Gao Fei, Jia Zhe, Shan Ya-Fei, Teng Yifei, Li Ying-di, Pu Xin-Ge
School of Safety Science and Engineering, Liaoning Technical University, Fuxin123008, China.
ACS Omega. 2022 Oct 26;7(44):39830-39839. doi: 10.1021/acsomega.2c03824. eCollection 2022 Nov 8.
The low-temperature oxidation spontaneous combustion of coal was caused by the active groups in its structure. The oxidation mechanism of carbon and oxygen functional groups in coal had been extensively studied, but there were few reports on the study of sulfur functional groups initiating the coal spontaneous combustion. To investigate the influence of organic sulfur functional groups on the spontaneous combustion of high-sulfur coal and explore its transformation characteristics, the low-temperature oxidation experimental system was used to study the spontaneous combustion tendency of coal with similar metamorphic degrees and different organic sulfur contents. The variations of element forms and organic sulfur functional groups were analyzed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy during low-temperature oxidation of coal and model compounds. The results showed that the forms of organic sulfur in coal mainly included mercaptans, thioethers, thiophenes, sulfones (sulfoxides), and sulfates, and the low-temperature oxidation of coal was not only related to the content of organic sulfur but also related to its type. The coal samples, which possess a low content of total sulfur and a small proportion of active organic sulfur groups such as mercaptans and thioethers, had a lower concentration of indicator gas and a smaller tendency to coal spontaneous combustion. After low-temperature oxidation, the content of mercaptan, thioether, thiophene, methyl(methylene), and pyridine in coal decreased, and the content of oxygen-containing groups such as sulfone (sulfoxide), sulfate, carboxyl, and nitrogen oxide increased. The elements of S, C, and N all changed to a high-valent state. In the oxidation reaction of model compounds, mercaptans were more reactive than thiophenes in the low-temperature region, and the oxidation of thiophene could direct form sulfone (sulfoxide), while the oxidation of mercaptan formed disulfide first. It is speculated that low-valence sulfur migrated to a high-valent state by providing sulfhydryl radicals (SH) and sulfur radicals (C-S) combined with active oxygen atoms. After the low-temperature oxidation reaction of model compounds, some organic sulfur existed in the form of aromatic sulfur or sulfur oxides and a small part of sulfur escaped as SO and HS gases in the solid oxidation product.
煤的低温氧化自燃是由其结构中的活性基团引起的。煤中碳氧官能团的氧化机理已得到广泛研究，但关于硫官能团引发煤自燃的研究报道较少。为研究有机硫官能团对高硫煤自燃的影响并探究其转化特性，采用低温氧化实验系统研究了变质程度相近但有机硫含量不同的煤的自燃倾向。利用X射线光电子能谱和傅里叶变换红外光谱分析了煤及模型化合物低温氧化过程中元素形态和有机硫官能团的变化。结果表明，煤中有机硫的形态主要包括硫醇、硫醚、噻吩、砜（亚砜）和硫酸盐，煤的低温氧化不仅与有机硫含量有关，还与其类型有关。总硫含量低且硫醇和硫醚等活性有机硫基团比例小的煤样，指标气体浓度较低，煤自燃倾向较小。低温氧化后，煤中硫醇、硫醚、噻吩、甲基（亚甲基）和吡啶的含量降低，砜（亚砜）、硫酸盐、羧基和氮氧化物等含氧基团的含量增加。S、C和N元素均转变为高价态。在模型化合物的氧化反应中，硫醇在低温区域比噻吩更具反应活性，噻吩氧化可直接形成砜（亚砜），而硫醇氧化首先形成二硫化物。推测低价硫通过提供巯基自由基（SH）和硫自由基（C-S）与活性氧原子结合迁移至高价态。模型化合物低温氧化反应后，部分有机硫以芳香硫或硫氧化物的形式存在，固体氧化产物中有一小部分硫以SO和HS气体形式逸出。
