Study On The Formation And Characteristics Of Particles With Biodiesel Blend Fuels In The Exhaust Gas Atmosphere

As a clean and renewable energy, biodiesel can be blended with diesel at any volume ratio (biodiesel blend fuels) that is applied to the diesel engine directly. The biodiesel blend fuels are used to improve combustion process and reduce particle emission for diesel engine, but NOx emission is increased. Combined with the technology of exhaust gas recirculation (EGR), it can be achieved the purpose of reducing NOx and particle emissions simultaneously. The physical and chemical properties of fuel are changed by using biodiesel blend fuels; the combustion environment is changed by using EGR technology. The change of fuel characteristics and combustion environment will lead to the effect of particle formation, which will have some influence on the morphology and structure of particles.This paper has seven chapters, the formation and characteristics of particles with biodiesel blend fuels in the exhaust gas atmosphere were studied by numerical simulation and experiment study. The article researched on the formation process of soot precursors in cylinder and the characteristics of soot formation during combustion Besides the morphology, structure, graphitization degree and oxidation activity of the particles were also analyzed. The coupled chemical kinetics model of biodiesel-diesel mixed fuel was established; the formation and evolution of living radical, carbon precursors such as acetylene and aromatic hydrocarbons were revealed. Based on the method of natural light, the evaluation parameter of soot generation was put forward,the mechanism of formation and oxidation of soot by biodiesel and EGR was discussed.Based on the analysis of the morphology of the particles, the evaluation index of the complexity of the particle profile was proposed. The change rules of the surface functional groups and oxidation activity of particles with biodiesel blend fuels were studied, the interacted relation between functional groups and oxidation activity was revealed.According to the rule of soot formation at different combustion stages, the development law of flame and soot formation characteristics with diesel (BO) and biodiesel blend fuels (B20, B50) in the exhaust gas atmosphere was analyzed by using high-speed photography in the constant volume combustion bomb. The evaluation parameters of soot generation are based on the integrated brightness of combustion flame, the mechanism of formation and oxidation of soot by biodiesel and EGR was discussed. The results showed that the oxygen entrainment of flame lift off length with biodiesel blend fuels was higher than that of diesel. The oxidation rate of particles was improved. And it was accelerated the oxidation of particles in the later stage of combustion. The results showed that soot was reduced remarkably. The formation and oxidation of soot were inhibited by using EGR technology. In the later stage of the diffusion combustion, the decreasing rate of combustion flame integrated brightness was decreased with the increase of exhaust gas concentration. The results showed that the decrease of soot oxidation rate was the main reason for the increase of soot emissions.The bench test of diesel engine was carried out, and pollutants of diesel engine were measured. At the same time, the combustion process was analyzed, and effects of biodiesel blend ratio and EGR rate on particle numerial concentration were mostly investigated. It was found that the particle number and size distribution showed unimodal orbimodal log-normal distribution. Under the same working condition, the number of nuclear particles increased obviously under the condition of small load, and the accumulated particles decreased under heavy load with the increase of biodiesel blend ratios. Compared with B0, the peak concentration of nuclear particles with B20 and B50 was increased by 16.6% and 82.8% under the condition of 25% load, and the peak concentration of accumulated particles was decreased by 33.5% and 80.3% under the condition of 75% load. With the introduction of EGR, the peak concentration of nuclear particles decreased obviously under the small load, and the peak concentration of accumulated particles increased obviously under the heavy load, and the average particle size of the particles increased.The soot formation of biodiesel blended fuels in the exhaust gas atmosphere was studied. A coupling chemical kinetic model of biodiesel-diesel mixed fuel was constructed containing the formation of aromatic hydrocarbons and soot. And effects of biodiesel blend ratio and EGR rate on living radical, carbon precursors such as acetylene and aromatic hydrocarbons were mostly investigated. It was found that the initial time of OH radical was in advance by using biodiesel blend fuels, and the flame was induced in advance. The mass fraction of C2H2 decreased, which inhibited the formation and growth of aromatic hydrocarbons,and led to a decrease in the production of Ai and A4. The soot mass fraction decreased gradually. With the increase of the EGR rate, the formation of the OH radical was decreased and the C2H2 was increased, the starting point of OH radical and C2H2 was delayed. The generation time of Ai and A4 was delayed, and the final production was dramatically increased. The mass fraction of soot was increased gradually. The time required for the formation and consumption of soot in the cylinder was prolonged with the technology of EGR.According to the state characteristics of particles with biodiesel blend fuels, the particle morphology, size distribution and basic carbon particle structure were studied by electron microscope and X-ray small angle scattering. A box-counting dimension was proposed to evaluate the complexity of the particle profile. The results showed that the particle group presented a cluster structure and the particle size range was between 20nm?80nm. The box-counting dimension of the particles increased with the increase of biodiesel mixing ratio, and decreased with the increase of EGR rate. The results showed that the degree of particle cluster was more compact after using biodiesel blend fuels, and the introduction of EGR would weaken the degree of particle cluster. The structure of basic carbon particles showed similar structures of the spherical carbon layer. With the increase of biodiesel blend ratio, it would lead to the fringe separation distance and fringe tortuosity of the basic carbon particles decreasing and the fringe length distance increasing slightly. With the increase of EGR rate, the fringe separation distance and fringe tortuosity of basic carbon particles were increased, and the fringe length distance was decreased gradually. The large volume and rough surface of particles would be formed with biodiesel blend fuels under the condition of exhaust gas.Influence of the chemical heterogeneity, graphitization degree and other parameters of particles have been investigated by means of Raman spectroscopy. The full width at half maximum of D1 peak increased gradually and the chemical heterogeneity of particles enhanced with the increase of biodiesel blend ratio and EGR rate. The ratio of ID1/IG gradually decreased with the increase of biodiesel blend ratio, and the ratio of ID3/IG increased slightly. The graphitization degree of particles increased, but had little effect on the content of amorphous carbon. The ratio of ID1/IG and ID3/IG gradually increased with the increase of EGR rate. With the introduction of EGR technology, the graphitization degree of particles decreased, but the content of amorphous carbon in particles increased.In order to reveal the relationship between the functional groups and oxidation activity of particles with biodiesel blend fuels, the distribution of functional groups of particles were analyzed and the variation rule of particle oxidation characteristic parameters was discussed by means of NEXAFS,TGA and other testing techniques. It was found that the content of oxygen-containing functional groups (C-OH, C=O) and aliphatic C-H functional groups on the surface of diesel particles increased with the increase of biodiesel blend ratio and EGR ratio. The activation energy of particles was between 54.25kJ/mol?87.17kJ/mol. With the increase of biodiesel blend ratio and EGR rate, the activation energy of particles decreases gradually. It was suggested that using biodiesel blend fuels and EGR technology could enhance the oxidation activity of particles, and further reduced the energy that particles were burnt out. The activation energy of the particles decreased with the increase of the content of the aliphatic C-H functional groups and the oxygen functional groups on the surface of the particles.Compared with the aliphatic C-H functional groups, the oxygen containing functional groups have a greater influence on the activation energy of particles, which was the main factor to improve the oxidation activity of the particles.