Biogas from Energy Crop Digestion Rudolf BRAUN Peter WEILAND Arthur WELLINGER Biogas from Energy Crop Digestion IEA Bioenergy
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Biogas from Energy Crop Digestion Rudolf BRAUN Peter WEILAND Arthur WELLINGER Biogas from Energy Crop Digestion IEA Bioenergy Task 37 - Energy from Biogas and Landfill Gas IEA Bioenergy aims to accelerate the use of environmental sound and cost-competitive bioenergy on a sustainable basis, and thereby achieve a substantial contribution to future energy demands. THE FOLLOWING natiONS ARE CURRENTLY MEMBERS OF TASK 37: Austria Rudolf BRAUN, [email protected] Canada Andrew McFarlan, [email protected] Denmark Jens Bo HOLM-NIELSEN, [email protected] European Commission David BAXTER, [email protected] Finland Jukka Rintala, [email protected] France Caroline MARCHAIS, [email protected] Olivier THEOBALD, [email protected] Germany Peter WEILAND, [email protected] Netherland Mathieu DUMONT, [email protected] Sweden Anneli PETERSSON, [email protected] Switzerland (Task leader) Arthur WELLINGER, [email protected] UK Clare LUKEHURST, [email protected] Rudolf BRAUN IFA-Tulln – Institute for Agrobiotechnology A-3430 Tulln, Konrad Lorenz Strasse 20 [email protected] Peter WEILAND Johann Heinrich von Thünen-Institut (vTI) D-38116 Braunschweig, Bundesallee 50 [email protected] Arthur WELLINGER Nova Energie CH-8355 Aadorf, Châtelstrasse 21 [email protected] IMPRESSUM Graphic Design: Susanne AUER Contents Biogas from Energy Crop Digestion Contents The world‘s energy supply – A future challenge 4 Development of energy crop digestion 4 Energy crops used in anaerobic digestion 5 Technology for anaerobic digestion of energy crops 5 Co-digestion and only energy crop digestion 7 Practical applications of energy crop digestion 8 An example of only energy crop digestion 8 An example of co-digestion of energy crops 9 An example of dry-fermentation of energy crops 11 Long term biogas plant operational experience 12 Significance and potential of energy crop digestion 13 Biogas yield per hectare of crops 13 Net energy yield per hectare of crops 14 Future significance of biogas from biomass 15 Citations 16 Further reading 17 Glossary, terms 17 Abbreviations 18 04 Biogas from Energy Crop Digestion The world‘s energy supply / Development of energy crop digestion the potential use of oats, grass and straw in New Zeal- The world‘s energy supply – and, resulting in methane yields of 170–280 m3.t-1 TS. Even water hyacinths and fresh water algae were shown A future challenge to result in medium methane yields between 150–240 m3.t-1 TS. In the USA a large project on microalgae Currently some 80% of the world‘s overall energy and kelp for aquatic raw material production was started. supply of about 400 EJ per year is derived from fossil Although the digestion of crop material was demon- fuels. Nevertheless roughly 10–15% of this demand is strated, the process was hardly applied in practice. Crop covered by biomass resources, making biomass by far the digestion was commonly not considered to be economi- most important renewable energy source used to date. cally feasible. Crops, plants, plant by-products and On average, in the industrialised countries biomass -waste materials were just added occasionally to stabilise contributes by some 3–13% to the total energy supplies, anaerobic waste digesters. but in developing countries this proportion is much With steadily increasing oil prices and improved higher. In quite a number of countries biomass covers legal framework conditions, “energy crop”-research and even over 50 to 90% of the total energy demand. development was again stimulated in the 1990s. In Ger- Biomass combustion is responsible for over 90% of many for example, the number of digesters using energy the current production of secondary energy carriers crops has increased from about 100 in 1990 to nearly from biomass. Liquid biofuels cover only a small part 4,000 in 2008 (Figure 1). and the most used are ethanol and biodiesel. Ethanol is The steady increase in energy crop digester applicati- produced from sugar- or starch crops, while biodiesel is ons in Germany and similarly in Austria, can be directly derived from vegetable oils or animal fats. attributed to the favourable supportive European and Currently biogas plays a smaller, but steadily growing National legal frameworks of eco-tariffs, paid for rene- role. Traditionally applied for sewage sludge treatment- wable energy. Depending on the electrical power capaci- and stabilisation purposes, energy recovery from biogas ty of the digestion plants, staggered feed in tariffs are was a welcome by-product. However, biogas has become guaranteed for the whole depreciation period of the a well established energy resource, especially through the investment. Similar subsidising systems exist for instance use of renewable biomass i.e. “energy crops”. Since about in Switzerland and France. Other European countries 1950, biogas production from manure and /or energy apply tax exemptions (e.g. Sweden) or certificates (e.g. crops, continued to develop as an important new farm UK) for renewable energies. enterprise. Development of energy crop digestion The idea to use dedicated plant bio- mass, the so called “energy crops” for methane production (biomethanation) is not new. Early investigations on the bio- methanation potential of different crops and plant materials have been carried out in the 1930s by Buswell in the USA and later on in the 1950s by Reinhold and Noa- Figure: 1: Number of Biogas Plants in Germany. ck in Germany. In 1980, Stewart described Development between 1990 and 2008 (Weiland, 2009) 05 Development of energy crop digestion Biogas from Energy Crop Digestion Energy crops used in anaerobic digestion Recent German practical experience showed mean metha- Numerous plants and plant materials have been ne yields of 348 m3/t VS for maize and 380 m3/t VS for tested for their methane formation potential. In principal barley (KTBL, 2009). many varieties of grass, clover, cereals and maize inclu- A comprehensive data bank on crop yields, ding whole plants, as well as rape or sunflower proved appropriate climate and growth conditions, feasible for methane production. Even hemp, flax, nettle, based on literature and own investigations, was miscanthus or potatoes, beets, kale, turnip, rhubarb and elaborated in the recent EU funded “CROPGEN” artichoke were tested successfully. Some practically used project <http://www.cropgen.soton.ac.uk/> “energy crops” are shown in photos 1 to 4. Depending on Crops are frequently used for digestion directly after numerous conditions, a fairly wide range of methane harvest. The harvest time can influence the bio-degrada- yields, between 120–658 m3.t-1VS, was reported in litera- bility and hence the methane yield from plants. Late ture from anaerobic digestion of different crops (Table 1). harvest usually is associated with higher cellulose content in the biomass, causing slower bio-degradation and less Tab. 1: Exemplary methane yields from digestion of various plants and methane yield. For a year – round availability of substra- plant materials as reported in literature (Data compilation after Braun, 2007) tes, the crops are most frequently stored in silage clamps. Under favourable circumstances, crops can also be dried, Methane yield (m3 per t volatile solids) by using for example surplus heat from a CHP. Maize (whole crop) 205 – 450 Barley 353 – 658 Wheat (grain) 384 – 426 Triticale 337 – 555 Oats (grain) 250 – 295 Sorghum 295 – 372 Rye (grain) 283 – 492 Technology for anaerobic digestion Grass 298 – 467 Alfalfa 340 – 500 of energy crops Clover grass 290 – 390 Sudan grass 213 – 303 Numerous technical solutions are offered by the Red clover 300 – 350 Reed Canary Grass 340 – 430 industry, which are all based on the same basic principle. Clover 345 – 350 Ryegrass 390 – 410 Six steps can be distinguished principally in energy crop Hemp 355 – 409 Nettle 120 – 420 digestion processes: Flax 212 Miscanthus 179 – 218 • Harvest and pre-processing of energy crops Sunflower 154 – 400 Rhubarb 320 – 490 as a substrate Oilseed rape 240 – 340 Turnip 314 • Storage of the substrate Jerusalem artichoke 300 – 370 Kale 240 – 334 Peas 390 • Feed regulation (dosage) and fermentation Potatoes 276 – 400 Chaff 270 – 316 (digestion) Sugar beet 236 – 381 Straw 242 – 324 • Treatment, storage and use of digestate Fodder beet 420 – 500 Leaves 417 – 453 • Treatment, storage and use of biogas Photo 1: Maize culture, most frequently used as energy crop, with a Photo 2: Miscanthus, a perennial plant, producing wide range of biomass yields per hectare from 9 up to 30 tons DM hectare yields in a wide range of 8–25 tons DM Photo 3: Sun flower field in early cultivation stage, used for oil Photo 4: Grass is frequently used as energy plant, enabling production as well as an energy crop in anaerobic digestion. 2–3 harvests per year under moderate climate conditions. Hectare yields of 6–8 tons DM are possible. Yields of 12–14 tons DM per hectare are obtained. 1 2 3 4 06 Biogas from Energy Crop Digestion Development of energy crop digestion Substrate preparation and dosage A wide range of annual and perennial plants may be used as energy crops (see Table 1). Nevertheless maize is Photo 6: Ensiling procedure most widely used by the majority of existing plants. of whole crop Standard combine harvesters are used, simultaneously chopped maize, chopping the whole maize plant (Photo 5) for the subse- using a front loader quent ensiling. Ideally the biomass used for ensilage should have to use bag silos. These could be e.g. 3.5 m diameter and TS contents of between 30–40%. Biomass materials 100 m length. A typical bag silo storage capacity is about with TS contents below 20% result in poor silage quali- 6,000 tons of silage. Bag silos are filled by packing ties, high leachate accumulation and subsequently poor machines, able to load up to 100 t/h, corresponding to biogas yields.