A system and a method for controlling NO_X emissions from a boiler that combusts carbonaceous fuels. The method includes the steps of (a) introducing carbonaceous fuel and combustion air into a furnace of the boiler for combusting the carbonaceous fuel in oxidizing conditions and producing flue gas including NO_X and CO, and (b) leading flue gas from the furnace to a catalyst section in a flue gas channel for converting, free from introducing an external agent for NO_X reduction, NO_X to N₂ and CO to CO₂ by using CO as the reductant of NO_X on a catalyst in the catalyst section.