IGNITION AND COMBUSTION OF BORON PARTICLES

This paper addresses the ignition and combustion of small boron particles in dry and wet atmospheres. The work is both experimental and theoretical in character. The paper summarizes much of our previously published work on this subject, provides some additional information relevant to that research, and addresses extinction and low-temperature oxidation phenomena, not explicitly covered in that previous work. Experiments were performed on the ignition and combustion of fine boron particles in hot wet gas at atmospheric pressure. In these experiments a steady nitrogen jet transporting the particles at low loading densities was injected coaxially into the combustion products of a flat-flame burner. Three powder samples, having radius around 0.05 μ, 3.5 μ, and 5.0 μ, were studied. The types of boron flames that existed for different flat-flame temperatures were identified and characterized. A yellow region of boron ignition and a white-glow region of boron combustion were observed, and their dimensions were measured. It was found that the yellow-region height strongly depends on the flat-flame temperature and is independent of the oxygen mole fraction in the product gas, while the thickness of the white-glow region is inversely proportional to the oxygen concentration and directly proportional to particle radius. Based on a theory for a one-step, Arrhenius, ignition process, overall rate parameters for ignition were extracted from measurements of the yellow-region height as a function of the flat-flame temperature. Scattering measurements were performed with an argon-ion laser demonstrating that particle sizes remained constant during ignition and decreased linearly with time during combustion. The results demonstrate that the combustion of particles of these sizes is controlled by chemical kinetics rather than by diffusion and that the surface reaction rate is kpO2, where pO2is the partial pressure of oxygen, and k = 0.0625 ± 0.0125 (mole/cm2-sec-atm). Early laser-ignition experiments in cold dry atmospheres, important work of Macek and coworker, are analyzed quantitatively for determining where boron oxidation occurs and what products are formed when a boron-oxide layer is present. These results provide needed dry-gas rate parameters for the ignition stage. The experiments on boron oxidation at low temperatures by Safaneef and coworkers are addressed for understanding why and how low-temperature boron oxidation occurs. Models of ignition and combustion of boron particles in both wet and dry atmospheres were developed, and their predictions were compared with available data. For the ignition stage the model involves equilibrium reactive dissolution of B in the thin B2O3(l) layer, surface attack of BO by O2(g) and by H2O(g) to form BO2(g) and HOBO(g), respectively, vaporization of B2O3, and later clean-surface attack of B by O2 to form O=B-B=O. For the combustion stage the model involves only the last of these processes. When the temperatures of the boron particles are lower than a cutoff temperature, diffusion-controlled reaction of O2 with boron at the B-B2O3(l) interface occurs, which leads to thickening of the boron oxide layer, so that the rate of heat loss by gas-phase conduction competes favorably with that of heat generation and cools the particle, preventing ignition. The cutoff temperatures are predicted to increase from 7650 K to 1730 K with pO2, increasing from 1 atm to 14 atm. Extinction of boron combustion in relatively cold atmosphere is caused by the rapidly increasing rate of heat loss with decreasing particle size in the late combustion stage. The quenching diameter decreases with increasing oxygen partial pressure and environment temperature. Since good agreement with experiment was obtained, these models provide a basis for calculation of ignition and combustion of boron in propulsion applications.