Inhibition of hydrogen oxidation by HBr and Br2

The high-temperature bromine chemistry was updated and the inhibition mechanisms involving HBr and Br2 were re-examined. The thermochemistry of the bromine species was obtained using the Active Thermochemical Tables (ATcT) approach, resulting in improved data for, among others, Br, HBr, HOBr and BrO. Ab initio calculations were used to obtain rate coefficients for selected reactions of HBr and HOBr, and the hydrogen/bromine/oxygen reaction mechanism was updated. The resulting model was validated against selected experimental data from the literature and used to analyze the effect of HBr and Br2 on laminar, premixed hydrogen flames. Our work shows that hydrogen bromide and molecular bromine act differently as inhibitors in flames. For HBr, the reaction HBr + H ⇌ H2 + Br (R2) is rapidly equilibrated, depleting HBr in favor of atomic Br, which is the major bromine species throughout the reaction zone. The chain-breaking steps are then H + Br + M → HBr + M (R1), Br + HO2 → HBr + O2(R7), and Br + Br + M → Br2 + M (R8). In Br2-doped flames, the reaction Br2 + H ⇌ HBr + Br (R9) is far from equilibration and serves to deplete H in the reaction zone by competing with H + O2 → O + OH. The inhibition is augmented by recombination of Br (R8). If the inlet Br2 mole fraction exceeds about 20%, reactions (R8) and (R2) are both reversed, now acting to promote chain branching and increase the flame speed. According to the present model, cycles involving HOBr are not important for generation or removal of chain carriers in these flames.