The Formation And Reduction Mechanism Of PAHs During Medical Waste Incineration

Medical waste is designated as a hazardous waste in the resource conservation and recovery act.Incineration is one of the main methods used for medical waste disposal.However,the medical waste incineration cannot avoid releasing persistent toxic organic pollutant?such as PAHs and dioxin?into environment.Thus,how to reduce the PAHs formation and its toxicity during medical waste incineration is urgently to be solved for the harmlessness,reclamation and safety disposal of medical waste.On the basis of the catalysis,adsorption,and heat transmission of porous alumina bed materials,porous alumina was used as an alternative bed material to suppress PAHs formation and reduce its toxicity during medical waste fluidized-bed incineration.The research contents and the key findings of this dissertation are as follows:Both a thermo-gravimetric analyzer?TGA?and the micro fluidized-bed reactor?MFBR?were used to study the reaction kinetics during medical waste pyrolysis/incineration process.As the size of conventional silica bed materials is increased from 160¹80?m to 250³20?m,the heat transfer coefficient of the silica bed materials are decreased,and the reaction activation energies during medical waste pyrolysis/incineration are increased.Meanwhile,the reaction activation energies and conversion rate during medical waste incineration under the bed materials of 180²50?m silica,nonporous alumina and porous alumina were also decreased systematically.The reaction activation energies during medical waste pyrolysis under the bed materials of 160¹80?m silica,180²50?m silica,250³20?m silica,nonporous alumina and porous alumina were in the sequence as47.6,52.7,57.3,44.8 and 47.4 kJ/mol.As for the medical waste incineration,the reaction activation energies under the bed materials of 160¹80?m silica,180²50?m silica,250³20?m silica,nonporous alumina and porous alumina were76.5,86.5,91.8,71.0 and 62.1 kJ/mol,respectively.Therefore,porous alumina can be used as an alternative bed material of reducing the reaction activation energy during medical waste incineration.In order to obtain the PAHs formation mechanism during medical waste incineration,the gas evolution profiles of the released gas during medical waste pyrolysis/incineration was studied by a thermogravimetric analyzer coupled with mass spectrometer?TG-MS?and a thermogravimetry coupled with gas chromatography-mass spectrometry?TG-GC/MS?.Meanwhile,thecorrelation of gaseous hydrocarbon?small hydrocarbon molecules,monocyclic?and PAHs in the flue gas were also investigated during medical waste incineration in a fluidized-bed reactor.The results indicated that the unsaturated hydrocarbon and monocyclic such as Ethylene?C₂H₄?,Ethyne?C₂H₂?,Vinylacetylene?C₄H₄?,1,3-butadiyne?C₄H₂?,Benzene?C₆H₆?,Toluene?C₇H₈?,Styrene?C₈H₈?,phenylacetylene?C₈H₆?,biphenyl(C₁₂H₁₀)and Naphthalene(C₁₀H₈)are the precursors of PAHs.Meanwhile,It was also postulated that the hydrogen-abstraction-carbon-addition?HACA?and monocyclic addition/cyclization?MAC?mechanisms are the main contributors for the 2⁴ rings PAHs formation.Whereas,the HACA mechanism plays an important role in the 5⁶ rings PAHs formation.Therefore,it is a significant pathway to suppress PAHs formation by reducing gaseous hydrocarbon?unsaturated hydrocarbon and monocyclic?in the flue gas.In addition,it was also found that the CO,CH₄,C₂H₄,Benzene,Styreneconcentration correlated well with the total PAHs concentration?R²>0.8?,and thus can be used as surrogate indicators for PAHs emission.In order to discover the reduction mechanism of PAHs suppressed by porous alumina bed materials during the medical waste incineration,experimental comparisons of the medical waste incineration under silica,nonporous alumina and porous alumina bed materials were carried out in both micro fluidized-bed reactor?MFBR?and semi-pilot scale fluidized-bed combustor?FBC?.Meanwhile,the effects of catalysis,adsorption,and heat transmission of porous alumina bed materials on the distributions and concentrations of the gaseous hydrocarbon?small hydrocarbon molecules,monocyclic?and PAHs in the flue gas were also conducted.It was postulated that there were three mechanisms caused PAHs reduction.Firstly,the low heat transfer coefficient would decrease the heat transmission rate between the bed materials and medical waste,which caused the reduction of the medical waste decomposition rate.Hence,less gaseous hydrocarbon?small hydrocarbon molecules,monocyclic?and PAHs were formed.When the bed materials of160¹80?m silica was replaced by 250³20?m silica,the total PAHs concentration in the flue gas was reduced by 32.9%.Secondly,the oxidization of the gaseous hydrocarbon might be promoted by the catalytic of alumina bed materials,which led to produce less gaseous hydrocarbon,and therefore suppressed the PAHs formation.When the bed materials of 180²50?m silica was replaced by nonporous alumina,the total PAHs concentration in the flue gas was reduced by 29.2%.Lastly,porous alumina bed materials possessed more BET area than that of the conventional silica sand,which could absorb more gaseous hydrocarbon and prolong the residence time of hydrocarbon in the diluted zone of FBC.Thus,the PAHs formation was suppressed.When the bed materials of nonporous alumina was replaced by porous alumina,the total PAHs concentration in the flue gas was reduced by 20.9%.The effects of operating parameters?such as temperature,resident time and excess air ratio?on the PAHs and TEQ concentrations reduced by porous alumina bed materials were also studied in a semi-pilot scale fluidized-bed combustor.The results demonstrated that operating parameters?temperature,resident time and excess air ratio?were significantly affected the distributions and concentrations of the PAHs in flue gas.Meanwhile,it also influenced the reduction of PAHs and PAHs TEQ by porous alumina bed materials.The maximum removal efficiency of PAHs and PAHs TEQ by porous alumina bed materials is obtained when the temperature is800^oC,the flue gas resident time is 1.8s,and the excess air ratio is 1.5.