Biogas basically consists of methane and carbon dioxide. In addition, it also contains unwanted contaminants, especially hydrogen sulfide (H2S). Hydrogen sulfide mainly occurs because of the degradation of sulfur-containing proteins (e.g. methionine and cysteine) or the desulfurization of sulfates. The fermentation of manure and other organic wastes shows typical H2S concentrations in the range of 2,000 to 6,000 ppm) in the biogas.
H2S impair the utilization of the biogas in CHP stations as it leads to formation of sulfur oxides which corrode the metal parts of the plant and drastically reduce the endurance of the engine oil in biogas motors.
In order to remove hydrogen sulfide from the biogas, the biological desulfurization is used beside various physico-chemical methods. While supplying oxygen, microorganisms oxidize H2S into elementary sulfur and sulfate. This conversion may either occur directly in the gas chamber above the fermentation substrate or in a down-stream biofilter after the digester.
Most of the sulfide-oxidising microorganisms belong to the family of Thiobacillus. For the microbiological oxidation of sulfide it is essential to add stoichiometric amounts of oxygen to the biogas. Depending on the concentration of hydrogen sulfide this corresponds to 2 to 6 % air in biogas. The bacteria are capable of oxidizing H2S at low pH levels.
The simplest method of desulphurization is the addition of oxygen or air directly into the digester or in a storage tank serving at the same time as gas holder. Thiobacilli are ubiquitous and thus systems do not require inoculation. They grow on the surface of the digestate, which offers the necessary micro-aerophilic surface and at the same time the necessary nutrients. If the desulfurization of the biogas is carried out directly in the digester, the microorganisms need to be provided with sufficient surface for the immobilization.
Beside H2S the bacteria need carbon, different minerals (N, P, K) as well as trace elements (Fe, Co, Ni) and oxygen. Depending on the temperature, the reaction time, the amount and place of the air added the hydrogen sulfide concentration can be reduced by 95 % to less than 50 ppm. Yellow clusters of sulfur are formed during the process. Measures of safety have to be taken to avoid overdosing of air in case of pump failures. Biogas in air is explosive in the range of 6 to 12 %, depending on the methane content).
Biological H2S removal is attractive because of low energy and chemical usage requirements, easy and automated operation, long life expectancy of system elements, potential for elemental sulfur recovery, and no solid waste stream requiring discharge or disposal. The key to obtaining an efficient reaction is to provide an ideal habitat for the growth of sulfide-oxidizing bacteria, to the exclusion of competing microbes, which normally predominate in aerobic treatment processes. Efficient removal of H2S requires media with enough surface area to maintain a large population of sulfide-oxidizing microbes.
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