From the article:
Hydrogen sulfide is a highly toxic and corrosive gas which is one of the larger pollutants of the atmosphere. The burning of the gas releases sulfur dioxide which is also a corrosive and toxic gas. Cleaning of this gas is necessary not only for pollution abatement but also for effective use of the gas mixtures like natural gas, biogas etc for end use application like power generation in IC engines where the presence of hydrogen sulfide at higher concentration corrodes the internal parts of the engines in a very short operational duration of run. This paper addresses the removal of hydrogen sulfide from biogas generated by ETP’s and STP’s by a patented ISET process which is a modified liquid redox process developed at CGPL, IISc, Bangalore and which has been tested on commercial scale quite extensively and which has installed power generation capacity of 4.8 MWe.
The hydrogen sulfide concentration is brought down from the designed level (volume percentage of biogas) to less than 50 ppm and the hydrogen sulfide is oxidized to elemental sulfur that is precipitated and is filtered. The process consists of two stage countercurrent scrubbing of hydrogen sulfide from biogas using a chelated polyvalent metal ion which undergoes reduction, followed by removal of precipitated sulfur in the filtration unit and the oxidation of the reduced metal ion using oxygen in the air, for recycling.
As a case study, one of the installation at USWL, which has recorded around 20,000 hours after its installation is considered. The system has scrubbed around 3.6 million m3 of biogas and generated 5.1 million units of electrical energy during the period. At an average concentration of around 4.5 % hydrogen sulfide in the raw gas, the amount of H2S removed is 0.16 million m3 and 200 tons of sulfur precipitated. Use of the scrubber has helped in improving the overall efficiency of conversion to electricity and reduced the emissions. The operating cost works out to be 2.5 US cents per kWh.