Investigation of overall pyrolysis stages for Liulin bituminous coal by

Subscriber access provided by University of Newcastle, Australia

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

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

Energy & Fuels

2

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

3

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

4

a

5

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

6

b

7

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

8

mechanism, Meso-scale structure theory

9

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

10

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

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

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

19

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

1

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

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

1

ACS Paragon Plus Environment

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

20

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

21

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

22

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

23

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

24

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

25

meso-scale structure theory.

26

1. Introduction

27

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

28

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

29

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

30

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

31

3

32

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

33

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

34

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

35

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

36

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

37

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

38

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

39

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

40

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

41

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

2


Page 2 of 27

Page 3 of 27

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

42

Energy & Fuels

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

43

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

44

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

45

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

46

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

47

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

48

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

49

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

50

MD simulations.8

51

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

52

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

53

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

54

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

55

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

56

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

57

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

58

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

59

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

60

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

61

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

62

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

63

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


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

Page 4 of 27

64

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

65

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

66

temperature have not been reported to our best knowledge.

67

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

68

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

69

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

70

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

71

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

72

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

73

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

74

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

75

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

76

simulations to investigate further reaction mechanisms during coal pyrolysis process.

77

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

78

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

79

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

80

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

81

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

82

advantage of the new features of VARxMD

83

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

84

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

85

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

34

are described in section 2.

4


Four pyrolysis stages

Page 5 of 27

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

Energy & Fuels

86

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

87

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

88

2. Method

89

2.1 ReaxFF MD simulation details

90

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

91

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

92

equation (1).35

93

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

(1)

94

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

95

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

96

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

97

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

98

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

99

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

100

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

101

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

102

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

103

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

104

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

105

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

106

(proximate and ultimate analysis,

107

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

13

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

5


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

108

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

109

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

110

found elsewhere.20

111

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

112

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

113

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

114

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

115

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

116

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

117

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

118

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

119

2.2 Chemical reactions analysis details

120

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

121

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

122

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

123

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

124

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

125

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

126

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

127

chemical reactions allows for uncovering the coal pyrolysis stages.

6


Page 6 of 27

Page 7 of 27

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

Energy & Fuels

128

3. Results and Discussions

129

Divided pyrolysis stages and the underlying cleavage of bridge bonds

130

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

131

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

132

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

133

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

134

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

135

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

136

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

137

roughly divided into three major stages.

138

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

139

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

140

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

141

configuration transformation are the major events observed accompanied with small radical

142

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

143

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

144

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

145

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

146

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

147

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

148

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

149

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


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

Page 8 of 27

150

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

151

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

152

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

153

and the recombination dominated stage.

154

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

155

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

156

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

157

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

158

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

159

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

160

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

161

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

162

are shown in Figure 1 (b).

163

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

164

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

165

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

166

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

167

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

168

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

169

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

170

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

171

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


46

The

Page 9 of 27

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

Energy & Fuels

172

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

173

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

174

condensation and cross-linking occur simultaneously at this stage.

175

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

176

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

177

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

178

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

179

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

180

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

181

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

182

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

183

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

184

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

185

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

186

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

187

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

188

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

189

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

190

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

191

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

192

the coal matrix.

193

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


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

Page 10 of 27

194

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

195

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

196

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

197

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

198

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

199

coal into liquid fuels.

200

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

201

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

202

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

203

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

204

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

205

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

206

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

207

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

208

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

209

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

210

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

211

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

3

Among them,

212

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

213

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

214

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

215

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


Page 11 of 27

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

Energy & Fuels

216

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

217

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

218

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

219

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

220

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

221

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

222

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

223

When temperature increases further, the violent competition between decomposition and

224

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

225

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

226

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

227

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

228

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

229

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

230

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

231

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

232

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

233

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

234

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

235

simulation.

236

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

237

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


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

238

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

239

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

240

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

241

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

242

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

243

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

244

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

245

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

246

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

247

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

248

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

249

criteria.

250

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

251

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

252

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

253

thermolysis.49,

254

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

255

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

256

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

257

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

258

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

259

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

50

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

12


Page 12 of 27

Page 13 of 27

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

Energy & Fuels

260

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

261

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

262

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

263

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

264

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

265

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

266

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

267

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

268

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

269

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

270

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

271

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

272

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

273

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

274

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

275

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

276

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

277

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

278

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

279

amount of radicals.

280

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

281

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


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

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


Page 14 of 27

Page 15 of 27

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

Energy & Fuels

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


Page 16 of 27

Page 17 of 27

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

Energy & Fuels

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


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

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.

390

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

391

2.

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


Page 18 of 27

Page 19 of 27

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

Energy & Fuels

Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Markham, J. R. Very Rapid Coal Pyrolysis. Fuel 1986, 65 (2),

392

3.

393

182−194.

394

4.

395

Pyrolysis - Experiment and Theory. Energy Fuels 1987, 1 (2), 138−152.

396

5.

397

Devolatilization. Energy Fuels 1988, 2 (4), 405−422.

398

6.

399

Fuels 1990, 4 (1), 42−54.

400

7.

Gavalvas, G. R. Coal Pyrolysis; Coal Science and Technology, Vol. 4; Elsevier Scientific: New York, 1982; pp 1−3.

401

8.

Li, X.; Mo, Z.; Liu, J.; Guo, L. Revealing Chemical Reactions of Coal Pyrolysis with GPU-enabled ReaxFF

402

Molecular Dynamics and Cheminformatics Analysis. Mol. Simulat. 2015, 41 (1−3), 13-27.

403

9.

404

Chemistry to Engineering. Nuclear Instruments & Methods In Physics Research Section B-Beam Interactions with

405

Materials And Atoms 2011, 269 (14), 1549−1554.

406

10. Kaledin, A. L.; van Duin, A. C. T.; Hill, C. L.; Musaev, D. G. Parameterization of Reactive Force Field: Dynamics

407

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

408

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

409

Am. Chem. Soc. 2012, 134 (6), 3042−3053.

410

12. Russo Jr, M. F.; Li, R.; Mench, M.; van Duin, A. C. T. Molecular Dynamic Simulation of Aluminum–Water

411

Reactions Using the ReaxFF Reactive Force Field. Int. J. Hydrogen Energy 2011, 36 (10), 5828−5835.

412

13. Agrawalla, S.; van Duin, A. C. T. Development and Application of a ReaxFF Reactive Force Field for Hydrogen

413

Combustion. J. Phys. Chem. A 2011, 115 (6), 960−972.

Serio, M. A.; Hamblen, D. G.; Markham, J. R.; Solomon, P. R. Kinetics of Volatile Product Evolution in Coal

Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. General-Model of Coal

Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Cross-Linking Reactions During Coal Conversion. Energy

Russo, M. F., Jr.; van Duin, A. C. T. Atomistic-Scale Simulations of Chemical Reactions: Bridging from Quantum

19


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

414

14. Castro-Marcano, F.; van Duin, A. C. T. Comparison of Thermal and Catalytic Cracking of 1-Heptene from ReaxFF

415

Reactive Molecular Dynamics Simulations. Combust. Flame 2013, 160 (4), 766−775.

416

15. Beste, A. ReaxFF Study of the Oxidation of Lignin Model Compounds for the Most Common Linkages in

417

Softwood in View of Carbon Fiber Production. J. Phys. Chem. A 2014, 118 (5), 803−814.

418

16. Liu, X.; Li, X.; Liu, J.; Wang, Z.; Kong, B.; Gong, X.; Yang, X.; Lin, W.; Guo, L. Study of High Density

419

Polyethylene (HDPE) Pyrolysis with Reactive Molecular Dynamics. Polym. Degrad. Stab. 2014, 104, 62−70.

420

17. Castro-Marcano, F.; Kamat, A. M.; Russo, M. F., Jr.; van Duin, A. C. T.; Mathews, J. P. Combustion of an Illinois

421

No. 6 Coal Char Simulated Using an Atomistic Char Representation and the ReaxFF Reactive Force Field. Combust.

422

Flame 2012, 159 (3), 1272−1285.

423

18. Castro-Marcano, F.; Russo, M. F., Jr.; van Duin, A. C. T.; Mathews, J. P. Pyrolysis of a Large-Scale Molecular

424

Model for Illinois No. 6 Coal Using the ReaxFF Reactive Force Field. J. Anal. Appl. Pyrolysis 2014, 109, 79−89.

425

19. Zheng, M.; Li, X.; Liu, J.; Guo, L. Initial Chemical Reaction Simulation of Coal Pyrolysis via ReaxFF Molecular

426

Dynamics. Energy Fuels 2013, 27 (6), 2942−2951.

427

20. Zheng, M.; Li, X.; Liu, J.; Wang, Z.; Gong, X.; Guo, L.; Song, W. Pyrolysis of Liulin Coal Simulated by

428

GPU-Based ReaxFF MD with Cheminformatics Analysis. Energy Fuels 2014, 28 (1), 522−534.

429

21. Zheng, M.; Wang, Z.; Li, X.; Qiao, X.; Song, W.; Guo, L. Initial Reaction Mechanisms of Cellulose Pyrolysis

430

Revealed by ReaxFF Molecular Dynamics. Fuel 2016, 177, 130−141.

431

22. Zhang, T.; Li, X.; Qiao, X.; Zheng, M.; Guo, L.; Song, W.; Lin, W. Initial Mechanisms for an Overall Behavior of

432

Lignin Pyrolysis through Large-Scale ReaxFF Molecular Dynamics Simulations. Energy Fuels 2016, 30 (4), 3140−3150.

433

23. Chen, B.; Diao, Z.-J.; Zhao, Y.-L.; Ma, X.-X., A ReaxFF Molecular Dynamics (MD) Simulation for the

434

Hydrogenation Reaction with Coal Related Model Compounds. Fuel 2015, 154, 114−122.

435

24. Salmon, E.; van Duin, A. C. T.; Lorant, F.; Marquaire, P.-M.; Goddard, W. A. III. Early Maturation Processes in 20


Page 20 of 27

Page 21 of 27

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

Energy & Fuels

436

Coal. Part 2: Reactive Dynamics Simulations Using the ReaxFF Reactive Force Field on Morwell Brown Coal Structures.

437

Org. Geochem. 2009, 40 (12), 1195−1209.

438

25. Qian, Y. A.; Zhan, J. H.; Lai, D. G.; Li, M. Y.; Liu, X. X.; Xu, G. W., Primary Understanding of Non-Isothermal

439

Pyrolysis Behavior for Oil Shale Kerogen Using Reactive Molecular Dynamics Simulation. Int. J. Hydrogen Energ. 2016,

440

41 (28), 12093−12100.

441

26. Chen, B.; Diao, Z.-j.; Lu, H.-y., Using the ReaxFF Reactive Force Field for Molecular Dynamics Simulations of the

442

Spontaneous Combustion of Lignite with the Hatcher Lignite Model. Fuel 2014, 116, 7−13.

443

27. Li, G.-Y.; Ding, J.-X.; Zhang, H.; Hou, C.-X.; Wang, F.; Li, Y.-Y.; Liang, Y.-H., ReaxFF Simulations of

444

Hydrothermal Treatment of Lignite and Its Pmpact on Chemical Structures. Fuel 2015, 154, 243−251.

445

28. Zhan, J.-H.; Wu, R.; Liu, X.; Gao, S.; Xu, G., Preliminary Understanding of Initial Reaction Process for

446

Subbituminous Coal Pyrolysis with Molecular Dynamics Simulation. Fuel 2014, 134, 283−292.

447

29. Hong, D.-k.; Shu, H.-k.; Guo, X.; Zheng, C.-g., Molecular Dynamics Simulations Study of Brown Coal Pyrolysis

448

Using ReaxFF Method. In Clean Coal Technology and Sustainable Development: Proceedings of the 8th International

449

Symposium on Coal Combustion, Yue, G.; Li, S., Eds. Springer Singapore: Singapore, 2016, pp 59−67.

450

30. Zhang, J.; Weng, X.; Han, Y.; Li, W.; Cheng, J.; Gan, Z.; Gu, J., The Effect of Supercritical Water on Coal Pyrolysis

451

and Hydrogen Production: A Combined ReaxFF and DFT study. Fuel 2013, 108, 682−690.

452

31. Wang, H.; Feng, Y.; Zhang, X.; Lin, W.; Zhao, Y., Study of Coal Hydropyrolysis and Desulfurization by ReaxFF

453

Molecular Dynamics Simulation. Fuel 2015, 145, 241−248.

454

32. Li, W.; Zhu, Y.-m.; Wang, G.; Wang, Y.; Liu, Y., Molecular Model and ReaxFF Molecular Dynamics Simulation of

455

Coal Vitrinite Pyrolysis. J. Mol. Model 2015, 21 (8), 188.

456

33. Bhoi, S.; Banerjee, T.; Mohanty, K., Molecular Dynamic Simulation of Spontaneous Combustion and Pyrolysis of

457

Brown Coal Using ReaxFF. Fuel 2014, 136, 326−333. 21


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

458

34. Liu, J.; Li, X.; Guo, L.; Zheng, M.; Han, J.; Yuan, X.; Nie, F.; Liu, X. Reaction Analysis and Visualization of

459

ReaxFF Molecular Dynamics Simulations. J. Mol. Graphics Modell. 2014, 53, 13−22.

460

35. van Duin, A. C. T.; Dasgupta, S.; Lorant, F.; Goddard, W. A. ReaxFF: A Reactive Force Field for Hydrocarbons. J.

461

Phys. Chem. A 2001, 105 (41), 9396−9409.

462

36. Brenner, D. W. Empirical Potential for Hydrocarbons for Use in Simulating the Chemical Vapor-Deposition of

463

Diamond Films. Phys. Rev. B 1990, 42 (15), 9458−9471.

464

37. Brenner, D. W.; Shenderova, O. A.; Harrison, J. A.; Stuart, S. J.; Ni, B.; Sinnott, S. B. A Second-Generation

465

Reactive Empirical Bond Order (REBO) Potential Energy Expression for Hydrocarbons. J. Phys-Condens. Mat. 2002, 14

466

(4), 783−802.

467

38. Rappe, A. K.; Goddard, W. A. III. Charge Equilibration for Molecular-Dynamics Simulations. J. Phys. Chem. 1991,

468

95 (8), 3358−3363.

469

39. Chenoweth, K.; van Duin, A. C. T.; Goddard, W. A. III. ReaxFF Reactive Force Field for Molecular Dynamics

470

Simulations of Hydrocarbon Oxidation. J. Phys. Chem. A 2008, 112 (5), 1040−1053.

471

40. Mueller, J. E.; van Duin, A. C. T.; Goddard, W. A. III. Application of the ReaxFF Reactive Force Field to Reactive

472

Dynamics of Hydrocarbon Chemisorption and Decomposition. J. Phys. Chem. C 2010, 114 (12), 5675−5685.

473

41. Zheng, M.; Li, X.; Guo, L. Algorithms of GPU-Enabled Reactive Force Field (ReaxFF) Molecular Dynamics. J.

474

Mol. Graphics Modell. 2013, 41, 1−11.

475

42. Mattsson, T. R.; Lane, J. M. D.; Cochrane, K. R.; Desjarlais, M. P.; Thompson, A. P.; Pierce, F.; Grest, G. S.

476

First-Principles and Classical Molecular Dynamics Simulation of Shocked Polymers. Phys. Rev. B 2010, 81 (5). 101.

477

43. Given, P. H.; Marzec, A.; Barton, W. A.; Lynch, L. J.; Gerstein, B. C. The Concept of a Mobile or Molecular Phase

478

within the Macromolecular Network of Coals: A Debate. Fuel 1986, 65 (2), 155−163.

479

44. Boudou, J. P.; Espitalie, J.; Bimer, J.; Salbut, P. D. Oxygen Groups and Oil Suppression during Coal Pyrolysis. 22


Page 22 of 27

Page 23 of 27

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

Energy & Fuels

480

Energy Fuels 1994, 8 (4), 972−977.

481

45. Chatterjee, K.; Stock, L. M.; Zabransky, R. F., The Pathways for Thermal-Decomposition of Aryl Alkyl Ethers

482

during Coal Pyrolysis. Fuel 1989, 68 (10), 1349−1353.

483

46. Hodek, W.; Kirschstein, J.; van Heek, K.-H. Reactions of Oxygen Containing Structures in Coal Pyrolysis. Fuel

484

1991, 70 (3), 424−428.

485

47. Miura, K., Mild Conversion of Coal for Producing Valuable Chemicals. Fuel Process. Technol. 2000, 62 (2–3),

486

119−135.

487

48. Li, J.; Ge, W.; Wang, W.; Yang, N.; Huang, W. Focusing on Mesoscales: From the Energy-Minimization Multiscale

488

Model to Mesoscience. Current Opinion in Chemical Engineering 2016, 13, 10−23.

489

49. Gavalas, G. R.; Cheong, P. H. K.; Jain, R. Model of Coal Pyrolysis .1. Qualitative Development. Ind. Eng. Chem.

490

Fund. 1981, 20 (2), 113−122.

491

50. Gavalas, G. R.; Jain, R.; Cheong, P. H. K. Model of Coal Pyrolysis .2. Quantitative Formulation and Results. Ind.

492

Eng. Chem. Fund. 1981, 20 (2), 122−132.

493 494

23


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

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

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

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

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

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

506 507

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

508

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

509

using VARxMD

26


Page 27 of 27

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

Energy & Fuels

510 511

a

512 513

b

514

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

515

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

516

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

517 518

a

b

519

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

520

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

521

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


Investigation of overall pyrolysis stages for Liulin bituminous coal by

Recommend Documents