Organic materials added to an anaerobic digester commonly contain sulphur components. During the anaerobic digestion process, the sulphur is converted into hydrogen sulphide (H2S), which is absorbed by the biogas.
When the biogas produced in an AD plant goes on to be combusted in a gas engine or boiler, the H2S will give off sulphur dioxide (SO2) and sulphuric acid (H2SO4). It is for this reason that the level of H2S in biogas must be reduced to acceptable limits before being combusted. Key biogas engine manufacturers, including Jenbacher, Caterpillar, and MWM request this level is no higher than 250ppm, as higher levels of H2S can degrade the state and performance of expensive equipment.