Natural gas liquefaction using the high-pressure potential in the gas transmission system

This article concerns natural gas liquefaction using high-pressure potential, which is available in pressure letdown stations. The article proposes an integration of a pressure letdown station with a natural gas liquefaction line which enables partial exergy recovery and decreasing natural resources consumption to produce LNG. Exergy recovery is carried out by a replacement of the pressure reduction valve by a turboexpander, which can recover exergy from high-pressure natural gas flowing through a pressure reduction stage. The recovered energy may be used to drive a natural gas liquefaction unit, coupled with the reduction station. A case study concerning an existing pressure letdown station includes two chosen model of a turboexpanders with low and high internal efficiency and several natural gas liquefaction units possible to integrate. The varying size of the liquefaction unit corresponds to a different degree of utilization of energy generated in the expander. Turboexpander produces electric power supplying the liquefaction unit, however, it requires the use of additional energy to heat the gas before the reduction stage, which increases the thermoecologic cost of natural gas transferred to distribution network. Energy, exergy and thermoecological cost analysis was carried out for three system design configurations and for six sizes of the liquefaction line with different turboexpander efficiency. The first configuration included a basic configuration with the pressure letdown station and the liquefaction unit, the second configuration also included an integration with a waste heat source (ICE exhaust gases), and the third configuration used multi-stage expansion. Energy efficiency of the integrated expansion-liquefaction system varies from 35.40% to 66.64%, while its exergy efficiency ranges from 15.75% to 46.33% depending on the size of the liquefaction unit and the gas liquefaction method. It was proved that it is possible to reduce thermo-ecological cost of LNG by 8.2%. Reducing raw material consumption needed for LNG production increases natural gas thermo-ecological cost by only 1%. A preliminary economic analysis based on prices of energy carriers was done. It was found that the boundary price was estimated at 0.0246 €/kWh for one of chosen systems.