Biogas
A gas mixture of roughly 50–70% methane and 30–50% CO2 produced by anaerobic digestion of organic matter such as manure, food waste, sewage sludge and energy crops.
Biogas is the product of anaerobic digestion or the natural decay of waste in landfills. Its heating value depends on methane content: at 60% CH4 it carries about 21–23 MJ/m3, roughly 6 kWh/m3, versus about 10 kWh/m3 for natural gas. Trace components include hydrogen sulphide (100–3,000 ppm), ammonia, siloxanes and water vapour, which must be removed before use in engines.
Typical biogas yields (m3 per tonne of fresh matter): cattle slurry 20–30, pig slurry 15–25, food waste 100–150, maize silage 180–220, used cooking fat up to 600 or more. Most plants burn biogas in gas engines for CHP, reaching 38–43% electrical and 40–45% thermal efficiency; a 500 kWe farm plant needs roughly 2.2 million m3 of biogas a year at 8,000 operating hours.
Biogas can also be upgraded to biomethane. Its climate benefit is largest when it captures methane that would otherwise escape from manure storage or landfills; leakage from digesters (1–3% of output) erodes it. Germany has around 9,000 plants; China and India have millions of household digesters. Estimate output with the biogas calculator.
Related terms
All terms →- Anaerobic digestion
The microbial breakdown of organic matter in the absence of oxygen, producing biogas and a nutrient-rich digestate usable as fertiliser.
- Biomethane (RNG)
Biogas upgraded to more than 95% methane so that it meets natural gas quality standards and can be injected into the gas grid or used as vehicle fuel.
- CHP (Combined Heat and Power)
Simultaneous generation of electricity and useful heat from one fuel input, reaching total efficiencies of 75–90% versus about 35–60% for power-only plants.
- Methane leakage
Unintended release of methane from oil, gas and coal supply chains through leaks, venting and incomplete flaring, often expressed as a percentage of gas produced.
- Calorific value (HHV / LHV)
The heat released by complete combustion of a unit quantity of fuel; the higher (gross) value includes condensation heat of water vapour, the lower (net) value excludes it.