Features of Combustion of Hydrogen–Methane–Air Fuels Over Surfaces of Noble Metals

Abstract

This chapter is aimed at the establishment of the regularities of catalytic ignition mainly of H2–methane mixtures; in a series of experiments methane blends were replaced with ethane, ethylene and n-pentane. It was shown that in the reactor, treated with ignitions, the ignition temperature of the mixture 70% H2 + 30% methane with air over rhodium surface is 62 °C. The result indicates the potential of using rhodium catalyst to lower markedly the catalytic ignition temperature of the fuels based on hydrogen–methane mixtures. The critical condition for volume reaction is revealed: the volume process occurs at 45% H2, but it is missing at ≤ 40% H2. If H2 ≤ 40%, only a slow surface reaction occurs; this phenomenon is qualitatively described by our calculations. It is revealed that the effective activation energies both of “upper” and “lower” limits of H2 + methane oxidation over the range of linearity are roughly equal (2.5 ± 0.6) kcal/mol; it means that the key reactions, responsible for the occurrence of “upper” and “lower” ignition limits are almost certainly the same. It was shown that the obtained values of effective activation energy are in mutual agreement and are characteristic of the surface nature of Rh action; Rh is more catalytically active than Pd. It was shown that under conditions of our experiments not the chemical nature of the catalyst but that of C2 hydrocarbon in the mix with H2 is the determining factor of catalytic ignition. The catalytic ignition limits of synthesis gas over Rh/Pd are qualitatively different from the dependencies for combustible hydrogen-hydrocarbon. Long delay periods of catalytic ignition of hydrogen—n-pentane mixes (tens of seconds) and the absence of the dependence of the periods on the initial temperature allow us to conclude that the catalytic ignition of hydrogen—n-pentane mixes is determined by the transfer of the molecules of the hydrocarbon blend to the surface of the catalytic wire. It was shown that the ignition limits of 2H2 + O2 and (80% H2 + 20% CH4)stoich + O2 mixes over Pt wire do not depend on the applied voltage without discharge up to 1200 V. We showed that for (80% H2 + 20% CH4)stoich + O2 mixes the application of an electric field (1200 V) leads to the disappearance of Pt containing particles from the reaction volume formed by decomposition of volatile platinum oxide in gas phase, which indicates that these particles are charged. This may be due to the chemiionization phenomenon observed in the combustion of hydrocarbons and then adsorption of the charged particles onto the Pt containing particles.