LNG comes from dry natural reservoirs, mainly constituting about 85–90% of methane (CH4) with very small percentages of ethane and propane. Its critical temperature is about 73°C. It is found in coal reserves in tight sands and is trapped in geo-pressurised zones within the earth. LNG is obtained when natural gas (NG) is placed under very high pressure. On the release of high pressure, LNG converts to NG. At atmospheric pressure and above freezing temperature, LPG is in gas form.
Due to the low critical temperature of NG, it cannot be liquefied at ordinary temperature. To liquefy NG, it must be cooled to cryogenic (ultra-low) temperatures. It must be stored in well-insulated containers so as to hold it in liquid form. The pressure must be maintained at about 50–60 bar. For automobiles, a pressure of 220–250 bar is required. The NG used in automobiles is known as CNG. NG used for domestic and industrial purposes is supplied in pipes and is hence known as piped natural gas (PNG). The calorific value of LNG is 40.7–41.2 MJ/m3.
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