Investigation of Overall Pyrolysis Stages for Liulin Bituminous Coal by Large-Scale ReaxFF Molecular Dynamics

Abstract

Deep understanding of the detailed coal pyrolysis process is very important for clean coal utilization. The overall stages in coal pyrolysis were investigated by ReaxFF MD simulations of large-scale coal models combined with reaction analysis of a cheminformatics approach. Analysis of slow heat-up ReaxFF molecular dynamics (MD) simulations shows that the Liulin coal pyrolysis process can be divided into four stages based on the thermal cleavage of bridge bonds: the activation stage of the coal structure (Stage-I), the primary pyrolysis stage (Stage-IIA), the secondary pyrolysis stage (Stage-IIB), and the recombination dominated stage (Stage-III). The transition from the dominant cleavage of the ether bridged bond into breaking of the aliphatic bridged bonds corresponds to the transition of Stage-IIA to Stage-IIB in Liulin bituminous coal pyrolysis. Further investigation of the relationship between radicals and gas production suggests that temperatures for the transition of gas generation rates can be used as indicators for pyrolysis stage transitions, namely H 2 O for Stage-I and Stage-IIA, and CH 4 for the primary and secondary pyrolysis reactions, provided such production rate transitions could be detected experimentally. In addition, the compromise between the competition reactions of decomposition and recombination as well as radical generation and consumption plays a significant role along the entire pyrolysis process, and the slight differences of the reactions in competition determine the yield, species, and distribution of final pyrolyzates, which seems consistent with the mesoscale structure theory.

Reaction Mechanisms in the Thermal Decomposition of CL-20 Revealed by ReaxFF Molecular Dynamics Simulations

“…The trajectory internal for analysis is 500 fs. VARxMD was developed to analyze the trajectories from ReaxFF MD simulations automatically by Liu et al 25 , which has played unique roles in investigating the reaction mechanisms of large scale reactive system with complex chemistry, including pyrolysis of coal 26 , polyethylene 27 , lignin 28 , cellulose 29 , pnitrophenol 30 as well as combustion of RP3 31 and bio-oil 32 and soot formation 33 . Therefore, VARxMD was used to obtain the at 3000 K To have an insight into the overall chemical events, the temporal evolutions of CL-20 and major intermediates and products generated during the thermal decomposition at 3000 K are depicted in Fig.…”

Simulating the Geological Fate of Terrestrial Organic Matter: Lignin vs Cellulose

“…Some kerogen models were also produced by a simple liquid quench approach using reactive molecular dynamics (RMD) simulations 13 using reactive force fields such as ReaxFF. 34 Driven by the need to understand the conversion of organic wastes into energy resources (coal, biofuels, etc...) 35,36 and/or manufacturable carbons, [37][38][39][40] RMD has also been extensively used in recent years to investigate the pyrolysis -at least at early stages -of various forms of organic matter, including lignin and cellulose, [41][42][43][44] coals, [45][46][47][48][49] kerogen 50,51 and other fossil OM. 52,53 Zheng et al studied the early stages of cellulose pyrolysis using isothermal MD simulations and observed the quick fragmentation of the polymer into small (less than 9 C atoms) molecules, water being the most abundant.…”

Timescale prediction of complex multi-barrier pathways using flux sampling molecular dynamics and 1D kinetic integration: Application to cellulose dehydration

“…Since the development of ReaxFF, an important field of application has been the transformation of organic products undergoing either lab pyrolysis -for the production of bio-sourced materials or fuels [4][5][6][7][8] -or geological burial 4,[9][10][11][12][13][14][15][16][17] . In these studies, the pyrolysis of some form of organic matter (OM) -including coals 9,11,12,15,17 , immature kerogen 14,16 , wood derivatives like lignin and cellulose [5][6][7][8] , or other fossil OM 10,13 -is simulated at either constant or increasing temperature and the distributions of created byproducts (gas or liquid) are analyzed and discussed as a function of the type of organic matter 9,13,16 , the temperature 4,8-10, [12][13][14]17 or heating rate 6 , or the time 5,11,13,17 . Some authors report detailed chemical mechanisms (i.e., sequences of individual chemical reactions) of the early pyrolysis stages [4][5][6][7]…”