Techno-economic analysis of expander-based configurations for natural gas liquefaction

The use of liquefied natural gas (LNG) as a marine fuel is rapidly growing because of the possible economic advantages over conventional fuels and stricter environmental regulations. Production of LNG is energy-intensive because of the required temperature level of around -160°C. Three main types of refrigeration cycles have been developed. The present work focuses on the comparison of six expander-based configurations, which in spite of the higher power consumption are more compact, flexible and easier to operate. They are optimised from a thermodynamic perspective: the exergetic efficiency ranges between 15.5 % and 30 % for a specific power consumption from 2570 kJ/kg down to 1340 kJ/kg.
Multi-objective optimisations are performed to simultaneously minimise the net power consumption and the heat transfer conductance as an indicator of the required heat transfer area. The latter ranges between 50 kW/K and 275 kW/K.
A trade-off between the power consumption and heat transfer area is found, which justifies a further economic analysis. A simplified economic analysis is set based on a discounted cash flow model. The unitary profit ranges between 0.3 and 0.85 DKK per kg of produced LNG. The most profitable expander-based configuration is the dual-refrigerant cycle with nitrogen in the bottoming cycle. Finally, the influence of the cost correlations on the economic outcome is assessed: the compressors represent the major costs, which leads to the coincidence of the thermodynamic and economic optima.