Process Study On SO₂ Reduction Coupling With Activated Carbon And Pyrolysis Gas To Elemental Sulfur

Sulfur is important chemical engineering materials which can be used to can be used as a raw material for almost all S-containing products.Scarcity of sulfur in China causes that the external dependence degree of sulfur is above 60%.At the same time,coal is the primary energy sources in China and the conbustion of coal results in serious environment pollution.Currently,sulfur element is recycled in forms of liquid SO2 or sulfuric acid by recycling desulfrization technology.However,the applications of liquid SO2 are relatively limited and sulfuric acid is difficult to be stored and transported.Therefore,elemental sulfur is the ideal form of recovered sulfur,which can remit SO2 pollution and sulfur shortage in China.Activated carbon adsorption desulfrization technology,as a green and recycling desulfrization technology,can achieve enrichment of SO2.Activated carbon and the pyrolysis gas released from activated preparation can both be used as reducing agent and activated carbon can also be used as catalyst because of abundant pore texture and surface functional groups.Therefore,it is presented a new sulfur recycling technology which uses activated carbon and pyrolysis gas as raw material.In this work,theoretical calculations were performed to determinethe equilibrium thermodynamics of SO2 reduction and the influence of factors in fixed bed and drop tube furnace system respectively were evaluated experimentally.The process design and economic calculation of SO2 reduction technology were also discussed and provide foundation for industrial application.Thermodynamic paramaters of SO2 reduction and other side reactions were calculated by factsage6.4.Effects of temperature,reducing agent/SO2 molar ratio and existence of CO2 and H2O on thermodynamic equilibrium were also predicted.As thermodynamic calculation,SO2 reductions with C,CO and H2 are exothermic reaction and can proceed spontaneously at lower temperatures.The temperature and reducing agent/SO2 molar ratio can both largely change the thermodynamic equilibrium.The reducing agent/SO2 molar ratio conformed to stoichiometric ratio and lower temperature are beneficial to the generation of elemental sulfur,and higher ones nagetively affect the elemental sulfur yield due to more S-containing byproduct. The contrast between equilibrium and experimental results indicated that SO2 reduction is difficult to occur due to limited kinetics and higher temperature make reaction achieving equilibrium easier.CO2 and H2O both cause equilibrium reversal movement,especially H2O lead to large amount of H2S production through converse Claus reaction.However,H2S with equivalent amount to H2O can eliminate the negative effects caused by reversal movement of Claus reaction.Effects of temperature,CO/SO2 molar ratio,retention time and charateristic of carbon materials on SO2 reduction with CO in an carbon bed were studied and reaction mechanism was also discussed.It is indicated that wirh increasing temperature,CO/SO2 ratio and rentention time,SO2 conversion increase.After SO2 conversion completed,futher increases in temperature and CO/SO2 ratio lead to dramatically decrease in S yield and selection but prolonging rentention time would not cause reaction efficiency decreasing-Carbon materials with abundant pore texture and high transition metal oxides are the ideal catalyst of CO-SO2 reaction.SO2 reduction with CO conformed to COS intermediate mechanism.Sulfone and R-O-S-S-R’ sulfur forms at the beginning of reaction and produces COS with CO.COS combines with carbon surface and produces thiophene sulfur or sulfide,and then reacts with SO2 to form elemental sulfur.The pyrolysis gas also contains H2,CO2 and H2O,except CO.Effects of temperature,H2/SO2 molar ratio,retention time,CO/H2 proportion and concertration of CO2 and H2O on SO2 reduction with H2 and mixed gas in an ativated carbon bed were discussed.It is presented that the starting temperature of H2-SO2 reaction is 600℃ and the temperature decreases about 100℃ under activated carbon actalysis.Effects of temperature and H2/SO2 ratio is similar to CO-SO2 reaction but prolongding retention time would cause increase in H2S generation.CO has higher SO2 reduction activity than H2 but SO2 reductions with CO and H2 do not impact mutually.Effects of CO2 and H2O are consistent with equilibrium prediction.In drop tube furnace system,earbothermic redution of SO2 in gas-solid two-phase flows was studied.It is indicated that,higher temperature and C/S ratio increase SO2 conversion with few amount of S-containing generated.With increasing amount of H2 and CO,SO2 conversion and initial reaction rate increase but amount of COS and H2S were also increase.CO2 and H2O cause more CO and H2 genarating and also have nagetive effects on S yield.In order to provide industrialization foundation of SO2 reduction,the moving granular activated carbon bed reduction process and powder activated carbon CFB(circulating fluidized bed)reduction process were presented.The economic evalvuation of two SO2 reduction processes taking 300MW unit as examples show that the balances of payments can achieved in the operational process.