NOx Suppression Techniques for an Indurating Furnace

Abstract

Techniques for suppressing NOx in an indurating furnace include the use of a premix burner, the use of a staged fuel injector structure to enhance fuel lean conditions in the downdraft, and the use of a Venturi mixture structure in the downcomer.

1 . An apparatus comprising:
 an indurating furnace structure defining drying, heating, and cooling stations;   a conveyor to convey pelletized material through the drying, heating and cooling stations;   a downcomer configured to direct a downdraft toward the heating station; and   a premix burner configured to form premix from fuel gas and combustion air and to inject the premix into the downcomer for combustion of the premix in the downdraft.   
     
     
         2 . An apparatus as defined in  claim 1  further comprising a reactant supply and control system configured to supply the premix burner with fuel gas and combustion air in amounts for premix to have a lean fuel-to-oxidant ratio. 
     
     
         3 . An apparatus as defined in  claim 1  further comprising a reactant supply and control system configured to supply the premix burner with fuel gas, unheated air from the ambient atmosphere, and heated downdraft air from the downcomer, and wherein the premix burner is configured to form the premix from the fuel gas, the unheated air, and the heated downdraft air. 
     
     
         4 . An apparatus as defined in  claim 3  wherein the reactant supply and control system is configured to supply the premix burner with the fuel gas, the unheated air, and the heated downdraft air in amounts for the premix to have a lean fuel-to-oxidant ratio. 
     
     
         5 . An apparatus as defined in  claim 1  wherein the downcomer defines a vertical passage for the downdraft and has a recessed portion defining a combustion zone recessed from the vertical passage, and wherein the burner is configured to inject the premix into the recessed combustion zone to form a flame projecting through the recessed combustion zone toward the vertical passage. 
     
     
         6 . An apparatus as defined in  claim 5  further comprising a reactant supply and control system configured to supply the burner with fuel and combustion air at rates for the premix to form a flame that projects fully through the recessed combustion zone and into the vertical passage. 
     
     
         7 . An apparatus as defined in  claim 5  further comprising a reactant supply and control system configured to supply the burner with fuel and combustion air at rates for the premix to form a flame that projects only partially through the recessed combustion zone. 
     
     
         8 . An apparatus comprising:
 an indurating furnace structure defining drying, heating, and cooling stations;   a conveyor to convey pelletized material through the drying, heating and cooling stations;   a downcomer configured to direct a downdraft toward the heating station;   a premix burner configured to form premix from fuel gas and combustion air and to inject the premix into the downcomer for combustion of the premix in the downdraft; and   a reactant supply and control system configured to supply fuel gas and combustion air to the premix burner and simultaneously to inject staged fuel gas without combustion air into the downcomer separately from the premix injected by the premix burner.   
     
     
         9 . An apparatus as defined in  claim 8  wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location downstream of the premix burner. 
     
     
         10 . An apparatus as defined in  claim 8  wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the premix burner. 
     
     
         11 . An apparatus as defined in  claim 8  wherein the downcomer defines a vertical passage for the downdraft and has a recessed portion defining a combustion zone recessed from the vertical passage, and wherein the premix burner is configured to inject premix into the recessed combustion zone to form a flame projecting through the recessed combustion zone toward the vertical passage. 
     
     
         12 . An apparatus as defined in  claim 11  wherein the reactant supply and control system is configured to supply the premix burner with fuel and combustion air at rates for the premix to form a flame that projects fully through the recessed combustion zone and into the vertical passage. 
     
     
         13 . An apparatus as defined in  claim 11  wherein the reactant supply and control system is configured to supply the premix burner with fuel and combustion air at rates for the premix to form a flame that projects only partially through the recessed combustion zone. 
     
     
         14 . An apparatus as defined in  claim 11  wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the recessed combustion zone. 
     
     
         15 . An apparatus as defined in  claim 11  wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location downstream of the recessed combustion zone. 
     
     
         16 . An apparatus as defined in  claim 8  further comprising a Venturi mixer structure that has a mixer flow passage and is arranged for the injected premix to entrain staged fuel gas into the mixer flow passage. 
     
     
         17 . An apparatus as defined in  claim 16  wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the Venturi mixer structure and also at a location downstream of the Venturi mixer structure. 
     
     
         18 . An apparatus as defined in  claim 8  wherein the reactant supply and control system is configured to supply fuel gas and combustion air to the premix burner in amounts for the premix to have a lean fuel-to-oxidant ratio, while simultaneously injecting staged fuel gas without combustion air at a rate that is stoichiometric with the premix supplied to the premix burner. 
     
     
         19 . An apparatus as defined in  claim 8  wherein the reactant supply and control system is configured to supply the premix burner with fuel gas, unheated air from the ambient atmosphere, and heated downdraft air from the downcomer, and wherein the premix burner is configured to form the premix from the fuel gas, the unheated air, and the heated downdraft air. 
     
     
         20 . An apparatus as defined in  claim 19  wherein the reactant supply and control system is configured to supply the fuel gas, the unheated air, and the heated downdraft air to the premix burner in amounts for the premix to have a lean fuel-to-oxidant ratio, while simultaneously injecting staged fuel gas without combustion air into the downcomer at a rate that is stoichiometric with the premix supplied to the premix burner. 
     
     
         21 . An apparatus as defined in  claim 8  wherein the premix burner port has a central axis and is configured to inject the premix into the downdraft along the axis, and the reactant supply and control system comprises an array of secondary fuel ports that are spaced radially from the burner port and configured to inject streams of second stage fuel gas into the downdraft in directions parallel to or inclined toward the axis. 
     
     
         22 . An apparatus comprising:
 an indurating furnace structure defining drying, heating, and cooling stations;   a conveyor to convey pelletized material through the drying, heating and cooling stations;   a downcomer configured to direct a downdraft toward the heating station;   a burner configured to inject fuel gas and combustion air into the downcomer for combustion in the downdraft;   a reactant supply and control system configured to supply fuel gas and combustion air to the burner and simultaneously to inject staged fuel gas without combustion air into the downcomer separately from the burner; and   a Venturi mixer structure that has a mixer flow passage and is arranged for the injected fuel gas and combustion air to entrain staged fuel gas into the mixer flow passage.   
     
     
         23 . An apparatus as defined in  claim 22  wherein the reactant supply and control system is configured to inject staged fuel gas without combustion air into the downcomer at a location upstream of the Venturi mixer structure and also at a location downstream of the Venturi mixer structure. 
     
     
         24 . An apparatus as defined in  claim 22  wherein the burner is a premix burner configured to form premix from fuel gas and combustion air and to inject the premix into the downcomer, and the reactant supply and control system is configured to supply the burner with fuel gas and combustion air in amounts for the premix to have a lean fuel-to-oxidant ratio while simultaneously injecting the staged fuel gas at a rate that is stoichiometric with the premix supplied to the burner.