Flue gas temperature control system

Claim 1

In a pulping liquor recovery boiler exhaust duct system including a pulping liquor recovery boiler in which flue gases are produced by the burning of pulping liquor, an evaporator positioned downstream from said recovery boiler to receive said flue gases from said recovery boiler, an electrostatic precipitator positioned downstream from said evaporator to receive said flue gases from said evaporator for the removal of particulate matter from said flue gases, and an exhaust stack for exhausting said flue gases into the atmosphere, a flue gas temperature control system for use in maintaining the temperature of said flue gases above their dew point as they pass through said exhaust duct system from said evaporator to said electrostatic precipitator and as they pass through said electrostatic precipitator comprising, in combination: air heating means for heating air to a temperature in excess of the dew point of said flue gases, said air heating means including a heater housing, a gas burner mounted in said heater housing, a gas supply line for supplying gas to said gas burner, and a gas flow control valve in said gas supply line for controlling the flow of gas to said gas burner; temperature control means positioned within said exhaust duct system for controlling the temperature of the heated air produced by said air heating means; conduit means including a first conduit branch connecting said air heating means to said exhaust duct system between said evaporator and said electrostatic precipitator and a second conduit branch connecting said air heating means to said electrostatic precipitator for conducting said heated air directly into contact with the flue gases passing through said exhaust duct system and through said electrostatic precipitator; damper means positioned in said conduit means for regulating the quantity of said heated air which is conducted into contact with the flue gases; and blower means positioned within said exhaust duct system between said electrostatic precipitator and said exhaust stack for inducing the flow of said flue gases through said exhaust duct system and said electrostatic precipitator and for inducing the flow of said heated air into contact with said flue gases in said exhaust duct system and in said electrostatic precipitator to maintain said flue gases at a temperature above their dew point to prevent condensation within said exhaust duct system and said electrostatic precipitator.