Technical Report

Biogas from Crop Digestion

October 2011

This brochure is a revision of the 2009 IEA Bioenergy Task 37 “Biogas from Energy Crop Digestion” technical brochure.

1.1 The world’s energy supply – A future challenge

Currently about 80 % of the world’s overall energy supply (ca. 400 EJ per year) is derived from fossil fuels. Biomass is by far the most important renewable energy source used to date, supplying 10-15 % of energy supply. On average, in industrialised countries biomass contributes 9-13% of the total energy supply, but in developing countries this proportion is much higher. In SubSahara Africa biomass supplies 70 to 90% of the total energy demand.

Biomass combustion is responsible for over 90% of the current production of energy from biomass. Liquid biofuels (e.g. ethanol and biodiesel) contribute only a small portion of biomass energy. First generation ethanol is produced from sugar or starch crops, while biodiesel is derived from vegetable oils or animal fats.

Currently biogas plays a smaller, but steadily growing role. Energy recovery from biogas by anaerobic digestion (AD) has been a welcome by-product of sewage sludge treatment for a number of decades. However, biogas has become a well established energy resource, especially through the use of biomass residues or crops. Since the 1950’s, biogas production from manure and / or crops has continued to develop as an important new farm enterprise in countries such as Austria, Denmark and Germany.

1.2 Development of crop digestion

The concept of crops for methane production (anaerobic digestion, biogas, methanisation or biomethanation) is not new. Early investigations on the biomethanation potential of different crops and plant materials were carried out in the 1930’s in the USA (Buswell and Hatfield, 1936), in the 1950’s in Germany (Reinhold and Noack, 1956), and in the 1980’s in New Zealand (Stewart et al., 1984). Although the digestion of crop material was demonstrated, the process was hardly applied in practice. Crop digestion was not considered to be economically feasible. Crops,, crop by-products and waste materials were occasionally added to stabilise anaerobic waste digesters.

In the 1990’s steadily increasing oil prices and improved legal framework conditions, stimulated crop research and development. In Germany for example, the number of digesters using crops was 100 in 1990. At the end of 2010 approximately 6,000 biogas plants were in operation in Germany (figure 1). The majority use a mixture of manure and crops; 90-95 % of all plants (between 5,400 and 5,700 plants) use crops. Several biogas plants employ mono-digestion.

The steady increase in crop digesters in Germany can be directly attributed to the favourable supportive national legal framework coupled with the tariffs paid for renewable energy. Staggered feed-in tariffs (which depend on the electrical power capacity of the biogas plants) are guaranteed for the whole depreciation period of the investment. Feed-in tariffs also exist in other countries, for instance in Switzerland, the Netherlands and France. Other European countries apply tax exemptions (e.g. Sweden) or a choice of certificates and feed-in tariffs (e.g. UK) for renewable energy. France, Switzerland or Sweden do not offer subsidies specifically for crop digestion.

1.3 Crops used in anaerobic digestion

Numerous plant species and plant residues have been tested for their methane potential. In principal, many varieties of grass, clover, cereals and maize, including whole plants, as well as rape and sunflower proved feasible for methane production. Hemp, flax, nettle, potatoes, beets, kale, turnip, rhubarb and artichoke have all been tested successfully. Some crops used for digestion are shown in Photos 1 to 4.

The literature typically refers to methane production in terms of m3 .t-1 Volatile Solids (VS). Volatile Solids refer to that portion of solids that are organic or dry and ash free; solids that can either combust or biodegrade. For example, 1 t of Volatile Solid has an energy value of about 19 GJ while 1 mn 3 of methane (CH4) has an energy value of ca. 38 MJ. Thus for conservation of energy the maximum production of methane is 500 m3 .t-1 VS (500 mn 3 CH4 * 38 MJ/mn 3 = 19,000 MJ = 19 GJ = 1 t VS). This value may increase for example due to the presence of alcohols and acids in silage liquors. Depending on specific process conditions, a fairly wide range of methane yields, between 120-658 m3 .t-1 VSadded, is reported in the literature from anaerobic digestion of different crops (Table 1). Recent German practical experience showed mean methane yields of 348 m3 .t-1 VS for ensiled maize and 380 m3 .t-1 VS for whole plant ensiled barley (KTBL, 2009). A comprehensive data bank on crop yields, appropriate climate and growth conditions was elaborated in the recent EU funded “CROPGEN” project (Cropgen, 2011).

Crops may be used for digestion directly after harvest. For year round availability of substrates, crops are frequently stored in silage clamps. Grass, for example, may be ensiled in a clamp or pit or it may be baled. In Irish conditions pit silage has a dry solids (DS) content of approximately 22% while bale silage has a dry solids content of about 30%. In drier climates such as Austria, grass is wilted (partially dried after cutting) prior to collection from the field and the resulting silage can have a dry solids content of up to 40%. The time of harvest varies for differing crops. Grass may be cut between two and five times in a season; the first harvest is as early as May (in the northern hemisphere). Sugar beet is harvested later than most crops, typically between November and January. Staggered harvest improves the possibility for co-digestion of fresh crops and reduces the amount of storage capacity required. The time of harvest can influence bio-degradability, and hence the methane yield. Late harvest (with longer growing period) usually leads to higher lignin content in grasses (Figure 2), causing slower bio-degradation and lower methane yield. Work in Ireland indicated a yield of 440 m3 CH4. t-1 VSadded for grass silage from an early harvest of perennial rye grass (Thamsiriroj & Murphy, 2011) though average values from the scientific literature are lower, reflecting the fact that later cuts have a higher fibre content.