Biomass can be defined as any accumulation of biological materials, that can be used as a source of energy. Examples of biomass are wood and crop residues.
Like fossil fuels, biomass can be burned directly to provide energy in the form of heat. It can also via decomposition processes be converted to methane (natural gas) and liquid fuels like methanol and ethanol.
The process of converting biomass to methane (or other fuels) is called bioconversion. In the absence of air, digestion of the organic materials in biomass by anaerobic bacteria produces biogas, which is about 60%–70% methane. The most suitable starting materials are sewage sludge or manure.
A number of cities in the Arctic and Subarctic burn municipal trash, which is generally about 40% waste paper, as a source of energy.
The biogas is used for cooking, lighting, and heating, or to generate electricity. Production of biogas is economically feasible only in locations where there is a large concentration of the starting materials.
Although it is unlikely that bioconversion will ever become a major source of energy, it is a useful way to supplement other sources. It is particularly valuable as a means of converting plant and animal agricultural wastes, and waste paper in municipal garbage, to usable energy.
The forests of the circumpolar North present an enormous reservoir for this renewable form of energy. Burning pellets can be seen as carbon neutral as it releases the exact amount of carbon that would have been emitted in nature anyway through decomposition.