INHIBITION OF HYDROGEN-AIR FLAMES BY HYDROGEN HALIDES

The effects of hydrogen bromide, hydrogen iodide and hydrogen chloride on the limits of flammability of hydrogen-air mixtures have been studied over a wide range of mixture composition and final flame temperature by varying the oxygen content of the air. The efficiency of these compounds in preventing flame propagation was found to decrease in the order: HI > HBr ≫ HCl, and it is shown that the iodide and bromide act as chemical inhibitors, whereas the chloride acts predominantly as a thermal diluent. The primary inhibiting step in rich limit mixtures has been identified as the reaction of hydrogen atoms with the chemical inhibitor: H + HX = H2 + X, which is in competition with the rate-controlling branching reaction: H + O2 = OH + O. It is suggested that the relative efficiencies of the three halides reflects the reactivity with hydrogen of the halogen atom formed in the first reaction. The probability of further reaction of bromine or iodine atoms with hydrogen is discussed and it is shown that even if this reaction occurs, the hydrogen atom regenerated will preferentially react with inhibitor and thus be effectively lost from the point of view of the propagation of the flame. © 1968.