Effects Of CaO/Fe₂O₃ On The Emission 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 lead to an enhanced production of sewage sludge.Thermal treatment is considered as a promising method to deal with sewage sludge because it can diminish 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 been converted in resource or energy by the method.However,the emission of trace elements and the generation of aromatics cause secondary pollutions,which shall be paid enough attentions.Understanding the transformation of trace elements and evolution of aromatics during sewage sludge is extremely important and fundamental to solve the problems.Calcium and iron compounds are abundant in sewage sludge,prat of which could transform to calcium oxide?CaO?and iron oxide?Fe₂O₃?.And these two oxides further affect the product distribution by catalysis effect or reactions with other materials.Exploring the effects of calcium oxide and iron oxide on the evolution of the two pollutants could help better understand the generation mechanisms and then put forward methods to inhibit pollution.The emission behaviors of three typical volatile trace elements arsenic?As?,cadmium?Cd?and lead?Pb?were monitored and the generation routes of aromatics were investigated in current study.Then,effects of CaO and Fe₂O₃ on the emissions of the two kinds of materials were studied.Moreover,through the co-pyrolysis of models 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 the type of sewage sludge and reaction condition?like temperature,heating rate,residence time?on the emissions of trace elements were explored through detection of trace elements in char from pyrolysis of sewage sludge on the fixed bed.The results showed that temperature is the crucial factor to influence the emissions of Cd and Pb,but shows little effects on the emissions of As.The enhancement of residence time promotes the release of organics and 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 corrections were observed between the release amount of arsenic and organics during sludge pyrolysis.The effects of heating rates in the emission of trace elements dependent on 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 converse effect for high-ash containing sludge.By the combination 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 explored 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 convertes 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 in heavy components first increase and then decrease from 450? to 850?.Under the combined effects of decomposition and aromatization,the amount of aromatics produced from sludge reach the peaks at 650?.By the comparision of bio-oils from sludge and the mixuture,it was found that CaO and Fe₂O₃ exhibit significantly 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 are able to react with NaAsO2 to inhibit the emission of arsenic.Nevertheless,the product of CaO and NaAsO2 is more difficult to be volatile or decompose at high temperatures than that from CaO and NaAsO2.The addition of CaO and Fe₂O₃ can promote the cracking of long-chain structural molecules by chain scission and aromatization to produce small aromatic molecules.Meantime,they also lead to the generation of long-chain aromatics at 450? and 650? by the combining effects of aromatization and polymerization.CaO enhances the aromatic degree of liquids at all temperatures while Fe₂O₃ only shows obvious effects at 850?.Both the two oxides could lead to the enhancement of aromatics from sludge at low temperatures.CaO can facilitate the generation of aromatics with large rings at 850?,while Fe₂O₃ also leads to the enhancement of single aromatic ring structural molecules.The tests of models 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.What is more,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 are also improved.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 transforms 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.Different from the effects of CaO,the addition of Fe₂O₃ leads the increase of aromatics with 1² ring and aromatics with more than 4 rings in the heavy components.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.Catalysis effects shown by CaO/Fe₂O₃ on the evolution of protein and lipids could be responsible for this phenomena.