Effects of Pressure and Oxygen Concentration on Ignition and Combustion of Boron in Oxygen/Nitrogen Mixture Streams

Abstract

An experimental study on the spontaneous ignition and combustion of small lumps of boron was performed by using the stagnation region of impinging O 2/N 2 mixture streams over a wide range of ambient pressure and oxygen concentration of the streams. Two types of ignition processes, the gas-phase ignition with a green–white flame and the surface ignition without a flame, were observed. The critical ignition temperature for gas-phase ignition decreased with decreasing pressure. With decreasing oxygen concentration, the ignition temperature slightly decreased until the surface ignition occurred, and then increased abruptly. During combustion with a flame, the sample appearance, the sample temperature, emission intensity distributions of specified species for boron combustion, and the mass burning rates were measured. It was found that BO, which was generated heterogeneously on the boron surface, and BO 2, which was generated in the gas phase, reacted in the flame close to the surface to finally form gaseous B 2O 3, which condensed at the periphery away from the flame. The qualitative characteristics of the flame structure did not depend on pressure and oxygen concentration. The thickness of the flame decreased with decreasing oxygen concentration. The burning rates of boron were found to decrease with decreasing pressure and oxygen concentration. The ignition and combustion mechanism is discussed: BO produced from heterogeneous reaction with combustion products plays a crucial role in determining the gas-phase combustion or the surface combustion. The dependence of the ignition temperatures on pressure and oxygen concentration is explained in terms of the balance between the rates of production and evaporation of B 2O 3.