Abstract:

Compared to traditional fixed source flue gas capture technologies, CO₂ direct air capture (DAC) has advantages such as flexible positioning and wide application. However, due to the extremely low concentration of CO₂ in the atmosphere (only around 0.04%), the high energy consumption of DAC has become the primary obstacle to its commercialization. Focusing on the energy consumption issue of adsorption based DAC, theoretical analysis and case studies have been conducted. The ideal minimum work for CO₂ separation of DAC was calculated to be 19.64 kJ·mol^-1 (temperature of 298.15 K, capture ratio of 50%, purity of 95%), which is 3.5 times that of flue gas capture technology under the same conditions. At a regeneration temperature of 393 K, the second law separation efficiency of temperature vacuum swing adsorption cycle (TVSA) is 22.75%. The processes of adsorption, evacuation, regeneration, condensation, compression, etc., are mainly driven by mechanical and thermal energy. The mechanical energy during the evacuation process accounts for only about 3% of the total energy consumption. The heat energy during the condensation process can be recovered through a regenerative cycle. The mechanical energy of the compression process is included in DAC in some studies and is determined by the target pressure. The flow mechanical energy during the adsorption process accounts for over 90% of the mechanical energy consumption, which is 5.43 GJ·t^-1 when using a conventional reactor. Structured reactors can reduce pressure drop to 1/1000, and reducing flow rate can also improve resistance and enhance capture ratio. The thermal energy during the regeneration process accounts for the main part of DAC energy consumption, about 50%—80%. The regeneration temperature, the mass ratio of the reactor to the adsorbent, and the strength of the adsorbent's adsorption of H₂O can all cause a multiple change in the heat consumption. Based on the analysis of the process energy consumption, recommendations for optimizing the energy consumption of adsorption DAC in terms of reactor design, circulation mode and operating parameters, natural environment and energy sources are given.

Key words: CO₂ capture, energy consumption, adsorption, desorption, greenhouse gas

摘要：

CO₂直接空气捕集（DAC）技术相对于传统的固定源烟气捕集技术具有位置灵活、应用广泛等优势，但由于大气中CO₂浓度极低（仅为0.04%左右），DAC技术的高能耗成为阻碍其商业化的首要难题。聚焦吸附法DAC技术的能耗问题，先后进行理论分析和案例引证。DAC技术的CO₂分离理想最小功为19.64 kJ·mol^-1（温度298.15 K，捕集率50%，纯度95%），为同等条件下烟气捕集技术的3.5倍。再生温度393 K时变温真空吸附循环（TVSA）第二定律分离效率为22.75%。吸附、排空、再生、冷凝、压缩等过程主要通过机械能和热能推动。其中排空过程机械能仅占3%左右；冷凝过程热能可以通过回热循环回收；压缩过程机械能由目标压力决定，在部分研究中计入DAC能耗。吸附过程流动机械能受反应器压降主导，床层厚度减小和吸附剂有序堆积均能够改善流动损耗问题。再生过程热能占DAC能耗的主要部分，为50%~80%，再生温度、反应器与吸附剂的质量比、吸附剂对H₂O吸附性的强弱，均能造成热耗的成倍变化。在分析过程能耗的基础上，给出了吸附法DAC在反应器设计、循环方式及操作参数、自然环境及能量来源等方面的能耗优化建议。

关键词: 二氧化碳捕集, 能耗, 吸附, 脱附, 温室气体