Catalytic combustion at millisecond contact times

This thesis describes experimental and computational studies of the catalytic combustion of hydrocarbon fuels over noble metal catalysts at contact times less than 10 milliseconds and temperatures over 1000{dollar}spcirc{dollar}C. Although catalytic combustion is practiced quite widely today, most conventional combustion catalysts operate at much cooler temperatures and contact times on the order of 1 second. The maximum operating temperature of conventional catalysts is limited by the thermal stability of the catalyst support which sinters at temperatures above 850{dollar}spcirc{dollar}C causing permanent deactivation of the catalyst. Because the rate of reaction on the catalyst is highly dependant upon the catalyst temperature, operation at higher temperatures would allow for operation of the catalyst at much shorter contact times resulting in significant savings in reactor volume. This is accomplished by using a low surface area catalyst where the catalytic metal is placed directly upon a thermally stable substrate which will not sinter at high temperatures. This results in a catalyst that has very little low temperature activity, but enhanced high temperature stability. These low area catalysts were first used in the fuel rich regime as partial oxidation catalysts for reaction systems such as the partial oxidation of methane to syngas and the oxidative dehydrogenation of alkanes to olefins. The focus of this thesis is to investigate the use of these new low surface area, high temperature, short contact time catalytic reactors in the fuel lean regime where the complete oxidation products carbon dioxide and water are the only expected products. This thesis examines the experimental use of the short contact time reactor for several combustion applications. These applications are: (1) A catalytic combustor for light alkanes. (2) A catalytic incinerator to remove volatile organic compounds from air streams. (3) A methane fired catalytic radiant heater.; In addition, a mathematical model of the short contact time catalytic reactor is developed to describe the interaction of homogeneous and heterogeneous chemistry in these reactors. Finally, the catalysts used in the experiments were analyzed with scanning electron microscopy and x-ray surface mapping and calculations of catalyst lifetime in combustion atmospheres were made based on equilibrium vapor pressure calculations.