Anaerobic digesters at dairy farms, wastewater treatment plants and landfills produce valuable biogas. However this gas contains hydrogen sulfide (H2S) which must be removed to protect engines and catalysts. Even trace amounts can damage catalysts used for hydrogen production. The most well known H2S treatment system is an iron sponge. Since the H2S removal capacity of the iron sponge is in the range of 80-90%, a second removal system must be used for applications requiring high level sulfur removal.
Impregnating Granular Activated Carbon (GAC) with alkaline or oxide solids enhances the physical adsorptive characteristics of the carbon with chemical reactions. Sodium hydroxide, sodium carbonate, and potassium hydroxide are common impregnants that can more than double activated carbon’s adsorption capacity of sulfur compounds. Since on-site regeneration of saturated impregnated GAC is currently not available it has to be disposed off-site, making it an expensive solution. The direct destruction of H2S and other sulfur components will be more economical than the use of adsorbents.
Microwave energy disassociates H2S into H2 and sulfur in the GAC bed (H2SàH2+S). The addition of oxygen will enhance the microwave disassociation reaction of H2S by the reaction H2S+O2àH2O+S, which is exothermic. This direct destruction of H2S will be more convenient and economical than using adsorbents.
Under previous California Energy Commission Energy Innovation Small Grants (#07-08 and #10-05), CHA demonstrated that H2S could be completely destroyed in a GAC bed or copper oxide /zinc oxide bed with microwave energy. Microwave desulfurization provides the most economical method for complete removal of H2S from biogas for H2 production. For streams with high sulfur levels, it can be combined with existing sulfur removal technologies to achieve complete removal.
The following figure shows the H2S destruction efficiency as a function of microwave power at different ratios of H2S to oxygen flow. It shows the effects of microwave power level and oxygen addition on H2S destruction efficiency. Since biogas already contains about one percent oxygen, the reaction of H2S with oxygen will be primary reaction for microwave desulfurization of biogas. This technology will be demonstrated as part of a recently awarded CEC grant at Tollenaar Holstein Dairy in Elk Grove, California.