基于ReaxFF MD模拟的低阶煤热解产物演化规律及反应机理

Product evolution and reaction mechanism of low-rank coal pyrolysis based on ReaxFF MD simulation

热解是实现煤炭资源清洁高效利用的重要途径,深入认识煤热解过程中挥发分自由基的变化规律对调控热解产物至关重要,但实验方法难以捕捉其细节。本文选用经典的褐煤分子模型,结合反应分子动力学(ReaxFF MD)模拟探究了低阶煤热解过程中挥发分自由基的演化规律及反应机理。ReaxFF MD模拟结果表明,挥发分产物的收率随升温速率的增大而增加,较高的升温速率抑制了气体产物的生成、提高了焦油产物的收率,但焦油的重质化严重。含氧官能团的裂解是煤热解的触发机制,热解过程主要分为活化(800~1200K)、热解(1200~2400K)和缩聚(2400~2800K)三个阶段。在高温缩聚阶段,焦油片段之间更容易交联,进而发生缩聚反应形成焦炭,并伴随着气体生成,导致焦油收率降低,气体和焦炭产率增加。因此,改善焦油收率和品质的关键是促进焦油片段的裂解,抑制其缩聚。分析了气相产物的形成机理,CO_(2)主要来自羧基和酯基的裂解;甲氧基侧链和桥键裂解形成·CH_(3)和·CH_(2)自由基并捕获·H,最终形成CH_(4)分子;焦油的二次热解和缩聚释放大量·H和H_(2),·H之间进一步反应生成H_(2);而煤中的硫醚结构与含氮支链裂解后,进而被·H自由基稳定为H_(2)S和NH_(3)。这些从分子层面获得的机理认识,可为实验或工业调控热解产物提供重要的参考依据。

Pyrolysis is an important way to achieve the clean and efficient utilization of coal resources,and an in-depth understanding of the changes of volatile radicals during coal pyrolysis is crucial to the regulation of pyrolysis products,but it is difficult to capture the details of experimental methods.A classical lignite molecular model was used to investigate the evolution of volatile radicals and the reaction mechanism during the pyrolysis of low-rank coal in combination with reactive molecular dynamics(ReaxFF MD)simulations.The simulation results showed that the yield of volatile products increased with the increase of heating rate,and the faster heating rate inhibited the formation of gas products and increased the yield of tar products,but the degree of heavy tar was serious.The cracking of oxygencontaining functional groups was the triggering mechanism of coal pyrolysis,and the pyrolysis process was mainly divided into three stages:activation(800-1200K),pyrolysis(1200-2400K)and condensation(2400-2800K).In the high-temperature condensation stage,the tar fragments were more easily cross linked with each other,and then the condensation reaction occured to form char,which was accompanied by gas generation,leading to a decrease in tar yield and an increase in gas and char production.Therefore,the key to improve tar yield and quality was to promote the cleavage of tar fragments and inhibit their polycondensation.The formation mechanism of gas-phase products was analyzed.CO_(2) is mainly produced by the cleavage of carboxyl and ester groups;the cleavage of methoxy side chains and bridge bonds forms·CH_(3) and·CH_(2) radicals,which capture·H and finally form the CH_(4) molecule;the secondary pyrolysis and condensation of tar release a large amount of·H and H_(2),and the further reaction between·H produces H_(2);the thioether structure and nitrogen-containing branch chains in coal are decomposed,and then stabilized to H_(2)S and NH_(3) by·H free radicals.These mechanisms obtained from the molecular level can provide important references for experimental or industrial regulation of pyrolysis products.