Research on the spontaneous combustion of coal caused by sulfur has always been focused on pyrite in coal but has rarely considered the influence of organic sulfur. In this paper, coal samples, rather than model compounds, were used to study the influence of organic sulfur content in coal on its spontaneous combustion process. The results of X-ray photoelectron spectroscopy and thermogravimetry, differential scanning calorimetry, and mass spectrometry indicate that organic sulfur in Shuiyu clean coal exists in forms of mercaptan, thioether, sulfone (sulfoxide), and thiophene. With the decrease of organic sulfur content, the characteristic temperature points and the peak values of the exothermic curves in the process of coal oxidation spontaneous combustion all shifted toward higher temperatures. The ignition activation energy of coal also increased, and the initial and peak gas evolution temperatures of the oxidation products shifted toward higher temperatures. These findings suggest that the reduction of organic sulfur content can inhibit the oxidation process and spontaneous combustion tendency of coal. This effectively reveals the mechanism of the spontaneous combustion of coal and is of great significance to future studies in this field.
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 SO2 and H2S gases in the solid oxidation product.
Coal samples from the Shanxi Shaping coal mine were selected to investigate the occurrence of H2S in low sulfur coal seams. The adsorption mechanism of coal to H2S was explored, and an occurrence equation for H2S in coal seams was fitted through adsorption experiment results. The results showed that under ambient temperature and pressure conditions, the H2S adsorbed by coal reached equilibrium within 24 h. The increase in H2S concentrations and the moisture content of coal samples resulted in an increase in the adsorption capacity of H2S. Chemical adsorption of H2S by the coal also occurred. The total sulfur content in the coal increased, and water promoted the conversion from H2S to sulfur in coal. After adsorption, most of the H2S remains in the coal structure in the form of inorganic sulfur, such as sulfur hydride, iron sulfide sulfur, and monomeric sulfur, and a small proportion of H2S is bonded in the structure of the coal in the form of organic sulfur such as thiophene, C-S-C, and C-SH. Therefore, the higher the total sulfur content in coal, the greater the occurrence of H2S. The total amount of H2S increased exponentially with the concentration of free H2S and the moisture content of coal at equilibrium. This meant that the total amount of H2S in the coal seam could be estimated by fitting an equation according to the concentration of free H2S and the moisture content of coal seams. The concentration of free H2S decreased linearly with the increase in moisture content of the coal, therefore, the concentration of H2S in space could be reduced by injecting water into coal seams.
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