Removal of Hydrogen Sulfide from Biogas Using Cow-manure Compost

The two-part objective of this study was to determine currently available H2S removal technologies suitable for use with farm biogas systems, and to test the feasibility of utilizing on-farm cow-manure compost as an H2S adsorption medium. Integrated farm energy systems utilize anaerobic digesters to provide a waste treatment solution, improved nutrient recovery, and energy generation potential in the form of biogas, which consists mostly of methane and carbon dioxide, with smaller amounts of water vapor, and trace amounts of H2S and other impurities. Hydrogen sulfide usually must be removed before the gas can be used for generation of electricity or heat. Biogas has remained a virtually untapped resource in the United States due to many factors, including relatively high gas processing costs. There are many chemical, physical, and biological methods currently available for removal of H2S from an energy gas stream. Dry based chemical processes have traditionally been used for biogas applications and remain competitive based on capital and media costs. Iron Sponge, Media-G2, and potassium-hydroxideimpregnated activated-carbon systems are the most attractive, with estimated capital costs of $10,000-$50,000 and media costs of $0.35-$3.00/kg H2S removed. These processes are simple and effective, but also incur relatively high labor costs for materials handling and disposal. Other significant drawbacks include a continually produced solid waste stream and growing environmental concern over appropriate disposal methods. Additions of air (2-6%) to the digester headspace, or iron compounds introduced directly in the digester, show promise as partial H2S abatement methods, but have limited and inconsistent operational histories. Liquid based and membrane processes require significantly higher capital, energy and media costs, and do not appear economically competitive for selective H2S removal from biogas at this time. Commercial biological processes for H2S removal are available (Biopuric and Thiopaq) that boast reduced operating, chemical, and energy costs, but require higher capital costs than dry based processes. Initial testing of cow-manure compost indicates that it has potential as an effective and economic matrix for H2S removal. Polyvinyl-chloride (PVC) test columns were constructed and a 2:1 biogas-to-air mixture passed through the columns containing anaerobically digested cow-manure compost. The most significant trials ran for 1057 hours with an empty-bed gas residence time near 100 seconds and inlet H2S concentrations averaging 1500 ppm, as measured by an electrochemical sensor with 40:1 sample dilution. Removal efficiencies over 80% were observed for a majority of the run time. Elimination capacities recorded were between 16 – 118 g H2S/m bed/hr. This is significant considering only minimal moisture, and no temperature or pH controls were implemented. Temperature in the bed varied from 19-43°C and the moisture contents in the spent column ranged from 41-70%, with pH values from 4.6 to 6.9. It is not clear whether the major mechanism for sulfur removal from the gas stream was biological, chemical or physical, but it is known that the sulfur content in the packing increased by over 1400%, verifying sequestration of sulfur in the compost. These initial results indicate that future work is warranted for examining the suitability of cow-manure compost as a biofiltration medium for use with biogas.