多塔串并联二氧化碳置换强化低浓度煤层气甲烷浓缩过程研究

China is rich in coal resources. However, the oil and gas resources are poor. The natural gas reserve is about 50 trillion cubic meters, and the coal mine methane is about 30 trillion cubic meters. The recovery and utilization of coal mine methane (CMM) provide a number of significant energy, economic and environmental benefits. For the enrichment of the low-concentration CMM, the separation of CH4/N2 is very difficult due to the similarity of two gases in physicochemical properties, and the available adsorbents have a poor adsorption CH4 capacity and CH4/N2 selectivity. Therefore, the objective of this work is to develop a novel multi-bed (connection in series or parallel) vacuum pressure swing adsorption (VPSA) process using the principles of displacement chromatography, in which the separation of CH4/N2 is significantly enhanced by the action of CO2 displacement. And the novel process is expected to be a promising technology for the enrichment of the low-concentration CMM with low energy consumption and high efficiency. A rigorous mathematical model is developed to demonstrate the CO2 displacement mechanism for the adsorbed CH4 and N2 in the column packed with the prepared adsorbent materials. Develop and validate a mathematical model for multi-bed (connection in series or parallel) (VPSA) process with CO2 displacement step, which contains mass balance, energy balance and momentum balance, as well as multi-component adsorption equilibrium model. A novel CO2 displacement multi-bed (connection in series or parallel) VPSA process is developed for the enrichment of the low-concentration CMM. In order to maximize the methane purity, recovery and productivity and minimize the cost and energy consumption, the operation conditions are optimized by experiments and simulation results. The method for the design and optimization of CO2 displacement multi-bed (connection in series or parallel) VPSA process will provide a strong theoretical and technical support on the utilization of low-concentration CMM.

我国是一个多煤少气贫油的国家，天然气储量约50万亿立方、煤层气储量约30万亿立方，煤层气有效利用对改善我国能源结构和大气环境具有重要意义。本课题针对现有吸附材料CH4吸附容量低、CH4/N2选择性差并难以分离的技术难点，引入置换色谱概念，创新性地提出了多塔串并联置换强化吸附浓缩煤层气甲烷新方法。该方法通过二氧化碳置换强化CH4/N2吸附分离，解决低浓度煤层气甲烷难以分离浓缩及高能耗问题。建立非线性多组分填充塔内竞争吸附数学模型，结合实验研究，探讨固定床内CO2置换强化CH4/N2分离的机理。建立并验证包含质量、热量、动量传递和多组分竞争吸附的多塔置换强化CH4/N2吸附分离过程数学模型。开发多塔串并联置换真空变压吸附分离CH4/N2工艺流程，通过实验与模拟手段，构建多塔置换真空变压吸附分离过程设计优化基本方法，为煤层气资源开发与工程应用奠定理论基础。

我国煤层气资源储量十分丰富，开采利用情况却不容乐观，约有2/3的井下抽采煤层气浓度低于30%，由于缺乏先进的甲烷浓缩分离技术而难以利用，大部分直接被排入大气中，不仅造成资源和能源的极大浪费，也会加剧温室效应。由于CH4和N2的物理性质极为接近，将二者有效分离是低浓度煤层气浓缩技术的难点。变压吸附技术，由于采用连续快速切换的吸附、脱附操作，使物理性质相近的CH4和N2混合物分离成为可能，具有富集低浓度煤层气甲烷的应用前景。该技术的核心主要有两点：（1）制备具有良好吸附分离CH4/N2性能的吸附剂材料；（2）加强吸附、脱附过程基础理论研究与多种操作模式的工艺组合优化。因此，针对吸附材料和吸附工艺两个关键影响因素。.本课题针对低浓度煤层气中CH4/N2难分离以及传统变压吸附工艺受吸附剂CH4吸附量低、CH4/N2选择性差限制，从而导致的低浓度煤层气甲烷浓缩效果不理想的难题，对CH4/N2分离的活性炭吸附材料进行深入研究，阐明炭材料微观结构与CH4/N2吸附分离性能构效关系，形成CH4/N2分离用炭材料吸附剂设计策略，并批量制备高效椰壳活性炭，其比表面积为1227.2m2/g，在298K，100kPa下，对CH4的平衡吸附量为1.6mol/kg，对N2的平衡吸附量为0.5mol/kg，平衡选择性为3.2。创新性将置换色谱概念引入吸附分离过程，探究置换强化CH4/N2分离过程中固定床内部分各组分分布特征，揭示床层长度和空速对分离过程影响规律。在置换真空变压吸附基础上，研发出七步CO2/CH4置换真空变压吸附新工艺，可将含CH4浓度为14.27%~39.11%的低浓度煤层气和含CH4浓度为32.84%~62.41%的低浓度生物质气联合富集纯化，得到甲烷纯度达到95%以上，CH4的回收率在75%左右，同时获得95%左右的高浓度N2副产品和75%左右的CO2副产品，得到的高纯度甲烷产品气能够直接并入天然气管网。进一步开发了多塔串联吸附分离CH4/N2新工艺，可将低浓度的甲烷气（33%CH4）连续浓缩，获得高纯度甲烷产品气，产品气纯度可接近100%，提浓效果优异。本项目研发的吸附材料和新工艺，实现了低浓度煤层气甲烷高效分离浓缩，为低浓度甲烷气资源开发与实际应用奠定了坚实的理论与应用基础，对缓解我国能源供需矛盾、改善能源结构，实现“碳达峰、碳中和”战略等具有重要意义。