A novel biogas liquefaction process adopting dual nitrogen expansion cycle with carbon dioxide pre-cooling.

With the increasing global demand for renewable energy, biogas produced by the fermentation of organic biomass, mainly composed of CH₄ and CO₂, has garnered significant attention for its efficient conversion and utilization. In the field of biogas liquefaction, the propane pre-cooling and mixed refrigerant cycle (C3/MRC) technology is widely employed due to its high efficiency and mature engineering applications. However, the process suffers from several drawbacks, including high complexity, sensitivity to fluctuations in feed gas
composition, and relatively high operational costs. Considering the variability inherent in biogas fermentation processes, this paper proposes a novel biogas liquefaction process based on CO₂ pre-cooling and a dual-stage nitrogen expansion refrigeration cycle. To evaluate the adaptability of the process, the effects of varying feed gas flow rates and CH₄ concentrations in the feedstock are investigated across different biogas sources. Additionally, an optimization model is developed using specific energy consumption (SEC) as key indicator, aiming to minimize energy consumption per unit of liquefied biogas. The process optimized by the particle swarm optimization (PSO) algorithm achieves the SEC of 0.69 kWh/kg, representing a 4.2% reduction compared to baseline design, and the liquefaction rate (LR) of 98.6%, representing a 6.2% improvement over conventional C3/MRC process. The techno-economic analysis shows that the unit production cost is 0.153 EUR/Nm³ and the estimated payback period is approximately 2.1 years. The results demonstrate that the proposed liquefaction process offers high efficiency, operational safety, and simplicity, making it suitable for liquefying biogas under varying conditions.