A combustor for a gas turbine includes a diffusion flame combustion zone, a catalytic combustion zone and a post-catalytic combustion zone. At start-up or low-load levels, fuel and compressor discharge air are supplied to the diffusion flame combustion zone to provide combustion products for the turbine. At mid-range operating conditions, the products of combustion from the diffusion flame combustion zone are mixed with additional hydrocarbon fuel for combustion in the presence of a catalyst in the catalytic combustion zone. Because the fuel/air mixture in the catalytic reactor bed is lean, the combustion reaction temperature is too low to produce thermal NO.sub. x. Under high- load conditions, a lean direct injection of fuel/air is provided in a post-catalytic combustion zone where auto ignition occurs with the reactions going to completion in the transition between the combustor and turbine section. In the post- catalytic combustion zone, the combustion temperature is low and the residence time in the transition piece is short, hence minimizing thermal NO.sub.x.
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