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Chapter 1 Evaluation of Combined Heat and Power Technologies for Wastewater Treatment Facilities Anaerobic digestion can involve large process vessels (with volumes often exceeding one million gallons), in which bacteria, in the absence of oxygen, break down volatile solids, releasing biogas. The material that remains after the digestion process is referred to as biosolids. Most anaerobic digesters are operated at mesophilic temperatures between 95 and 100 degrees F (35 to 38 degrees C). Thermophilic digesters operate at temperatures between 124 and 138 degrees F (51 to 59 degrees C). The digestion of the volatile solids causes the formation of methane-rich biogas, more commonly known as digester gas. The composition of digester gas is primarily methane (CH4) and carbon dioxide (CO2). Given its high methane content, digester gas is highly flammable and must be carefully managed. In the past the most common method of managing ever-evolving digester gas was to combust it in a flare. 1.3 CHP Advantages The purpose of this section is to provide a more detailed discussion of the primary advantages of operating a digester gas-fueled CHP system at a POTW. Keep in mind that an anaerobic digestion process at a typical POTW can support a synergistic relationship with a CHP system for the following reasons: Anaerobic digestion processes provide a “free” source of fuel for CHP systems CHP systems can supply all of the heat required by the anaerobic digestion process CHP systems can offset a significant portion of a POTW’s electric power demand POTWs are usually designed and constructed to operate for several decades and can therefore provide a long-term fuel supply to the CHP system. 1.3.1 Heat Recovery An adequate and reliable supply of sufficient process heat is critical to the anaerobic digestion process. While available digester gas and the POTW’s electrical needs are important considerations when selecting a CHP system, useable heat must also be given careful consideration. Given the continuous heat requirements of the anaerobic digestion process, it is common for POTWs to select and operate CHP systems with the production of useable heat as the primary objective, with onsite electric power generation being an added benefit. All CHP systems presented in this document incorporate anaerobic digester heating. Depending on the local weather conditions at the POTW and the specific anaerobic digestion treatment process employed, as well as several other factors, a CHP system might provide all of the heat required to operate the anaerobic digestion process. In the event that the CHP system does not provide adequate heat, the shortfall can be met by backup natural-gas-fired boilers. Should a CHP system provide heat in excess of the anaerobic digestion requirements, the additional heat may be sufficient to heat some onsite facilities or treatment processes. Water heating and space heating are other options for using excess CHP heat. Depending on a POTW’s specific needs, heat can also be used to produce chilled water and thus maximize the overall CHP system heat use. Currently, absorption chillers are the most common and well proven method of converting heat energy into mechanically refrigerated chilled water. Excess CHP system heat, if hot enough, can be used, via an absorption chiller, to produce 45 degree Fahrenheit chilled water for building space cooling during warm months. An additional benefit of cooling with absorption chillers is the use of water as the refrigerant and the elimination of conventional refrigerants (i.e., chlorofluorocarbons, or CFCs) that are expensive and can be harmful to the earth’s ozone layer and contribute to global warming. 1-4PDF Image | Combined Heat and Power Technologies for Wastewater Facilities
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