Ideally, any CH4 generated during wastewater treatment and the digestion of solid wastes like manure and domestic sewage sludge should be recovered and used as an energy source. There are incentives from governments to promote the use of biologically generated CH4 as a replacement for natural gas. However, in most cases the biogas generated in digesters is used for electricity generation with combined heat and power (CHP) generators. In most European countries electricity from biogas is considered as renewable and is subsidized. The heat generated through biogas combustion in CHP systems is often used to warm the input of the digester and digester content, and the surplus can be used for drying the solid fraction after separation of the digested manure. It may also be used to heat commercial buildings or residential areas. In the latter case it can be more profitable to install the CHP near the heat-demanding area and transport the biogas instead of transporting the heat.
Direct use of the biogas in the national natural gas grid, or use as transport fuel in cars, buses and trains, is becoming more and more popular. For direct use of the gas, it must first be purified to (more or less) natural gas properties (FNR, 2005). Flaring of CH4 has to be applied if recovery is not possible. The emissions mitigation advantage of flaring - the conversion of CH4 (GWP 21-25) to CO2 and water vapour - is that total emissions in terms of CO2-eq decrease considerably.
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