Amine scrubbing (amine absorption)

Amine Scrubbing is a chemical absorption process for upgrading raw biogas to pipeline-grade biomethane, in which an aqueous solution of an alkaline amine — most commonly monoethanolamine (MEA), methyldiethanolamine (MDEA), or piperazine-activated MDEA — selectively absorbs carbon dioxide and hydrogen sulphide while letting methane pass through. The CO₂-laden solvent is then heated in a regenerator (stripper) to release pure CO₂ and recycle the lean amine back to the absorber. The technology was originally developed for natural gas sweetening and has been adapted at scale to biogas since the 2000s.

A typical Indian CBG amine plant operates in a continuous loop:

• Absorber column — raw biogas enters at the bottom, lean amine sprayed from top, contact at 30–50 °C and 1–3 bar
• Regenerator — rich amine heated to 100–120 °C by a reboiler, CO₂ stripped overhead
• Heat exchangers — lean/rich amine economiser recovers thermal energy
• Solvent reclaimer — periodic distillation to remove degradation products and heat-stable salts

Amine scrubbing achieves the highest methane purity of any commercial upgrading technology — routinely above 99% CH₄ with methane slip below 0.1%, comfortably meeting the IS 16087 specification (90% CH₄ minimum) used for Indian CBG sold to gas marketers. The CO₂ off-gas is also high-purity (above 98%), enabling food-grade or industrial CO₂ recovery as a co-product.

The trade-offs are real. Amine systems carry higher capex than water scrubbing (₹3–5 crore for a 5,000 Nm³/day plant) and significantly higher opex due to thermal energy demand — regeneration typically consumes 3.5–4.5 MJ per kg CO₂ removed, often supplied by burning a fraction of the produced biogas. Amine degradation also generates corrosive heat-stable salts, requiring stainless steel construction and periodic solvent replacement. For these reasons, amine scrubbing is the dominant choice for large Indian CBG plants above 10 TPD where the purity premium and CO₂ co-product justify the cost, while smaller plants favour water scrubbing or membranes.