下行床中煤/甲烷催化共炼高选择性生成小分子芳烃和烷烃的研究

The lignite, which accounts for 55 percent of our country coal resources, will become low efficiency or environmental pollution if direct combustion or gasification due to the high volatiles and low combustion energy. And it is mainly comprised of the aromatic macromolecular structure. Therefore, from the viewpoint of theory, the lignite can be decomposed to high selectively produce the small molecule arenes and alkanes like as refining oil. The main process of refining coal is pyrolysis. However, the productivity of tar is low and mainly composed with the polyclinic aromatic hydrocarbons because of the scarcity of H and the condensation reaction among the tar radical in the pyrolysis. In this application, the hydro-pyrolysis of lignite is coupled with the reaction of methane catalytically decomposed in the downer by using the high temperature semi-coke as the heat carrier and the catalyst of decomposing methane, which solve the limitation of H scarcity and limit the condensation reaction among the tar. Meanwhile, the tar is directly decomposed into the small molecule arenes and alkanes by using the non-noble metal modified HZSM-5 catalyst, which are packed a new reactor of catalytic reaction and dust filter in packed bed and regeneration in the fluidized bed. The goal of this project is to solve the problem of the low efficiency and environmental pollution in the process of direct combustion or gasification of lignite by coupling the hydro-pyrolysis with the methane decomposed, and the chemical products of small molecule arenes and alkanes with the low dust content are obtained.

褐煤占据我国煤炭资源55%以上，源于其高挥发分和低燃烧值，使其直接燃烧或气化其效率低污染严重。褐煤的主要由芳环大分子结构组成。因此，从理论上讲褐煤分子像炼油一样可进行定向剪切使其高选择性生成小分子芳烃和烷烃。“炼煤”的主要技术为煤热解。然而，源于其过程中烃类自由基生成时缺H的限制和自由基间二次缩合反应的影响，使其焦油产率低且主要产物为稠环芳烃。本项目提出在下行床中以高温半焦为热载体和甲烷裂解催化剂，实现煤快速加氢热解和甲烷催化裂解制氢耦合，从而解决了其过程中缺氢的限制和抑制自由基间的二次缩合反应。同时，通过下行床出口耦合新型地固定床催化除尘-流化床再生反应器，使用非贵金属改性的HZSM-5催化剂来完成焦油分子的在线加氢定向裂解，使其高选择性地生成小分子芳烃和烷烃。该项目通过煤与甲烷“共炼”解决了褐煤直接燃烧或气化效率低污染严重问题，并获得了低含尘量的小分子芳烃和烷烃等化合物。

褐煤先热解后燃烧已经被认为是褐煤清洁利用的关键技术。然后，源于煤缺氢限制，使热解存在油气产率低和半焦产品应用困难。在该研究中，提出了甲烷半焦催化裂解制氢，褐煤流化床内加氢热解来提高煤焦油产率和解决半焦产物燃烧困难的问题。褐煤热解是一个复杂物理化学过程，定量表征煤热解过程反应动力学是预测过程关键。该研究中假设煤热解是多个平行一级反应，并在相同转化率下发生同一反应，从而通过求解相关非正定矩阵来获得每个反应活化能，指前因子和质量分率等动力学参数。该模型具有以下优点：一、模型预测更为准确；二、计算时间降低一个数量级；三、更易与CFD理论组份输运方程耦合。基于该模型分析的结果显示，对于褐煤热解过程而言，仅需要几或十几个一级反应就可对复杂煤热解过程进行预测，同时当煤颗粒直径小于3mm时其内扩散和颗粒内热导对热解过程影响可忽略，当气固间传热系数大于200w.m-2.s-1时，其传热影响可忽略，因此流化床被建议。首先使用高压热重研究褐煤加氢热解过程，实验结果显示当氢气压力从常压提高为2.0Mpa时，褐煤转化率从41%增加为78%，煤焦油增加15%，甲烷气化产率从7%提高为24.5%，半焦碳结构芳香度和石墨化程度增加，且孔隙度与惰性组份相比提高一倍，比表面达480m2/g，其燃烧性能较好。开展了煤加氢热解的常压流化床和高压固定床实验，并同时进行加氢催化热解过程。实验结果显示在压力2.0Mpa，温度800oC时，与惰性气体相比氢气可以将气相产率提高一倍和焦油产率提高40%左右，且氢气使用可明显地降低气相中CO2产率，增加CH4产率，轻质化焦油，沥青含量降低2%，增加单环芳烃含量。为了进一步提高液气产率和轻质化焦油，开发Fe-Ca、Co-Ca和Ni-Ca三类双金属催化剂，结果显示：增加焦油产率依次为：Ni-Ca>Co-Ca>Fe-Ca，提高气相产率依次为：Co-Ca>Ni-Ca>Fe-Ca。三类催化剂均可使气相中CO和CO2产率上升，CH4产率下降，并降低焦油中稠环芳烃和沥青馏分。Co-Ca增加焦油中BTX单环芳烃和萘类化合物，降低PCX酚类化合物。高压加氢热解有望成为褐煤清洁利用的关键技术。