AGRICULTURAL RESIDUES FOR DECENTRALIZED ENERGY PRODUCTION V. Skoulou, A.Zabaniotou Aristotle University of Thessaloniki, Dept. of Chemical Engineering, University Box 455, University Campus , 541 24, Thessaloniki, Greece E-mail: [email protected]
; [email protected]
ABSTRACT. This paper describes combustion, pyrolysis and gasification as a potential olive stones exploitation method, and presents a comparison between those treatments, when utilised as a source of renewable energy. The aim of the present work is to strengthen the interest in agricultural residues potential for energy production in Greece. Combined with technical, economic and environmental data, the paper focuses on the benefits that thermochemical technology of fluidised bed gasification is able to offer, either in investigation or in future technological application for alternative exploitation of agricultural residues that mainly remain unexploited and otherwise could be a potential pollution source for the environment.
1. INTRODUCTION Environmental pollution and alarming consumption rates of fossil fuels, in combination with constantly raising energy demands indicate the necessity of competitive renewable fuel exploitation in a sustainable way. Additionally, diverse topography and tradition in agriculture activities lend Greece the opportunity to exploit almost any form of its renewable energy source. Combustion of selected agricultural residues (e.g. olive kernels) is already applicable, in many Greek islands, for closed space heating and cooking. Additionally, difficulties that many rural areas face in order to achieve connection with a central power supply system and unstable petroleum prices acted as the driving force for biomass exploitation for energy production purposes. Residues from olive oil production industry (cuttings, fresh and pressed kernels, leaves etc) are accumulated annually and in huge amounts leaving unexploited their attractive energy content [1, 2]. A small percentage of them exploited, worldwide, for energy production mainly by combustion [ 3-4], while research is extended in fields like co-combustion with coal , anaerobic digestion  and composting. Greece, as the third olive oil productive country in the world - after Spain and Italy – shows an annual production of olive kernel that reaches 300 - 400.000 ton /yr . Olive kernel production is shown in Figure 1.
Evia and Central Greece 12% Attica
Creta island 27%
Aegean islands 9% Thrace 0% Epirus Macedonia 1% Thessaly 2% 4%
Ionian islands 9%
Figure 1. Annual production of olive kernels in Greek prefectures (%)  Exploiting a percentage of its huge agricultural residues stock, almost 2.730 plants during year 2002 (cotton gins, olive kernel factories, wood industries, rice mills etc.) use their agricultural residues for energy production. Especially combustion of olive kernels, olive wood and cuttings produced 9.300 ΤOE (2002), while olive kernels alone produce 2.800 ΤOE same year. This kind of exploitation is detected mainly locally and especially in olive productive areas all over the country and the produced energy is utilised for heat production purposes, greenhouse heating, drying processes etc, even in hotel and hospitals heating (Crete Island). Some industries that use olive kernels for energy production are shown in the Table 1. Thermochemical treatment of agricultural residues has a long history in Mediterranean countries with considerable scientific research [4,7,82
11,13,14-20], while special interest is devoted to olive kernel exploitation in those countries [1012,21-23].Especially, olive kernel exploitation seems valuable not only for economic but also for environmental protection reasons. 2. FRESH AND PRESSED OLIVE KERNEL CHARACTERISTICS. The first residue of olive oil production is called fresh olive kernel and composed of a thin flesh residue - that is left from first step press-, has a quite high percentage of moisture and looks like a pulp. This fresh olive kernel residue is led to olive kernel factories, where it is separated from flesh. Then olive kernel oil of first (from flesh) and second (from olive kernel) quality is produced and the main byproduct of the process is pressed olive kernel. This pressed olive kernel has moisture of about 50%, before drying, a woody part of almost 45% and small amounts of oil, organic and inorganic compounds reaching 5%. Organic compounds like acids, aldeydes etc. contained in small percentages in olive kernels, are the reason for bad odor and emissions from olive kernel factories. Moreover, when those compounds are released to the environment bring on several serious environmental problems, due to their phytotoxicity. In Table 2 a typical ultimate and proximate analysis of dried pressed olive kernel shows its attractive characteristics in terms of energy productive purposes. TABLE 1. Industries exploiting olive kernels for energy production in Greece (2002)  Area
Installed Thermal electric power Power (MWe) (MWh/y)
Engines 6*1,356 MW Heraclio,
Fluidized bed combustion, Steam turbines
Kreta island Meligalas
2.633 plants all over greek territory
TABLE 2. Ultimate and proximate analysis of olive kernel. Ultimate analysis
Heating value HHV (Kcal/kg)
2.1 Pressed Olive kernel exploitation technologies. Low moisture (>15%) and relatively high heating values of dry pressed olive kernel (5661,5 Kcal/Kg), combined with locality in production and huge amounts, make it an attractive form of agricultural residue for its local thermochemical treatment in decentralized, probably modular, small energy production systems. Olive kernels, at the moment, are exploited through conventional combustion mainly from the same factories where they are produced, to cover their energy consumption needs, especially in drying processes. Today, the cost of dried pressed olive kernel reaches 0,046 €/Kg (while in 2001 was around 1€/Kg) a quite satisfying price for its sale. But under conditions of international competition and problems of tracing amounts of benzopyrenes -components that are assumed carcinogenic- in olive kernel oils of Spain, Italy and Greece, its value reached the present prices, something that also entrained downwards and the price of olive kernels as a fuel. But still dry pressed olive kernel is utilised as an excellent fuel and in comparison with the present high petroleum prices (0,577 €/lt) olive kernel cost is reduced almost 12 times (0,046 €/Kg). Except combustion, olive kernels are also exploited through composting. Mixing olive kernels with olive mill wastewaters gives a fertilizer of very good quality, ecological and with an open future in Greece that demand in compost, according to an estimate, will raise at levels of 500.000 tons up to 2010.At the same time, olive kernels are in use for eliminating toxicidy of olive mill wastewaters and for raising natural soil fertility that usually falls due to extensive agricultural activity . Some other uses of olive kernels, that are still under scientific research, are: production of active carbons [11-12] furfural products , heavy metal adsorption and cleaning of aquatic solutions , mixing with thermoplastics for production of a ‘new generation’ material for container production  etc.Finally, the part of pressed olive kernel that contains proteins (flesh) is utilized as animal food with high nutritional value, while its thermochemical treatment is not recommended, due to its high moisture content. In particular, when flesh of olive kernels used as animal food for goats and replaced 100% e.g alfalfa, raised animal’s weight and enriched its milk and meat in the antioxidant component eleyropaine. Lately, also, special interest is focused in isolation and use of similar valuable antioxidants, for pharmacy and cosmetic, from olive mill residues and wastewaters. 3. OLIVE KERNEL EXPLOITATION FOR ENERGY PRODUCTION 3.1 Combustion Direct combustion of dried pressed olive kernel is the simplest exploitation method for energy production. Its moisture should be up to 20%, and the fact that pressed olive kernels come from olive kernel factories are already shattered and dry, make them an ideal fuel for combustors and boilers. Combustion takes place after drying and before pyrolysis and gasification into the combustion chamber. But, even though combustion constitutes an easy way for energy and heat production, gives rise to several environmental problems related mainly to atmospheric pollution (ash production, gaseous emissions CO, SOx etc). Most combustion systems accomplish low efficiencies (40%) and as a result there are considerable heat losses to the environment. Special olive kernel combustors are quite common in Greece today and in some cases replace common central heating systems with petroleum combustors. Their combustion chamber is slightly different 4
and consists of a coil that carries the dried pressed olive kernel into the furnace, while a blower supplies excess air to sustain the combustion. Some difficulties in dried pressed olive kernel storage and management appear to be in the way of storage and place and it mustn’t get wet. Olive kernel combustors are usually designed in order to, also, work with conventional fuels or similar kind of biomass. Technology that also dominates in large power production systems from olive kernels is integrated combined electricity and heat production systems. Heat that released from olive kernel combustion is conjucted in a steam cycle with power production of 5-20 MWe. The most known technology of combustion is grade firing while, under more strict environmental obligations to EE commitments, fluidized bed combustion is considered more attractive. After several trials to more economic entrance of olive kernel combustion to the market, co combustion with conventional fuels (e.g coal) is promoted. Advantages of this practise are: reduction of fossil fuel consumption, ensuring annual feeding of the mixture of fuels, environmental protection and reduction of atmospheric pollution due to gaseous pollutants. The cost of an olive kernel combustor is slightly higher than a petroleum combustor (e.g. for power 40-50.000 Kcal/hr, efficiency 60-70 % costs almost 2.500 €) . But olive kernel’s low cost and its relatively high heating content remain an attractive solution and e.g. an olive kernel combustor with power of 120.000 Kcal/hr that works 10 hr/day for 3 months/yr has an operating cost 10 times lower that a petroleum combustor under the same conditions. 3.2. Pyrolysis Biomass slow pyrolysis takes place under high temperature (up to 500 oC), in environments with no oxygen contents, that prevent biomass combustion. Also, temperature, that is not high enough to break long bonds in carbon chain, impose production of tars in significant quantities. Finally, and after biomass devolatilization, char (black solid residue) is produced and shows different properties than the parent biomass Material. Process parameters, which have the largest influence on the pyrolysis products are: particle size, temperature and heating rate. The process conditions can be optimised to maximize production of pyrolytic char, oil or gas, all of which have an attractive potential use as fuels. The char is an attractive by-product, with applications as soil amendments and production of activated carbons  or even as sorbent for air pollution control as well as for wastewater treatment. Recent scientific research activities also focus in flash pyrolysis that used for biooil production, followed by less gas and char production. 3.3. Gasification Gasification takes place under high temperatures (850-1200oC) under the presence of air in lower quantities of that needed for stoichiometric combustion and produces a mixture of gases with Η2 content between 3-7%. During first gasification steps, drying and pyrolysis take place, and right after production of synthesis gas is converted, due to conversion of CO and H2O, to CO and H2. Imposing higher temperatures than pyrolysis, biomass bonds break down and reform in a gas mixture of permanent gases - mainly H2, CO, CO2 and CH4 and small amounts of light hydrocarbons. Percentages of those gases depend on several factors like e.g. gasification medium, quality of olive kernel (heating value, proximate and ultimate analysis), heating rate and temperature of gasification etc.Gasification with air gives lower percentage in hydrogen content in syngas mixture in relation to steam gasification and that happens due to steam reforming of CH4 to H2 and CO.Lower heating value of syngas production is caused due to the N2 content of air that dilutes further syngas. A composition of olive kernel gasification with air mixture is shown in Table 3. Syngas is not recommended for distribution through pipes but it is suitable, after thorough cleaning, for 5
exploitation in ICE, turbines and fuel cells, systems that are attractive for energy production and are going to be a significant step in energy production from biomass. As it concerns fluidized bed reactors, the problem that appears to be a drawback in their efficient utilization, is tar production. Tar is formed in a temperature range between 700-900oC and disturbs bed fluidization. Another critical point in large scale application of thermochemical treatments for biomass is gas cleaning from tar and other suspended solids that come from fluidized bed or chars. By-products from the different thermochemical treatments e.g. gasification against combustion gives better limits in CO2 emissions and it is widely known that is considered CO2 "neutral" with respect to air pollution problems. It doesn’t increase CO2 concentration in atmosphere, as the carbon dioxide released from gasification, is already the inherent amount that biomass gained from atmosphere with photosynthesis. Gasification under certain practise (Integrated Gasification Combined Cycle) gives higher efficiencies (45-50%) compared with 25-35% that usually is achieved via combustion. And at the end pyrolysis can lead to biofuel utilization, with the advantages that a liquid fuel is able to offer (easy storage for short time and easy transport) or char material exploiting as active carbon. TABLE 3. Gas mixture from olive kernel gasification with air  Component
3. CONCLUSIONS Greece‘s opportunity to exploit its agricultural residues -and especially olive kernels stocks- seems to be very attractive. Moreover under national commitments on EU obligations over alignment with Kyoto protocol, gaseous emissions abatement and climate change prevention, Greece could exploit olive kernels in a more environmental friendly way that combustion for accomplishing decentralized energy production. Optimization of well known olive kernel combustion technique and combination of the latest knowledge on thermochemical treatment of gasification through a closed integrated cycle with internal combustion engine, gas and steam turbines and evening future fuel cells, could probably lead to an economic reasonable and technologic viable way of sustainable energy production. Some weaknesses like low repeatability, high capital costs, big volumes that generated in rural areas could be solved through co-combustion / gasification of olive kernels and conventional fuels or other kinds of solid wastes, decentralized and modular form of energy production systems and a very good established waste management/logistics system. Acknowledgments. Authors are thankful to the Ministry of Development, General Secretariat of Research and Technology and the European Commission for funding this research under PENED2003.
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