System Study and CO2 Emissions Analysis of a Waste Energy Recovery System for Natural Gas Letdown Station Application

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Babasola, Adegboyega
Direct Internal Reforming Fuel Cell , City Gates Waste Energy Recovery System , Natural Gas Pipeline Transmission , Combined Heat and Power System , Molten Carbonate Fuel Cell , Fuel Cell
A CO2 emission analysis and system investigation of a direct fuel cell waste energy recovery and power generation system (DFC-ERG) for pressure letdown stations was undertaken. The hybrid system developed by FuelCell Energy Inc. is an integrated turboexpander and a direct internal reforming molten carbonate fuel cell system in a combined circle. At pressure letdown stations, popularly called city gates, the pressure of natural gas transported on long pipelines is reduced by traditional pressure regulating systems. Energy is lost as a result of pressure reduction. Pressure reduction also results in severe cooling of the gas due to the Joule Thompson effect, thus, requiring preheating of the natural gas using traditional gas fired-burners. The thermal energy generated results in the emission of green house gases. The DFC-ERG system is a novel waste energy recovery and green house gas mitigation system that can replace traditional pressure regulating systems on city gates. A DFC-ERG system has been simulated using UniSim Design process simulation software. A case study using data from Utilities Kingston’s city gate at Glenburnie was analysed. The waste energy recovery system was modelled using the design specifications of the FuelCell Energy Inc’s DFC 300 system and turboexpander design characteristics of Cryostar TG120. The Fuel Cell system sizing was based on the required thermal output, electrical power output, available configuration and cost. The predicted performance of the fuel cell system was simulated at a current density of 140mA/cm2, steam to carbon ratio of 3, fuel utilization of 75% and oxygen utilization of 30%. The power output of the turboexpander was found to strongly depend on the high pressure natural gas flowrate, temperature and pressure. The simulated DFC-ERG system was found to reduce CO2 emissions when the electrical power generated by the DFC-ERG system replaced electrical power generated by a coal fired plant.
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