Steam temperature control with overfire air firing

In a fossil fuel-fired steam generator having a vertically elongated furnace with a gas outlet at the upper end thereof, steam generating tubes lining the walls of said furnace, a gas exit duct connected to the gas outlet of said furnace for conveying gases therefrom, superheater surface located in said exit duct, and means for conveying steam generated in said steam generating tubes through said superheat surface in heat exchange relationship with the gases passing through said exit duct;

• a method of firing said furnace comprising:
• a. injecting fuel into the lower region of said furnace remote from the gas outlet thereof;
• b. introducing a first portion of air into the lower region of said furnace in the immediate vicinity thereof whereupon combustion of the fuel is initiated, said portion of air being a quantity less than the stoichiometric amount for the fuel introduced thereto;
• c. introducing a second portion of air into said furnace in an intermediate zone below the gas outlet thereof and above and spaced apart from said first zone, said second portion of air being a quantity sufficient to substantially complete combustion of the fuel introduced into said first zone;
• d. measuring the outlet temperature of the steam conveyed through said superheat surface;
• e. comparing said measured superheat steam outlet temperature to a desired superheat steam outlet temperature and establishing a signal indicative of a high or a low superheat steam outlet temperature;
• and
• f. regulating the outlet temperature of the steam conveyed through said superheat surface by selectively directing said second portion of air into said furnace at a downward angle with the horizontal away from the gas outlet of said furnace in response to a signal indicative of a high superheat steam outlet temperature, and at an upward angle with the horizontal towards the gas outlet of said furnace in response to a signal indicative of a low superheat steam outlet temperature.

A fossil fuel-fired steam generator comprising:

• a vertically elongated furnace;
• steam generator tubes lining the walls of said furnace;
• a gas exit duct connected to the gas outlet of said furnace for conveying gases therefrom;
• superheat surface located in said exit duct;
• means for conveying steam generated in said steam generating tubes through said superheat surface in heat exchange relationship with the gases passing through said exit duct;
• stationary firing means for injecting fuel into said furnace in a region remote from the gas outlet of said furnace;
• first air means for introducing air into said furnace in the immediate vicinity of the fuel;
• second air means spaced apart from and above said first air means for introducing additional air into said furnace remote from said firing means;
• means for selectively proportioning the air introduced to said furnace between said first and second air means;
• means for measuring the outlet temperature of the steam conveyed through said superheat surface;
• means for comparing said measured superheat steam outlet temperature to a desired superheat steam outlet temperature and establishing a signal indicative of a high or a low superheat steam outlet temperature;
• and means for selectively directing the air introduced through said second air means into said furnace at a downward angle with the horizontal away from the gas outlet of said furnace in response to a signal indicating a high superheat steam outlet temperature, and at an upward angle with the horizontal towards the gas outlet of said furnace in response to a signal indicating a low superheat steam outlet temperature.