The Formation And Evolution Of Polycyclic Aromatic Hydrocarbons (PAHs) In The Process Of Sewage Sludge Pyrolysis

Sludge,as a by-product of wastewater treatment,has a high volatile content on its drying base,a considerable calorific value,and good potential for recycling and energy recovery.Pyrolysis reaction,as an energy recovery type thermochemical reaction,is widely used in the industry as one of the recovery and treatment technologies of sludge.Studying the generation and evolution of PAHs in the process of sludge pyrolysis treatment will be helpful to effectively prevent and reduce the environmental risk of PAHs pollutants and build a theoretical foundation for the optimization of sludge energy cleanup process.The thesis mainly focuses on the investigatons into generation and distribution of 16 PAHs which that are preferentially controlled by the U.S.Department of Environmental Protection under the conditions of different pyrolysis conditions,different sources of sludge,and different pretreatment methods.It was aimed to explore the sources of PAHs production.The quantitative analysis of 16 PAHs in pyrolysis products was carried out by solid phase extraction and gas chromatography mass spectrometry.A comprehensive study of PAHs in pyrolysis products was conducted from the aspects of mass fraction of 16 PAHs,TEQ,and distribution of PAHs with different rings.The pyrolysis temperature has a significant effect on the evolution of PAHs during the pyrolysis of sludge.During the pyrolysis of sludge,the PAHs tended to be concentrated in the liquid phase product,followed by the gas phase product.All 16 PAHs(?16PAHs)were detected in the liquid phase product.The total mass ratio of ?16PAHs in the liquid phase product at 650°C was the highest.It is 96%,mainly including low-ring PAHs naphthalene(NaP),terpene(Acp)ring PAHs(Pyr),flex(CHR)high-ring PAHs benzo(a)quinone(BaP),benzo(ghi)(The mass ratio of ?16 PAHs in the gas phase product at 750 °C is the highest,which is more than 21.3%.The content of PAHs in the solid phase product is very small,only 1% at 650 °C.At the pyrolysis temperature of 850 °C,the PAHs content and the toxicity equivalent in the liquid phase reached a maximum of 15.25 mg·kg-1 and 1.129,respectively.The gas-phase products mainly consisted of naphthalene(NaP),amphetamine(Acp),alum(Flu),and anthracene(Ant)PAHs.No high-ring PAHs were detected.The presence of ?16PAHs in sludge from four different sources,including domestic sewage sludge,food processing sewage sludge,industrial mixed sewage sludge,and printing and dyeing sludge,was detected,but their distribution content was different.The content of free PAHs in the dried samples of domestic sewage sludge was the highest(1.59 mg·kg-1),and the content of free PAHs in dried sludge of food processing was the lowest(0.59 mg·kg-1).The seven-sludge sludge toxicity equivalent(0.178)is slightly higher than Lin'an sludge(0.167).The amount of PAHs produced in the pyrolysis process of sludge is closely related to the volatilization content and pyrolysis temperature of the sludge itself.In addition,low PAHs dominated in the pyrolysis products of different sources,and the percentage of PAHs in the high proportion of plutonium was different.For example,the proportion of PAHs in pyrolysis liquid products of industrial mixed sludge accounted for up to 37.91%(850 °C),the lowest proportion of pyrolysis liquid products in domestic sewage sludge was 19.50%(850 °C).The degree of sludge drying also affected the PAHs content distribution in the pyrolysis products.As the moisture content decreased,the PAHs content in the liquid phase product continued to increase.When the sludge was completely dried and pyrolyzed,its liquid phase PAHs content increased significantly.It increased by 2.45 times(450°C)and 7.74 times(850°C).With the addition of wood flour(4:1)and the pyrolysis of the sludge,although the heating value increased,the PAHs produced by pyrolysis were less than those without the addition of about 43%(450°C)and 69%(850°C).The deashing treatment removes the inorganic components in the sludge and enriches the organic components in the sludge.The content of ?16PAHs in the liquid phase product is increased by 2.2 times.When KCL and Na2CO3 were added as catalysts to co-pyrolyze with sludge,the effect of PAHs on cracking was similar.The content of PAHs in liquid products decreased by 43% under high temperature conditions.Sludge contains some small amounts of stable PAHs,and a comprehensive comparative analysis of the formation of PAHs in the pyrolysis products.It was found that PAHs during the pyrolysis of the sludge is mainly produced by the cracking,polycondensation and aromatization of organic components.