Thiopaq process for H2S removal
H2S is an acidic gas, therefore it can be effectively scrubbed with caustic solution, but the efficiency will be low because CO2 in the biogas will consume caustic as well. The Thiopaq process introduces a following biological regeneration stage, where caustic is regenerated by stripping off the absorbed CO2 and oxidation of sulfide. The product, element S is separated from the caustic solution with a clarifier.
With reference to the sketch below, in the scrubber, the gas containing H2S is brought into contact with the washing water in counter currently. Absorption of H2S under slightly alkaline conditions (pH 8-9) enables a chemical reaction with hydroxide ions:
H2S(gas) + OH- == HS-(liquid) + H2O
In the bioreactor the sulphide is oxidised into elemental sulphur by autotrophic colourless sulphur bacteria:
HS-(liquid) + 1/2 O2 == S (solid) +OH-
The hydroxide used in the scrubber is regenerated in the bioreactor. Since the washing water entering the scrubber at the top is sulfide-free, a high concentration difference between the liquid and gas phase makes it possible to obtain a very high H2S removal efficiency, exceeding 99.5%. Both the small bleed stream (consisting of sodium salts) and the produced Sulphur is free of sulfide, so discharge is no problem.