Cold utilization systems of LNG: A review

Natural gas is one of the cleanest forms of fossil fuels and its usage is rising since the importance of environment-friendly conversion systems has been seen. Liquefied natural gas (LNG) and pipeline technologies are two present methods for the natural gas transportation between the exporter and importer. This review presents the LNG cold utilization systems which are discussed on the applications such as separation processes, cold food storage, cryogenic carbon dioxide (CO2) capture, power generation, etc. The minimum power consumption and flow rate are detected as 0.45 kWh/kg and 8 ton/hour, respectively for the air separation processes. Exergetic and energetic efficiencies are increased by nearly 40% of those of the conventional separation processes. In the desalination system, it is inferred that the LNG cold utilization has provided nearly 50% energy saving while 2 kg ice melt water is produced by 1 kg equivalent LNG cold energy in the freeze seawater desalination. For the cold storage systems, payback periods are found less than 5 years by using the LNG cold energy. Energetic and exergetic efficiency are increased significantly by using the LNG cryogenic exergy. Moreover, it is used in the cryogenic CO2 capture systems, and there is no dramatic decrement in the overall efficiency values while the other CO2 methods decrease the overall efficiency. The LNG cold utilized power generation systems are discussed in detail with different alternatives including the gas, steam, combined and Stirling systems. The minimum thermal efficiencies are found nearly 40% and 20% for the gas and steam cycles. The selection of working fluids in the cycles is also discussed. Lastly, the importance of economic and environmental aspects of LNG cold utilization systems is discussed. Basic modeling equations are presented for the designed LNG cold utilization cycle with respect to the thermoeconomic and exergoenvironmental methods.