Integrating an ORC into a natural gas expansion plant supplied with a co-generation unit

The paper presents a thermoeconomic analysis of a proposed novel system for natural gas expansion, based on the integration of gas expanders, ICE engine and an organic Rankine cycle (ORC). The objective of a natural gas expansion plant is to generate electric power while reducing the natural gas pressure. In contrast to the throttling process in pressure regulators, gas expanders allow one to utilize the physical exergy of the pressurized gas. However, the necessity of gas pre-heating bound to thy hydrate formation problem decrease the performance of existing expansion plants. Pre-heating systems based on cogeneration modules with ICE-engines have better performance that boiler systems, however, as shown in authors' previous studies, they are burdened with a relatively high irreversibility bound to an excessive temperature difference between the engine exhaust gases and the pre-heated natural gas. The proposed system integrates an ORC between the exhaust gases and natural gas. Pre-heating of natural gas is carried out partially directly by the co-generation module, by means of the engine cooling cycle, and partially indirectly, by means of the engine exhaust gases supplying heat for the ORC, while the ORC condenser is coupled with the first stage of gas pre-heating. It has been demonstrated that the integration of ORC increases the system exergy efficiency to 0.526 compared to the reference case of 0.498. The performance ratio (electric output related to local fuel input) of the system reaches a favourable value of 0.77. The performed detailed thermoeconomic analysis points out the occurrences of irreversibilities and depicts the process of cost formation in particular devices.