Mechanism of organic Ca transformation during coal hydropyrolysis: insights from ReaxFF molecular dynamics simulations

doi:10.11949/0438-1157.20250054

Abstract

Abstract:

To reveal the migration and transformation mechanisms of organic Ca in coal during hydropyrolysis, this study constructed a macromolecular structural model of coal containing carboxylic acid-bound Ca. The reactive force field molecular dynamics (ReaxFF MD) simulations were employed to track the atomic-level trajectory of Ca during hydropyrolysis under different heating modes. Hydropyrolysis experiments were conducted by using Ca-loading treatment, and the content of Ca in the resulting products was analyzed by using an inductively coupled-plasma optical emission spectroscopy (ICP-OES). The simulation results show that during the heating simulation process, organic Ca migrates to inorganic Ca, gaseous hydrocarbon Ca, and tar Ca at high temperature. Atomic Ca is challenging to release directly from char and readily combines with generated H₂O molecules. Under constant-temperature simulations, as the temperature increases, the migration of organic Ca intensifies, with substantial conversion of coke-associated Ca into inorganic Ca and Ca in gaseous hydrocarbons. At lower temperatures (1600—2200 K), organic Ca requires more time to transition, whereas at higher temperatures (2500—2800 K), this process accelerates significantly. By analyzing the evolution of Ca in the pyrolysis experiment, the simulation results are preliminarily verified. In addition, the effect of macerals on the migration behavior of organic Ca was also discussed. Vitrinite, containing abundant aliphatic chains, undergoes thermal decomposition to generate more small molecular fragments capable of binding with Ca. In contrast, inertinite exhibits relatively weaker organic calcium binding capacity due to its compact aromatic structure and limited active sites available for interaction. Ultimately, the transition mechanism of organic Ca during hydropyrolysis was elucidated: organic Ca exhibits strong binding capacity and readily combines with small molecules after decomposition. It is predominantly converted into inorganic small molecules, followed by gaseous hydrocarbons and tar molecules. Additionally, Ca migrates and transforms between these forms under high temperatures. These results can provide scientific reference and theoretical support for pollution control and product optimization of high alkali coal in the low and medium temperature pyrolysis industry.

Key words: organic Ca, Zhundong coal, hydropyrolysis, migration and transformation, molecular simulation, experimental validation

摘要：

为揭示煤中有机Ca在煤加氢热解过程中的迁移转化机制，构建了含羧酸Ca的煤大分子结构模型，采用反应分子动力学（ReaxFF MD）模拟手段，基于不同升温模式从原子水平追踪了Ca在煤加氢热解过程中的运动轨迹；通过人为加载Ca型煤进行加氢热解实验，采用电感耦合等离子体光谱仪（ICP-OES）测定了产物中Ca元素的含量。模拟结果表明，在升温模拟过程中，有机Ca在高温下向无机Ca、气态烃Ca、焦油Ca迁变。原子Ca很难单独从焦炭中释放，并且极易和生成的H₂O分子结合。在恒温模拟过程中，随着温度的升高，煤加氢热解过程中有机Ca迁变行为加剧，焦炭Ca大量向无机Ca和气态烃中的Ca转化。在较低温度下（1600~2200 K），需要持续一定时间有机Ca才会发生迁变，而在较高温度下（2500~2800 K）相应发生迁变过程需要的时间更短。通过分析热解实验中Ca的演化规律，初步验证模拟结果。此外，还探讨了显微组分对有机Ca迁移行为的影响，镜质体含有较多脂肪链，能热分解更多小分子碎片与Ca结合，而惰质体因芳香结构致密且活性位点较少，有机Ca的结合能力略弱。最终，从原子水平揭示了有机Ca在煤加氢热解的迁变机制：有机Ca的结合能力很强，极易与分解后的小分子物质结合。有机Ca可以直接向无机小分子、气态烃与焦油分子中转化。其中，Ca首先倾向于进入无机小分子，其次是气态烃，最后是焦油。同时，赋存在三者之间的Ca会在高温下持续迁移转化。研究结果可为高碱煤在中低温热解产业中污染控制与产物优化提供科学参考和理论支持。

关键词: 有机Ca, 准东煤, 加氢热解, 迁移转化, 分子模拟, 实验验证

CLC Number:

TQ 530.2

Xiaoling WANG, Shaoqing WANG, Yungang ZHAO, Fangzhe CHANG, Ruifeng MU. Mechanism of organic Ca transformation during coal hydropyrolysis: insights from ReaxFF molecular dynamics simulations[J]. CIESC Journal, 2025, 76(8): 4297-4309.

王小令, 王绍清, 赵云刚, 常方哲, 穆瑞峰. 基于ReaxFF MD模拟的煤加氢热解有机Ca转化机制研究[J]. 化工学报, 2025, 76(8): 4297-4309.

Figures/Tables 15

Table 1 The proximate analysis， ultimate analysis， and mean maximum vitrinite reflectance （Ro） of samples

样品	工业分析/%			R_o/%	元素分析/%
样品	M_ad	A_ad	V_daf	R_o/%	C_daf	H_daf	O_daf^①	N_daf	S_t,d
ZDV	15.14	2.93	48.57	0.54	72.92	4.27	21.57	0.61	0.63
ZDI	14.33	3.95	31.73	0.37	78.66	3.10	17.01	0.63	0.60

Fig.1 Image of various macerals of coal samples

Table 2 Content of Ca in loading Ca experiment and molecular model of coal

样品	Ca的含量/%
样品	实验加载Ca	模型中的Ca
ZDV-Ca	1.98	1.53
ZDI-Ca	2.02	1.61

Fig.2 Aggregate structural model of coal containing calcium-system (ZDV-Ca and ZDI-Ca, containing 600 H2 molecules)

Fig.3 ReaxFF MD simulation of coal hydropyrolysis in containing Ca systems

Fig.4 Relative content of different forms of Ca during hydropyrolysis

Fig.5 The reaction pathways of atomic Ca in hydropyrolysis process of ZDV-Ca

Fig.6 The reaction pathways of H2OCa in hydropyrolysis process of ZDV-Ca

Fig.7 The reaction pathways of H2OCa in hydropyrolysis process of ZDI-Ca

Fig.8 Time evolution of different forms of Ca concentrations during the ReaxFF MD simulation

Fig.9 The distribution of H2OCa during the ReaxFF MD simulation

Fig.10 The distribution of atomic Ca during the ReaxFF MD simulation

Table 3 The content of Ca in char and tar products of ZDV-Ca hydropyrolysis at 500， 600， 700℃

样品与产物	Ca的含量/(mg/kg)
样品与产物	Char（焦炭基准）	Char（原煤基准）	Tar（原煤基准）
ZDV-Ca	19716.5	19716.5	—
ZDV-Ca-500	27262.0	17338.6	43.3
ZDV-Ca-600	27313.2	15732.4	2.0
ZDV-Ca-700	33422.3	17780.7	57.2

Fig.11 The content of different forms and the extraction rate of Ca in coal hydropyrolysis experimental char

Fig.12 The migration and transformation pathways of organic Ca to inorganic Ca

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