Pyrolysis Simulation Of Fugu Subbituminous Coal By ReaxFF Molecular Dynamics

Coal pyrolysis refers to the initial reaction step in most coal conversion processes,which plays a significant role in efficient and clean utilization of coal,especially for low-rank coals.Coal pyrolysis is well accepted as a radical driven process that involves myriad coupled reaction pathways with vast free radical intermediates generated.The heterogeneous nature of coal and the complexity of the pyrolysis process have made it very difficult to access the comprehensive mechanisms,even with the state-of-the-art experimental approach.When combined with molecular dynamics?MD?,the ReaxFF reactive force filed based on bond order can describe the dynamic evolution of bond breaking and forming smoothly with accuracy close to the widely used Density Functional Theory?DFT?method with very much reduced computational costs.The overall pyrolysis process and the underlying chemical reactions of Fugu subbituminous coal are studied in the thesis by combining large-scale ReaxFF MD simulations and solid pyrolysis experiments with in situ synchrotron vacuum ultraviolet photoionization time-of-flight mass spectrometry?SVUV-PI-TOF-MS?.The main results are summarized as the following.The pyrolysis experiments of Fugu subbituminous coal were performed using the solid pyrolysis apparatus with the volatile species detected by SVUV-PI-TOF-MS at the National Synchrotron Radiation Laboratory?NSRL?in Hefei,China.The dynamic profiles of major light gases and aromatic-based tar products with temperature were obtained by comparison of major pyrolyzates at varied temperature conditions?500,600,700,and 800 ??.The experimental results reveal that the existence of phenolic hydroxyl groups and enlargement of aromatic nucleus can both promote the reactivity of alkyl substituents in aromatic rings.A strategy was proposed for constructing large and reasonable coal molecular models with multi-components manually based on the limited conventional characterization data of coal samples(proximate and ultimate analysis,¹³C NMR,and solvent extraction experiments).Both the structure diversity of macro coal molecules and model construction efficiency were considered in the proposed strategy.Local structure variation and average strategy for elemental composition are combined in the construction of macro coal molecules,which allows for structure diversity and easy match in a multi-component coal model with experimental characterization data of coal samples.Following the proposed strategy,a multi-component molecular model was constructed for Fugu subbituminous coal.The constructed Fugu coal model is a large model containing 23,898 atoms.It consist of 75 macromolecules with 20 varied average structures for structural diversity and 29 varied small compounds to capture the mobile phase.The element composition,aromaticity and averaged aromatic nucleus size of the Fugu coal model constructed are in good agreement with the experimental characterization of Fugu subbituminous coal.Slow heat-up?1 K/ps?and long-time?2000 ps?isothermal simulations were performed using the constructed large Fugu coal model with the GPU-enabled ReaxFF MD code of GMD-Reax to investigate the effects of heating rate and temperature on the coal pyrolysis.The reaction analysis code of VARxMD was used for uncovering the associated reaction mechanism in coal pyrolysis.Analysis of the heat-up ReaxFF MD simulation trajectories shows that Fugu coal pyrolysis process can be divided into three stages:the activation stage of coal structure?Stage-1?,the primary pyrolysis?Stage-?A?and the secondary pyrolysis stage?Stage-?B?,and the recombination dominated stage?Stage-??.The element distribution and migration among pyrolyzates in coal pyrolysis were analyzed.The content of C element in pyrolyzates are ranked as heavy tar>light tar>gas,which indicate that C element migrates to heavy tar and non?volatile molecules.While H element migrate to gas products.The content of O element in pyrolyzates are ranked as gas>light tar>heavy tar.The N element tends to stay in coal molecules that will migrate to gas products at stage-III of high temperature.By taking advantage of VARxMD for reaction analysis for ReaxFF MD simulations,detailed reaction pathways for the early gas product generation of CO₂,H₂O,H₂ and CH₄ in the activation and primary pyrolysis stages of Fugu subbituminous coal are revealed on the basis of heat-up simulations,which is hardly accessible experimentally or by other computational approach.As expected,most of CO₂ molecules are generated from the cracking of carboxylic acid or ester in coal unit structures.The early generation of H₂O is associated with the H atom detached from carboxyl groups.The H atom attacks the O atom of another carboxyl group through the hydrogen bonding or reacts with hydroxyl group in the phenolic structure will lead to the formation of H₂O at Stage-I of coal activation.In primary pyrolysis?Stage-IIA?,the H₂O can be generated by the reaction of H atom detached from the carboxyl group with hydroxyl group of alcohol side chains of phenyl structure,or by the combination of H atoms from the hydroxyl group of different phenolic structures.Although the early generation of H₂ was observed at about 450 K,the rapid generation of H₂ occurs at high temperature from two H atoms that are generated in varied reactions.All the H₂ molecules at coal activation stage?Stage-I?are generated by the combination of two H atoms both detached from the carboxyl groups.In addition to the same pathway observed in the activation stage,H₂ generated in primary pyrolysis stage?Stage-IIA?is through the H abstraction reactions from methyl group,hydroxyl group and aliphatic rings by the H atom which is dropped from the carboxyl groups.In the secondary pyrolysis stage?Stage-?B?,the H₂ molecules can be generated from the combination of two H atoms both detached from the hydroxyl groups of phenolic structures.It is important to note that the rapid generation of H₂ molecules at very high temperature is through the combination of two H atoms detached from diverse sources in the coal pyrolysis system,of which the pathways are hard to list.The methyl radical ·CH₃ generated from O-CH₃ is observed as the only important radical that contribute to the generation of CH₄ molecules that are formed by H abstraction of CH₃ radials.It can be concluded that the gas product generation of CO₂,H₂O,H₂ and CH₄ in the activation and primary pyrolysis stages are closely associated with carboxyl and methoxyl groups,indicating the critical role of oxygen-containing groups in the initialization of subbituminous coal pyrolysis.Furthermore,the effects of elevated simulation temperature and simulation time on pyrolyzate profiles and yields were first investigated by the long time?2000 ps?isothermal ReaxFF MD simulations.Compared with reported experiments of fluidized bed pyrolysis,the coal tar yield is over-predicted,while the gas yield is slightly under-estimated.By comparing the weight profiles between the short and long duration time at high and low temperature conditions,it is found that to shorten the simulation time from 2000 ps to 250 ps,an increase of 400 K in average for simulation temperature is needed in range of 1200-2200 K to get similar pyrolyzate profiles that will result in the over-prediction for the yields of gas and tar,as well as under-prediction for the non-volatile yield.What obtained can be used in refining simulation strategy for coal pyrolysis study.The proposed strategy for fast constructing large and reasonable coal models manually with varied chemical structures is practical for construction of large multicomponent molecular models for coal pyrolysis study with ReaxFF MD.Combining with the reasonable and large coal molecular model,the ReaxFF MD simulation approach is useful for getting an overall scenario of coal pyrolysis and deeper insight into the complex pyrolytic reaction of Fugu subbituminous coal.The method can be applied in studying pyrolysis reaction mechanism of other low rank coals for their better utilization.