates of formation of new resources. The problem with non-renewable energies, in addition to limited resource, is that they cause environmental pollution. Burning fossil fuel produces CO2 and other atmospheric pollutants.

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2.1.2 Energy situation in Germany Germany is one of the world's largest energy consumers. Germany imports most of the energy to meet its energy needs because it has limited local energy resources (except for coal and natural gas). The total primary energy consumption in Germany for the year 2001 was 14501*1015 Joule (AGEB, 2002). The current energy consumption is primarily based on petroleum (39,5 %), natural gas (21.5 %), and hard coal and lignite (13.1 and 11.2 % respectively) – the main causes of the greenhouse effect and climatic changes. Behind them are nuclear power (12.9 %), and hydro and wind power (together just under 0.8 %). Slightly less than 2 % was provided by the other renewable energies such as solar energy and biomass (AGEB, 2002). In the case of final energy used by the consumer, transportation is the dominant sector (30 %). Households account for 29 % of overall final energy consumption, followed by industry (26 %), and crafts, trade, and services at 16 % (BMWi, 2002).

2.1.2.1 Electrical energy More than a third of primary energy consumption in Germany is converted to electricity in power stations. In 2001, Germany generated 561.5 billion kilowatt hours of electricity (BMWi, 2001). Nearly 100 % of electricity needs in Germany are met by domestic production. The main shares are accounted for by atomic energy (33 %), lignite coal (29 %), and hard coal (22 %). Natural gas (7 %) and renewable energies (7 %) presently have only slight importance; oil no longer plays a role of any significance (2 %) (VDEW, 2002). The electricity generated by hydropower and wind energy represents 54.8 % and 31.8 % of the electricity produced from regenerative sources respectively. The remaining 13.4 % is generated by other regenerative energy sources (VDEW, 2002).

2.1.2.2 Renewable energy The German Government supports and encourages energy production from renewable resources to reduce the environmental pollution from fossil energy. The German government is also hoping to use renewable energy sources to compensate for the loss of atomic power

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through better conservation and new technology, particularly renewable resources. The Federal Environment Minister has stated that up to 3/5 of nuclear power could be replaced by wind energy by 2030, though only a few of the additional plants have been built yet. Germany's main renewable resource is wind power. The opening of the Europe's largest wind farm in Paderborn, Germany in 2001 has increased Germany's total wind power electricity capacity to 700 MW. The use of renewable energy sources for electricity production would increase by 6 % in 2000 to 21 % in 2020. Electricity production from biomass (including biogas) would expand twelve-fold relative to its 1999 level to reach a level of 4.5 % of the total electricity production. Electricity production from wind energy would rise to 10 % in 2020. Under today's conditions, many renewable energies cannot compete with conventional energy sources for electricity generation. Electricity production costs with wind-driven units are two to three times higher and with solar units twenty-five times higher than average electricity generation costs featured by conventional power stations (about 3 cents per kWh).

2.1.2.3 Energy policy Reduction of carbon emissions is a major German international and domestic policy objective. The total CO2 emissions in Germany declined by 15.4 % to some 859 million tons from 1990 to 1999. Energy-related CO2 emissions fell in the same time period by 15.6 % to roughly 833 million tons (UBA 2002). The 1991 Act on Feeding Electricity from Renewable Energies into the Public Grid which sought to promote the production of electricity from renewable energy sources was improved and replaced on April, 2000, by the Renewable Energy Act (REA). The REA considerably improves and expands the feed-in provisions to include all renewable energies. In the future, regenerative forms of energy like wind power, solar energy, and biomass will have a greater role in energy supply. The federal government and the operators of nuclear power plants signed an agreement in June, 2001, which serves as a basis for the controlled termination of the use of nuclear power in Germany to reduce the environmental pollution. Nuclear power is approximately a third of Germany's total electricity production. The replacement of CO2-free nuclear power by fossil energy sources results in additional CO2 emissions and the emission of other climate-relevant trace gases that have an impact on the climate. Therefore, coverage of the replaced nuclear energy is expected with renewable energy resources.

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Also, one of the German policies to reduce the environmental pollution is to save energy use by increasing the energy taxes (eco-tax). At the same time encouraging and supporting energy from renewable resources (REA, 2000). In a study supported by the Federal Ministry of Economics and Technology it was found that the eco-tax will be continued and increased (it was assumed that the increase will be for example in 2020 to 11 fold for gasoline, double for natural gas and three times for electricity).

2.1.3 Energy from biomass The world's energy markets rely heavily on the fossil fuels coal, petroleum crude oil, and natural gas as sources of energy, fuels and chemicals. Biomass is the only other naturallyoccurring, energy-containing carbon resource known that is large enough to be used as a substitute for fossil fuels.

Energy crops

Harvest residues

Organic by-products

Wastes

Harvest, collection, preparation Preparation

Transport

Thermochemical process

Storage

Physical-chemical process Pressing/extraction

Coalif.

Gasif.

Pyrolysis Esterification

Coal

Solid fuel

Producer gas Pyrol. oil

Gas fuel

Plant oil

PME

Biochemical process Alcoholic ferment.

Anaerobic ferment.

Ethanol

Biogas

Aerobic ferment.

Fluid fuel

Burning Electrical energy

Heat energy

Heat-mechanical conversion

Power

Heat

Source: Kaltschmitt, et al., 2000

Figure 1: Energy life cycle from biomass

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Biomass resources include any organic matter available on a renewable basis, including dedicated energy crops and trees, agricultural food and feed crops, agricultural crop wastes and residues, wood wastes and residues, aquatic plants, animal wastes, municipal wastes, and other waste materials. Kaltschmitt, et al., 2001, (Figure 1) divided biomass into four main groups; energy crops, harvest residues, organic by-product (manure), and wastes (i.e., slaughter house wastes, sludge). Energy from biomass (organic matter) is the sun's energy stored through photosynthesis. That energy is released when biomass is used. The technologies of releasing this energy include a variety of thermal and thermochemical processes for converting biomass by combustion, carbonization, gasification and liquefaction, physical-chemical processes to convert biomass to oil, and the microbial conversion of biomass to obtain gaseous and liquid fuels by fermentative methods (i.e., biomass can substitute all forms of energies; solid, liquid and gas). Solid biomass can be burned directly or after carbonization to produce energy (e.g., heat and steam for electricity production). Biomass can also be used to produce energy in the form of alternative transportation fuels. The two most common bio-fuels (liquid) are Ethanol and Biodiesel. Even gas can be produced from biomass for generating energy. Through the gasification process, biomass can be converted into a combustible gas (producer gas) which can be burned directly for space heat or drying, or it can be burned in a boiler to produce steam. The degradation of biomass in an oxygen-free environment (anaerobic fermentation) also produces a gas (biogas) that can be burned or combusted to produce electricity. All sources of biomass (except lignin) are suited for anaerobic fermentation, but produce different biogas yields depending on their properties. For energy production biomass must be prepared and made available. Usually in the energy production process transportation is essential. In many cases biomass must be treated mechanically (e.g., chopped up) before it can be used as energy source. Often, storage of biomass is important to adapt the amount of biomass to the energy required.

2.1.3.1 Energy crops Many different types of crops can be grown for the specific-purpose of energy production. There are two main factors that determine whether a crop is suitable for energy use. A good energy crop has a very high yield of dry material per unit of land (dry matter ton/hectare) which reduces land requirements and lowers the cost of producing energy from biomass.

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Similarly, the amount of energy to be produced from a biomass crop must be more than the amount of energy required to grow the crop. Also it is important to clarify, if such a crop is convenient for the process used (or vice versa ) and if there is another energy crop that could potentially produce more energy with the same process. The key to the large-scale production of energy from biomass is to grow suitable crop species at costs that permit the biomass to be grown as a profitable energy crop. Just as with human and animals, proper plant nutrition is fundamental to satisfactory plant growth and crop production. It has been established that plants need sixteen essential plant nutrients for satisfactory growth and development (Helsel, 1987). All sixteen nutrients are vital, but they are required in different quantities by different crops. Carbon, hydrogen, and oxygen are obtained from water and air. The remaining thirteen nutrients are supplied by the soil. The most important nutrients that each plant needs are nitrogen, phosphorus, and potassium. Deficiency of even a single nutrient would adversely affect the agronomic efficiency of other plant nutrients and crop productivity. However, soils have limited reserves of these nutrients. Consequently, in a continuous cropping system where harvested products remove nutrients from the soil, these nutrients need to be continuously replenished in order to maintain soil fertility and crop productivity. The main sources of plant nutrients include organic manure, plant residues, biological nitrogen fixation, and chemical fertilizers.

2.1.3.2 Biogas production Biogas is the mixture of gases, mainly methane, CH4, and carbon dioxide, CO2, resulting from the anaerobic fermentation of organic matter. It contains 50-70 % methane and can be used as a fuel for heating or electrical power generation. Production of biogas from agricultural plants offers several environmental benefits including production of renewable energy (CO2-neutral), and the nutrient rich stabilized liquor (NH4+ is directly available for plants). Further, anaerobic digestion may also assist in reducing and destroying pathogens to acceptable levels, reducing greenhouse gas emissions, and aid in reducing odors often associated with storing and handling liquid wastes. Biogas production, when compared to other biomass energies, has the advantage that it can be produced from specially grown energy crops as well as from organic waste products. There is still lack of information of biogas yield from anaerobic fermentation of various organic substrates and on the influence of different operational regimes.

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2.2 Anaerobic fermentation The biological treatment processes of organic materials can be classified into two major groups; anaerobic and aerobic. The anaerobic fermentation can be defined as the use of biological processes, in the absence of oxygen, for the breakdown of organic matter by conversion to methane, carbon dioxide, trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide, and a nearly stable residue. The organic fraction of almost any form of biomass, including sewage sludge, animal wastes and industrial effluents, can be broken down through anaerobic digestion.

2.2.1 Principles of anaerobic fermentation Knowledge of the fundamentals of the anaerobic fermentation is useful in the design and operation of efficient digesters, and in understanding how unfavorable (upset) conditions can occur and how to avoid them. Anaerobic fermentation is a complex process consisting of a mixed biological system in which organic materials such as carbohydrates, lipids, and proteins are utilized by microorganisms in their normal metabolic activities. It occurs in four basic steps as the result of the activity of a variety of microorganisms (Figure 2). Initially, the hydrolysis group, which is the enzymatic breakdown of large complex organic molecules, converts organic material to a form that can be directly utilized by the anaerobic organisms. The second group (acidogenesis) utilizes the simple organic molecules to form organic acids and alcohol as well as carbon dioxide and hydrogen. The product of the acidogenesis is then degraded by the third group (acetogenesis) to acetate, carbon dioxide and hydrogen. The Methane-producing (methanogenesis) anaerobic bacteria utilize these products and complete the decomposition process. Provided that anaerobic degradation is possible at all, complex organic matter can be transformed into methane and carbon dioxide according to the Equation 1.

a bö æ æn a bö æn a bö C n H a O + ç n - - ÷ H 2 O ® ç - + ÷CO2 + ç + - ÷CH 4 b è 2 4ø è 2 8 4ø è 2 8 4ø

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[1]

polymer fat, protein, carboydrates, etc. Hydrolysis

monomer amino acids, fatty acids, sugar Acidogenesis

carboxylic acids - alcohols Acetogenesis

acetate

-

H2, CO2

Methanogenesis

biogas Source: Bryant, 1977

Figure 2: Stages of anaerobic fermentation

2.2.1.1 Hydrolysis Hydrolysis is the first and often the rate-limiting step. In the hydrolysis stage, polymeric compounds are converted by extra-cellular enzymes to soluble smaller substrate molecules. The polymeric components in substrate that need to be hydrolyzed can be found in different physical states, in particles, dissolved or emulsified. Particles are the most commonly found. Sanders, et al., (2000) showed that the hydrolysis rate of particulate substrates is limited by the amount of surface available to the hydrolytic enzymes. Hydrolytic reactions generally limit the amount of methane produced during anaerobic digestion of biomass (Chynoweth, et al., 1987).

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2.2.1.2 Acidogenesis Acidogenesis (fermentative bacteria) converts soluble organic material mainly to acetate, propionate, butyrate, H2 and CO2. Certain fermentative reactions proceed only at low H2 concentrations and depend on H2 removal by H2-oxidising bacteria (methanogenesis). The resulting low H2-concentration accompanies formation of acetate as the major soluble product. When H2 removal by methanogenesis is less efficient, H2 blocks electron disposal by proton reduction, and fermentative bacteria must channel electrons to other disposal sites (Chynoweth, et al., 1987).

2.2.1.3 Acetogenesis Acetogenic organisms are the vital link between hydrolysis/acidogenesis and the methanogenesis in anaerobic digestion. Acetogenesis provides the two main substrates for the last step in the methanogenic conversion of organic material, namely hydrogen and acetate. Both the acidogenesis and acetogenesis produce the methanogenic substrates, acetate and H2CO2. The important distinction between these two types is that the fermentative bacteria have the possibility of using various electron acceptors for the disposal of electrons. The acetogenesis is an obligate proton-reducer and can utilize only protons as electron acceptors and only when the H2-concentration is low. At very low H2 concentrations, however, methanogenesis from H2 and CO2 becomes unfavorable (Chynoweth, et al., 1987). The oxidations catalyzed by obligate proton reducers, however, yield only small amounts of energy, provided that the concentration of the produced hydrogen is kept low (Zehnder, 1988).

2.2.1.4 Methanogenesis Methanogenesis is the last stage of the fermentation process. The two major methanogenic intermediate products are acetate and H2-CO2. Approximately 70 % of the fermenter methane comes from acetate and the remainder from CO2 reduction to CH4. Methanogenesis are strictly anaerobic bacteria. However, there are marked differences in oxygen-sensitivity among the methanogenesis. The oxidation-reduction potential required for methanogenesis may be as low as –300mV or even lower (Large, 1983).

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In the presence of electron acceptors such as metal oxides [Fe(OH), MnO2], nitrogen oxides (NO3, NO2), or oxidized sulfur compounds (SO4, SO3), methanogenesis may be inhibited and/or altered (Zehnder, et al., 1982). Methanogenic bacteria are more sensitive to changes in temperature than other organisms present in the digester. This is due to the faster growth rate of the other groups, such as acetogens, which can achieve substantial catabolism even at low temperature. Ammonia acts as a strong inhibitor of the formation of methane from H2 and CO2. It has only a minor effect on the formation of methane from acetate. The inhibition of hydrogen consumption leads to an inhibition of propionate breakdown, which acts as an inhibitor of the acetate-consuming methanogens (Wiegant and Zeeman, 1986). The most notable feature of the anaerobic process is that its successful operation depends on the interaction of metabolically different bacteria. To maintain an anaerobic stable treatment system, the nonmethanogenic and methanogenic bacteria must be in a state of dynamic equilibrium. To establish and maintain such a state, all the parameters affecting the process performance should be monitored and kept within the acceptable range. The most important of these parameters are pH, alkalinity, temperature, nutrients, retention time, C:N-ratio, toxic material, and organic loading.

2.2.2 Factors affecting the anaerobic fermentation A variety of factors affect the rate of digestion and biogas production.

2.2.2.1 Type of substrate Substrate composition is a major factor affecting the anaerobic process which affects methane yield and production rates. The microbial populations involved in anaerobic digestion require nutrients to grow and multiply. C:N:P:S ratio at a rate of 600:15:5:3 will be sufficient, since the nutrients requirements are very low due to low biomass formation (Weiland, 2001). At the same time, the balance of carbon and nitrogen in a feed material is important. It is often suggested that an optimum C:N ratio is between 20:1 and 30:1. If there is too little nitrogen present, the bacteria will be unable to produce the enzymes which are needed to utilize the carbon. If there is too much nitrogen, then it can inhibit the growth of the bacteria through NH3 toxic concentration (Braun,1982).

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Generally, celluloses are resistant to hydrolysis by enzymes or acids because of their structure and the lignin barrier. Carbohydrates are converted to equal amounts of methane and carbon dioxide, methanol and lipids to more methane than carbon dioxide, formic acid and oxalic acid to more carbon dioxide than methane (Gujer, et al., 1983). The biogas yield changes with different substrates e.g., fat (1200-1000 Nl/kg oTS), carbohydrate (700-800 Nl/kg oTS), protein (600-700 Nl/kg oTS ), biowaste (350-500 Nl/kg oTS), and lignin (ca. 0 Nl/kg oTS) (Weiland, 2001).

2.2.2.2 pH and buffer capacity pH is one of the first indication factors of inhibition in anaerobic reactors. The optimum pH for anaerobic digestion is normally in the range of 7 –8 (Chynoweth, et al., 1987). Low pH levels, for example, can be a sign of digester imbalance. As volatile acid concentrations increase, the pH in the digester decreases. At pH levels below 6, the acidic conditions produced can become toxic to methane bacteria. However, high pH can also become a problem if high levels of ammonium are generated at high organic loading rates. A factor which tends to prevent the pH conditions from changing is the alkalinity due to bicarbonate. The bicarbonate ion (HCO3-) concentration is directly proportional to the carbon dioxide content in the gas and the pH. Thus if the organic matter is being broken down too quickly for the methane bacteria to utilize the acetate and the released carbon dioxide, there will be an excess of carbon dioxide in the gas, and hence a greater concentration of CO2 dissolved in the liquid as bicarbonate. This will tend to prevent the pH from falling. There are two main operational methods for correcting an unbalanced, low pH condition in a digester. The first approach is to stop the feed and allow the methanogenic population time to reduce the fatty acid concentration and thus raise the pH to an acceptable level of at least 6.8. The second method involves the addition of chemicals to raise the pH and provide additional buffer capacity. An advantage of chemical addition is that the pH can be stabilized immediately, and the unbalanced populations allowed to correct themselves more quickly. Calcium hydroxide (lime) is often used (Marchaim, 1992).

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2.2.2.3 Temperature Anaerobic fermentation can occur under three temperature ranges; the psychrophilic range (1020ºC), the mesophilic range (33-37ºC), and the thermophilic range (50-55ºC). Conventional anaerobic digesters are commonly designed to operate in either the mesophilic or thermophilic range. The rate of digestion increases with increasing temperature. In the thermophilic range, decomposition and biogas production occur more rapidly than in the mesophilic range. However, the process is highly sensitive to disturbances such as changes in feed materials or temperature. While all anaerobic digesters reduce the viability of weed seeds and disease-producing (pathogenic) organisms, the higher temperatures of thermophilic digestion result in more complete destruction. On the other hand, most biogas plants in agricultural sector work under mesophilic range to reduce high heating costs (Weiland, 2000) which is the highest running cost of the biogas plant. All bacterial populations in digesters are fairly resistant to short-term temperature upsets, up to about two hours, and return rapidly to normal gas production rates when the temperature is restored. However, numerous or prolonged temperature drops can result in unbalanced populations, and lead to low pH problems (Marchaim, 1992).

2.2.2.4 Toxicity effects Toxic compounds affect digestion by slowing down the rate of metabolism at low concentrations, or by poisoning or killing the organisms at high concentrations. Excess quantities of organic and inorganic substances, including volatile fatty acids, ammonia, metal ions and antibiotics can create toxicity for the anaerobic bacteria. Although all groups involved in digestion can be affected, the methanogenic bacteria are generally the more sensitive. At concentrations between 1500 and 3000 mg/l of total ammonia nitrogen, and a pH greater than 7.4, the ammonia concentration may inhibit methane production (Braun, 1982). At concentrations above 3000 mg/l, ammonia becomes toxic regardless of pH. It is generally recommended that the concentration of total ammonia nitrogen be maintained below 2000 mg/l. Ammonia toxicity is often a problem of feedstocks with a high protein content.

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High concentration of volatile acids such as acetate, propionate or butyrate, are associated with toxicity effects. Although low concentrations of metal ions such as copper, zinc, and chromium stimulate the bacterial activity, they cause disruption at high concentrations (Seyfried, et al., 1990). Antibiotics can also inhibit or completely stop methane production. In addition, a small amount of oxygen (> 0.1 mg/l O2) act toxic on the fermentation process (Weiland, VDI 2001). In order to control and adjust operation, to minimize toxic effects, it is important to identify inhibition in its early stages. The two main indicators of inhibition are: Reduction in methane yield and increase in volatile acids concentration, generally occurring when the total volatile acids (expressed as acetic acid) exceed the normal range of about 250 to 500 ppm (mg/l).

2.2.2.5 H2S The sulfide species equilibrates among different forms, hydrogen sulfide, H2Sg in the gas phase and H2Ssol in aqueous phase, hydrosulfide, HS-, and sulfide, S2-, according to reaction in Equation 2.

H 2 S gas Û H 2 S sol Û H + + HS - Û 2 H + + S 2 -

[2]

Sulfide in anaerobic reactors normally originates in biological reduction of sulfate with hydrogen. Sulfate reducing organisms have a complex competition function with hydrogen use and lead to an accumulation of acetic acid (Polomski, 1998). Sulfide level is normally inhibitory to anaerobic organisms at levels of above 100 mg/l and completely inhibits at levels above 200 mg/l. The associated form (H2S) is thought to be the inhibitory agent at 50100 mg/l. Polomski (1998) reported that H2S-concentration of 85 mg/l will inhibit the methanogenic bacteria to 50 %. Weiland (2001) reported that a lower concentration of 50 mg/l is toxic. Moreover, hydrogen sulfide, H2S, is highly corrosive to metal, particularly copper, Cu, iron, Fe, and mild steel (Constant, et al., 1987). This will reduce the utility of biogas that contains a certain amount of H2S as fuel. The solution to this problem will be discussed in Chapter (2.4.3).

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2.2.2.6 Loading rate and retention time The loading rate is the term used to designate the daily amount of organic substance fed into the digester in relation to the digester’s total volume. If the loading rate is too low, the bacteria will exhibit a lower metabolic activity, and very small quantities of gas will be produced. If the loading rate is too high, an overload situation will be produced in which volatile fatty acids (VFA) build up, gas production drops, and the proportion of carbon dioxide rises. The loading rate changes with substrate type. The retention time (residence time) is the time needed for the fed substrate to remain in the digester under proper reaction. The retention time of a digester is calculated by dividing the total capacity of the digestion tank by the rate at which organic matter is fed into it. If the retention time is too short, the bacteria in the digester are washed out faster than they can reproduce, so that fermentation practically comes to a standstill. The longer a substrate is kept under proper reaction conditions, the more complete its degradation will become. But the reaction rate will decrease with increasing residence time. The disadvantage of a longer retention time is the increasing reactor size needed for a given amount of substrate to be treated.

2.2.2.7 Mixing Mixing is important in anaerobic fermentation to ensure adequate contact between bacteria and substrate and also to help strip gas out of the liquid. Another reason for using mixing is to reduce scum formation which reduces the overall capacity of the digester. It was reported early (Dague, et al., 1970) that intermittent mixing resulted in an increased biogas production and increased COD and solids reduction, compared to continuous mixing. Recently, Stroot, et al., (2001) reported that minimally mixed digesters demonstrated a much more stable operation than digesters that were continuously and vigorously mixed; vigorous, continuous mixing inhibited relationships between syntrophs and their methanogenic partners, possibly by disrupting the side by side position between these organisms. Mixing is normally carried out by either mechanical agitation, digester contents re-circulation, or gas re-circulation.

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2.3 Co-fermentation The growing awareness of the pollution problems, associated with inadequate management of animal manure and organic waste, emphasizes the need for appropriate solutions to deal with the problem. An integrated process to solve this problem is co-fermentation which is defined as the co-digestion of mainly animal manure and other types of suitable organic waste in biogas plants. Recently, numerous agricultural and non-agricultural organic wastes have been introduced to farm digesters as co-substrates. The additional feedstocks applied are mainly derived from the agro- and food industries as well as from municipalities (biogenic wastes). Typical feedstocks are: ·

food remains from large kitchens, hospitals, etc.

·

flotation slimes, fat separation sludges, spent edible oils etc.

·

animal wastes from slaughterhouses and rendering plants.

·

source separated, organic fraction of municipal solid waste.

·

organic wastes from the food processing industries, biochemical industries, etc.

The feedstocks for anaerobic digestion vary considerably in composition, homogeneity, fluid dynamics and biodegradability. For example, the dry matter content varies widely. In intensive animal farming, pig and cow slurries are reported to contain dry matter contents in the range of 3 to 12 %. Chicken manure contains 10 to 30 % TS. (Braun, 1982; Wellinger, 1991). Some agro-industrial wastes may contain less than 1 % TS, while others contain high TS contents of more than 20 %. This results in some substrates being able to be fermented only when mixed with other substrate or diluted (Weiland, 1998). In addition, the overall nutrient ratio in waste materials is of major importance for the microbial biodegradation process. The C:N-ratio in wastes can vary in a considerably wide range between ca. 6 (e.g., animal slurries) and more than 500 (e.g., wood shavings). Furthermore, the feedstocks have a different distribution of organic macromolecules like proteins, fats and carbohydrates in which their degradation is of great importance. The degradation of feedstocks with high fat contents increases VFA considerably, whereas that of high protein content leads to large amounts of ammonia (NH4+).

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Thus, a combination of two or more substrates, i.e., co-fermentation, will optimize the degradation properties of the feedstocks and hence increase the methane yield. It has been reported that the performance of digesters could be considerably improved by means of cosubstrate addition and hence increase degradation efficiency and biogas production (Weiland, 1998, Kaparaju, et al., 2001). An additional advantage of co-fermentation is the ecological disposal of organic waste and at the same time gain of energy. However, co-fermentation application is not always useful and could have some disadvantages (Table 1).

Table 1: Some benefits and risks of the application of co-fermentation Benefits for agriculture

Risks for the agriculture

Income from wastes disposal

Entry of heavy metals

Gain of renewable energy

Entry of organic harmful substances

Free nutrient input

Excess of nutrients

Supply of humus

Introduce diseases

Improve digestion quality of manure

Entry of unwanted matters

Free manure treatment

Disorder in digester

Source: Weiland, 1997

2.4 Biogas plants 2.4.1 Requirements of biogas plants Generally, there are two main different types of digesters for producing biogas; storage technique (batch) and flow technique (continuous or semi-continuous feeding). Both techniques will be described in Chapter (2.4.2). The most commonly and used one is the flow technique. Therefore, biogas plants in this chapter will be referred to this technique. The principle layout of biogas plant is shown in Figure 3. The substrate is collected (after preparing) in a collecting tank near the digester and then fed semi-continuously by a pump to the digester (input). The completely gas-tight digester is made mainly of steel or concrete. It is isolated to keep the optimum temperature (by heating) for the organisms inside the digester constant, i.e., 37°C or 55°C.

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The digester content is mixed with a mixer at least several times a day to ensure homogeneity. The outputs of the digester are biogas and digested effluent. The latter is stored in a storage tank to be used later on as a fertilizer. Biogas is collected in gas holder, possible for short periods without compression, to be used at a constant rate. A gas handling system is needed to remove biogas from the digester (gas holder) and transports it to the end-use, such as an engine or boiler. Gas handling includes: piping; gas pump or blower; gas meter; pressure regulator.

Figure 3: Principle layout of biogas plant

Digester operation requires successful start-up. The general start-up procedure initially involves addition of an inoculum containing the micro-organisms required for digestion to the reactor. The biomass feedstock is then added to the culture at a low loading rate in order to ensure stable digester performance. As the micro-organisms in the culture grow, a balance between the groups of organisms performing several metabolic functions is maintained. The biomass loading rate is gradually increased until the planned feed loading is achieved. The rate of start-up is affected by the characteristics of the inoculum, the biomass feed, the configuration and size and operating conditions of the digester. It takes sometimes up to 2-3 months.

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The input and output of the digester identify the technique used in the biogas plant and sometimes require additional treatments. For example, fermentation of manure does not require additional treatment (Weiland, 1999). On the other hand, co-fermentation and industrial or agricultural wastes require supplementary pretreatment such as sorting, chopping, shredding, sieving, pasteurization before they can be fed to the digester. The end product, biogas, can contain corrosive and disturbing components such as water vapor, ammonia and hydrogen sulfide. The use of biogas defines the degree of elimination which ranges from null, i.e., direct use (cooking) to total removal (in fuel cell). A summarized category of several techniques used in biogas plants is shown in Table 2.

Table 2: Technical options in biogas plants Stage

Technique option

Preparation of Wastes

· Shredding · Sieving · Sorting · Pasteurization · One-/two step fermentation · Meso-/thermophilic fermentation · Dis /continuous fermentation · Wet-/dry fermentation Effluent · Dewatering · Composting Biogas · Condensate removal · Sulfur removal

Fermentation

Preparation of Products

Source: Weiland, 1999

There are also serious safety and environmental considerations associated with biogas plants because methane is a potent greenhouse gas and forms explosive mixtures when mixed with air. This can be achieved by installation of sensors and safety equipment. Biogas flares are also used to safely burn biogas that is surplus to the demand of energy recovery plant or where recovery plant fails.

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2.4.2 Types of biogas reactors Anaerobic digesters (reactors) are made out of concrete, steel, brick, or plastic. They are shaped like silos, troughs, basins or ponds, and may be placed underground or on the surface. There are numerous designs and configurations of anaerobic digesters, and each has its advantages and drawbacks. There are two basic types of digesters: batch and continuous. Batch-type digesters are the simplest to build. Their operation consists of loading the digester with organic materials and allowing it to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated. In a continuous digester, organic material is constantly or regularly fed into the digester. Unlike batch-type digesters, continuous digesters produce biogas without the interruption of loading material and unloading effluent. They may be better suited for large-scale operations. There are many types of continuous digesters; the most common types are: continuously stirred tank reactor (CSTR), up-flow anaerobic sludge blanket (UASB), plug-flow reactor, attached film reactors (anaerobic filter, fluidized-bed), two-phase reactor. The most commonly used reactors in agriculture are batch reactors, continuously stirred tank reactors and two phase reactors. Therefore, only these types will be described briefly.

2.4.2.1 Batch reactor The batch reactor is the simplest type of digestion used. It represents a non-continuous process that is particularly suited for seasonally produced biomass feeds and for feeds with very high solids content. In batch processes, feed is added to an inoculum derived from a previous digester or other source and then placed in the batch reactor and allowed to digest anaerobically over a period of time depending upon the feed material. The fermentation is allowed to proceed until gas production stops or becomes negligible. The reactor contents are usually heated and maintained at the desired temperature; they are agitated occasionally and this releases bubbles of gas from the feed material.

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A batch reactor is very simple to run, since little attention needs to be paid to it between starting up and emptying out. The main use of batch reactors is to assess the digestibility of a particular waste before a full-scale unit is built. A major disadvantage of batch digestion is that it is relatively unstable and uncontrollable due to change in the bacterial population during the course of the fermentation. These changes can lead to population imbalance, digester failure, and variations in the quantity and composition of product gas.

2.4.2.2 Continuously stirred tank reactor The continuously-stirred tank reactor (CSTR) is the most common design used in wastewater and farm applications treating feeds with > 3 % solids. It is an adaptation of a batch reactor in which the substrate is continuously pumped into the reactor while the reaction mixture is removed at the same rate. The CSTR is usually heated, mixed continuously or intermittently, and fed intermittently rather than continuously. Since it is possible in such a system that some substrate molecules can move through the reactor unchanged (washout of unreacted solids and active micro-organisms at higher loading rates), the gas yield will usually be lower than in a batch process. By mixing, a good contact between biomass and the organic material to be digested is maintained. CSTR is the simplest among the continuous reactors and has the following advantages: uniform distribution of the substrate throughout the digester, good control of process parameters may be established, prevention of scum layer formation when properly mixed, easily modeled and does not require high operating costs.

2.4.2.3 Two-phase reactor Generally, anaerobic digestion is carried out by two groups of bacteria (nonmethanogenic and methanogenic) that differ significantly with respect to their requirements (Chynoweth, et al., 1987). In a one-phase reactor (e.g., CSTR), all the bacteria groups are working in the same reactor in which the reaction condition is not ideal for each bacteria. In two-phase reactor the hydrolysis/acid formation and methane formation are physically separated so that each reaction can take place under optimal conditions.

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If the hydrolysis is the rate-limiting step, the first phase must be optimized so that the hydrolysis have enough time for the decomposition of biopolymers. In case of light hydrolyzed substances the first step occurs quickly before the generation of the methanogenesis and the separation between the two processes can be achieved. Otherwise, from economical consideration, the two phase process will not be optimal (Edelmann, 2001). The two-phase technology has numerous benefits: hydrolysis and acidification occur quicker than in conventional systems; the common problems of foaming in single-tank systems are reduced by the destruction of biochemical foaming agents before they reach the methane forming reactor; and the biogas produced is typically rich in methane. On the other hand, the two-stage systems are the most complex, and most expensive, of all systems.

2.4.3 Purification of biogas The use of the product gas will determine, in most cases, the extent of gas cleanup. A natural gas substitute fuel would have high requirements including moisture removal, carbon dioxide and hydrogen sulfide scrubbing. The requirements of using biogas in a fuel cell (low temperature) is higher and include even the removal of slight concentration of irrelevant gases. Hydrogen sulfide, H2S, is the most important factor limiting the utility of biogas as fuel. It is highly corrosive to metal, particularly copper, iron, and mild steel (Constant, et al., 1987). Moreover, biogas is usually saturated with water vapor, and humidity is an important factor for accelerating the rate of corrosion. Hydrogen sulfide can be oxidized into sulfur oxides, SO2 and SO3, during biogas combustion and by reaction with water vapor leads to the formation of sulfurous, H2SO3, and sulfuric acid, H2SO4. These acids are also corrosive to metals especially when, after condensation, they accumulate locally. The removal of H2S may also be necessary because of its own toxicity (1000 ppm H2S is fatal). The moisture content of the biogas depends on the temperature of the gas as it is collected. Generally, the biogas leaves the methane digester saturated with water. Accumulation and freezing of water has to be systematically avoided throughout the whole system. Also, when biogas is compressed, water removal is required to prevent the storage devices from corrosion. The removal of carbon dioxide for some applications is not necessary because its elimination will only increase the biogas calorific value. However, its removal is essential for use, as fuel for automobile or feed to natural gas networks, for quality assurance and volume reduction

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(Werner, 2001). Whenever biogas is compressed, carbon dioxide has to be eliminated because it becomes a highly corrosive agent under these conditions. In general, biogas can be purified by biological, chemical, physical and physical-chemical methods. Table 3 lists the techniques that can be used for the treatment of biogas in relation to the compound to be removed.

Table 3: Overview of techniques used for biogas treatment Compound removed

Technique

Principle

H2S

Air oxygen dosing FeCl3 dosing to digester slurry Adsorption to Fe2O3 pellets Absorption with caustic solution Absorption with iron solution Membrane separation Biological filters Activated carbon Molecular sieves Demister Cyclone separator Moisture trap Water tap Adsorption to silica Glycol drying unit Pressure swing adsorption Membrane separation Absorption techniques

biological chemical physical-chemical physical-chemical physical-chemical physical biological physical-chemical physical physical physical physical physical physical physical physical-chemical physical physical-chemical

Water

CO2

Source: Schomaker, et al., 1999

2.4.4 Storage of biogas Methane can only be liquefied at temperatures lower than –82.5°C (critical point), therefore, the suitable form for storing methane or biogas is in a gas form that needs large space. A biogas plant generally includes a gas storage system for balancing out the fluctuations in gas production, quality, and consumption. For economical reasons, the volume of the gas holders is usually limited to daily production of biogas.

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The gas storage devices can be classified according to their running pressure. In low pressure gas holders, the gas is kept at a pressure below the maximum of 50 mbar. A common example for low pressure gas holder is a gas bag and digester head space with a foil membrane. Their volume capacity ranges between 1 and 1000 m³ (Edelmann, 2001). High pressure gas holders (separate gas cylinders or tanks) are usually used when biogas has to be compressed and used as fuel. In 30 to 50 liter cylinders biogas can be compressed to a pressure of 200 to 300 bar (Edelmann, 2001). Moreover, a drastic purification of the biogas is necessary to avoid the corrosive action. A floating digester cover can also be used for gas storage as well as for gas collection. This is simply a pontoon cover which floats on the liquid surface and has skirt plates extending down into the liquid to provide a seal. The weight of the floating cover provides a pressure head and allows the gas to be withdrawn as it is needed.

2.5 Biogas as energy source

2.5.1 Gas composition and quality Biogas composition is dependent on the type of feedstocks and to some extent on the technique used in the digestion process (Weiland, 2000a). The feedstocks used for anaerobic digestion vary considerably in composition, homogeneity and biodegradability. Biogas is primarily composed of methane (CH4) and carbon dioxide (CO2), with smaller amounts of hydrogen sulfide (H2S) and ammonia (NH3). Slight concentrations of hydrogen (H2), nitrogen (N2), carbon monoxide (CO) and oxygen (O2) are occasionally present in the biogas. Moreover, the biogas is usually saturated with water and might contain dust particles. CH4 gas is considered as a valuable fuel. The gas is non-toxic, non-odorous, and is lighter than air. When burned, CH4 is converted into a molar equivalent amount of CO2 and water. CO2 is an inert colorless, odorless gas and is heavier than air. CO2 is mildly toxic, is an asphyxiate. A higher CO2 concentration in the biogas results in a lower calorific value of the biogas. H2S is a colorless gas. Since H2S is heavier than air, it might cause extra danger at low levels. At low concentrations this gas has the typical smell of rotten eggs. In addition to its toxicity H2S is corrosive, which can cause problems during combustion of the biogas. Water vapor, although a harmless product, becomes corrosive in combination with the NH3, CO2 and especially the H2S of the biogas. The maximum water content of the biogas is governed by the gas temperature.

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When water saturated biogas leaves the digester, cooling of the gas will result in condensation of water. The quality of biogas is determined from the percentages to these constituents. Table 4 shows some characteristics of biogas components.

Table 4: Characteristics of biogas components Characteristics Calorific value (kWh/m³) Ignition temperature (°C) Critical pressure (bar) Critical temperature (°C) Density (kg/m³) Density relation (gas/air) Flame speed (cm/s laminar)

CH4

CO2

H2 S

10 650 47 -82,25 0.72 0.55 47

75 31 1,98 1,5 -

6,3 270 90 100 1.54 1,2 73

Natural gas 10 650 0.7 0.54 39

Biogas (65% CH4) 6 700 75-89 -82,5 1.2 0.9 25

Source: Beitz, 2000 / Anonym, 1998

2.5.2 Biogas application Biogas has numerous end-use applications compared to other renewable energy resources. It has a calorific value typically between 50 % and 70 % of natural gas. The calorific value of biogas ranges between 17 and 25 MJ/m3 depending on the amount of methane in biogas. Biogas can be used for cooking (in developing countries), heating, mechanical power generation, and electrical power generation. Treated biogas can also be supplied to the local natural gas net. Biogas can be burned in boilers with high efficiency (79 %) to produce hot water and steam used for other industrial uses. Two types of biogas engines are in common use for electricity generation; internal combustion engine and gas turbine engine. In either case, the engine drives a generator which produces an alternating current for use locally or to supply the national grid. For the electrical use of biogas only 35 % of the energy value is converted to electrical energy, the remaining 65 % is thermal and waste energy. An excellent way to extract the maximum benefit from biogas is to make use of the waste heat that results from generating electricity, using a combined heat and power (CHP) plant. Waste heat recovery can increase the energy efficiency of the system by 40 to 55 percent to reach 85 %. This heat can be used to heat the digester and/or provide hot water or space heat to the facility.

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Ongoing research and development is focusing on the use of stirling engine, microturbines and fuel cells for converting biogas to electricity. Stirling engine may be used in CHP plant to increase the overall efficiency up to 95 %. Microturbines are high-speed, small-scale (typically less than 100 kW) gas-driven turbine systems that produce electricity efficiently, have low emissions and require little maintenance. Fuel cells can convert fuel to electricity at efficiencies close to 40 %. These techniques are still under research and are expensive. In the case of direct-gas use, water and H2S removal will be the appropriate treatment steps. If the gas is to be used by gas engines, or if it is upgraded to a natural gas quality, the biogas composition should comply with the appropriate requirements. For upgrading biogas to natural gas quality removal of CO2, H2S, NH3, water and dust is essential in order to achieve the required quality. If it is worthwhile to install a digester, it is equally worthwhile to find the most efficient use for the gas. Obviously this depends in the first instance on how much gas is produced. At the same time as considering the uses for the gases, we should take stock of all fuel requirements in order to find the least wasteful system of energy use.

2.6 Principles of system evaluation Performance evaluation is one of the core activities of mankind. Some time ago the performance of any system was mainly evaluated only from the economical side of view. Nowadays, it is essential to consider energetic, ecological and social aspects for evaluation, even from the economical point of view. The social evaluation will not be considered in the thesis.

2.6.1 Description and methods of analysis systems The evaluation and analysis of systems with energetic and ecological balances is not simple (Moerschner, et al., 1997). Hartmann, et al., (1995) referred this to many reasons. First, the methods of calculation and analysis are not clearly defined and standardized. Secondly, are the different values of various energy inputs and outputs. For example fuel and electrical energy can not be directly compared with each other. Moreover, assessment of by-products in biomass processes is problematic, especially when the by-product is not used in an energetic form but as fertilizer.

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The first important step in evaluating any system is to define its boundaries. Through the selected boundaries can be determined whether a parameter could be considered in the balancing or not, which can strongly affect the balancing results. Reinhardt (1993) classified the system boundaries into subjects, time, and space limitations. The subject limitation is given through the considered element (material, energy, information). The time limitation defines the time interval of the input and output variables (vegetation period). The space limitation keeps the system with in a defined space (continent, region, field, etc.). In establishing evaluation it is also important that the comparable energy sources have the same end use, especially when comparing bioenergy fuel with fossil fuel. For example, electricity production from biogas has to be compared with that fossil fuel from which the same amount of electricity is produced. Moreover, within the scope of bioenergy source production, it is needed to evaluate the by-products. Normally, it is aimed to assess the by-product within the life-cycle of bioenergy production.

2.6.2 Energy balance Energy production and use are normally presented in the form of an energy balance in which the individual energy sources are shown in a way that they can be compared and added on the basis of physical aspects such as calorific value. A major aim of energy analysis is to estimate the total quantity of energy required to produce a good or service to a final user. The simplified way of energy balance takes into account only the efficiency of converting the main stream of direct production energy into useful energy losing in the converter of energy losses as a part of direct production energy (FAO, 1984). This method is usually applied when investigating simple mechanisms. The general way of balance takes into account all components of energy balance creating a complex chain of energy conversion. This can be achieved in terms of energy outlay (output) and energy gain (input). The growing of crops as a source of energy or fuels is potentially a renewable and sustainable system of harnessing solar energy. However, this potential is realized only if the energy obtained from the crop material exceeds the fossil fuel energy inputs used in growing the crop and in any processes used to convert it to a desired fuel form. This thesis will emphasize energy balancing for agricultural crops.

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Energy input In the determination of the energy input of a process, not only the raw materials for energy gain, but also all the basic material within the considered system will be considered (Reinhardt, 1993) i.e., the total production chain of direct and indirect energy sources inside that system. According to Heyland & Solansky (1979) energy inputs can be differentiated for direct and indirect types. The energy flux scheme in Figure 4 clarifies the system boundaries selected for the included field cropping experiments and the resulting energy fluxes. In modern agriculture, human power is insignificant relative to the total energy use. Therefore, it will not be considered in energy balancing analysis in this thesis. Since the solar energy is renewable and free it will also not be taken into account.

Energy Input Direct

Energy Output Indirect

Solar energy Seeds PPA Diesel oil

Min. Fertilizer Org. Fertilizer

Main product By-product

Fuel oil Machines Electricity

Losses

Equipments Buildings Other plants

Human

Source: Kalk, et al., 1995

Figure 4: Energy flux scheme

Direct energy input Direct energy used in crop production includes electricity, heating fuel and machinery fuel used in crop production, grain drying, transportation of farm products and personal energy use (Biermann, et al., 1999). Two methods can be used for evaluation of direct energy sources. They can either be evaluated only through their calorific value for the generation of efficient

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energy (Werschnitzky, et al., 1987). In this case losses occurring during the conversion of primary energy into the final energy form are not considered. Or they can be evaluated by considering all energies involved in the production and handling process of the used direct energy source (Reinhardt, 1993). For example, for diesel oil as energy source Reinhardt (1993) assumed an additional calorific value of 11.5 % for diesel production chain. Tables 5 and 6 show some energy sources and their calorific values.

Table 5: Mean calorific value of fossil energy sources Energy source Hard coal Lignite coal Gasoline Diesel Natural gas Lubricant

Calorific value [MJ/kg] 29.78 8.49 43.54 42.71 50.2 54

Electricity

Density [kg/l] 0.8-0.9 0.7-1 0.72-0.8 0.84 0.79 (kg/m³) 0.893 3.6 MJ/kWh

Source : Beitz, 2000 / Patyk, et al., 1997 / Mauch, 1996

Table 6: Mean calorific value of some bio-energy sources Energy source Biogas Methane Ethanol Rape oil RME

Calorific value [MJ/kg] 17.9-20.8 37.82 26.8 36.0 37.2

Density [kg/m3] 1.05-1.2 0.72 0.79 (kg/l) 0.95 (kg/l) 0.88 (kg/l)

Source: Hartmann, 1995 / VDI, 2001

The amount of direct energy used (mainly fossil fuel) depends on the types of equipment (indirect energy) used, since they determine their consumption. Fossil fuel is used in agriculture for irrigation, machinery (for whole growing process; including fertilizers, harvest), transport, storage and food processing. It is a major part of energy input. Because of the strong variation in the machinery, even for the same work, used in agriculture and because of the different ground type and ground situation it is not possible to summarize the fuel consumption 45

throughout the field work in a standardized form. So, in establishing energy balance each field must be analyzed separately according to the plant and machinery present. The diesel consumption of different tractors for different applications is shown in Table 7.

Table 7: Diesel consumption of different tractors and applications Power (kW)

32a

Application Hard field work Normal field work Simple field work Transportation Running without load

6.21 3.69 2.01 3.05 0.70

52b 60c Diesel consumption [kg Diesel/h] 8.76 10.03 4.98 5.51 2.52 2.99 3.97 4.14 0.69 0.86

74a

14.84 9.48 6.54 8.06 1.15

Source: Gräf, et al., 1994a / Krahl, 1993b / Wörgetter, et al., 1993c

Indirect energy input In addition to energy consumed directly in agriculture, many farm inputs have indirect energy components. Indirect energy consists of the energy used in the production of most of the farm inputs paid for by operators, including commercial fertilizers, plant protection agents, machinery, seeds, transportation, farm buildings. Fertilizers fall into two categories: organic and inorganic. Organic fertilizers are primarily crop residues, manure and organic waste. Inorganic fertilizers are man-made compounds based on the essential elements; nitrogen, phosphorus and potassium. Energy consumption for mineral fertilizers has a great affect on agriculture energy balancing. According to Mudahar, et al., (1987), energy used in the production of fertilizers accounts for about 40 % of total energy used in agricultural plant production in developed countries. Most of this energy was consumed in the production of nitrogen, phosphorus and potassium fertilizers. Another recent study (Biermann, et al., 1999) reported that the portion of mineral nitrogen fertilizer may come up to 50 % of the whole input of fossil energy. Nitrogen fertilizer is by far the most important chemical fertilizer in terms of the amount of plant nutrient used and even more so in terms of energy requirements. Data about the energy consumption of mineral fertilizers differs widely. Average energy requirements for production of individual fertilizers are reported in Table 8.

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Table 8: Energy input in fertilizer production Fertilizer Nitrogen Phosphorus Potassium Lime

Energy consumption [MJ/kg] Only production Total * 42.4 49.1 8.73 17.7 8.41 10.5 1.03 2.39

Source: Patyk, et al., 1997, * include supply and transportation

Little information is found in literature about the energetic evaluation of organic fertilizers other than for mineral fertilizers (Hülsbergen, et al., 1997). The simplest and most common means of estimating the value of organic materials is to consider them as substitutes for mineral fertilizers. When the organic wastes of acceptable quality are returned to agricultural soils on a regular basis they increase not only the soil fertility but also contribute greatly to the overall maintenance of soil structure and productivity, and reduce the need for mineral fertilizers (Helsel, 1987). Such values are often highly variable depending on the origin of the material and the mode/method of processing and storage. Plant protection agents (PPA) – herbicides, insecticides and fungicides – are used to protect agricultural crops. They essentially substitute for human and mechanical labor otherwise needed to control weeds, insects and fungus. Their manufacturing requires considerable energy. Green (1987) calculated mean energy consumption values for various PPA groups: 265 MJ/kg for herbicides, 214 MJ/kg for insecticides and 173 MJ/kg for fungicides. According to Gaillard, et al., (1997), each plant protection agent should be evaluated separately. Although the total energy content of PPA is about several times more than that of fertilizers, their application rates per hectare is very small and range between 0.3 to 4 liter (Kaltschmitt, et al., 1997). Therefore the total energy input for application of PPA is clearly less than that of fertilizer. The energy input for seed material includes energy used for breeding, production, transportation and storage. It varies within a wide range depending on the type of seed. For example 0.7 MJ is needed for 1 kg potato while for 1 kg sugar beets 30.14 MJ is used (Kaltschmitt, et al., 1997). Since the energy balance for agriculture is usually expressed per hectare, the amount of seeds per hectare play an important role in the energy input for seeds.

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Energy input for machinery manufacturing covers the whole energy used for technical construction, equipment and tractors. Due to the variation in the production it is not simple to estimate a generalized value. The energy needed for a finished part step alone ranges between 3.96 and 10.08 GJ/ton finished product (Oheimb, 1987). According to Diepenbrock, et al., (1995) the part of indirect energy input for agricultural building is not easy to estimate.

Energy output Generally, energy output includes all products that leave the system. According to Heyland, et al., (1979), agricultural outputs can be divided into main products, by-products and residues. The energy output can by categorized under these groups. Residues are that part of output products from the main- or/and by-product that can not be used. This part is considered as losses. The energy in the main- and by-product is produced in three forms, namely: food, feed and renewable energy resource. These can be used directly or after some management (Figure 5). Food is normally produced only as a main product whereas feed and renewable energy are produced in agriculture as main- and by-products.

Main product food direct use

forage

ren. energy direct use

ensiling

treatment

treatment use

use by-product

use by-product

Fertilizers

Figure 5: Energy output scheme

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by-product

The energetic value of agricultural products depends mainly on their constituents which differ from one plant to another. The main contents of agricultural products and their calorific value are shown in Table 9. Food as an output source of energy can be used directly by humans or after being treated (e.g., vegetable oil). In the latter case, additional energy will be required to produce the final output. In this case, the main agricultural product will considered as a subproduct (stage) for the final output and the additional energy needed for the treatment must be regarded as additional input energy. Feed and renewable energy sources can also be used directly or after additional treatment (Figure 5). The energy output from renewable biomass energy can be in the form of heat, mechanical power and electricity.

Table 9: Physical calorific values of main biomass constituents Constituent Protein Fat Fiber Nitrogen-Free extract (NFE) Lignin

Calorific value [MJ/kg] 23.04 38.96 17.59 17.17 26.39

Source: Greef, et al., 1993

2.6.3 Ecological balance Ecological assessment is an important component of any product system by assessing the impacts of those energies and materials uses and releases to the environment. The assessment includes the entire life cycle of the product or activity, encompassing extracting and processing raw materials, manufacturing, distribution, use, re-use, maintenance, recycling and final disposal, and all transportation involved (Lindors, et al., 1995). A standardized technique for measuring and comparing the environmental consequences of providing, using and disposing of a product is the Life Cycle Assessment (LCA). LCA addresses environmental impacts of the system under study in the areas of ecological systems, human health and resource depletion. It does not address economic or social effects.

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LCA has become a widely recognized method in the public and private sectors for analyzing and assessing the environmental performance of product and process systems. According to ISO 14040 (1997) the LCA shall include definition of goal and scope, inventory analysis, impact assessment and interpretation of results, as illustrated in Figure 6.

Goal and scope definition Direct applications: Inventory analysis

Interpretation

- Product development and improvement - Strategic planning - Public policy making - Marketing - Other

Impact assessment

Source: ISO 14040, 1997

Figure 6: Phases of life cycle analysis (LCA)

Definition of goal and scope The definition of the goal and scope is the critical part of an LCA due to the strong influence on the results of the LCA. They contain the following main issues: goal, scope, functional unit, system boundaries, and data quality. The goal definition has to define the intended use of the results and users of the result. The definition of the scope sets the limits of the assessment - what is included in the system and what detailed assessment methods are to be used. The functional unit determines equivalence between systems. Its primary purpose is to provide a reference to which the inputs and outputs are related. The system boundaries define the processes/operations (e.g., manufacturing, transport, and waste management processes), and the inputs and outputs to be taken into

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account in the LCA. The input can be the overall input to a production as well as input to a single process - and the same is true for the output. According to Patyk, et al., (1997) the system boundary defines also the geographical and time related-boundaries. The quality of the data used is naturally reflected in the quality of the final LCA. It is important that the data quality is described and assessed in a systematic way that allows others to understand and control for the actual data quality.

Inventory analysis According to ISO 14041 (1998) a life-cycle inventory consists of data collection and calculations to quantify the inputs and outputs of a product life-cycle. The data have to be collected from all single processes in the life cycle. These data can be quantitative or qualitative. The quantitative data are important in comparisons of processes or materials, but often the quantitative data are missing or the quality is poor (too old or not technologically representative etc.). Life-cycle inventory data are often incomplete or inaccurate, largely because the resources necessary to gather high-quality data are not available. Sometimes inventory data cannot be obtained because it is proprietary. Sometimes the inventory data that is available is aggregated so that air and waterborne emissions are quantified as totals rather than given as quantities for individual constituents, making a thorough impact assessment impossible. Moreover, the data are subject to obsolescence; the use of obsolete data can therefore cause distortions. Another source of uncertainty in life-cycle inventories is the use of "average" or "typical' values instead of values specific to a facility or region. For example, when evaluating the energy requirements of a product, both the raw materials and the inputs and outputs from the generation and use of the energy flow must be included. The inputs and outputs of energy generation vary widely among the different methods of power generation.

Impact assessment In assessing impacts, life-cycle inventory data are combined with information on the environmental characteristics of the inputs and outputs for the product's life cycle, so that the varying potency of effects from different emissions are accounted for. It is also a follow-up of the decisions made in the goal and scoping phase. 51

Impact assessment contains many categories (ISO, 14042). They are selected in order to describe the impacts caused by the considered products or product systems. The impact categories considered are: ·

Resource depletion

·

Land use

·

Global warming

·

Stratospheric ozone depletion

·

Acidification

·

Eutrophication

·

Human- and ecotoxicological impacts

·

Photochemical oxidant formation

Resource depletion: The current generations are using up resources that will make future generations unable to develop or maintain a quality of life equivalent to our own. Resource depletion includes water resources, fossil fuels, and mineral resources. The issue here is whether the resources used in producing agricultural products can be replaced or renewed. Fossil fuel burning is a classic example of a resource that cannot be replaced. On the other hand, water used for irrigation and other uses can be, to some extent, replaced.

Land use: Land use and transformation can be seen from two perspectives (Finnveden, 1996): ·

Land as a resource for humans, i.e., area for food production

·

Land use related to ecosystem and landscape degradation, landscape fragmentation, desiccation, habitat alterations and impacts on, e.g., biodiversity

It is an unavoidable effect of agriculture that we replace natural ecosystems with crops. Without question, agricultural activity has led to the loss of more species and habitats than any other human activity. We need to farm to feed our populations, but there are ways to help maintain species diversity and healthy ecosystems in agricultural settings.

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Greenhouse effect: The greenhouse effect is a result of reflecting the heat radiation by carbon dioxide (CO2), and other greenhouse gasses (e.g., methane (CH4), dinitrogen oxide (N2O), chlorofluorocarbons etc.). The problem with greenhouse gases is that over the past hundred years (since the industrial revolution), the concentration of greenhouse gases in the atmosphere, especially CO2, has increased significantly. That is because we are burning lots of fossil fuel to produce power and heat. The possible consequences of the greenhouse effect include an increase of the temperature level leading to melting of the polar ice caps, resulting in elevated sea levels. The increasing temperature level may also result in regional climate changes. The greenhouse effect is normally quantified by using global warming potentials (GWP) for substances having the same effect as CO2 in reflection of heat radiation. GWP for greenhouse gases are expressed as CO2-equivalent, i.e., their effects are expressed relative to the effects of CO2. GWP for some known greenhouse gases are shown in Table 10.

Table 10: Global warming potentials (GWP) given in kg CO2-eq./kg gas Substance

Formal

Carbon dioxide Methane Dinitrogen oxide CFC-11 HCFC-22 HFC-23

CO2 CH4 N2O CFCl3 CHF2Cl CHF3

20 years 1 62 290 5000 4300 9200

GWP 100 years 1 25 320 4000 1700 12100

500 years 1 7.5 180 1400 520 9900

Life time, years 150 10 120 50 13 250

Source: Albritton, et al., 1996

Many of the greenhouse gases that derive from agricultural products come from the use of fossil fuels to provide power and transportation, not only on-farm but also in food processing and storage. But there are also significant on-farm sources of greenhouse gases. Ruminants such as cows, goats and sheep produce large quantities of methane as a by-product of their digestion. Methane and dinitrous oxide also come from rice fields and from manure management practices.

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Stratospheric ozone depletion: The thin layer of ozone protects all life on earth from the harmful effects of ultra-Violet radiation (UV radiation). Too much UV radiation causes skin cancer and cataracts, and also is very detrimental to plants. There are several man-made compounds (freons and other halogenated compounds) that act to destroy ozone in the atmosphere. These compounds are usually used in refrigeration (many agricultural products are refrigerated to maintain their freshness) and in fire suppression, although they also have some other applications as solvents. Over the years, the release of these compounds through leaks in refrigeration systems and the like has led to the creation of ozone holes at the North and South Poles. Just like greenhouse gases, a scale has been developed that compares the strength of different ozone depleters to a standard, in this case to CFC-11.

Acidification: Acidification is the process by which acid gases are deposited in air. This deposition can be as dry deposits, or it can be as rain, snow, fog or other precipitation. Acidification alters soil chemistry, leading to toxic effects on plants. It also can cause lakes and rivers to become acidified, killing many of the organisms that live there. The potential effects are strongly dependent on the nature of the receiving ecosystem. Some soils have a high neutralization capacity, and relatively little occurs as a result of acid deposition. Some soils have very little soil neutralization capacity, and cause the dying off of trees and other plants.

Table 11: Acidification potentials (AP) for acidifying substances Substance

formula

Sulfur dioxide Sulfur trioxide Nitrogen dioxide Nitrogen oxide Hydrogen chloride Nitric acid Sulfuric acid Phosphoric acid Hydrogen fluoride Hydrogen sulfide Ammonia

SO2 SO3 NO2 NO HCL HNO3 H2SO4 H3PO4 HF H2S NH3

Source: Hauschild, et al., 1998

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AP kg SO2/kg 1 0.80 0.70 1.07 0.88 0.51 0.65 0.98 1.60 1.88 1.88

Acid gases are primarily derived from combustion processes in transportation and in heating and electricity generation. Lindfors, et al., (1995) recommend that the following substances should be considered: SO2, NOx, NH3 and HCl but other substances having a proton releasing effect also have to be considered (Table 11). The acidification potential (AP) can be estimated as SO2 equivalents. The acidification potentials for acidifying substances are given in Table 11.

Eutrophication: Eutrophication is the enrichment of aquatic ecosystems with nutrients leading to a deterioration of the water quality and a reduction in the value of the utilization of the aquatic ecosystem. Eutrophication of aquatic and terrestrial ecosystems can be caused by surplus nitrogen, phosphorus and degradable organic substances. The primary effect of surplus nitrogen and phosphorus in aquatic ecosystems is the growth of algae. The secondary effect is the decomposition of dead organic material (e.g., algae) and anthropogenic organic substances. The decomposition of organic material is an oxygen consuming process leading to decreasing oxygen saturation and sometimes anaerobic conditions. The anaerobic conditions in the sediment at the bottom of lakes or other inland waters may furthermore result in production of hydrogen sulfide (H2S) which may lead to incidents and liberation of toxic hydrogen sulfide to the surrounding water. Excessive use of fertilizers on farms, and poor manure management mean that nutrients are released from the farm into the air and the waterways.

Human- and ecotoxicological impacts: These impacts are exactly what they sound like. They are based on the release of toxic substances into the air or the water. The approach is to calculate the concentrations of toxic substances in air and water at the property or field boundaries and report both individual substances exceeding the no-effects level. Although every toxic substance has a different mechanism of action, and different responses in different species, there is no consensus in the scientific literature as to how to combine these different effects. Human toxicological impacts depend on exposure to and effects of chemical and biological substances. The potential effect on humans depends on the actual emission and fate of the specific substances emitted to the environment as for ecotoxicological impacts. Human toxicological effects can be: acute toxicological effects, irritation, allergenic reactions, genotoxicity, carcinogenicity, neurotoxicity and teratogenicity.

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Photochemical smog: Smog results from the action of sunlight on volatile organic compounds in the presence of nitrogen oxide (NOx). The reactions are complex, but the outcome is the creation of ozone and other noxious chemicals ("smog" as a local impact and "tropospheric ozone" as a regional impact). The limiting factors for the production of smog in agricultural areas are the presence of sunlight and nitrogen oxides. Nitrogen oxides come primarily from the burning of fossil fuels (in farm). Ozone is toxic to all life. It causes mutations that can lead to cancer and to birth defects and premature aging. Exposure of plants to ozone may result in damage of the leaf and finally the whole plant. Exposure of humans to ozone may result in eye irritation, respiratory problems, and chronic damage of the respiratory system.

Interpretation Interpretation is the fourth phase in life cycle assessment. Its aim is to reduce the number of quantified data and/or statements of the inventory analysis and/or impact assessment to the key results to facilitate a decision making process based on, among other inputs, the LCA study. This reduction should be robust to uncertainties in data and methodologies applied and should give an acceptable coverage and representation of the preceding phases. Interpretation contains the following main issues (ISO 14043, 1998): ·

Identification of significant environmental issues

·

Evaluation

·

Conclusions and recommendations

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3

Materials and Methods

This chapter provides the details of the methods and materials used to accomplish the objectives of this research. The experimental approach is explained first. Secondly, the methods used for the evaluation of the energetic and ecological balance are described.

3.1 Laboratory experiments The laboratory investigations in this thesis include the operation of bench-scale anaerobic digestion systems under controlled laboratory conditions. Two main experiments were conducted in the laboratory of the Institute of Technology and Biosystems Engineering of the Federal Agricultural Research Centre (FAL), Braunschweig-Germany. The first experiment has been carried out with forage beets silage (FBS). In the second experiment sugar beets silage (SBS) were examined. Both experiments were run approximately in the same manner. Most chemical analyses were performed according to standard methods.

3.1.1 Type of reactors used Within the scope of this thesis two different types of reactors were used, namely discontinuous reactors (batch reactors) with a total number of 17 reactors and 16 semi-continuous reactors. 11 batch reactors and 12 semi-continuous reactors have been carried out under mesophilic temperature (37 °C) and the rest were operated under thermophilic temperature (55 °C).

3.1.1.1 Batch reactors The mesophilic batch experiments were performed in 30-liter glass reactors. The reactors had a cylindrical shape (inside diameter is 25 cm) and a half sphere at the top of the reactor (the height of the half sphere is 7 cm) with a gas sampling port at the center. The biogas generated in the reactor was collected via a tube attached to the gas sampling port in a special gas bag from TESSERAUX company. A sketch of this batch test setup is illustrated in Figure 7.

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Gas sampling port

Gas bag

Figure 7: Batch reactor

The reactors were filled with substrate only at the beginning and covered gas-tight with the half spherical cover. Then they were placed in a temperature controlled chamber (37 °C). To keep the temperature for the batch experiments within the thermophilic range of 55 °C semicontinuous reactors with water jacket were used as batch reactors which were filled only once at the beginning. The water jackets were connected to the thermostat heated to 57 °C. Biogas production and composition were also measured. The batch experiments used for the determination of maximum possible degradation efficiency and methane yield.

3.1.1.2 Semi-continuous reactors For semi-continuous experiments 20-liter acrylic glass reactors were used. The reactors had a cylindrical shape (inside diameter is 24 cm). The biogas generated in the reactor was collected via a tube attached to the gas sampling port using similar gas bags to that used for batch experiments. A sketch of a semi-continuous reactor is illustrated in Figure 8.

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Agitator

Gasbeutel

Gas bag

Waste sampling port

Gas sampling port Feeding port

Treatment unit

Figure 8: Semi-continuous reactor

Feeding of new material and withdrawal of digested substrate (or sample for analysis) were done through the sampling port on the top of the reactor (Figure 8). The sampling port was installed so that its lower end was placed under the treated substrate surface to prevent air from entering into the digester. The temperature in the reactor was kept within the required range (37 °C or 55 °C) by using two methods; by placing the reactors in temperature controlled room and by using water jackets (circulating hot water around the outside of the reactor). In the temperate room it was only possible to achieve the mesophilic temperature range (37 °C). The reactor content was mixed using electrical motor. The stirrer was placed into the reactor through the cover on which a water cup was installed to prevent the entrance of air. Programmable timers were used to control the operation. Samples were taken periodically for substrate analysis.

3.1.2 Necessary analysis Several types of analyses were performed to determine the stability and performance of anaerobic digestion processes: these include gas production and composition, reduction in organic matter, pH, alkalinity, volatile acids, and other parameters such as oxidation-reduction potential, FOS/TAC, phosphorus concentration, and ammonium concentrations.

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The analyses were used for samples, including analysis of reactor effluent samples as well as characterization of the feed substrates. Analytical methods from Standard Methods for the Examination of Water and Wastewater (Deutsche Einheitsvorschrift, 2001) were used in most cases. The description of the analytical techniques employed in the determination of the parameters mentioned are discussed in the following sections.

3.1.2.1 Gas production and composition The ultimate products of biomethanogenesis are methane (CH4), carbon dioxide (CO2) and small amount of hydrogen sulfide (H2S). Gas production and composition are an indicator of microbial activity. Methane content is probably one of the best indicators of overall digester performance and its economic efficiency. During stable digestion with a non-variable feed, relatively stable gas production with a predictable methane content can be expected. When gas production or composition is not stable, there is an indication of digester instability. Low methane content is an initial indicator of the inhibition of methanogenic bacteria, as the amount of gas produced and its composition are relatively simple to measure. The effect of H2S was already discussed in (2.2.2.5). High amounts of H2S is an indicator for the accumulation of toxic sulfide which affects the methanogenesis. Since CH4, CO2 and H2S constitute the main components of biogas (with small amount of trace gases < 1 %), decrease in the total measured percentage (< 95 %) will indicate a leakage in the system. The biogas produced in the experiments was analyzed for composition by using three measuring devices installed serially. Methane and carbon dioxide content were measured using instruments from the MAIHACK (Type FINOR) and SIEMENS (Type ULTRAMAT1) companies (Manual book, 1988; 1990). The devices utilized infrared-absorption measuring method for determining the gases concentration. Hydrogen sulfide was measured using an instrument from the HARTMANN&BRAUN company (Type Radas 2) which based is on the NDUV measuring method (Manual book, 1997). The volume of gas was then measured by a gas meter from the RITTER company. A graphical representation of this system is illustrated in Appendix 1.

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3.1.2.2 pH As described in 2.2.2.2 pH is considered as one of the first control parameters in anaerobic reactors. Deviation in pH from the optimum range is an indication of unstable conditions in the anaerobic process. The pH-value of all the examined substrates was measured at the beginning of the experiment. Since the two main substrates in the research (FBS and SBS) have a lower pH-value than that for optimal condition in anaerobic digestion, it was necessary to measure the pH value regularly. The pH of the reactors contents was determined by pH-meter (Type pH 91) from the WTW company. The normal limits of accuracy reported for this method are plus or minus 0.1 unit.

3.1.2.3 Dry matter (TS) and organic dry matter (oTS) Dry matter is defined as the mass which remains when the water content is removed by drying in an oven at 105°C until it maintains a constant mass. Within the scope of this thesis the dry matter is expressed as total solids (TS). The concentration of total solids (TS) in a substrate is a critical factor in materials handling and in digester design and operation (Chynoweth, et al., (1987). It affects the hydraulic retention time and the feed heating requirements for anaerobic digestion and hence the capital and operational costs of anaerobic digestion. Although anaerobic fermentation is possible up to 50 % TS-content, only up to 20 % is applied in a continuously automated process because of pumping ability. Dry matter (TS) is a mixture of inert inorganic matter (e.g., metals and mineral matter) and organic matter (oTS) including the biodegradable part. The determination of oTS is achieved by heating the dry matter in a muffle furnace at 550°C for 6 hours, where oTS is defined as the solids lost after ignition. The organic dry matter (oTS), sometimes expressed as volatile solids (VS), analyses are used as a basis for determining loading rates and evaluating digester performance. TS and oTS can be measured using the Equations 3 and 4 below.

TS =

m( dry ) * 100% m( wet )

[3]

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oTS =

m( dry ) - m( Ash ) *100% m( wet )

[4]

where: TS oTS m(wet) m(dry) m(ash)

= dry matter = organic dry matter = wet mass = dry mass = remaining mass after ignition

3.1.2.4 Chemical oxygen demand (COD) Chemical oxygen demand (COD) has been cited as an important parameter in the evaluation of anaerobic fermentation. It provides a gauge of the completeness of the digestion process. COD is a measure of the oxygen equivalent of that portion of organic matter that is susceptible to oxidation by a strong chemical oxidant. The chemical oxygen demand (COD) was determined using the cuvette test from the DR.LANGE company (Type LCK 514). The analysis sample was diluted according to its measuring range. Then 2 ml from the diluted sample was poured into the cuvette and then heated to 148 °C for two hours in pulping block. Through this time COD is measured by oxidizing the organic matter with potassium bichromate in boiling sulfuric acid. As a result, and due to the release of Cr3+ the color of sample become green. The intensity of the green color was evaluated by the photometer from the DR.LANGE company, Type ISIS 9000 (Dr. Lange, 2001). For each sample three analyses were applied. The value of COD can be calculated from the following equation (Equation 5).

COD =

color intensity * dilution (g) weighted sample (g)

[5]

The unit of COD is mg of oxygen per liter of substrate examined. COD expresses the number of mg O2 required to oxidize the organic matter present in 1 liter of water or substrate examined by chemical means.

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Due to the following reaction 1g COD corresponds 0.35 liter CH4. Since 10 % of the degradable COD is needed for biomass generation only 0.32 liter CH4 will be considered for 1 g COD (ATV-FA, 1993). With this method the theoretical methane production can be estimated. CH4 + 2O2 1 mole CH4 2 mole O2

1 g COD @

® CO2 + H2O @ 22,4 liter @ 2 x 32g ® 64g

22.44 l CH 4 64 g O2

@ 0 .35 l

CH 4 g O2

[6]

Most compounds are almost completely oxidized with the exception of certain aromatic compound (ATV-FA 7.5, 1993). This method does not differentiate between organic or volatile solids which may or may not be biodegradable and thus gives an overestimate of the amount of the material available for biological treatment.

3.1.2.5 Ammonium-nitrogen (NH4-N) In anaerobic fermentation most of the organic nitrogen will be converted to ammoniumnitrogen. Ammonium (NH4+) is an important parameter for the anaerobic process. It is considered as a nitrogen source for the bacteria growth, but high concentration may inhibit the digestion process (see 2.2.2.4). Ammonium-nitrogen was determined according to the distillation procedure. About 5 ml sample was weighted in analysis flask. Additionally, 50 ml boric acid (2 %) and 200 hl mix indicator were mixed in 250 ml Erlenmeyer-flask. The two flasks were placed in the distillation instrument form the GERHARDT company, Type Vapodest 12. After adding NaOH to the sample flask the distillation took 7 minutes through which the ammonium transferred to the Erlenmeyer-flask. Finally, the Erlenmeyer-flask was titrated with 0.1 n sulfuric acid (H2SO4) until the color changed to light violet.

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The calculation of ammonium-nitrogen concentration was done according to the following equation (Equation 7).

NH 4 -N

=

consumption of (H 2 SO4 ) * 1.4 = g NH 4 -N / kg sample weighted sample (g)

[7]

3.1.2.6 Total nitrogen (Total-N) Total nitrogen is composed of ammonium-nitrogen and organic bounded nitrogen. Through anaerobic digestion no change will take place in the amount of total nitrogen; only trace amount of ammonia has been registered in biogas (Wellinger, 1991). Measuring total nitrogen is an important parameter for the application of digester effluent as fertilizer. The established method for the measurement of total-ammonium is the Kjeldahl method. Soluble samples (about 5 g, depending on the concentration) were weighted in flasks and filled with small amount of distilled water. A Kjeltab (5 g Potassium Sulfate + 5 mg Selenium), stones and anti-foaming agent were added to the sample. Then the sample was acidified with 10 ml concentrated sulfuric acid and brought in a pulping block from the GERHARDT company to be heated up to 410 °C for three and half hours. Through this process the nitrogen compounds were converted to ammonium sulfate. The samples then were titrated in instrument from the GERHARDT company (Type Vapodest 6). In Vapodest 6 the samples were first alkalized with sodium hydroxide whereby the ammonium sulfate was converted to ammonia and then distillated in boric acid-indicatorsolution. The concentration of total-ammonium is equivalent to the acid consumption. The calculation of the total-N was done by the Vapodest 6 instrument after giving the values of the samples weight to it. Samples were generally analyzed in duplicate.

3.1.2.7 Phosphate-phosphorus (PO4-P) Phosphorus is considered in addition to nitrogen an important nutrient for anaerobic microorganisms. It is also an essential parameter for the application of effluent as fertilizer. Since through the anaerobic degradation no phosphate exhausts in gas form, the concentration of phosphate-phosphorus in effluent will only be affected by the feed type.

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Preparation and pulping of PO4-P were done similarly to the procedure used for total-nitrogen. The soluble samples were alkalized to pH value range between 3-4 using sodium hydroxide and then filtered and diluted into 250 ml graduated flask. 5 ml of sample was added to a cuvette test from the DR.LANGE company (Type LCK 049). The concentration of PO4-P is then evaluated by a photometer from the DR.LANGE company, Type ISIS 9000 (Dr. Lange, 2001). For each sample three analyses were applied. The value of PO4-P can be calculated from Equation 8.

PO4 -P =

color int ensity * 250 = weighted sample (g)

g PO4 -P / kg sample

[8]

3.1.2.8 FOS/TAC The relation of volatile organic acid (FOS) to total anorganic carbon (TAC) characterizes the stability of the anaerobic digestion process. Accumulation of FOS (formic acid, acetic acid, propionic acid, lactic acid and butyric acid), which are formed in the acidogenesis and converted in the acetogenesis and methanogenesis to methane and carbon dioxide, will inhibit the anaerobic process. In a steady anaerobic process the decrease in pH value, which can be a result of an increase in organic acid concentration, is prevented as long as bicarbonate ion (HCO3-) concentration is sufficient for the buffer capacity. The TAC plays an important role since it defines the buffer capacity. Generally, a FOS/TAC value > 0.3 indicates an overloaded process. For the determination of FOS and TAC the samples were first centrifuged at 15000 * g for 20 minutes. The supernatant were diluted at a ratio of 1 to 3 and then titrated with 0.05 molar sulfuric acid to pH value of 5.0 using the titrator from the SCHOTT company (Type 154). From the consumption of H2SO4 the TAC can be calculated according to the following equation (Equation 9).

TAC = 250*

20 ml *consumption of (H 2 SO4 )to pH 5 weighted sample (ml)

[9]

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With further titration from pH 5.0 down to 4.4 the FOS value can be obtained from the next equation (Equation 10).

FOS = [[consumption (H 2 SO4 )to pH 4 .4*

20 ml * 1.66 ] - 0 .15 ] * 500 weighted sample (ml)

[10]

3.1.2.9 Volatile fatty acids (VFA) The concentration of VFA has been recognized for a long time to be an important control parameter for anaerobic digestion. It is a key piece of information for understanding and controlling the anaerobic process. Inhibition of the final stages of methanogenesis and generation of VFA in a rate higher than that of the methanogenesis consumption will increase the concentration of VFA in the digester medium. High concentration of VFA will result in unstable anaerobic process and lead to break down of the whole process. The VFA concentration was determined by High Performance Liquid Chromatography (HPLC). Refractive Index (RI) and Ultra-Violet (UV) detectors were used for the determination. The (RI) detectors measure the ability of sample molecules to bend or refract light. The (UV) detectors measure the ability of a sample to absorb light. In addition to organic acids (formic acid, acetic acid, propionic acid, lactic acid and butyric acid) volatile alcohol can also be determined using this method. The disadvantage of this system, is that it recognizes only substances that already calibrated in the standard sample. For determination of VFA the samples were acidified to pH value of 3 by adding sulfuric acid (10 %) and then diluted at a ratio of 1:1 with 0.05 molar sulfuric acid. The samples were then centrifuged at 15000 * g for 20 minutes. The supernatants were diluted again at a ratio of 1 to 10 and then filtered into the sample vials which they were stored in the refrigerator and laterally analyzed by chromatography from the SHIMADZU Company (IR-Detector Type 8110, UV-Detector Type SPD-6AV). The determination has been completed within 80 minutes using a 30 cm long, 7.8 mm i.d. column (Aminex HPX-87H Ion). The eluent (moving phase) used was 0.05 molar sulfuric acid with a flow rate of 0.8 ml/min.

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3.1.3 Controlling and measuring parameters The main criteria used to monitor and evaluate the performance of anaerobic digestion of different feeds or new digester reactor designs in addition to laboratory analysis are methane yield and production rate, organic loading rate, retention time, and degradation efficiency.

3.1.3.1 Loading rate and hydraulic retention time One of the most important parameters in digester operation is the organic loading rate (OLR) which expresses the amount of solid organic material per unit volume that will be introduced into the digester and which, in turn, can be converted to methane. Too low OLR as well as too high OLR will cause problems in the anaerobic fermentation. It is preferable to run the digester with high loading rate in a way that no accumulation to volatile acids could occur. The organic loading rate (OLR) was expressed here in oTS and was calculated according to the following equation (Equation 11).

OLR =

m(oTS) VR*t

OLR m VR t

= = = =

[11]

where: organic loading rate [g-oTS/l*d] mass of organic material [g] reactor volume [l] time [d]

The retention time is a very important factor in digester operation. It is calculated by dividing the total capacity of the digestion by the rate at which organic matter is fed into it. The retention time can be accurately defined in batch reactors. For continuous systems, it can be differentiated between hydraulic retention time (HRT), solids retention times (SRT) and microorganism retention times (MRT). The simplest form is HRT and is determined by calculating the number of days required for displacement of the fluid volume of the culture.

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In a completely mixed continuously stirred tank reactor (CSTR) all three forms are similar and can be measured by the following equation (Equation 12).

HRT = SRT = MRT =

VR V&feed

[12]

where: VR = & feed = V

reactor volume [l] daily substrate volume fed to reactor [l]

Longer retention time promotes greater biodegradation of less biodegradable materials and enhances digester stability. Longer SRT and MRT can be established in several reactor designs without increasing the HRT (Chynoweth, P., et al., 1987). From an economical point of view low retention time is desired. This is achieved by a high loading rate. On the other hand too low retention time will wash the bacteria out of the digester before they can reproduce, which in turn arrests the whole process. Therefore, economically the HRT must be chosen with high OLR so that there will be sufficient time for the bacteria reproduction.

3.1.3.2 Methane yield Methane yield is the quantity of methane produced relative to the quantity of organic matter added to the reactor, and is usually reported as l/g [or m³/kg] added. The organic matter added can be expressed in term of COD or oTS. Methane yield is one of the parameters for determining unbalance in the digester at least when the amount and the composition of the feed remains more or less constant. But it is often too late for corrections when the production drops. Methane yield is calculated on the basis of normalized conditions to ensure that data are comparable and can be measured from the following equation (Equation 13).

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Y (CH 4) =

V (CH 4) m( feed )

[13]

where: Y(CH4) V(CH4) m(feed)

= = =

methane yield [l/g] methane production within specific time [l/d] organic matter added in the same period of time [g/d]

3.1.3.3 Methane productivity Methane productivity refers to the quantity of methane generated in volumes of methane per volume of active reactor per day. It is an important factor in the economical of the biogas plants. The goal is to produce methane at high production rates to create a more economical system. Increasing the loading rate (decrease in HRT) will increase the methane production rate until a point where hydraulic retention time is not sufficient to prevent the washout of bacteria from the reactor. Then any increase in OLR will have an opposite effect i.e., reduction in methane production rate. The methane productivity was obtained by correcting the biogas production to standard temperature and pressure and measured according to the following equation (Equation 14).

Ρ(CH4) =

V(CH4) VR

[14]

where: P(CH4) = V(CH4) = VR =

methane productivity [l/l*d] methane production rate [l/d] reactor volume [l]

3.1.3.4 Degradation efficiency Organic degradation efficiency is an important performance parameter in anaerobic digestion because it influences methane yield and is related to the quantity of process residues requiring further processing or disposal. It shows how far the substrate added to the digester is degraded.

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The degradation efficiency can be measured throughout gas phase or fluid phase. The degradation efficiency from the gas phase is based on the assumption that 0.35 l methane will be produced from 1 g COD. The part of the degradable COD needed for new biomass generation will be disregarded. The degradation efficiency can be calculated according to Equation 15.

η(gas) =

V(CH4) * 100 0.35l/gCOD * (m(feed)/ρ(feed))* COD(feed)

[15]

where: h(gas) V(CH4) m(feed) r(feed) COD(feed)

= = = = =

degradation efficiency throughout gas phase [%] daily methane production [l/d] daily organic matter added [g/d] feed density [g/l] feed chemical oxygen demand [g/l]

The degradation efficiency from fluid phase can be calculated according to the following equation (Equation 16).

η(fluid) =

COD(feed) * m(feed) - COD(effl.) * m(effl.) COD(feed) * m(feed)

* 100

[16]

where: h(fluid) m(feed) m(effl.) COD(feed) COD(effl.)

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= = = = =

degradation efficiency throughout fluid phase [%] daily organic matter added [g/d] daily organic matter removed [g/d] feed chemical oxygen demand [g/l] effluent chemical oxygen demand [g/l]

3.1.4 Substrate properties As it has been mentioned in the beginning of this chapter two main experiments have been conducted within the scope of this thesis. The first experiment (EXP1) has been carried out with forage beets silage (FBS). In the second experiment (EXP2) sugar beets silage (SBS) were examined. In both experiments cow manure and inoculum were used in addition to the main substrates. The two experiments were run approximately in the same manner. Large amounts of the main ensiled substrates were stored and examined so that the same substrate with the same properties could be used for the whole experiment. Cow manure (CM) was collected once every month to drive the experiments for at least several weeks with constant properties. However, there was no significant change in the CM batch for each experiment and the inoculum (IN) used in each experiment had the same characteristics.

3.1.4.1 Experiment with forage beets silage (EXP1) The first experiment was carried out for forage beets silage (FBS) and a mixture (FBS:CM) of FBS and screened cow manure (CM) at a volumetric ratio of 1:1. The FBS can be described as pulp, homogeneous and pumpable. The manure was screened with rotational cylinder (cylinder diameter is 26 cm, screen dimension is 12.5 x 0.75 mm) to prevent plugging the laboratoryscale system. The CM used in the mixture was obtained from the FAL experimental station. The FBS was brought from a farm in Bad-Bentheim, Germany, and stored at 4 °C temperature. For starting up the experiments inoculum from different sources was used. As inoculum for the mesophilic batch experiments (INmesophile) sludge from the sewage plant in Gifhorn was applied. Fermented cow manure (FCM) was used as inoculum for mesophilic semi-continuous experiments. It was obtained from the FAL biogas plant. For the thermophilic semi-continuous experiments sludge from the biowaste treatment plant in BraunschweigWatenbuettel was applied as inoculum (INthermophile). The characteristics of the substrates and inoculum used in EXP1 are shown in Table 12. The FBS1 was used throughout the mesophilic experiments and only at the beginning of the thermophilic experiments. Then the thermophilic experiments were continued with the second forage beets silage batch (FBS2). The second batch (FBS2) was first applied on the 42nd experiment day for only the reactors with FBS and on the 56th experiment day for the reactors with mixture FBS:CM.

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Table 12: Characteristics of the substrates and inoculum used in EXP1 Substrate FBS1 FBS2 CM FCM IN (mesophile) IN (thermophile)

TS [%] 10.6 11.2 3.1 2.5 3.5 7.9

oTS [%] 9.4 9.8 1.9 1.5 2.1 5.1

COD [g/kg] 138 159 31 24 33 87

Total-N [g/kg] 1.64 1.84 2.37 2.34 2.13 4.95

NH4-N [g/kg] 0.45 0.35 1.50 1.57 2.23

PO4-P [g/kg] 0.42 0.25 0.45 0.44 0.60 0.17

pH [-] 3.6 3.3 7.7 7.8 7.6 -

The pH of the ensiled forage beets was measured to have a value of £ 3.6 through which high concentrations of organic acids were indicated in FBS. The concentrations of organic acids of FBS compared with CM are shown in Table 13.

Table 13: Concentration of organic acids in substrates (EXP1) Substrate FBS1 FBS2 CM

Lactic acid [g/kg] 34.0 39.4 0.02

Acetic acid [g/kg] 11.1 12.9 0.02

Propionic acid [g/kg] 8.0 6.2 0.0

Additional experiments were carried out for FBS mixed with plant denaturation agent (PDA) to examine their effect on the digestion process. Three different types of PDA (Tieröl, Arcotal and Arbin) were examined. These could be found in low concentrations in forests and landscaping for the protection from red- and roedeer biting. Arcotal and Arbin were procured from the Stähler Agrochemie company. They are classified as dangerous to health and poisonous to fish. Arcotal is considered to be poisonous to algae and consists of 28 % active agent Thriam. Arbin is composed mainly of fats and oils whereas Tieröl consists mainly of fatty acid nitrile. Tieröl was obtained from the Carl Ullmann GMBH company. Additional sludge from the sewage plant in Gifhorn was used as inoculum for starting the experiments with PDA.

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3.1.4.2 Experiment with sugar beets silage (EXP2) The second experiment (EXP2) was carried out for sugar beets silage (SBS) and a mixture (SBS:CM) of SBS and screened cow manure (CM) at a weight ratio of 20 % to 80 %. The mixture percentage was chosen so that its dry matter be in the same range of the mixture SBS:CM from the first experiment so that they can be significantly compared. The EXP2 was carried out only under mesophilic conditions (37 °C). Sugar beets from the FAL experimental station was cleaned, chopped and then ensiled in two containers (200 liter each). All ensiling factors were ideal (Wenner, 1986) and no additives were needed. The SBS can be described as hard pulp, not easy to mix and difficult to pump. The CM used in the mixture was obtained from the FAL experimental station. Sludge from the sewage plant in Gifhorn was used as inoculum (IN-S) for starting the experiments. Table 12 shows the characteristics of the substrates and inoculum used in EXP2.

Table 14: Characteristics of the substrates and inoculum used in EXP2 Substrate SBS CM IN-S SBS:CM

TS [%] 21.1 3.0 2.5 6.6

oTS [%] 20.5 2.1 1.9 5.8

COD [g/kg] 247 39 20 79

Total-N [g/kg] 1.62 1.96 2.05 1.89

NH4-N [g/kg] 0.20 1.08 1.18 0.90

PO4-P [g/kg] 0.40 0.39 0.55 0.39

pH [-] 3.4 7.6 6.5

The low pH value of the ensiled sugar beets (< 3.4) was an indication of high concentrations of organic acids. The concentrations of organic acids of SBS and that of mixture SBS:CM are shown in Table 15. Because of the high sugar content in sugar beets high amount of saccharose and alcohol were expected to be in the silage substrates. The concentration of saccharose and alcohol in SBS and SBS:CM are shown in Table 16.

Table 15: Concentration of organic acids in substrates (EXP2) Substrate SBS SBS:CM

Lactic acid Formic acid [g/kg] [g/kg]

7.97 7.30

2.88 0.15

Acetic acid [g/kg]

8.67 3.95

Propionic acid Butyric acid [g/kg] [g/kg]

4.71 1.21

1.02 0.81

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Table 16: Concentration of alcohol and sugar in substrates (EXP2) Substrate

Fructose [g/kg]

Glucose [g/kg]

Ethanol [g/kg]

Methanol [g/kg]

SBS SBS:CM

60.0 -

10.0 -

7.5 1.4

1.44 1.0

3.1.5 Description of the experiments As mentioned before two experiments (EXP1 & EXP2) have been conducted in a series. EXP1 (with FBS) has been carried out in the period from July, 2000 to October, 2001 followed by EXP2 (with SBS) which has been carried out in the period from December, 2001 to May, 2002. In both experiments batch- and semi-continuous reactors were applied. Batch reactors were operated for approximately 8 weeks whereas the semi-continuous reactors were operated until the maximum loading rate was reached. The batch reactors were filled with substrate (mixed with inoculum) only at the beginning of each experiment. At the beginning the reactors gassed with N2 to assure optimum anaerobic conditions. The reactors were hand-shaken daily to improve homogeneity in the digester. Biogas (methane) amount and composition were also measured daily. To avoid over foaming, the reactors were not completely filled (leaving up one third to half reactor as headspace). The headspace of batch reactors significantly affects the percentage of the methane produced specially with low total biogas production. The measured methane was therefore corrected according to equation 17.

CCH 4 (korr) =

VCH 4(t 2 ) + HS*CCH 4(t 2 ) - HS*CCH 4(t 1 ) VBiogas(t 2 )

where: t C VCH4 HS Vbiogas

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= = = = =

point of time [t2>t1] methane concentration [%] methane in gas bag [l] head space [l] biogas in gas bag [l]

[17]

No correction was needed for the head space of the semi-continuous reactors because of the daily constant biogas production (methane) which corrects the error automatically. The semicontinuous reactors were started up with 100 % inoculum and then gassed with N2 to assure optimum anaerobic conditions. The following is a description of the methodology for sampling and measuring EXP1 and EXP2.

3.1.5.1 Experiments with forage beets silage (EXP1) In EXP1 5 batch experiments and 7 semi-continuous experiments have been carried out for FBS and a mixture of FBS:CM. All experiments were run in duplicate.

3.1.5.1.1 Batch experiments The batch reactors were filled with a mixture of inoculum and the tested substrates (FBS & FBS:CM) in a ratio of 2:1 oTS. Inoculum was used to apply enough bacterial concentration in the digester. Batch reactors with pure inoculum have been also started to determine the methane produced from inoculum. This value has been subtracted from the methane production of the other batch experiments to determine the specific methane production of the FBS and FBS:CM. Table 15 shows details of the substrates weights used in batch reactors (EXP1).

Table 17: Weights of substrates mixture used in batch reactors (EXP1)

Thermophilic

Mesophilic

Reactor M(G1) M(G2) M(G3) M(G4) T(G5) T(G6) T(G7) T(G8) T(G9) T(G10)

IN 20 000 20 000 18 045 18 045 13 000 13 000 10 220 10 220 8 922 8 922

Weight [g] FBS 1 955 1 955 2 780 2 780 -

FBS:CM 4 078 4 078

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The mesophilic reactors were each charged with 20 liters of the relevant mixture whereas the thermophilic reactors were each charged with 13 liters. After introduction of the desired amount of mixture a certain amount of NaOH was added to adjust the pH value to the optimal range (7.5) for the bacteria and then flushed with nitrogen gas and closed. The mesophilic batch tests were located in the temperature controlled chamber (37 °C) whereas for the thermophilic condition semi-continuous reactors with water jackets were used as batch reactors. The produced biogas volume and composition were measured daily. The fermented substrates were analyzed only at the end of the batch experiments.

3.1.5.1.2 Semi-continuous experiments Four mesophilic semi-continuous reactors were started by seeding them with fermented cow manure (FCM) as inoculum. The other four thermophilic reactors were seeded with (INthermophile). Two reactors of each method were fed with FBS and the other two with mixture FBS:CM of the considered concentration. The operating volume of the reactors was kept constant at 15 liters. Before starting the experiments the oxygen in the head space of the reactors was replaced with nitrogen and the inoculum were allowed to stand overnight before starting feeding. Then the reactors were fed stepwise increasing the loading rate starting with 1 g oTS/l*d and with 0.5 g oTS/l*d interval. The reactors were fed in this manner until process failure occurred as evidenced by the decrease in methane production and reduction in substrate utilization as measured by chemical analysis (COD). Each loading rate step has was until a steady state condition was attained. Once steady-state operation was achieved at a particular loading rate (OLR), the system parameters were measured over a period for that OLR. The initial parameters used in determining whether the steady state condition was achieved were: stable biogas production and composition and the pH value which were measured daily. These parameters were later confirmed with the analytical methods that were done only after changing the loading rate. The reactors were fed on a once-daily basis. The quantity of daily feed introduced to the reactors was measured according to OLR. The amount of feed and their corresponded OLR and HRT are shown in Table 18.

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Table 18: Hydraulic retention times and loading rates of EXP1

1 8 15 29 36 50 60 79

Thermophilic FBS Mixture T(K5) & T(K6) T(K7) & T(K8)

[g/l*d]

mfeed [g/d]

HRT [d]

[g/l*d]

mfeed [g/d]

HRT [d]

Day

Day

Mesophilic FBS Mixture M(K1) & M(K2) M(K3) & M(K4)

1.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0

159.5 318.9 398.6 478.4 558.1 637.8 717.5 -

94.1 47.0 37.6 31.5 26.9 23.5 20.9 -

1.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0

264.2 528.3 660.4 792.5 924.5 1056.6 1188.7 1320.8

56.8 28.4 22.7 18.9 16.2 14.2 12.6 11.4

1 8 15 22 36 42 50 56 64 79 92 96 102 106 108 111 112 121 126 132 137 156

OLR

OLR

[g/l*d]

mfeed [g/d]

1.0 2.0 2.5 3.0 3.5 3.5 4.0 4.0 4.2 4.5 4.7

166.8 333.9 417 500.4 583.8 533.9 610.2 610.2 640.7 686.5 716.9

OLR

HRT [d]

OLR [g/l*d]

89.9 1.0 44.9 2.0 35.9 2.5 29.9 3.0 25.7 3.5 28.1 3.0 24.6 3.2 24.6 3.2 23.4 3.5 21.8 3.7 20.9 4.0 4 days not fed 3 457.6 32.8 2.5 3.5 533.9 28.1 3.0 4 610.2 24.6 3.5 4.3 655.9 22.8 3.7 4.5 686.5 21.8 3.85 4.2 640.7 23.4 4.0 3.8 579.7 25.9 4.2 3 days not fed 4.2 3.5 533.9 28.1 4.3 3.5 533.9 28.1 4.5

mfeed [g/d]

HRT [d]

267.9 535.8 669.8 803.7 937.7 803.7 857.3 776.6 849.5 898.0 970.8

55.9 27.9 22.4 18.7 15.9 18.7 17.5 19.3 17.6 16.7 15.5

606.7 728.1 849.5 898.0 934.4 970.8 1019 1019 1043 1092

24.7 20.6 17.6 16.7 16.1 15.5 14.7 14.7 14.4 13.7

(bold numbers are for the second substrate batch)

The gas produced during each feeding period was measured daily. Samples were collected when changing OLR for analysis of TS, oTS, COD, NH4-N, total-N, PO4-P, FOS/TAC and VFA. The pH was measured every 2-3 days. The temperature inside the reactors was controlled in the range of 55 °C with the thermostatically controlled re-circulating water (water jacket). Under mesophilic conditions the reactors were located in temperature controlled chamber (37 °C). The reactors content was mixed by mechanical agitation. The mixer operated for 3 minutes every 45 minutes. The analysis samples were taken after a period of 5 minutes of mixing.

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Four additional semi-continuous experiments were performed to indicate the effect of three plant denaturation agents (PDA) on the anaerobic process. Three reactors were fed with FBS displaced with small amount of PDA (Tieröl, Arcotal and Arbin). The fourth reactor was applied only with FBS for comparison. The reactors were operated similar to the previous reactors. They were started with 1 g oTS/l*d with PDA concentration of 0.5 g PDA per kilogram FBS. The PDA concentration was raised twice at constant OLR to reach a concentration of 1.5 g PDA per kilogram FBS. The concentration of PDA fed into the digester was then increased indirectly by increasing OLR. Table 19 provides a summary of the operation parameters of the reactor with PDA. All the semi-continuous reactors for EXP1 were operated without any problems.

Reactors

Table 19: Operational plan of semi-continuous reactors with PDA

days 1 - 28 29 - 46 47 - 70 71 - 81 82 - 87 88 - 100

Mesophilic M(K09): pure FBS M(K10): FBS with Tieröl M(K11): FBS with Arcotal M(K12): FBS with Arbin OLR [g/l*d]

mfeed [g/d]

HRT [d]

PDA [g/kg]

1.0 1.0 1.0 1.5 2.0 2.5

160 160 160 240 320 400

93.8 93.8 93.8 62.5 46.9 37.5

0.5 1.0 1.5

Remark On 8th and 15th days of the experiments an additional 0.75 g PDA were added to the corresponding reactors

The HRT and OLR are both a function of organic matter fed to the anaerobic digester. Decreasing HRT will cause an increase in OLR. Economically, low HRT and high OLR are required to reduce the reactor volume and hence biogas plant costs. To achieve low HRT the semi-continuous experiments were started with high HRT and stepped down until the lowest HRT was reached. The lowest HRT has been reached where there was no wash up of bacteria. HRT, and hence OLR, were changed approximately every 2 weeks (after steady state was reached). The HRT relationship with OLR for the mesophilic experiments as well as thermophilic experiments is shown in Figures 9 and 10. 78

hydraulic retention time [d]

100

FBS 1

80

(FBS:CM) 1

60

40

20

0 0

1

2

3

loading rate

4

5

6

[g/l*d]

Figure 9: Hydraulic retention time and loading rate for mesophilic experiments (EXP1)

50

hydraulic retention time [d]

FBS 1 FBS 2

40

(FBS:CM) 1 (FBS:CM) 2

30

20

10

0 0

1

2

3

4

5

loading rate [g/l*d]

Figure 10: Hydraulic retention time and loading rate for thermophilic experiments (EXP1)

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3.1.5.2 Experiments with sugar beets silage (EXP2) In EXP2 three batch experiments and two semi-continuous experiments have been carried out under mesophilic condition for SBS and a mixture of SBS:CM. All experiments were run in duplicate with exception of inoculum batch experiments which were run in triplicate. All the reactors were placed in temperature controlled chamber (37 °C).

3.1.5.2.1 Batch experiments The batch reactors in EXP2 were filled with mixture of inoculum (IN-S) and the tested substrates (SBS & SBS:CM) and analyzed similarly to that of the batch reactors of EXP1. Since pH value of the mixture was within the optimal range no additional NaOH was needed. The reactors were each charged with only 15 liters of the relevant mixture to have enough head space in case of over-foaming. Table 20 shows details of the substrates weights used in batch reactors (EXP2).

Table 20: Weights of substrates mixture used in batch reactors (EXP2)

Mesophilic

Reactor M(G11) M(G12) M(G13) M(G14) M(G15) M(G16) M(G17)

IN-S 15044 15015 15035 14374 14369 1968 1968

Weight [g] SBS 642 643 -

SBS:CM 2063 2056

3.1.5.2.2 Semi-continuous experiments The four mesophilic semi-continuous reactors were started by seeding them with inoculum (IN-S). Two of the reactors (M(K13) & M(K14)) were fed with SBS and the other two (M(K15) & M(K16)) with mixture SBS:CM. The reactors were fed and analyzed in similar fashion to the semi-continuous reactors of EXP2 (3.1.5.1.2). Therefore, the procedure of feeding and sampling that is only different to that in EXP2 will be described here.

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In comparison to EXP1 some difficulties occurred throughout this experiment (EXP2). Due to the high oTS of the SBS (18-20 %) feeding the reactors M(K13) and M(K14) was not easy and smooth, care must be taken to prevent oxygen from entering into the system. This problem arose with the increase in OLR. This problem was solved by feeding the reactor slowly and very carefully. On the 92nd day, one of the reactors with SBS, M(K14), had to be changed due to a crack in the reactor. The reactor content was changed to a new reactor followed by filling the head space with nitrogen to remove oxygen. The new reactor could operate normally within two days parallel to the duplicate reactor M(K13). Another problem resulted from the feeding of low pH of SBS; the reactors foamed heavily directly after feeding. This problem was solved by increasing the mixing time to 20 minutes after feeding and for 10 minutes every 45 minutes. The amount of feed and the corresponded OLR and HRT are shown in Table 21.

Table 21: Hydraulic retention times and loading rates of EXP2

Day

1 16 41 58 72 95 114 130 142

Mesophilic SBS Mixture M(K13) & M(K14) M(K15) & M(K16) OLR mfeed HRT OLR mfeed HRT [g/l*d] [g/d] [d] [g/l*d] [g/d] [d] 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

73.3 109.9 148.9 186.2 156.7 277.1 323.2 401.8

204.6 136.5 100.7 80.6 58.4 54.1 46.4 37.3

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

269.8 404.7 519.0 662.5 793.7 864.9 985.2 1110.2 1277

55.6 37.1 28.9 22.6 18.9 17.3 15.2 13.5 11.7

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hydraulic retention time [d]

250

SBS

200

(SBS:CM)

150

100

50

0 0

1

2

3

4

5

6

loading rate [g/l*d]

Figure 11: Hydraulic retention time and loading rate for EXP2

HRT, and hence OLR, were changed approximately when a steady state was reached. The HRT relate to OLR for the second experiments are shown in Figure 11.

3.2 Systems evaluation The entire spectrum of preparation possibilities of biogas energy source from biomass and the fossil energy sources that they can substitute will be reviewed in this thesis. The cycle of biogas energy that can be extracted potentially from forage and sugar beets was investigated. By parameter selection, only those that under the present basic conditions were chosen. Two Scenarios were used in the evaluation; Scenario 1 compares biogas with natural gas and Scenario 2 compares electricity (and heat) from biogas with conventional electricity and heat (Figure 12).

82

Biogas cycle from beets

Fossil fuel cycle

Seed Ferti. PPA Mac.

Exploration

Fuel Haulage Chopping Pumping Storing

Transport Refining

Fermentation:

Heating, stirring reactor, pumping

Biogas

Heating oil

Transportation

compare

(Scenario 1)

Burning

CHP

Heat Electricity

Fuel

compare

(Scenario 2)

Electricity

Heat

Figure 12: Energy production cycles of biogas and fossil fuel

Although the beets yield is a function of climatic conditions, soil and crop rotation, these parameters have been considered in this thesis as independent parameters. High forage and sugar beets yield of 120 and 65 t/ha per year, respectively, were assumed. To present more specific results from the assessment of energy and ecological balance two farm Models were assumed; Model 1 represents 20 hectares whereas Model 2 represents 80 hectares. For simplifying the assessment, the cycle of forage beets was divided into three main stages: Stage 1 – production of forage and sugar beets Stage 2 – preparation of beets to the biogas plant Stage 3 – Production of biogas.

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3.2.1 Method of energy balance As described previously energy consumption by the agricultural sector can be categorized into either direct or indirect energy use. The effective direct energies considered in the first stage are the consumption of diesel and oil to operate tractors and farm machinery; no irrigation pumps, drying equipment, refrigeration, and trucks for transportation were needed. According to Reinhardt (1993) not only the energy content of diesel and oil will be considered but also the energy included in their production. This will result in an additional energy value of 11.3 % which will increase the diesel energy factor to 46.97 MJ/kg (39.45 MJ/l). The amount of diesel used for all field work for forage beets is considered to be 125 l/ha (105 kg/ha) (Stephen, 1999). The consumption amount of lubricant oil will be assumed to be 4 % of diesel consumption (Oheimb, 1987). For high forage beets yield fertilizer is needed to recover the withdrawal of nutrients. This can be done by chemical and organic fertilizers. The amount of chemical fertilizers needed for high forage and sugar beet yield per hectare are 275 and 220 kg N, 130 and 180 kg P2O5, 350 and 380 kg K2O, and 700 kg CaO (KTBL-Taschenbuch 2002). The reactor effluent can be used as organic fertilizer to substitute, to some extent, the high energetic value mineral fertilizers. The reactor effluent has the advantage of returning minerals that are not contained in chemical fertilizer. Since the anaerobic digestion increases the availability of nutrients to plant and 10 % of the organic nitrogen can be available by soil microorganisms, in addition to the return of leaves to soil, it was assumed that about 75 % of the mineral fertilizers can be recovered through the forage beets cycle. The unused leaves can return 60 – 100 kg N, 30 – 40 kg P2O and 140 – 160 kg K2O to soil (BAL, 1992). According to Kaltschmitt, et al., (1997) a mean energy consumption factor of 241 MJ/kg active substance will be used in the energy balance for plant protection agents (PPA) – herbicides, insecticides and fungicides. The energy factor for production of seed material is considered to have the value 30.14 MJ/kg (Kaltschmitt, et al., 1997). For machinery and equipment manufacturing an energy factor of 70 MJ/kg is assumed in this thesis (Kalk and Hülsbergen, 1996). This value is dependent on the area used (e.g., 20 ha) and the lifetime of the machines. Energy input for silos and fermentation reactor will be estimated. Two Scenarios were used in the evaluation of the energy output. In Scenario 1 the production of biogas was evaluated. In the second Scenario (Scenario 2) the production of electrical and heat energies were evaluated.

84

3.2.2 Method of ecological balance Ecological evaluation in this thesis was achieved due to the Life Cycle Assessment (LCA) method. The assessment includes the entire life cycle of biogas production from forage and sugar beets and their application. Since in practice the biogas is burned in engines (not fed to natural gas net) for the energetic use, only Scenario 2 was considered in the ecological assessment. The ecological balance includes the energies and materials used in the energy balance. The following parameters were considered in the ecological evaluation: ·

Resources depletion

·

Land use

·

Global warming

·

Stratospheric ozone depletion

·

Acidification

·

Eutrophication

·

Human- and ecotoxicological impacts

·

Photochemical oxidant formation

In the balancing system natural gas and electrical energy from fossil fuel were substituted by biogas and energy generated from biogas. As land a set-aside agricultural land was considered. The equivalents of CO2, CH4 and N2O emission were evaluated as greenhouse effect. For stratospheric ozone depletion N2O-emission was calculated. SO2-equivalents for SO2, NOx, NH3 and HCl were estimated as acidification parameters. In the eutrophication assessment only the nitrogen compounds (NOx and NH3) were considered. Moreover, NOx and NMHC emission were measured as photochemical smog. Additionally, air harmful substances were estimated under human- and ecotoxicological aspects.

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4

Results and discussion

4.1 Experimental results

4.1.1 Mesophilic experiments

4.1.1.1 Batch experiments The batch experiments were done primarily for examination of the suitability of the digested substrates. They were also used for the determination of maximal possible degradation efficiency and methane yield. Within the scope of this thesis, 11 batch reactors were operated under mesophilic condition.

4.1.1.1.1 Inoculum The accumulative methane curves of mesophilic FBS batch experiments and mesophilic SBS batch experiments of the inoculum are given in Figures 13 and 14, respectively. The accumulative methane represents the total methane production from the fermented substrate at the measured time. The shapes of the curves show an expected increase of methane production. Figures 13 and 14 cannot be compared directly with each other since the inoculum used was different (had different characteristics). Figure 13 shows the curves progression of reactors M(G1) & M(G2). The two curves show identical value up to the 13th day of experiment. They then started to move along different gradients. The reason for this variation is most probably due to inhomogeneity of inoculum in the two reactors. Moreover, the Figure shows that until the 44th day of the experiments, the methane accumulative curves were still increasing. This means that the degradation is not completed and the difference between the curves might be corrected later. The methane yield of inoculum within the considered period of the experiment (35 days) is 0.4 and 0.47 l CH4 per kilogram inoculum for reactors M(G1) and M(G2), respectively. This value is low and indicates that the degradable anaerobic parts in inoculum are small. Since the

86

methane yield per kilogram inoculum used to be subtracted from other batch experiments to indicate the specific methane production of the substrate, the mean value of the curves was used.

methane yield [l/kg inoculum]

0.7 0.6 M (G1) inoculum

0.5

M (G2) inoculum

0.4 0.3 0.2 0.1 0.0 1

6

11

16

21

26

31

36

41

time [day]

Figure 13: Methane accumulative curve of inoculum used for mesophilic FBS batch experiments

0.9

methane yield [l/kg inoculum]

0.8

M(G11) inoculum M(G12) inoculum M(G13) inoculum

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1

6

11

16

21

26

31

36

41

46

time [day]

Figure 14: Methane accumulative curve of inoculum used for mesophilic SBS batch experiments

87

The run of the curves for reactors M(G11), M(G12) & M(G13) in Figure 14 is similar and indicates that the inoculum in the three reactors was homogeneous. They produce a methane yield of 0.47 to 0.49 CH4 per kilogram inoculum throughout the considered period of the experiments. In contrast to the curves in Figure 13, the horizontal shifting of the curves in Figure 14 indicates that the biogas production is negligible. This indicates that all the degradable materials were consumed by the anaerobic bacteria. Although the two Figures can not be compared directly, the curves outward of Figure 14 and that inward of Figure 13, specially in the first two weeks, show that the inoculum used in EXP2 is more active.

4.1.1.1.2 Forage beets silage Figure 15 illustrates the accumulative methane curves of mesophilic forage beets silage (FBS) batch experiments. Generally, the two reactors (M(G3) &M(G4)) show similar results. The run of the curves shows a sudden increase in the methane production on the 3rd day and then takes a calm phase up to the 8th day after which it started to increase again to reach a maximum value on the 14th day. The progression of the curves can be shown more clearly in Figure 16 which shows the daily methane production. The explanation of the first peak is due to the very light degradable substances in the substrate (i.e., Glucose) which are directly converted from the bacteria methane.

60

methane yield [l/kg FBS]

max. theoretical methane yield

50 40

M(G3) FBS M(G4) FBS

30 20 10 0 1

6

11

16

21

26

31

36

time [day]

Figure 15: Accumulative methane of mesophilic FBS batch experiments

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The next phase (up to 10th day) can be described as an adaptation phase; the bacteria need some time to adapt to the new substrate. After the 8th day, the bacteria have adapted to the milieu and could convert the light degradable substances to methane. In this phase 85 % of the organic matter has been degraded. After a second calm phase the methane production started to increase again (23rd day of the experiment). This increase could be a result of the conversion of the hard degradable substances to methane. After the 27th experiment day, no methane production was reported in Figure 16. This indicates that all degradable substances were converted.

methane [l/kg FBS per day]

10

8 M(G3) FBS M(G4) FBS

6

4

2

0 1

6

11

16

21

26

31

36

time [day]

Figure 16: Daily methane production of mesophilic FBS batch experiments

From Figure 15, the maximum amount of methane that could be produced from 1 kg FBS is 47 l per kilogram FBS. This value is high and when compared to the stoichiometric calculation (1 gram COD produces 0.32 liter methane) it shows a high degradation efficiency of about 90 % which will make the application of FBS in biogas plant very interesting. These results have obviously lower values than those obtained in agricultural engineering institute Bornim (Linke, 1999). They have obtained more than 90 liter methane from 1 kg FBS which is not possible. The results reported by Beck (2001) substantiate the results obtained in this research. Experiments of FBS with manure could not be done due to technical reasons.

89

4.1.1.1.3 Sugar beets silage The results of the experiments with sugar beets silage (SBS) under mesophilic batch conditions are demonstrated in Figures 17 and 18. Figure 17 represents the accumulative methane curves whereas Figure 18 shows the daily methane production. The direct increase in the methane production in the starting phase of the curves for reactors M(G14) and MG(15) indicates that microorganisms did not need time to acclimatize to the substrate (Figure 18). This is because of the active inoculum used (Figure 14).

100

methane yield [l/kg SBS]

max. theoretical methane yield

80

60 M(G14) SBS

40

M(G15) SBS

20

0 1

6

11

16

21

26

31

36

41

46

time [day]

Figure 17: Accumulative methane of mesophilic SBS batch experiments

The continuous increase in methane production has reached its maximum value of 9.28 and 10.7 liter methane per kilogram and day on the 7th day for reactors M(G14) and M(G15), respectively (Figure 18). Then the methane production decreased gradually until the 10th day on which the light degradable substances were degraded. After the 18th (15th) day the bacteria started to convert the hard substances and continued to produce methane at a very low rate during the last days. The maximum theoretical methane production of SBS according to its COD was calculated to be 86.5 liter per kg SBS. Throughout the batch experiments the maximum methane production from 1 kg SBS were 74.6 and 85.8 l methane for reactors M(G14) and M(G15), respectively (Figure 17). Although both results show noticeable

90

variation, they agree in that SBS have a high degradation efficiency. The difference in the degradation efficiencies (86.7 % and 99 %) could be a result of the inhomogeneity of the samples applied (only ca. 640 g sample with high dry matter in 15000 g reactor content).

methane [l/kg SBS per day]

12 10 M(G14) SBS M(G15) SBS

8 6 4 2 0 1

6

11

16

21

26

31

36

41

46

time [day]

Figure 18: Daily methane production of mesophilic SBS batch experiments

4.1.1.1.4 Sugar beets silage with manure Figures 19 and 20 illustrate the results of the mesophilic sugar beets silage with cow manure (FBS:CM) for the batch experiments. Figure 19 shows the accumulative methane curves whereas Figure 20 reflects the daily methane production. Generally, the progressive curves of reactors M(G16) and M(G17) are similar to those of pure SBS experiments (Figure 17 and 18). The degradation of light degradable substances in the first phase of the process was proceeded by the degradation of the hard substances. The maximum theoretical methane yield on the basis of the stoichiometric calculation for the SBS:CM is 27.9 liter per kilogram SBS:CM. In Figure 19 the batch reactors show a maximum methane value of 18.1 and 16.69 l methane per kg mixture for reactors M(G16) and M(G17), respectively. From maximum methane production the degradation efficiencies were calculated to be 64.87 % and 59.8 %. These values are less than the degradation efficiency of pure SBS. 91

The reduction in efficiency is due to small degradation efficiency of manure which is normally about 50 % (Wellinger, 1991). However, the addition of manure to SBS gave more stability to the degradation process as it can be seen in the smooth curve progression of Figure 19.

methane yield [l/kg (SBS:CM)]

30 25

max. theoretical methane yield

20 15 10

M(G16) SBS:CM (1:4) M(G17) SBS:CM (1:4)

5 0 1

6

11

16

21

26

31

36

41

46

time [day]

Figure 19: Accumulative methane of mesophilic SBS:CM batch experiments

methane [l/kg (FBS:CM) per day]

3.0 2.5 M(G16) SBS:CM (1:4)

2.0

M(G17) SBS:CM (1:4)

1.5 1.0 0.5 0.0 1

6

11

16

21

26

31

36

41

46

time [day]

Figure 20: Daily methane production of mesophilic SBS:CM batch experiments

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The results of batch experiments of forage beets silage (FBS) in comparison with sugar beets silage (SBS) are summarized in Table 22. The higher dry matter of SBS resulted in higher methane yield per fresh material (FM) than FBS. Comparing the methane yield of FBS and SBS on the basis of organic dry matter, the FBS has produced higher methane yield than SBS. However, both FBS and SBS have shown high degradation efficiency. The lower methane yield and lower degradation efficiency of the mixture SBS:CM is due to the addition of manure.

Table 22: Mean results of the batch experiments (mesophile) Methane yield [l/kg-FM]

Methane yield [l/kg-oTS]

Degradation efficiency [%]

FBS

47

468

95

SBS

83

400

92

SBS:CM (1:4) *

17

294

62

Substrate

*

FBS:CM (1:1)mesophile results are not available

4.1.1.2 Semi-continuous experiments

4.1.1.2.1 Methane productivity The importance of methane productivity or methane production rate has already been discussed in 3.1.3.3. It plays a major role in the economic efficiency of biogas plants. The higher methane productivity, the smaller the reactor volume – the lesser costs.

4.1.1.2.1.1 Forage beets silage Figure 21 shows the methane productivity with different loading rates for reactors M(K1) and M(K2) with pure forage beets silage (FBS). Generally, the methane productivity in both reactors is similar confirming the result obtained. In the starting phase, the curves of Figure 21 show irregular methane production with the constant loading rates. This is similar to the results obtained from the batch experiments. This means that the bacteria have not yet completely

93

adapted to the new substrate (FBS). FBS could not be degraded totally. The daily feed of FBS into the reactors coupled with insufficient degradation of FBS results in lower methane production. The adaptation period took about two weeks after which the microorganisms started to degrade the accumulated substances. This explains the high methane production in the beginning with organic loading rate of 2.5 g oTS/l*d. After the 31st day, the methane productivity increased with increasing OLR at a constant rate: this is a sign of stable process. The methane productivity started to decrease with increase in OLR when it reached an OLR of 4.5 g oTS/l*d (Figure 21).

methane productivity [l/l*d]

M(K1) FBS M(K2) FBS OLR-oTS

2.0

5

4 1.5 3 1.0 2 0.5

oTS-loading rate [g/l*d]

6

2.5

1

0

0.0 1

11

21

31

41

51

61

71

81

time [day]

Figure 21: Methane productivity of FBS in semi-continuous reactors

The reduction in methane productivity is a sign that the fed FBS were not totally degraded and with continual feeding the process becomes over loaded and disturbed as it can be seen in the Figure. Therefore, maximum methane productivity of FBS that can be reached in stable system is 1.92 l/l*d with an OLR of 4 g oTS/l*d. It is also important to notice that after one to two days without feeding the bacteria needs some time to acclimatize again to its last stage before feed break. This can be seen clearly in Figure 21 in the methane reduction after feed break (in Figure 21 no feed º OLR line without dots).

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4.1.1.2.1.2 Forage beets silage with manure The results of methane productivity from forage beets silage with manure (FBS:CM) in Figure 22 is similar to the experiments with pure FBS (Figure 21). Reactors M(K3) and M(K4) show identical results with different loading rates. After the adaptation period the methane productivity increased with increasing OLR at a constant rate indicating a stable process. The maximum methane productivity of FBS:CM could be reached with OLR of 4.5 g oTS/l*d which is higher than the maximum OLR of FBS and has the amount of 1.75 l/l*d.

methane productivity [l/l*d]

M(K3) FBS:CM (1:1) M(K4) FBS:CM (1:1) OLR-oTS

2.0

6 5

1.5

4 3

1.0

2 0.5

oTS-loading rate [g/l*d]

7

2.5

1 0

0.0 1

11

21

31

41

51

61

71

81

91

time [day]

Figure 22: Methane productivity of FBS:CM in semi-continuous reactors

The comparison between mean methane productivity at constant loading rate of pure FBS and the mixture FBS:CM is represented in Figure 23. The Figure shows the linear relationship between methane productivity and OLR. The reduction in methane productivity at high OLR due to inhibition of process is also distinguished. The maximum loading rate within which methane from FBS can be produced under a stable condition is 4 g oTS/l*d. This value is raised to 4.5 g oTS/l*d when mixing FBS with manure. Although, reactors with FBS:CM can achieve higher OLR than those with pure FBS their methane productivity is still lower than that produced by reactors with pure of FBS. This is due to manure’s low methane production.

95

methane productivity [l/l*d]

2.5

2.0

FBS FBS:CM (1:1)

1.5

1.0

0.5

0.0 0

1

2

3

4

5

6

oTS-loading rate [g/l*d]

Figure 23: Methane productivity of FBS and FBS:CM under different OLR

4.1.1.2.1.3 Sugar beets silage Reactors M(K13) and M(K14) in Figure 24 represent methane productivity with different loading rates for pure sugar beets silage (SBS). The curves progression of the two reactors is identical until the 85th day after which a variation from the expected value is noticed in both reactors. This variation is most probably due to difficulties in feeding the reactors. SBS with pH value equal to 3.6 resulted on one hand in high foaming in reactors after feeding, which then made it necessary to mix the reactors for a long time. On the other hand, it was not easy to feed the reactors with homogenate SBS (due to its structure and high TS concentration ca. 20 %). The methods used in the experiment to reduce this effect were: feeding the reactors carefully and weighting the SBS more attentively which produced improved results. Due to the active inoculum used, as batch results showed, no adaptation time was needed for the bacteria and it started producing a constant rate of methane from the beginning. The methane productivity increased with increasing OLR at a constant rate. The critical reduction in methane productivity at OLR of 4.5 g oTS/l*d is a sign of overloading the process indicating, in addition to analytical results, that to OLR of 4.0 g oTS/l*d the reactors can be operated under a steady state condition.

96

methane productivity [l/l*d]

M(K13) SBS

2.0

6

M(K14) SBS OLR-oTS

5

1.5

4 3

1.0

2 0.5

oTS-loading rate [g/l*d]

7

2.5

1 0.0

0 1

11

21

31

41

51

61

71

81

91

101 111 121 131

time [day]

Figure 24: Methane productivity of SBS in semi-continuous reactors

4.1.1.2.1.4 Sugar beets silage with manure Figure 25 represents the methane productivity of sugar beets silage mixed with manure (SBS:CM) at different loading rates throughout the entire experiments. The progressive curves of reactors M(K15) and M(K16) show similar results to those obtained from reactors M(K13) and M(K14) with pure SBS. It is also noticeable after OLR of 3.5 g oTS/l*d that the methane productivity fluctuated around the expected value which can be attributed to the same reasons explained in the previous section (4.1.1.2.1.3). The maximum methane productivity for SBS:CM reached in these experiments is 1.91 l/l*d at an OLR of 5 g oTS/l*d. This value is not accurate because the experiments were stopped prior operating for enough days under OLR of 5 g oTS/l*d (before reaching steady state). Therefore, OLR of 4.5 g oTS/l*d will be considered as the maximum OLR at which the maximum methane productivity can be reached under stable condition. In light of this the maximum methane productivity will then be 1.58 l/l*d. Figure 26 shows the results of mean methane productivity at a constant loading rate of pure SBS and SBS:CM comparatively. Figure 26 shows similar results to those found Figure 23 with the only difference being in the maximum methane production values measured. The inlinearity of the curves of SBS and SBS:CM with OLR is due to the fluctuation in methane production rate with constant OLR. 97

8 7

M(K15) SBS:CM (1:4)

2.0

6

M(K16) SBS:CM (1:4) OLR-oTS

5

1.5

4 1.0

3 2

0.5

oTS-loading rate [g/l*d]

methane productivity [l/l*d]

2.5

1 0.0

0 1

16

31

46

61

76

91

106

121

136

time [day]

Figure 25: Methane productivity of SBS:CM in semi-continuous reactors

methane productivity [l/l*d]

2.5

2.0 SBS SBS:CM (1:4)

1.5

1.0

0.5

0.0 0

1

2

3

4

5

6

oTS-loading rate [g/l*d]

Figure 26: Methane productivity of SBS and SBS:CM under different OLR

98

4.1.1.2.2 Biogas composition Throughout the experiments the biogas composition has shown constant values under steady state conditions. These are shown in Table 23. Exceptions were noticed in the starting phase which showed minor variation and in the end phase when the reactors were over loaded which showed an acute decrease in methane production. Moreover, low variations were also recorded directly after increasing OLR. The concentration of H2S was within the safe range for anaerobic digestion so that no inhibition occurred, even for the mixture SBS:CM with its high H2S concentration of 850 ppm. Additionally, for standard biogas application no purification was needed for the energetic use of the produced biogas except for SBS:CM. The methane content obtained in this research is much lower than the 73 % which is reported by Linke (1999). The value obtained by Linke seems to be unrealistic; forage beets composed mostly from carbohydrates which cannot result to such high methane content of more than 70 %.

Table 23: Biogas composition (mesophilic experiments) CH4 (%)

CO2 (%)

H2S (ppm)

FBS

53 ± 1

46 ± 1

100

FBS:CM (1:1)

55 ± 1

44 ± 1

100

SBS

53 ± 1

46 ± 1

350

SBS:CM (1:4)

56 ± 1

43 ± 1

850

Reactor

4.1.1.2.3 Degradation efficiency The degradation efficiency was calculated according to equation 15 in 3.1.3.4. Figure 27 shows the mean degradation efficiency of the semi-continuous mesophilic experiments according to the gas phase. The degradation efficiency according to the fluid phase is not considered because the results obtained were not representative. The reason is that the effluent analysis is a main parameter in the calculation which is strongly affected by the inoculum used. This means it can take a long time until the influent displace the inoculum completely under a constant loading rate, and then the effluent can represent the degradable value of influent.

99

degradation efficiency [%]

120 100 80 60 40

FBS SBS FBS:CM (1:1) SBS:CM (1:4)

20 0 0

1

2

3

4

5

6

oTS-loading rate [g/l*d]

Figure 27: Degradation efficiency of substrates with different loading rates according to gas phase

Generally, the Figure shows that the degradation efficiency of pure substrates is higher than that of mixtures. This is because of the added amount of manure which has lower degradation efficiency. In this context, the degradation efficiency of the mixture SBS:CM is lower than that of the mixture FBS:CM because of the higher amount of manure in the SBS:CM mixture. Pure FBS and SBS show on the other hand nearly identical values. The unrealistic degradation efficiency (> 100 %) of SBS with OLR 3 g oTS/l*d is due to the unexpected high amount of methane production in this period (Figure 24). The run of the curves for degradation degree of FBS and FBS:CM shows the expected values; with increasing loading rate more organic materials will be removed from the reactors resulting in a less effective process. This does not apply to SBS because they have high HRT due to their high oTS and therefore shows stable degradation efficiency. The fluctuation in the degradation efficiency of SBS:CM is most probably due to the variation in the methane production which could falsify the results. Figure 27 shows clearly that beets silage have high degradation efficiency of more than 90 % at high OLR. After OLR for more than 4 g oTS/l*d for pure substrates and more than 4.5 g oTS/l*d for FBS:CM the degradation degree fell sharply down which is a sign of an over loaded system. The degradation degree curve of SBS:CM will be expected to follow in the same manner unless the experiment was not ended.

100

4.1.1.2.4 Laboratory analyses

4.1.1.2.4.1 Chemical oxygen demand The average chemical oxygen demand (COD) of effluent measurements obtained during different loading rates are shown in Figure 28. For all the experiments, the COD of effluent of mono-substrate were lower than those mixed with manure. This is due to the lower degradation efficiency of manure which results in less degradable substances. Since the COD of inoculum is low (20-24 g/kg), any increase above this value will be due to the effect of influent. The curves progressions show an expected increase in COD with increase in OLR. However, the effluent COD value is also a function of reactor content (inoculum) and the actual value can be considered when all the inoculum is replaced by the influent. This can be done by operating the reactors for long time. Because this is not a common practice under experimental condition (due to the length of time it takes), the longer feeding the reactors under constant OLR the less is the effect of reactor content until full replacement. Therefore, the trend was to consider the latest measured value.

40

COD [g/kg effluent]

35 30 25 20 15

SBS:CM (1:4) SBS

10

FBS:CM (1:1) FBS

5 0 0

1

2

3

4

5

6

oTS-loading rate [g/l*d]

Figure 28: Chemical oxygen demand of effluent at different OLR.

101

4.1.1.2.4.2 Total and ammonium-nitrogen The average total nitrogen (Total-N) and ammonium-nitrogen (NH4-N) concentrations obtained during EXP1 (with FBS) and EXP2 (with SBS) are shown in Figures 29 and 30, respectively. As explained in section 3.1.2.6 no change will take place in the amount of total nitrogen through anaerobic digestion. In spite of this statement both Figures show variation in total nitrogen concentration. The reason for that is the effect of reactor content. The slight decrease of total nitrogen concentration in Figure 29 is because of the replacement of the reactor content material with influent material which have lower Total-N. The concentration of Total-N of FBS:CM and FBS, are 2.1 and 1.84 g/kg, respectively, compared to 2.34 g/kg for inoculum. Similarly, the slight decrease of SBS and the increase of SBS:CM Total-N concentration in Figure 30 is due to the change of reactor content with influent. The Total-N concentration tends to run at a constant rate which is due to the long experiment period.

N - concentration [g/kg effluent]

2.4 2.1 1.8 1.5 1.2

Total-N FBS:CM (1:1) Total-N FBS

0.9

NH4-N FBS:CM (1:1)

0.6

NH4-N FBS

0.3 0.0 0

1

2

3

4

5

oTS-loading rate [g/l*d]

Figure 29: Total-N and NH4-N concentration of FBS and FBS:CM effluent.

102

On the other hand, in anaerobic fermentation most of the organic nitrogen will be converted to ammonium-nitrogen (NH4-N). An expected NH4-N concentration increase through both experiments cannot be recognized in Figures 29 and 30. An explanation for this as mentioned above is the higher NH4-N concentration of the reactors content. Therefore, the curves progression explaining that the NH4-N concentration measured moves primarily from reactors concentrations range to actual range of the degradable influent. This can be seen clearly in Figure 30 (EXP2) at which the NH4-N concentration tend to have constant rate indicating that all original reactor content were displaced. This stage was reached after 110 experiment days. However, NH4-N concentrations of EXP1 in Figure 29 had not reached their actual value at the end of the experiments on the 85th day. The decreasing trend of the curves explains this result. Nevertheless, it could be expected that the curves would tend to show horizontal progression in short time. Therefore, the last measured concentrations of 0.91 and 0.75 g/kg for FBS:CM and FBS, respectively, were considered to be effluent concentration. The effluent NH4-N

N - concentration [g/kg effluent]

concentration of SBS:CM and SBS were measured to be 1.2 and 0.3 g/kg, respectively.

3.0

Total-N SBS:CM (1:4) Total-N SBS NH4-N SBS:CM (1:4) NH4-N SBS

2.5 2.0 1.5 1.0 0.5 0.0 0

1

2

3

4

5

6

oTS-loading rate [g/l*d]

Figure 30: Total-N and NH4-N concentration of SBS and SBS:CM effluent.

103

4.1.1.2.4.3 Phosphate-phosphorus Since no phosphate exhausts in gas form through anaerobic degradation, the concentration of phosphate-phosphorus in effluent will only be affected by the substrates fed to the reactors. The average PO4-P concentrations obtained during EXP1 (with FBS) and EXP2 (with SBS) are shown in Figure 31. The decrease of PO4-P concentration of FBS and FBS:CM was due to the effect of reactor content dilution with influent material which had low concentration of PO4-P. However, phosphate is generally quite insoluble, so it is associated with the solids portion of the substrate. Until complete replacement of reactors content occurred, variation of PO4-P concentration between reactor content concentration and influent concentration were expected. This can explain the fluctuation of SBS and SBS:CM PO4-P concentration in the beginning phase. Moreover, settling of phosphate could happen during sampling. However, the curves’ progression in Figure 31 tends to show stable results at the end of the experiments. These results can represent the PO4-P effluent concentration of the substrates

PO4-P concentration [g/kg effluent]

used.

0.8 SBS SBS:CM (1:4) FBS:CM (1:1) FBS

0.6

0.4

0.2

0.0 0

1

2

3

4

5

6

oTS-loading rate [g/l*d]

Figure 31: PO4-P concentration of effluent for mesophilic EXP1 and EXP2

104

4.1.1.2.4.4 FOS/TAC and volatile fatty acids As described before, the relation FOS/TAC characterizes the stability of anaerobic digestion process. Generally, a FOS/TAC value of less than 0.3 refers to a stable process. Figure 32 shows the FOS/TAC-values at different OLR under mesophilic condition.

0.8 FBS FBS:CM (1:1) SBS SBS:CM (1:4)

FOS/TAC - value [-]

0.7 0.6

limit of stability

0.5 0.4 0.3 0.2 0.1 0 0

1

2

3

4

5

6

oTS-loading rate [g/l*d]

Figure 32: FOS/TAC-value with different OLR throughout the mesophilic experiments

At the beginning the FOS/TAC-value recorded was too high and started to decrease during the experiment to reach a stable value. The high FOS/TAC-value at low OLR was due to the adaptation of the bacteria to the process so that they could not degrade all the organic acids. This result is confirmed with the HPLC results in Figures 33, 34, 35 and 36. Then the reactors were operated under stable condition showing FOS/TAC-value within the stability limit up to OLR of 4 g oTS/l*d for FBS and SBS and up to 4.5 g oTS/l*d for FBS:CM and SBS:CM. The high FOS/TAC-value for OLR of more than 4 g oTS/l*d for FBS and SBS and more than 4.5 g oTS/l*d for FBS:CM and SBS:CM coupled with the results of HPLC explain the lower methane productivity at the same OLR and subsequently the break down of the process.

105

Because of the high concentration of total volatile fatty acids (VFA) of raw FBS and SBS eight volatile fatty acids were identified and quantified during the steady state operation (lactic, formic, acetic, propionic, i-butyric, n-butyric, i-valeric, n-valeric, and capronic acid) of which acetic acid and propionic were the predominant volatile acids. Figures 33, 34, 35 and 36 are a summary of the measured concentration for FBS, FBS:CM, SBS and SBS:CM, respectively.

VFA - concentration [g/kg]

2.0 1.6

lactic acid acetic acid

1.2

propionic acid

0.8

0.4 0.0 1

1.5

2

2.5

3

3.5

4

4.5

5

oTS-loading rate [g/l*d]

Figure 33: VFA concentration of FBS throughout mesophilic experiments

VFA - concentration [g/kg]

0.7 0.6 0.5 lactic acid

0.4

acetic acid propionic acid

0.3 0.2 0.1 0.0 1

1.5

2

2.5

3

3.5

4

4.5

5

oTS-loading rate [g/l*d]

Figure 34: VFA concentration of FBS:CM throughout mesophilic experiments

106

Generally, all Figures show similar results; high concentration of VFA in the starting phase due to the adaptation to the system and high concentration at ORL 4 g oTS/l*d for pure substrate and 4.5 g oTS/l*d for mixture substrate which indicate an over-loaded system. There was problem with HPLC at the end of EXP2, therefore the results of SBS and SBS:CM for OLR 4 and 4.5 oTS/l*d could not be measured. Nevertheless, it can also be expected that at higher OLR the VFA concentration will increase.

0.9

VFA - concentration [g/kg]

0.8

methanol butyric acid propionic acid

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1

1.5

2

2.5

3

3.5

4

4.5

oTS-loading rate [g/l*d]

Figure 35: VFA concentration of SBS throughout mesophilic experiments

1.0

VFA - concentration [g/kg]

0.9 methanol butyric acid propionic acid

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1

1.5

2

2.5

3

3.5

4

4.5

oTS-loading rate [g/l*d]

Figure 36: VFA concentration of SBS:CM throughout mesophilic experiments

107

Because of the high sugar content in sugar beets the concentration of another eight substances (fructose, glucose, methanol, ethanol, i-propanol, n-propanol, i-butanol, and n-butanol) were quantified after OLR 2 g oTS/l*d. Figures 35 and 36 show that the very high concentration of fructose and the concentration of glucose and methanol measured in the influent substrates were totally degraded through the anaerobic digestion process. The concentration of methanol was reduced through the digestion process and seems to be constant with increased OLR. It should be noted that this statement is not enough and the results of OLR 4 and 4.5 were needed, which failed, to correctly describe the process. This will be the same for butyric acid concentration.

4.1.1.2.4.5 pH One of the important indication factors in anaerobic reactors is the pH-value. Since the main substrates used in this thesis have a pH-value lower than the optimum pH for anaerobic digestion it was necessary to control the pH regularly. Figure 37 represents the pH-values of the reactors during the experiments. Generally, the Figure shows that the pH-values were within the optimal range for anaerobic digestion. This indicates that the substrates can be used in anaerobic digestion regardless of their low pH-value and without additional pH corrector additives such as NaOH.

8.0 7.5

pH-value [-]

7.0 6.5 6.0

FBS FBS:CM (1:1) SBS SBS:CM (1:4)

5.5 5.0 4.5 4.0 0

20

40

60

80

100

120

140

time [day]

Figure 37: pH-value of reactors for mesophilic EXP1 and EXP2 108

160

4.1.1.2.5 Experiments of FBS containing plant denaturation agents Figure 38 represents the results of methane productivity with different loading rates for the experiments of FBS containing plant denaturation agents (PDA). Similar to the previous experiments the methane production has varied in the beginning during the adaptation period and then showed constant rate. The gradual increase in the PDA concentration in FBS from 0.5 to 1.5 g per kg FBS at constant OLR which is equivalent to an increase from 0.0795 to 0.239 g PDA per reactor and day, has not shown any change in the methane productivity. However, the additional amount of PDA added to the reactors on the 7th and 14th days of the experiment has clearly affected the digestion process of the reactor with FBS-Arcotal which results in the high acid concentration in Figure 40 and hence low methane productivity. On the other hand the reactors with FBSTieröl and FBS-Arbin could not be affected by the additional amount of PDA and showed no accumulation of acids as shown in Figures 39 and 41 and hence no change in methane production.

5 FBS FBS(Tieröl) FBS(Arcotal) FBS(Arbin) oTS-OLR

1.2 1.0

4

3

0.8 0.6

2

0.4

oTS-loading rate [g/l*d]

methane productivity [l/l*d]

1.4

1 0.2 0.0

0 1

11

21

31

41

51

61

71

81

91

101

time [day]

Figure 38: Methane productivity of FBS mixed with PDA in semi-continuous reactors

109

Figure 38 also shows that only with higher OLR (> 2 g oTS/l*d), which results in higher PDA concentration (> 0.032 g PDA per liter reactor and day), all the reactors with PDA were over loaded. The high concentration of propionic acid (> 1.6 g/kg) in all reactors indicates the break down of the reactors (Figures 39, 40 and 41). The acetic acid concentration of the reactor with FBS-Tieröl in Figure 39 shows that it increased gradually with low concentration throughout the experiment. It is assumed that the concentration will keep increasing as a result of addition of Tieröl without considering the effect of the propionic acid on the 94th experiment day. The result concludes that the stated amount of Tieröl, which is less than 0.2 g/kg forage beets (BLE, 2002), will not affect the digestion process.

VFA - concentration [g/kg]

2.0

1.6 lactic acid acetic acid propionic acid

1.2

0.8

0.4

0.0 14

18

82

94

time [day]

Figure 39: VFA concentration of FBS mixed with Tieröl

The effect of PDA-Arcotal on the digestion process is clearly seen in Figure 40. The additional amount of Arcotal on the 8th and 14th experiment days results in high propionic concentration which were degraded throughout the experiment. On the other hand, the acetic acid concentration increased during the experiment to reach a concentration of 1.4 g/kg on the 94th experiment day regardless of the high propionic acid concentration.

110

Contrary to Tieröl and Arcotal, the results of Arbin in Figure 41 show that neither the additional amounts of Arbin on the 8th and 14th days nor the low concentration have resulted in an organic acid accumulation. Generally, low concentration of Tieröl and Arbin (< 0.024 g per liter reactor and day) show no inhibition affect on the digestion process whereas Arcotal shows significant effect. Increasing the OLR, the PDA concentration per reactor increased, which resulted in the breakdown of the digestion process.

VFA - concentration [g/kg]

2.0 lactic acid acetic acid

1.6

propionic acid

1.2

0.8

0.4

0.0 14

18

82

94

time [day]

Figure 40: VFA concentration of FBS mixed with Arcotal

VFA - concentration [g/kg]

2.0

1.6

lactic acid acetic acid propionic acid

1.2

0.8

0.4

0.0 14

18

82

94

time [day]

Figure 41: VFA concentration of FBS mixed with Arbin

111

4.1.2 Thermophilic experiments

4.1.2.1 Batch experiments

4.1.2.1.1 Inoculum The methane accumulative curves of thermophilic FBS batch experiments of the inoculum are represented in Figure 42. The shape of the curves signifies the pattern of methane production over the examined period and shows an expected progression. The curves progression of reactors T(G5) & T(G6) are identical which is an indication of inoculum homogeneity in the two reactors. Until ending the experiments on the 44th day the methane accumulative curves were still increasing which is a sign of unfinished degradation. The methane yield of inoculum within the considered period of the experiment (33 day) is 3.0 l CH4 per kilogram inoculum for each reactor. This value compared to inoculum used for the mesophilic experiments is too high and indicates that the inoculum is active. Similar to the mesophilic experiments, the methane yield per kilogram inoculum is subtracted from other batch experiments to indicate the specific methane production of the substrate.

methane yield [l/kg inoculum]

4.0 T(G5) inoculum

3.5

T(G6) inoculum

3.0 2.5 2.0 1.5 1.0 0.5 0.0 1

6

11

16

21

26

31

36

41

time [day]

Figure 42: Methane accumulative curve of inoculum used for thermophilic FBS batch experiments

112

4.1.2.1.2 Forage beets silage The results of batch experiments for the thermophilic forage beets silage FBS are shown in Figures 43 and 44. Figure 43 shows the accumulative methane curves of the reactors T(G7) and T(G8). Generally, the curves progression are similar to that of FBS under mesophilic condition. In the first phase the light degradable substances were converted to methane (85 % of organic matter). After the 15th day the hard degradable substances were converted to methane. On the 27th day no methane production was reported which indicates that all the degradable substances were degraded. Figure 44 shows the curves progression more clearly as daily methane production. The main difference in the presented results of the thermophilic batch experiments to that of the mesophilic, is that the thermophilic bacteria did not need any adaptation time to adapt to the milieu. This is mainly referred to the very active inoculum used. It is also important to ascertain here that the delay in the mesophilic batch experiments was due to the less active inoculum. From Figure 43 the amount of methane that can be produced from 1 kg FBS is 45 and 43 l per kilogram FBS for reactors T(G7) and T(G8), respectively. This value represents ca. 90 % degradation efficiency.

50

methane yield [l/kg FBS]

max. theoretical methane yield

40

30 T(G7) FBS

20

T(G8) FBS

10

0 1

5

9

13

17

21

25

29

time [day]

Figure 43: Accumulative methane of thermophilic FBS batch experiments

113

9

methane [l/kg FBS per day]

8 7

T(G7) FBS

6

T(G8) FBS

5 4 3 2 1 0 1

6

11

16

21

26

31

time [day]

Figure 44: Daily methane production of thermophilic FBS batch experiments

4.1.2.1.3 Forage beets silage with manure The results of the thermophilic forage beets silage mixed with manure (FBS:CM) batch experiments are shown in Figure 45 and 46. Figure 45 shows the accumulative methane curves whereas Figure 46 shows the daily methane production. The Figures are duplicates from Figures 43 and 44. The curves progression in the two experiments is similar, but they differentiate in that only 78 % of the organic matter is degraded at the beginning for the mixture of FBS with cow manure, and within a short time the degradable materials available for the anaerobic bacteria were consumed. The maximal theoretical methane value on the basis of the stoichiometric calculation for the FBS:CM is 29 liter per kilogram FBS:CM. The maximal measured values are 23 and 22 l methane per kg mixture for reactors T(G9) and T(G10), respectively. From these values the COD degradation efficiency is calculated to have a value of ca. 85 %. The reduction in the efficiency compared to that of pure FBS is due to the low degradation efficiency of manure which is normally around the range of 50 % (Wellinger, 1991).

114

35

methane yield [l/kg (FBS:CM)]

max. theoretical methane yield

30 25 20 15

T(G9) FBS:CM (1:1)

10

T(G10) FBS:CM (1:1)

5 0 1

5

9

13

17

21

25

29

time [day]

Figure 45: Accumulative methane of thermophilic FBS:CM batch experiments

methane [l/kg (FBS:CM) per day]

5

4 T(G9) FBS:CM (1:1) T(G10) FBS:CM (1:1)

3

2

1

0 1

6

11

16

21

26

31

time [day]

Figure 46: Daily methane production of thermophilic FBS:CM batch experiments

The mean results of the batch experiments of forage beets silage (FBS) under mesophilic and thermophilic temperatures are listed in Table 24. The Table shows no variation in the batch experiments under mesophilic and thermophilic conditions.

115

Table 24: Mean results of batch experiments (FBS) under different temperatures Substrate

Methane yield [l/kg-FM]

Methane yield [l/kg-oTS]

Degradation efficiency [%]

FBSmesophile

47

468

95

FBSthermophile

45

468

95

FBS:CM (1:1)thermophile

23

400

85

4.1.2.2 Semi-continuous experiments

4.1.2.2.1 Methane productivity

4.1.2.2.1.1 Forage beets silage The methane productivity of the thermophilic experiments for reactors T(K5) and T(K6) is shown in Figure 47. The curves progression are similar to the results produced from FBS under mesophilic condition, i.e., with increasing OLR the methane productivity increases. After reaching the maximum loading rate of 4 g oTS/l*d (under stable condition) the methane productivity has decreased with increasing in OLR.

6

T(K5) FBS T(K6) FBS OLR-oTS

2.0

5 4

1.5 3 1.0 2 0.5

1

0.0

0 1

21

41

61

81

101

121

141

161

time [day]

Figure 47: Methane productivity of FBS in semi-continuous reactors

116

oTS-loading rate [g/l*d]

methane productivity [l/l*d]

2.5

Feeding the reactors at 3 g oTS/l*d OLR after 5 days feeding-break after the 103rd experiment day caused an over loading of the process. Therefore, it was necessary, after an additional rest day to degrade the accumulated substances, to start feeding again with low OLR in increasing stepwise. This method also was not sufficient to operate the reactors again with its last obtained OLR and led to process disruption (less methane production). The reason is that the thermophilic bacteria could not adapt to the high increase in OLR. An attempt was made to avoid the accumulation by reducing OLR in the reactors: it did not improve the situation. It was therefore necessary to stop feeding until the accumulated substances were degraded and then return to feeding with low OLR (3,5 g oTS/l*d) until a steady state was reached. The methane productivity obtained during this period (31 days) was within the normal methane production range at the same OLR. Anyhow, this result is not enough to describe the stability of the digestion process and must be compared with the results in section 4.1.2.2.4.4.

methane productivity [l/l*d]

2.5

thermophilic

2.0

mesophilic

1.5

1.0

0.5

0.0 1

2

2.5

3

3.5

4

oTS-loading rate [g/l*d]

Figure 48: Methane productivity of FBS at different OLR

Figure 48 demonstrates the methane productivity of FBS under thermophilic condition compared to mesophilic conditions. It is clear from Figure 48, starting from OLR of 2.5 g oTS/l*d, that the thermophilic methane productivity increased slightly above the mesophilic methane productivity until both reached their maximum at OLR of 4 g oTS/l*d. The maximum measured methane productivity under thermophilic condition was 2.1 l/l*d. 117

This result reflects that under thermophilic condition (55 °C) methane can be more efficiently produced than under mesophilic condition (37 °C). On the other hand, the results under thermophilic conditions reveal that the digestion process is very sensitive to any disruption which can occur in real plants. Moreover, the thermophilic process consumes more heating energy than the mesophilic which increases the input cost.

4.1.2.2.1.2 Forage beets silage with manure Similar to the results of pure FBS Figure 49 shows the results of methane productivity from forage beets silage with cow manure (FBS:CM) for reactors T(K7) and T(K8). Increasing OLR resulted in increasing the methane productivity to reach its maximum 4.5 g oTS/l*d. The feeding of FBS:CM after the 43rd experiment day did not produce the expected amount of methane which is a sign of substances accumulation (see result of FOS/TAC, Figure 57). This disruption might be due to the jump from OLR 3 to 3.5 g oTS/l*d coupled with short HRT. Therefore, to prevent over loading the OLR was first reduced to 3 g oTS/l*d and then increased in shorter intervals which helped to overcome the disruption. Feeding the reactors with lower OLR after feeding break on the 108th experiment day was successful and no disruption was recorded. This indicates that the addition of manure to FBS increases the stability of the process even under thermophilic condition.

7 T(K7) FBS:CM (1:1)

6

T(K8) FBS:CM (1:1)

2.0

OLR-oTS

5

1.5

4 3

1.0

2 0.5

oTS-loading rate [g/l*d]

methane productivity [l/l*d]

2.5

1 0.0

0 1

21

41

61

81

101

121

141

161

time [day]

Figure 49: Methane productivity of FBS:CM in semi-continuous reactors

118

4.1.2.2.2 Biogas composition The biogas produced throughout the thermophilic experiments for FBS and FBS:CM under steady state conditions, has shown similar composition to that of the mesophilic experiments. Table 25 shows the measured biogas compositions under thermophilic conditions. Variation in the biogas composition was noticed when the operated conditions were not stable. This was the case when the OLR increased, or at the starting phase, or under over loading phase, or even after one day feeding’s break. The percentage of methane in biogas obtained is similar to the results found by Beck, et al., (2002).

Table 25: Biogas composition (thermophilic experiments) Reactor

CH4 (%)

CO2 (%)

H2S (ppm)

FBS

53 ± 1

46 ± 1

100

FBS:CM (1:1)

55 ± 1

44 ± 1

100

4.1.2.2.3 Degradation efficiency The degradation efficiency of the semi-continuous thermophilic experiments was evaluated in a similar way to mesophilic experiments. Figure 50 shows the mean degradation degree according the gas phase. The lower degradation efficiency of FBS:CM is due to the lower degradation efficiency of manure. With exception of the unrealistic degradation efficiency (> 100 %) in the starting phase in Figure 50 the curves show horizontal progression. This indicates that all the organic materials were degraded before they were removed from the reactor despite the increase in OLR. The high degradation efficiency with high OLR is an advantage of thermophilic experiments over the mesophilic experiments. The very high degradation efficiency at the beginning can be attributed to the active methane productivity of inoculum.

119

degradation efficiency [%]

120 100 80 60 FBS

40

FBS:CM (1:1)

20 0 1

2

2.5

3

3.5

4

4.5

oTS-loading rate [g/l*d]

Figure 50: Degradation efficiency of substrates with different loading rates

4.1.2.2.4 Laboratory analyses

4.1.2.2.4.1 Chemical oxygen demand The average chemical oxygen demand (COD) of effluent of FBS and FBS:CM for different loading rate throughout the thermophilic experiments is shown in Figure 51. As explained before, the effluent COD value is a function of reactor content (inoculum) and the actual value can be considered when all the inoculum is replaced by the influent. This can explain clearly the run of Figure 51 which shows that the COD throughout the experiment, beginning with high COD, decreased rapidly until the 80th experiment day for FBS:CM and 120th experiment day for FBS after which tended to maintain a constant low value. The low COD is a result of continued biological stabilization. During the COD decline period it was not easy to calculate the actual COD value of the degraded substrate because of the high COD of inoculum. Therefore, the longer the reactor operated under steady state condition, the closer the COD got to the actual value.

120

80

COD [g/kg effluent]

FBS

60

FBS:CM (1:1)

40

20

0 0

50

100

150

200

time [day]

Figure 51: COD of FBS and FBS:CM throughout the experiment

4.1.2.2.4.2 Total and ammonium-nitrogen Figure 52 presents the average total nitrogen (Total-N) and ammonium-nitrogen (NH4-N) concentrations for FBS and FBS:CM throughout the thermophilic experiment. Similar to the explanation of the mesophilic experiments, no considerable change took place in the amount of total nitrogen throughout the anaerobic digestion. The decrease seen in this Figure is due to the replacement of high Total-N concentration of inoculum with the concentration of FBS and FBS:CM. The horizontal curve progression of Total-N indicate steady state condition and the ending of the reactor replacement effect. The concentration of Total-N for FBS and FBS:CM was measured at the end of the experiment to be 2.1 and 2.49, respectively. Generally, because of the conversion of organic nitrogen to ammonium-nitrogen in anaerobic digestion the concentration of NH4-N was expected to increase. The decrease in NH4-N in Figure 52 was due to reactor replacement explained before. The longer feeding the reactors the less is the effect of replacement, which can be shown in the horizontal trend of the curves in Figure 52. Therefore, the measured NH4-N concentration in this period can represent the actual effluent concentration. The effluent NH4-N concentration for FBS and FBS:CM was measured at the end of the experiment to have a value of 0.94 and 1.34 g/kg, respectively. These values are higher than the concentration in FBS and FBS:CM and can improve the fertilizer efficiency of the effluent.

121

N - concentration [g/kg effluent]

5 Total-N FBS Total-N FBS:CM (1:1)

4

NH4-N FBS NH4-N FBS:CM (1:1)

3

2

1

0

0

50

100

150

200

time [day]

Figure 52: Total-N and NH4-N concentration for effluent of FBS and FBS:CM

4.1.2.2.4.3 Phosphate-phosphorus Figure 53 shows the average PO4-P concentrations of FBS and FBS:CM throughout the thermophilic experiment. As explained before the concentration of phosphorus in effluent will only be affected by the substrates fed to the reactors. The Figure shows the expected result. At the beginning of the experiment PO4-P concentrations were in the range of the inoculum. During the experiment the concentration of PO4-P was directed towards the concentration of influents. The PO4-P concentration on the 176th experiment day were measured to be 0.23 and 0.34 g/kg for FBS and FBS:CM, respectively. These values were considered to represent the effluent concentration.

122

PO4-P concentration [g/kg effluent]

0.4 FBS FBS:CM (1:1)

0.3

0.2

0.1

0.0 0

50

100

150

200

time [day]

Figure 53: PO4-P concentration of effluent for FBS and FBS:CM

4.1.2.2.4.4 FOS/TAC and Volatile fatty acids As mentioned before FOS/TAC–value of 0.3 is considered as a limit value for determining the stability of the anaerobic process. For the thermophilic experiments with FBS the FOS/TACvalues were measured within the stable range ( 0.1 Euro/kWh). However, an assessment of energy balance, ecological balance and economy of biogas production from forage and sugar beets came out in favor of forage beets.

151

8

Zusammenfassung

Sowohl die begrenzten Ressourcen an fossilen Energien als auch die durch Gewinnung und Nutzung fossiler Brennstoffe hervorgerufenen Umweltbelastungen geben in der letzten Zeit der Energieproduktion aus erneuerbaren Rohstoffen einen hohen Stellenwert. Eine der momentan verfügbaren und verwendbaren alternativen Energiequellen ist die anaerobe Vergärung (Biogasproduktion) von nachwachsenden Rohstoffen, die von der deutschen Bundesregierung gefördert wird. Futter- und Zuckerrüben sind aufgrund ihres hohen organischen Trockenmassegehalts (oTS), der pro Hektar erzielt wird, sehr energieeffiziente Pflanzen, die daher für eine Biogasproduktion besonders geeignet erscheinen. Sie können einfach siliert und gelagert werden und stehen somit ganzjährig für die Biogasproduktion zur Verfügung. Bisher liegen aber keine abgesicherten Daten über die anaerobe Vergärung von Futter- und Zuckerrüben vor. Zudem stellt die Kombination von Silierung und Fermentation eine neuartige Verfahrenskette dar. Das erste Ziel dieser Arbeit war die genaue Untersuchung der anaeroben Vergärung von Futter- und Zuckerrübensilage (FRS & ZRS). Hierzu wurden Untersuchungen im Labor (17 diskontinuierliche Reaktoren und 16 quasi-kontinuierliche Reaktoren) durchgeführt, um den maximalen Abbaugrad von Futter- und Zuckerrübensilage, sowie den Biogasertrag, den Methan- und H2S–Gehalt des Biogases unter verschiedenen Bedingungen zu bestimmen. Die Möglichkeit der Kofermentation von Rüben mit Rindergülle (RG) wurde ebenfalls in dieser Arbeit untersucht. Der Einfluss des Vergärungsprozesses auf verschiedene Eigenschaften des Gärrückstandes

(chemischer

Sauerstoffbedarf,

Ammonium-Stickstoff-Gehalt,

flüchtige

organische Säuren, etc.) wurde bestimmt. Das zweite Ziel dieser Arbeit bestand in der Untersuchung der Energiebilanz, Ökobilanz und Ökonomie beim Wechsel von fossilen Energieträgern zur Energieproduktion auf Basis von Biogas. Dabei sollten möglichst optimierte Bilanzmethoden definiert werden, mit welchen alle beeinflußten und meßbaren Parameter berücksichtigt werden. Die Ergebnisse zeigen, dass FRS und ZRS grundsätzlich für Mono- und Kofermentation geeignet sind. Durch Beimischung von Rindergülle sinkt der Abbaugrad und infolgedessen auch die Methanausbeute, jedoch wird gleichzeitig die Prozeßstabilität erhöht. Die Ergebnisse der Batch-Versuche sind in der folgenden Tabelle dargestellt.

152

Durchschnittliche Ergebnisse der Batch-Versuche Methanausbeute [l/kg-FM]

Methanausbeute [l/kg-oTS]

Abbaugrad [%]

FRS

47

468

95

ZRS

83

400

92

ZRS:RG (1:4)

17

294

62

FRS

45

468

95

FRS:RG (1:1)

23

400

85

Thermophile Temperatur

Mesophile Temperatur

Substrate

In den quasi-kontinuierlichen Versuchen betrug die maximal erzielbare Raumbelastung unter stabilen Prozessbedingungen für Futter- und Zuckerrüben 4 g oTS/l*d. Durch die Beimischung von Gülle stieg dieser Wert auf 4.5 g oTS/l*d. Die folgende Tabelle zeigt die Methanproduktivität bei maximaler Raumbelastung sowie den H2S- und Methan-Gehalt, die für die Ökonomie der Biogasanlage von Bedeutung sind.

Durchschnittliche Ergebnisse der quasi-kontinuierlichen Versuche Methanproduktivität [l/l*d]

Methangehalt [%]

H2S Gehalt [ppm]

FRS

1.92

53 ± 1

100

ZRS

1.80

53 ± 1

350

FRS:RG (1:1)

1.76

55± 1

100

ZRS:RG (1:4)

1.58

56 ± 1

850

FRS

2.10

53 ± 1

100

FRS:RG (1:1)

1.90

55 ± 1

100

Thermophile Temperatur

Mesophile Temperatur

Substrate

Die Versuchsergebnisse zeigen eine Zunahme des NH4-N-Gehalts um bis zu 55 %, wodurch die Effizienz bei der Verwendung der Gärrückstände als Wirtschaftdünger verbessert wird. Die durch den Silierungsprozess hervorgerufene hohe Konzentration von organischen Säuren in

153

den Gärsubstraten beeinflusste den pH-Wert des Vergärungsprozesses nicht, was auf eine hohe Pufferkapazität des Gärsubstrates hinweist. Die Arbeit gibt ebenfalls Informationen über den Einfluss von Denaturierungsmitteln auf den Vergärungsprozess. Von der energetischen Seite her betrachtet wurde festgestellt, dass bei alleiniger Betrachtung der elektrischen Energieproduktion aus Futterrüben (Zuckerrüben) ein Output/Input–Faktor von 1,36 gegenüber einem Output/Input–Faktor von 0,32 bei fossilen Energieträgern erzielt werden kann. Mit Berücksichtigung der Wärmeproduktion erhöht sich der Output/Input–Faktor der Biogasenergie auf 3,06. Ein Ersatz von fossilen Energieträgern kann durch den höheren Output/Input–Faktor und die positive Biogasenergie zu Einsparungen von 280 (264) GJ fossile Energie pro Hektar und Jahr führen. Von Vorteil ist weiterhin, dass die Verwendung der ganzen Pflanze und die Benutzung der Gärrückstände als Wirtschaftsdünger die Biogasgewinnung durch eine einfache und geschlossene Energiebilanz kennzeichnet. Im Vergleich zu fossiler Energie ergibt sich bei der Verwendung von Biogas aus Rüben eine bessere Ökobilanz. Durch die Produktion von Biogas aus Futter- bzw. Zuckerrüben kann im Vergleich zur Verwendung von Fossilenergie pro Hektar die Freisetzung von 21 bzw. 20 Tonnen CO2-Äquivalenten vermieden werden. Generell hat die anaerobe Fermentation einen sehr geringen negativen Einfluss auf die globale Umwelt. Aufgrund der ökonomischen Analyse ist die Biogasproduktion aus Futter- bzw. Zuckerrüben derzeit wirtschaftlich noch nicht empfehlenswert. Durch die Nutzung der Wärmeenergie sowie bei Erhöhung der derzeitigen Einspeisevergütung (> 0.1 Euro/kWh) kann die Biogasproduktion aus Futter- bzw. Zuckerrüben jedoch wirtschaftlich werden. Grundsätzlich ist die Energie- und Ökobilanz sowie die Ökonomie der Biogasproduktion aus Futterrüben vorteilhafter im Vergleich zu Zuckerrüben.

154

9

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163

10 Appendix Appendix 1: Gas measuring system used in the experiments

3

2 1 4

11

10

5 6

Exhaust air

9

8

1. Gas measurement bag 2. Condensed water separator 3. Pumps 4. Three way valve 5. Needle valve 6. Humidity controller 7. CH4 – Measure device Type MAIHACK 8. H2S – Measure device Type HARTMANN&BRAUN 9. CO2 – Measure device Type SIEMENS 10. Gas meter Type RITTER 11. Bypass pathway (to quick empty the gas bag)

164

7

Appendix 2: Properties of different types of biogas engines Characteristics Efficiency [%] Life time Maintenance Operating hour Invest. Cost Power [kW]

Benzine motor Gas-Otto 22-27 low high 10000 low 5-30

Injection Diesel engine 28-35 medium high 30000-40000 medium 30-150

Gas motor ³ 35 high low 80000 high > 150

Source: Weiland, 2000

165

166

0.408 0.408 -

0.0025** 0.13664

188.33 188.33 -

85** 3155.5

g/MJ 188.33 74.4 8.479 271.209 58.753

120 120 -

203040 564

MJ 67500 104700 104700

203040

0.18 0.534 0.534 0.13664 -

4921 368.12 368.12 3155.5 -

4.7 110 4 110 -

g/MJ 0.408 0.003 0.123 0.534 0.1235

0.2 7.45 2.07 1.38 0.29

1774.6 2829 1117 617 284

CH4 g/kg

6 275 130 350 700

CO2 g/kg

g/MJ 0.00694 0.001 0.00021 0.029927 0.00125

kg/ha

kg/ha 0.46845 0.1047 0.021987 0.595137 0.2538

0.223344 0.1854376 0.41 1.56 1.38

0.00 0.00 0.00

0.0252 4.1525 0.00494 0.1715 0.0133 -3.26 0.007097 0.0009867 0.0000359 0.0361669 1.15

N2O

0.0011** 0.32879

0.00694 0.00694 -

1.51 0.00897 0.00897 0.32879 -

4.2 15.1 0.038 0.49 0.019

g/kg

52.03 6.94 58.97

19.10 2157.99 153.52 282.91 208.13 -2101.91 25.42 42.28 1.54 342.73 1131.70 0.466 0.466 -

17.4 2.037 2.037 0.8967 -

6.6 5.16 12 0.27 0.11

g/kg

kg/ha

kg/ha 31.455 2.1987 3.18288 36.83658 0.0881194

0.081216 0.5057388 0.59 1.96 1.45

0.12 0.02 0.14

0.0396 1.419 1.56 0.0945 0.077 -2.36 0.08178 0.22407 0.008148 0.098637 1.24

SO2

17342.56 0.0004** 1840,97 0.8967 1840.97 3031.64 1190.67 Fossil energy emission g/MJ kg/ha g/MJ 199.166 0.466 13550.68 74.795 7831.04 0.021 11.6212 1216.74 0.0304 22598.46 61.7625 10900.80 0.000434

85.4** 3264.13

200.75 200.75 -

3182.6 7847.3 1180.9 808.3 297.3 -7600.34 5408.7 384.3 384.3 3264.13 -

CO2-equivalent g/kg kg/ha

Source: (Kaltschmitt, et al.,1997 / Patyk, et al., 1997) from (AGE, 1995 / Fritsche, et al., 1995) * electricity emissions, ** units are in g/MJ, *** 80% fuel efficiency assumed

Electricity Heat (burning) *** Heat (production) *** Sum fossil (kg/ha) Natural gas burning

Beets production Seed (kg) N (kg) P2O5 (kg) K2O (kg) CaO (kg) Reactor effluent Plant PA (kg) Diesel (kg) Lubricant (kg) Fuel burning (kg) Machines (kg) Sum Biogas production Chopping (MJ/t) * Pumping (MJ/t) * Silo (m³) Sum Energy production Heating, stirring, etc. CHP Gas burning (MJ) Heating oil (kg) Sum Total 20ha (kg/ha) Total 80h (kg/ha)

Amount 1/ha.a

Appendix 3a: Emissions from biogas production (FBS) and fossil life cycle

0.07846

g/MJ 0.044 0.0143 0.00455

0.017** 9.821

0.044 0.044 -

2.66 0.3023 0.3023 9.821 -

2.6 2.8 1.42 0.42 3

g/kg

kg/ha

kg/ha 2.97 1.49721 0.476385 4.943595 15.930518

3.45168 5.539044 8.99 10.90 5.36

0.01 0.00 0.01

0.0156 0.77 0.1846 0.147 2.1 -2.40 0.012502 0.033253 0.0012092 1.08031 1.89

CO

0.1287

g/MJ 0.183 0.03 0.019

0.028** 33.733

0.183 0.183 -

6.92 1.558 1.558 33.733 -

12.9 15.8 8.58 1.15 0.52

g/kg

kg/ha 12.3525 3.141 1.9893 17.4828 26.131248

5.68512 19.025412 24.71 30.15 11.13

0.05 0.01 0.05

0.0774 4.345 1.1154 0.4025 0.364 -4.67 0.032524 0.17138 0.006232 3.71063 5.39

NOx kg/ha

0.00351

g/MJ 0.00472 0.003 0.0102

0.0025** 5.551

0.00472 0.00472 -

0.29 0.679 0.679 5.551 -

kg/ha 0.3186 0.3141 1.06794 1.70064 0.7126704

0.5076 3.130764 3.64 4.38 1.25

0.00 0.00 0.00

0.0048 0.15675 0.0689 0.0455 0.0357 -0.23 0.001363 0.07469 0.002716 0.61061 0.74

NMHC kg/ha

0.8 0.57 0.53 0.13 0.051

g/kg

167

0

0.00031

g/MJ 0.00167

kg/ha 0.112725 0 0.032457 0.145182 0

0 1.661544 1.66 2.01 0.35

-

2.946

0.00 0.00 0.00

0.003 0.026125 0.0663 0.01645 0.014 -0.09 0.0002021 0.008635 0.000314 0.32406 0.35

0.00167 0.00167

0.043 0.0785 0.0785 2.946 -

0.5 0.095 0.51 0.047 0.02

0.00164

g/MJ 0.01083 0.001 0.00093

0.0001** 0

kg/ha

g/kg

HCL kg/ha

kg/ha 0.731025 0.1047 0.097371 0.933096 0.3329856

0.020304 0 0.02 0.47 0.47

0.00 0.00 0.00

0

g/MJ 0.0111 0 0.000048

0 0.020069

5.55E-06 5.55E-06 -

kg/ha

0.00 0.00 0.00

0.012 1.83975 0.00156 0.000665 0.000644 -1.38 0.000752 0.0002020 0.0000073 0.0022076 0.48

NH3

0.16 0.001836 0.001836 0.020069 -

2 6.69 0.012 0.0019 0.00092

g/kg

0 0.0113190 0.01 0.49 0.48 Fossil energy emission kg/ha g/MJ kg/ha 0.74925 5.55E-06 0.0003746 0 0.00E+00 0 0.0050256 2.00E-05 0.002094 0.7542756 0.0024686 0 0 0

0.00 0.00 0.00

0 0 0.0009821 0.0005539 0.00 0.02 0.02

0.0111 0.0111 -

0.0036 0.1 0.0006 0.63525 0.068 0.0187 0.1443 0.021 0.00273 0.2975 0.074 0.0259 0.665 0.013 0.0091 -1.31 -0.04 0.004418 0.21 0.000987 0.004554 0.00213 0.0002343 0.0001656 0.00213 0.0000085 0 0.0009821 0.000108 0.45 0.02

Dust

0.01083 0.01083 -

0.94 0.0414 0.0414 0 -

0.6 2.31 1.11 0.85 0.95

g/kg

Source: (Kaltschmitt, et al.,1997 / Patyk, et al., 1997) from (AGE, 1995 / Fritsche, et al., 1995) * electricity emissions, ** units are in g/MJ, *** 80% fuel efficiency assumed

Electricity Heat (burning) *** Heat (production) *** Sum fossil (kg/ha) Natural gas burning

Beets production Seed N P2O5 K2O CaO Reactor effluent Plant PA Diesel Lubricant Fuel burning Machines Sum Biogas production Chopping* Pumping* Silo Sum Energy production Heating, stirring, etc. CHP Gas burning Heating oil Sum Total 20ha (kg/ha) Total 80h (kg/ha)

Partikel g/kg kg/ha

Appendix 3a: continue

0.00019

g/MJ 0.000183 0.0001 0.000069

0.00009** 0.4697

0.000183 0.000183 -

0.013 0.004697 0.004697 0.4697 -

0.067 0.021 0.032 0.0081 0.0035

0 0.11102

0.0001027 0.0001027 -

0.0056 0.003587 0.003587 0.11102 -

kg/ha

0 0.0626153 0.06 0.08 0.01

0.00 0.00 0.00

0.0001002 0.002035 0.001079 0.00077 0.000616 -0.00 0.0000263 0.0003946 0.0000144 0.0122122 0.01

Benzol

0.0167 0.0074 0.0083 0.0022 0.00088

g/kg

kg/ha g/MJ kg/ha 0.0123525 0.0001027 0.0069323 0.01047 0 0 0.0072243 0.00002 0.002094 0.0300468 0.0090263 0.0385776 0.00015 0.030456

0.0182736 0.2649108 0.28 0.34 0.07

0.00 0.00 0.00

0.000402 0.005775 0.00416 0.002835 0.00245 -0.01 0.0000611 0.0005167 0.0000188 0.051667 0.05

Formaldehy g/kg kg/ha

0.0905

g/MJ 0.603 0.042 0.043

0.02** 24.55 24.57

0.603 0.603 -

22.73 3.13 3.13 24.55 -

19.48 28.7 18.05 1.07 0.46

kg/ha 40.70 4.40 4.50 49.60 18.38

7.14 13.85 20.99 27.02 13.18

0.07 0.07 0.14

0.12 7.89 2.35 0.37 0.32 -8.20 0.11 0.34 0.01 2.70 5.89

SO2-equivalent g/kg Kg/ha

100

1.638

H2S ppm kg/ha

168

4921 368.12 368.12 3155.5 -

188.33 188.33 -

85** 3155.5

g/MJ 188.33 74.4 8.479 271.209 58.753

4.47 105 4 105 -

65 65 -

191520 532

MJ 63700 98750 98750

191520

1774.6 2829 1117 617 284

4 220 180 380 700

CO2 g/kg

g/MJ 0.408 0.003 0.123 0.534 0.1235

0.0025** 0.13664

0.408 0.408 -

0.18 0.534 0.534 0.13664 -

0.2 7.45 2.07 1.38 0.29

CH4 g/kg

g/MJ 0.00694 0.001 0.00021 0.00815 0.00125

0.0011** 0.32879

0.00694 0.00694 -

1.51 0.00897 0.00897 0.32879 -

4.2 15.1 0.038 0.49 0.019

g/kg

kg/ha

kg/ha 0.442078 0.09875 0.0207375 0.5615655 0.2394

0.210672 0.17491628 0.39 1.33 1.15

0.00 0.00 0.00

0.0168 3.322 0.00684 0.1862 0.0133 2.65 0.0067497 0.00094185 0.00003588 0.03452295 0.94

N2O

28.19 3.76 31.94

12.49 1726.40 212.56 307.15 208.13 1840.68 24.18 40.36 1.54 342.73 1034.86 0.466 0.466 -

17.4 2.037 2.037 0.8967 -

6.6 5.16 12 0.27 0.11

g/kg

16358.59 0.0004** 1736.52 0.8967 1736.52 2803.32 1066.80 Fossil energy emission g/MJ kg/ha g/MJ 200.75 12787.83 0.466 74.80 7386.01 0.021 11.62 1147.59 0.0304 21321.43 61.7625 11828.75 0.000434

85.41** 3264.13

200.75 200.75 -

3123.60 7847.25 1180.91 808.30 297.33 7600.34 5408.70 384.34 384.34 3264.13 -

CO2-equivalent g/kg kg/ha

Source: (Kaltschmitt, et al.,1997 / Patyk, et al., 1997) from (AGE, 1995 / Fritsche, et al., 1995) * electricity emissions, ** units are in g/MJ, *** 80% fuel efficiency assumed

Electricity Heat (burning)*** Heat (production)*** Sum fossil (kg/ha) Natural gas burning

Beets production Seed (kg) N (kg) P2O5 (kg) K2O (kg) CaO (kg) Reactor effluent Plant PA (kg) Diesel (kg) Lubricant (kg) Fuel burning (kg) Machines (kg) Sum Biogas production Chopping (MJ/t) * Pumping (MJ/t) * Silo (m³) Sum Energy production Heating, stirring, etc. CHP Gas burning (MJ) Heating oil (kg) Sum Total 20ha (kg/ha) Total 80h (kg/ha)

Amount 1/ha.a

Appendix 3b: Emissions from biogas production (SBS) and fossil life cycle kg/ha

kg/ha 29.6842 2.07375 3.002 34.759950 0.08311968

0.076608 0.4770444 0.55 1.92 1.44

0.07 0.01 0.07

0.0264 1.1352 2.16 0.1026 0.077 2.61 0.077778 0.213885 0.008148 0.0941535 1.29

SO2

0.07846

g/MJ 0.044 0.0143 0.00455

0.017** 9.821

0.044 0.044 -

2.66 0.3023 0.3023 9.821 -

2.6 2.8 1.42 0.42 3

g/kg

kg/ha

kg/ha 2.8028 1.412125 0.4493125 4.664238 15.0266592

3.25584 5.224772 8.48 10.36 5.13

0.01 0.00 0.01

0.0104 0.616 0.2556 0.1596 2.1 2.35 0.0118902 0.0317415 0.0012092 1.031205 1.87

CO

0.1287

g/MJ 0.183 0.03 0.019

0.028** 33.733

0.183 0.183 -

6.92 1.558 1.558 33.733 -

12.9 15.8 8.58 1.15 0.52

g/kg

kg/ha 11.6571 2.9625 1.87625 16.4959 24.648624

5.36256 17.945956 23.31 28.61 10.67

0.05 0.01 0.05

0.0516 3.476 1.5444 0.437 0.364 4.37 0.0309324 0.16359 0.006232 3.541965 5.25

NOx kg/ha

0.00351

g/MJ 0.00472 0.003 0.0102

0.0025** 5.551

kg/ha 0.300664 0.29625 1.00725 1.604164 0.6722352

0.4788 2.953132 3.43 4.17 1.22

0.00 0.00 0.00

0.0032 0.1254 0.0954 0.0494 0.0357 0.23 0.0012963 0.071295 0.002716 0.582855 0.74

NMHC kg/ha

0.00472 0.00472 -

0.29 0.679 0.679 5.551 -

0.8 0.57 0.53 0.13 0.051

g/kg

169

0

0.00031

g/MJ 0.00167

kg/ha 0.106379 0 0.0306125 0.136992 0

0 1.567272 1.57 1.92 0.36

-

2.946

0.00 0.00 0.00

0.002 0.0209 0.0918 0.01786 0.014 0.11 0.00019221 0.0082425 0.000314 0.30933 0.36

0.00167 0.00167 -

0.043 0.0785 0.0785 2.946 -

0.5 0.095 0.51 0.047 0.02

Partikel Kg/ha

HCL kg/ha

g/kg

NH3 kg/ha

0.019152 0 0.02 0.46 0.46

0.00 0.00 0.00

0.0111 0.0111 -

0.00 0.00 0.00

5.55E-06 5.55E-06 -

0.00 0.00 0.00

0 0 0 0 0.0009821 0.000522477 0.020069 0.010676708 0.00 0.01 0.02 0.39 0.02 0.38 Fossil energy emission g/MJ kg/ha g/MJ kg/ha g/MJ kg/ha 0.01083 0.689871 0.0111 0.70707 5.55E-06 0.000353535 0.001 0.09875 0 0 0.00E+00 0 0.00093 0.0918375 0.000048 0.00474 2.00E-05 0.001975 0.880459 0.711810 0.002329 0.00164 0.3140928 0 0 0 0

0.0001** 0

g/kg

0.0024 0.1 0.0004 2 0.008 0.5082 0.068 0.01496 6.69 1.4718 0.1998 0.021 0.00378 0.012 0.00216 0.323 0.074 0.02812 0.0019 0.000722 0.665 0.013 0.0091 0.00092 0.000644 1.27 0.04 1.11 0.0042018 0.21 0.0009387 0.16 0.0007152 0.004347 0.00213 0.00022365 0.001836 0.00019278 0.0001656 0.00213 0.00000852 0.001836 0.000007344 0 0.0009821 0.000103121 0.020069 0.002107245 0.44 0.02 0.38

Dust kg/ha

0.01083 0.01083 -

0.94 0.0414 0.0414 0 -

0.6 2.31 1.11 0.85 0.95

g/kg

Source: (Kaltschmitt, et al.,1997 / Patyk, et al., 1997) from (AGE, 1995 / Fritsche, et al., 1995) * electricity emissions, ** units are in g/MJ, *** 80% fuel efficiency assumed

Electricity Heat (burning)*** Heat (production)*** Sum fossil Natural gas burning

Beets production Seed N P2O5 K2O CaO Reactor effluent Plant PA Diesel Lubricant Fuel burning Machines Sum Biogas production Chopping* Pumping* Silo Sum Energy production Heating, stirring, etc. CHP Gas burning Heating oil Sum Total 20ha (kg/ha) Total 80h (kg/ha)

g/kg

Appendix 3b: continue

0.00019

g/MJ 0.000183 0.0001 0.000069

0.00009** 0.4697

0.000183 0.000183 -

0.013 0.004697 0.004697 0.4697 -

0.067 0.021 0.032 0.0081 0.0035

kg/ha 0.0116571 0.009875 0.00681375 0.028346 0.0363888

0.0172368 0.2498804 0.27 0.32 0.07

0.00 0.00 0.00

0.000268 0.00462 0.00576 0.003078 0.00245 0.01 0.00005811 0.000493185 0.000018788 0.0493185 0.05

Formaldehy g/kg kg/ha

0.00015

g/MJ 0.0001027 0 0.00002

0 0.11102

0.0001027 0.0001027 -

0.0056 0.003587 0.003587 0.11102 -

kg/ha 0.00654199 0 0.001975 0.008517 0.028728

0 0.05906264 0.06 0.07 0.01

0.00 0.00 0.00

0.0000668 0.001628 0.001494 0.000836 0.000616 0.00 0.000025032 0.000376635 0.000014348 0.0116571 0.01

Benzol kg/ha

0.0167 0.0074 0.0083 0.0022 0.00088

g/kg

0.0905

g/MJ 0.603 0.042 0.042

0.02** 24.55 24.57

0.603 0.603 -

22.73 3.13 3.13 24.55 -

19.48 28.7 18.05 1.07 0.46

kg/ha 38.41 4.15 4.15 46.7061 17.33

13.96 13.06 27.02 32.77 19.71

0.04 0.04 0.08

0.08 6.31 3.25 0.41 0.32 7.72 0.10 0.33 0.01 2.58 5.67

SO2-equivalent g/kg kg/ha

350

ppm

5.39

H2S kg/ha

Erklärung

„Hiermit versichere ich, daß ich die vorliegende Dissertation selbständig und ohne unerlaubte Hilfe angefertigt und andere als die in der Dissertation angegebenen Hilfsmittel nicht benutzt habe. Alle Stellen, die wörtlich oder sinngemäß aus veröffentlichten oder unveröffentlichten Schriften entnommen sind, habe ich als solche kenntlich gemacht. Kein Teil dieser Arbeit ist in einem anderen Promotions- oder Habilitationsverfahren verwendet werden.“

Braunschweig, 2003

.......................................... Elhussein Abdien Hassan

170