The Effects of Refueling System Operating Pressure on LNG and CNG Economics

Abstract

Natural gas (NG) liquefaction and compression are energy intensive processes which make up a significant portion of the overall delivered price of liquefied NG (LNG) and compressed NG (CNG). Increases in system efficiency and/or process changes which reduce the required amount of work will improve the overall economics of NG as a vehicle fuel. This paper describes a method of reducing the delivered cost of LNG by liquefying the gas above ambient pressures. Higher pressure LNG is desirable because OEM NG engine manufacturers would like NG delivered to the engine intake manifold at elevated pressures to avoid compromising engine performance. Producing LNG at higher pressures reduces the amount of work required for liquefaction but it is only practical when the LNG is liquefied on-site. Using a thermo-economic approach, it is shown that NG fuel costs can be reduced by as much as 10% when producing LNG at higher pressures. A reduction in the delivered cost is also demonstrated for CNG produced on-site from high pressure LNG.