Biogas (raw biogas)

Biogas is a renewable gaseous fuel produced by the anaerobic microbial breakdown of organic matter — agricultural residues, animal manure, food waste, energy crops, sewage sludge, or industrial organic effluent — in sealed reactors known as digesters. Raw biogas is a mixture dominated by two gases: methane (CH4) at 55–65% and carbon dioxide (CO2) at 35–45%, with trace contaminants including hydrogen sulphide (50–5,000 ppm), water vapour (2–7%), ammonia, siloxanes (from sewage feedstocks), and nitrogen. The methane fraction carries the energy; the remaining components are either inert (CO2, N2) or harmful to downstream equipment and must be removed for high-value applications.

The energy content of raw biogas is typically 21–24 MJ per normal cubic metre, equivalent to roughly 0.6 litres of diesel per Nm3 or about 60% of the calorific value of pure natural gas. This makes biogas directly usable in three pathways: direct combustion in boilers, kilns, and cookstoves with minimal cleanup; combined heat and power (CHP) in gas engines producing 1.8–2.2 kWh of electricity per Nm3 plus recoverable heat; and upgrading to biomethane by removing CO2 and impurities to produce a gas with 95–99% methane content, which can be compressed into bio-CNG cylinders, blended into natural gas grids, or used as vehicle fuel under SATAT.

India's biogas sector spans three scales. Household biogas under the National Biogas Programme uses small fixed-dome or floating-drum digesters (1–25 m3) on dairy farms and rural households for cooking gas. Industrial biogas at distilleries, dairies, and food processing units captures methane from existing effluent treatment to displace coal or furnace oil. Commercial CBG plants under SATAT compress upgraded biogas to 200–250 bar for sale to oil marketing companies at notified prices, with over 100 plants commissioned and 5,000+ planned. Biogas captured from anaerobic digestion also represents avoided methane emissions — a greenhouse gas roughly 28 times more potent than CO2 over a 100-year horizon — making it both a renewable fuel and a climate mitigation lever.