Energy and exergy analysis of reciprocating natural gas expansion engine based on valve configurations

Natural gas pressure should be reduced in city gate stations before consuming. Part of the physical exergy of this high pressure gas is wasted if throttling valves are employed for pressure reduction. Reciprocating natural gas expansion engine (RNGEE) could be utilized to recover most of the physical exergy. In this study, a single acting RNGEE is investigated thermodynamically in order to optimize the ports opening/closing times. For this purpose, cylinder and slide valves and two types of piston valves have been modeled and compared. Based on an exergy analysis, a genetic algorithm has been developed to optimize the valves timing. Moreover, effects of the pressure ratio on the exergetic efficiency and power generation of the RNGEE have been studied numerically. Methane was modeled as a real gas by employing AGA8 equation of state. Results showed that beside importance of exergy efficiency optimization, inlet process period has also critical impacts on engine performance. Moreover, power generation is almost the same while using cylinder or flange valves (∼1986 kW/kg) with exergy efficiencies of 83.6% and 82.7% respectively. In contrast, slide and piston valves are found to have lower power generation (1746 kW/kg and 1753 kW/kg respectively) with the exergy efficiency of ∼72%.