Conceptual design and analysis of a boil-off gas re-liquefaction process driven by a multi-stage active magnetic regenerative cryocooler

Abstract

Considering the significant share of natural gas in the world energy sources reducing the emission in distribution chain is very important and managing the boil-off gas has a substantial role in this context. A novel process for boil-off gas re-liquefaction based on an active magnetic regenerative cryocooler integrated with a cold energy storage unit is proposed and analysed. The active magnetic regenerative refrigeration section of the system consists of nine active magnetic regenerative cryocooler units, each comprising five stages and producing a 37 K temperature span using permanent magnets with a magnetic field strength of 1.5 Tesla. Propane is selected as the heat transfer fluid due to its wide availability and cost-effectiveness. The system includes a closed-loop heat sink cooled by a nitrogen-enriched vapour-compression unit operating at a maximum pressure of 12 bar. A computational one-dimensional model is applied to simulate thermodynamic and heat transfer processes in the regenerators. Key performance parameters including specific energy consumption, exergy efficiency, coefficient of performance, and liquefaction capacity are reported, and finally a comprehensive economic analysis is conducted to evaluate the costs through the proposed process. The proposed system achieves a specific energy consumption of 0.7816 kWh/kgLNG, an exergy efficiency of 0.4192, a coefficient of performance of 0.3396. An 11.7% reduction in specific energy consumption, a 44% improvement in coefficient of performance, 29% increase in exergy efficiency in comparison with currently in use systems and a liquefaction capacity of approximately 90% demonstrate the potential of the active magnetic regenerative refrigeration-based re-liquefaction system to provide an efficient and cost-effective alternative to traditional boil-off gas re-liquefaction systems. The proposed re-liquefaction system provides advantage of eliminating the need for cold boxes and multi-stream heat exchangers, thereby reducing process complexity, lowering reliance on in-port maintenance, and significantly improving scalability. Additionally , the economic analysis shows that, compared with the most widely used commercial LNG carrier re-liquefaction system, it enables noticeable downsizing, with purchased costs of the compressors, heat exchangers, and turbo expanders reduced by approximately 12.6%, 36.6%, and 12.6%, respectively.