Study On Pressurized Pyrolysis Of Municipal Sewage Sludge And Characteristics Of Products

The large amount of sewage sludge（SS）produced from municipal wastewater treatment plants without treated properly will take up land and cause serious problems.The high moisture,organic matter and micro-organisms as well as the pathogenic bacteria,parasitic ovum and heavy metals in SS will cause potential harm to environmental and human health.The need for proper treatment and disposal of SS is urgent.Pyrolysis is a potential and developing technology for SS management.The SS is pyrolyzed in an oxygen-free environment to obtain pyrolysis gas with energy added value,tar and char,the harmless and resourceful disposal will be carried out while achieving the effect of volume reduction and treatment.Pyrolysis technology has been widely studied as an effective method for SS treatment and disposal.However,there are still some problems to be solved,such as the problem of low-quality tar and syngas.In response to this problem,this paper carried out sludge pressurization pyrolysis experiments and studied the effect of different final pyrolysis temperatures（FPT）,pyrolysis pressures and residence times（RT）on the product mass distribution and pyrolysis gas composition.Furthermore,the pressure influence on the properties of tar and char were also detected and analyzed.The main contents and conclusions of this study are as follows（1）Thermogravimetric analysis（TGA）was conducted on the SS raw material.The SS pyrolysis process was divided into three stages,namely,the drying stage,primary and secondary pyrolysis weight loss stage.The degradation of organic materials and the precipitation of volatiles occurred in the main reaction stage in 200⁵20oC.（2）Pressurized pyrolysis experiments were carried out in a high-pressure reactor.The effects of FPT,pyrolysis pressure and RT on the mass distribution of SS pressurized pyrolysis products were investigated.With the increase of FPT,the tar yield increased and then decreased,gas yield increased,meanwhile the char yield decreased.As for the effect of pressure,it was found that the tar yield obtained the highest harvest at atmospheric pressure.With the increase of pressure,the yield of tar was obviously reduced,while the yield of the gas products increases obviously.The longer residence time under pressurized condition was of little significance to the reaction.（3）The effects of three factors on the composition of gas were also studied.CO and H₂were the main rising gas components at higher temperature with the decrease of CO₂,and CH₄production was suppressed.With the increase of pyrolysis pressure,the content of CH₄increased significantly and reached the highest content at 7.0 MPa.RT has little effect on the gas component.The low heating value（LHV）of combustible gas increased with the increase of pressure.（4）The composition and properties of pyrolysis tar obtained under different pressures were analyzed by FTIR and GC-MS.The N-H and O-H functional groups in tar were broken,and the content of oxygen-containing compounds such as acids,alcohols,and amides in tar decreased with the elevated pressures.Pressure promoted the aromatization reaction of chain hydrocarbons,such as hydrocracking of chain hydrocarbon and free radical cyclization and dehydrogenation reactions.The quality of tar was improved by increasing the pyrolysis pressure.（5）From the perspective of char utilization,the physicochemical properties,LHV,heavy metal accumulation,functional groups and microscopic surface structure of chars derived from different pressures were analyzed.With the increase of pressure,volatile content char decreased,while ash and fixed carbon content increased.Considering the most of heavy metals contained in SS were fixed in char,the potential pollution in land application needed to be controlled.FTIR results showed that the increase of pressure promoted the fracture of C-H,C=C and COO-functional groups.SEM showed the expansion of the internal structure of char,pressure promoted the development of pore structure.The functional groups and microstructure of char are worth further study for its catalytic and adsorption applications.