Molecular model and pyrolysis simulation of Zaoquan coal

doi:10.11949/j.issn.0438-1157.20181218

Abstract

Abstract:

This work studied the macromolecular structure model of Zaoquan coal from Ningdong, China by means of various characterizations such as industrial analysis, elemental analysis, X-ray photoelectron spectroscopy and ¹³C nuclear magnetic resonance (NMR), combined with computer-aided techniques. After annealing dynamics simulation and fully geometric structural optimizations, the bond length, bond angle and the spatial configuration of coal molecular structure were changed significantly compared with the initial one. Also the arrangement mode of the aromatic layers became nearly parallel. The calculated spectra of Fourier transform infrared and ¹³C NMR agreed well with those in experiments, which further confirmed the obtained coal molecular model. Based on the molecular model, the effects of final temperature and heating rate upon chemical behavior of coal pyrolysis were studied by using the reactive force field molecular dynamics simulations. It was shown that the reaction rate was gradually increased as temperature increased. The heating rate was rather significant for gas generation during coal pyrolysis. In simulations, most of the fragments produced were below C₁₅, while the species of macromolecules were the minority. As heating rate increased, the gas, liquid and solids products were all decreased. In addition, according to the results of reaction molecular dynamics simulation, the formation mechanism of CO₂ in the pyrolysis process was traced, and three different CO₂ formation paths were obtained.

Key words: coal, model, quantum chemical calculation, molecular modeling, pyrolysis

摘要：

以宁东枣泉煤为研究对象，使用工业分析、元素分析、X射线光电子能谱、¹³C固体核磁等表征手段和计算机辅助，构建获得枣泉煤大分子结构模型。经过分子动力学退火动力学模拟和几何结构全优化，与初始结构相比键长、键角发生明显改变，立体构型显著，芳香层片之间近似平行的排列方式明显。获得的傅里叶变换红外和¹³C固体核磁的实验与计算谱图总体吻合较好，进一步证明了构建模型的合理性。使用反应分子动力学方法模拟枣泉煤的热解过程，考察不同热解终温和升温速率对热解行为的影响。结果发现，随着温度的升高，反应速率逐渐加快。不同升温速率对枣泉煤热解过程中气体的产生有显著影响。在动力学模拟中大多产生C₁₅以下的碎片，大分子的种类则并不多。随着升温速率的增加，气、液、固三相产物整体上都呈现下降的趋势。此外，还根据反应分子动力学模拟结果追踪了热解过程中CO₂的形成机理，获得了三种不同的CO₂形成路径。

关键词: 煤, 模型, 量子化学计算, 分子模拟, 热解

CLC Number:

TQ 530

Wei FENG, Hongfeng GAO, Gui WANG, Langlang WU, Jingqin XU, Zhuangmei LI, Ping LI, Hongcun BAI, Qingjie GUO. Molecular model and pyrolysis simulation of Zaoquan coal[J]. CIESC Journal, 2019, 70(4): 1522-1531.

冯炜, 高红凤, 王贵, 吴浪浪, 许靖钦, 李壮楣, 李平, 白红存, 郭庆杰. 枣泉煤分子模型构建及热解的分子模拟[J]. 化工学报, 2019, 70(4): 1522-1531.

Figures/Tables 12

Table 1 Physicochemical properties of coal sample

Proximate analysis/%			Ultimate analysis/%					Content/%(mass)
M_ad	A_ad	V_daf	C_daf	H_daf	O_daf	N_daf	S_daf	Vitrinite	Exinite	Inertinite
6.58	2.24	25.49	72.81	3.85	20.31	0.83	0.35	29.00	0.00	71.00

Table 2 Atomic ratio of coal sample

H/C	O/C	N/C	S/C
0.63	0.21	0.01	0.00

Fig.2 13C NMR spectra of coal sample

Table 3 Percentage of structural parameters of coal sample

Sample	f_a	f_a ^C	f_a ^＇	f_a ^N	f_a ^H	f_a ^P	f_a ^S	f_a ^B	f_al	f_al ^*	f_al ^H	f_al ^O
ZQ	73.84%	3.37%	70.47%	29.75%	40.72%	7.73%	6.48%	15.54%	26.16%	10.44%	9.98%	5.74%

Fig.3 XPS spectra of coal sample

Table 4 XPS C 1s,O 1s,N 1s data of coal sample

Elemental peak	Functionality	Binding energy/eV	Molar content/%
C 1s	C—C, C—H	284.43	38.14
	C—O	285.67	36.50
	C $?$ O	286.79	18.97
	COO—	289.75	6.39
O 1s	inorg oxygen	530.58	2.93
	C $?$ O	531.84	24.07
	C—O ^-	533.06	58.84
	COO ^-	534.60	9.75
	adsorbed oxygen	536.04	4.41
N 1s	pyridinic nitrogen	399.10	23.00
	pyrrolic nitrogen	400.37	39.12
	quatemary nitrogen	401.48	24.72
	oxidized nitrogen	403.32	13.16

Table 4 XPS C 1s,O 1s,N 1s data of coal sample

Elemental peak	Functionality	Binding energy/eV	Molar content/%
C 1s	C—C, C—H	284.43	38.14
	C—O	285.67	36.50
	C $?$ O	286.79	18.97
	COO—	289.75	6.39
O 1s	inorg oxygen	530.58	2.93
	C $?$ O	531.84	24.07
	C—O ^-	533.06	58.84
	COO ^-	534.60	9.75
	adsorbed oxygen	536.04	4.41
N 1s	pyridinic nitrogen	399.10	23.00
	pyrrolic nitrogen	400.37	39.12
	quatemary nitrogen	401.48	24.72
	oxidized nitrogen	403.32	13.16

Fig.4 3D model of coal molecular structure

Fig.5 Experimental and calculated FT-IR (a) and 13C NMR (b) spectra of coal sample

Fig.6 Time evolution of potential energy during coal pyrolysis at various temperature conditions (1cal=4.18 J)

Fig.7 Various products of coal pyrolysis at different temperature

Fig.8 Various products of coal pyrolysis with different heating rate

Table 5 Formation mechanism of CO2 in coal pyrolysis simulation

Path	Formation reaction
1
2
3

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