Agricultural biogas plants utilise organic materials found on farms to generate biogas, a renewable fuel source and in turn renewable power through cogeneration / combined heat and power. The plant may be designed to accept energy crops that have been grown specifically to input into the digestion facility or agricultural wastes. These crops are typically ensilaged and stored in clamps or hoppers and are continuously fed into the digester throughout the year. In India biogas plants may be called gober gas plants.
Energy Crops and Feedstocks for Biogas Production
Typical energy crops for biogas production can include:
- Maize
- Grass
- Wheat
- Rye
- Triticale
- Crassuleacean acid metabolism (CAM) plants such as pineapples
Alternatively, other organic materials such as waste products may be used including:
- Vegetable waste.
- Glycerol – from biodiesel manufacture
- Palm oil mill effluent
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Agricultural Biogas Plants
Agricultural biogas plants typically consist of several low digester tanks built either from concrete or metal. They are often topped by a twin-skinned gas storage bag, giving them a characteristic appearance. The majority of biogas will be produced by the first digestion tank with a lower gas yield being attained in the secondary digestate storage tank.
A useful approximate rule of thumb is that for 1 acre (0.405 hectares) of whole crop maize will produce enough gas to generate 1kW of electrical power – i.e. it takes 500 acres of whole crop maize silage to produce sufficient feedstock for a 500kWe digester / gas engine combination.
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Economics of Agricultural Biogas
Typically, biogas plants derive revenue from a gate fee for processing a waste material, a revenue stream from the sale of electricity and heat (or biomethane) and a possible revenue stream from the sale of soil improver.
Agricultural biogas plants are often more cost sensitive due to the lack of a gate fee revenue stream. There is also a cost associated with growing the feedstock for the digester. Due to these facts there are essential economic considerations:
- The efficiency of the biogas CHP engine (conversion efficiency to electricity and heat)
- The availability of the CHP engine (number of running hours per year)