关键词: 低阶煤, 热解, 反应机理, 挥发分自由基, 反应力场, 分子动力学

Abstract:

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 oxygen-containing 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. CO₂ is mainly produced by the cleavage of carboxyl and ester groups; the cleavage of methoxy side chains and bridge bonds forms ·CH₃ and ·CH₂ radicals, which capture ·H and finally form the CH₄ molecule; the secondary pyrolysis and condensation of tar release a large amount of ·H and H₂, and the further reaction between ·H produces H₂; the thioether structure and nitrogen-containing branch chains in coal are decomposed, and then stabilized to H₂S and NH₃ by ·H free radicals. These mechanisms obtained from the molecular level can provide important references for experimental or industrial regulation of pyrolysis products.

Key words: low-rank coal, pyrolysis, reaction mechanism, volatile radicals, reaction force field, molecular dynamics