hydrogen sulfide (hydrogen sulfide)

Hydrogen sulfide (H2S) is a colourless, highly toxic, corrosive, and flammable gas produced as a metabolic by-product of sulfate-reducing bacteria during the anaerobic digestion of sulfur-containing organic matter. It is naturally present in raw biogas at 500-5,000 ppm (0.05-0.5% by volume) — the exact concentration depends on feedstock sulfur content. H2S removal is mandatory before biogas can be compressed, upgraded, or used in engines or burners because of its severe corrosivity, toxicity, and regulatory limits.

Toxicity is acute. H2S effects at various concentrations:

• 0.5-1 ppm: detectable rotten-egg odour.
• 10-20 ppm: eye and respiratory irritation; OSHA 8-hour TWA limit.
• 100-150 ppm: olfactory paralysis (odour no longer perceptible).
• 500 ppm: respiratory failure within minutes.
• 1,000 ppm: immediate loss of consciousness; rapid death.

The loss of odour warning at 100+ ppm is the deadliest feature of H2S — workers may not realise they are in a lethal atmosphere. Indian plant safety protocols mandate continuous H2S monitoring with 10 ppm warning and 15 ppm alarm in confined biogas-handling areas, and SCBA respirators for entry to atmospheres with potential H2S above 100 ppm.

Corrosion damage is the operational driver for H2S removal. In the presence of moisture, H2S forms sulfuric acid which attacks steel pipes, compressor cylinders, valves, and engine components. Untreated 2,000 ppm H2S biogas can corrode through 6 mm steel pipe in 3-5 years and destroys CHP engine bearings, exhaust valves, and oil within 1,000-3,000 operating hours.

Indian H2S removal technologies in CBG plants:

• Iron sponge (iron oxide on wood chips): 99%+ removal; cheap capex; spent media must be replaced every 3-12 months and disposed of as hazardous waste.
• Activated carbon: high efficiency, regenerable; suitable as polishing stage downstream of bulk removal.
• Biological scrubbers (Thiobacillus): 95-99% removal; no chemicals; lowest opex; sensitive to operating conditions.
• Chemical wet scrubbing (NaOH, Fe-chelate): high efficiency; consumable cost and disposal burden.
• In-situ air dosing into digester headspace: 60-90% removal; cheapest; risks oxygen ingress.

IS 16087:2016 limits H2S in CBG to 16 mg/Nm3 (about 11 ppm). The trade-off in H2S removal strategy is capex vs opex vs operability. A two-stage system — biological scrubber for bulk removal followed by activated carbon polishing — is now standard in Indian SATAT plants because it combines low operating cost with reliable compliance.