Thermodynamic Optimization of a Natural Gas Throttling Pressure Difference Power Generation System

Abstract Natural gas often experiences a pressure change during extraction, gathering, transportation and utilization processes. For example, when natural gas extracted from well, it is throttled to decrease the pressure for transportation, restricted by the pipeline transportation capability; and high-pressure urban gas also needs to be throttled for distributing to customers. The throttling operation not only waste the pressure energy, but also reduces the gas temperature due to the Joule-Thomson effect, which poses a risk of hydrate formation and damages the pipeline. To prevent the temperature falling to the hydrate point, additional heat is required, which causes the increase of carbon dioxide emission. However, if the pressure difference of natural gas throttling is used for power generation, and parts of the electricity is used to compensate for the heating process, a certain amount of power is output through this prosses instead of consume energy. It is an effective way to improve energy efficiency and reduce carbon dioxide emission. In this work, a thermodynamic model of turbine power generation system was established by using the pressure difference between high-pressure end and low-pressure end of the pipelines. In addition, to prevent the formation of hydrates, a thermodynamic model of heating system according to the heat pump principle was established. In the heating system, a specific refrigerant was compressed to increase the temperature in the compressor, which was driven by the electricity generated from the turbine process, and the high temperature refrigerant was used to heat the nature gas. The operating parameters were simulated through Aspen Plus software for the pre-turbine heating and post-turbine heating system. Then the exergy analysis was carried out, and some different schemes was proposed and analyzed to optimize the operating parameters in terms of the maximum exergy efficiency. Finally, the two heating routes were evaluated. It is revealed from the results that the pre-turbine heating of natural gas has higher exergy and generates more power than that of post-turbine heating gas. Although post-turbine heating system reduces the compressor power, the overall power generation efficiency is lower than that of pre-turbine heating. The optimal operating parameters were obtained through exergy efficiency analysis and optimization.