宁东煤热解初始段硫迁移的分子动力学模拟

Molecular dynamics simulation of sulfur migration in the initial pyrolysis period of Ningdong coal

为探究硫元素在煤热解过程中的迁移规律和污染物的形成,以宁东煤为研究对象,采用X射线光电子能谱、^(13)C固体核磁等表征手段结合工业分析、元素分析对宁东煤内部元素种类、化学价态以及成键方式进行分析,构建了2种含硫低阶宁东煤干燥无灰基下的化学结构模型。在Material Studio软件中经过分子动力学退火模拟和几何结构优化得到了立体结构明显的化学结构,保证了每个模型盒子中有4个硫原子,以正确归纳煤热解过程硫的迁移路径和产物。采用ReaxFF力场在参考温度2500 K下进行200 ps的分子动力学模拟,发现热解产物分布与参考模拟结果吻合较好,证明了模型构建的合理性。通过归纳硫醇和硫酚存在形式的S原子分别在煤热解过程初段的转变特性,发现2种存在形式的硫原子均进入焦油产物中,但具有不同的迁移速率和稳定链结构。这属于硫在煤热解初始阶段的迁移,可解释实际低温煤热解时煤焦油中含硫较高的现象。需提高模拟参考温度或延长计算时间,才能确定煤热解过程硫迁移的二次反应行为。

In order to explore the migration law of sulfur element and the pollutant formation in the process of coal pyrolysis,and taking Ningdong coal as the research object,X-ray photoelectron spectroscopy,^(13)C nuclear magnetic resonance(^(13)C-NMR),industrial analysis and element analysis were used to analyze the internal element types,chemical valence states and bonding modes of Ningdong coal,and two types of low rank chemical structure models of Ningdong coal which contains sulfur but no ash and water content were constructed.After molecular dynamics annealing simulation and geometry structure optimization in Material Studio software,the chemical structure with obvious stereoscopic structure was obtained,which ensured that there were four sulfur atoms in the model box,to correctly summarize the sulfur migration path and productions during the coal pyrolysis process.After that,ReaxFF force field was used to simulate in 200 ps molecular dynamics at reference temperature of 2500 K.It is found that the distribution of pyrolysis products is in good agreement with the reference simulation results,which proves the rationality of the model.Thus,the transformation characteristics of sulfur atoms in the form of mercaptan and thiophenol during initial period of coal pyrolysis was summarized.Although both types sulphur eventually migrate to tar products,there have different migration rates and stable chain structures.The migration of sulfur in the initial stage of coal pyrolysis corresponds to the reason of high sulfur content in coal tar during the actual low temperature coal pyrolysis.Increasing the simulation reference temperature or extending the calculation time is required to determine the deeper secondary reaction behavior of the sulfur migration during coal pyrolysis process.