Biogas routes and climate change

Biogas is a combustible gas produced when organic matter such as animal dung, human waste, kitchen waste and crop residue is decomposed by bacteria in the absence of oxygen (anaerobic digestion). Composition: methane (CH₄) 55–65%, carbon dioxide 35–45%, traces of H₂S, H₂ and N₂. Calorific value is about 20–25 MJ/m³. The by-product, slurry, is a rich organic fertiliser.

Four major routes for conversion of biomass

1. Direct combustion (heat, steam, power; briquettes and pellets).
2. Thermo-chemical conversion: gasification and pyrolysis, giving producer gas, bio-oil and charcoal.
3. Bio-chemical conversion: anaerobic digestion (biogas) and fermentation (ethanol).
4. Chemical conversion / extraction: transesterification of oils to biodiesel; Fischer–Tropsch synthesis of liquid fuels from syngas.

How biogas reduces climate change effects

• Replaces firewood, kerosene and LPG: less CO₂ from burning fossil fuel, and fewer trees cut (forests are carbon sinks).
• Captures methane: dung left in the open releases methane, which has a global warming potential about 25–28 times that of CO₂ over 100 years. In a digester, methane is collected and burnt to CO₂, which reduces the warming effect greatly.
• Biogenic carbon: the CO₂ released on burning was recently absorbed from the air by plants, so it is nearly carbon-neutral.
• Slurry replaces chemical fertiliser, whose production emits N₂O and CO₂.
• Reduces black carbon and smoke from traditional stoves.
• Nepal's biogas programme (supported by AEPC; first CDM-registered activity of Nepal) earns carbon credits for the avoided emissions.