Use of DigesterGas in Combined Heat and Power Cycle

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Daniels, Wade; DeMillard, Eric; Johnson, Charles; McCarrel, Jarrod
article description
The Dry Creek Waste Water Treatment Plant (WWTP) is a facility that process es raw sewage from Cheyenne, Wyoming. This facility uses anaerobic digestion as the method for stabilizing the sludge, prior to drying and disposal. Heating of the sludge is necessary for the anaerobic digestion process and is achieved by the use of a hot water bath heat exchanger. A portion of the digester gas produced by the facility, along with purchased natural gas, is currently combusted to heat water in the heat exchanger. The unused portion of the produced digester gas is flared to the environment. The digester gas produced by anaerobic digestion is a valuable resource, as it has a considerable amount of chemical energy stored within it that could be utilized. EnTec’s goal was to demonstrate that the digester gas being produced could provide value t o the WWTP. Electricity is approximately six times more valuable than natural gas per unit energy. Electricity produced by using energy within the digester gas would thus provide the most amount of value to the WWTP and would be utilized directly by the W WTP to offset their electricity consumption. EnTec has determined through thermodynamic and economic modeling that use of a gas turbine and generator is the most feasible means for producing electricity from the digester gas. Waste heat from the gas turbi ne engine will be utilized to preheat the sludge, creating a combined heat and power (CHP) system. With this application, it was determined through thermodynamic modeling that electricity could be produced between 400 - 500 kW from the gas turbine system. Ad ditionally, the amount of energy contained within the exhaust of the system will be able to entirely supplement the sludge heating demands for the WWTP. The future value gained for the WWTP from this project was estimated to be about 2.1 million dollars.