Study On Ignition And Combustion Mechanism Of Boron Particles

Boron has advantages in volumetric calorific value and gravimetric calorific value as a high-energy metallic fuel. Now it is thought as the most ideal metallic fuel used in solid propellant. However, the ignition temperature of elementary boron is too high. The heat value of boron particles can’t be released completely in limited time due to the long ignition delay. It’s very important to understand the ignition and combustion mechanism of boron particles, which is the basis to make full use of the energy of boron. Therefore, further and systematic studies on the basic theory of boron ignition and combustion are urgently needed. Ignition and combustion processes were studied deeply in this paper, including study on the thermodynamic processes, the thermal oxidation characteristics, the ignition and combustion characteristics, and the kinetic models.In order to study the reaction mechanism of boron in a multi-component system, Factsage was used to calculate the thermodynamic processes of boron reaction in complex systems. The competitive reaction mechanism was analyzed. The results revealed that the reaction of B with O2was the dominant reaction of B/O2/N2system, and very small amount of B involved in the reaction with N2. For B/O2/H2O system, the reaction priority of B with O2was higher than that with H2O. B was found to firstly react with O2, follwed with the reaction between its product and H2O. For B/O2/CO2system, it was shown that B only reacted with the O2in system, which did not react with CO2at all. Elevating temperature was not conducive to the energy release of system, whereas increasing the environment pressure was helpful to improve energy release.In order to investigate the thermal reaction characteristics of boron, experiments were carried out in thermal balance. The dependence of thermal reaction on sample characteristics, particle size, ambient pressure, intial oxide thickness, flow rate, heat rate and reaction atmosphere were investigated. It was observed that all these factors affect the reaction characteristics. The results showed crystalline boron was more resistant to ignition compared to amorphous boron. The ignition temperature and reaction efficiency decreased as the particle size of boron powder decreased. As the ambient pressure increased from1atm to20atm, the ignition temperature decreased nearly50℃, whereas the combustion efficiency decreased a lot at high pressure. The oxygen was conducive to increasing the reaction rate of boron. But the presence of initial oxide layer was found to be unfavorable to the reaction. Kinetic analysis showed the effective activation energy of the oxidation of boron powder was influenced by particle size and ambient pressure. Boron powder with smaller particle size at higher pressure had a relatively low effective activation energy value.Experiments were carried out to study the combustion characteristics of boron. The effect of sample characteristics, particle size, intial oxide thickness, power of ignition source, mole fraction of oxygen, reaction atmosphere and flow rate were investigated. The results showed amorphous boron could ignite more rapidly, burn more intensely, and react more completely compared to crystalline boron. The increase of particle size slightly promoted the combustion of boron. The presence of initial oxide layer was found to be unfavorable to both ignition and combustion. With enhancing the power of ignition source, the propagation speed of the combustion flame of boron became faster. The oxygen was conducive to increasing the combustion intensity and efficiency of boron. Ignition and combustion of amorphous boron in nitrogen were observed, but the flame intensity was much weaker than that in oxygen atmosphere. When the flow rate was kept at a low level, the increase of flow rate could promote the ignition and combustion of boron. However, when the flow rate exceeded0.8L/min, the increase of flow rate would weaken the ignition and combustion process. The results also showed the increase of water vapor concentration was conducive to the ignition of boron. The ignition and combustion of boron particles were investigated at high pressures in the range of1-10atm. Observation results showed the combustion of2-μm amorphous boron particles occurred in two continuous stages. The ignition and combustion times of boron particles were measured. Ignition time decreased slightly with an increase of pressure, while the combustion time showed strong dependence on pressure.A comprehensive ignition model for single boron particles in oxygenated environment containing O2and H2O was developed. The micro characteristics of the boron oxide layer on the surface of boron particles at elevated temperatures were studied. Two typical species distribution of the surface oxide layer were detected. One was composed of three layers with a distribution of B2O3,(BO)n and B2O3, while the other was composed of two layers only with a distribution of (BO)n and B2O3, both according to the order from internal to external. In the model development, two submodels, submodel Ⅰ and submodel Ⅱ were developed with regard to the two different observed species distribution of the surface oxide layer respectively. For submodel I, it was supposed that both (BO)n and O2were the govern species diffusion into the liquid oxide layer, whereas for submodel II, only (BO)n was the govern species. The two submodels were combined into a new BD model, which consisted of four key kinetic processes, containing the evaporation of the liquid oxide layer, the overall surface reaction between oxygen from the environment and boron, the reaction between the inner boron core and oxygen, and the overall reaction of boron with water vapor, respectively. The prediction of the new model was in good agreement with experimental results, especially under high-pressure conditions. A combustion model was also developed, concerning the reaction mechanisms of boron both with O2(g) and H2O(g).