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EPA – COMBINED HEAT AND POWER TECHNOLOGY EVALUATION 5. OTHER DIGESTER GAS USES 5.1 Other Non-CHP Uses for Digester Gas Although combined heat and power (CHP) systems are becoming commonplace, flaring and boiler operation continue to be the most common methods for digester gas management/use, particularly for publicly owned treatment works (POTW) with wastewater flows less than about 10 million gallons per day (mgd). All POTWs that operate an anaerobic digestion process are equipped with flares for the safe destruction of unused digester gas. This is true even for POTWs that beneficially use the digester gas in some fashion, as it is necessary to safely dispose of the digester gas in the event that the beneficial use system is out of service or excess digester gas is produced. Flares safely dispose of digester gas by combusting the methane and emitting the products of combustion to the atmosphere in a safe location away from the anaerobic digesters. Aside from the elimination of the digester gas and largely oxidizing methane to its lowest global warming potential as carbon dioxide, there is no additional benefit associated with the operation of a flaring system. In fact, some flares are inefficient combustors that allow a portion of the methane to escape to the atmosphere. Additionally, many flares require a constant supply of pilot fuel, which adds to operating costs and emissions. Although a properly designed CHP system can provide all the heat necessary to sustain the anaerobic digestion process, utilizing CHP does not eliminate the need for a boiler. A backup boiler should be integrated into the CHP system to maintain heat requirements during routine maintenance or unexpected outages of the prime mover. Some non-CHP approaches to utilizing digester gas are presented in the sections below. Each alternative digester use requires some other means to meet the heat demands of the anaerobic digestion process. 5.1.1 DigesterGasPurificationtoPipelineQuality If properly treated and pressurized, digester gas can be marketed and sold to the local natural gas utility. Depending on the amount of digester gas produced and the location of the POTW, it may be possible to inject the pressurized and purified methane (CH4) from the digester gas into a natural gas pipeline. This option requires treating the digester gas to such an extent that the product is essentially pure methane. Purified digester gas, suitable for injection into a natural gas pipeline, is typically at least 95 percent CH4 and less than 3 percent carbon dioxide (CO2). To achieve pipeline quality gas, hydrogen sulfide (H2S) must also be removed from the digester gas. Water and H2S levels should not exceed 7 pounds per million cubic feet and 2.7 ppm, respectively in the purified methane. Prior to pipeline injection, the purified digester gas must be compressed to several hundred pounds per square inch (psi) to match the pipeline pressure. Table 5-1. King County, Washington – Digester Gas Purification To Pipeline Quality • CO2, H2S, and other water soluble contaminants are stripped from digester gas in vertical towers at high pressure by absorption into plant effluent • High pressure, purified methane is sold to the local natural gas utility • Onsite thermal needs are met by heat extracted from plant effluent by heat pumps • 2.4 million ft3/day capacity 5-1PDF Image | Combined Heat and Power Technologies for Wastewater Facilities
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