Biogas can be combusted to produce heat. When biogas is mixed with air with a 1/7 ratio, complete combustion occurs. For heating purposes, biogas can be used in gas-fired furnaces and hobs. Biogas can be used in stoves operated with liquefied petroleum gas by adjusting the nozzle diameters. When used in biogas stoves, a chimney system is required to allow any hydrogen sulfide gas to vent to the atmosphere.
Electricity only
Electricity generation is a relatively straightforward use for biogas, and it can be the most profitable. Biogas requires minimal investment in cleaning and upgrading. Electricity is easier to transport than heat and supply is easily measured.
Electricity storage, however, is not simple and connecting to the electricity network is costly.
Combined Heat and Power (CHP)
Combined heat and power (CHP) is the simultaneous production of useable heat and electricity. As the process of AD requires some heat it is suited to CHP and this is currently the most popular option for most plants. Whilst coal and gas-fired power stations have an efficiency of around 34% and 55% respectively, CHP plants can achieve overall efficiencies in excess of 80% at the point of use.
The ratio of heat to power varies dependent on the scale and technology, but typically 35-40% is converted to electricity, 40-45% to heat and the balance lost due to inefficiencies at various stages of the process. This typically equates to over 2kWh electricity and 2.5kWh heat per cubic meter, at 60% methane.
A generator producing electricity from a biogas plant can be connected to a transmission network, distribution system or even to the cables owned by the end customer.
Biomethane Injection
Biogas can be upgraded to biomethane and injected into a gas grid. This can be the national high pressure gas transmission grid or a local low pressure gas distribution network. To be used in the gas grid biogas needs to be cleansed of impurities, dried and upgraded to higher methane content (> 95%) so that it resembles the qualities of natural gas.
Biomethane for Transport
Biogas can be cleaned to remove impurities and upgraded to pure biomethane. It can then be used as a renewable transport fuel in vehicles designed to run on compressed natural gas (CNG) or liquefied natural gas (LNG).
Organic Fertilizers
While there are suitable inorganic substitutes for the nutrients nitrogen, potassium and phosphorous from organic fertilizer, there is no artificial substitute for other substances such as protein, cellulose, lignin, etc. They all contribute to increasing a soil's permeability and hygroscopicity while preventing erosion and improving agricultural conditions in general. Organic substances also constitute the basis for the development of the microorganisms responsible for converting soil nutrients into a form that can be readily incorporated by plants.
Due to the decomposition and breakdown of parts of its organic content, digested sludge provides fast-acting nutrients that easily enter into the soil composition, thus becoming immediately available to the plants. They simultaneously serve as primary nutrients for the development of soil organisms, e.g. the replenishment of microorganisms lost through exposure to air in the course of spreading the sludge over the fields. They also nourish actinomycetes (ray fungi) that act as organic digesting specialists in the digested sludge. (Preconditions: adequate aeration and moderate moisture).
