Study On The Formation,evolution,and Control Of Soot Particles In Alcohol-doped Ethylene Diffusion Flames

With the rapid development of the economy and society,energy and environmental issues have become increasingly prominent,which has been one of the important factors restricting social development.Soot particles produced by incomplete combustion of carbonaceous fuels are one of the main air pollutants,and they have implicated in adverse environment and human health consequences.The use of clean alternative biofuels and the development of advanced emission control technologies are the main effective ways to reduce soot emissions.Understanding the soot evolution process and the physicochemical properties of soot particles from alcohol-doped flames play an important role in reducing soot formation and developing soot removal technology,and could provide theoretical guidance for the application of bio-alternative fuels.On the basis of inverse diffusion burner platform,the effects of flame structures(inverse diffusion flame and normal diffusion flame),fuel molecular structures(four butanol isomers and three pentanol isomers),and combustion atmospheres(CO₂,N₂,He,H₂O and active molecule O₃)on soot evolution and variations of physicochemical properties including morphology,nanostructure,oxidation reactivity,graphitization degree,surface chemistry and element compositions were investigated.The soot particles were captured using thermophoretic sampling and quartz plate sampling technologies,and characterized by transmission electron microscopy(TEM),high-resolution transmission electron microscopy(HRTEM),thermogravimetric analyzer(TGA),Raman spectroscopy(Raman),X-ray diffraction spectroscopy(XRD),X-ray photoelectron spectroscopy(XPS),elemental analyzer,and surface area(BET)analyzer to explore the soot evolution process and control mechanism.Furthermore,the fringe characteristic parameters were extracted and calculated by compiling the HRTEM image processing program,and the relationship between the soot nanostructure and reactivity was established.Firstly,the effects of long-chain alcohol isomers on soot formation features were investigated.The variations of physicochemical properties of soot particles in butanol isomers-doped ethylene inverse and normal diffusion flames and pentanol isomers-doped ethylene inverse diffusion flames were studied by means of quartz plating sampling.The soot produced by inverse diffusion flames with butanol isomers contained irregularly shaped liquid-like material and solid particles.The soot nanostructure was highly heterogeneous showing amorphous carbon and fullerenic-like structures.When n-butanol,iso-butanol,sec-butanol,and tert-butanol was separately blended in the inverse diffusion flames,the soot graphitization degree and the fringe length increased,and the fringe tortuosity decreased,leading to a reduction in oxidation reactivity.Whereas in the normal diffusion flames as butanol isomers was individually added,the soot was composed of nearly spherical particles,and exhibited a typical core-shell structure.The soot oxidation reactivity gradually increased as the normal diffusion flame was successively added with iso-butanol,n-butanol,tert-butanol,and sec-butanol,which was due to the decrease in soot graphitization degree and fringe length,and the increase in fringe tortuosity.The soot particles formed in the inverse diffusion flame with pentanol isomers additions were also covered with liquid-like substances,and the nanostructure was highly heterogeneous.The additions of pentanol isomers could enhance soot reactivity through reducing the degree of graphitization.The effects of n-pentanol and iso-pentanol were more significant.Secondly,the spatial variation of soot physicochemical features was studied by a combination of thermophoretic sampling and quartz plate sampling methods,through collecting the particles in n-butanol-doped ethylene inverse and normal diffusion flames at different axial heights and radial positions.The soot particles in inverse diffusion flames avoided significant carbonization and oxidation,and the sizes of primary particles and aggregates increased with the increase of axial height.The soot graphitization degree was low,thus the oxidation reactivity was high.In normal diffusion flames,the diameter of the particles increased first and then decreased with the increasing flame height.In the lower flame region,the particles mainly experienced the surface growth process and aggregated by collision,while in the upper region the oxidation process dominated.The soot from normal diffusion flames had higher degree of graphitization,leading to lower reactivity.The addition of n-butanol to the flames with different types could enhance soot oxidation reactivity.The growth patterns of soot particles along the centerline and the boundary of the same flame type were similar.Then,the effects of combustion atmospheres on soot formation,evolution and control mechanism were further analyzed.The soot formation and evolution processes and physicochemical properties of particles in n-butanol added ethylene diffusion flames with different dilution gases including CO₂,N₂,He,H₂O and active molecule O₃ were investigated by thermophoretic sampling and quartz plate sampling technologies.The soot particles all experienced inception,surface growth,agglomeration and carbonization process in different atmospheres.Both CO₂ and H₂O diluent gases could inhibit soot formation in the inception process,mainly reducing the important H radical required for the formation of polycyclic aromatic hydrocarbons and soot by the chemical effects,which lead to a reduction soot generation.CO₂ mainly consumed H radical by participating in the reaction CO+OH?CO₂+H,while H₂O primarily took part in the chemical reaction H₂O+H?OH+H₂ to reduce the H concentration,which resulted in a decrease in the initial nuclear rate of soot.CO₂ could enhance the soot reactivity and reduce the carbonization degree.The higher the concentration of H₂O added in the flame,the higher the degree of graphitization of the soot,thus the oxidation reactivity was lower.Blending n-butanol in different atmospheres could improve the soot oxidation reactivity,because the addition of n-butanol reduced the degree of graphitization and increased the surface oxygen content.The formation of soot particles increased slightly with O₃ addition,but the surface oxygen content and the nanostructure disorder of the soot increased,which made the oxidation reactivity enhanced.Through the above analyses,the relationship between the soot nanostructure and oxidation reactivity in different conditions was finially established.There is a high linear correlation between the soot nanostructure and oxidation reavtivity.The oxidation reactivity was negatively correlated with the fringe length,and positively correlated with the fringe tortuosity.The soot with longer fringe length and smaller fringe tortuosity would show lower reactivity.