Black Hole-Inspired Optimal Design of Biomethane Liquefaction Process for Small-Scale Applications

Abstract

Biomethane is regarded as a promising renewable energy source, with great potential to satisfy the growth of energy demands and to reduce greenhouse gas emissions. Liquefaction is a suitable approach for long distances and overseas transportation of biomethane; however, it is energy-intensive due to its cryogenic working condition. The major challenge is to design a high-energy efficiency liquefaction process with simple operation and configuration. A single mixed refrigerant biomethane liquefaction process adopting the cryogenic liquid turbine for small-scale production has been proposed in this study to address this issue. The optimal design corresponding to minimal energy consumption was obtained through the black-hole-based optimization algorithm. The effect of the minimum internal temperature approach (MITA) in the main cryogenic heat exchanger on the biomethane liquefaction process performance was investigated. The study results indicated that the specific energy consumption of modified case 2 with MITA of 2°C was 0.3228 kWh/kg with 21.01% reduction compared to the published base case. When the MITA decreased to 1°C, the specific power of modified case 1 reduced to 0.3162 kWh/kg, which was 24.96% lower than the base case. In terms of exergy analysis, the total exergy destruction of the modified cases 1, 2, and 3 was 31.28%, 22.27%, and 17.51% lower than the base case, respectively. This study’s findings suggested that introducing the cryogenic liquid turbine to the single mixed refrigerant-based biomethane liquefaction process could reduce the specific energy consumption and total exergy destruction significantly. Therefore, this study could provide a viable path for designing and improving the small-scale biomethane liquefaction process.

Editorial: SDEWES science - The path to a sustainable carbon neutral world

“…Any enhancement in liquefaction process either in terms of cycle modification or in refrigerant selection will be useful in energy saving [63,64]. The study [65] presents an innovative biomethane liquefaction process that utilises CO2 as a precooling refrigerant. This CO2 precooling provides dual benefit; one is to overcome load on the main refrigeration cycle, and the other is its utilisation in a close loop that will contribute to manage CO2 in a safe and economical way.…”

Design and analysis of cascade liquefaction processes for coproducing liquid ethane and LNG

“…The common classification method of natural gas liquefaction processes is classifying them into cascade process, mixed refrigerant process, and expansion process. 23,24 Besides, natural gas liquefaction processes are also classified in other ways like the number or type of cycles. The world's first two industrial-scale LNG plants (the Camel project in Arzew, Algeria built in 1964 and the Kenai project in Alaska, USA, built in 1969) both adopted cascade process, 25 which provides cold energy by multiple independent vapor compression refrigeration cycles using single working fluids.…”

“…Considering that the temperature required for natural gas liquefaction is much lower than that for NGL recovery, the existing integrated LNG‐NGL recovery process is basically designed according to the conventional natural gas liquefaction process. The common classification method of natural gas liquefaction processes is classifying them into cascade process, mixed refrigerant process, and expansion process 23,24 . Besides, natural gas liquefaction processes are also classified in other ways like the number or type of cycles.…”

Absorption Chillers to Improve the Performance of Small-Scale Biomethane Liquefaction Plants

“…Mixed refrigerant cycles are commonly adopted in large-scale liquefaction plants [24], due to their large time response in the case of mass flow fluctuations with multiple potential start-ups during the year [25,26] and to the need for a pre-cooling section to achieve high efficiency, which is not compatible with small-scale systems. However, in the literature, some authors propose single refrigerant loop mixed refrigerant cycles to liquefy biomethane [27] and few manufacturers have been proposing this type of cycle (without precooling section) for larger digestion plants [28]. Obviously, mixed refrigerant cycles are more efficient than the Joule-Brayton cycle, due to the better match of the heat transfer curves [29].…”