Study On Soot Formation And Physic-Chemical Properties Of Soot In ABE Blended Fuels Diffusion Flame

Air pollution and fossil energy shortage are becoming more and more serious in China,while car ownership is growing rapidly,it is urgent to find some clean and high effective energy to ensure the sustainable development of our nation and society.In this direction,n-butanol has been considered to be the most promising biofuel for large-scale applications,because the combustion characteristics of n-butanol are similar to diesel,and it can reduce both soot particles and NOx emissions.Biological Fermentation and Purification of the mixed solution of Acetone-Butanol-Ethanol(ABE)from renewable biomass feedstock is the main method to produce bio-butanol,which uses some Clostridium bacteria to metabolize the sugars,organic acids,and polyalcohol under the anaerobic environment.However,the energy requirement of separating purity butanol from ABE is too high.Therefore,the direct application of ABE as a bio-alternative fuel has become a research hotspot in the fields of combustion,energy saving,emission reduction,environment,and safety,etc.The existing researches have confirmed that ABE reduces soot emission in the engine,and ABE has similar spray combustion characteristics to diesel.But few studies have been conducted on the basic combustion parameters of ABE fuel,such as laminar flame speed,ignition delay time,extinction limit,heat release efficiency,soot formation,etc.This work aims at studying the sooting tendency of ABE/Diesel blends,the variation in the soot oxidative reactivity upon the addition of ABE to diesel,the inhibition mechanism of ABE on soot formation in diffusion flame,and the effect of ABE addition on the formation or consumption of the polycyclic aromatic hydrocarbons(PAHs)in premixed flame.Specifically as follows:We designed a wick-fed burner to generate liquid fuel diffusion flame.The relationship between flame height and fuel consumption rate was investigated,and the smoke points of ABE/Diesel blends were measured.The sooting tendency of blend fuels was evaluated using the Threshold Sooting Index(TSI)and Oxygen Extended Sooting Index(OESI).It is observed that flame height versus fuel uptake rate starts with a linear relationship,then disjointed points appear,finally returns linear relationship again.The flame height corresponding to the first mutational point is close to the smoke point measured by the observation method.Normalized TSI and OESI values show an exponential relationship with the blending ratio,and the higher exponential index for ABE-diesel-blends indicates that ABE has a stronger ability to inhibit soot formation than butanol.The different oxygen content and C/H atomic ratio indicate that butanol and ethanol in ABE promote the ability to inhibit sooting tendency,while acetone plays an opposite role.The chemical kinetic mechanism of ABE oxidation was simplified using the Direct Relation Graph Method Based on Error Propagation(DRGEP),then the ignition delay time of toluene and butanol,the laminar premixed flame structure of ABE and butanol fuels,the laminar premixed flame velocity of ethanol and butanol were calculated using the simplified mechanism,the simulation results agree well with the experimental measurements.The characteristic parameters of soot formation in ethylene and ethylene/ABE diffusion flames were calculated by CoFlame program.The results show that the soot volume fraction decreases significantly along the pathline of maximum soot on the wings and centerline for the ethylene/ABE flame compared to that for ethylene flame,and the primary particle diameter first increases then decreases.The reduction along the pathline of maximum soot is smaller than that along the centerline.We analyzed the maximum soot nucleation rate at each axial height for the ethylene and ethylene/ABE flames,both have the same trend along pathline.At the same flame height,the methods of the H,H2,and C2H2 formation are changed,which reduces the rate of H-Abstraction-C2H2-addition reaction(HACA)and the concentration of PAHs involved in soot nucleation.Ultimately,the nucleation rate in ethylene/ABE flame is very low,and lower by 1?2 orders of magnitude compared to that in ethylene flame.The soot surface growth rate in ethylene/ABE flame decreases by 1-fold,and the peak rate changes slowly because of the distribution of C2H2 concentration.The reaction rate of soot and O2 in ethylene/ABE flame is about 30%lower than that in ethylene flame,while the reaction rate of soot and OH radical is slightly higher for the flame height above 7 cm.Alcohols are conducive to the formation of HO2 radical,then produces a higher concentration of OH radical,which offset the reduction in soot volume fraction and primary particle diameter.The zero-dimensional,constant volume and mass,and adiabatic model(SENKIN)in CHEMKIN software was used to study the decomposition paths of each composition in ABE fuel.The effect of ABE addition on PAHs formation and consumption was investigated by analyzing the rate of production(ROP).The results show that the attenuation proportion of maximum PAHs mole fraction decreases step by step with the increase of benzene rings in PAHs.The carbon flow rate of butanol and ethanol transfer to the C2H2 formation in ABE are higher than that of acetone.The PAHs content in flame decreases upon the addition of ABE because(1)the contribution rate of ABE/ethane-blend to the C2H2 formation is lower than that of purity ethane,and decreases the content of C2-C3 species which determine the benzene(A1)formation;(2)the decrease of C2H2 and A1 concentration reduce the HACA reaction rate,and shows an exponential decay trend along with benzene rings;(3)butanol and ethanol contribute to the formation of HO2 radical,then produce more OH radical,which accelerates PAHs consumption.The soot samples derived from ABE/diesel diffusion flame were collected offline,the thermogravimetric curves of soot particles were obtained by the comprehensive thermal analyzer(TGA).The activation energies of soot samples oxidation reaction were calculated by Kissinger-Akahira-Sunose(KAS)method,the results show that the oxidation activity of soot particles increases with the growth of blending ratio.Furthermore,the elemental composition,functional groups on soot surface,and microstructures of soot samples were measured.It is found that with the increase of ABE content in blends,the content of C=C double bond functional groups decreases,the content of oxygen-containing functional groups increases,and the O/C and H/C atomic ratios increases,which indicate that there are more reaction sites on soot surface and fewer oxygen requirements for soot oxidation.With the increases of ABE contents in blends,the internal structure of soot becomes more disordered,the size and stacking height of PAHs nanocrystallite decreases,the PAHs nanocrystallite layers and the curvature of PAHs nanocrystallite increases.The more disordered internal structure indicates that it can be more easily for oxygen to enter the soot,the reduction in PAHs nanocrystallite size determines the carbon number at the edge of soot increases,and the greater curvature demonstrates the energy required for bond breaking decreases.The variation in physical and chemical properties of soot indicates that soot particles derived from ABE/diesel blends diffusion flame are oxidized more easily.