Technical-economic evaluation of a small liquefaction plant for bio-LNG production

Lately, an increased attention has been dedicated to the study and development of suitable processes for small-scale bio liquefied natural gas (bio-LNG) production plants (from 5 to 500 tons of LNG per day) to improve the profitability of small gas fields and biogas plants with anaerobic digestion processes and landfills. In this paper, a technical-economic evaluation of a small bio-LNG production plant (10t/day of raw biogas processed) has been conducted. The basic plant configuration consists on a two-pressure levels Joule-Brayton reverse cycle, with nitrogen as working fluid. Liquefaction of bio-methane occurred after the upgrading with conventional processes. The system has been simulated and optimized in Aspen Hysys, determining the values of the various working parameters which minimized the energy specific consumption. Results showed that a bio-LNG plant requires 0.32 kWh/Nm3 of raw biogas without considering the upgrading process. An economic analysis of the liquefaction system has been conducted. This analysis showed that the efficiency of the cryogenic expander is the most critical component of the plant, both from the economic and technical point of view.