Exploration of Recovering Waste Heat in a Cascade Liquefied Natural Gas (LNG) Plant

Abstract

Abstract Natural gas (NG) has lower and cleaner emissions compared to other fossil fuels such coal and petroleum, thus NG gas has been widely used in recent years to replace coal, diesel, or other petroleum derived fuels to generate power. However, many nations in the world have limited or minimal NG resources, importing liquefied Natural Gas (LNG) has become the only method for countries where transporting natural gas through land-based piping is not sufficient or not available, for example, like Japan, South Korea, Taiwan, Pakistan, to name a few. LNG can be obtained when natural gas is refrigerated to the temperature around −260°F (−162°C). At this temperature, the original volume of the natural gas is reduced by 600 times. To reach this temperature, a multistage refrigeration system with intercooling and regeneration is required. The process of producing LNG is an energy intensive process, which demands a large amount of compression from multiple compressors. Each of these compressors is directly driven by a gas turbine. The gas turbine exhaust contains a large amount of useful energy, but often it is released into the atmosphere and ultimately wasted. The objective of this study is to identify methods to utilize this hot wasted exhaust and recover energy from it. This paper first provides a generic review of various existing commercial LNG production processes. Then a cascade LNG process is simulated to utilize up to four succeeding (cascading) vapor compression refrigeration loops to achieve the cryogenic temperature around −260°F (−162°C). This exhaust has been combined and the energy has been recovered through a Heat Recovery Steam Generator (HRSG). This recovered heat through HRSG has been utilized to run a Rankine cycle power plant to produce electricity. The results show that the wasted heat recovery system combined with a Ranking cycle can recuperate up to 47% of the wasted energy.