Homogeneous ignition of hydrogen-air mixtures over platinum

Abstract

The homogeneous ignition of hydrogen-air mixtures over platinum is studied at atmospheric pressure using a stagnation-point flow model with detailed gas-phase kinetics, surface kinetics, and transport phenomena. The momentum, energy, and mass balances are discretized using a second-order finite difference scheme. The obtained set of algebraic equations is then solved using Newton's method. An are-length continuation is employed to determine ignitions and to perform parametric studies. Inhibition of the homogeneous ignition caused by the catalyst is observed in agreement with published experiments. A local maximum in the homogeneous ignition temperature is found at the surface stoichiometric point at ∼15% H2 in air, a composition determined by surface reactions and multicomponent transport effects. The gasphase inhibition is induced mainly by product formation through the termination reaction H+O2+M→HO2+M and by depletion of the fuel below ∼15% H2 in air or of the oxidant above ∼15% H2 in air. Sensitivity analysis shows that desorption of radicals has a minor effect on gas-phase ignition of H2 in air. On the other hand, the dissociative adsorption of O2 through O2+2°→2O°, (° denotes adsorbed species or an adsorption site) has a strong influence on the homogeneous ignition temperature. Reaction path analysis reveals a change in the major surface reaction path for formation of H2O° from 2OH°→H2O°+O° when H2 in air is less than ∼15% to OH°+H°→H2O°+° when this composition is greater than ∼15%. Implications for homogeneous ignition of hydrocarbons near catalysts are also discussed.