Occurrence And Evolution Of Organic Sulfur In Coal And Its Influence On Coal Structure

In order to realize the clean utilization and comprehensive utilization of high-sulfur coal resources,the starting point is to study the formation causes of coal seams with different sulfur content in the same area and coal with large differences in organic sulfur content in different areas.Furthermore,the organic sulfur structure and coal structure characteristics in high-organic-sulfur coals with different coal rank were studied.Coal petrology and quality analysis and combined with contemporary advanced equipment analysis methods,including optical Electron Microscope,high-resolution transmission electron microscopy（HRTEM）,Fourier transform infrared absorption spectroscopy（FTIR）,Raman spectroscopy（Raman）,X-ray photoelectron spectroscopy（XPS）,X-ray diffractometer technology（XRD）,Fourier transform ion cyclotron resonance mass spectrometry（FT-ICR MS）and extraction.From the perspectives of coal geology,organic geochemistry and coal structure to explain the evolution characteristics of organic sulfur in coal with coal rank and its influence on coal organic structure,and provide a scientific theoretical basis for the clean and comprehensive utilization of high-sulfur coal resources in China.The reasons for the formation of high organic sulfur in coal were studied on No.4and No.11 coal seams of Shanxi Pingshuo Open-pit Mine and the No.6 coal seam of Guangxi Chonggu Mine.Among them,the No.4 coal seam in Shanxi Pingshuo is low-sulfur coal,the No.11 coal seam in Shanxi Pingshuo is high-sulfur coal and is dominated by organic sulfur,and the No.6 coal seam in Guangxi Chonggu Mine is super-high-organic sulfur（SHOS）coal.The variation characteristics of coal petrology and coal quality of each coal seam on the coal seam profile were analyzed.According to the characteristics of coal petrology and coal quality,combined with the geological background of coal formation,the reasons for the difference in sulfur content between the No.4 and No.11 coal seams in the same Pingshuo open-pit mine were explored,and explore the reason why the organic sulfur content in Guangxi Chonggu coal is much higher than that in Pingshuo No.11 coal.The research results show that the thickness of the No.11 coal seam is much smaller than that of the No.4 coal seam.The No.11 coal seam at the bottom of the Taiyuan Formation is thick in the middle and south and thinner at the northern edge.It is speculated that the No.11 coal seam may be formed at the bottom of the lake and in the area where the water flow is relatively gentle or still,and the plant structure is well preserved.Covered by the water caused the lack of oxygen,the bottom of the lake is in a reducing environment,which is conducive to the accumulation of organic sulfur in coal.The Chonggu coal in Guangxi is a marine deposit,and the sulfur source provided by the seawater is relatively sufficient.Funginite were also found in the Chonggu coal.It is speculated that the large amount of sulfate in seawater may be reduced to form organic sulfur in coal under the influence of bacteria.The FTIR and Raman structure parameters of high-and low-organic-sulfur coal with different coal ranks were studied,and it was found that the aromatic structure parameters of high-and low-organic-sulfur coal were not grow linearly as coal rank increases.When R_o>1.30%,the aromatic structure parameters of low-organic-sulfur coal are higher than those of high-organic-sulfur coal of the same coal rank.Within the studied coal rank range（0.26%<R_o<3.57%）,the chain hydrocarbon length of the high-organic-sulfur coal is smaller than that of the low-organic-sulfur coal in the same coal rank.The hydrocarbon generation potential of several SHOS coals involved in the samples is significantly higher than that of the other samples.The comparative analysis of Raman structure parameters shows that the graphitization degree of low-organic-sulfur coal is greater than that of high-organic-sulfur coal at the same coal rank,and organic sulfur acts as a heteroatom to inhibit the coalification.The aromatic lamella structure characteristics of SHOS coal and low organic sulfur coal with similar coal rank were studied by HRTEM and XRD.XRD analysis results show that when the coal rank is low（R_o≈0.70%）,the diameter of the aromatic layer of SHOS coal is significantly larger than that of low-organic-sulfur coal.Regardless of whether the R_o is 0.70%or 1.70%,the stacking thickness and effective stacking layers of the SHOS coal are significantly smaller than those of the low-organic-sulfur coal at the same coal rank.The average relative molecular mass of different coal samples calculated by TEM analysis is consistent with the XRD experimental results,that is,the larger the aromatic layer,the larger the average relative molecular mass.When the coal rank is low,the average relative molecular mass of SHOS coal is higher than that of the low organic sulfur coal of the same coal rank.The arrangement direction of the lattice fringes is related to the internal force between the coal molecules and the dispersion force of the sample to a certain extent.When the coal rank is low,the super-high organic sulfur content in the coal has the greatest effect on the average diameter（L_a）of the aromatic layers,which is the primary reason that the relative molecular mass of SHOS coal is higher than that of low-organic-sulfur coal of the same coal rank.The existing forms of organic sulfur in coal were analyzed by FTIR and XPS,and the calculation formulas of organic sulfur structural parameters S_aro1、S_aro2、S_aro1^*and S_aro2^* were proposed and their variations with coal rank were analyzed.S_aro1 and S_aro1^* are the ratios of thiophene to other non-thiophene organic sulfurs,and S_aro2 and S_aro2^* are the ratios of thiophene to total organic sulfur.When R_o<2.00%,S_aro1 and S_aro1^* increase slowly with the increase of coal rank,while S_aro2 and S_aro2^* increase rapidly with the increase of coal rank.When R_o>2.00%,S_aro1 and S_aro1^* increase rapidly with the increase of coal rank,while S_aro2 and S_aro2^* increase slowly with the increase of coal rank.It is inferred from the change of organic sulfur structure parameters that R_o=2.00%is a turning point.Most of the organic sulfur in coal has been converted into the structurally stable thiophene when R_o is 2.00%.Extraction combined with FT-ICR MS analysis to study the composition characteristics of different coal rank high organic sulfur coal extracts.It was found that the SHOS coal with the highest extraction rate had the least resins components,which is related to the formation of the super-high organic sulfur content in the coal.When the sulfur-containing substances enter the coal to form organic sulfur,they first react with the most active functional groups in resins,that’s consume a large amount of the resins in coal.It is found that the abundance of organic sulfur compounds in coal extracts is not proportional to the organic sulfur content due to the comprehensive influence of coal rank and coal-forming environment.The existence of S₃ organosulfur compounds was identified by FT-ICR MS study.Organosulfur compounds with a relative molecular mass greater than 500amu are also relatively common in the S₃ class,and the possible molecular structures of the organosulfur compounds with a relative molecular mass greater than 500 amu were constructed according to their molecular composition characteristics.