Waste Heat Recovery in LNG Liquefaction Plants

High thermal efficiency of LNG liquefaction plants is of importance in order to minimize feed usage and to reduce CO2 emissions. The need for high efficiency becomes important in gas constrained situations where savings in fuel auto consumption of the plant for liquefaction chilling and power generation can be converted into LNG production and also from the standpoint of CO2 reduction. This paper will provide a comprehensive overview of waste heat recovery approaches in LNG Liquefaction facilities as a measure to boost thermal efficiency and reduce fuel auto-consumption. The paper will cover types of heating media, the need and use of heat for process applications, the use of hot oil, steam and water for process applications and direct recovery of waste heat. Cogeneration and combined cycle approaches for LNG liquefaction will also be presented along with thermal designs. Parametric studies and cycle studies relating to waste heat recovery from gas turbines used in LNG liquefaction plants will be provided. The economic viability of waste heat recovery and the extent to which heat integration is deployed will depend on the magnitude of the accrual of operating cost savings, and their ability to counteract the initial capital outlay. Savings can be in the form of reduced fuel gas costs and reduced carbon dioxide taxes. Ultimately the impact of these savings will depend on the owner’s measurement of the value of fuel gas; whether fuel usage is accounted for as lost feed or lost product. The negative impacts include the reduction in nitrogen rejection that occurs with reduced fuel gas usage and the power restrictions imposed on gas turbine drivers due to the increased exhaust system back-pressure caused by the presence of the WHRU. When steam systems are acceptable, a cogeneration type liquefaction facility can be attractive. In addition to steam generation and hot oil heating, newer concepts such as the use of ORCs or supercritical CO2 cycles will also be addressed.