Upside-down type low NOx boiler

An upside-down type low NOx boiler comprising:

• a vertical combustion chamber;
• the vertical combustion chamber including:
• a high-temperature reductive combustion zone provided to an upper portion of the combustion chamber and surrounded by a refractory material and having a burner;
• a low-temperature oxidative combustion zone provided below the high-temperature reductive combustion zone and surrounded by a water wall and having a second-step combustion air nozzle;
• a combustion gas outlet port provided below the low-temperature oxidative combustion zone and communicated with a rear pass;
• and
• an ash discharge mechanism provided to a bottom portion of the combustion chamber and connected with a steam drum and a non-heated downcomer pipe for cooling the low-temperature oxidative combustion zone,
• wherein the burner combusts a low-quality fuel under a high-temperature reductive atmosphere in the high-temperature reductive combustion zone, so as to produce a combustion gas, and
• wherein the second-step combustion air nozzle supplies a second-step combustion air having a temperature lower than a temperature of the combustion gas, so as to further combust a partly combusted gas flowed downward from the high-temperature reductive combustion zone, under a low-temperature oxidative atmosphere,
• wherein the combustion gas flowed downward from the low-temperature oxidative combustion zone is flowed out from the combustion gas outlet port, and the combustion gas flowed out from the combustion gas outlet port is flowed upward in the rear pass and then passes through a super-heater tube, and
• wherein an ash that is accumulated on a furnace bottom portion of the combustion chamber is discharged, during the operation of the furnace, by using the ash discharge mechanism.