| Ammonia（NH3）has a mature infrastructure system for production,storage and transportation,and is considered to be a highly competitive alternative fuel for hydrogen storage.The mixing combustion of NH3 and hydrocarbon fuel is one of the important technological pathways to achieve the carbon emission reduction in the field of transportation power.However,there is a lack of understanding of the mechanism on the effects of NH3 addition on soot formation in hydrocarbon fuel flames,and the related studies are few.In this paper,the effects of NH3 addition on flame structure,the formation behavior of soot and its precursors,and particle characteristics were systematically investigated in ethylene（C2H4）laminar diffusion flames.The main research contents and conclusions are as follows:（1）The soot formation behavior in the C2H4 flames with different NH3 doping ratios was investigated by laser diagnostics and thermophoretic probe sampling combined with the transmission electron microscopy（TEM）,and the concentration distribution of soot,polycyclic aromatic hydrocarbons（PAHs）and OH radical,and the diameters of soot primary particles along the flame axis were measured and analyzed.The formation of PAHs and soot were strongly inhibited by NH3,and the inhibitory effect on PAHs became highly pronounced when the number of benzene rings increased.The diameters of soot primary particles and the concentration of OH radical decreased sharply with the addition of NH3,indicating that NH3 reduced soot formation by inhibiting the processes of PAHs growth,soot inception,and surface growth,instead of promoting the soot oxidation.A detailed NH3/C2H4/PAH mechanism was also constructed to explore the influence of NH3 addition on the concentrations of key species in the gas phase part.It was found that a large number of carbon atoms combined with nitrogen atoms of NH3 to form cyanide,which resulted in the reduction of“available carbon atoms”for soot formation in the NH3-doped flames,and consequently the low loading on PAHs and soot.（2）The soot morphology and nanostructure evolution with different NH3 doping ratios were further investigated.Results showed that the sizes of soot primary particles and aggregates,and the number of primary particles per aggregate decreased,and the peak position of which moved downstream of the flame with the addition of NH3,indicating that NH3 inhibited the processes of soot nucleation,surface growth,and particle aggregation.The changes of soot nanostructure further illustrated that NH3 caused the delay of soot formation.In addition,the trends of the diameters of soot primary particles and the projected area of soot aggregates at the different heights indicated that the soot development were affected by both thermal and chemical effects of NH3.（3）The thermogravimetric analysis,TEM analysis,Raman spectroscopy,and elemental analysis were applied to explore the impacts of NH3 addition on the oxidation reactivity,intrinsic structures,and chemical composition of soot particles at the growth and oxidation stages.Results showed that with the addition of NH3,the oxidation reactivity of soot improved,and the content of amorphous carbon and disordered carbon in particles decreased,i.e.,the degree of graphitization decreased.The mass of N element in soot increased with the addition of NH3,indicating that nitrogen-containing substances（such as N-PAH）may be an important component in the nucleation and growth process of soot.（4）By coupling the NH3/C2H4/PAH mechanism and the partitioned soot model,the chemical mechanisms of NH3 and hydrogen（H2）addition on the formation of the key intermediates and soot in C2H4 diffusion flames were calculated and compared in-depth.It was found that both NH3and H2 reduced soot formation by inhibiting the growth of PAHs.Differently,NH3 impeded the PAHs growth through the reaction of nitrogenous substances with C2H2,whereas H2 inhibited the PAHs growth by decelerating the HACA reaction due to the high concentration of H radicals in H2-doped flames. |
PDF Full Text Request