Hydrogen sulfide is the contaminant that quietly destroys RNG plants. Raw biogas carries anywhere from a few hundred to >10,000 ppmv H₂S (protein- and sulfate-rich feedstocks run highest). Pipeline injection typically demands ≤4 ppmv; vehicle-fuel and equipment-protection limits are often tighter still.
The most cost-effective sulfur strategy starts inside the digester with iron-salt dosing, then polishes downstream. In-digester ferric chloride (FeCl₃) or ferric sulfate (Fe₂(SO₄)₃) precipitates dissolved sulfide as insoluble iron sulfide, dropping gas-phase H₂S before it ever leaves the reactor:
2 Fe³⁺ + 3 HS⁻ → Fe₂S₃ ↓ + 3 H⁺ (ferrous route: Fe²⁺ + HS⁻ → FeS ↓ + H⁺)
Design dosing to the sulfur load, not a fixed rate. A practical starting point is an Fe:S molar ratio of ~1.5–3:1, trimmed by measured H₂S and feed sulfate. Feed-point selection and mixing matter: the iron must contact sulfide, so pair dosing with adequate digester agitation.
Technology | Principle | Best fit |
|---|---|---|
Iron sponge / iron oxide media | Sulfide reacts with Fe oxide on media | Small–mid plants; simple; media replacement cost |
Activated carbon (impregnated) | Adsorption / catalytic oxidation | Polishing to low-ppm before upgrading |
Biological scrubber (bio-trickling) | Sulfur-oxidizing bacteria convert H₂S to S⁰/sulfate | Steady, high loads; low chemical cost |
Chemical scrubber (caustic/oxidant) | Absorption + oxidation | High loads; requires chemical handling |
In practice, in-digester ferric dosing plus a downstream polishing stage is the field-standard combination: iron control removes the bulk load cheaply and protects the polishing media, which then guarantees pipeline-grade gas.