activated carbon (activated carbon)

Activated carbon is a highly porous form of elemental carbon manufactured by carbonising organic precursors — typically coconut shell, coal, wood, or peat — and then activating the char physically with steam or carbon dioxide at 800-1,000 deg C, or chemically with phosphoric acid or zinc chloride. The activation step develops an extensive network of micropores (under 2 nm), mesopores (2-50 nm), and macropores (above 50 nm), producing an internal surface area of typically 800-1,200 m2 per gram and as high as 2,500 m2/g in specialised grades.

This surface area enables adsorption — molecules from a gas or liquid stream are physically trapped on the pore walls by van der Waals forces and, for some functionalised grades, by chemisorption with surface oxygen groups. In biogas purification, activated carbon beds are the standard polishing step for removing residual hydrogen sulfide (H2S), siloxanes, volatile organic compounds (VOCs), mercaptans, and trace ammonia after bulk H2S removal by iron sponge or biological scrubbers. Typical Indian biogas plant configurations use 100-500 kg of activated carbon per Nm3/h of biogas flow.

Two categories serve biogas applications. Impregnated activated carbon — typically loaded with potassium iodide, potassium hydroxide, or potassium permanganate — converts H2S into elemental sulfur on the pore walls, achieving H2S removal to below 1 ppm at H2S inlet concentrations up to 200 ppm. Non-impregnated coconut-shell carbon is preferred for siloxane removal because its predominantly microporous structure suits the molecular dimensions of D4 and D5 cyclic siloxanes.

Operating economics depend on adsorption capacity (typically 100-300 mg H2S per gram of impregnated carbon at breakthrough), regeneration possibility, and disposal. Spent carbon saturated with sulfur is classified as hazardous waste under Indian Hazardous Waste Rules, requiring authorised treatment facilities for disposal — costs of Rs 25-40 per kg are typical. Some operators regenerate carbon thermally, but for siloxane-laden beds regeneration is uneconomic and single-use disposal is standard. Capital cost for a complete activated carbon polishing unit on a 5 TPD CBG plant is typically Rs 15-25 lakh.