PURPOSE: To reduce corrosion in the water tubes in a water wall due to the low NOx combustion by supplying a gas that contains oxygen into a furnace through a hole provided in the water wall when an oxygen partial pressure measurement device that is provided in the water wall near a burner detects that the oxygen partial pressure near the surface of the water wall pipe has become a specified value.

CONSTITUTION: A plurality of burners 3 are installed to a water wall 2 that forms a furnace 1. The water wall 2 is provided with an after-air port 4, secondary air duct 5, and side wall duct 7. The side wall duct 7 is provided with a port 8 that supplies a gas 6a containing oxygen into the furnace 1 and an oxygen sensor 9 which is a means to measure oxygen concentration in the surface layer section of a water wall pipe that will be described below. As the gas 6a that contains oxygen and is supplied from the port 8 into the furnace 1 the secondary air 6 used for the combustion of the burners 3 is branched to be supplied to a branch duct 7. When the partial oxygen pressure becomes below -20atm. a flow rate regulating valve 13 opens at (a) and (b), and the secondary air 6a is supplied into the furnace 1 through the port 8. With this arrangement it is possible to remarkably reduce the high temperature corrosion of the water wall that is caused by the combustion with low NOx.