CH₄ percentage (CH₄ percentage)

CH4 percentage is the volumetric concentration of methane in a biogas or upgraded biomethane stream, expressed as percent by volume (vol%). It is the single most important quality metric for biogas because methane is the only energy-carrying component — CO2, N2, water vapour, and trace impurities are all energy-neutral or energy-negative dilutents. Higher CH4 percentage means higher calorific value per Nm3 of gas, lower transport cost per unit energy delivered, and direct compatibility with natural gas infrastructure.

Typical CH4 percentages at different points of a CBG plant:

• Raw biogas from agricultural feedstocks (cattle dung, pressmud): 55-65% CH4, 35-45% CO2.
• Raw biogas from energy-dense food waste: 60-70% CH4.
• Raw biogas from sewage sludge: 60-65% CH4.
• After H2S removal (iron sponge, bio-scrubber): unchanged CH4 percentage (H2S is only 0.05-0.5% by volume).
• After CO2 removal (water scrubbing, PSA, membrane): 90-99% CH4.
• CBG (compressed bio-gas) compliant with IS 16087:2016: minimum 90% CH4.
• Commercial CNG (IS 15958:2009): minimum 89% CH4 (CBG and CNG are blendable when both meet 90%+).

CH4 percentage directly translates to lower heating value:

• Raw biogas at 60% CH4: 19-22 MJ/Nm3.
• Upgraded biomethane at 95% CH4: 33-34 MJ/Nm3.
• Pure methane: 35.8 MJ/Nm3.

Operators monitor CH4 percentage continuously through inline gas analysers — NDIR (non-dispersive infrared) for CO2 and CH4, electrochemical sensors for O2. Trends in CH4 percentage are diagnostic of process health:

• CH4 dropping from 60% to 50%: methanogen stress; acidogens producing more CO2 and H2; check pH and VFA immediately.
• CH4 holding at 60-65%: healthy digester.
• CH4 above 65-70%: outstanding feedstock or possible measurement error; verify with grab sample.

CH4 percentage in upgraded biomethane is a contractual specification under SATAT — OMCs reject lots below 90% CH4 and pay full SATAT price for 90%+ compliant gas. The trade-off in upgrading design is methane purity vs methane slip: pushing membrane or PSA systems toward 98-99% purity reduces methane slip (lost to atmosphere with CO2) but increases compressor energy and equipment size. Most Indian SATAT plants design for 95-96% purity with 1-2% slip, balancing IS 16087:2016 compliance headroom against energy and capex penalties of pushing higher purity.