Hetero-/homogeneous combustion and stability maps in methane-fueled catalytic microreactors

Abstract

The hetero-/homogeneous steady combustion and the stability limits of methane-fueled catalytic microreactors (Pt-coated) have been investigated numerically in a 1-mm-gap channel at pressures of 1 and 5 bar. Computations were carried out with a full-elliptic two-dimensional model for the gas- and solid-phases that included elementary heterogeneous and homogeneous chemical reaction schemes, heat conduction in the solid wall, surface radiation heat transfer, and external heat losses. Gas-phase chemistry extended the low-velocity stability limits due to the establishment of strong flames and to an even greater degree the high-velocity blowout limits due to the heat release originating primarily from the incomplete homogeneous oxidation of methane. When considering the same mass throughput, the stable combustion envelope at 5 bar was substantially wider than its 1 bar counterpart due to the increased reactivity of both catalytic and gaseous pathways at elevated pressures. Stable combustion could be sustained with solid thermal conductivities at least as low as 0.1 W/mK, while the stability limits reached their larger extent between 20 and 50 W/mK, a range that covers many practical metallic compounds. The stability limits of catalytic microreactors were wider than those reported for non-catalytic systems. Surface radiation heat transfer greatly impacted the microreactor energy balance and combustion stability. At conditions well-below the stability limits, surface radiation provided an efficient heat loss mechanism that moderated the surface temperatures, whereas close to the limits it could stabilize combustion by transferring heat from the hotter rear of the channel to the colder front. Investigation of smaller confinements has shown that gas-phase combustion could be sustained in catalytic microreactors with gaps as low as 0.3 mm.