Superadiabatic Combustion in Porous Media: Wave Propagation, Instabilities, New Type of Chemical Reactor

Abstract

This paper discusses theoretical and experimental results of filtration combustion of methane, hydrogen, and acetylene in porous media. Two general cases were studied: linear propagation of a slow, thermal combustion wave during fast fuel filtration, and a reverse unsteady-state combustion process, when the fuel flow direction is periodically switched from one end to the other. The intensive heat transfer between the heat releasing filtrating gas and the high thermal capacity, porous medium (through the highly developed internal solid surface) results in energy accumulation in the solid body and in the so called superadiabatic effect, when gas temperatures in the porous combustor can significantly exceed the adiabatic temperature of a feeding gas fuel. It was found that in such a superadiabatic combustor (SAC): (1) a very fuel lean gas mixture (e.g. very small concentration of CH4, C2H2, or H2 in air) can be burned, and (2) conversely even a very small amount of oxygen in such gas fuels as CH4 or H2S can support a combustion process associated with the high temperature pyrolysis, hydrogen production and hydrocarbon or hydrogen sulfide partial oxidation. The physical phenomena of superadiabatic combustion examined included: superadiabatic (SAC) wave propagation, SAC-wave velocity dependence on fuel concentration and gas flow rate, gasdynamic and kinetic thermodiffusion instabilities of SAC-waves, and cellular and double wave structures occurring as a result of the overheating instabilities. Possible SAC applications are discussed. These include: applications in chemistry and energy systems (particularly for a lean gas fuel burning), partial oxidation of very fuel rich gas mixtures, hydrogen production from fossil fuels, and applications for environmental control, in particular for air purification of volatile organic compounds (VOC).