97%–99% (97–99% CH4)

A methane purity range of 97–99% defines premium-grade biomethane — the quality typically required for direct injection into city gas distribution networks or transmission pipelines. This is at the top end of the Bureau of Indian Standards IS 16087 specification for compressed biogas, which sets a minimum of 90% methane, with most SATAT off-take agreements targeting 95–97% as the operating range. At 97–99% purity, the gas closely approaches the composition of fossil-origin natural gas (typically 92–98% methane in Indian city gas networks) and can be blended interchangeably.

Achieving 97–99% purity requires careful selection of upgrading technology. Amine scrubbing (MEA, MDEA) is the most reliable route, delivering 99%+ purity with methane slip below 0.1%. Membrane separation achieves 96–99% but typically requires multi-stage configurations with recycle, raising capex. Pressure Swing Adsorption (PSA) can hit 97% in single stage and 99% in two-stage configurations but at the cost of higher methane slip (2–4% in single-stage). Water scrubbing rarely exceeds 96% economically and is usually limited to plants targeting the 90–95% range.

Operating at the high end of the purity range has implications beyond product specification. Higher purity reduces volumetric requirements for storage and dispensing — every percentage point of CO₂ removed increases volumetric energy density. It also reduces corrosion risk in downstream equipment because residual CO₂ in the presence of water forms carbonic acid that attacks carbon steel pipework. The trade-off is energy cost: each incremental percent of CO₂ removal at the top end of the range requires disproportionately more compression energy or membrane area. Most SATAT plants therefore optimise for the minimum purity that satisfies their off-take contract rather than chasing the 99% mark unless pipeline injection is the end use.