Biogas is an energy-rich mixture of gases consisting mostly of methane and carbon dioxide and also small amounts of hydrogen sulfide and other substances. It is produced through the decomposition of agricultural waste, food waste, plant material, sewage and manure in the absence of oxygen, and its applications include heating, cooking, internal combustion engines and fuel cells.
Biogas is already widely used throughout much of the world, mainly on a small scale, and its use is expected to continue to grow rapidly because it can be a very convenient, relatively clean, and inexpensive source of renewable energy, particularly for farmers and others in isolated locations. Continuing technological advances, including with regard to storage and transport, are also contributing to this growth.
A major disadvantage of biogas is that it is difficult to produce on a large scale, especially on a scale sufficient for use in large urban areas. Another is that it can be very harmful to the environment, both because burning it produces carbon dioxide and because the methane from leaks of unburned biogas is extremely efficient at trapping heat in the atmosphere, an efficiency more than 25 times that of carbon dioxide.