Migration And Transformation Characteristics Of Sulfur/Arsenic/Lead And Its Release Relationship During Coal Pyrolysis

Sulfur is one of the main harmful elements restricting the combustion,conversion and utilization of coal.Sulfur undergoes a series of complex physical changes and chemical reactions during the coal utilization process,and is finally released into the atmosphere in the form of SO₂,SO₃,H₂S,etc.,causing environmental problems such as acid rain and visible plumes.Coal,in addition to the main elements（C,H,O,N,S）,contains some hazardous trace elements.During the processing and utilization processes of coal,some of these elements are released into the atmosphere with the emission of sulfur oxides,nitrogen oxides and dust.Arsenic and lead have potential carcinogenic effects,which have caused widespread concern in the society.Pyrolysis technology could realize partial gasification and liquefaction of coal to obtain gas,tar and high-calorific value char.As a smokeless clean fuel,char has a positive effect on improving the air quality in our country.Moreover,pyrolysis,as the basic process of coal thermal conversion and utilization,is very important for the combustion and gasification process of coal.Previous studies have found that the mode of occurrence of arsenic and lead in coal is closely related to sulfur.This means that there may be a specific relationship between the volatilization characteristics of arsenic and lead elements and sulfur during the pyrolysis process.Therefore,studying the migration and transformation characteristics of sulfur,arsenic and lead during coal pyrolysis has important guiding significance for effectively controlling the emission of sulfur,arsenic,and lead during the thermal conversion of coal.Based on the occurrence relationship of sulfur,arsenic and lead in coal,reveal the migration and transformation laws of sulfur,arsenic and lead during pyrolysis,clarify the correlation between the release behaviors of three elements,which provides a theoretical basis for the cooperative control of sulfur,arsenic and lead pollutant emissions during the thermal conversion of coal.In this study,three coals with representative sulfur content and sulfur form from Gansu Yaojie（GS）,Yunnan（YN）and Chongqing（CQ）were selected as raw materials.The combined application of chemical analysis method and X-ray photoelectron spectroscopy（XPS）has qualitatively and quantitatively analyzed the distribution of sulfur form in three raw coals.Sequential chemical extraction was used to determine the speciation of arsenic and lead in coal.Furthermore,with the help of electron probe microanalysis（EPMA）,the occurrence relationship of sulfur,arsenic and lead in coal was studied.The pyrolysis experiment of different coal samples was carried out through a fixed bed reactor.The transformation and migration behavior of sulfur at different temperatures and particle sizes were investigated.By analyzing the content and form of sulfur in char,the migration and transformation laws of sulfur during pyrolysis were revealed,and the relationship between the retention extent of sulfur in char with the particle size of raw coal and influence mechanism were clarified.At the same time,the volatilization and transformation characteristics of arsenic and lead during pyrolysis were investigated.Fact Sage thermodynamics software and fourier infrared spectroscopy（FTIR）were used to ascertain the influence of organic matter and minerals in coal on the volatilization of arsenic and lead.The migration and transformation laws of arsenic and lead during pyrolysis and their enrichment mechanism in char were clarified.Based on the occurrence correlation of the three elements（sulfur,arsenic and lead）in coal,the relationship of the release behavior of three elements during pyrolysis was discussed.The main conclusions obtained in this study are as follows:（1）The analysis of speciation of sulfur,arsenic and lead in raw coals shows that the occurrence of elements in different coals are quite different.（1）Sulfur is mainly present in coal in the form of pyrite and organic sulfur.In GS and YN coal,sulfur mainly occurs as organic sulfur.However,there is a big difference in the composition of organic sulfur in GS and YN coal.For GS coal,thiophene and sulfone are the main components,the proportion of which are 11%and 20%respectively.For YN coal,thiophene and sulfoxide are the dominant sulfur-containing compounds,followed by organic sulfide.Pyrite is the main sulfur-containing compound in CQ coal,and its proportion is as high as 79%.The organic sulfur composition is dominated by stable thiophene-S.（2）Arsenic and lead in coal are mainly associated with inorganic matter.For GS and YN coal,arsenic is mainly associated with pyrite,accounting for 32.8%and 29.1%of the total arsenic,respectively.Lead is mainly associated with carbonates,phosphates,sulfates and other minerals,the proportion of which are 57.4%and 69.5%respectively.For CQ coal,arsenic is mainly associated with clay minerals,followed by pyrite,the proportions of which are 47.3%and35.7%respectively.Lead is mainly associated with pyrite,accounting for 71.5%of the total lead.（3）EPMA analysis shows that arsenic and lead in all coals are significantly enriched in the distribution area of pyrite.（2）The study on the release and transformation characteristics of sulfur during pyrolysis shows that sulfur is more likely to be retained in char due to the different thermal stability of different sulfur forms and the mutual transformation of internal forms of sulfur.Rising the temperature and reducing the particle size could promote the decomposition of sulfur-containing compounds,the release amount of sulfur-containing gas gradually increases,thereby improving the desulfurization extent of char.For GS coal,the content of sulfur locate at surface of coal particles is higher than that of bulk phase.With the rise of temperature,the sulfur on the surface of the particles gradually decreases.For YN coal,the bulk sulfur is always higher than the surface sulfur,with the increase of pyrolysis temperature,both bulk sulfur and surface sulfur are enriched in different degrees.During the pyrolysis process,the main change is the mutual transformation of internal forms of sulfur.Especially at 400-800℃,the active sulfur produced by the decomposition of pyrite is almost completely captured by organic matter,and only a small amount of sulfur escapes from coal particles in gaseous form.Sulfur in CQ coal is mainly composed of pyritic sulfur.At 500℃,pyrite begins to decompose and produce pyrrhotite and active sulfur.Part of the active sulfur reacts with organic matter to form organic sulfide or thiophene.The remaining part of the active sulfur escapes as a gas.In addition,due to the high degree of metamorphism and the low content of active hydrogen of CQ coal,the initial decomposition temperature of pyrite is high,and remains in char in the form of pyrrhotite at 800℃.This is the main reason for the low desulfurization extent of CQ coal during pyrolysis.（3）The migration and transformation characteristics of arsenic and lead during pyrolysis were investigated,and the effects of pyrolysis temperature and speciation on the volatilization characteristics of arsenic and lead during pyrolysis were also investigated.The results show that temperature has a significant effect on the volatilization of arsenic.With the rise of pyrolysis temperature,the volatilization ratio of arsenic gradually increases.However,the volatilization ratio of arsenic changes in stages with temperature.Based on the analysis of arsenic species in char,it is found that there are significant differences in the thermal stability of different species of arsenic.Exchangeable arsenic and organic arsenic are easy to escape from coal with the release of volatiles.The thermal stability of arsenic associated with pyrite depends on the distribution of arsenic in the pyrite.Arsenic diffused in pyrite lattice is easily released with the decomposition of pyrite.In addition,arsenic could enter the pyrite structure by substitution for S atom and distributed in pyrite in the form of arsenopyrite（Fe As S）,which has high thermal stability.Arsenic combined with aluminosilicate has the highest thermal stability and hardly decomposes during pyrolysis.Furthermore,there are also mutual transformations of different species of arsenic during pyrolysis.After 500℃,the acid-soluble arsenic gradually transforms into organic arsenous and aluminosilicate bound arsenic.The effect of temperature on the volatilization characteristics of lead is phased.Below 500℃,rising the temperature promotes the decomposition of aliphatic structure and pyrite,thereby accelerating the volatilization of lead.At 500-700℃,the main change of lead is the mutual transformation between different species.The lead released from the decomposition of lead sulfate and pyrite fails to escape from the coal in time,and is captured by organic matter to form a new organic lead.At 700-800℃,the decomposition of galena is the main cause of the increase in lead volatilization ratio.At high temperatures,the melting of aluminosilicate is still an important factor in inhibiting lead volatilization,and a large amount of gaseous lead is fixed in char.（4）The carboxyl and hydroxyl functional groups play a key role in the enrichment of arsenic and lead in char.Qualitative and semi-quantitative analysis of functional groups in raw coal and char by FTIR shows that the oxygen-containing functional groups in coal are closely related to the volatility of arsenic and lead.Arsenic and lead ions interact with oxygen-containing functional groups selectively during pyrolysis.They preferentially form organic complexes with oxygen in the carboxyl group.When the active sites on the carboxylic acid are all occupied,they then interact with the hydroxyl functional group.（5）Fact Sage thermodynamics software was used to study the influence of sulfate,carbonate and aluminosilicate minerals in coal on the volatilization characteristics of arsenic and lead,and the possible reaction pathways were predicted.The results show that sulfate and carbonate minerals could inhibit the volatilization of arsenic,and their decomposition products chemically adsorb with gaseous arsenic and form corresponding arsenates（Fe As O₄,Ca₃（As O₄）₂ and Mg₃（As O₄）₂）.Aluminosilicate minerals containing Na/K elements are melted at high temperatures and adsorb gaseous arsenic and gaseous lead due to their low melting point,thereby inhibiting the volatilization of arsenic and lead.In addition,the experiment did not find the influence of sulfate and carbonate minerals on the volatilization characteristics of lead.（6）The simultaneous release of arsenic and sulfur is observed during the pyrolysis process.A characteristic peak is observed for the volatilization rate of arsenic and sulfur,and it almost coincides with the second weight loss peak of coal.For GS,this peak occurs at 400℃,which is mainly caused by the decomposition of organic arsenous and aliphatic organic sulfur.For CQ,the characteristic peak of arsenic and sulfur are observed at 550℃,which is generated through the decomposition of pyrite.