Abstract:

Biogas is an important renewable energy source, produced from organic matter, which has no net carbon emissions from a life cycle perspective. The development of biogas is an important way to alleviate China's natural gas supply shortage. However, the CO₂ content in biogas is generally higher than 20%, which poses negatively impacts on the calorific value, storage and transportation of biogas. Biogas liquefaction after CO₂ removal to produce LNG can significantly improve the calorific value of biogas and facilitate its storage and transportation. This study proposes an integrated process of cryogenic CO₂ removal under supercritical pressure and liquefaction for biogas to address the challenges in existing cryogenic CO₂ removal technology: the risk of freeze blockage in distillation and the high energy consumption associated with low-pressure de-sublimation. The de-sublimation of biogas under critical pressure not only facilitates the effective separation of CO₂ and CH₄, but also reduces the energy consumption of the subsequent methane liquefaction process. The proposed process is modeled in HYSYS and optimized by genetic algorithm coded in MATLAB. The results show that when biogas is cooled to -131℃ at 8.5 MPa, CO₂ can be removed to 0.5% by supercritical de-sublimation. As the CO₂ content increases, the specific power consumption and exergy efficiency of the system increases gradually. When the CO₂ content is between 10% and 30%, the specific power consumption of the system is 0.5295—0.6149 kWh/kg LNG, with the exergy efficiency reaching 57.5%—62.1%. Compared with CO₂ removal processes based on dual-pressure distillation, chemical absorption, low-pressure de-sublimation, the specific power consumption is reduced by over 60% with only 11.9% reduction in LNG density. The considerable reduction in power consumption implies that this study presents a novel approach for high-efficient LNG production from biogas, which holds significant implications for promoting economically viable biogas-to-LNG conversion.

Key words: CO₂ removal, carbon dioxide, methane, liquefaction, optimization

摘要：

大力发展沼气是缓解我国天然气供应紧张的重要途径，但是沼气中CO₂含量高达20%以上，严重影响热值和储运。沼气脱碳后制LNG可显著提高热值和储运便捷性。本研究提出一种沼气超临界压力低温脱碳-液化耦合流程，以解决现有沼气低温脱碳技术中精馏易冻堵、低压凝华能耗高的难题。采用HYSYS软件和遗传算法对所提出的流程进行了建模和优化，结果表明，沼气在8.5 MPa下冷却到-131℃时可将CO₂脱除至0.5%。当CO₂含量在10%~30%时，系统比功耗为0.5295~0.6149 kWh/kg LNG，系统㶲效率为57.5%~62.1%。与基于双塔精馏、化学吸收、低压凝华等的工艺相比，在LNG密度仅降低11.9%时，可实现比功耗降低60%以上。

关键词: 脱碳, 二氧化碳, 甲烷, 液化, 优化