Study On Torrefaction Pretreatment Of Municipal Sludge And Thermochemical Conversion Of Solid Products

Sewage sludge,a by-product of municipal sewage treatment,is rich in organic matter,nitrogen,phosphorus,potassium and other nutrients,as well as pathogens,toxic heavy metals and other pollutants.In recent years,thermochemical treatment has recently been considered as a promising method to improve the resource utilization and recycle of sludge owing to its good sterilization effect,rapid treatment speed and enormous stored energy in organic matter.The low quality of sludge feedstock limits its low cost and large-scale application,which also leads to inefficient use of subsequent processes and equipment operations.Torrefaction is one of the pretreatment methods used before thermochemical conversion,and it can improve the fuel quality,making it convenient for transportation,storage and subsequent thermochemical conversion.In this study,sludge was performed for atmospheric-pressure（AP）torrefaction and gas-pressurized（GP）torrefaction under different carrier gas atmospheres（pure nitrogen and 9 vol.%oxygen in nitrogen）,respectively.The torrefaction temperature was 200℃,250℃and 300℃,and the residence time of torrefaction was 30 min.The effects of different torrefaction pretreatment conditions on the basic characteristics,combustion characteristics and pyrolysis characteristics of solid products were studied.The result show that compared with the solid products from AP torrefaction,the GP torrefied sludge had lower H/C and O/C molar ratio,better hydrophobicity,larger surface area and lower bulk density.GP torrefaction can significantly promote the transformation of heavy metals（e.g.Pb,Zn and Cd）into the residual fraction.The secondary interaction between volatiles and solids in GP torrefaction via gas-pressure-induced deoxygenation behavior dominated the difference of derived-solid products from GP torrefaction and AP torrefaction.In addition,torrefaction temperature has a great influence on the combustion and pyrolysis characteristics of torrefied sludge.Comprehensive combustion characteristics of GP torrefied sewage sludge obtained at torrefaction temperature of 200℃（7.264～11.616）were much higher than those obtained at torrefaction temperature of 300℃（3.109～6.724）.The KAS and Coats-Redfern methods were followed to determine the activation energy（E_a）,and evaluate the kinetics of torrefied sludge pyrolysis.It was found that the coefficient of determination of the KAS and Coats-Redfern methods was better for GP-torrefied sludge at200℃ than that of GP-torrefied sludge at 300℃.Except that the higher heating value is slightly lower,the characteristics of oxidized torrefied sludge under atmosphere of 9.0%O₂-91.0%N₂ and non-oxidized torrefied sludge under pure N₂ atmosphere have little difference.The results of this study provide a reference for the introduction of waste heat flue gas with low oxygen content into sludge as the torrefaction carrier gas.