Sulfur Release And Transformation During Pyrolysis Of High Sulfur Coals

In this study,Py-MS,Py-GC and S K-edge XANES were used to investigate the effects of atmospheres with different CO₂ concentration on sulfur evolution and transformation of Yanzhou?YZ?coal with high inorganic sulfur content and Pingshuo?PS?coal with high organic sulfur content during pyrolysis under pure Ar,25%CO₂-Ar,50%CO₂-Ar,75%CO₂-Ar,85%CO₂-Ar and pure CO₂ atmospheres.Py-GC and S K-edge XANES were carried out to investigate the effects of atmospheres and temperatures on sulfur release and transformation during pyrolysis under pure CO₂atmosphere,and the effects of low oxygen content atmosphere on sulfur release during pyrolysis under 3%O₂-Ar atmosphere.The main conclusions are obtained as following:?1?When pyrolyzed under atmospheres with different CO₂ concentrations,the desulfurization ratios increase with increasing CO₂ concentration,but char yield ratios decrease.For YZ coal,after pyrolysis sulfur mainly distributes in gas phase and char,but sulfur in tar is less.With the increase of CO₂ concentration,sulfur transferred into gas phase increases,however sulfur in char and tar decreases;For PS coal,after pyrolysis,most of the sulfur is distributed in tar.With the concentration of CO₂ increasing,sulfur content in the gas phase?especially COS?increases,while the sulfur distributed in the other two phases decreases.Pure CO₂ can significantly reduce the release temperature of H₂S and COS.Pyrolysis atmospheres with different concentrations of CO₂?25%,50%,75%,85%?can make the release temperature of the sulfur-containing gas move to the high temperature.And it is favorable for stable organic sulfur to decompose under all CO₂ atmospheres at higher temperatures.By analyzing S K-edge XANES spectras of PS coal and its chars under atmospheres with different concentrations of CO₂,it can be seen that the peaks about 2473.2 eV under atmospheres with different CO₂ concentrations are all higher than that under Ar atmosphere.This indicates that sulfurs in the chars under these CO₂ atmospheres are mainly stable thiophenes,as other unstable sulfur forms can be removed or transferred to stable thiophenes during pyrolysis under these CO₂ atmospheres.?2?With temperature increasing,the desulphurization rates of coals gradually increase under pure Ar and pure CO₂ atmospheres,and char yield gradually decreases,but the sulfur removal ratios of all coals under pure CO₂ atmosphere are higher than that under pure Ar atmosphere.When YZ coal was pyrolyzed under Ar atmosphere,sulfur is mainly distributed in tar under Ar atmosphere.As the temperature rises,the sulfur content in tar increases.The trend of sulfur content in tars of YZ raw coal under pure CO₂ atmosphere is similar to that under Ar atmosphere.However,the trend of sulfur in tars of YZ deashed coal and YZ depyrited coal under pure CO₂atmosphere is opposite to that under Ar atmosphere,as their sulfur is mainly distributed in the gas phase.For PS coal,when pyrolyzed at low temperature under pure CO₂ atmosphere,sulfur is mainly distributed in char,but less than under Ar atmosphere.With temperature increasing,the sulfur in char decreases and the sulfur in the gas phase increases rapidly,which indicates that the CO₂ atmosphere is beneficial for sulfur in coal to transfer into the gas phase.By analyzing the forms of sulfur in YZ coal and its chars abtained at different pyrolysis temperatures,active organic sulfides and pyrite can decompose under these two atmospheres at the low temperatures.With the increase of temperature,a new absorption peak?FeS?is formed about 2469.7 eV after 400 ^oC for YZ raw coal and YZ deashed coal.The intensity of this peak reaches the maximum at 700 ^oC under Ar atmosphere,and at 600 ^oC under pure CO₂ atmosphere,then the intensity of peak decreases.The thiophenes peak including some sulfides in YZ raw coal decreases obviously at 400-600 ^oC,but it changes slightly after 600 ^oC under Ar atmosphere.While the thiophenes peak including some sulfides in YZ raw coal increases at 400-600 ^oC and slightly decreases after 600 ^oC under pure CO₂ atmosphere.?3?Sulfur removal is the highest under 3%O₂-Ar among those atmospheres,and sulfur form in the gas phase is mainly SO₂.The peak intensity of sulfate about 2481.6eV in chars of PS raw coal is significantly enhanced,because a large amount of SO₂is generated under 3%O₂-Ar atmosphere,then alkaline minerals in coals can absorb SO₂ to form stable sulfates remained in chars.