Hydrogen sulphide removal from biogas - Part 1
Hydrogen sulphide (H2S) is always present in biogas, although concentrations vary with the feedstock. The concentration of hydrogen sulphide in the gas is a function of the digester feed substrate and inorganic sulphate content. Wastes which are high in proteins containing sulphur-based amino acids (methionine and cysteine) can significantly influence biogas hydrogen sulphide levels.
Hydrogen sulphide removal techniques
The inorganic sulphate present in the feedstock of the digestion process will also be reduced by sulphate reducing bacteria in the digester and end up contributing to the sulphide level in the biogas. The hydrogen sulphide contained in biogas causes odours, corrosiveness, and sulphur emissions when the gas is burned. If the gas is to be used in internal combustion engines, turbines or fuel cells, the removal of hydrogen sulphide from the biogas may be required to protect the equipment.
Several treatment methods like
- iron sponge,
- iron oxide pellets,
- activated carbon,
- water scrubbing,
- NaOH scrubbing,
- biological removal on a filter bed,
are used for hydrogen sulphide removal from biogas. When comparing different technologies, one should not only take into consideration the cost of the adsorbent or chemicals used, but also parameters like adsorption capacity; labour requirements to do exchanges; vacuum trucks used to empty filters, disposal and transport cost of waste, etc. In certain cases, there can be a risk of spontaneous ignition of the spent or used adsorbent.
Hydrogen sulphide removal with the iron sponge process
Iron sponge for example consists of wood shavings or wood chips impregnated with hydrated iron oxide. When iron sponge is used for biogas purification and exposed to H2S and mercaptans, they will produce iron sulphides and iron mercaptides. The removal of H2S with iron sponge will result in iron sulphides formation:
2Fe2O3 + H2O + 6H2S --> 2Fe2S3 + 8H2O
When the spent iron sponge (iron sulphide) is then removed from the purification system and is exposed to the air, it may be re-oxidized to iron oxide and elemental sulphur, by the following exothermic reaction:
2Fe2S3 + 3O2 --> 2Fe2O3 + 6S
This last reaction is exothermic: with heat built up spontaneous and combustion can occur.
For more info on hydrogen sulphide removal and biogas treatment solutions in general, don’t hesitate contact us!