Hydrogen Sulfide

Hydrogen sulFide (H2S) is particularly harmful when biogas is used in internal combustion engines. Its chemical reactions and those of its combustion product - sulphur dioxide - lead to corrosion and wear on engines.

The acid which is formed corrodes engine parts in the combustion chamber, exhaust system and in various bearings. This is enhanced by frequent starts, short running times and the relatively low temperatures when starting up and after cutting off the engine. The water cooling system also provides the means (water needed to form sulphuric acid) for corrosion. Running engines with gas containing H2S can reduce the service time to the first general overhaul by about 10 - 15%. The sulphur content of biogas used in gas engines shortens the time between oil changes and overhauls. SO2 from combustion and water vapour both dissolve in the lubricating oil. The oil becomes acidic, and its properties change, losing its ability to lubricate and sometimes corroding metal components. Under continuous operating conditions, the interval between oil changes is reduced to 200 - 250 hours.

Methods for removing H2S from biogas

Lime scrubber:  One of the methods to de-sulphurise the incoming biogas is the use of Quick or Slaked Lime. The modus operandi for this is fairly simple. The incoming biogas is passed through a scrubber tower which is continuously filled with a lime water solution.  By monitoring the pH level of the lime water solution, one is able to determine when the solution needs to be replaced. The waste solution can be added back to the digester with new slurry input.

VOW Process: The supplier of the bioaugmentation bacteria mentioned in a previous section suggests that their product is able to eliminate the H2S in the incoming biogas through the use of their biocultures:

Bioaugmentation using VOW Biocultures results in:

• Better quality biogas (75 - 80% CH4 and no H2S)
• Higher value bio-solids
• Ability to cope with various waste streams (high solids and/or liquids) with predictable and consistent results
• Fast breakdown of wastes resulting in reduced infrastructure
• No odours associated with the process or outputs