Which Form of Iron Best Binds Hydrogen Sulphide?

Hydrogen sulphide is a recurring problem at every biogas plant. It corrodes gas engines, heat exchangers and pipework, and inside the digester it holds back the methanogens as well. Many plants fight it on two fronts at once: they inject air into the digester and add an iron compound to the feedstock. How well the iron works depends on the form in which it goes in. Iron chloride, iron hydroxide, iron oxide and metallic iron react with sulphide very differently, and each carries its own risks.

Why bind sulphur in the digester?

Hydrogen sulphide comes from feedstocks rich in protein and sulphur compounds. Raw biogas can contain up to 18,000 mg/m³, or about 1.32% by volume (Okoro & Sun, 2019). In the engine it burns to sulphur dioxide, which forms sulphuric acid with water, corroding the engine and degrading the lubricating oil quickly; hydrogen sulphide itself attacks pipework, compressors and gas holders (Bailon Allegue & Hinge, 2014).

Manufacturers therefore set tight limits. Depending on the type, gas engines need less than 500 ppm, and some less than 50 ppm (Bailon Allegue & Hinge, 2014); a more recent review puts the typical manufacturer limit for stationary power generation at 100 ppm (Vu et al., 2022).