Tobias Häggblom, Frans Launonen, and Lauri Bastman, Vahterus, Finland, explore the opportunities LNG regasification presents to improve energy efficiency through recovering the energy available in cold LNG.
L
NG is not a substance that occurs naturally on Earth. When natural gas is liquefied at temperatures of approximately -162˚C, its mixture of simple hydrocarbons of primarily methane mixed with small quantities of ethane, propane, and butane have a boiling point well below any temperature found naturally on the Earth’s surface. LNG represents a storage of energy, both in the sense that it is a hydrocarbon fuel that can be burned, but also in that it is an extremely cold liquid that has been artificially created at the expense of energy. During regasification, this energy is released. Sadly, it is typically entirely wasted: primary heat sources such as ambient air or seawater are often used, and after regasification are simply dumped back into the environment at a cooler temperature. However, this energy can be tapped for beneficial purposes, to improve the total LNG cycle energy efficiency and reach energy savings at locations physically removed from the LNG liquefaction itself. On a large scale, it is a refrigeration process with a virtual pipeline, with compressors located at the liquefaction plant and the consumer located at the regasification location. This represents an additional financial benefit to the LNG purchaser: reducing operating costs for any processes requiring media cooling. The production of LNG from natural gas requires substantial amounts of energy in a compressor-based
refrigeration cycle. For example, it has been calculated that for pure methane and a process pressure of 55 bar, the compression work alone consumes approximately 800 kJ per kg of liquefied methane product. In real processes, the energy expenditure is even higher, and in assorted studies energy consumption in the range of 1000 - 1500 kJ per kg of LNG appears usual. Compared to the lower heating value of LNG, the energy required by liquefaction represents approximately 3% of the energy contained in the LNG itself. With cold recovery, the overall efficiency of this process can be substantially increased, since around half of the energy can be reused. In a typical case, with logistics and regasification included, up to 25% of the energy content of LNG is consumed to deliver the gas to the end consumer. Much of this energy is wasted as ambient heat during the liquefaction process and transportation, but there is significant potential for energy recovery from the ‘cold energy’ stored in the LNG.
Practical applications of cold recovery
One of the most inexpensive and least complex ways of utilising the cold energy is to use it in boil-off gas (BOG) handling. When LNG is stored, the heat from the surroundings will make part of the LNG vaporise, since there is no such thing as a perfectly insulated tank. The sub-cooled LNG in the tank
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