Abstract: Described herein is a waste heat steam boiler which is capable of using as secondary combustion air, the exhaust gases from a steam turbine. The boiler has burners which are fed with primary combustion air by an air blower. This air is cool and can be used to keep part of the burners cool. The burners have throats supplied with secondary combustion air from a windbox which receives this secondary air either from the gas turbine or from a forced draught fan. The burner throats are adjustable between an open full quarl position when the secondary air is the gas turbine exhaust and a partially closed position where the effective quarl diameter is reduced when the secondary air is supplied by the forced draught fan.