Dear Shervin,

Some amount of hydrogen sulphide is formed in production of bio gas.Elevated hydrogen sulphide (H2S) concentrations can cause problems for the gas engines. Combustion leads to formation of sulfur dioxide leading to increasing corrosion and a significant drop in oil pH and subsequent more frequent oil changes.

Depending on the substrate composition more or less hydrogen sulfide (H2S) is produced. Proteins that carry sulfur compounds are microbially broken down and release sulfur. In an environment lacking oxygen hydrogen sulfide (H2S) is formed.

1.Biological Desulfurization - H2S Removal

  Biological desulfurisation utilizes the microbial process where microbes remove hydrogen sulfide (H2S) and convert it into sulfur.

 With farm material and substrates low in protein desulfurization is handled by air injection into the gasholder. Microbes take the oxygen and convert hydrogen sulfide (H2S) into sulfur.

2  H2S  + O2 -> S2 + 2 H2O

Sulfur oxidizing bacteria perform that task. Being autothrophic these species oxidize hydrogen sulfide as a source of energy.

Air is normally injected into the biogas digester with a controlled compressor. The total air going into the digester should not exceed 2-5% of the biogas (volume) produced at any one time. Higher levels can lead to incendive and explosive gas mixtures.Warning: Methane can form a highly incendive and explosive gas mixture in connection with air.

2.     Chemical desulfurization - precipitation with iron salts

Chemical desulfurization applies metal salts like FeCl2 to precipitate sulfur. Iron salts, like iron II chloride (FeCl2), help to keep hydrogen sulfide levels (H2S) low.

 With the addition of iron II chloride (FeCl2) into the digestion process sulfure can ? due to the high chemical affinity between iron and sulfur ? be pecipitated selectively. Iron stays in solution for a long time and reacts with sulfur when it becomes present. Due to this one dosing point is sufficient to secure a complete precipitation in the whole digester.

 Precipitation reaction FeCl2 + H2S    --> FeS + 2 HCl

 Only a small amount of FeCl2 can control and keep H2S-conzentrations at less than 200 ppm in order to minimize corrosion with the biogas engine burning the digester gas.

Regards,

Prem Baboo

Thanks Prem,

But I am specifically looking forward to biological desulfurization operational problems and its start-up.

How much sediments may be produced in its process?

Should we adjust pH?

How much nutrients or treated wastewater should be added for proper biological growth?

Should we change or backwash packing media?

Dear Shervin,,

The attached article may  helpful

Purification of biogas

The two main components of raw biogas are carbon dioxide and methane, but hydrogen sulphide (H2S) and water vapour are also present. This is a corrosive mixture. There are sulphur/sulphide oxidising microorganisms most, of which belong to the family of Thiobacillus. To remove the sulphur and not oxidise the methane, it is essential that (in the presence of these biological agents), precisely the right amount of oxygen is added. (Zafar, 2017). There are physical methods of removing most of the carbon dioxide and water vapour.

Zafar’s emphasis is on removing the carbon dioxide, the sulphur and the water vapour before the biogas is supplied as biomethane to the gas grid. Heesterman (2019) also emphasises that if the gas is then combusted with a marginal excess of oxygen, any small remaining amount of sulphur will be in the form of sulphur dioxide (SO2), which is very water soluble and is removed with the condensing water that arises from the combustion of methane, by cooling and compression, in order to obtain pure carbon dioxide as feedstock for the manufacture of synthetic carbohydrate fuels

References

Zafar, Salman (2017): “Biological Desulphurization of Biogas” Cleantechloops (Blogginghub) https://www.cleantechloops.com/biological-desulphurization-of-biogas/

Heesterman, A.R.G.: “Renewable energy supply and carbon capture: capturing all the carbon dioxide at zero cost” Clean Technologies and Environmental Policy August 2019, Volume 21, Issue 6, pp 1177–1191