Inhibition of hydrogen-air and hydrogen-nitrous oxide flames by halogen compounds

Abstract

The paper describes an experimental and theoretical study of the inhibitory effects of halogen compounds on hydrogen-nitrogen-oxygen and hydrogen-nitrogen-nitrous oxide flames. Studies were made of the effects of methyl bromide, ethylene, and hydrogen bromide on the rich flammability limits of hydrogen-nitrogen-nitrous oxide mixtures, and the results were compared with published data for the hydrogen-air system. The effects of CH 3 Br and HBr on the limit were considerably less in the nitrous oxide supported flame than in the air flame; but, when ethylene was the additive, the difference was not so great. The effect of trace additions of HBr on the burning velocities of some near-limit hydrogen-nitrogen-oxygen flames showed a considerable degree of inhibition, e.g., addition of 0.027% HBr to a flame containing 4.60% oxygen reduced the burning velocity from 9.2 to 6.4 cm sec −1 . The measured reduction in burning velocity has been used as an experimental criterion of the success or otherwise of possible reaction mechanisms which were the subject of parallel computer studies. The computer studies involved the solution of the flame-conservation equations to obtain burning velocities and temperature and composition profiles. It was concluded that most of the added HBr is removed by Reaction (xxiii) in the early stages of the flame to give an equilibrium mixture of H, Br, H 2 , and HBr over most of the reaction region. The temporary removal of H atoms in this way is not sufficient, however, to explain H+HO 2 =H 2 +O 2 (xii) H+HBr⇌H 2 +Br (xiii) H+HBr⇌H+Br 2 (xxiv) Br+Br+M=Br 2 +M (xxv) H+Br+M=HBr+M (xxvi) Br+HO 2 =HBr+O 2 (xxvii) the observed inhibition. For this, new chain-breaking steps such as Reactions (xxv)–(xxvii) must be added to the reaction mechanism for the uninhibited flame. The rate constants which must be assumed for Reactions (xxv)–(xxvii) are not unreasonable, e.g., 0.25≤ k 27 / k 12 ≤0.5.