PURPOSE: To prevent the drop of output of a gas turbine by providing spray means for spraying water toward heat transmission pipes on the upper portion of a coal economiser in equipment provided with an ammonia injector for reducing NOx contained in exhaust gas which is guided to a waste heat recovering heat exchanger.

CONSTITUTION: A waste heat recovering heat exchanger in a combined cycle generation plant is provided with a coal economiser having multiple heat transmission pipes 20, and in this case, the heat transmission pipes 20 are horizontally arrayed and provided so as to be interconnected to headers 22 in right and left hot boxes 21. Multiple flat-spray type spray nozzles 23 are disposed on the upper portion of the coal economiser in such a manner that their spray spreads in the axial direction of the heat transmission pipes 20. Thus, water is periodically sprayed from these spray nozzles 23, whereby ammonium sulfate produced by the reaction between ammonia and sulfurous acid gas, which are jetted to reduce NOx contained in exhaust gas, is washed out and removed from among fins 17.