Biogas upgrading (biomethane production)

Biogas upgrading is the post-digestion treatment step that converts raw biogas — typically 55–65% methane, 30–40% carbon dioxide, and trace contaminants — into pipeline-quality biomethane suitable for use as Compressed Biogas (CBG) under the SATAT scheme. The Bureau of Indian Standards specification IS 16087 requires CBG to contain a minimum of 90% methane, with hydrogen sulfide below 20 mg/Nm³, water dew point below minus 4°C, and oxygen under 0.5%. Most Indian SATAT plants target 95–97% methane purity to ensure consistent CNG-equivalent calorific value of around 49 MJ/kg.

Four upgrading technologies dominate the Indian market. Water scrubbing uses pressurised water to dissolve CO₂ and H₂S and is the lowest-capex option but consumes 0.2–0.3 kWh per Nm³ of raw gas. Pressure Swing Adsorption (PSA) uses carbon molecular sieves at 6–10 bar to selectively retain CO₂ — capex is moderate but methane slip can reach 2–4%. Membrane separation uses polymeric hollow-fibre modules; capex is higher but plot area and operator skill demands are lowest. Amine (MEA/MDEA) scrubbing achieves the highest purity (above 99%) and the lowest methane slip (under 0.1%) but adds chemical handling complexity.

Pre-treatment is non-negotiable. Hydrogen sulfide must be removed first using iron-sponge or activated-carbon beds because it corrodes upgrading equipment and poisons adsorbents. Moisture is knocked out with chillers or silica gel beds to protect compressors downstream. Key trade-offs that decide technology choice are capex versus opex, methane slip (each percentage point lost is both a revenue loss and a Scope-1 greenhouse-gas penalty under PAT/CCTS), and footprint. For a 5 TPD CBG plant, total upgrading capex typically falls between ₹3.5 crore and ₹6 crore depending on technology and redundancy.