10 are disposed about the rotor axis within a turbine casing 11 to supply the total hot gas flow needed to drive the turbine the catalytic combustion system includes a combustor basket 12 a catalytic unit 13 and a transition duct 14 which directs the hot gas to the annular space through which it passes to be directed against the turbine blades the combustor 12 is mounted on the casing 11 and preferably provided with a primary and plural six secondary sidewall fuel nozzles 18 and 20 fuel supplied through the primary nozzle 18 is mixed with primary air and burned in a primary combustion zone to provide hot gas for driving the turbine or for preheating a downstream fuel air mixture to the level required for catalytic reaction the supplemental use of the primary burner in the system 10 also enables compensating primary fuel supply increases to be made for dropoff in catalytic activity with operation time the ratio of conventional combustion to catalytic combustion is sufficient under all operating conditions to achieve the needed combustion assistance without the production of an unacceptable no x penalty a fuller explanation of the combustion operation of the system is presented in the aforementioned copending patent applications gases flow downstream within the basket 12 from the primary combustion zone to the entry to a secondary zone where the secondary fuel nozzles 20 inject fuel preferably with respective surrounding jets of atomizing air through sidewall scoops 21 for mixing with the primary gas flow the resultant mix expands as it passes through an outwardly flared diffuser 24 which forms an end portion of the basket 16 it then enters a catalytic reaction element.