Investigation Of Secondary Reactions For Low-rank Coal Tar By Using Reactive Molecular Dynamics (ReaxFF MD)

Deep investigation of secondary reactions for low-rank coal tar during coal pyrolysis is vital to high quality chemical production and upgrading process of tar,which plays an important role in efficient and clean coal conversion and utilization.The heterogeneous nature of coal tar products as well as the complexity of radical-driven process have made it a big challenge to access the comprehensive mechanisms of secondary reactions for coal tar,even with the state-of-the-art experimental approaches.A new methodology of ReaxFF MD combining GPU-enabled high performance computing and cheminformatics analysis can shed light on dynamic profiles of structure transformation and reaction details of coal tar,complementing available experimental observation.In order to explore secondary reactions for coal tars with different composition,three large-scale tar models for low-rank coal were constructed in this thesis based on the ReaxFF MD simulations of the largest Naomaohu coal model built in the previous work.The coal tar models abstracted from slow heat up simulation trajectories of coal pyrolysis process represent tar products from primary pyrolysis stage,the initial stage of secondary pyrolysis and the large stage of secondary pyrolysis,respectively.The isothermal GMD-Reax simulations were performed at different conditions to explore temperature effects on secondary reactions of coal tar model abstracted from primary pyrolysis stage of raw coal.The simulation results were analyzed with the aid of unique capability in VARx MD for automated analysis of detailed chemical reaction and visualization of reaction sites.The analysis of ReaxFF MD trajectories obtains the evolution tendencies of products(char,tar and gas),major bridge behavior,dynamic profiles of hydrocarbon ring groups,the distribution of oxygen-containing functional groups as well as the generation pathway of major gases as a function of time and temperature during secondary pyrolysis process of low-rank coal tar.Particularly,the char formation process from heavy tar was revealed,which shows a few of recombination or cross-linking reactions take place at-CH₂-O-reactive sites at the initial process of secondary reactions while those occur at-CH₂-CH₂-sites at late secondary pyrolysis stage,dominating the char formation.In addition,it is confirmed that five-and seven-membered hydrocarbon ring groups are the precursor intermediate structure for char formation,which is consistent with the previous work in literature.These detailed structure information about coal char are hardly accessible by experiments or other computational methods.Furthermore,the paper first explores the effects of the composition and structure in coal tar formed from different pyrolysis stage of NMH coal tar on the secondary reactions.It is obtained that the secondary reactions of coal tar from lower pyrolysis stage produce much more small gases than those from relatively higher pyrolysis stages,especially the gases with high H/C ratio.The conversion rates from heavy tar decrease with pyrolysis stage of coal tar products,but the yields of char formation increase pyrolysis stage,which indicates parts of light tar products take part in recombination or cross-linking reactions.By using high-performance computing and cheminformatics analysis for reaction details,the comprehensive process of secondary reactions of low-rank coal tar was investigated in this work.It is a necessity for the optimization of coal valorization into targeted products under pyrolytic conditions,which is helpful for modulating the nature of chemical species and upgrading of the tar products.