>99.9% (>99.9% CH4)

A methane purity specification of greater than 99.9% (>99.9% CH₄) represents the strictest grade of biomethane or methane used in industrial practice, with combined non-methane impurities (CO₂, N₂, O₂, H₂S, moisture, VOCs) held below 1,000 parts per million by volume. This grade is well beyond what Indian CBG sales contracts require — the IS 16087 specification used for CBG sold to oil marketing companies under the SATAT scheme calls for only 90% CH₄ minimum, while pipeline-grade natural gas typically targets 95–98% CH₄.

Applications requiring >99.9% purity are narrow but real:

• Laboratory and analytical use — gas chromatography, calibration standards, NMR carrier gas
• Semiconductor manufacturing — chamber-purge applications where any impurity contaminates wafers
• Specialty chemical feedstocks — methane-to-methanol or methane-to-syngas processes where impurity build-up poisons catalysts
• Food and beverage — though more typical for CO₂, food-contact gas streams sometimes require ultra-pure methane

Achieving >99.9% from raw biogas (typically 55–65% CH₄, balance CO₂ with trace H₂S, siloxanes, moisture) requires a multi-stage train: a primary upgrading step (amine scrubbing or PSA, taking purity to 97–98%), followed by polishing through cryogenic distillation, deep-bed adsorption, or membrane permeation. Capex and opex escalate sharply at each polishing stage. A standard amine-scrubbed CBG plant producing 99% CH₄ for vehicle fuel might cost ₹4 crore for a 5 TPD plant; adding a polishing step for >99.9% purity can add ₹1.5–3 crore plus ongoing solvent and energy costs.

In the Indian CBG market, almost no operator targets >99.9% purity because there is no premium customer willing to pay for it. The economics favour producing pipeline or vehicle-grade gas in volume rather than ultra-pure specialty product in small quantities, unless a captive industrial offtaker (a fertiliser plant, for example) has been pre-contracted.