Energy & Fuels, Vol.26, No.2, 1259-1267, 2012
Exergy Evaluation of the Arctic Snohvit Liquefied Natural Gas Processing Plant in Northern Norway-Significance of Ambient Temperature
The first Arctic liquefied natural gas (LNG) plant was evaluated using the exergy method. The well stream is separated into flows of tradable products, substances for deposit or reinjection, pollutants for capture, and discharged products. Some natural gas is used for fuel in the on-site combined heat and power (CHP) plant. The exergy of the delivered products was 95.1% of the feed stream exergy, as most of the hydrocarbons were flowing through the plant without chemical change. The consumed exergy was the thermomechanical exergy of the feed stream, mainly as a result of its pressure, and the chemical exergy of the CHP fuel. The exergy efficiency taken as the ratio of the desired exergy change, owing to separation, cooling, and compression to the consumed exergy was 23.2%. Here, separation accounted for 1.9%, while compressed CO2 accounted for 0.7%. The separation exergy was expressed as the total change of the mixing term of the chemical exergy across the processing plant. The exergy losses were 37%, 52%, and 11% in the processing plant, gas turbines, and heat recovery unit, respectively. To assess the effects of low ambient temperature, the actual plant at 4 degrees C was compared to a "twin" with the same material and energy flows and then to another with the same overall exergy efficiency. The former approach required an exergy efficiency of 25.6% at 20 degrees C and 28.1% at 36 degrees C, while the latter resulted in 10.9% and 19.9% less fuel consumption at 4 degrees C compared to 20 and 36 degrees C, respectively. These values represent the benefits of the cold climate compared to a similar plant in a tempered or a tropical climate.