A Feasibility Study of Employing an Internal Combustion Engine and a Turbo-expander in a CGS

Most of the natural gas reaches its end users through transmission and distribution pipelines. Transmission pipelines operate at high pressures. In a place of consumption or at passing into a lower pressure pipeline the pressure of the gas must be reduced. This pressure reduction takes place in places which called City Gate Stations (CGS). In CGSs the pressure must be reduced from 5-7 MPa to 1.5-2.0 MPa (usually to 1.7 MPa) into high pressure intrastate pipeline, then to approximately 0.3 MPa into medium pressure intrastate pipeline. In this study, based on a comprehensive program, Inlet and outlet properties of natural gas flow and daily flow rate through a typical CGS were measured and recorded for a whole year. Based on this data recording, the amount of electricity which can be produced from natural gas pressure has been calculated when utilizing a turbo-expander. The amount of obtainable work could be increased significantly through the use of a CHP system. The idea is to utilize an additional prime mover as a heat source while simultaneously generating additional work along a turbo expander. The additional prime mover was chosen to be an internal combustion engine driving an additional generator. Some Internal combustion engines have been added to preheat the natural gas as well as generating electricity. Preheating the gas rise the amount of electricity generation as well as preventing hydrate forming in natural gas stream. The amount of electricity generation for both systems have been calculated and compared. The economics of such a system are very complex, and there are many variables that must be considered. Some of the major considerations are total installed cost, the load factor or capacity factor, the value of the electricity, and, where preheat is required, the cost differential between the electricity produced and the fuel used. Any design must make assumptions about these variables and then the final design must be a series of compromises, which will yield the optimum combination. The result shows that a considerable amount of energy can be produced in a CGS. The result also showed that if a gas turbine utilized along to the turbo-expander, the amount of electrical energy can be doubled comparing when only a turbo-expander is installed.Keywords: