| As a clean energy,bodiesel can be mixed with diesel without major changes for the diesel engine.Compared with diesel,biodiesel can significantly reduce particulate emissions,but its oxygen-containing properties will lead to nitrogen oxide(NO_X)emissions.Exhaust Gas Recirculation(EGR)is used as a NO_X reduction technique,the secondary combustion of the exhaust will cause particulate emissions increased.In this paper,EGR technology is applied to the biodiesel combustion.By changing the composition of the mixture and combustion atmosphere in the cylinder,the microstructure and oxidation characteristics of particles are observed.In this paper,testing bench is set on a high-pressure common rail diesel engine under0,15%and 25%EGR rates with B0 and B20 fuels.The effects of B0 and B20 on particle size distribution,graphitization degree,surface functional group and oxidation rate at different EGR rates were analyzed by using particle analysis test instruments.(1)The MOUDI sampler is used to classify the particle sizes of B0 and B20 with EGR rates of 0,15%and 25%to obtain mass concentration size distribution and modal distribution of particles.Compared with B0 particles,the mass concentration size distribution of B20 shifts to smaller size,the aggregated particle emissions increase,while the average particle size decreases.When burning same fuel,the distribution of mass concentration shifts to the larger particle size with the increases of EGR rate,the number of coarse particles raise and the average size increases.The effect of EGR rate on the average particle size of particulate matter is higher than that of biodiesel blending.(2)By observing the electron micrographs and Raman spectra five-band fitting figures of B0 and B20 particles under different EGR rates,the morphology of aggregates,the accumulation of primary carbon particles and changes of microstructure parameters were analyzed.The agglomerate of particles becomes spherical,chain-like and dendritic,the size of primary carbon particles are mainly in the range of 23-60 nm.Compared with B0 particles,the agglomeration of B20 particles increases and the microstructure becomes more complicated,the chemical heterogeneity and amorphous carbon of exhaust particles are improved,but the degree of graphitization decreases.With the increase of EGR rate,the degree of agglomeration decreases,the chemical heterogeneity gradually decreases,the degree of graphitization increases while the content of amorphous carbon decreases.(3)By observing the infrared spectrum of B0 and B20 particles under different EGR rates,the changes of surface functional groups and its content were analyzed.It indicates that the surface functional groups of particles are basically unchanged.Compared with B0particles,the content of the aliphatic hydrocarbon increases.For the same fuel,the content of aliphatic hydrocarbon functional group decreases with the increment of EGR rate.The higher the content of functional groups,the lower the degree of graphitization.(4)The oxidative activity and the microstructure change during weightlessness were studied by a thermogravimetric analyzer combined with high-power transmission electron microscopy.The results show the mass change of the particles under different conditions flattens after a rapid change.With the increase of EGR rate,the duration of complete oxidation increases gradually,the oxidation rate of particulate matter decreases,and the reactivity of particulate matter decreases.The oxidation speed of B20 particles in different weightlessness stages is faster than that of B0 particles.The B0EGR0 particles are oxidized from the surface layer to the inner core,while the B20EGR0 particles are oxidized from the inner core to the surface layer and it shows a hollow capsule-like structure.However,surface oxidation and internal oxidation are simultaneously occurred for the oxidation of B0EGR15 particles. |
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