Effects Of CaO/Fe₂O₃ On The Release Of Trace Elements And Aromatic Evolution Of Bio-oils During Sewage Sludge Pyrolysis

The development of urbanization and the increasing requirement for a high quality of water resource lead to a rapid increase of sewage sludge production.Thermal treatment is considered as a promising method to dispose sewage sludge because it can disinfect the bacteria in the sample as well as reduce the biological activity of the trace elements in the solid.What is more,the organics in sludge could also be converted into energy resource by the method.However,the emission of trace elements and the generation of aromatics cause secondary pollutions,which shall be paid enough attention.Understanding the transformation of trace elements and the evolution of aromatics during thermal treatment of sewage sludge is extremely important and fundamental for solving above problems.Calcium and iron compounds are main inorganic components in sewage sludge,part of which could transform to calcium oxide?CaO?and iron oxide?Fe₂O₃?.These two oxides further affect the product distribution by both theircatalytic effects and acting as reactantsin the process.Investigating these effects of calcium oxide and iron oxide is critical to understand the formation mechanism of pollutants and then put forward to control strategies.The release behaviors of three typical volatile trace elements such as arsenic?As?,cadmium?Cd?and lead?Pb?,as well asthe generation routes of aromatics in pyrolytic oil were investigated in the current study.Then,effects of CaO and Fe₂O₃ on the emissions of the three materials were studied.Moreover,through the co-pyrolysis of model compounds and the oxides,the inhibitory mechanisms of the oxides on arsenic emissions and the effect mechanisms of oxides on aromatics generation were further explored.This study is of great importance for the harmless thermal disposal and resource utilization of sewage sludge.Effects of sewage sludge types and reaction conditions?temperature,heating rate and residence time?on the emissions of trace elements were studied through quantification of trace elements in char from pyrolysis of sewage sludge on the fixed bed.The results showed that the temperature played a crucial role in affecting the emissions of Cd and Pb,but shows little effects on the emissions of As.The increase of residence time promotes the release of organics and the emission of trace elements.The release rate of Cd and Pb is lower than that of organics,but similar with that of arsenic.Strong linear correlations were observed between the release amount of arsenic and organics during sludge pyrolysis.The effects of heating rates in the emission of trace elements are dependent on the type of sewage sludge.It was found that the high heating rates promote the emission of trace elements in sludge with low ash contents,but show an inverse effect for high-ash containing sludge.By the combination characterization of ultraviolet fluorescence,gas chromatograph-mass spectrometer and Fourier transform ion cyclotron resonance mass spectra,the aromatic evolution of bio-oils derived from sewage sludge was revealed from both a global and molecular level.Lipids are the main composition in the liquids derived from pyrolysis of sewage sludge at 450?,in which aromatics containing nitrogen and oxygen atoms are also observed.The increasing temperature causes the cracking of heavy components and oxygen containing molecules by chain scission effect and deoxygenation.Moreover,it also converts chain structural molecules into aromatics.Aromatic compounds are found to be abundant in the liquids produced at 850?,where large amounts of aromatics containing nitrogen atoms and aromatics containing oxygen atoms are observed.The amount of heavy components first increase and then decrease from 450? to 850?.With the synergetic effects of decomposition and aromatization,the amount of aromatics reaches the peaks at 650?.By the comparison of bio-oils from sewage sludge mixed with or without CaO,we found that CaO and Fe₂O₃ significantly exhibit inhibitory effects on the emission of arsenic but show little effect on the emission of cadmium and lead.The addition of these two materials leads to the capture of 14.0-57.0% and 7.4-51.1% of arsenic,respectively.The inhibitory effect is more remarkable at high temperatures with the addition of CaO.This is because that CaO could react with As₂S₃ and As₂O₃ to generate thermal stable arsenic compounds,but Fe₂O₃ could not.Both CaO and Fe₂O₃ can react with NaAsO₂ to inhibit the emission of arsenic.Nevertheless,the product of CaO and NaAsO₂ is more difficult to be volatilised or decomposed at high temperatures than that from CaO and NaAsO₂.By comparing the composition of bio-oils derived from the pyrolysis of sludge and the mixture of sludge and CaO,it was found that CaO significantly increased the aromatic degree of bio-oils from 450 to 850?,but Fe₂O₃ only increased it at 850?.GC-MS and FT ICR-MS results suggest that both of CaO and Fe₂O₃ could promote the cracking of long-chain structural molecules by chain scission and aromatization to produce small aromatic molecules.In order to comprehend the impact mechanisms,model compounds experiments using soybean protein and octadecanoic acid were conducted.The results suggest that high temperature promote the condensation of both lipids and protein,but the condensation degree for lipids is more obvious.The model compounds experiemnts demonstrated that temperature promotes the aromatization of lipids and protein at high temperature and the effects are more obvious for liquids.CaO can decrease the reaction temperature for stearic acid and facilitate them to convert into aromatics.In addition,CaO can lead to the cracking of protein,thus improving the amount of single aromatic structural aromatics at low temperatures.Besides,the amount of molecules with long chains in heavy components and polycyclic aromatic hydrocarbon like phenanthrene,fluoranthene and pyrene also increases.Fe₂O₃ inhibits the decomposition of stearic acid and shows little promoting effects on the aromatic degree of bio-oils at low temperatures.At 850?,Fe₂O₃ inhibits the formation of single aromatics and converts them into PAHs.At the same time,Fe₂O₃ also inhibits the aromatization of heavy components.During the pyrolysis of protein,Fe₂O₃ promotes the cracking of protein and the aromatic degree of bio-oils.From the above,it could be concluded that the elevation of temperature promotes the emission of trace elements and the aromatics.Fe₂O₃ and CaO can react with arsenic compounds like As₂S₃ and NaAsO₂ to inhibit the emission of arsenic from sewage sludge.Meantime,the addition of the oxides facilitates the generation of aromatics to various degrees.Catalytic effects behaved by CaO/Fe₂O₃ on the evolution of protein and lipids could be responsible for this phenomena.