Investigation of Overall Pyrolysis Stages for Liulin Bituminous Coal by

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Investigation of overall pyrolysis stages for Liulin bituminous coal by large-scale ReaxFF MD Mo Zheng, Xiaoxia Li, Fengguang Nie, and Li Guo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03243 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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Energy & Fuels

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Investigation of overall pyrolysis stages for Liulin bituminous coal by large-scale ReaxFF MD

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Mo Zhenga,*, Xiaoxia Lia,b,*, Fengguang Niea, Li Guoa,b

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a

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Academy of Sciences, Beijing 100190, P. R. China

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b

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KEYWORDS: Coal pyrolysis stage, Bridge bond behavior, Radical behavior, Competition

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mechanism, Meso-scale structure theory

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ABSTRACT: Deep understanding of the detailed coal pyrolysis process is very important for clean

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coal utilization. The overall stages in coal pyrolysis were investigated by ReaxFF MD simulations of

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large-scale coal models combined with reaction analysis of cheminformatics approach. Analysis of

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slow heat-up ReaxFF MD simulations shows that Liulin coal pyrolysis process can be divided into

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four stages based on the thermal cleavage of bridge bonds: the activation stage of coal structure

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(Stage-I), the primary pyrolysis stage (Stage-IIA), the secondary pyrolysis stage (Stage-IIB) and the

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recombination dominated stage (Stage-III). The transition from the dominant cleavage of ether

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bridged bond into breaking of the aliphatic bridged bonds corresponds to the transition of Stage-IIA

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to Stage-IIB in Liulin bituminous coal pyrolysis. Further investigation of the relationship between

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radicals and gas production suggests that temperatures for the transition of gas generation rates can

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be used as indicators for pyrolysis stage transitions, namely H2O for Stage-I and Stage-IIA, CH4 for

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State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

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ACS Paragon Plus Environment

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the primary and secondary pyrolysis reactions, provided such production rate transitions could be

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detected experimentally. In addition, the compromise between the competition reactions of

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decomposition and recombination as well as radical generation and consumption plays a significant

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role along the entire pyrolysis process, and the slight differences of the reactions in competition

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determines the yield, species, and distribution of final pyrolyzates, which seems consistent with the

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meso-scale structure theory.

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1. Introduction

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Coal pyrolysis refers to the thermal decomposition in an inert atmosphere or in a vacuum, which is

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an important intermediate stage in most of coal conversion processes such as gasification,

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combustion and liquefaction.1 Coal pyrolysis is a complex process involving multiphase reactions,

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complex reaction pathways, highly unstable intermediates, as well as heat and mass transfer effects.2,

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3

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product evolution and reaction events with the extended model by Serio and Solomon et al.1, 4 The

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proposed scheme consists of three stages: coal metaplast generation, primary pyrolysis and

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secondary pyrolysis. As coal is heated, the disruption of hydrogen bonds and vaporization of species

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with non-covalent interaction in coal chemical structures are the first steps occurring prior to the

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primary pyrolysis. The weakest bridges can break to produce molecular fragments that abstract

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hydrogen from hydroaromatics or aliphatics during primary pyrolysis. Some fragments will be

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released as tar if they could evaporate and escape from coal particles, while other fragments will

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recombine with each other to produce molecules that are too large to evaporate.5 Other chemical

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events occurred during coal pyrolysis are decomposition of functional groups in coal

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macromolecular structures to release CO2, H2O and light aliphatic gases, and also cross-linking

A rough scheme on the stage of bituminous coal pyrolysis was developed as a basis to describe

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reactions as well accompanied with methyl and carboxyl groups dropping off.6

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Though coal pyrolysis stages could be speculated from experimental observations, the direct

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information about chemical processes that control product profiles and pyrolysis stages still lacks,

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because coal pyrolysis is a radical-driven process that cannot be readily investigated

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comprehensively by experimental techniques so far.7 Fortunately, the strategy of atomistic modeling

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simulations with large-scale coal models sheds new light on the pyrolysis stage investigation, which

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was facilitated by significantly improved computing capability by parallel computing using graphics

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processing unit (GPU) and by reaction details revealed using cheminformatics analysis in ReaxFF

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MD simulations.8

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ReaxFF is a reactive force filed based on the bond-order concept. ReaxFF has a comprehensive

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parameterization of atomic, bonding, angle, and torsion properties, which can fully address the bond

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breaking, forming and polarization effects in complicated chemistry environment.9 Because of its

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close accuracy to Density Functional Theory (DFT), and particularly no pre-designation of multiple

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reaction pathways required, ReaxFF MD has been applied for reaction mechanism investigations in a

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wide variety of materials10-12 and fuels13-16 including complex coal17-20 and biomass utilization,21, 22

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which demonstrates its feasibility and potential in investigating complex reaction mechanisms. Table

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1 lists coal models and simulation results obtained using ReaxFF MD in recent years. By employing

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ReaxFF MD simulations, most of the obtained results 17, 18, 23-26 on coal transformation are molecular

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number or weight evolution tendencies of major pyrolyzates (char, tar and gas),18, 24, 25, 27-29 product

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lists in terms of chemical formula,18, 24, 28, 29 evolution trends and generation pathways of small gases

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(H2O, CO2, CO, H2 and light aliphatics),25, 26, 30-33 and small radicals behaviors (HO· and ·CH3).26, 29

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Particularly, the thermal cleavage evolution of cross-links including alkylethers, alkyl thioethers, aryl 3


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ethers and arylthioethers were reported 18 with focus on the behavior of cross-links containing S. But

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the overall pyrolysis stage and reaction details at each stage for coal thermolysis process at high

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temperature have not been reported to our best knowledge.

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A large-scale coal model with 28,351 atoms was constructed in our previous work to investigate

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product profiles and chemical reactions in Liulin coal pyrolysis with the ReaxFF MD simulation

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approach.20 The evolution tendencies of major products (char, tar and gas) and representative

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pyrolyzates (CO2, CO and naphtahlenes) as a function of time and temperature were obtained, which

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is in broad agreement with experimental results observed from the literature and Py-GC/MS

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experiments. With all the detailed reactions available for Liulin coal pyrolysis from the simulations,

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as well as the continuous development of VARxMD (Visualization and Analysis of Reactive

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Molecular Dynamics) for reaction analysis,34 we see the potential to describe pyrolysis stages for

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coal thermolysis at atomic level, which trigged us to revisit the previous results and perform more

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simulations to investigate further reaction mechanisms during coal pyrolysis process.

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In this paper, the overall pyrolysis stage for bituminous coal divided based on the thermal

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cleavage of bridge bonds and representative indicators for transition temperatures of different stages

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are obtained for the first time by using large-scale ReaxFF MD simulations and cheminformatics

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based reaction analysis. Low heat-up ReaxFF MD simulations of the Liulin coal model with 28,351

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atoms constructed previously were performed. Simulations and reaction analysis details by taking

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advantage of the new features of VARxMD

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proposed on the basis of bridge bond cleavage and relationships among the coal pyrolysis stage,

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product distributions, bridge bond trends, radical behaviors and chemical reactions are described in

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section 3, which extends the understanding on the coal pyrolysis by ReaxFF MD approach and

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are described in section 2.

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Four pyrolysis stages

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demonstrates further the potential of large-scale ReaxFF simulations in investigation of coal

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pyrolysis mechanism. The conclusions are summarized in the last section.

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2. Method

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2.1 ReaxFF MD simulation details

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ReaxFF is a reactive force field that can simulate chemical reactions by introducing the dynamic

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bond-orders and partial charges of atoms. Interatomic interactions between all atoms are stated in

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equation (1).35

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EReaxFF (rij , rijk , rijkl , qi , BOij ) = Ebond + Elp + Eover + Eunder + Eval + E pen + Ecoa + Etors + Econj + EHbond + EvdWaals + ECoulomb

(1)

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Where Ebond, EvdWaals, ECoulomb, Eval, Etors, and EHbond represent the interaction energy of bonded,

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van der Waals, Coulombic, valence angle, torsion and hydrogen bond respectively. The other terms

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introduced for energy corrections are Elp for the presence of lone pair, Eover and Eunder for over and

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under coordination of atoms with respect to their valency, Epen as penalty energy to stabilize a three

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body system where the centered atom has two double bonds, Ecoa and Econj for conjugated chemical

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bonds. The atomic connectivity evolution in ReaxFF MD are determined using the bond-order

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formalism by Tersoff and Brenner,36, 37 meanwhile with polarization effects accounted by dynamic

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charge updated with Electronegativity Equilibration Method (EEM) proposed by Rappe and

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Goddard,38 which makes it suitable for exploring complex chemical reactions in coal pyrolysis. A

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more detailed description of ReaxFF force field could be found in the work by van Duin et al.13, 39, 40

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As one of the largest coal model ever simulated by the ReaxFF MD method, the Liulin

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bituminous coal model with 28,351 atoms was constructed based on a combination of experiments

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(proximate and ultimate analysis,

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are presented in Figure S1, S2 and Table S1 as Supporting Information.20 The GPU-enabled ReaxFF

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C NMR analysis) and the classical coal model of Wiser, which

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MD simulations (GMD-Reax)41 were used to perform simulations to investigate temperature effects

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on the product distribution. The model construction and GMD-Reax simulation details could be

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found elsewhere.20

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In order to explore the overall pyrolysis stages, the underlying reactions and the behavior of

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dominant radicals, a heat-up simulation from 500 to 2500 K with a relatively low heating rate of 2

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K/ps was carried out with a time-step of 0.25 fs to integrate Newton’s equation of motion by the

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velocity-Verlet algorithm. The Berendsen thermostat was employed to maintain the temperature

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equilibrium with a 0.1 ps damping constant. The parameters of ReaxFF force field used were

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developed by Mattsson et al.42 and provided in Supporting Information. It took about 14.8 days to

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simulate 1 ns heat-up condition using GMD-Reax running on a CentOS 5.4 server with an Intel Xeon

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E5620 2.4 GHz, 2 GB RAM with a C2050 GPU card attached.

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2.2 Chemical reactions analysis details

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It is a big challenge for obtaining the complex reaction information from ReaxFF MD simulation

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results of the large-scale coal pyrolysis. VARxMD34 is a unique tool dedicated to the chemical

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reaction analysis and visualization from the trajectories obtained in ReaxFF MD simulations. Based

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on chemical structure properties including functional groups, elemental composition, substructure

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searching etc., the new functions for categorizing species and chemical reactions developed recently

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have been integrated into VARxMD. Therefore, VARxMD was employed to analyze ReaxFF MD

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simulation results,8, 22, 34 particularly its capability of automated classification of species and complex

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chemical reactions allows for uncovering the coal pyrolysis stages.

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3. Results and Discussions

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Divided pyrolysis stages and the underlying cleavage of bridge bonds

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Solomon et al. point out that coal pyrolysis is a complex process involving char, tar and gas

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generation.1 In order to keep consistent with our previous work,20 coal pyrolysis products obtained

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from ReaxFF MD simulations are classified into char, tar and gas based on the molecular weight.

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C40+ compounds with molecular weight great than 700 a.m.u are considered as char, while C14−C40

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and C5−C13 fragments of 80−700 a.m.u as heavy and light tar products respectively. The fragments of

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C0−C4 are considered as gas products in this paper. Based on the evolution tendency of weight

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percentage for char, tar and gas as shown in Figure 1 (a), the simulated pyrolysis process can be

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roughly divided into three major stages.

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The first pyrolysis stage (Stage-I) occurs at 500−1400 K, where the Liulin coal model has no

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obvious weight loss and few products are detected by VARxMD. The activation and conformation

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adjustment of coal macromolecular structures occur at this stage, in which reversible reactions and

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configuration transformation are the major events observed accompanied with small radical

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generation. When temperature increases to 1400−2400 K, the weight percentage of C40+ fragments

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decreases rapidly, meanwhile the amount of heavy tar increases constantly with temperature. The

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light tar products (C5−C13 fragments) appear relatively late at 1800 K, and its amount increases

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continuously with temperature. It is the second pyrolysis stage (Stage-II) where most important

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reactions occur to produce major pyrolyzates in tar and gas. The last pyrolysis stage (Stage-III)

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occurs at temperature higher than 2400 K. The C40+ and C14−C40 compounds reach their minimum

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and maximum at this stage while the amounts of light tar (C5−C13 fragments) and gases (C1−C4

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molecules + inorganic gas) keep growing with the increase of temperature, which suggests 7


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recombination reactions happen frequently to have large molecules generated. Therefore, Liulin coal

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pyrolysis process can be divided into three stages on the basis of the evolution profile of products in

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weight percentage with temperature: the activation stage of coal structures, the major pyrolysis stage,

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and the recombination dominated stage.

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It is generally recognized that the primary thermolysis reactions usually occur at the weakest

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part of the coal macromolecular structure,4, 43 such as the aliphatic and ether bridge bonds44-46 to

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generate large amount of intermediate fragments prompting pyrolysis process. Thus, the

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investigation of bridge bond cleavage during pyrolysis process is of great interest and very important

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for deep understanding of the underlying chemical reactions in coal pyrolysis. With the aid of

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VARxMD, the comprehensive process of bridge bond breaking from ReaxFF MD simulations of coal

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pyrolysis was obtained. The amount evolutions with temperature for the bridge bonds of -O-CH2-

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(Car-O-CH2-Car), -O- (Car-O-Car), -CH2-CH2- (Car-CH2-CH2-Car), -CH2- (Car-CH2-Car), and Car-Car

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are shown in Figure 1 (b).

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Except for a few back-and-forth reactions occurring in -O-CH2- linkage, almost none of the

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bridge bonds break at Stage-I, thus the weight of C40+ fragments almost keeps same at low

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temperature. At Stage-II of 1400−2400 K, the C-O bonds in the -O-CH2- and -O- bridges are found

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breaking first at 1400 K, leading to their amount decrease significantly with temperature. The

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cleavage of ether bridge bonds is the dominant reactions at Stage-II, corresponding to the obvious

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weight loss of coal in Figure 1 (a), which agrees with the experimental observations.45,

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aliphatic linkages start to break at around 1700 K for monomethylene bridge bonds and 1800 K for

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the dimethylenes’ and decrease rapidly with temperature, contributing to the light tar generation in

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Figure 1 (a). The numbers of -O-CH2-, -O-, -CH2-CH2- and -CH2- bonds decrease continuously into 8


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The

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almost zero at the end of Stage-II. Besides depolymerization reactions, the increasing number of

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Car-Car bonds starts from 1800 K, indicating that lots of repolymerization reactions such as

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condensation and cross-linking occur simultaneously at this stage.

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What obtained from the cleavage trends of bridge bonds shows that 1800 K is a transition

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simulation temperature from which the dominant ether linkage breaking reactions will become fade

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away, meanwhile the cleavage of aliphatic bridges and some of ether bonds becomes significant,

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accompanied with recombination reactions of produced intermediate fragments occurring at the same

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time. Therefore, Stage-II can be divided further into two sub-stages based on the bridge bond

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evolution, namely Stage-IIA at 1400−1800 K and Stage-IIB at 1800−2400 K. Stage-IIA can be

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considered as the primary pyrolysis stage, which is characterized by the absolute dominant cleavages

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of -O-CH2- and -O- bridges. Although it is challenging and controversial to distinguish the primary

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and secondary pyrolysis or select calibration products in the secondary pyrolysis from the

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experiments, Stage-IIB can be considered as the secondary pyrolysis stage from the atomistic point

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of view, which is characterized by the starting of fast cleavages of aliphatic bridges and the forming

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of Car-Car bonds, responsible for the starting generation of light tar that contributes to the continuous

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weight loss in Figure 1 (a). It should be noted that ReaxFF-NVT-MD simulation implies an ideal heat

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transfer without resistance, which leads to simultaneous occurring of continuous breaking of coal

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matrix, secondary cracking of volatiles and recombination reactions at Stage-IIB. It is difficult to

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distinguish the occurring sequences of these reactions. Thus, the secondary pyrolysis stage defined in

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the paper is a bit off the definition proposed by Tromp 47 that refers to the cracking of volatiles out of

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the coal matrix.

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There are very few ether or aliphatic bridge bonds in the system of Liulin coal pyrolysis when 9


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temperature increases to higher than 2400 K, resulting in the coal char formation process mostly

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associated with the fast formation of Car-Car linkage at Stage-III. The phenomenon indicates that the

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recombination of Car-Car might be the initial step for coal char formation during coal pyrolysis at

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high temperature. If an inhibitor for the Car-Car bond connection at high temperature could be found,

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coal pyrolysis would become a simple and effective method for coal utilization by converting solid

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coal into liquid fuels.

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To further illustrate the divided pyrolysis stages in Figure 1, the evolving trends of

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decomposition and recombination reactions are also obtained and shown in Figure 2. Coal pyrolysis

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is a very complex process, which consists of radical dominated reactions, hydrogen abstraction and

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hydrogen addition reactions, volatile product cracking reactions and so on.1,

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decomposition of coal macromolecular structure into tar products (C5−C40 together with heavy and

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light tar) or tar products into smaller fragments, and recombination of tar fragments with each other

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producing char molecules (C40+) are two major reaction categorizations. With the aid of VARxMD,

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the competition tendencies of decomposition and recombination reactions involved by C5+ fragments

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in coal pyrolysis were obtained from the heat-up ReaxFF MD simulation and shown in Figure 2,

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where the yellow line represents the distribution of decomposition reactions and blue line for that of

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recombination reactions along the coal pyrolysis stages, in terms of a proportion ratio calculated by

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the number of decomposition reactions or recombination reactions divided by their total numbers.

3

Among them,

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As shown in Figure 2, the decomposition reactions compete intensively with recombination

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within the whole pyrolysis stage. At Stage-I, the simulation temperature is too low to provide enough

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energy for complete breaking of bridge bonds in coal structure, leading to the small reaction number

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(the reaction number at Stage-I is between 2-15, see Figure S3 of Supporting Information). Most 10


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reactions occurring at this stage can be considered as back-and-forth reactions (the reaction list of

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500-1000 K is provided as Table S2 in Supporting Information), which means a bond in a molecule

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breaking at this output moment will form in its parent structure at next or a later output moment (see

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examples of Reaction-5 of 540-560 K and Reaction-1 of 560-580 K in Table S2). The reaction

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numbers of decomposition and recombination are very close and no dominant can be found, leading

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to the small weight loss in Figure 1 (a). But the fluctuations within 4 wt% still exist in the weight

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percentage of five pyrolyzates, which is shown in Figure S4 of Supporting Information.

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When temperature increases further, the violent competition between decomposition and

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recombination continuous. But it is clearly observable that decomposition reactions dominate within

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the whole Stage-II and its occupied ratio is significantly more than that of recombination reactions,

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i.e., the reaction number of decomposition is greater than that of recombination reactions, which

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leads to the rapid weight loss and large amounts of tar generation at Stage-II. The interesting

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observation is that the proportion evolution tendency of depolymerization reactions increases with

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temperature at Stage-IIA, while decreases significantly at Stage-IIB. Such a transition right at the end

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of Stage-IIA for the two types of competitive reactions confirms that the simulation temperature of

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1800 K is indeed the transition temperature from the primary pyrolysis stage to secondary pyrolysis,

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leading to the consequent growing trend for a dominant role of recombination reactions to produce

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char fragments in coal pyrolysis process at high temperature. It could be speculated that the occupied

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proportion of repolymerization reactions would be more than 50% in an longer-time ReaxFF MD

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simulation.

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It should be noted that the thermal decomposition reactions are not solely a dominant

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mechanism governing the stages of Liulin coal pyrolysis. The competitions between the very close 11


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portion occupied by the decomposition and recombination reactions leading to the compromise

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between them determine the yield, species, and distribution of final products. Decomposition

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reactions produce large amount of relative small fragments (tar and gas) released from coal structural

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cluster, meanwhile the recombination reactions contribute to the amount increasing tendency of char

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or coke products because of Car-Car bond formation. In other words, the final product distributions

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are co-determined by the decomposition and recombination mechanisms together. Therefore, the

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compromise tendency with time and temperature along the competition between decomposition and

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recombination is a key factor governing the coal pyrolysis stages. The compromise phenomenon in

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coal pyrolysis simulations seems consistent with the meso-scale structure theory proposed by Li et

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al.48 that the structure transition of a complex system is the result of a compromise between two

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competitive of mechanisms existing jointly, which is critical to evaluate their respective variational

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criteria.

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Behaviors of small radicals and their effects on Liulin coal pyrolysis stage

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Coal pyrolysis is considered as the radical-driven process with a large number of active radicals,

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intermediate compounds and chemical reactions generated during an extremely short period of

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thermolysis.49,

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simulations. Using VARxMD, all the reactions with small radicals involved (C0 and C1 radicals) were

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picked from the total 8915 reactions and 35,884 species of the heat-up ReaxFF MD simulation.

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These reactions were further categorized into two reaction types for radical generation and

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consumption. It turned out that the evolution tendency of small radicals has good relationships

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corresponding to the detailed pyrolysis stages in Figure 1 (b) and provide further support to the

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defined transition temperature of the primary pyrolysis stage and secondary pyrolysis stage.

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It is of great importance to observe the radical behaviors in coal pyrolysis

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As shown in Figure 3 (a), the pink line represents the amount evolution of small radicals with

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temperature in Liulin coal pyrolysis obtained from the 2 K/ps heat-up ReaxFF MD simulation. At

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Stage-I, the number of small radicals increases significantly with temperature. The small radical

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amount increases continuously at Stage-IIA but at a much slower rate. Its maximum number appears

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right around the transition temperature from Stage-IIA to Stage-IIB. The amount of small radicals

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decreases rapidly with temperature at Stage-IIB and Stage-III. The radical evolution tendency

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indicates that although almost no cleavage of bridge bonds occurring at low temperature prior to the

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primary pyrolysis (see Figure 1 (b)), large amount of radicals have been generated, meanwhile

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accompanied with functional groups dropping off from side chains. The unstable state of these small

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radicals is retained at low temperature, thus allows for their prompting the consequent primary

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pyrolysis at Stage-IIA and secondary pyrolysis process at Stage-IIB.

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Figure 3 (a) presents the reaction amount evolution of small radicals during the four coal

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pyrolysis stages, the blue line for small radical generation, while the yellow line for their

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consumption. The evolution profile indicates that small radical generation reactions compete

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intensely with their consumption reactions within the four simulated stages. The very small excess

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amount of radical generation reactions over their consumption reactions at Stage-I leads to the

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amount increasing trend of small radicals generated (the pink line in Figure 3 (a)). The generation

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reactions of small radicals compete fiercely with their consumption reactions at the whole Stage-II,

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the slightly larger amount of radical consumption reactions at later stage leads to the decreasing

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amount of radicals.

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The Figure 3 (b) shows the evolving tendency for the proportion of small radical involved

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reactions in the total reactions, which reveals obviously the important role of small radicals in coal 13


Energy & Fuels

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282

pyrolysis. At Stage-I and Stage-IIA of 500−1800 K, about 70-100% reactions have small radicals

283

involved, reflecting their significant role in promoting the pyrolysis process at the activation and

284

primary pyrolysis stages. When temperature increases to 1800 K, the portion of small radical

285

involved reactions decreases with temperature in the secondary pyrolysis and recombination

286

dominated stages. Particularly, the contrast of low portion of radical involved reactions and

287

considerable amount of radicals available at high temperature might suggest that small radicals play

288

more significant role in accelerating thermal decomposition reactions other than in that of

289

recombination reactions.

290

Figure 4 displays the amount evolving tendencies of major radicals and their corresponding gas

291

products to illustrate further the radical effects on the relationship of radicals and products in coal

292

pyrolysis. The numbers of HO· and ·CH3 radicals increase with temperature at Stage-I in Figure 4 (a),

293

meanwhile only a small amount of H2O molecules are released from the coal macromolecular

294

structure. It is very interesting to have observed that the amount of HO· reaches its maximum at

295

around 1400 K (transition temperature between Stage-I and Stage-IIA), while the ·CH3 radical

296

reaches its first peak amount at 1800 K (transition temperature from Stage-IIA to Stage-IIB).

297

Accordingly, the rapid amount increasing of H2O starts at the beginning of Stage-IIA, while there is a

298

transition to the rapid generation of CH4 at around 1700 K, close to the end of Stage-IIA. The good

299

consistency between the amount evolution of radicals and gas products indicates not only that the

300

radical consumption contribute mostly to small gas product generation, but also that the production

301

rate transition of small gas products occurred could be used as indicators for pyrolysis stage

302

transition provided that could be detected experimentally.

303

When temperature increases further, the amount of HO· decrease very quickly to almost zero at 14


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304

Stage-III. Meanwhile, the amount of ·CH3 radical increases from a local minimal at around 2000 K

305

within Stage-IIB into another local maximal at 2400−2500 K. This observation is consistent with

306

what Solomon et al.6 pointed out that cross-linking reactions may occur accompanied by substitution

307

reactions in which the attachment of a larger molecule to coal cluster structure releases the methyl

308

group. The re-increasing trend of ·CH3 radical amount might suggest 1900−2000 K is the initial

309

temperature for cross-linking reactions. The phenomenon indicates that the cross-linking reactions in

310

coal pyrolysis start to happen in the middle stage of the secondary pyrolysis where the re-increasing

311

generation of ·CH3 radicals can be regarded as an indicator for cross-linking reactions. The small

312

radical behaviors observed in the simulations validates that the coal pyrolysis is indeed a

313

radical-driven process.

314

4. Conclusions

315

It is a challenging computational task to simulate and analyze a complex process like coal pyrolysis

316

involving thousands of elementary reactions and volatile species and with the paucity of molecular

317

information on intermediate species. In this paper, a heat-up ReaxFF MD simulation with a relatively

318

low rate of 2 K/ps was performed using a large-scale Liulin coal model with 28,351 atoms

319

constructed in the previous work to explore the overall stages in pyrolysis process. The reaction

320

analysis tool VARxMD was employed to detect the behavior of important intermediates and

321

categorized reactions from the ReaxFF MD simulation trajectories.

322

Analysis of the weight evolving tendencies of main pyrolyzates (char, tar and gas) shows that

323

Liulin coal pyrolysis can be divided into three major stages: the activation of coal structure at

324

500−1400 K (Stage-I), the pyrolysis stage at 1400−2400 K (Stage-II) and the combination reaction

325

dominated stage at temperature higher than 2400 K (Stage-III). With the aid of VARxMD, the 15


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

326

cleavage trends of bridge bonds are obtained and indicate that Stage-II can be divided further into the

327

primary pyrolysis stage at 1400−1800 K (Stage-IIA) and the secondary pyrolysis stage at 1800−2400

328

K (Stage-IIB). The transition from the dominant cleavage of ether bridged bonds into that of the

329

aliphatic bridged bonds corresponds to the boundary of Stage-IIA and Stage-IIB in Liulin bituminous

330

coal pyrolysis. Aromatic-aromatic bond forming and cross-linking reactions also occur

331

simultaneously at Stage-IIB, which makes the secondary pyrolysis stage even complex.

332

Particularly, large amount of small radicals are generated at very low temperature from the side

333

chain functional groups of coal macromolecular structure dropping off. These radicals can keep their

334

active state until the temperature increasing to a specific value (1400 K, transition temperature of

335

Stage-I and Stage-II) and then prompt the reactions in the primary and secondary pyrolysis. Further

336

investigation of the relationship between radicals and gas production suggests that the temperature

337

transition of gas generation rate can be used as the indicators for pyrolysis stage boundaries, namely

338

H2O for Stage-I and Stage-IIA, CH4 for the primary and secondary pyrolysis stage, provided such

339

rate transitions could be detected experimentally.

340

The competition trends of decomposition and recombination reactions as well as small radical

341

generation and consumption are also obtained, which indicates that the compromise between the

342

competition mechanisms plays a very important role along the entire pyrolysis process, and very

343

small amount difference of these reactions determines the yield, species and distribution of final

344

pyrolyzates. The compromise of the competition mechanisms in coal pyrolysis seems consistent with

345

the meso-scale structure theory.

346

It should be noted that although the large-scale ReaxFF MD simulation combined with reaction

347

analysis of cheminformatics can obtain the overall pyrolysis stages of Liulin coal, the distinguishable 16


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348

boundary temperatures for every pyrolysis stage is a big challenge for the mechanism investigation

349

in coal pyrolysis. As an example, 1400 K is the apparent transition temperature from Stage-I to

350

Stage-II, but the temperatures ranging from 1750 to 1850 K can be considered as both to the primary

351

pyrolysis stage (Stage-IIA) and to the secondary pyrolylsis stage (Stage-IIB). Because of the

352

heterogeneous nature of coal structure and complexity of coal pyrolysis, the intermediate fragments,

353

radicals and chemical reactions around the transition temperature of different pyrolysis stages are

354

extremely complex but significantly important for investigating reaction mechanisms. Further

355

investigations on how to detect and track the dominant intermediates and classify the reactions

356

occurring around the transition temperature of different stages are still needed in future work.

357

ASSOCIATED CONTENT

358

Supporting Information

359

The Supporting Information is available free of charge

360

The parameters of ReaxFF force field used in the heat-up simulations with a very low heating

361

rate of 2 K/ps (ffield.reax). The unimolecular model of Liulin coal constructed based on Wiser model

362

(Figure S1). The Liulin coal model with formula C14782H12702N140O690S37 (Figure S2). The number

363

evolving tendency of decomposition and recombination reactions involved by C5+ fragments at four

364

coal pyrolysis stages obtained from the heat-up ReaxFF MD simulation at 2 K/ps by VARxMD

365

(Figure S3). The weight percentage evolution of major pyrolyzates with temperature at Stage-I of

366

500-1400 K obtained from the heat-up ReaxFF MD simulation of 2 K/ps (Figure S4). The

367

comparison of structure properties between coal sample experiments and constructed coal models

368

(Table S1). Reactions involved by C5+ fragments at 500-1000 K obtained from the heat-up ReaxFF

369

MD simulation of 2 K/ps using VARxMD (Table S2). 17


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370

AUTHOR INFORMATION

371

Corresponding Author

372

*Phone: 86-10-82544936 Fax: 86-10-62561822 E-mail: [email protected].

373

*Phone: 86-10-82544944 Fax: 86-10-62561822 E-mail: [email protected].

374

Present Address

375

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese

376

Academy of Sciences, No. 1 Zhongguancun North Second Street, Beijing 100190, P.R. China

377

Author Contributions

378

The manuscript was written through contributions of all authors. All authors have given approval to

379

the final version of the manuscript.

380

Notes

381

The authors declare no competing financial interest.

382

ACKNOWLEDGMENTS

383

This work was co-supported by the National Natural Science Foundation of China (21373227,

384

91434105), China’s State Key Laboratory of Multiphase Complex Systems (COM2015A003), and

385

the National Key Research and Development Plan (2016YFB0600302-02). The authors thank Prof.

386

Jinghai Li (CAS Member) for the insightful discussion on the mechanism compromise trend in coal

387

pyrolysis from the point view of meso-scale structure theory.

388

REFERENCE

389

Solomon, P. R.; Serio, M. A.; Suuberg, E. M. Coal Pyrolysis - Experiments, Kinetic Rates and Mechanisms. Prog.

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Energy Combust. Sci. 1992, 18 (2), 133−220.

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Solomon, P. R.; Fletcher, T. H.; Pugmire, R. J. Progress in Coal Pyrolysis. Fuel 1993, 72 (5), 587−597. 18


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of the Nb6O19Hx ((8-X)-) Lindqvist Polyoxoanion in Bulk Water. J. Phys. Chem. A 2013, 117 (32), 6967−6974.

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11. Pitman, M. C.; van Duin, A. C. T. Dynamics of Confined Reactive Water in Smectite Clay-Zeolite Composites. J.

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493 494

23


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495

496

Page 24 of 27

The table used in the paper is put here

Table 1 Coal models investigated using ReaxFF MD and their main results published these years Coal model 26

Model Scale

Main results

Objective

Product distribution

< 500 atoms

Spontaneous

Number

(50 O2)

(O2: 100 atoms)

combustion

CH2O, CO2, H2O and H2O2

Wiser 30

< 2000 atoms

Supercritical

(400 H2O)

(H2O: 800 atoms)

water effect

Hatcher

31

evolution

of

reactants,

CO2 formation pathway

· HO2 consumption and formation H2 generation pathways

Number evolution of H2

H radical-rich water cluster effect

< 3000 atoms

Hydropyrolysis

Number evolution of gases

(400 H2)

(H2: 800 atoms)

desulfurization

S composition

Wulfum 27

< 15,000 atoms

Hydrothermal

Distribution of char, tar and gas

(4334 H2O)

(H2O: 13,002 atoms)

treatment

H-bond distribution

Wiser

Reactions

Desulfurization pathways Examples

of

cleavage,

cross-linking,

H-release,

H-capture

reactions Pyrolysis process divided into

Oil shale kerogen 25

Weight evolution of char, tar and gas 17, 160 atoms

Pyrolysis

Number evolution of small gases (H2, H2O, NH3, CH4, etc)

three stages: initial stage, major pyrolysis stage, late stage Bond breaking sites Examples of reaction pathways for typical structures

Number evolution of char, tar and gas Hatcher 28

< 2000 atoms

Subbituminous

Number evolution of small gases

Formation pathways

coal pyrolysis

(CO2, H2O, CO, CH4 and H2)

(CO2, H2 and CO)

of

gases

of

gases

of

small

Product list in terms of chemical formula Vitrinite 32 Hatcher 29 Morwell

2052 atoms

< 3000 atoms

Vitrinite

Number evolution of gases (CH4,

Formation pathways

pyrolysis

CO2 and C2H6)

(CO2, H2 and CH4)

Brown

coal

pyrolysis

Product list in terms of chemical formula Weight evolution of char, tar and gas

Brown coal 33

< 7000 atoms

Combustion

Number evolution of gases (CO2,

(1000 O2)

(O2: 2000 atoms)

and pyrolysis

CO, H2, H2O, HCHO and O2)

Illinois No. 6 coal char

17

(14,000 O2)

Number evolution of gases (CO, 35, 458 atoms (O2: 28,000 atoms)

Coal

char

combustion

CO2, H2O, H2 and O2) Number

evolution

of

5-,

6-,

7-membered rings

Number

radicals (HO· and · CH3) —— Examples

2692 atoms

Early

formula

maturation

Weight evolution of char, tar and gas 2D structures of example products 24


of

C-O

bond

dissociation Conversion

pathways

of

6-membered ring into 5- and 7-membered rings

Product list in terms of chemical Morwell 24

evolution

——

Page 25 of 27

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Energy & Fuels

Product list in terms of chemical Illinois No. 6 coal model 18

~ 50,000 atoms

Illinois No. 6 coal pyrolysis

formula 2D structures of example fragments Number evolution of gases (CO, CO2 and H2O)

497 498

25


Number evolution of cross-links at 2000 K Examples dissociation

of

C-S

bond

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499

Page 26 of 27

Figures used in the paper are grouped here

500 501

a

b

502

Figure 1 Pyrolysis stage for Liulin coal pyrolysis obtained from the heat-up ReaxFF MD simulation

503

of 2 K/ps: (1) three stages divided based on the weight percentage evolution of major pyrolyzates

504

with temperature; (b) four detailed stages divided based on the number evolution of bridge bonds in

505

coal model with temperature

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Figure 2 Competition tendency of decomposition and recombination reactions involved by C5+

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fragments at four coal pyrolysis stages obtained from the heat-up ReaxFF MD simulation of 2 K/ps

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using VARxMD

26


Page 27 of 27

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Energy & Fuels

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a

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b

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Figure 3 Small radical behaviors at four coal pyrolysis stages obtained from the heat-up ReaxFF MD

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simulation at 2 K/ps by VARxMD: (a) reactions for small radical generation and consumption, and

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the radical amount generated; (b) portion of small radical involved reactions in all the reactions

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a

b

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Figure 4 Amount evolving tendency of radicals and their corresponding gas products at four

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pyrolysis stages obtained from the heat-up ReaxFF MD simulation at 2 K/ps for coal pyrolysis by

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VARxMD: (a) HO· and ·CH3; (b) H2O and CH4 27


Investigation of Overall Pyrolysis Stages for Liulin Bituminous Coal by

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