Biogas and LFG always contains hydrogen sulphide (H2S). This is usually in the range of 1,000 to 6,000 ppm but may also reach 2% Vol (20 000 ppm) or more. H2S, Ammonia (NH3) and Silicone compounds often need to be reduced – either for reasons of environmental protection or because of the corrosion hazard to engines and upstream processes.
For relatively small amounts of H2S and Siloxanes, activated carbon (AC) filtering can be employed. This process involves routing the gas through a bed of activated carbon where contaminants and impurities in the gas are exposed to the filter media and captured through the absorption process. Carbon is then replaced once saturated.
The non-chemical biological process is usually preferred and this involves injecting a limited amount of air (10 – 15% in relation to the gas throughput) into the process. Bacteria known as Thiobacillus Spp convert the H2S to elementary sulphur and sulphuric acid.
In order for the bacteria to grow it only needs:
• Oxygen
• A small amount of added nutrients and trace elements (readily available in South Africa)
• A growing surface