Abstract: A process and system of operation for combusting nitrogen-containing fuels with resulting low NO.sub.x emissions. A fuel-rich mixture of the fuel is combusted in a catalytic bed which is carried by a support structure. Active or passive means are provided for limiting transfer of heat from the bed into the support structure for minimizing formation of NO.sub.x during combustion. The active means for limiting the heat transfer includes, in different embodiments, electrical resistance back heating, induction back heating, exhaust gas back heating, and combustion fired back heating. The passive means for limiting the heat transfer includes, in different embodiments, a body of heat insulating material between the bed and support structure, or the insulating effect of the outer cells of the bed within a recess in the support structure.