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

This article concerns natural gas liquefaction using high-pressure potential, which is available in pressure reduction stations. The article proposes an integration of a pressure reduction station with a natural gas liquefaction line which enables partial exergy recovery. 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 reduction stage. This recovered energy may be used to drive a natural gas liquefaction unit, coupled with the reduction station. A case study concenring an existing pressure reduction station includes one chosen model of a turboexpander and several natural gas liquefaction units possible to integrate. The varrying 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.Energy and exergy analysis was carried out for three system design variants and for six sizes of the liquefaction line. The first variant included a basic configuration with the pressure reduction station and the liquefaction unit, the second variant also included integration with a waste heat source (ICE exhaust gases), and the third variant used multi-stage expansion. Energy efficiency of integrated expansion-liquefaction system varies from 35.40% to 66.46%, while exergy efficiency ranges from 15.75% to 45.56% depending on the size of the liquefaction unit and the gas liquefaction method. A preliminary economic analysis based on prices of energy carriers was done. It was found that the boundary price was estimated at 0.105PLN/kWh for one of chosen systems.