Emission Characteristics And Formation Mechanism Of Polycyclic Aromatic Hydrocarbons During Used Mineral Oil Combustion And Pyrolysis

The combustion and pyrolysis of used mineral oil（UMO）is an important way to realize its volume reduction and safe reclamation.However,the toxic polycyclic aromatic hydrocarbons（PAHs）,which has detrimental influence on human health,environment and the utilization of products,can be produced during this process.In view of the deficiencies of the research on PAHs formation and emission during the thermal disposal of UMO,the emission characteristics and formation mechanism of PAHs during the combustion and pyrolysis of UMO were deeply studied in this paper.It provides guidance for the optimization of thermal disposal process of UMO and the control of PAHs.The results of UMO combustion in the drop tube furnace showed that high similarities on the emission characteristics of PAHs from different UMO combustion were observed in spite of the varied contents and species of PAHs in the five kinds of UMO.2-and 3-ring PAHs made up more than 70%of the total PAHs in flue gas,with naphthalene being the most predominant species.Over 85%of the emitted PAHs was partitioned into gas phase,while PAHs in the particulate phase were more toxic.PAHs produced from UMO combustion mainly came from the incomplete combustion of fuel.PAHs in the original mineral oil had little influence on PAHs formation.High temperature（excess air coefficientα=1）was beneficial to reduce PAHs emission,but can lead to the enrichment of highly toxic PAHs in flue gas.Raising excess air coefficient was more conducive to reduce the production of PAHs in the particulate phase.More than 95%of PAHs in flue gas can be eliminated whenαwas increased from 0.8 to 1.4 at 900°C.It was found that the different collocations of temperature and heating rate gave rise to various kinds of valuable products during used engine oil（UEO）pyrolysis.The main components of the gaseous product were methane and C₂～C₄ olefins.The pyrolytic oil obtained can be used as base oil,fuel oil or chemical feedstocks.Increasing temperature and heating rate can lead to more PAHs formation and the conversion of small PAHs to heavy PAHs.Comparatively,distillation dominated in the slow pyrolysis,and chemical reactions occurred markedly at the high temperature（≥500°C）conditions of medium and fast heating modes for both UEO evolution and PAHs formation.Over 92%of the total PAHs produced was enriched into pyrolytic oil during different UMO pyrolysis.The primary PAHs species in the pyrolytic oil were different at different heating rate conditions,while the main PAHs species in all gaseous products were naphthalene,phenanthrene and pyrene.The naphthene component of UMO was the prominent factor influencing PAHs formation during pyrolysis.However,the contributing factor of aromatics to PAHs formation was about fourfold of the naphthenes.Based on the established PAHs predicting index method,the trend of PAHs production from different UMO pyrolysis can be made a good estimation.CO₂ can reduce PAHs formation through depressing the dehydrogenation step of PAHs growing reactions and had a better inhibiting performance on small PAHs formation.The results of Reax FF MD simulation revealed that the thermolysis transformation of mineral oil was staged by the decomposition of oil to small species firstly,and then the polymerization of small substances.PAHs formation was dominated by the hydrogen abstraction and vinyl radical addition reaction during mineral oil pyrolysis.In addition,the polycyclization of side-chain groups attached to the carbon cluster was an important route to form heavy PAHs.CO₂ can reduce PAHs formation by inhibiting dehydrogenation and vinyl/acetylene addition reactions during pyrolysis.The oxidation of intermediates by CO₂can also lead to PAHs reduction under this high temperature scenario.In the combustion process,the oxygenolysis of reactants and intermediates gave rise to less PAHs formation.