Abstract: A multi-stage topping combustor unit having rich burn, and lean burn zones followed by a quench/dilution zone. Combustible fuel gas is supplied only to the rich burn zone, and an oxygen-containing air-flue gas mixture is supplied to all three zones in a fixed flow proportioning that establishes a desired temperature profile through the combustor unit that minimizes combustor NOx and CO emissions. The combustor unit casing is cooled by an augmented air and oxygen stream discharged into the rich burn zone only, or into both the rich and lean burn zones. The cooling air flow is regulated by a control device/valve that allows the cooling air flow, and hence the augmenting oxygen entering the rich burn zone or the rich and lean burn zones, to be reduced as the fuel gas flow/firing temperature is reduced.