Management of the gas-phase and surface chemistry in methane-fueled catalytic micro-combustors

Design guidelines for the management of gas-phase and surface chemistry in methane-fueled catalytic micro-combustors were developed. To find out how the interplay between gas-phase and surface chemistry is affected by different operating conditions, computational fluid dynamics (CFD) simulations were performed using a two-dimensional model with detailed chemistry and transport. The effects of feed composition, inlet velocity, combustor dimension, and heat loss were studied to determine the relative role of gas phase and surface chemistry and to understand the important factors controlling chemistry. To delineate the homogeneous-heterogeneous coupling mechanisms, comparisons were made between results for cases where only heterogeneous reactions were allowed, only homogeneous reactions were allowed, and where both mechanisms were allowed. The competition and synergism between gas-phase and surface chemistry were delineated for obtaining design insights. It was shown that the contribution of gas-phase chemistry depends strongly upon the operating conditions; it decreases with decreasing channel size, increasing the heat loss to the surroundings, and the composition away from the stoichiometric point. The crucial factor controlling chemistry is the maximum temperature. To minimize gas-phase chemistry, temperatures should be kept as low as possible in the stable operating regime. Gas-phase chemistry can be either promoted or inhibited by surface chemistry, depending on flow velocity; it can be sustained in catalytic micro-channels (as low as 0.2 mm) well below the quenching distance due to the promoting effect induced by surface heating. Surface chemistry alone can occur under appropriate operating conditions such as compositions, flow velocities, and heat exchange/heat loss rates. Contribution diagrams were constructed and design recommendations were finally made. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.