Biomethane liquefaction: A systematic comparative analysis of refrigeration technologies

This work presents a comparative technical and economic analysis of alternative refrigeration technologies for the production of liquefied biomethane (a liquid biofuel also known as bio-LNG). These processes are designed for biogas plants (size: 1–10 tCH4/day), where they can be installed downstream of the biogas upgrading step, to bring biomethane (essentially pure CH4)from the gaseous to the liquid phase, at −152 °C and 2 bar. Five technologies are considered: liquid nitrogen vaporization (benchmark); reverse Rankine cycle with mixed refrigerant; reverse Brayton cycle; Claude cycle; reverse Stirling cycle. The energetic and economic analyses are carried out, under a consistent methodology (reference production capacity of 4.6 tCH4/day), based on Aspen Plus process modelling and simulation for the calculation of the heat and mass balances, linked with ad-hoc Matlab algorithms for equipment sizing and cost estimation, and adopting the PGS-COM numerical optimizer for the selection of the optimal process conditions. The Rankine cycle with mixed-refrigerant turns out to be the best option, with an electric consumption of 3061 kJ/kgCH4, and a levelized biomethane liquefaction cost of 6.3 €/GJLHV. However, for liquid nitrogen supply costs lower than 66 €/tN2, liquid nitrogen vaporization, the least capital-intensive option, becomes the favourite solution. Finally, sensitivity analysis on the key variables is carried out to give a broader perspective to the techno-economic assessment.