In a combustion zone a fuel injector is immersed in a mixture of oxidant and products of combustion having a temperature of about 2000 degrees F. or higher. In order to maintain rapid and stable combustion, it is desirable to avoid excessive absorption of thermal energy from this mixture. To that end, the present invention provides means for impeding transfer of heat to the fuel injector from the adjacent mixture, such that portions of the mixture immediately adjacent the fuel injector may be kept at a temperature of approximately the ash-fusion temperature of the fuel, or higher, while the interior of the fuel injector is kept at a temperature substantially below the ash-fusion temperature. This means for impeding heat transfer preferably comprises at least one material having a thermal conductivity substantially lower than that of the fuel injector and, in a preferred embodiment, consists essentially of slag formed from noncombustible-mineral constituents of the fuel.