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EPA – COMBINED HEAT AND POWER TECHNOLOGY EVALUATION 4. OTHER CONSIDERATIONS 4.1 Gas Treatment and Appurtenances For optimum performance, digester-gas-fueled combined heat and power (CHP) systems generally require fuel pretreatment. The most significant impurities common to digester gas are hydrogen sulfide (H2S) and siloxanes. In addition, moisture removal should be included in most digester gas treatment systems. This chapter discusses proven and emerging technologies for treating digester gas for use in a CHP system. 4.1.1 HydrogenSulfide Sulfur, in particulate form or as sulfide or sulfate, is a common wastewater constituent. The three most common sulfur species in digester gas are H2S, COS (carbonyl sulfide), and CS2 (carbon disulfide). Typically the H2S species accounts for more than 98 percent of the sulfur found in digester gas. H2S is a corrosive, combustible, and malodorous compound that forms in wastewater and sewage sludge under anaerobic conditions. A common method of limiting H2S concentrations is the addition of iron salts, typically ferric and ferrous chloride, into wastewater collection systems, at treatment plant headworks or the feed to anaerobic digesters. Because of the high cost of chemicals and constant evolution of H2S within anaerobic digesters, it is usually necessary to treat digester gas and further reduce H2S levels prior to combustion. H2S in raw, wet digester gas can be very corrosive. In particular, H2S reacts with water to form sulfuric acid (H2SO4), which is extremely corrosive to CHP machinery. High H2S concentrations in digester gas can turn engine and compressor oil acidic, resulting in more frequent oil changes and increased maintenance. Unless removed from digester gas fuel, H2S can lead to CHP equipment corrosion, increased maintenance and associated maintenance costs, and a shorter lifespan of the CHP equipment. The presence of H2S in digester gas adversely affects the quality of combustion emissions. The combustion of H2S, as well as COS and CS2, produces sulfur dioxide (SO2), which is the most common form of SOx (oxides of sulfur). Therefore, the removal of various sulfur species from the digester gas causes a reduction in SOx emissions to the same degree as was removed from the gas. This is an important air permitting consideration because air permits often limit SOx. Iron sponge treatment is one common method for removing H2S from digester gas at publicly owned treatment works (POTW). In this process the saturated digester gas flows through process vessels containing wood chips impregnated with hydrated ferric oxide (Fe2O3 · H2O). The H2S reacts with the ferric oxide to form iron sulfide (Fe2S3) according to the following equation: Fe2O3 · H2O + 3H2S Fe2S3 + 4H2O This process requires the digester gas to be saturated with water vapor and is therefore typically the first step in the fuel treatment process, before moisture is removed. Certain iron sponge systems are capable of media regeneration. The addition of oxygen and water to the iron sulfide, in the absence of digester gas, produces hydrated ferric oxide and elemental sulfur, and thus partially regenerates the iron sponge media. Eventually the iron sponge media will become exhausted and regeneration is no longer an option. Once exhausted, the nonhazardous, iron sulfide-laden, wood chips can be safely disposed of at most municipal landfills. 4-1PDF Image | Combined Heat and Power Technologies for Wastewater Facilities
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