Composition of solid and gaseous primary combustion products of boron-based fuel-rich propellant

The investigation of the primary combustion products of boron-based fuel-rich propellants is conducive to understanding the mechanism of the two-stage combustion process and provides important information for the prediction and organization of secondary combustion processes. The present investigation methods include numerical simulation and experimental analysis. The former deviates considerably from the actual conditions. The latter is currently a reliable and practical method, although it is difficult to quantify when the components are complex. In this work, SEM, LPSA, XRD, XPS, IC, ICP-MS, AAS, EDS, and chemical analysis methods were employed to qualitatively and quantitatively analyze the 3 solid products (aggregations, condensed-phase products, and flocculent products) at 1, 5, and 10 bar. GC, GC-MS, and IC combined with chemical tests were adopted to investigate the gaseous products (including chlorine-containing gas) for the first time. The solid products primarily included B2O3, B(HO)(3), unburned boron, KB5O8(H2O)(4), KCl, C, NH4Cl, NH4Mg(H2O)(6)Cl-3, B13C2, Fe3O4, Mo, MgO, Mg(OH)(2), MoO3, C3N4, BN, MgCO3(H2O)(3) (or MgCO3), MoO2, Fe-4(Fe(CN)(6))(3), and FeB (or BFe4), and the first 7 were dominant. The reaction degree of boron was less than 50 wt% (increasing with increasing pressure), indicating the significance of the secondary combustion process. The reacted boron was likely to react with O and C but not with N. The generated B2O3 vapor easily combined with H2O to form B(HO)(3) or reacted with KCl (from the thermal decomposition of KP) and H2O to produce KB5O8(H2O)(4) after cooling. There was less B13C2 (its content increased with increasing pressure, resulting in increases in particle size), since it was easily oxidized to generate B2O3. NH4Cl was produced by the oxidation-reduction reaction between the NH3 and HClO4 released from large-sized AP, and some of it crystallized with MgCl2 and H2O to form NH4Mg(H2O)(6)Cl-3. The gaseous products primarily included N-2, H-2, CO, CH4, CO2, C2H4, C2H2, HCl, C4H6, C3H6, Cl-2, C6H6, and other hydrocarbons. N-2 and H-2 had the highest content. The N-2 content increased with increasing pressure, while the content of the combustible gases decreased, indicating that high pressure promotes the propellant reaction. The results also show that high pressure was conducive to the low-temperature decomposition of AP (producing more HCl and Cl-2) and the reaction of boron and hydrocarbons (generating more B13C2), while it had an adverse effect on the C reaction (producing less COx and mostly CO). Nevertheless, the increasing pressure had little effect on the combustion product components but did affect the content within the pressure range.