Mechanism Of Pyrolysis Desulfurization Of Coal-like Organic Sulfur-containing Model Compounds Enhanced By Iron Powder

For high-sulfur coal rich in organic sulfur,the effective removal organic sulfur in coal is a hard problem,and it is also an urgent problem to be solved in the clean and efficient utilization of coal.Our research group found that adding iron powder in the pyrolysis process can significantly improve the removal efficiency of organic sulfur in high-sulfur low-rank coal.However,the migration and removal mechanisms of sulfur in organic sulfur in coal during the pyrolysis process of adding iron powder have not been elucidated.Therefore,based on the organic sulfur groups contained in Shengli lignite,three kinds of organic sulfur-containing model compounds（OSMC）were selected in this study.Conventional pyrolysis and pyrolysis enhanced by iron powder were compared to obtain the optimal condition for the desulfurization performance of OSMC.The desulfurization law and removal mechanism of sulfur of OSMC under the pyrolysis enhanced by iron powder were elucidated.The pyrolysis mechanism of removal of organic sulfur enhanced by iron powder is expected to provide a new direction and idea for the efficient removal of organic sulfur from low-rank coal,oil,and solid waste.Diphenyl sulfone（DPS）,dibenzothiophene（DBT）,and phenyl disulfide（PDS）were selected to replace sulfone sulfur,thiophene sulfur and thioether sulfur in lignite,respectively,according to the occurrence forms of organic sulfur in high-sulfur Shengli lignite.The three kinds of OSMC were loaded on the activated carbon（AC）by the impregnated method,which was used to carry out the research.The single factor optimization method was used to explore the influence of the main operating factors on the desulfurization effect of OSMC in the pyrolysis process on the tube furnace.The sulfur reduction rate was selected as the evaluation index of the pyrolysis desulfurization performance,and the desulfurization performance of OSMC under the optimal test conditions of the two pyrolysis methods were compared.X-ray photoelectron spectroscopy（XPS）,infrared spectroscopy（FTIR）,gas mass spectrometry（GC-MS）,gas chromatography（GC）and thermogravimetric mass spectrometry（TG-MS）and other instruments were employed to analyze the components of pyrolysis products（solid matter,liquid phase products,and sulfur-containing gas）of activated carbon loaded with organic sulfur-containing mode compounds（OSMC/AC）.According to the analysis results,the possible pyrolysis reaction pathways during the process of pyrolysis enhanced by iron powder were established.The reaction mechanism of iron powder enhanced pyrolysis removal of organic sulfur was revealed.The weight loss of Shengli lignite and three kinds of OSMC/AC in the process of conventional pyrolysis and pyrolysis enhanced by iron powder at different heating rates was determined by means of a thermogravimetric analyzer.Based on the thermogravimetric test data,the pyrolysis kinetic behaviors of the four samples were analyzed by distributed activation energy model（DAEM）,Kissinger-Akahira-Sunose（KAS）method,Flynn-Wall-Ozawa（FWO）method and master curve method.The main conclusions are as follows:The sulfur reduction rates of activated carbon loaded with diphenyl sulfone（DPS/AC）,dibenzothiophene（DBT/AC）,and phenyl disulfide（PDS/AC）increased from 65.81%,50.34%,and 32.97%under the conventional pyrolysis to 86.40%,59.86%,and 72.89%under the pyrolysis enhanced by iron powder,respectively.It was found that the desulfurization performance of pyrolysis enhanced by iron powder in removing sulfide sulfur was the best,followed by sulfone sulfur,and thiophene sulfur was the weaker.After the pyrolysis enhanced by iron powder for three kinds of OSMC/AC,the sulfur content of activated carbon was lower than that of conventional pyrolysis,the sulfur content of iron powder increased,and iron sulfur compounds（Fe S/Fe S₂）that could react with hydrochloric acid to generate H₂S were formed on the surface of posted-pyrolysis iron powder.It was indicated that iron powder during pyrolysis not only has the role of strengthening desulfurization,but also has the role of fixing sulfur.When DPS/AC was treated under the pyrolysis enhanced by iron powder,the O=S=O radicals were generated by the fracture of the C-S bond in the DPS structure.Most of O=S=O radicals reacted preferentially with Fe to generate Fe S/Fe S₂,and a small part escapes as SO₂ gas.DPS was also reduced by active hydrogen in AC to form diphenyl sulfide,then undergoing thermal cleavage to generate DBT and PDS.When DBT/AC was treated under the pyrolysis enhanced by iron powder,one C-S bond in the DBT structure was broken,and then combined with active hydrogen to obtain 2-phenylthiophenol,which was further cleaved to generate biphenyl and·SH.Most of·SH reacted with Fe preferentially to form Fe S/Fe S₂,while a small part of·SH reacted with AC to form SO₂ gas.In addition,the two C-S bonds in the DBT structure were broken to form biphenyl radical and·S·,·S·also preferentially reacted with Fe to form Fe S/Fe S₂,and a small amount of·S·reacted with AC to generate SO₂ gas.When PDS/AC was treated under the pyrolysis enhanced by iron powder,the S-S bond in the PDS structure was broken to form a phenylthio radical,which acquired active hydrogen to generate thiophenol,and its C-S bond was fractured to obtain benzene radical and·SH.Most of·SH reacted preferentially with Fe to form Fe S/Fe S₂,a small part of·SH reacted with active hydrogen to form H₂S,and a very small part of·SH reacts with AC and was released in the form of COS and SO₂.The average activation energy in the main pyrolysis stage of the pyrolysis enhanced by iron powder of lignite and DBT/AC calculated by DAEM or KAS/FWO method was lower than that of conventional pyrolysis.However,the average activation energy of DPS/AC and PDS/AC in the main pyrolysis stage of pyrolysis enhanced by iron powder was higher than that of conventional pyrolysis.However,it was inferred that the addition of iron powder during pyrolysis can reduce the activation energy of sulfur-containing bond thermal fracture in OSMC structure.Based on KAS/FWO method,the kinetic mechanism functions in the main pyrolysis stage of conventional pyrolysis（heating rate of 15K/min）for Shengli lignite,DPS/AC,DBT/AC,and PDS/AC（provided by master curve method）were（1-α）^10.06,（1-α）^1.6,（1-α）^1.94,and（1-α）^3.33,respectively.The kinetic mechanism functions in the main pyrolysis stage of pyrolysis enhanced by iron powder（heating rate of 15K/min）for Shengli lignite,DPS/AC,DBT/AC,and PDS/AC（provided by master curve method）were（1-α）^8.93,（1-α）^1.63,（1-α）^1.81,and（1-α）^3.57,respectively.And the kinetic reaction mechanisms of the main pyrolysis stage of the same sample under two pyrolysis methods were different.