Research On Formation And Evolution Characteristics Of Soot Particles In Msw Incineration Flames

With the rapid development of China’s economy,the output of municipal solid waste(MSW)has increased significantly.Under the background of the “Double Carbon” policy in China,incineration has increasingly become the preferred proposal method for the harmlessness,reduction,and resource treatment of MSW.Soot particles are important solid-phase products formed during the incomplete combustion of MSW.They not only affect the incineration efficiency and radiative heat transfer inside the furnace,but also play an important role in the formation process of other pollutants especially dioxins,which have adverse effects on the human health and atmospheric environment.Therefore,it is of great significance to study the formation mechanism of soot particles in MSW incineration flames.In this paper,combustion experiments were conducted using single-particle and fixed bed combustion experimental devices,and multiple analytical methods such as High-resolution transmission electron microscopy(HRTEM),X-ray diffraction spectroscopy(XRD),Raman spectroscopy(Raman),and Thermogravimetric analysis(TGA)were used to systematically characterize the physical and chemical properties of soot particles formed during the combustion of MSW with complex components.Meanwhile,the formation and evolution process of soot particles in the combustion flame and the effect of MSW composition and experimental parameters on soot formation were investigated in this paper.The main results are as follows:Firstly,the physicochemical properties and oxidative reactivity of soot particles from the combustion of typical single components of MSW were studied.The results showed that the soot particles mainly took the form of agglomerates with fractal characteristics in the combustion flame,and their internal structures exhibited a typical “core-shell” shape.Compared with biomass components such as packaging paper,plastic components(polyethylene,polyvinyl chloride,and chemical fiber)generated soot particles with larger deposition quality and particle size,and the aggregate structure of these soot particles was tighter;Quantification of the nanostructure in soot by fringe analysis showed that biomass components generated soot particles with more regular fringe distribution,indicating the higher degree of graphitization and lower oxidation activity.It can be seen that the physical and chemical properties of soot were greatly affected by the differences in MSW components,and plastic components were considered the main source of soot emissions during MSW incineration.Further correlation analysis between the physicochemical parameters showed that the nanostructure was highly related to the graphitization degree and oxidation activity,while the correlation between the nanostructure and surface oxygen-containing functional groups was poor,indicating that the oxidative reactivity of soot particles mainly depended on the nanostructure.The larger the fringe tortuosity and separation distance of soot particles,the higher the disorder level of the nanostructure,the lower the degree of graphitization,and the stronger the oxidation reactivity.To further study the effects of MSW composition and experimental parameters on the nanostructural characteristics of soot particles,a single-particle combustion device based on a Hencken flat flame burner was used.And the soot samples were collected using thermophoretic sampling and deposition sampling methods.The results showed that with the increasing proportion of plastics(volatile content)in the mixed waste,the soot particles were formed a lot with larger deposition quality,particle size,and fractal dimension.And in terms of the nanostructure,the fringe length got shorter,while the fringe tortuosity and separation distance both increased.It was significantly found that the 30% proportion of plastics in MSW was considered the key turning point for the changes in both deposition quality and nanostructure of soot particles.When investigating the effect of typical experimental parameters on the soot properties,the results showed that the increase of ambient temperature and oxygen concentration both resulted in the increase of flame temperature,which further reduced the size of soot particles by enhancing the carbonization and oxidation processes;however,the increase of ambient temperature made the nanostructure of soot particles be more ordered,indicating the higher degree of graphitization and lower oxidative reactivity,while the increase of oxygen concentration showed the opposite effect.For diffusion flames from the combustion of mixed MSW,as the flame height increased,the evolution process of soot particles can be divided into three stages,namely the nucleation stage,surface growth stage,and carbonization and agglomeration stage;along with this process,the mean particle size of soot particles first increased and then decreased,while for the nanostructure,the fringe length became longer,while the fringe tortuosity and separation distance both decreased,revealing the evolution of soot particles from nascent to be mature.Then,aiming at the volatile mineral components in municipal solid waste,the release characteristics of atomic alkali metals during the single-particle combustion of typical combustible components were explored firstly by flame emission spectroscopy(FES).The results showed that the emission spectrum of MSW combustion flame was composed of the continuous spectrum of soot radiation and the characteristic spectrum of alkali metal(Na and K)atoms,which confirmed that soot particles were the main participating radiation medium in the flame.The FES method was modified according to the Beer-Lambert law,and the quantitative relationship among the flame temperature,alkali metal spectral intensity,and alkali metal concentration was further established.According to the concentration curves of alkali metals,it can be known that the release characteristics of Na and K were similar,both featuring threestage processes including volatilization,char,and ash reaction stage.And the overall release characteristics of Na and K were bimodal,but the gas phase concentration of K was significantly higher than that of Na.The results showed that the release peak concentration of alkali metals was affected by their initial content in MSW,with a strong positive correlation between them.It revealed that the higher the alkali metal content in MSW,the stronger its release ability during combustion.Based on the understanding of alkali metal release characteristics,typical water-soluble potassium chloride was added to the mixed waste using the impregnation method,and then the influence of K content on soot physicochemical characteristics was investigated.The results showed that the increase in K content led to a higher concentration of gas phase K in the volatile flame.In terms of the nanostructure,as the K content in the mixed waste increased to 5%,the size of soot particles decreased and became uniform,and the fractal dimension increased slightly,while the fringe tortuosity and separation distance increased,indicating that the nanostructure developed to be more disordered with the lower degree of graphitization.The thermogravimetric analysis showed that the oxidative reactivity of soot particles was enhanced,with the apparent activation energy decreasing from 167 k J/mol to 124 k J/mol.Combined with the release characteristics of alkali metals during MSW combustion,the intrinsic relationship between the K release path and the evolution of soot formation in the combustion flame was explored.The interaction between K and soot resulted in the inhibition of soot formation and catalyzation of soot oxidation.Finally,a fixed bed combustion device was built,and the characteristics of soot particles formed during the combustion process of MSW were further studied.The influence of typical experimental parameters and fuel properties on the generation of soot particles was explored using the orthogonal test method.It was found that the excess air coefficient and proportion of plastics in MSW were respectively the main experimental parameter and fuel property.And based on this finding,the experiment was subsequently conducted to study the formation mechanism of soot particles during the combustion of high calorific value MSW under optimal combustion conditions.Py-GC/MS results showed that the mature soot particles were composed of a highly aromatic core and external chains with nitrogen and oxygen atoms.And with the increasing plastic proportion in MSW,the relative content of PAHs also increased.PAHs was the important precursors of soot particles,and the increase in PAHs relative content agreed well with the increases in the soot particle size and deposition quality.In addition,the disordered soot and PAHs were also important carbon sources for the synthesis of dioxin.High contents of chlorine and copper elements were meanwhile found in soot particles.Chlorine element can be seen as the chlorine source for dioxin synthesis,while copper could be seen as the catalyst to promote the formation of dioxins.