Study On The Formation Behavior Of Pyrolytic Products And Transformation Mechanism Of Sulfur During Pyrolysis Of Sulfur-containing Organic Solid Waste

With the growth of the Chinese economy and the acceleration of urbanization,organic solid waste’s output is also increasing sharply.As an efficient way to treat organic solid waste,pyrolysis is receiving more and more attention and research.However,the source of organic solid waste is very complicated,and there is plenty of sulfur-containing organic solid waste.The sulfur-containing components may produce sulfur-containing pollutants during the pyrolysis process,which will affect the clean utilization of subsequent products.Therefore,research on the control of the migration and transformation of sulfur is significant during the pyrolysis process of sulfur-containing organic solid waste.This article selected two typical sulfur-containing organic solid wastes,waste tires and organic sludge,as the research object.Then studied the generation characteristics of pyrolysis gas,tar,and char under different heating conditions（especially for wide heating rates）.The law of migration and transformation of sulfur during the pyrolysis process was discussed,and the effective mechanism of inorganic oxides as additives on sulfur fixation during the pyrolysis process of organic solid waste was also explored.The main research conclusions are as follows:（1）The pyrolysis of waste tires could be roughly divided into three stages:moisture removal stage（<180℃）,cracking reaction stage（180-540℃）,and carbon residue decomposition stage（>540℃）.The non-sulfur components in the pyrolysis gas of waste tires were mainly composed of H₂,CH₄ and CO.With the heating rates increased from60℃/min to 6000℃/min,the bond in macromolecular organic component might cleavage at a higher heating rate,producing more·CH₃ and H·groups,resulting in the significant increase of pyrolysis gas.The production of H₂ and CH₄ had approximately doubled.Simultaneously,aliphatic sulfur,thiophene,sulfoxide,sulfone,and other organic sulfur components in waste tires had poor thermal stability,thus easy to crack during pyrolysis.The total proportion of organic sulfur after pyrolysis had dropped by about 32-40%.Organic sulfur might form·SH groups in the pyrolysis process,and which could combine with the H·and·OH groups to generate H₂S and SO₂.The reaction process was significantly affected by the heating rate,with the increase of the heating rate,the production of sulfur-containing pyrolysis gas,mainly included H₂S and SO₂,had shown an increasing trend,especially for H₂S,which had increased by about 1.5 m L/g.The increased production of sulfur-containing pyrolysis gas such as H₂S and SO₂ directly led to an increase in the proportion of sulfur transferred into the pyrolysis gas,and the proportion of sulfur remaining in the pyrolysis char was reduced by about 6-8%.In addition,part of the sulfate reacted with volatiles to generate inorganic sulfides,which reduced the proportion of sulfate by about 3-8%after pyrolysis,while the proportion of inorganic sulfides increased.（2）The composition of organic sludge was quite different from waste tires,the oxygen content was about 33-54%higher than that of waste tires.Compared with waste tires,the non-sulfur components in sludge pyrolysis gas contained more CO₂,which was about 15-38m L/g.The thermal stability of organic sludge was poor,and its pyrolysis process could divide into three stages:moisture removal stage（<127℃）,cracking reaction stage（127-557℃）,and carbon residue decomposition stage（>557℃）.The increasing heating rate would shift the weight loss peak to the high temperature direction,but the heating rate change had little effect on the weight loss rate.Meanwhile,the content of aliphatic sulfur,thiophene sulfur,sulfone and other organic sulfur in the organic sludge were reduced by about 34-64%after pyrolysis.The·SH group formed by the decomposition of the organic sulfur component could combine with H·and·OH to generate sulfur-containing pyrolysis gas such as H₂S and SO₂.Besides,the proportion of inorganic sulfide in pyrolysis char increased slightly after pyrolysis,basically fluctuating within the range of 1-2%.The proportion of sulfate increased first and then decreased with the increase of the final temperature of pyrolysis.The turning point was about 425℃,and it might be due to the reaction of some sulfates with hydrogen sulfide in the volatile matter at high temperature to generate a small amount of SO₂.（3）The interaction between inorganic components and sulfur-containing solid waste in the pyrolysis process could promote the fixation of sulfur in pyrolysis char and restrain its transfer behavior to pyrolysis gas and tar.The addition of calcium oxide（calcium oxide and the mass ratio of raw materials was 2.5-10%）increased pyrolysis char yield by about 15-25%.When the proportion of calcium oxide increased,the pyrolysis tar yield showed a downward trend.It might be due to the immobilization of H₂S,CO₂ and SO₂ by calcium oxide in the volatile matter.Simultaneously,the addition of calcium oxide might promote the cracking of organic components in solid waste.Then forming small molecular gases,which including high-value combustible gases like H₂ and CO.Besides,the addition of calcium oxide could effectively absorb and fix H₂S,CH₃SH and other gases generated during the pyrolysis of sulfur-containing organic solid waste.It could be fixed in the pyrolysis char in the form of inorganic calcium sulfide,and the fixation rate increased by about 5-25%.However,the addition of calcium oxide also introduced oxygen,which would increase the amount of produced SO₂ produced.Generally,the addition of calcium oxide could effectively reduce the sulfur converted into the pyrolysis gas and tar,and promoted its fixation in the pyrolysis char in the form of inorganic calcium sulfide.When the mass ratio of calcium oxide to raw material exceeded 5%,the fixation rate of sulfur in pyrolysis coke was basically above 70%.