Comparative evaluation of a natural gas expansion plant integrated with an IC engine and an organic Rankine cycle

Abstract

The aim of the paper is to propose and evaluate an innovative exergy recovery system for natural gas expansion, based on the integration of an internal combustion engine (ICE) and an organic Rankine cycle (ORC), and to compare it with other alternatives. Natural gas expansion plants are a substantial improvement to the conventional gas pressure reduction stations, based on the throttling process, since the available physical exergy of pressurized gas is converted into mechanical energy by means of an expansion machine (turbine or piston expander) instead of being lost in the throttling process. However, due to the hydrate formation problem the gas has to be pre-heated prior to the expansion, which diminishes the system performance. An efficient method for performing this pre-heating is by the proposed system that comprises an ICE and an ORC: Pre-heating of natural gas is carried out partially directly by the co-generation module, via the engine cooling cycle, and partially indirectly, by means of the engine exhaust gases, which supply heat for the ORC, while the ORC condenser is connected with the lowest stage of natural gas pre-heating. Other alternatives are the use of an ICE without ORC, the use of a boiler, and even expansion in a throttling valve.The paper evaluates the performance of the aforementioned four configurations by means of both energy and exergy analysis. Several alternative performance indicators have been defined, calculated and discussed. Sources of irreversibilities have been identified by means of exergy analysis methodology. The fuel-product approach was used to define the exergy efficiency. Except for the reference system with a throttling valve, the analyzed systems achieve favourable values of exergy efficiency (up to 52.6%), and outstanding performance ratios (0.69–0.77), relating the energy fluxes of generated work to the fuel locally combusted in the system.