Bio-CNG (Compressed Biogas (CBG))

Bio-CNG is renewable methane produced from organic waste through anaerobic digestion, purified to natural-gas-grade quality (95% minimum CH4), and compressed to 200–250 bar for use as a vehicle fuel or industrial gas. The term is functionally interchangeable with Compressed Biogas (CBG) — both refer to the same product specification, with 'bio-CNG' emphasising the fossil-CNG-equivalent end use and 'CBG' the policy/regulatory designation used in SATAT and CPCB documents. In Indian retail markets, oil marketing companies sell the product under both names depending on regional branding.

The bio-CNG production pathway has four stages. Anaerobic digestion of organic feedstock — agricultural residues, livestock manure, press mud, food waste, sewage sludge — generates raw biogas at 55–65% methane and 35–45% CO2. Upgrading using water scrubbing (most common in India at ~60% of installations), pressure swing adsorption (~25%), membrane separation, or chemical (amine) scrubbing removes CO2 and H2S, raising methane content to above 95%. Drying using glycol or refrigerated dryers brings the dew point below -10 degC to prevent condensation in cylinders. Compression in two- or three-stage reciprocating compressors raises pressure from near-atmospheric to 200–250 bar for storage in cascade cylinder banks or direct dispensing.

The product must meet IS 16087:2016 for fuel-grade bio-CNG: minimum 90% methane (operators typically target 95–97% for margin), maximum 4% CO2, maximum 7 mg/Nm3 H2S, water dew point -10 degC at delivery pressure, and total sulphur below 20 mg/Nm3. These specifications match CNG so that bio-CNG can use identical vehicle engines, fuel-station infrastructure, and tax treatment. The energy content of bio-CNG at 97% CH4 is approximately 47 MJ/kg or 35 MJ/Nm3 — equivalent to 1.4 litres of petrol or 1.1 litres of diesel per kg. Indian retail markets price bio-CNG comparable to CNG, with the methane producer receiving a SATAT-notified ex-plant rate (currently around Rs 54 per kg) plus a state-level VAT/SGST treatment that varies. The carbon footprint of bio-CNG, accounting for digester energy and methane slip, is 70–85% lower than fossil CNG on a well-to-wheel basis.