Worldwide energy crisis makes it necessary to optimize consumption. One fundamental way to reduce energy consumption is recovery of waste energy. This can be applied in oil, gas and petrochemical industries in which large amounts of energy waste occur. One of this can be referred to gas traortation in which there is a pressure reduction at natural gas pressure reducing stations. Natural gas is traorted for longer distances through transit pipeline at high pressures. In a consumption place or at passing into a lower pressure pipeline, the pressure of the gas must be reduced. Supplying a proper natural gas pressure in combustion system of a power plant or steam boilers, the natural gas pressure should be reduced in multi stages. This is performed through pressure reduction stations and through throttle valves, where the isenthalpic expansion takes place without producing any energy. This reduction in pressure leads to energy loss and chilling of gases. A non-negligible amount of pressure energy is wasted through an irreversible process and throttling of natural gas lowers its potential energy. Most gases cool during the expansion, due to Joule-Thompson effect. Thus the reduction in the pressure can be used in power generation or chilling systems. In this study, based on usage of an expansion turbine for achieving pressure reduction, enhancement of energy recovery was analyzed. The replacement of the gas-throttling process by the process of its expansion in turbo expanders makes it possible to convert the pressure of the natural gas into the mechanical energy, which can be transmitted to a loading device, for example, an electric generator. Expansion turbines are usually coupled with a generator in one power pack. From thermodynamics relations involving enthalpy, entropy and also by using the first law of thermodynamic, supposing the expansion turbine undergoes an isentropic process, the work can be achieved. In this thesis, based on a performed modeling, a simulation program was carried out by using MATLAB software, in order to predict the behavior of natural gas (as a gas mixture) in high pressure, and calculate electrical power production by using an expansion turbine and the required energy for preheating of the gas. This is for prevention of having a two-phase flow system. The amount of generated power by the expansion turbine system was presented by simulation for various input pressure and temperature options. Furthermore, two technical study of recycling lost exergy of natural gas in Neka and Shazand pressure reduction stations was carried out. Then the feasibility study and exergy analysis of the proposed unit cycle was presented. The model was validated and its error was estimated to be 1% by comparing the simulation results of installed expansion turbines in Neka and the real specifications of the turbines. The simulation results of pressure reduction station in Shazand, have shown the possibility of installing two 9 MW expansion turbines. Accordingly, the total required heat for the inlet gas will be supplied by the lost heat of the main cycle, boilers blowdown. Exergy analysis has shown that the exergy efficiency of the proposed unit is 75%. Key Words: Pressure reduction station, expansion turbine, electricity generation, exergy analysis, modeling.
