PURPOSE: To protect a duct from being clogged and prevent heating energy loss resultant from a rise in the temperature again by connecting a spared water-cooled duct, which branches from its lower part, to a water-cooled duct and installing an opening/closing means of an exhaust gas line to the lower part of the water-cooled duct.

CONSTITUTION: When a water-cooled duct 21A is defined as a fist water-cooled duct, a spared water-cooled nozzle 21B which branches from its lower part is installed and connected to a line. Opening/closing means 22A and 228 which serve for an exhaust gas passage are installed to the lower part of the water- cooled ducts 21A and 21B respectively. If the water-cooled ducts are repeatedly switched over and deposited ashes are repeatedly eliminated, there will be no clogging of the ducts attributable to the deposited ashes. What is more, no suspended operation will be called for when an attempt is made to remove the deposited ashes. Therefore, there will be no heating energy loss due to cooling and reheating.