基于分子动力学的焦炭溶损反应及其机理研究

doi:10.11949/0438-1157.20230207

摘要/Abstract

摘要：

为从分子层面研究焦炭溶损反应及其机理，借助分子动力学和第一性原理构建了一种包含石墨化碳和非石墨化碳的焦炭分子模型，研究了电荷密度和孔径大小对焦炭溶损反应及其机理的影响，并通过实验对模拟结果加以验证。结果表明，所建模型真密度在焦炭实测密度范围内；焦炭石墨化程度越高，碳原子周边电荷密度越小，其溶损反应程度越低，实验结果与模拟结果相近；CO₂在孔径大于50 Å（1 Å=1×10^-10 m）气孔内以扩散为主，其扩散表观活化能最小，为131.24 kJ/mol；在孔径小于2 Å气孔内以吸附为主，其吸附能的绝对值最大，为192.54 kJ/mol；焦炭溶损反应机理包括焦炭分子生成石墨化碳和非石墨化碳原子的裂解反应、碳原子之间的异构化反应以及碳原子与CO₂的氧化反应，在氧化反应过程中，碳原子与CO₂反应生成的烯酮式结构会因其稳定性差裂解成CO，完成反应过程。

关键词: 焦炭, 溶损反应, 机理, 分子模拟, 电荷密度, 孔径, 吸附

Abstract:

In order to study the coke dissolution loss reaction and its mechanism at the molecular level, a molecular model of coke containing graphitized carbon and non-graphitized carbon was constructed by molecular dynamics and first principles. The accuracy of the model was verified by comparing the true density of the molecular model. The influence of charge density and pore structure on the coke dissolution loss reaction was analyzed, and the mechanism of the reaction was also discussed. The results show that the true density of the model is within the range of the measured true density of coke. The higher the graphitization degree of coke, the smaller the charge density around carbon atoms, and the lower the degree of dissolution loss reaction. The experimental results are similar to the simulation results. CO₂ is mainly diffused in pores above 50 Å, with the lowest diffusion activation energy (131.24 kJ /mol), while it is mainly adsorbed in pores below 2 Å, with the highest adsorption energy (-192.54 kJ/mol). The mechanism of coke dissolution loss reaction includes the cracking reaction of graphitized carbon and non-graphitized carbon atoms, the isomerization reaction between two kinds of carbon atoms and the oxidation reaction between two kinds of carbon atoms and CO₂. In the process of oxidation reaction, the ketene structure generated by the reaction of two kinds of carbon atom and CO₂ will be cracked into CO due to its instability, which completes the reaction process.

Key words: coke, dissolution loss reaction, mechanism, molecular simulation, charge density, pore diameter, adsorption

中图分类号:

TQ 526.1

陈吉, 洪泽, 雷昭, 凌强, 赵志刚, 彭陈辉, 崔平. 基于分子动力学的焦炭溶损反应及其机理研究[J]. 化工学报, 2023, 74(7): 2935-2946.

Ji CHEN, Ze HONG, Zhao LEI, Qiang LING, Zhigang ZHAO, Chenhui PENG, Ping CUI. Study on coke dissolution loss reaction and its mechanism based on molecular dynamics simulations[J]. CIESC Journal, 2023, 74(7): 2935-2946.

图/表 17

表1 配煤及基本性质

Table 1 Schemes of blending coal and quality of blending coal

样品编号	JM/kg	FM/kg	1/3JM/kg	SM/kg	QM/kg	A_d/%	V_daf/%	S_t,d/%	G	Y/mm
c1	28	11	17	7	7	9.30	28.27	0.83	80	16.4
c2	28	11	15	7	9	9.42	28.85	0.84	79	16.1
c3	28	11	13	7	11	9.61	29.34	0.86	76	15.6
c4	28	11	10	7	14	9.74	29.78	0.89	77	15.2

表2 四种焦炭的基本性质

Table 2 Basic properties of four kinds of coke

样品编号	性质/%（质量分数）
样品编号	A_d	V_daf	M₄₀	M₁₀	CRI	CSR
c1	12.59	1.06	78.2	6.3	22.9	60.7
c2	12.61	1.08	76.7	6.5	23.3	60.2
c3	12.65	1.07	75.2	6.7	23.8	59.9
c4	12.73	1.05	74.3	6.7	24.3	59.4

表3 四种焦炭的元素分析

Table 3 Element analysis of four coke

样品编号	含量/%（质量分数）
样品编号	C_d	H_d	O_d^①	N_d	S_d	P_d
c1	97.04	0.56	0.49	1.11	0.79	0.01
c2	97.18	0.49	0.54	0.99	0.78	0.02
c3	97.20	0.51	0.50	1.02	0.76	0.01
c4	97.17	0.54	0.48	1.04	0.75	0.02

图1 焦炭XPS谱图

Fig.1 XPS spectrum of coke

图2 不同碳原子电荷密度的分子结构

Fig.2 Molecular structure with different carbon atom charge densities

图3 焦炭分子模型

Fig.3 Coke molecular model

表4 分子模型、焦炭中石墨化碳原子与非石墨化碳原子含量对比

Table 4 Contents of graphitized carbon atoms and non-graphitized carbon atoms in the model and coke

项目	石墨化碳	非石墨化碳
实验值	72.1%	27.9%
模拟值	71.5%	28.5%
误差	-0.83%	2.15%

图4 焦炭模型对CO2的稳定吸附构型

Fig.4 Stable adsorption configurations of CO2 on the proposed coke model

图5 三种不同气孔的模型

Fig.5 The models of three different pores

表5 三种模型的气孔大小与分布

Table 5 Pore size distribution of three models

类别	0~5 Å/%	5~10 Å/%	10~15 Å/%	15~25 Å/%
5A	54.21	45.79	0	0
5B	61.79	33.21	3.57	1.43
5C	65.61	25.39	5.11	3.89

图6 CO2在三种模型内的扩散系数

Fig.6 Diffusion coefficient of CO2 in the three models

图7 三种模型的应力-应变曲线

Fig.7 Stress-strain curves of three models

表6 焦炭溶损反应机理

Table 6 Reaction mechanisms of coke solution loss

基元反应	频率
cokesp³C	104
cokesp²C	67
sp³C—sp³C 2sp³C	89
2sp³C sp²C sp²C	7
sp²C sp²C sp³C + sp²C	8
sp³C + CO₂ sp³CO + CO	38
sp²C + CO₂ sp²CO + CO	11
sp³C + CO₂ sp³CCO	94
sp²C + CO₂ sp²CCO	36
sp³CCO sp³C + CO	92
sp²CCO sp²C + CO	31

图8 焦炭溶损反应机理

Fig.8 Reaction mechanisms of coke solution loss

图9 四种焦炭XRD谱图

Fig.9 XRD patterns of four coke samples

表7 焦炭的晶格参数

Table 7 Lattice parameters of coke

样品	状态	2θ₀₀₂/(^o)	d₀₀₂/nm	L_a/nm	L_c/nm	石墨化度
c1	反应前	26.41	0.34	5.81	3.07	0.62
c1	失重30%	26.38	0.34	6.19	3.01	0.71
c2	反应前	26.03	0.34	5.76	2.62	0.61
c2	失重30%	26.26	0.34	5.56	2.68	0.69
c3	反应前	26.30	0.34	6.23	3.46	0.63
c3	失重30%	26.37	0.34	6.07	3.58	0.71
c4	反应前	26.22	0.34	5.01	2.46	0.60
c4	失重30%	26.27	0.34	5.31	2.62	0.72

图10 四种焦炭孔径分布

Fig.10 Pore size distribution of four coke samples

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