90% (90% methane)

90% methane by volume is the minimum methane concentration that compressed biogas must achieve under IS 16087:2016, the Bureau of Indian Standards specification for bio-CNG used as automotive fuel. This threshold is not arbitrary — it is the concentration at which biomethane becomes interchangeable with fossil CNG in standard spark-ignition and dual-fuel engines without modification, recalibration, or warranty implications. Falling below 90% creates an inconsistent air-fuel ratio, causing rough idling, knock, reduced power output, and accelerated catalyst poisoning.

Raw biogas leaves an anaerobic digester at 55-65% methane, with the balance being carbon dioxide (35-45%), water vapour, hydrogen sulfide (200-3,000 ppm), nitrogen, and trace siloxanes. Reaching 90%+ requires an upgrading step — typically water scrubbing, pressure swing adsorption (PSA), membrane separation, or amine scrubbing — to strip CO2 and remove H2S below 10 ppm. PSA and membrane plants commonly achieve 96-98% methane, providing comfortable headroom above the IS 16087:2016 floor.

The standard also bounds other parameters: CO2 must not exceed 4% by volume, H2S must be below 16 mg/Nm3 (around 10 ppm), moisture below 5 mg/Nm3, and total sulfur below 40 mg/Nm3. Together these limits ensure the energy density (calorific value) sits at 47-52 MJ/kg, the Wobbe Index falls within the natural gas range, and corrosion in storage cylinders and dispensers is suppressed. CBG meeting these specifications can be blended with fossil CNG in pipelines and cascade banks without separation.

For producers, the 90% threshold drives upgrading-equipment sizing and operating cost. Every additional percentage point of methane recovery raises capital cost (larger membranes or longer PSA cycles) but reduces methane slip — the fraction lost in the off-gas stream — which directly affects plant economics. Most SATAT-registered plants target 92-94% methane as the sweet spot between recovery efficiency and capex.