Analysis Of Solid Products And Distribution And Transformation Of Polycyclic Aromatic Hydrocarbons From Thermal Chemical Treatment Of Municipal Sludge

The increasing depletion of traditional energy sources has made it an inevitable trend for humans to develop and use new energy sources.In recent years,sludge treatment has become a difficult and hot topic for solid wastes.The presence of polycyclic aromatic hydrocarbons(PAHs)in sludge has greatly limited the application of sludge as a resource.Thermochemical treatment is currently a popular method in sludge treatment and disposal technologies.Therefore,the study of the conversion and distribution of PAHs during the thermochemical treatment process can help us to understand the conversion mechanism of PAHs in sludge and how to reduce them in solids content,which can as the theoretical basis for studying the PAHs in the products and provide new ideas for the application of sludge recycling.In view of the fact that there are still many deficiencies in the study of PAHs in the hydrothermal reaction and pyrolysis process,this subject has carried out the following researches:On the one hand,the effects of temperature,residence time and the volume fraction of cosolvent ethanol on the hydrothermal carbonization process of the sludge were studied.Three temperature gradients and three different residence times were set for the experiment,which were 180,220,260°C and 30 min 60 min,90 min,respectively.At the same time,the five different ethanol volume fractions were 0%,30%,50%,80%,and 100% at a temperature of 260°C and a residence time of 90 min.The thermogravimetric analysis of sludge and its solid products can be used to investigate the mass loss of organic matter in the sludge,and it can be inferred whether there is an inevitable relationship between the reduction of these organic substances and the change of PAHs.The changes in the changes in the functional groups of the curves and infrared curves are largely consistent with the following changes in the concentration of TPAHs.The change of concentration of PAHs in the hydrothermal reaction and the distribution of solid-liquid phase show that the concentration of TPAHs in the solid phase product decreases with increasing temperature and residence time,while the concentration of TPAHs in the liquid phase product gradually decreases.Hydrothermal technology can indeed inhibit the formation of solid product PAHs during thermochemical treatment.The products with the highest content in the solid phase product are An and Phe,and the dominant ones in the liquid phase product are Nap,An and Phe.With increasing temperature,the concentration of 2-ring PAHs increases,while the concentration of 3-ring PAHs and 4-ring PAHs decreases with increasing temperature,especially 3-ring PAHs.The concentration of the 5-ring and 6-ring PAHs has a slight increase during this process.There was a slight increase in concentration of PAHs in different solid and liquid phases at different residence times.The concentration of all PAHs in the solid phase product increased with the increase in the ethanol volume fraction.Except for Nap,Ace,Acey,and Fl,the concentration of all PAHs in the liquid phase product increased with increasing ethanol concentration,except for Fl,Phe,An,Ace,Pyr,Chr,BaF.The concentration of TPAHs in solid phase at the 0-30% ethanol fraction decreased.When the ethanol concentration increased to 80%,the TPAHs concentration gradually increased.When the ethanol volume fraction increasing to 100%,the TPAHs decreased again.The concentration of TPAHs in the liquid phase product continuously decreases when the volume fraction of ethanol is 0-30%.When the volume fraction increases continuously to 100%,the concentration of TPAHs continuously increases.The experimental results show that PAHs have the lowest concentration in solid and liquid products at 260°C-30%.Since the solid products(pyrolytic carbon)and liquid products(pyrolytic oil)produced during the pyrolysis process all have higher calorific value,they all have higher energy application prospects.Therefore,in order to understand the PAHs of the solid and liquid phase distributions and transformations under conditions of sludge pyrolysis,including the five temperature gradients and three different residence times were set up for experiments at 300,400,500,600 and 700°C,and 30 min,60 min,and 90 min,respectively.The changes of the solid product properties and 16 kinds of PAHs concentrations under different pyrolysis conditions were studied.The thermogravimetric analysis of the sludge and its pyrolyzed solid product does not provide a good indication of its changing pattern.From the inf rared spectrum of the pyrolysis-char,it can be indicated that the decomposition of aliphatic hydrocarbons can be accelerated,and the concentration of PAHs in the pyrolysis products is greatly affected by the degradation of macromolecular substances such as proteins and lipids in the sludge with increasing temperature and residence time.The change of PAHs concentration and the distribution of solid-liquid phase in the pyrolysis reaction of sludge showed that the concentration of TPAHs gradually increased with the increase of temperature and residence time.Among the solid phase products,Phe and Fl are the main polycyclic aromatic hydrocarbons.Among the liquid products,Nap is the most important polycyclic aromatic hydrocarbon.The concentration of 2-ring PAHs in pyrolytic carbon decreases with increasing temperature.3-ring,4-ring,5-ring and 6-ring PAHs increase with increasing temperature,especially 3-ring PAHs;all concentrations of PAHs in pyrolysis oil increase with increasing temperature.The residence time is consistent with all PAHs concentrations and increases with increasing residence time.