Greenhouse gas emission by liquefaction and transportation of liquefied natural gas

Choosing typical natural gas fields in overseas, which are exporting Liquefied Natural Gas (here after, LNC) to Japan, the authors conducted detailed investigations on the emission of carbon dioxide and methane by extraction, liquefaction and transportation stages for the production of LNG. From the natural gas fields, approximately 16% of the total LNG imported by Japan is produced. According to the obtained results, 6.7g of carbon dioxide is released from the extraction, liquefaction and transportation stages for the natural gas production of 1 MJ. Main part of the carbon dioxide emission occurs at the liquefaction process. For the liquefaction process, more than 10% of the extracted natural gas is used for cooling energy source. The improvement of the efficiency of the liquefaction process greatly contributes to reduce the amount of carbon dioxide release by LNG production. Because of the recent improvement of LNG tankers, the boil-off rate is becoming less than 0.15% per day and it brings the emission of natural gas during the transportation almost negligible. The result shows that the possibility of long distance transportation without large environmental impacts as well as the loss of loaded LNG. Though LNG requires much energy for its liquefaction process, assuming a model gas field location, it can be estimated that carbon dioxide release by LNG is less than that by domestic coal in Japan if combustion process of each fuel is considered.