Effect of carbon dioxide concentration on the combustion characteristics of boron agglomerates in oxygen-containing atmospheres

In ramjet combustion chambers, carbon dioxide (CO2) produced by the combustion of carbonaceous fuel enters the chamber together with boron agglomerates. In order to investigate the effect of CO2 concentration present in an oxygen-containing atmosphere on the combustion characteristics and oxidation mechanisms of boron agglomerates, we used a laser ignition system, an X-ray diffractometer (XRD), and a thermogravimetric-differential scanning calorimetry (TG-DSC) combined thermal analysis system. Single-particle boron was tested in the laser-ignition experiments as the control group. The ignition experiment results showed that with a fixed O2 concentration of 20%, when the particle temperature reaches the melting point of boron, increasing CO2 content causes the combustion process of boron agglomerates to transition from single-particle molten droplet combustion to porous-particle combustion. Furthermore, XRD analysis results indicated that the condensed-phase combustion products (CCPs) of boron particles in a mixed atmosphere of O2 and CO2 contained B4C, which is responsible for the porous structure of the particles. At temperatures below 1200 °C, the addition of CO2 has no obvious promotion effect on boron exothermic reaction. However, in the laser-ignition experiment, when the oxygen concentration was fixed at 20% while the CO2 concentration increased from 0% to 80%, the maximum temperature of boron agglomerates rose from 2434 to 2573 K, the self-sustaining combustion time of single-particle boron decreased from 396 to 169 ms, and the self-sustaining combustion time of boron agglomerates decreased from 198 to 40 ms. This study conclusively showed that adding CO2 to an oxygen-containing atmosphere facilitates boron reaction and consumption pathways, which is beneficial to promoting exothermic reaction of boron agglomerates at relatively high temperatures.