Iron sponge for H2S removal
Iron sponge is a low cost material used to absorb H2S in the biogas. Following reaction happens in the reactor, in which sulfur is retained as solid ferric sulfide.
Fe2O3 • H2O + 3 H2S = Fe2S3 + 4 H2O
The used iron sponge can be regenerated when exposed to air:
Fe2S3 + 3/2 O2 + 3 H2O = Fe2O3 • H2O + 2 H2O + 3 S
The overall reaction is (ferric oxide plays a catalyst role in this reaction):
3 H2S + 3/2 O2 = 3 H2O + 3 S
The reactor can be designed as batch mode with two or more reactors. When the iron sponge in one reactor is being replaced, other reactors can continue operation.
The used iron sponge can also be replaced in a semi-continuous mode with the aid of loading and unloading hoppers. The unloading hopper is at the bottom, where dirtiest biogas is fed into the reactor, and the loading hopper is at the top, where fresh material is added into to ensure the best H2S removal.
The regeneration process can happen in-situ or off-site. For some landfill gas, the oxygen contained could be enough for regeneration. As the regeneration reaction is exothermic, the temperature needs to be carefully controlled. Localised over heating could be a fire hazard.
The iron sponge will be less effective every time after regeneration, mainly because of the element sulfur taking up most surface area. Once sulfur weights over 25% of the iron sponge, it needs to be replaced with fresh material. Do follow the instruction on the disposal of used iron sponge, which could be a fire hazard.