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Agricultural Biogas & Cogeneration / CHP

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 & 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

Alternatively other organic materials such as waste products may be used including:

  • Slurry.
  • Manure.
  • Vegetable waste.
  • Glycerol – from biodiesel manufacture.
Agricultural biogas CHP schematic, www.clarke-energy.com

Electrical output from biogas plant feedstocks, www.clarke-energy.com

Agricultural Biogas Plants

Agricultural biogas plants typically consist of a number of low digesters 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.

An 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.

Economics of Agricultural Biogas

Agricultural biogas plants typically generate returns via the sale of electricity alone, this means that the gas engine is of particular importance for the success of the plant. Gate fees as a charge for the acceptance of waste materials may be low or none-existent. If the farmer grows energy crops to feed into the plant then there is a cost associated with producing the feedstock. These two factors make it essential for the farmer to have an engine with the maximum levels of availability (running time per year) and the highest levels of electrical efficiency, in order to convert the gas to the maximum level of electrical output.

CHP Importance and Maintenance Contracts

The GE Jenbacher gas engine is known for having the highest levels of electrical efficiency and availability on the market. When coupled with a contractual maintenance agreement with Clarke Energy, it will give peace of mind to the customer that they will achieve the highest levels availability and hence consistent returns from their biogas plant.