Liquefied natural gas supply chain using liquid air as a cold carrier: Novel method for energy recovery

Abstract

A novel concept for energy integration across the liquefied natural gas (LNG) supply chain is proposed by using liquid air as a medium for recycling cold energy. This allows the natural gas liquefaction and LNG regasification stages to be integrated despite the long distance between the two sites. LNG carrier ship is used to convey liquid air on its return journey when it would otherwise be empty. On its return to the liquefaction site, the cold energy stored in the liquid air can then be harnessed to liquefy natural gas for transportation. At the regasification site, LNG cold energy is used to regenerate liquid air for the ship’s return journey. In this way, LNG cold energy, which is generally wasted, is used in the proposed LNG supply chain. As a result, the energy requirement for liquefying natural gas can be reduced significantly, and the waste LNG cold energy recovered. The proposed mixed refrigerant with liquid air (MRLA) process design has an energy requirement of 865.82–917.20 kJ/kg-LNG, which is 26.1–30.2% lower than that for a single mixed refrigerant (SMR) process. The proposed process exhibits even lower energy requirements with a much lower capital cost when compared to the propane precooled mixed refrigerant (C3MR) process (974.0–992.0 kJ/kg-LNG), known as one of the most efficient natural gas liquefaction processes. The results of exergy analysis for the proposed processes show that the cold energy supply from liquid air achieves a reduction in the energy requirement. Furthermore, a techno-economic study reveals that an 18.6–22.6% reduction in total costs for LNG liquefaction can be achieved and economically attractive to be implemented in commercial natural gas liquefaction plants, given the minimal infrastructural changes necessary. Overall using this novel approach, the liquefaction and regasification stages are integrated despite the physical separation of the individual processes over long distances. This significantly enhances the LNG supply chain by not only improving its energy performance but also contributes to the utilization of cold energy by using the empty LNG ship on the return journey.