Catchcrop2biogas - Potential and optimization of biogas production from catch crops

PROJECT No. 10683 Catchcrop2biogas Potential and optimization of biogas production from catch crops Project period 01/10-2011 – 31/03-2014 Instituti...
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PROJECT No. 10683

Catchcrop2biogas Potential and optimization of biogas production from catch crops Project period 01/10-2011 – 31/03-2014

Institution responsible for the project: Section for Sustainable Biotechnology Aalborg University Copenhagen, A.C. Meyers Vænge 15, 2450 København, Denmark. CVR no.: 29102384 Authors: Hinrich Uellendahl, AAU Cph Beatriz Molinuevo-Salces, AAU Cph Birgitte K. Ahring, AAU Cph Søren Ugilt Larsen, AgroTech

3 April 2014 Financed by

Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

Preface This is the final report of the project “Catchcrop2biogas - Potential and optimization of biogas production from catch crops”, which was granted by the ForskEL program (no. 10683). The project was carried out from October 2011 to March 2014 by the Section of Sustainable Biotechnology at Aalborg University Copenhagen (SSB-AAU) in collaboration with AgroTech and Biokraft A/S. The report comprises the main results and conclusions of the project. For more detailed information please refer to the different articles in the list of dissemination of results at the end of the report. A detailed evaluation of the agricultural aspects of using catch crops for biogas production is submitted as supplementary report by Søren Ugilt Larsen from AgroTech (in Danish). Copenhagen, 3 April 2014

Hinrich Uellendahl, Associate Professor, Section for Sustainable Biotechnology, Aalborg University Copenhagen

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

Content Preface ................................................................................................................................................. 3 Content................................................................................................................................................. 4 Brief summary ...................................................................................................................................... 5 Implementation ................................................................................................................................... 9 The work packages of the project ...................................................................................................... 10 Results ................................................................................................................................................ 12 WP 1 Screening of catch crops ....................................................................................................... 12 WP 2 Lab-test optimization of 2-3 suitable candidates ................................................................. 16 WP 3 Large-scale testing of 1 most promising catch crop ............................................................. 19 WP 4 Evaluation ............................................................................................................................. 26 Dissemination of project results ........................................................................................................ 30 References.......................................................................................................................................... 31

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Catchcrop2biogas project

Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Brief summary The main objective of the Catchcrop2biogas project was to evaluate the utilization of different catch crops as feedstock for biogas plants in the frame of sustainable biomass production for bioenergy production. The biomass yield of the catch crops, their specific methane yield and their ability of binding nutrients were screened to identify the catch crops with highest potential for biogas production (WP 1).Co-digestion of catch crops with manure was carried out in order to identify the optimal process parameters for a continuous biogas process (WP 2). Harvest, storage, transport and feeding of the catch crops to the biogas plant were studied in large-scale to identify the technical operation of using catch crops as feedstock for manure-based biogas plants (WP 3). Finally, the biogas potential of catch crops including prerequisites and optimization strategies for achieving a stable and economically feasible biogas process were evaluated (WP 4). WP1 Screening of catch crops. More than 60 samples of different catch crops were screened for their potential to be used for biogas production and with regards to their environmental benefit. The basis for the evaluation was the achievable methane yield per hectare YCH4,ha, which is the product of the biomass yield Ybiomass and the specific methane yield of the catch crop YCH4,VS: (

)

(

)

(

)

The screening showed that the biomass yield of catch crops is highly variable and often too low to justify harvest of the biomass. Catch crops provided biomass yields around or below 1 t of dry matter per hectare in most of the cases. However, biomass yields of more than 3 t of dry matter per hectare were obtained in some cases. The biomass yield of a catch crop depends on several parameters. These include time of establishment, time of harvest and fertilization. It was observed that earlier establishment) favored generally higher biomass yields. A later harvest may lead to higher biomass yields; however, the biodegradability of more mature may decline due to lignification of the plant. Fertilization may also improve biomass yields up to 77% but the effect of fertilization on biomass yield varies considerably depending on the soil type, the climate conditions and the ability of the crop to uptake nutrients. Grass species used as catch crops, with perennial ryegrass and Italian ryegrass among the most widely used, are often presenting a proper establishment resulting in high biomass production but they may develop too well and, hence, reduce the yield of the main crop. Among faster developing species, oil seed radish and white mustard are the most used catch crops in recent years and have a vigorous growth, rapid root development and an efficient uptake of nutrients so they are desirable in terms of their environmental effect. The specific methane yield of the different catch crops were in the range of 229-474 m3-CH4/t-VS. It was observed that the specific methane yield is determined by the catch crop species the temperature and rainfall during the growth period and the total nitrogen (TN) content of the biomass, discriminating the samples by the different locations where they were cultivated. Lower biomass yields were correlated with higher specific methane yields, presumably due to a lower

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

plant maturity resulting in lower lignin content. The lignin content coupled with the bioavailability of cellulose and hemicellulose determines the actual biodegradability and thereby the specific methane yield of the biomass. A positive correlation was observed between the specific methane yield and the total nitrogen content of the biomass, i.e. clayey soils coupled with high rainfall favored the assimilation of nutrients in the catch crops, which positively affected the specific methane yield of those crops. Thus, the location (i.e. climate conditions and soil types) was identified as a key parameter influencing both biomass yield and specific methane yield of catch crops. Oil seed radish (Raphanus sativus var. oleiformis) and white mustard (Sinapis alba) appeared to be the most promising catch crops for biogas production in Denmark. However, grass species such as ryegrasses sown into the main cereal crop in spring may have some other benefits for biogas production due to a proper establishment, lower moisture and ash content. Therefore, Italian ryegrass (Lolium multiflorum) and oil seed radish were selected for further studies. Also, nitrogen fixing catch crops such as clover species may be advantageous since nitrogen availability may be limiting for catch crop growth in some situations. With a generally high biodegradability and specific methane yield of the different catch crops, the economic feasibility of catch crops for biogas production depends rather on the biomass yield of the catch crop per hectare than on the specific methane yield. WP 2 Lab-test optimization of 2-3 suitable candidates The potential inhibitors or deficiencies during anaerobic co-digestion of catch-crops and manure were studied for oil seed radish and Italian ryegrass. Methane yields for the co-digestion of manure and oil seed radish were in the range of 271 to 483 m3-CH4/t-VS while methane yields for the co-digestion of Italian ryegrass and manure were in the range of 195 to 263m3-CH4/t-VS. The rapid degradation observed for oil seed radish could imply a risk of acidification in the reactor due to volatile fatty acids (VFA) accumulation. However, the high buffer capacity of manure would counteract that effect, leading to a stable process. Therefore, with regards to the specific methane yield, the high biodegradability of oil seed radish would make this catch crop more valuable as a co-substrate with manure than Italian ryegrass giving higher yields in the biogas process. The continuous co-digestion of manure and catch crops in a semi-continuous reactor set-up was investigated using oil seed radish as co-substrate. The average methane yield at a hydraulic retention time of 20 days was 384 ± 58 m3-CH4/t-VS, when replacing 50% of the volatile solids of the manure by oil seed radish. This represented a 1.63-fold increase of the methane yield compared to manure alone at the same organic loading rate of 1 g-VS L-1 d-1. Adaptation of the process to the catch crop was reflected in a graduate increase of the methane yield throughout two hydraulic retention times (40 days) and no inhibition was detected for the long-run operation. From the reactor test it can be concluded that oil seed radish can be successfully used as a cosubstrate for manure-based biogas plants in a mixture of 50% with manure (on VS basis). Full process adaptation is reached after 1-2 hydraulic retention times and no inhibition is observed for continuous operation.

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

WP 3 Large-scale testing of the most promising catch crop The technical feasibility of using catch crops as feedstock for manure-based biogas plants was tested in large-scale. This included harvest, transport, storage and feeding of the catch crops to the biogas plant. On the basis of the large scale test the costs for these different processes for supplying the catch crops to the biogas plant were determined and a cost-benefit model was developed, including the biomass yields and specific methane yields of the tested catch crops. . Silage was tested as a storage method that would guarantee the availability of feedstock for biogas production during the whole year. Oil seed radish and clover grass were successfully preserved as silage for up to 252 days while Italian ryegrass did not properly ensile during the test, probably due to a very high dry matter content. In general, the proper storage of the feedstock as silage could be controlled by determining dry matter content prior to ensiling, by avoiding soil particles in the bales and by chopping the biomass before ensiling. The anaerobic digestion of the ensiled samples showed a more rapid degradation in the beginning, which could represent an economical advantage for biogas plants since lower hydraulic retention times would be needed when feeding ensiled samples to the reactor. No effect of the duration of storage time process on the final methane yield was detected, being specific methane yields in the range of the fresh catch crops (288-376, 305-370 and 389-473 m3-CH4/t-VS for oil seed radish, Italian ryegrass and grass clover silage samples, respectively). It is worth mentioning that especially for silage samples the dry matter content determined by drying at 105°C must be corrected by adding the dry-oven losses of volatile organic compounds (such as organic acids) to the standard dry matter determination to avoid overestimation of the specific methane yield. The feasibility of solid-liquid separation of catch crops with high water content was tested as possible means to reduce transportation costs to the biogas plant. The test of a commercial screw press on oil seed radish showed, however, that the savings in transportation costs from leaving the liquid fraction on the field would be lower than the loss of methane sales from the easily degradable organic matter of the liquid fraction. Therefore, this method is not suggested to gain economic advantages for biogas production from catch crops. A new agricultural strategy was tested with the aim of increasing the biomass yield of catch crops. by harvesting the catch crop together with the stubble left of the main crop. Biomass yields in the range of 3.2 and 3.6 t-TS/ha were obtained for different blends of catch crops and stubble. Catch crops contributed in the range of 3-10% of those biomass yields. This relatively low contribution from catch crops was probably due to the dry summer and the lack of available nitrogen, which may have limited the growth of the catch crops. Specific methane yields were in the range of 172234 m3-CH4/t-VS due to the high proportion of straw in the biomass, which has a generally lower degradability than catch crops. Based on the test stubble heights of 40-55 cm are recommended, while the standard height of 13 cm was shown to be too low for increasing the biomass yield of catch crops. The analysis of the specific methane yield of straw showed that the yield was higher for the straw samples that were later harvested, indicating an increase in biodegradability for straw when it stayed longer on the field. This may be due to microbial hydrolytic activity that may occur already on the field. The higher biodegradability was, however, not reflected by a change in the chemical composition of straw for the different harvest times.

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

WP 4 Evaluation of catch crops as feedstock for an economically feasible biogas process The investigations performed during the project reveal that certain catch crops may be a valuable feedstock for biogas production when cultivated under the right conditions. While the specific methane yield of catch crops is generally high, it is rather the biomass yield which determines if the use of catch crops for biogas production is economically feasible. Biomass yields of catch crops should be at least 2 t-TS/ha with a resulting methane yield of 660 m3-CH4/ha in order to obtain an economically feasible process. Biomass yields from catch crops up to 3 t per hectare. However, the biomass yields of the catch crops investigated in the project presented a wide variation, depending on the species, climate conditions (rainfall and temperature) and soil types. For favorable conditions, methane yields of up to 800 m3 per ha were found. In order to increase the biomass yield of catch crops, it is recommended to start the growth period as early as possible e.g. by early establishment. In many cases, the yield of catch crops may be limited by the availability of nutrients, so that fertilization of the catch crop could increase the biomass production. Also, strip harvesting of the main crop prior to full maturity may allow more light to penetrate to the catch crop in July-August, and the stubble from strip harvesting may be harvested together with the catch crop in October and, thus, contribute to increase the overall yield. . For using catch crops as feedstock for biogas production, it has to be kept in mind, that for Denmark catch crops are mandatory to grow in certain proportion and thereby already established on 210,000 ha (2011) due to their positive environmental effects by reducing nutrient leaching to the groundwater. Besides, catch crops can improve the soil quality.. Furthermore, the use of catch crops biomass for energy purposes would not compete with food/feed while contributing to improve the greenhouse balance. However, from the farmer´s point of view, biogas production from catch crop would only be of interest if it is economically feasible. The economic profit from the sale of the biogas should, therefore, compensate the harvest, transportation, handling and storage costs of catch crops. Silage was demonstrated as a proper storage method that would guarantee catch crops as feedstock during the whole year. The organic matter degradation occurring during the ensiling process resulted in a rapid methane formation that would represent an economic advantage for biogas plants by reducing the hydraulic retention times in the reactors. Separation of the solid fraction by using a screw press to reduce transportation costs was not worth due to the loss in methane potential of the liquid fraction. Harvesting catch crops together with the straw from the previous main crop would increase the total biomass yield up to 9 times. The catch crops would represent only around 10% of the total biomass yield, but this strategy could improve the quality of the silage since the biomass would be drier when harvested together with straw and run-off of leachates can be avoided. Finally, the co-harvest of straw and catch crops could be a good alternative to using straw alone as feedstock for biogas plants.

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

Implementation Energy from renewable sources is being promoted nowadays and, according to the Danish Energy Agreement from 2012, 35% of Denmark´s energy is expected to come from renewable sources by the end of 2020. This agreement improves the conditions for biogas production by increasing the financial support for the investment of new biogas plants from 20 to 30% [1]. Anaerobic digestion is already a well-established technology in Denmark. Currently, there are 23 centralized and about 60 farm-scale manure-based biogas plants in operation. The Danish government aims to increase the use of manure for energy production by 2020, from the current 5% to up to 40% of the total manure produced in the country [2]. The low yield of manure when digesting it alone coupled with the low availability of high yielding substrates for co-digestion make the identification of new potential biomass feedstock of major interest. Catch crops are grown as supplementary crops after the harvest of the main crop with the primary purpose of binding nutrients in the soil, hence diminishing pollution to the aquatic environment. Nitrate leaching is an important cause of eutrophication of surface water bodies in the North of Europe. Accumulated nitrogen in the soil, field management during autumn, weather conditions (especially the excess of rainfall during winter) and soil type play a major role on nitrate leaching [3,4]. Catch crops improve, moreover, the soil quality by reducing soil erosion, adding organic matter and reducing the need of application of fertilizer in the following growing season [5]. Based on the Nitrates Directive in Denmark, it is mandatory to grow catch crops on farms larger than 10 ha. [6]. Thus, catch crops constitute a by-product of sustainable crop production that can be potentially used as a biomass resource for bioenergy production without interfering with the production of food and fodder crops. Moreover, biogas production from energy crops in codigestion with manure shows the highest potential for the reduction of greenhouse gas emissions compared to other biofuels due to synergistic effects of substitution of fossil fuel and avoiding greenhouse gas emissions of untreated manure [7]. Previous studies have identified catch crops as possible feedstock for biogas production with a simultaneous positive impact on the cultivation of other crops [8-9]. The methane potential (specific methane yield based on volatile solids (VS)) of different catch crops obtained in previous works is in the range of 250-450 m3 t-1 of VS [10-12]. The use of catch crops as feedstock for biogas plants is, however, scarcely investigated in detail. The Catchcrop2biogas project evaluated the biogas potential of a variety of catch crops and identified the most suitable candidates fxfor biogas production in Denmark. The most appropriate methods for the cultivation of catch crops and for feedstock supply including transportation, storage, and feeding of catch crops to the biogas plant were also studied.

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

The work packages of the project The project was divided into four work packages, namely the Screening of catch crops (WP1), the Lab-test optimization of 2-3 suitable candidates (WP2), the Large-scale testing of 1 most promising catch crop (WP3), and the Evaluation of the use of catch crops for biogas production (WP4). SSB-AAU was responsible for performing work package WP1 in cooperation with AgroTech. The large-scale test (WP3) was performed at Biokraft A/S on Bornholm, in coordination with Bornholms Landbrug. An additional large-scale test was performed on Jutland, which was coordinated by AgroTech and Videncenter for Landbrug (VfL). The evaluation (WP4) was performed by SSB-AAU and AgroTech, based on the data acquainted from all partners during the project. The tasks of the different work packages and the respective milestones are listed in Table 1.1 and 1.2, respectively. Table 1.1. Work packages of the Catchcrop2biogas project

WP 1 Screening of catch crops WP 1.1 Literature study and evaluation of previous field trials on catch crops with respect to biomass yield and nutrition quality (AgroTech) WP 1.2 Evaluation of different catch crops suitability for agricultural needs in Denmark (AgroTech) WP 1.3 Biogas potential screening of different catch crops from previous field trials (SSB-AAU) WP 1.4 Batch test for identification of the change in biogas potential due to different harvest times (SSB-AAU) WP 2 Lab-test optimization of 2-3 suitable candidates WP 2.1 Batch test for identification of the change in biogas potential due to storage of catch crops (SSB-AAU) WP 2.2 Identification of potential inhibitors from catch crops during anaerobic digestion (SSB-AAU) WP 2.3 Continuous reactor test in lab-scale with co-digestion of catch crops together with manure (SSB-AAU) WP 3 Large-scale testing of 1 most promising catch crop WP 3.1 Test at Biokraft’s biogas plant with addition of catch crops in co-digestion with manure (Biokraft A/S) WP 4 Evaluation WP 4.1 Calculation of biogas yield per hectare from catch crops under optimized conditions (AgroTech, SSB-AAU) WP 4.2 Influence of the concept on Agriculture in Denmark (AgroTech) WP 4.3 Technical aspects for large scale operation of catch crops for biogas production (Biokraft A/S)

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

Table 1.2. Milestones of the Catchcrop2biogas project

WP 1 Screening of catch crops M 1.1 Identification of 2-3 candidates of suitable catch crops in Denmark with high potential for biogas production M 1.2 Identification of best harvesting times of suitable catch crops in Denmark with high potential for biogas production WP 2 Lab-test optimization of 2-3 suitable candidates M 2.1 Identification of optimal process parameters for continuous biogas process of catch crops WP 3 Large-scale testing of 1 most promising catch crop M 3.1 Identification of biogas yield and technical operation for a large scale co-digestion process with catch crops WP 4 Evaluation M 4.1 Identification of best practice and energy yield from using catch crops for biogas production in Denmark

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

Results WP 1 Screening of catch crops WP 1.1 Literature study and evaluation of previous field trials on catch crops with respect to biomass yield and nutrition quality (AgroTech) The cultivation of catch crops with respect to biomass yield was studied in field trials performed by AgroTech in the years 2010, 2011 and 2013[13, 14]. Twelve different catch crop species were grown in 4 different locations in Jutland, Denmark, namely Holstebro (Hb), Horsens (Hs), Aabenraa (Aa) and Haderslev (Hd) . In these field trials, catch crops provided biomass yields of more than 3 tTS/ha in certain cases, but in most cases the yield was around or below 1 t-TS/ha. A biomass yield of 3 tons of dry matter per hectare (t-TS/ha) was calculated in a recent report to be the limit for a cost efficient use of catch crops for biogas production [15]. The screening showed that the biomass yield of catch crops depends both on factors that the farmer cannot modify, such as soil type and climate (WP 1.4), and factors that may be moderated by management, e.g. choice of catch crop species, time of establishment, time of harvest of the main crop and fertilization of the catch crop. Early establishment of the catch crop is of vital importance for a reasonable biomass yield, and earlier harvest of the main crop may also increase the biomass yield of the catch crop. Fertilization of the catch crop may improve the biomass yield as well. More specifically, the biomass yield of Italian ryegrass (IR) and oil seed radish (OSR), were increased by up to 77% and 52%, respectively, when applying 100 kg of nitrogen per ha, if compared with unfertilized fields. However, the effect of fertilization on biomass yield varied considerably for different locations, possibly because other factors such as lack of water, which also would affect nitrogen recovery from catch crops [16]. The biomass yield could also be increased by harvesting the catch crop biomass together with straw from the cereal main crop with high grain stubble (WP 3). WP 1.2 Evaluation of different catch crops suitability for agricultural needs in Denmark (AgroTech) Many plant species have been screened over the years in terms of their suitability as catch crops. Catch crops can be divided into two main groups, namely slow developing species that are suitable for under sowing in the main crop in the spring and faster developing species that are more suitable for sowing a few weeks prior to harvest or immediately after harvest of the main crop. The slow developing species may e.g. comprise various grass species, and perennial ryegrass and Italian ryegrass are among the most widely used catch crop species, with Italian ryegrass having a more rapid growth during the early stages. An advantage of sowing in the spring is that there is a relatively high probability for proper establishment of the catch crop, which is a prerequisite for a high biomass production. On the other hand, it can be a disadvantage that the spring-sown catch crops may develop too well and, hence, reduce the yield of the main crop. The faster developing species may include many dicot species including a number of cruciferous species. In recent years, the main focus has been on oil seed radish and white mustard, which have both been found to have a vigorous growth, rapid root development and an efficient uptake of nutrients from the soil during the autumn and, hence, being desirable in terms of their environmental effect. Surveys of

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Catchcrop2biogas project

Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

the use of catch crops in Denmark in 2011 and 2012 indicated that grass species and cruciferous species were used in approximately equal areal proportions and comprised the far majority of the total catch crop area [17, 18]. Grass species used as catch crops were predominant on sandy soils where spring cereals are often grown whereas cruciferous species were predominant on the more clayey soils where there is a larger proportion of winter cereals. WP 1.3 Biogas potential screening of different catch crops from previous field trials (SSB-AAU) The 66 samples of the different catch crops, grown and harvested by AgroTech in the field trials during the years 2010 and 2011 were screened for their specific biogas potential. Specific methane yields in the range of 229-474 m3-CH4/t-VS were obtained, which corresponded well with those reported in previous works for the different catch crops species. The specific methane yield per ton of organic matter (t-VS) and the resulting methane yield per ton of biomass (t-biomass), based on the VS content, are shown in Table 2. Table 2. Organic matter (VS) content, and methane yields obtained for the different catch crops. Field trial 2010 Methane yield

VS content Catch crop species

Field trial 2011

3

3

Methane yield

VS content 3

t-VS /t-biomass

m -CH4 /t-VS

m -CH4 /t-biomass

t-VS /t-biomass

m -CH4 /t-VS

m3 -CH4 /t-biomass

0.09-0.10

356-378

33.5-39.3

0.09-0.16

368-474

32.3-73.9

Brassicaceae Raphanus sativus

Oil seed radish

Brassica napus

Rapeseed

0.12

362-377

44.9-46.6

0.11-0.19

368-448

45.2-74.3

Brassica rapa

Turnip rape

0.14

289

43.3

n.d.

n.d.

n.d.

Brassica oleracea

Kale

0.12

373

44.8

n.d.

n.d.

n.d.

Sinapis alba

White mustard

0.12-0.14

251-298

35.6-34.6

0.09-0.20

239-369

33.4-57.9

Hemp

0.19

263

50.2

n.d.

n.d.

n.d.

Sunflower

0.11

269

30.1

n.d.

n.d.

n.d.

0.14-0.15

383-407

55.4-57.2

n.d.

n.d.

n.d.

Cannabaceae Cannabis sativa Asteraceae Helhiantus annus Graminaceae Avena sativa

Oat

Lolium sp.

Ryegrass

0.16

413

65.0

n.d.

n.d.

n.d.

Secale cereale

Rye

0.15

407

61.6

n.d.

n.d.

n.d.

Lupinus arboreus

Lupin

n.d.

n.d.

n.d.

0.09-0.14

229-327

30.6-47.2

Phaseolus sp.

Bean

n.d.

n.d.

n.d.

0.10-0.13

289-320

29.2-38.8

Fabaceae

n.d. not determined

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Catchcrop2biogas project

Principal Component Analyses (PCA) was applied to the data to identify the main variables affecting the specific methane yield of catch crops and find the most suitable catch crops as possible co-substrate for full-scale biogas production in Denmark. The PCA was employed to ten variables affecting the specific methane yield of twelve different catch crops species [21]. In general, the variables that differentiated the samples were the temperature, rainfall, specific methane yield and total nitrogen content in the biomass (TN), discriminating the samples by the different locations where they were harvested. Moreover, it was observed that catch crops species presented a major importance on the specific methane yield since the catch crop species were grouped according to their respective botanical families. In this way, the wide specific methane yield ranges within the same catch crop species (Table 2) may be due to the different lignocellulosic composition and the content of secondary metabolites. Secondary metabolites such as glucosinolates in the species belonging to Brassicaceae botanical family or the alkaloids produced in Lupinus sp. have been reported to affect the anaerobic digestion process, thus leading to lower specific methane yields in some cases. The biochemical composition of the catch crop biomass is correlated to the maturity of the plant and the capacity of the crop to assimilate nutrients (crop species). The lignin content coupled with the bioavailability of cellulose and hemicellulose determines the actual biodegradability and the specific methane yield of the biomass. It is not possible to degrade lignin under anaerobic conditions, but several pretreatments can be used to break lignin bonds and increase the rate of cellulose and hemicellulose utilization and, consequently, the specific methane yield. The different climate conditions and soil types may lead to differences in biomass chemical composition, which could be responsible for the wide specific methane yield ranges observed. WP 1.4 Batch test for identification of the change in biogas potential due to different harvest times (SSB-AAU) Due to a low variation in harvest time of the samples, a correlation between biogas potential and harvest time could not be performed. However, the location was found to be a key parameter not only for the biomass yield but also for the specific methane yield and the correlations were investigated in more detail for ten different catch crops in two locations, namely Holstebro (Hb) and Aabenraa (Aa). Several parameters affecting plant growth and anaerobic digestion process were evaluated [20]. The following catch crops or crop mixtures were evaluated: White mustard (C1), white mustard and common vetch (C2), turnip rape and common vetch (C3), perennial rye and Persian clover (C4), yellow lupine (C5), oil seed radish (C6), winter ryegrass and winter vetch (C7), triticale and winter vetch (C8), lupine (C9) and bean (C10). Biomass yields and plant heights were up to ten times higher in Holstebro than in Aabenraa. The higher rainfall registered in Aabenraa coupled with the more sandy soil in that location increased nutrient leaching diminishing nutrient availability for the plants, which was also reflected in a lower content of nitrogen in the soil. This combined with a generally lower fertility of sandy soils resulted in low biomass yields and low plant heights. On the contrary, the fine sandy clay loam soil in Holstebro retained nutrients, which were efficiently used by the catch crops increasing biomass yields and plant heights. The specific methane yields were in the range of 229-450 m3 per t of VS.

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Catchcrop2biogas project

A positive correlation was observed relating specific methane yield to total nitrogen (TN) content (Figure 1). Clayey soils coupled with high rainfall could favor the assimilation of nutrients in the catch crops. This would be reflected in a higher TN in the biomasses of the crops and it positively affected the specific methane yield of those crops. The higher proportion of TN (proteins) in some catch crop biomass could have led to a higher methane yield, since the stoichiometric methane yield of proteins is 490 m3-CH4/t-VS, whereas for carbohydrates it is 415 m3-CH4/t-VS. It was observed that the specific methane yield decreased with increasing lignin concentration in the biomass (Figure 1). The lower specific methane yields obtained for catch crops grown in Holstebro could be a consequence of the higher lignin concentration in those catch crops compared to the ones grown in Aabenraa. The methane yields per hectare (m3/ha) of the catch crops were in the range of 367-812 and 78566 m3/ha for those catch crops grown in Holstebro and Aabenraa, respectively. The methane yields per hectare of the catch crops grown in Aabenraa were in all cases lower than 700 m3 of CH4 per hectare, which was earlier set as the threshold for an economically feasible use of catch crops as feedstock for manure-based biogas plants in Denmark [15]. The high rainfall observed in Aabenraa during the growing season coupled with the sandy soil could have been the reason for lower plant growth, thereby reducing biomass yield and methane yield per hectare.

Figure 1. Correlation of total nitrogen (TN) and lignin content to specific methane yieldfor Holstebro (open squares) and Aabenraa (closed squares) samples.

Milestone 1.1 Identification of 2-3 candidates of suitable catch crops in Denmark with high potential for biogas production Based on the screening of the different catch crops samples including their biomass yield per hectare, their specific methane yield per ton of organic matter (WP 1.3) and their suitability for agricultural needs (WP 1.2), oil seed radish (Raphanus sativus var. oleiformis) and white mustard (Sinapis alba) appear to be the most promising catch crops for biogas production in Denmark. However, grass species such as ryegrasses sown into the main cereal crop during spring may have some other benefits for biogas production due to a lower moisture and ash content (WP 1.1).

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Catchcrop2biogas project

Therefore, Italian ryegrass (Lolium multiflorum) and oil seed radish (Raphanus sativus var. oleiformis) were selected for further studies. Also, nitrogen fixing catch crops such as clover species may be advantageous since nitrogen availability may be limiting for catch crop growth in some situations. Milestone 1.2 Identification of factors influencing the use of catch crops for biogas production in Denmark Based on the screening of different catch crops in WP 1.3, the location (i.e. soil and weather conditions) was identified as important key parameter for the biomass yield and also for the specific methane yield. The differences in climate conditions and soil quality significantly affected the biomass yield and consequently the methane yield per hectare. White mustard (C1, C2) and oil seed radish (C6) presented the highest methane yields per hectare. As all catch crops generally show high biodegradability expressed in high specific methane yields, the economic feasibility of using catch crops for biogas production is rather dependent on their biomass yield per hectare than on their specific methane yield.

WP 2 Lab-test optimization of 2-3 suitable candidates WP 2.1 Batch test for identification of the change in biogas potential due to storage of catch crops (SSB-AAU) The influence of the storage as silage on the specific methane yield was investigated for Italian ryegrass and oil seed radish in batch tests. The main findings were that the initial degradation was faster than in fresh samples, so the specific methane yield after 20 days of incubation slightly higher in silage samples compared to fresh samples. The effect of silage on the specific methane yields was investigated in more detail for oil seed radish, Italian ryegrass and clover grass from the harvest of 2012 (WP 3). WP 2.2 Identification of potential inhibitors from catch crops during anaerobic digestion (SSB-AAU) Based on the results obtained in WP 1, Italian ryegrass (IR) and oil seed radish (OSR) were selected to study potential inhibitors or deficiencies during the anaerobic co-digestion of catch-crops and manure. Two sets of experiments were carried out using the same methodology; the first one for co- digestion of oil seed radish with manure and the second one for co-digestion of Italian ryegrass with manure. The effect of two parameters, namely substrate/inoculum ratio (So/Xo) and percentage of volatile solids (VS) provided by catch crop biomass over specific methane yield were investigated for both catch crops. A central composite design followed by response surface methodology was applied for each catch crop co-digestion in order to determine the effect of both parameters over the specific methane yield. After 103 days of anaerobic digestion, specific methane yields for the co-digestion with OSR were in the range of 271 to 483 m3-CH4/t-VS for the different mixtures. The highest values corresponded to the experiment with a So/Xo ratio of 0.98 and catch crop percentage of 50%. The lowest specific methane yields were observed in the experiments that contained a high proportion of OSR, which seemed to be an easily biodegradable substrate. The high hydrolysis rate of OSR 16

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Catchcrop2biogas project

could have resulted in total volatile fatty acids (TVFA) accumulation, which led to an overload of the system for methanogenic microorganisms, which needed up to 70 days to start producing methane. pH values were, however, despite a possible TVFA accumulation, not affected with values around 7.6 at the end of the experiments, indicating the high buffer capacity of swine manure. Methane yields for IR were in the range of 195 to 263m3-CH4/t-VS. IR presented generally lower specific methane yields than OSR. In this case, the set-ups with high IR content presented the lowest specific methane yields, probably due to an incomplete conversion into methane due to the higher lignocellulosic content of IR compared to OSR. In this case, TVFA were rapidly consumed and methane production started after a short lag phase of 5 days, indicating rapid adaptation of the microorganisms to the substrate. The results showed that the use of OSR as co-substrate could imply a risk of acidification in the reactor due to TVFA accumulation. However, methane yields in OSR co-digestion were up to 1.52 fold higher than in IR co-digestion, due to the higher biodegradability of OSR. The high biodegradability and the high methane yield obtained from OSR makes this catch crop a more valuable feedstock, leading to a higher biogas production than IR. Therefore, OSR was chosen as co-substrate for the continuous experiments to investigate the co-digestion of this catch crop with manure in a semi-continuous reactor set-up. WP 2.3 Continuous reactor test in lab-scale with co-digestion of catch crops together with manure (SSB-AAU) Oil seed radish was selected as co-substrate for lab-scale reactors to study the effect of catch crop addition to manure anaerobic digestion and to identify the optimal process parameters for continuous biogas process from catch crops. Two continuous stirred tank reactors (CSTR) with a working volume of 3 L were used. One reactor was used for anaerobic co-digestion of manure and oil seed radish (OSR) (reactor R1) and the other for anaerobic digestion of manure alone (reactor C). Agitation was provided by a mechanical stirrer and the temperature was maintained at 37 ± 2 ºC with a temperature-controlled water bath. Biogas production (quantified by water displacement) was measured every weekday. The reactors were initially filled with mesophilic anaerobic sludge. After two days, the reactors were fed manually once every weekday. In order to adapt the system, 30% OSR was fed into R1 during the first week of experiment, and then the proportion of OSR was increased up to 50%. Reactor C was constantly fed with manure during the whole experimental period. Organic loading rate (OLR) and hydraulic retention time (HRT) were set for both reactors to 1 ± 0.3 g-VS L-1 d-1 and 20 days, respectively. The process was evaluated for the period of three consecutive HRTs. The content of methane in the biogas was measured every weekday. In order to keep a constant and equal influent concentration of VS in both reactors, substrates for R1 and C were prepared every weekday by diluting the substrate with water.

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Catchcrop2biogas project

Figure 2. Methane yield and TVFA concentration for semi-continuous reactor experiment with co-digestion of manure and oil seed radish during start-up and periods HRT 1-3; the ratio of OSR/manure in reactor R1 is given in percentage for the different periods; control reactor C was fed only with manure. Horizontal lines indicate the average methane yields in the corresponding period

The methane yields achieved for the co-digestion of manure and OSR in reactor R1 and for manure alone in control reactor C, are shown in Figure 2. A gradual increase in the methane yield during HRT 2 and HRT 3 indicated eventually the adaptation of the inoculum to oil seed radish. During HRT 3, the average methane yields were 384 ± 58 and 236 ± 44 m3-CH4/t-VS, for R1 and C, respectively. In this final period (HRT 3), the replacement of 50% of the volatile solids of the manure by OSR, improved the methane yield by 1.63-fold. During the HRT 3 the co-digestion reactor (R1) achieved 1.6, 1.3 and 1.2-fold removal of total solids (TS), volatile solids (VS) and chemical oxygen demand (COD), respectively, compared to the control reactor. This is indicating a higher biodegradability of the organic matter from the oil seed radish, which is also reflected by the higher methane yields. This is also in accordance with low lignin content (1.70-1.98 % of TS) of the OSR used in the present study as analyzed previously [20]. Two of the major inhibitors of the anaerobic co-digestion processes of manure and easily degradable substrates, namely ammonia and TVFA overload, were evaluated. No inhibition was detected in any of the reactors.

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Catchcrop2biogas project

M 2.1 Identification of optimal process parameters for continuous biogas process of catch crops Oil seed radish could be successfully used as a co-substrate for manure-based biogas plants in a mixture of 50% with manure (on VS basis). Oil seed radish is a highly biodegradable material that, as a co-substrate could supply carbon to the co-digestion feedstock, thus enhancing the carbon/nitrogen ratio and improving methane yields. The process may need an adaptation time of 1-2 hydraulic retention times.

WP 3 Large-scale testing of 1 most promising catch crop Two large-scale tests were performed during the project, the first one on different catch crops cultivated on Bornholm in 2012 and the second one on Jutland in 2013 with earlier cultivation of catch crops and harvest together with straw of the main crop of catch WP 3.1 Test at Biokraft’s biogas plant with addition of catch crops in co-digestion with manure (Biokraft A/S). The production costs were specified for the large-scale test based on the costs for harvest (1000 – 1500 DKK/ha), transportation (42 DKK/t) and storage (25 DKK/t), as given by Bornholms Landbrug and Biokraft A/S. No additional costs were accounted for the establishment of the catch crops, taking that amount of catch crop into consideration that it is currently mandatory to cultivate, which is typically 10-14% of the farmland. Also, no additional costs were accounted for transferring the digested slurry including nutrients from the biogas plant back to the field where catch crops were harvested. The resulting costs per m3 of methane in relation to the biomass yield are shown in Figure 3 for different scenarios, depending on the harvesting costs, dry matter concentration (TS) and the specific methane yield of the catch crop. As can be seen, the reduction of the harvest costs from 1500 to 1000 DKK has the main impact on reducing the overall production costs. With the higher harvest costs biomass yields of 3 t-TS/ha and more are needed so the production costs are lower than the earlier revenue for biogas production of 0.78 DKK/kWh-el. For the newly introduced revenue of 1.15 DKK/kWh-el, however, biomass yields below 3 t-TS/ha and in some cases below 1.5 t-TS/ha would be sufficient to pay back the production of biogas from catch crops. With a VS/TS ratio of 83% (as average measured for the catch crop samples) and a specific methane yield of 400 m3-CH4/t-VS a threshold of 1.5 and 2.0 tTS/ha would be equivalent to a methane yield of 500 and 660 m3-CH4/ha, respectively. This relativizes the threshold of 3.0 t-TS/ha that was formerly calculated by Hvid [13]. The catch crops grown on Bornholm for the first large-scale test in summer 2012 showed biomass yields lower than 1 t-TS/ha. Since the costs for the harvest of catch crops for the large-scale test would exceed by far the revenue of biogas production it was decided to reduce the amount of harvested catch crops to about 1 hectare per catch crop species and to focus on the technical performance of harvest, storage, transport and feeding of the catch crops to the biogas plant. This test was performed on three different catch crops, oil seed radish (OSR), Italian ryegrass (IR) and clover grass (CG).

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Catchcrop2biogas project

Figure 3. Production costs for biogas (methane) from catch crops in relation to the biomass yield for different harvest costs, dry matter (TS) concentration and specific methane yield.

Storage of catch crops: Changes of catch crops during silage. The special cultivation regime of catch crops coupled with a general short growing season under Northern climate conditions promotes the need to store the biomass. Silage as a storage method makes the feedstock available for biogas production during the whole year. About 1 ton of the three catch crops, oil seed radish (OSR), Italian ryegrass (IR) and clover grass (CG), was harvested and wrapped for silage in November 2012 (Figure 4).

Figure 4. Harvest and wrapping of oil seed radish on Bornholm for silage(November 2012).

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Catchcrop2biogas project

The ensiled samples were transported to Biokraft’s biogas plant and four samples, corresponding to four different silage bales, were taken from each ensiled catch crop. The first set of ensiled samples, named OSR-1, IR-1 and CG-1, were obtained after 86, 129 and 128 days of ensiling, respectively. The second set of ensiled samples, named OSR-2, IR-2 and CG-2, were obtained after 106, 149 and 148 days of ensiling, respectively. The third set of ensiled samples, named OSR-3, IR3 and CG-3, were obtained after 153, 196 and 195 days of ensiling, respectively. The fourth set of samples, named OSR-4, IR-4 and CG-4, was obtained after 210, 253 and 252 days of ensiling, respectively. All silage samples were studied with respect to their composition and specific methane yield. For all samples obtained during the large-scale test on Bornholm, the VS content (70% of TS), was lower than in the catch crops analyzed during the screening in WP 2, indicating that a high amount of soil particles were blended with the crops when harvesting the plants with low height. The content of organic acids and ethanol that are the typical fermentation end products produced during ensiling are displayed in Table 3. Table 3. Content of total solids (TS), lactic, acetic, propionic, butyric acids and ethanol in different catch crop samples (OSR: oil seed radish, IR: Italian ryegrass, CG: clover grass) after different duration of ensiling (1: 86-128 d, 2: 106-148 d, 3: 1353-195 d, 4: 210-252 d). Standard deviation in brackets. Sample Uncorrected TS

Corrected TS*

Lactic acid

Ethanol

Acetic acid Propionic acid Butyric acid

g/kg WW

g/kg WW

%TS

%TS

%TS

%TS

%TS

OSR 1

207.6 (11.5)

217.5 (11.4)

5.20 (0.07)

0.49 (0.05)

1.96 (0.01)

0.00 (0.00)

0.32 (0.00)

OSR 2

221.0 (13.1)

232.3 (12.8)

5.03 (0.40)

0.94 (0.04)

1.85 (0.01)

0.00 (0.00)

0.30 (0.00)

OSR 3

209.5 (4.6)

219.6 (5.0)

5.24 (0.43)

0.57 (0.04)

1.92 (0.09)

0.00 (0.00)

0.32 (0.00)

OSR 4

308.3 (45.3)

323.2 (46.2)

3.91 (0.96)

0.24 (0.06)

2.52 (0.14)

0.49 (0.03)

0.22 (0.00)

IR1

661.6 (51.52)

670.1 (50.8)

0.73 (0.12)

0.42 (0.02)

0.48 (0.05)

0.00 (0.00)

0.13 (0.00)

IR2

458.8 (42.8)

469.9 (42.9)

0.68 (0.04)

1.18 (0.00)

0.85 (0.01)

0.00 (0.00)

0.17 (0.00)

IR3

560.4 (26.7)

571.7 (28.5)

1.25 (0.37)

0.77 (0.13)

0.62 (0.04)

0.00 (0.00)

0.16 (0.01)

IR4

579.0 (0.0)

592.4 (2.9)

0.29 (0.07)

1.45 (0.41)

0.63 (0.06)

0.00 (0.00)

0.14 (0.00)

CG1

333.1 (14.8)

344.4 (15.5)

3.19 (0.39)

0.87 (0.01)

0.97 (0.02)

0.00 (0.00)

0.26 (0.00)

CG2

327.7 (6.5)

337.5 (5.5)

2.02 (0.01)

0.79 (0.15)

0.94 (0.14)

0.18 (0.25)

0.31 (0.04)

CG3

354.1 (24.9)

365.5 (24.3)

3.15 (0.10)

0.81 (0.05)

0.88 (0.00)

0.00 (0.00)

0.27 (0.01)

CG4

381.0 (9.9)

395.3 (8.7)

4.22 (0.55)

0.77 (0.00)

1.12 (0.07)

0.00 (0.00)

0.26 (0.00)

* Corrected according to Porter and Murray, 2001. WW stands for wet weight

For oil seed radish (OSR) and clover grass (CG) a generally high lactic acid production indicated together with the visual inspection of the samples, a good quality of the silage. The silage of Italian ryegrass (IR) showed, however, poor quality, both by low lactic acid production, a pH above 5 and

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Catchcrop2biogas project

a texture of the samples that resembled the fresh cut IR. The low fermentation of IR may be due to the high TS content of IR (460-660 g/L). Regarding CG, a slower fermentation was observed and the pH was slightly higher, probably due to the buffer capacity of legumes caused by a higher ammonia formation during the ensiling process from the proteins content in legumes. Based on these results, the following recommendations should be taken into account to improve the quality of OSR, IR and GC silage and use them for feedstock at the biogas plant: -

The contamination of the feedstock with soil particles must be avoided to reduce wear and tear of the feeding equipment. Chopping to a length of 1-3 cm would be required for all biomass samples before feeding to the biogas plant. Determination of the TS content before ensiling and adjustment of the biomass-TS, if necessary, would be recommended to enhance the quality of the silage.

The specific methane yields for OSR, IR and CG silage samples in this study varied from 288-376, 305-370 and 389-473 m3-CH4/t-VS, respectively. No significant difference was observed for the individual catch crop sample during ensiling, meaning that the storage time as silage had no effect on the specific methane yield of the biomass. Main methane formation occurred rapidly during the first days of anaerobic digestion without inhibition or lag time for all crop species investigated. More specifically, 80% of the specific methane yield was achieved in around 20, 24 and 14 days, for OSR, IR and CG, respectively. This would be interesting for the biogas plants from an economic point of view. The rapid methane formation when digesting ensiled samples would permit to reduce the hydraulic retention time in the biogas reactors, obtaining economic advantages for the overall process. It is worth mentioning that in this study total solid (TS) content in the different silage samples was corrected with the volatile compounds that are lost during oven drying. The determination of total solids by oven drying at 105°C is not accurate for samples that contain volatile compounds, which is the case of ensiled samples. The amount of volatile compounds that are lost during oven drying depends on the temperature and TS content in the sample. More specifically, 100% of the ethanol, 89.2% of the lactic acid and 37.5% of the TVFA are lost during oven drying at 100°C [21]. In the present study, the corrected TS values were 5%, 2% and 3% higher for OSR, IR and CG, respectively, when adding the values of volatile components lost during the oven drying to the standard calculated TS (Table 3). Transportation of catch crops: Evaluation of solid-liquid separation of catch crops. Solid-liquid separation of oil seed radish (OSR) was tested as means for reducing transportation costs by only supplying the separated solid fraction to the biogas plant while leaving the liquid fraction on the field. As separation method a commercially available screw press with 3 different mesh sizes (4.5 cm, 3 cm and 1.5 cm) was used. The cost-benefit calculations for the separation were performed based on mass balance of the screw press in- and output and determination of the methane yield of the different fractions. This included also the calculation of the methane potential loss in the liquid fraction when left on the field (Table 4).

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Catchcrop2biogas project

Table 4. Transportation costs and total income for non-separated oil seed radish (first row) and for the solid fraction -separated by the screw press using different mesh sizes. Mesh size

Biomass yield

Percentage of solid fraction

Biomass to transport

Transportation cost

Methane yield

VS content

Methane production

Energy

Income

cm

t ha -1

%

t ha -1

DKK ha -1

m 3 t-VS -1

% of biomass

m 3 ha -1

k Wh ha -1

DKK ha -1



8.5



8.5

357

368

10.2

319

3191

3669

4.5

8.5

71

6.0

253

320

11.1

214

2142

2465

3

8.5

72

6.1

257

266

11.72

191

1908

2194

1.5

8.5

62.5

5.3

223

247

13.92

183

1827

2101

The results show that the TS and VS content of the solid fraction increased with a smaller mesh size. However, even for the smallest mesh size (1.5 cm) the TS concentration of the solid fraction was still low (18%) and not much higher than for the non-separated OSR (14%). The final methane yield of the non-separated OSR was 368 m3-CH4/t-VS and in the range of 247-320 m3-CH4/t-VS for the separated fiber fraction (Table 4). It was observed that the smaller the mesh size of the screw press used, the lower the methane yield obtained in the solid fraction. This indicated that using a smaller mesh size resulted in pressing more liquid with soluble organic matter out while a smaller amount of the solid fraction remained with a higher content of fibers with a lower methane yield. The methane yield of the liquid fractions was in the range of 477-524 m3-CH4/t-VS, thereby 1.6fold higher than of the solid fractions. The calculation of the transportation costs was based on the transportation costs per ton of biomass as given earlier by Biokraft A/S (42 DKK/t). The income from the sale of biogas was calculated based on the energy value of methane (10 kWh/m3-CH4) and the current feed-in tariff for biogas in Denmark (1.15 DKK/kWh). As can be seen in Table 4, the transportation costs (223357 DKK/ha) represented around 10% of the income from the sale of biogas (2101- 3669 DKK/ha) for all cases. This income per hectare from the sale of biogas was up to 1.5 times lower for the separated solid fraction than for non-separated oil seed radish and the potential income of the liquid fractions represented 15-20% of the total income. This means that the separation resulted in a reduction of the transportation cost, but the loss in the potential income from the biogas of the liquid fraction would be much higher. Therefore, the results achieved from the solid-liquid separation of oil seed radish in a screw press do not suggest this method to gain economic advantages.

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Catchcrop2biogas project

2nd large-scale test: Harvest of catch crops together with straw to increase biomass yields Based on the findings of the screening of the catch crops in WP 1 and of the first large-scale trial on Bornholm, that the economy of using catch crops for biogas production is very dependent on the biomass yield per hectare, a new agricultural strategy was studied to improve the biomass yield of catch crops. The idea is to cultivate the catch crops earlier in between the main crop, to harvest the main crop at a higher height and finally to harvest the catch crops together with the straw/high stubble of the main crop (Figure 5).

I Figure 5. Sketch of the new agricultural strategy of harvesting cereal straw/high stubble and catch crops together.

In an additional field trial the biomass yield and specific methane yield of seven blends of catch crops together with the straw/high stubble from the previous main crop (spring wheat) were evaluated (Table 5). Biomass yields in the range of 3.2 and 3.6 t-TS/ha were obtained for the different combinations of catch crop and stubble. The catch crops contributed in the range of 310% to those yields, when subtracting the biomass yield of the wheat straw alone (first row in Table 5). This relatively low contribution from catch crops was probably due to the dry summer and the lack of available nitrogen, which may have limited the growth of the catch crops. The highest contribution of the catch crops (0.4 t TS/ha) was observed for the blend of perennial ryegrass and white clover (Table 5). Specific methane yields (after 57 days of anaerobic digestion) were in the range of 172-234 m3-CH4/t-VS, which is much lower than the specific methane yields observed for the catch crops alone. However, the methane yield of all blends of catch-crop and stubble were significantly higher than of the wheat stubble alone (159 m3-CH4/t-VS). The blend of fescue and stubble achieved the highest methane yield of 234 m3-CH4/t-VS, representing a 51% higher yield compared to stubble alone (Table 5). The resulting methane production per hectare of the catch crop and stubble blend was in the range of 471-745 m3-CH4/ha, which is a bit higher than for catch crops alone as identified in the first large-scale test The yields are, however, still only giving a small economic benefit, with the highest value for the fescue and stubble blend. The effect of the higher biomass yield of the straw and catch crop blend is counteracted by the lower biodegradability and thereby lower specific methane yield of the wheat straw in the blends. The benefit of the new agricultural strategy of harvesting the stubble together with the catch crops is, however, higher than of the harvest of catch crops or straw alone. Therefore, this new strategy may be a good alternative for economically feasible supply of catch crops and straw for biogas production

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Catchcrop2biogas project

Table 5. Biomass yield, methane yield and methane yield per hectare for the different blends of catch crops and wheat stubble (no catch crop). Catch crop

Biomass yield Biomass yield Biomass yield Methane yield Volatile solids Total Catch crop Total

Methane yield per hectare

t TS ha -1

t TS ha -1

%VS of TS

t VS ha -1

m3 -CH4 t-VS-1

m3 -CH4 ha -1

3.2 3.6 3.3 3.4 3.2 3.4 3.5 3.3

_ 0.4 0.1 0.2 0 0.2 0.3 0.1

92.8 93.3 94.3 95.9 91.4 95.8 96.3 95.6

3.0 3.4 3.1 3.3 2.9 3.3 3.4 3.2

158.7 172.0 239.3 194.9 179.9 176.8 186.4 165.4

471.3 577.3 745.0 635.1 525.9 575.6 628.3 521.8

No Perennial ryegrass + white clover Fescue Red clover Oil seed radish Oil seed radish 50N Oil seed radish + winter vetch Oil seed radish + red clover

The effect of stubble height on biomass yield and specific methane yield was studied by harvesting the main crop at different stubble heights (13, 40 and 55 cm) and later harvest of the stubble together with the catch crop, ryegrass in this case. Ryegrass biomass represented around 10% of the total biomass obtained in both cases. The stubble heights 40 and 55 cm both gave a biomass yield of 0.32 t-TS/ha whereas no biomass was obtained at 13 cm stubble height. There was no significant difference of the specific methane yield between the stubble heights 40 and 55 cm Finally, wheat stubble was harvested at 3 different times in order to study the effect of harvest time on the composition and specific methane yield of wheat straw. Biomass yields were 3.4, 2.7 and 2.8 t-TS/ha for the wheat straw harvested on the 6th September, 27th September and 31st October, respectively. While biomass yields were slightly declining for the later harvest times, the specific methane yield increased as the wheat straw stayed longer on the field with 143, 171 and 203 m3-CH4/t-VS for the three harvest dates, respectively. No significant difference in biomass composition was detected for the wheat stubble samples harvested at different times (Table 6). Moreover, no difference was found when comparing the composition of wheat straw with the blend of ryegrass and wheat straw, indicating that biomass the blend was mainly containing straw. Table 6. Composition analysis of wheat stubble at different harvest times, of Wheat stubble and ryegrass, of oil seed radish (OSR) and of ryegrass. Wheat stubble 06-09-2013

Wheat stubble 27-09-2013

Wheat stubble 31-10-2013

Wheat stubble +Ryegrass

OSR 1

OSR 2

Organic matter (VS) g/100 g-TS

93.54 (0.00)

96.11 (0.01)

95.99 (0.00)

96.05 (0.00)

81.8

76.77

83.87

83.1

Ash

g/100 g-TS

6.70 (0.48)

3.70 (0.14)

4.20 (0.13)

4.46 (0.29)

19.31

19.02

11.98

13.1

Klason lignin

g/100 g-TS

22.03 (0 09)

19.71 (0.05)

20.90 (0.08)

20.23 (0.22)

12.46

12.45

20.63

17.64

Glucose fraction

g/100 g-TS

37.46 (1.27)

43.58 (0.08)

37.28 (2.21)

37.15 (0.43)

14.76

17.19

21.45

20.72

Xylose fraction

g/100 g-TS

19.33 (1.02)

21.14 (0.05)

18.56 (1.04)

18.36 (0.54)

6.26

6.68

8.38

9.69

Arabinose fraction

g/100 g-TS

1.53 (0.02)

0.98 (0.03)

4.01 (0.00)

1.42 (0.04)

1.61

0

4

4

Residuals

g/100 g-TS

12.95

10.88

15.06

18.38

45.59

44.65

33.55

34.84

Units

Ryegrass 1 Ryegrass 2

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

M 3.1 Identification of biogas yield and technical operation for a large scale co-digestion process with catch crops Silage was demonstrated as a proper storage method that would guarantee catch crops as feedstock during the whole year. The organic matter degradation occurring during the ensiling process resulted in a rapid methane formation that would represent an economic advantage for biogas plants by reducing the hydraulic retention times in the reactors. A reduction in the transportation cost was obtained when removing water from catch crops biomass, but the reduction was not worth the loss in methane potential per ton of water removed. Therefore, this method is not suggested to gain economic advantages for methane production from catch crops. Harvesting catch crops together with the straw from the previous main crop would increase total biomass yields up to 9 times, but the catch crop would represent only around 10% of the total biomass yield. However, the final biomass yield and, consequently, the economic sustainability of this strategy are strongly influenced by climate conditions and soil quality. Moreover, this strategy could improve the quality of the silage since the higher dry matter content in the blend with straw would more suitable for silage without the risk of leachate.

WP 4 Evaluation WP 4.1 Calculation of biogas yield per hectare from catch crops under optimized conditions (AgroTech, SSB-AAU) The Catchcrop2biogas project clearly demonstrates that both the biomass yield and the specific methane yield of the catch crops are key parameters for an economically feasible biogas production from catch crops, where the energy yield per hectare have to pay-back the costs for harvest, transport and storage. The biomass yield of organic matter has been found to vary largely with levels ranging from nearly no harvestable yield up to approx. 3 t-TS per hectare. Although there can be big differences in yield between catch crop species, the ranking of yield between species may differ between sites, and in many cases the biomass production may vary more between sites than between species. This indicates that the biomass yield in catch crops is often limited by various environmental factors rather than by the genetically determined yield potential of the catch crops. In order to optimize the biomass yield of catch crops, it is important to start the growth period as early as possible e.g. by early establishment, either by spring-sowing or by sowing during July prior to the harvest of the main crop. In many cases, the yield of catch crops may be limited by the availability of nutrients, and fertilization of the catch crop may increase the biomass production. Also, strip harvesting of the main crop prior to full maturity may allow more light to penetrate to the catch crop in July-August, and the straw/stubble from strip harvesting may be harvested together with the catch crop in October and, thus, contribute to an increase in the overall yield. This system, however, has to be further tested. The methane potential of catch crops has been found to range from approx. 230 to 470 m3 per ton organic matter. In general, catch crops belonging to Brassicaceae and Graminaceae families presented the highest methane yields, except for white mustard, whereas Cannabis sp.,

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

Helianthus sp., Lupinus sp. and Phaseolus sp. presented low methane yields, regardless of the location.The generally high degradability makes, therefore, catch crop biomass a suitable feedstock for biogas production. The specific methane yield is mainly related to the catch crop species, which is determined by its specific composition. It was specifically observed that the total nitrogen content of the biomass was positively correlated to the specific methane yield of the catch crops. Furthermore, climate conditions (rainfall and temperature) had an influence not only on the biomass yield, but also on the specific methane yield. As the screening has shown that the biomass yield of the different catch crop species varies generally more than their methane potential, the energy yield per hectare of harvested catch crops depends rather on the biomass yield than on the methane potential of the catch crop. Therefore, the catch crops with the highest potential for biogas production should be chosen on the basis of high biomass yields and these catch crops should be cultivated in those locations where high biomass yields can be achieved. Finally, the agricultural strategy should target on high biomass yields by for example early establishment of the catch crop. Then methane yields of more than 660 m3 per hectare can be achieved. Methane yields per hectare in the same range (up to 745 m3) were obtained when harvesting straw/stubble together with catch crops. Although the catch crops only represented around 10% of the total biomass yield when harvested together with straw, the methane yield of the mixture was significantly higher than of straw alone, making the combined harvest of straw and catch crop a good alternative to the harvest of straw alone. Overall it can be concluded that the choice of an appropriate catch crop species coupled with a suitable agricultural strategy to achieve high biomass yields would make the catch crops a valuable feedstock for biogas plants in Denmark. WP 4.2 Influence of the concept on Agriculture in Denmark (AgroTech) The use of catch crops in agriculture has multiple purposes of which the most important is to reduce leaching of nitrogen from the root zone to the aquatic environment. Moreover, they may have a positive effect on soil fertility, increase the content of soil organic matter, enhance the tillage of the soil as well as reduce the occurrence of weeds and diseases. Conversely, catch crops may also have negative effects, e.g. by reduced possibilities for weed control and harvest of the main crop or by loss of nitrogen as ammonia and nitrous oxide. The positive environmental effect of catch crops has led to a statutory demand of ‘compulsory catch crops’ on typically 10-14 % of the farm land in Denmark. Although the farmer has the option to choose other alternatives such as ‘short-term catch crops’ (‘mellemafgrøder’, grown from July 20th to September 20th) or reduced nitrogen fertilization, most farmers choose to grow catch crops with an overall area of approx. 210.000 ha of catch crops in 2011. Hence, there is a considerable area already existing where catch crops are grown and where only harvest, transport and storage are necessary to supply this biomass for biogas production. Based on the present knowledge on yield and quality of catch crops, the agricultural aspects of harvesting catch crops for biogas production has been evaluated, i.e. as an alternative to leaving the biomass on the field. Since catch crops are generally grown for other purposes than for food and feed, the utilization of this biomass for biogas production is not expected to compete for land with the production of crops for feed and food etc.

27

Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

By harvesting the above-ground biomass of catch crops in autumn, the risk of mineralization and leaching of nitrogen during the winter is reduced and the nutrients may, on the other hand, be recirculated to subsequent crops in the following spring in a more accessible form. Harvest of catch crops will reduce the contribution of biomass to maintain the pool of soil organic matter. On the other hand, it can be assumed that the use of the biomass for energy production leads to an improved greenhouse gas balance, since the main proportion of biomass incorporated in the soil would be degraded within a few years anyway. Overall, the use of catch crops for biogas production is, therefore, expected to mainly have positive effects on the environment and climate. In general, catch crops can be harvested and transported by traditional equipment for forage production, but harvest during October may coincide with e.g. harvest of maize forage and the last harvest of grass for forage. In certain cases, harvest may be impeded by precipitation and potential risk of damaging the soil structure. Catch crop biomass can be stored as forage either at the farm or at the biogas plant. There may, however, be a risk of leachate run-off from the biomass, which may be prevented by aeration on the field, storage on top of dryer silage, mixing with straw or harvest of straw together with high stubble. From the farmer’s perspective, harvest of catch crops for biogas production is only of interest if it is economically profitable, either by sale of biomass to a biogas plant or by improvement of the economy in the farmer’s own biogas plant. In this case, the economic analysis performed during the large-scale tests of the project can be transferred to the farmer’s economy where the harvest constitutes the largest cost. Other costs comprise transport, yield reduction in the main crop, and either costs due to export of nutrients from the farm or costs due to bringing back nutrient to the field. In general, a rather high biomass yield is required to ensure profitability for the farmer, and there is a need for increasing the biomass yield and achieving a stable yield over several years to make the use of catch crops for biogas economically attractive for the farmer. Also, there is a need for adjusting the legal regulation e.g. by improving the possibilities for fertilizing catch crops. If the biomass yield of catch crops can be increased and the costs for harvest etc. can be reduced to a degree that ensures profitability by using the biomass for biogas production, it can have farreaching perspectives using this resource to produce renewable energy. WP 4.3 Technical aspects for large scale operation of catch crops for biogas production (Biokraft A/S) Based on the large-scale test on Bornholm, the following technical requirements for supplying catch crops as feedstock to the biogas plant can be formulated: -

During harvest, the contamination of the feedstock with soil particles must be avoided to reduce wear and tear of the feeding equipment at the biogas plant. The biogas plant should be equipped with a feeding device for solid biomass (for example screw conveyor) Chopping of the catch crop feedstock to a length of 1-3 cm would be required before feeding to the biogas plant. Determination of the TS content before ensiling and adjustment of the biomass-TS, if necessary, would be recommended to enhance the quality of the silage for storage.

28

Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

M 4.1 Identification of best practice and energy yield from using catch crops for biogas production in Denmark Catch crops are grown on large areas in Denmark, primarily to reduce the leaching of nutrients to the aquatic environment. Since catch crops are compulsory to grow, and since the biomass is normally just incorporated in the soil, the use of catch crops for biogas production is not expected to conflict with production of food and feed etc. By harvesting the catch crop biomass rather than leaving it on the field, it may contribute to the production of renewable energy and reducing the emission of greenhouse gasses. Moreover, the nutrients in the biomass can be recirculated to the field in the subsequent growing season in a more accessible form. Also, the biomass quality has been found to be of adequate or even high quality for biogas production. Overall, the concept of using catch crops for biogas is, therefore, very meaningful. The main obstacle for an extensive use of catch crops for biogas, however, is that the biomass yield per hectare varies considerably, and often it is too low to ensure profitability of harvesting, transporting and storing the biomass for biogas production. Therefore, the concept must be developed further, primarily by achieving higher and more stable yields of catch crops. This may be done by enhancing the development and growth by e.g. earlier establishment, strip harvesting of the main crop prior to maturity and fertilization of the catch crop. Additionally, the yield per hectare may be increased by harvesting catch crops together with straw from the main crop. Another way of making the use of catch crops to biogas economically feasible may be to reduce the costs for harvest and transport. Finally, it is relevant to evaluate each individual field prior to harvesting to determine if the biomass yield appears to be sufficiently high to justify harvest of the catch crop. A detailed evaluation of the agricultural aspects of using catch crops for biogas production can be found in the supplementary report “Efterafgrøder til biogas – landbrugsmæssig vurdering” by Søren Ugilt Larsen, AgroTech (in Danish).

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

Dissemination of project results  S.U. Larsen, 2011. Oversigt over landsforsøgene. Videncenter for Landbrug. S. 235 – 237.  B. Molinuevo-Salces, B.K. Ahring, and H. Uellendahl, 2012. Biogas production from catch crops. Nordic Biogas Conference, Copenhagen, (Poster presentation).  H.S. Østergaard and S.U. Larsen, S.U. 2012. Udbytter i efterafgrøder med fokus på produktion af biogas. Planteavlsorientering nr. 105, Landbrugsinfo 16/3 2012.  B. Molinuevo-Salces, B.K. Ahring, and H. Uellendahl. 2013. Catch crops as an alternative biomass feedstock for biogas plants. International Anaerobic Digestion Symposium “BiogasWorld”, Berlín, 23-25 April, (Oral presentation).  B. Molinuevo-Salces, S. Larsen, B.K. Ahring, and H. Uellendahl, 2013. Biogas from Italian ryegrass and Oil seed radish: Effect of nitrogen application on biomass yield and methane production. 21st European Biomass Conference and Exhibition (EU BC&E), Copenhagen, June, (Poster presentation).  B. Molinuevo-Salces, B.K. Ahring, and H. Uellendahl. 2013. Factors influencing the feasibility of using catch crops for biogas production. 13th World Congress on Anaerobic Digestion in Santiago de Compostela, Spain, June. (Oral presentation).  B. Molinuevo-Salces, S. Larsen, B.K. Ahring, and H. Uellendahl, 2013. Biogas production from catch crops: Evaluation of biomass yield and methane potential of catch crops in organic crop rotations, Biomass and Bioenergy 59, 285-92.  B. Molinuevo-Salces, S. U. Larsen, R. Biswas, B. K. Ahring and H. Uellendahl. 2013. Key factors for achieving profitable biogas production from agricultural waste and sustainable biomass. 13th World Congress on Anaerobic Digestion in Santiago de Compostela, Spain, June. (Oral presentation).  T. Skøtt. 2013. Efterafgrøder kan sætte skub I produktion af biogas. Forskning i Bioenergi 46, 20.  B. Molinuevo-Salces, Férnandez-Varela, R. and H. Uellendahl, 2014. Key factors influencing the potential of catch crops for methane production, Environmental Technology, DOI:10.1080/09593330.2014.880515.  B. Molinuevo-Salces, S.U. Larsen, B.K. Ahring, and H. Uellendahl. 2014. Biogas Production from Catch Crops: a Sustainable Agricultural Strategy to Increase Biomass Yield. 22st European Biomass Conference and Exhibition (EU BC&E), Hamburg. (Oral presentation).  E. Fog. 2014. Halm og efterafgrøde til biogas. Plantekongres 14.-15. januar 2014, Herning kongrescenter. (Oral presentation).  B. Molinuevo-Salces, B.K. Ahring, and H. Uellendahl. 2014. Anaerobic digestion of catch crops: Optimization and adaptation to a CSTR treating manure. (Article in preparation).  B. Molinuevo-Salces, B.K. Ahring, and H. Uellendahl. 2014. Storage of catch crops to produce biogas: Effect of the silage process on methane yield. (Article in preparation).  Larsen, S.U. (2014). Efterafgrøder til biogas – landbrugsmæssig vurdering. Supplementary report for the Catchcrop2biogas project, 2011-2014. AgroTech, February 2014.

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

References [1] Danish Energy Agency. Publikationer [Internet]: Accelerating Green Energy Towards 2020. 2013, p. 12. Available at: http://www.ens.dk/sites/ens.dk/files/dokumenter/publikationer/downloads/accelerating_gree n_energy_towards_2020.pdf [2] Triolo JM, Ward AJ, Pedersen L, Sommer SG. Characteristics of animal slurry as a key biomass for biogas production in Denmark. In: Matovic MD, editor. Biomass now-sustainable growth and use. InTech [Internet], p. 20. Available from: http://www.intechopen.com/books/biomassnow-sustainable-growth-and-use/characteristics-of-animal-slurry-as-a-key-biomass-for-biogasproduction-in-denmark [3] Kronvang B, Jeppesen E, Conley DJ, Sondergaard M, Larsen SE, Ovesen NB, et al. 2005. Nutrient pressures and ecological responses to nutrient loading reductions in Danish streams, lakes and coastal waters. J Hydrol 304(1-4):274-88. [4] Askegaard M, Olesen JE, Rasmussen IA, Kristensen K. 2011. Nitrate leaching from organic arable crop rotations is mostly determined by autumn field management. Agric Ecosyst Environ 142(3-4):149-60. [5] Talgre L, Lauringson E, Makke A, Lauk R. 2011. Biomass production and nutrient binding of catch crops. Zemdirbyste 98(3):251-8. [6] EC Council Directive 91/676/EEC. Concerning the protection of waters against pollution caused by nitrates from agricultural sources. Off J EU 1991; L 375:0001-8. [7] Thyø KA, Wenzel H. 2007. Life Cycle Assessment of Biogas from Maize Silage and from Manure; Institute for Product Development, Lyngby, Denmark. http://www.geotekno.com/htmlarea/life%20cycle%20assessment%20report.pdf [8] Aebiom 2009. A Biogas Road Map for Europe. The European biomass association. Aebiom brochure published October 2009. [9] Niemeläinen O, Jauhiainen L, Kontturi M, Nissinen O, Vuorinen M, and Markku Niskanen M. 2007. Undersown catch crops as a source of biomass for energy. Proceedings NJF Seminar 405: Production and Utilization of Crops for Energy, Vilnius, Lithuania, 25-26 September 2007,p. 109110. [10] Weiland P. 2003. Production and energetic use of biogas from energy crops and wastes in Germany. 7th FAO/SREN Workshop. Appl Biochem Biotech 109(1-3):263-74. [11] Amon T, Amon B, Kryvoruchko V, Machmuller A, Hopfner-Sixt K, Bodiroza V, et al. Methane production through anaerobic digestion of various energy crops grown in sustainable crop rotations. Bioresource Technol 2007;98(17):3204-12. [12] Raposo F, De la Rubia MA, Fernández-Cegrí V, Borja R. 2011. Anaerobic digestion of solid organic substrates in batch mode: an overview relating to methane yields and experimental procedures. Renew Sustain Energy Rev 16(1):861-77.

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Section for Sustainable Biotechnology A.C. Meyers Vænge 15, 2450 Copenhagen SV

Catchcrop2biogas project

[13] Oversigt over Landsforsøgene (2011). Efterafgrøder til biogas. I: Forsøg og undersøgelser i Dansk Landbrugsrådgivning. Videncentret for Planteproduktion, Planteproduktion. S.235-237. https://www.landbrugsinfo.dk/Planteavl/Landsforsoeg-og-resultater/Oversigten-ogtabelbilaget/Sider/pl_oversigten_2011_afsnit_N_Goedskning.pdf?download=true

[14] Oversigt over Landsforsøgene (2012). Efterafgrøder til biogas. I: Forsøg og undersøgelser i Dansk Landbrugsrådgivning. Videncentret for Planteproduktion, Planteproduktion. S.234-236. https://www.landbrugsinfo.dk/Planteavl/Landsforsoeg-og-resultater/Oversigten-ogtabelbilaget/Sider/pl_oversigten_2012_afsnit_N_Efterog_mellemafgroeder.pdf?download=true [15] Hvid, S.K. (2012). Efterafgrøder til biogas er ikke rentable med aktuelle priser på biogas. Planteavlsorientering 092, Landbrugsinfo, 16/2 2012. https://www.landbrugsinfo.dk/Oekonomi/Produktionsoekonomi/Planteavl/Analyser-ogberegninger/Sider/pl_po_12_092.aspx [16] Molinuevo-Salces B, Larsen SU, Ahring BK, Uellendahl H. 2013. Biogas from Italian ryegrass and Oil seed radish: Effect of nitrogen application on biomass yield and methane production”. 21st European Biomass Conference and Exhibition (EU BC&E). Copenhagen, June 3-7,2013. [17] Østergaard, H.S. (2011). Undersøgelse af etableringen af mellem- og efterafgrøder i 2011. Planteavlsorientering nr. 065, Landbrugsinfo, 17/7 2013. https://www.landbrugsinfo.dk/planteavl/afgroeder/efterafgroeder/sider/pl_po_11_065.aspx [18] Østergaard, H.S. (2013b). Undersøgelse af etableringen af mellem- og efterafgrøder i 2012. Planteavlsorientering nr. 167, Landbrugsinfo, 17/7 2013. https://www.landbrugsinfo.dk/planteavl/afgroeder/efterafgroeder/sider/Undersoegelse-afetableringen-af-mellem-og-efterafgroeder-2012_pl_po_13_167.aspx [19] Molinuevo-Salces B, Férnandez-Varela R, and Uellendahl H. 2014. Key factors influencing the potential of catch crops for methane production, Environmental Technology ,DOI:10.1080/09593330.2014.880515. [20] Molinuevo-Salces B, Larsen S, Ahring BK, Uellendahl H. 2013. Biogas production from catch crops: Evaluation of biomass yield and methane potential of catch crops in organic crop rotations, Biomass and Bioenergy 59, 285-92. [21] Porter MG, Murray RS. 2001: The volatility of components of grass silage on oven drying and the inter-relationship between dry-matter content estimated by different analytical methods. Grass Forage Sci, 56, 405-411.

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