基于CO2“浓缩-凝华”机制的低能耗“膜分离-深冷”耦合沼气纯化机理及特性研究

Clean and efficient utilization of energy is one of the hot topics in environment and energy field. Biogas, as a promising renewable energy, has attracted more and more attention. Biogas upgrading to produce bio-natural gas is a significant strategy to improve its heating value and electrical efficiency, and which could be also used as a green vehicle fuel. However, high energy consumption, high methane loss and low bio-natural gas purity are the critical challenges. To obtain both of the high purity CH4 (bio-natural gas) and CO2 (dry ice) products, membrane-cryogenic hybrid biogas upgrading technology will be investigated in this work. The proposed hybrid process can facilitate the subsequent bio-natural gas liquefaction and high purity CO2 utilization. The cryogenic gas separation performance of two kind hollow fiber membranes (polyimide and polysulfone) will be studied, and the objective is to overcome the limitation of “trade-off” between selectivity and permeability. Meanwhile, the energy and material transfer properties of cryogenic CO2 desublimation will be explored, and the heat integration performance between membrane and cryogenic units will be also evaluated. The research emphasis includes: 1) the preparation and characterization of different membrane skins (nodular and dense), 2) the mechanism of gas permeation under two feed manners (bore and shell) at low temperature. Finally, high purity CH4 (bio-natural gas) and CO2 (dry ice) will be simultaneously gathered based on the integration of membrane and Stirling cryogenic cycle. The research will provide a significant theoretical and technical guidance for low-cost biogas upgrading and bio-natural gas production technology.

能源清洁、高效利用乃当今热点。沼气作为一种丰富的可再生能源，近年来引起了广泛关注。将其提纯制取高品质生物天然气，燃烧热值可显著提升，用作汽车燃料。但现有沼气纯化技术存在能耗高、CH4损失率大且纯度低等难题。据此，申请人提出“膜分离-深冷”耦合沼气纯化技术，拟实现高纯度CH4（生物天然气）与CO2（干冰）的双效分离，具有低温CH4易于液化和高纯度CO2易于资源化的双重优点。课题将重点研究不同膜材料在深冷条件下的气体吸附-扩散机理，克服常温膜分离选择性与渗透性的“trade-off”效应，探讨膜浓缩后CO2深冷相变分离的能、质传递特性，优化膜与深冷分离单元间热整合方式及效率。通过具有不同皮层结构的中空纤维膜材料制备及表征，阐述低温条件下管程、壳程两种进气方式对膜材形貌与渗透性能的影响机制，确立膜分离与深冷凝华的一体化耦合方式，为其在低能耗沼气纯化制备生物天然气技术中的应用提供理论和技术依据。

结合项目团队在CO2膜分离等领域的优势，聚焦碳减排国家重大需求，成功研发新型CO2膜分离材料，完成了低温膜材料设计、制备及评价等相关方面的研究，达到以下研究成果：（1）开发了在低温环境下具有高渗透性和选择性的6种混合基质膜材料，研究了不同混合基质膜材料在不同温度条件下的气体吸附-扩散机理，克服常温膜分离选择性与渗透性的“trade-off”效应。项目团队利用混合配体法将氨基引入ZIF-8框架结构，利用聚乙二醇二缩水甘油醚在ZIF-8-NH2表面预先锚定了Pebax胶囊层，构建了胶囊状ZIF-8，将其分散至Pebax基质后，Pebax胶囊层在膜中形成“填充剂-胶囊层-基质”型界面微结构，保障了ZIF-8-NH2填充剂的功能性发挥，气体分离性能突破了2008年Robeson上限。（2）考察了低温环境下的气体分离表现，揭示了低温下气体分离机制，提出了强化分离过程的后处理方法，探讨了膜浓缩后CO2深冷相变分离的能、质传递特性，优化膜与深冷分离单元间热整合方式及效率，确立膜分离与深冷相变的一体化耦合工艺。结果表明，在面对高浓度排放源高炉烟气时，该耦合系统能够实现90%的CO2捕集率目标和97.05%的CO2产品纯度。同时，相较于传统的耦合碳捕集工艺，该耦合系统具有更低的捕集能耗和捕集成本，分别节省28.85%和17.11%。（3）项目执行期间，项目团队共计发表中英文论文11篇，其中SCI索引8篇，项目期内申请中国发明专利5项，培养博士研究生9人、硕士研究生22人。