煤中的小分子物质是以游离或镶嵌的形式赋存在煤大分子结构中并且分子量低于500的有机化合物，其对煤化学性质和物理性质都有重要影响。煤层气开发是集能源利用、环境保护和煤矿安全于一体的新兴能源产业，近年来备受关注。煤层气中主要成分甲烷基本以吸附态赋存在煤结构。探索小分子物质对煤吸附甲烷的影响，对查明煤中甲烷富集机理和赋存状态具有重要的理论意义，也可为煤层气资源勘探与开发及矿井瓦斯灾害防治提供科学依据。

      本文以神府煤为研究对象，使用NMP-CS₂萃取和去离子水反萃取法分离富集了神府煤中主要小分子物质，采用正己烷、苯和四氢呋喃对富集小分子物质的反萃取物进行逐级萃取实现小分子物质的组分分离，通过GC/MS测试了不同类型小分子物质的组成，研究了小分子物质萃取前后煤孔结构的变化特征。通过向萃余煤中加入不同种类的小分子模型化合物，研究了小分子物质对煤结构吸附甲烷的影响。结果表明：NMP-CS₂混合溶剂对神府煤样进行萃取的最大萃取率为14.2%。煤中小分子物质主要包含脂肪烃类（31.01%）、芳烃类（12.1%）和杂原子类（51.71%）；其中，脂肪烃类小分子主要是正构烷烃及少量烯烃，芳烃类主要是含有一个苯环的芳香烃和少量萘和蒽等，杂原子类主要是含氧和含氮化合物。样品对甲烷吸附量大小顺序依次为：原煤 > 添加芳烃类小分子的萃余煤 > 添加烷烃类小分子的萃余煤 > 添加杂原子类小分子的萃余煤 > 萃余煤。含烷烃、芳烃和杂原子三类小分子的反萃取物吸附甲烷量最大，随着烷烃和芳烃的去除，小分子物质对甲烷的吸附能力逐渐降低。证实了小分子物质对煤吸附甲烷有促进作用，其中芳烃类小分子的促进作用最大，杂原子类小分子促进作用最小。

       利用分子模拟方法构建并优化了含有不同种类小分子的5个煤晶胞模型，研究了甲烷分子在煤微孔中的吸附状态。模拟结果表明：小分子对煤吸附甲烷有促进作用，其中芳烃类小分子的促进作用最大，杂原子类小分子最小。当孔径小于0.85 nm时，甲烷在微孔中的吸附呈微孔填充机制。随着微孔孔径的增大，煤对甲烷的吸附能力也增强。小分子的加入增加了煤的比表面积，扩大了煤中孔径。添加芳烃类小分子可以提高可进入孔的占比，而添加杂原子类小分子会降低可进入孔的比例。径向分布函数计算结果表明各元素对甲烷吸附的强弱顺序依次：N > O > C > H。模拟结果和实验测试有很好的一致性。

       实验和模拟结果揭示了煤中小分子物质增强煤吸附甲烷吸附能力的微观机理。其中芳烃和烷烃类小分子增强甲烷吸附能力主要是通过扩大煤中微孔的孔径，增加微孔的数量来实现，而杂原子类小分子增强甲烷吸附能力是通过增强煤分子与甲烷分子相互作用来实现，这两种方式共同作用，增强小分子对煤甲烷吸附的促进作用。

       The small molecular substances in coal are organic compounds with a molecular weight of less than 500 that exist in the macromolecular structure of coal in a free or inlaid form, which have important effects on the chemical and physical properties of coal. CBM development is an emerging energy industry that integrates energy utilization, environmental protection, and coal mine safety, and has attracted much attention in recent years. Methane, the main component of coalbed methane, is basically present in the coal structure in an adsorbed state. Exploring the influence of small molecular substances on coal adsorption of methane has important theoretical significance for identifying the enrichment mechanism and occurrence status of methane in coal, and can also provide scientific basis for the exploration and development of coalbed methane resources and the prevention and control of mine gas disasters.

        In this paper, the main small molecular substances in Shenfu coal were separated and enriched by NMP-CS₂ extraction and deionized water back extraction, and the small molecular substances were separated by step extraction with n-hexane, benzene and tetrahydrofuran. The change characteristics of pore structure of coal before and after extraction. By adding different kinds of small molecular model compounds to the raffinate coal, the effect of small molecular substances on the adsorption of methane by coal structure was studied. The results showed that the maximum extraction rate of NMP-CS₂ was 14.2%. The small molecular substances in coal mainly include aliphatic hydrocarbons (31.01%), aromatics (12.1%) and heteroatoms (51.71%); among them, the small molecules of aliphatic hydrocarbons are mainly n-alkanes and a small amount of olefins, aromatics are mainly aromatic hydrocarbons with a benzene ring and a small amount of naphthalene and anthracene, and heteroatoms are mainly oxygen-containing and nitrogen-containing compounds. The order of methane adsorption capacity is as follows: raw coal > raffinate coal with aromatic small molecules > raffinate coal with alkane small molecules > raffinate coal with heteroatom small molecules > raffinate coal. With the removal of alkanes and aromatics, the adsorption capacity of small molecules to methane gradually decreased. It is confirmed that small molecules can promote the adsorption of methane by coal, among which aromatic small molecules have the largest promotion effect and heteroatom small molecules have the smallest promotion effect.

        Five coal cell models containing different kinds of small molecules were constructed and optimized by molecular simulation method, and the adsorption state of methane molecules in coal micropores was studied. The simulation results show that small molecules can promote the adsorption of methane by coal, among which aromatic molecules have the largest promotion and heteroatom molecules have the smallest. When the pore size is less than 0.85 nm, the adsorption of methane in the micropores presents a micropore filling mechanism. With the increase of pore size, the adsorption capacity of coal to methane also increases. The addition of small molecules increases the specific surface area of coal and expands the pore size of coal. The addition of aromatic small molecules can increase the proportion of accessible pores, while the addition of heteroatom small molecules can reduce the proportion of accessible pores. The results of radial distribution function calculation show that the order of methane adsorption is N > O > C > H. The simulation results are in good agreement with the experimental results.

        The experimental and simulation results reveal the mechanism of the small and medium molecular matter in coal to enhance the adsorption capacity of methane. Among them, aromatics and alkanes molecules enhance the adsorption capacity of methane mainly by expanding the pore size of micropores in coal and increasing the number of micropores. The heteroatom molecules enhance the adsorption capacity of methane by enhancing the interaction between coal molecules and methane molecules. These two ways work together to enhance the promotion of small molecules on the adsorption of coal methane.

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