Kinetics of high-temperature B/O/H/C chemistry

Homogeneous gas phase boron-oxygen-hydrogen-carbon (B/O/H/C) combustion chemistry is studied to characterize general mechanistic behavior of high-temperature (1800 K < T < 3000 K) B/O/H/C reacting systems and to identify critical reaction rate constants for future experimental evaluation. The chemistry is numerically modeled with a reaction mechanism comprising 19 chemical species and 58 forward and reverse elementary steps. Reaction flux/pathway techniques and sensitivity analysis theory of isothermal systems are used to identify the important reactions of the mechanism. The results include the effects of mixture temperature, pressure, oxygen content, hydrogen content, and carbon content on the reaction dynamics of B, BO, BO₂, B₂O₂, B₂O₃, HBO, and HBO₂. For example, hydrogen addition is observed to accelerate both the oxidation of intermediates and the heat release rate, as well as to alter the dominant suboxides and reaction products. From the sensitivity analysis calculations, a group of 14 key boron-containing reactions are identified and suggested for future elementary reaction studies. To date, only one of these reactions has been studied experimentally. Lastly, several important conclusions are drawn about the combustion process of particulate boron. For example, the current homogeneous calculations have shown that the sensitivity of the species concentration profiles to the chosen initial species speciation (for a fixed number of moles of each element) is nearly independent of this speciation for reaction times greater than a few microseconds, suggesting that the identity of the species evolving from a reacting boron particle is not eritical to the surrounding gas-phase combustion process. Furthermore, as noted above, hydrogen containing species have a significant impact on accelerating the gas-phase combustion. However, larger quantities of hydrogen promote the formation of HBO₂, which is thermodynamically favored over gaseous B₂O₃ as the temperature is lowered. Consequently, as the combustion gases are cooled, gas-phase boron can be trapped as HBO₂.