A review of biological removal of H2S from gas streams

One of most important air pollutant gases is H2S which could cause corrosion in equipment, exacerbation of air pollution, harmful effect on health etc [1]. This gas can be easily found in sour gases (gases resulted from burning of fossil fuels), natural gas, outcome gas of Amine process, gas flow come coke, sour wastewater and out flow of Claus treatment unit. There are various physio-chemical processes like Amine process, fixed-bed process etc. For removal of H2S from gas flows. Biological processes can also be used for removal of H2S in ambient temperature and pressure, Investment cost, wastewater treatment and energy consumption in these processes reach to a minimum. They have no environmental concerns and they are totally safe [2]. According, the use of biological process for H2S removal from gas flows has attracted attention recently. In these processes, H2S can be totally (environment standard level) removed from gas flows without using chemical and toxic solvents. Also there will be minimum requirement for equipment and they will result in production of elemental sulfur. Biological process is consisted of two steps: First step: Absorption of H2S in absorption column using buffer solution. H2S Absorption: H2S + OH- → HS- + H2O H2S Absorption: H2S + CO32- → HS- + HCO3Second step: biological recovery of absorbing solution using bacteria inside the reactor. The recovered solution is moved back to absorption tower for absorbing [3]. Sulfur production HS- + ½ O2 → 1/8 S8 + OH- Sulfate production HS- + 2O2 + OH- → SO42- + H2OPolysulfide production 2HS- + S8 → 2 HS- 5Nearly 99% of H2S in gas flows can be removed by this process. Noting that some of H2S in the bioreactor turns into sulfate, a part of out flows gas enters another bioreactor to reduce sulfate to sulfide and the out flows of this bioreactor returns to the first one. SO42- + 4 H2 → HS- + OH- + 3H2O