Method for reducing burner exhaust emissions

Abstract

NO.sub.x exhaust emissions from furnace or boiler burners are reduced by combusting fuel with low excess air (about 5 percent more air than the theoretically required minimum air volume). The fuel is combusted so that the maximum flame temperatures are less than the theoretically obtainable maximum temperature. This is achieved by cooling flames in the chamber through contact with combustion chamber walls and by combusting central portions of the flames in two stages. Burners in a vertically lower burner bank are initially fired with insufficient air and burners in an upper burner bank are fired with additional air to make up for the air deficiency of the lower burners. The additional air is intermixed with the uncombusted fuel from the lower burners and such fuel is burned at a later stage downstream of and spaced from the burners. The additional air supplied from the burners in the upper row is measured to provide the above stated total of 5 percent excess air for the furnace.

Description

BACKGROUND OF THE INVENTION

Industrial furnaces, boilers, steam generators, and the like are a main source of air pollution. Attempts are continuously being made to reduce such industrial air pollution without compromising the efficiency of the furnaces and boilers.

A major industrial pollutant discharged by furnaces and boilers (hereinafter sometimes collectively referred to as "furnaces") are the oxides of nitrogen (hereinafter NO.sub.x). NO.sub.x emission levels are inversely related to the furnace oxygen concentration and the flame temperatures. To a much lesser extent it also depends on the nitrogen content of the fuel.

There are a number of methods for reducing NO.sub.x which include increasing the furnace cooling surface, lowering the combustion air temperature by increasing the air supply in the furnace, recirculating the flue gas through the furnace burners, operating the burners with low excess air, and two stage firing and off-stoichiometric or biased (hereinafter off-ratio) firing of the burners. Each method has certain advantages and disadvantages over the others.

Increasing the furnace cooling surface is effective but requires a high initial investment and cannot usually be used for modifying existing furnaces. Lowering the combustion air temperature reduces the efficiency of the furnace. Flue gas recirculation is highly effective but again requires a high initial investment and is ill adapted for modifying existing furnaces. Low excess air firing is effective in reducing NO.sub.x emissions and enhances the efficiency of the unit but requires a very accurate distribution of the air in the combustion chamber. Two stage and off-ratio firing provide good results for lowering NO.sub.x emissions but traditionally require more excess air and thus reduce the efficiency of the furnace unless the subsequently furnished additional air is evenly distributed in the combustion chamber. A failure in the even distribution of the additional air and low excess air operation leads to an incomplete combustion and the emission of particulates and smoke, highly undesirable pollutants.

It has been recognized that a combination of low excess air firing and off-ratio combustion aids in reducing NO.sub.x emissions and is, therefore, highly desirable. Problems are usually encountered in evenly mixing non-combusted fuel from off-ratio firing and the necessary additional air to effect the complete combustion of the fuel. This has been especially acute in modifying existing furnaces which have a low or intermediate number of about 10 or less burners.

Large scale utility type furnaces as employed in electric generating plants and the like use relatively large numbers of burners, usually well in excess of ten, which are arranged in two or more vertically spaced rows. Furthermore, their combustion chambers are very large. Under such conditions it is possible to eliminate one or more burners and discharge therefrom additional air only for off-ratio firing and a truly two-stage combustion in which the additional air is derived from one or more auxiliary air supplies. By properly selecting spaced apart burner locations for the additional air discharge an even distribution of the additional air can be obtained.

However, this approach is not suitable for smaller furnaces as are commonly found in industrial heating, boiler or steam generating plants which as a rule employ ten burners or less arranged in only two vertically spaced apart rows or burner banks. If, in such a situation, an existing burner were replaced with an air nozzle the additional air would be unevenly distributed. Consequently, portions of the combustion chamber would have too much air and other portions would have insufficient air. The furnace would have an inefficient and incomplete combustion yielding high pollutant levels and an economically unsound operation, thereby defeating the purpose for combining low excess air firing with off-ratio combustion.

SUMMARY OF THE INVENTION

The present invention provides a method for reducing the emission of NO.sub.x from furnaces without compromising the efficiency of the furnace, increasing the emission of other pollutants such as smoke or particulates, or requiring expensive equipment and controls which may make the method economically unfeasible. The invention is ideally suited for modifying small and intermediate size existing furnaces so that they can be operated at reduced NO.sub.x emission levels without increasing the emission of other pollutants above levels for conventional low excess air firing.

Broadly speaking, this is accomplished by operating the furnace with low excess air, that is with a total air volume of no more than about 5% in excess of the theoretically required air volume to effect a complete combustion of the fuel and as contrasted with a normal excess air volume of between 12 -15%, and by further maintaining the maximum flame temperatures in the combustion chamber of the furnace below the theoretically obtainable maximum flame temperature for low excess air operation. The temperature is maintained at a relatively lower value through the off-ratio firing of some of the centrally disposed furnace burners and burning non-combusted fuel at a downstream location from the burners with additional air provided from other burners.

The burners are arranged in at least two vertically spaced apart rows, an upper and a lower burner row, with the burners in the lower row being operated off-ratio and with the additional air required for completely combusting all fuel discharged by the burners in the lower row being provided from the corresponding burners in the upper rows. The additional air volume is carefully controlled so that the overall excess air for the furnace does not exceed 5%. Burners disposed on the sides of the burner banks are operated with low excess air without off-ratio firing since their flames are immediately cooled by the surrounding furnace walls so that the maximum theoretically obtainable flame temperatures are never reached.

The present invention is adapted for use with relatively small furnaces such as industrial furnaces having ten or less burners arranged in at least two vertically spaced banks of aligned burner pairs. The present invention is further particularly well suited for modifying existing small and intermediate size industrial furnaces for low NO.sub.x emission level operation since such modification can be economically accomplished.

Instead of selecting one or more of the burners as an air inlet only the present invention selectively controls the air-to-fuel discharge of at least some of the burners over at least selected portions thereof. For operational purposes aligned burners in the upper and lower burner bank are constructed so that the lower burner discharges more fuel than can be combusted with its air supply for an off-ratio operation while the corresponding burner in the upper bank is constructed so that it discharges additional air that is directed into the flame of the lower burner so that the additional air can combine with uncombusted fuel for burning downstream at points spaced from the burners.

To avoid an excessive cooling of the flames from burners adjacent the furnace walls, that is the side burners of each bank, such burners are operated with low excess air at least over burner portions which direct flames towards the furnace walls to prevent an incomplete combustion of such flames due to excessive cooling. Off-ratio firing is employed on burners between the side burners and can also be employed on inwardly facing portions of the side burners, or in instances in which there is only one burner in each bank over the lower burner portion facing the upper burner. In this manner the combustion temperature in the chamber is held at less than the theoretically obtainable temperatures because the combustion process is drawn out and generated heat has time for dissipation and absorption before all fuel is combusted. Consequently, the NO.sub.x emission level is lowered due to the relatively low oxygen level and the relatively low combustion temperature in the furnace.

The present invention also provides apparatus for operating furnaces in accordance with the above-outlined method which generally comprises at least two vertically spaced burners arranged in an upper and a lower burner bank. Each burner has a fuel discharge nozzle and means for mixing discharged fuel with combustion air. Means is provided for discharging from at least a portion of the burners in the lower bank more fuel than can be completely combusted with air discharged by such burners. Means is further provided for discharging from the corresponding burner in the upper bank additional air, or less fuel than can be completely combusted with the air discharged by such burner to effect an even admixture of the non-combusted fuel discharged by the burners in the lower bank with the additional air. This fuel--additional air mixture is subsequently subjected to a secondary combustion at locations spaced from the burners whereby the flame temperatures are maintained below the maximum theoretically obtainable temperature.

In the preferred embodiment of the invention each bank has a plurality of burners and the fuel discharge nozzles are constructed to discharge the desired fuel quantity in predetermined fixed patterns. Each fuel discharge nozzle includes an end member which has a plurality of fuel discharge apertures that are drilled through the end member in a quantity and size so that the nozzle discharges a controlled, predetermined volume of fuel. The apertures are further positioned and oriented so that they form the desired spray pattern. Thus, the fuel discharge apertures in the end member of a nozzle can be rearranged to spray more or less fuel in one or the other direction as may be dictated by the particular location of a burner.

The fuel discharge aperture arrangement is utilized for discharging the additional air from the upper burners in the direction of the fuel discharged from the lower burners so as to burn uncombusted fuel at a point spaced from the burners. Since the relative positioning and arrangement of the burners in the banks is critical and in order to attain the desired fuel and air discharge patterns and to effect an intimate mixture of the excess fuel and excess air discharged by the various burners, the present invention provides a color coding that is applied to the different burners to assure their proper replacement after removal for cleaning and maintenance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevational view of furnace burners arranged and constructed in accordance with the present invention;

FIG. 2 is a fragmentary side-elevational view, in section, of a burner constructed in accordance with the present invention and is taken on line 2--2 of FIG. 1;

FIG. 3 is a schematic side-elevational view through a furnace having burners constructed and arranged in accordance with the present invention and illustrates the interfacing of flames from burners in the lower burner bank with flames from burners in the upper burner bank of the furnace;

FIG. 4 is a schematic representation of a burner arrangement and construction for one upper and one lower burner;

FIG. 5 is a view similar to FIG. 4 but illustrates an arrangement for two upper and two lower burners;

FIG. 6 is an arrangement similar to FIG. 4 that illustrates three vertically spaced burner banks each having four burners;

FIG. 7 is a diagram illustrating the NO.sub.x emission levels as a function of the excess air volume;

FIG. 8 is a diagram illustrating the reduction in the NO.sub.x emission level obtained from off-stoichiometric firing of a burner;

FIG. 9 is an enlarged front-elevational view of a fuel nozzle constructed in accordance with the present invention and is taken on line 9--9 of FIG. 2; and

FIG. 10 is a fragmentary side-elevational view, in section, of the nozzle and is taken on line 10--10 of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIGS. 1 through 3, a schematically illustrated upright furnace 2 has a floor 4, vertical sidewalls 6 and an exhaust duct 8 through which exhaust gases from a combustion chamber 10 of the furnace are emitted to the atmosphere. The furnace is of an intermediate, say industrial size, and in the illustrated embodiment comprises a lower and an upper bank 12 and 14, respectively, of five burners 16 each. The burners in each bank are vertically aligned with burners in the other bank and they are concentrically disposed in openings 18 of the furnace sidewalls. Each burner includes a fuel discharge nozzle 20 that is fluidly connected with fuel supply (not separately shown) via a conduit 22. Pressurized combustion air is provided through an annual space 24 for admixture with fuel discharged by the nozzle and for combusting such fuel in chamber 10. The combustion air is provided from wind boxes (not separately shown) via suitable ducts, directional guides and combustion air flow control means which do not form part of this invention. See U.S. Pat. No. 2,818,109 or 2,838,103 for exemplary constructions of combustion air supplies.

As is schematically illustrated in FIG. 3, the fuel air mixture and flame in combustion chamber 10 forms upwardly curved lower and upper flame zones 26 and 28 from the burners in the lower and the upper burner banks 14, 16, respectively. In the chamber the flames are cooled through contact with the furnace sidewalls 6 as well as heat exchangers (not shown) that may be present in the furnace and the combustion gases from the flames are guided towards exhaust duct 8 so that they leave the combustion chamber after they have been cooled to the desired temperature by the transmission of heat energy to the heat exchange components of the furnace.

As already indicated above, the present invention reduces NO.sub.x emissions by employing low excess air, that is by employing no more than about 5 percent excess air over the theoretically required volume of air to fully combust the fuel, and through off-ratio firing. Referring briefly to FIGS. 7 and 8, the diagram of FIG. 7 illustrates the NO.sub.x reduction attained from low excess air firing. As compared to standard excess air firing, (15 percent excess air) low excess air reduces the NO.sub.x emission by factor of approximately one-third. In addition, thereto and as shown in the diagram of FIG. 8, the off-ratio firing of a burner, as further described hereinafter, at a stoichiometric ratio of 0.9 as compared to the standard ratio (standard excess air) of 1.15 reduces NO.sub.x emissions for that burner by an additional 40-45 percent. Consequently, to minimize NO.sub.x emission in accordance with the present invention, the furnace as a whole is operated with low excess air and, in addition, off-ratio firing is employed to the greatest extent possible.

Referring again to FIGS. 1 through 3, the burners 16 in each burner 12, 14 are identified as side burners 30 which are the burners proximate sidewalls 6 of the furnace and central burners 32 which are the three burners disposed intermediate the side burners. The flames emanating from the side burners are in close proximity to the furnace walls and are, therefore, immediately cooled. Since off-ratio firing involves a combustion process in which the maximum flame temperature is less than the attainable maximum and since the flames from the side burners are subject to very significant and immediate cooling, the side burners are operated with low excess air without off-ratio firing to prevent the possibility of a flame-out or of an incomplete combustion due to excessive flame cooling. Thus, for the side burners a one-third NO.sub.x emission level reduction is obtained from its low excess air operation.

The flames of the central burners 32 are not subjected to the rapid cooling of the side burner flames; the lower central burners 32 (1) are therefore operated off-ratio with only 90 percent of the theoretically required minimum air (hereinafter 0.9 air). The upper central burners 32u are divided into a first, lower segment 34 and second upper segment 36, the first segment being the portion of the upper burner adjacent the lower burners.

Referring now also to FIGS. 9 and 10, the two segments of the upper burners are defined by an end plate or member 38 of fuel nozzle 20 which has a generally circular configuration and which covers a similarly circular interior space 40 of the nozzle which, in turn, communicates with nozzle conduit 22. A plurality of apertures 42 are drilled in the end plate so that pressurized fuel can be discharged from the interior space of the nozzle into the combustion chamber of the furnace. Generally speaking, the fuel discharge apertures are arranged, that is, are oriented and distributed so that the nozzle sprays and atomizes the fuel in conical pattern as is schematically indicated by cone 44 in FIG. 2.

For the dual-segment upper central burners 32u, the segments are defined by the distribution, orientation and size of the apertures. FIGS. 9 and 10 are an illustration of such a dual segment nozzle. It comprises a plurality of evenly spaced and distributed apertures 42a in the upper nozzle segment 36 and one or more equally spaced apertures 42b in the lower nozzle segment 34. The apertures in the lower segment have substantially lesser density and/or diameter so that for a given combustion air flow through annular gap 24 less fuel is discharged over the angle .alpha. of the lower segment than can be combusted with the air volume entering through the air gap. In other words, over the angular extent of the lower segment, additional air is discharged. As is best seen in FIGS. 1 and 2, apertures 40b in the lower nozzle segment are arranged so that the projected cone surface as indicated by the lower leg 44a of cone 44 does not fully intercept the air flow through the lower portion of air gap 24 and a given amount of air volume enters through an arcuate air gap section 46 as is schematically illustrated by the arrow 48 in FIG. 2. This additional air is directed generally downwardly and into the flame from the corresponding central burner 32(1) in lower bank 12. The additional air is thus admixed with uncombusted fuel discharged by the lower central burners due to their off-ratio firing and the resulting insufficient air supply. As is schematically illustrated in FIG. 3, the additional air from the upper central burners mixes with lower flame zone 26 along their common interface 50.

To assure complete combustion of all fuel, and low excess air operation of the total furnace, that is of all burners combined, the fuel and air volumes and the direction in which they are discharged are closely controlled. In the exemplary arrangement illustrated in FIG. 1, the lower central burners are operated with 0.9 air or with 85.5 percent of their 1.05 theoretical low excess air requirement. The necessary additional air to assure a complete combustion of the fuel discharged by the lower central burners, is obtained by restricting the fuel discharge over the lower segment 34 of upper central burners 32u so that the arcuate air gap section 46 supplies the necessary additional air for the lower central burners. In the example, and with a 5 percent overall excess air operation for the furnace, in which the lower burners are fired with 0.9 air and in which .alpha.=150.degree.for the lower section of the upper burners, 14.3 percent of the air supplied to the upper burners is used for combusting the unburned fuel of the lower burner (1.05 .times. 0.143 = 0.15 additional air). This is obtained by drilling end plate 38 of the lower burner segment 34 so that 66 percent of that segment operates with normal 5 percent excess air while the remaining 34 percent of the lower segment operates with 100 percent air. In terms of the drawings, arcuate air gap section 46 occupies 34 percent of the lower segment and through it the additional air required by the lower burners is supplied. It will, of course, be apparent that the angle .alpha., the fuel discharge volume and the air gap section 46 of the lower segment 34 of the upper burners 32u can be varied to suit particular applications as long as the required overall additional air volume is supplied.

In operation the side burners 30 are self-sustaining, that is they have sufficient air to fully combust their fuel and they operate with low excess air to reduce their NO.sub.x emission level as above described. Fuel discharged from the upper segment 36 of the upper burners 32u is also fully combusted since the upper segments also operate with 5 percent low excess air in the same manner as do the side burners. All of the lower burners 32(1) operate off-ratio with only 0.9 air. The necessary additional air for a full combustion of the fuel from the lower burners being supplied through arcuate air gaps 46. That additional air gradually mixes with the lower flame zone 26 through interface 50 in an upward direction as is schematically illustrated by the phantom lines in FIG. 3, so that all fuel is fully combusted when it reaches the top of combustion chamber 10 with low excess air with the lower center burners being additionally operated off-ratio.

Referring now briefly to FIG. 6, a burner arrangement similar to that illustrated in FIG. 1 but having three vertically stacked burner banks 54, 56 and 58 is illustrated. Side burners 60 are again operated with 1.05 low excess air (LEA). Central burners 62 of the lowermost bank 54 are operated with 0.9 air. Central burners 64 of middle bank 56 are divided into upper and lower segments 66 and 68, respectively. The lower segment 68 is operated so that it provides the additional air required for unburnt fuel from the lowermost burners 62 as above described while the upper segments 66 are operated with 0.9 off-ratio air. Central burners 70 of uppermost bank 58 are also divided into upper and lower segments 72 and 74, respectively with the lower segment again providing the required additional air for fully combusting all fuel discharged by the off-ratio operated upper segment 66 of burners 64 in the middle row. The upper segments 72 of central burners 70 in the uppermost bank 58 are operated with 1.05 low excess air in the same manner in which the upper central burners 32u of the embodiment of FIG. 1 are operated. The arrangement illustrated in FIG. 6 provides the same advantages of NO.sub.x emission reduction as does the embodiment shown as FIG. 1 by operating some of the burners off-ratio and supplying the necessary additional air from other burners, the remainder of which is operated with low excess air.

Referring now to FIG. 4, an arrangement for use with only two vertically spaced apart upper and lower burners 76 and 78 is illustrated. In the arrangement shown in FIG. 4, it is assumed that there occurs a significant heat loss from the flame discharged by the lower burner in a downward direction towards the furnace floor. Consequently, only an upwardly directed segment 80 of the lower burner is operated off-ratio with 0.9 air while the remaining segment 82 is operated with 1.05 low excess air. The necessary additional air for unburnt fuel from the upper segment of the lower burner is provided by lower segment 84 of the upper burner 76 while a remaining upper section of that burner is operated with 1.05 low excess air. If heat losses in a downward direction are not very large, the portion of the lower burner that is fired off-ratio can, of course, be enlarged so that, for example, only the sides of the burner are operated with 1.05 air.

Referring briefly to FIG. 5, yet another exemplary arrangement of burners in accordance with the present invention is illustrated. This arrangement comprises two vertically spaced banks 86, 88 of two burners 90 each. Outer portions 92 of the burners are operated with 1.05 low excess air while inner portions 94 of the lower burners are operated off-ratio 0.9 air. Accordingly, inner portions 96 of the upper burners are constructed to provide the additional air necessary for fully combusting all unburnt fuel discharged from the inner portions 94 of the lower burners.

In all above-described embodiments of the invention, the essentials of the operation are the same. The vertically lower burners or at least portions thereof which are not subject to excessive and rapid cooling are operated off-ratio to gain the above-noted reduction in NO.sub.x emissions from off-ratio firing. The additional air required for combusting fuel discharged by the lower burners and not burnt due to the insufficient air supply is provided by the most convenient portion of the adjacent upper burner which assures an intimate intermixing of the additional air with the unburnt fuel and a complete fuel combustion before discharge to the atmosphere. Those portions of the burners or those burners the flames of which are subject to rapid cooling, and the burners in the uppermost row are operated with 1.05 low excess air.

Referring again briefly to FIGS. 1 through 3, the installation of the burners and their relative orientations are, of course, of great importance to assure a proper operation of the furnace through an even and thorough admixing of all fuel and all available air to attain a complete combustion of the fuel. Since it is difficult to judge the fuel and air discharge capacities and orientations of the burners from the arrangement of the fuel discharge apertures in end plate 38 of the fuel discharge nozzle 20, it is preferred that the nozzles be color coded so that they can be installed in the proper burner opening 18 without tedious checking. Thus, burner end faces and/or backsides of the burners are differently colored as is schematically illustrated in the drawings by the indicated grating. Thus, the side burners 30 receive one, the lower central burners 32(1) receive another, and the upper burners 32u receive yet a third color. Additionally, proper orientation devices such as interengaging protrusions and grooves (not shown) in the nozzles and the associated supporting structures can be provided so that the nozzles, and particularly the upper central nozzles 32 are always properly oriented.

Claims

I claim:

1. A method of operating an existing industrial furnace at a low NO.sub.x emission level, the furnace including at least two vertically spaced burners, the method comprising the step of modifying the combustion process in the furnace with the steps of firing at least a portion of one of the burners with an off-ratio air supply, firing at least a portion of the other burners with a sufficient additional air supply to provide for a low excess air firing of all burners, and combusting all fuel discharged by the burners by first combusting a portion of the fuel adjacent the burner and thereafter combusting another portion of the fuel at a location spaced from the burners.

2. A method according to claim 1 including the step of firing at least a portion of at least one of the burners with about 5 percent excess air.

3. A method of modifying an existing furnace for operating it at a low NO.sub.x emission level, the furnace having a plurality of burners arranged in at least two vertically spaced rows, each burner having a fuel discharge nozzle and an annular air supply concentrically arranged about the nozzle, the method comprising the steps of adapting at least a portion of at least one burner in the lower bank to discharge more fuel than can be completely combusted with the air supply for such burner to thereby leave a quantity of unburnt fuel, and adapting a portion of at least one burner in the upper bank proximate the adapted burner in the lower bank to discharge a volume of additional air over and above the air required by such burner for completely combusting said unburnt fuel, and adjusting the air supplied by at least said one burner in the upper bank to provide a total of no more than about 5% excess air for all fuel discharged by the burners.

4. A method for combusting fuel discharged into a combustion chamber from at least one lower nozzle and at least one upper nozzle disposed above the lower nozzle, the method comprising the steps of discharging fuel from each nozzle in a generally circular pattern, discharging combustion air concentrically with respect to the discharged fuel from each nozzle for admixture and combustion with the fuel, limiting the air-to-fuel ratio in at least a section of the lower nozzle adjoining the upper nozzle to less than the stoichiometric ratio, injecting additional air in excess of the stoichiometric ratio over a section of the upper nozzle adjoining the lower nozzle, and combusting some of the fuel discharged from the section of the lower nozzle with additional air from the section of the upper nozzle at a point spaced from and downstream of the nozzles to effect a complete fuel combustion at a relatively low temperature to thereby reduce the emission of NO.sub.x.

5. A method according to claim 4 including outer nozzles disposed on each side of each of the upper nozzle and the lower nozzle, and including the step of discharging from the outer nozzles a quantity of fuel and a quantity of air to effect a low excess air combustion of fuel discharged by the outer nozzles.

6. A method according to claim 5 wherein the steps of discharging air comprise the step of discharging approximately 5 percent more air than the air required to effect a theoretically complete combustion of all discharged fuel.

7. A method according to claim 4 wherein the step of discharging air at the section of the lower nozzle comprises the step of discharging approximately 10 percent less air than the air required to effect a theoretically complete combustion of the fuel discharged by such section.

8. A method according to claim 7 wherein the step of discharging additional air from the section of the upper nozzle comprises the step of discharging from the upper nozzle a quantity of additional air sufficient to supply approximately 5 percent more air in the chamber than theoretically required to effect a complete combustion of all fuel discharged from the section of the lower nozzle.

9. A method for reducing the level of NO.sub.x emissions in exhaust gases of a furnace comprising the steps of discharging fuel and combustion air into a combustion chamber of the furnace from an upper and a lower bank of burners, providing a total air volume discharged into the chamber of no more than about 5 percent in excess of the theoretically required air volume to effect a complete combustion of the fuel, and maintaining the maximum gas temperature within the chamber at less than the theoretically attainable maximum gas temperature by discharging less air than the theoretically required minimum amount of air from at least portions of central burners in the lower bank facing the upper bank, and discharging additional air from central burners in the upper bank over and above the air volume required to effect a complete combustion of fuel discharged by such burners, the additional air being discharged over a portion of the central burners in the upper bank adjoining the lower bank to effect an interfacing of fuel-rich mixture discharged at the lower bank and air-rich mixture discharged at the upper bank downstream of the burners.

10. A method according to claim 9 wwherein the step of discharging additional air comprises the step of discharging sufficient additional air to provide an overall excess combustion air for the central burners of about 5 percent above the theoretical minimum to effect a complete combustion of fuel discharged by such burners.

11. A method for reducing NO.sub.x in exhaust gases from furnaces, boilers and the like comprising the steps of arranging a plurality of burners in at least two vertically spaced burner banks, providing an overall air supply to the burners of no more than about 5 percent above the required theoretical minimum air volume for effecting a complete combustion of fuel discharged from the burners, mixing approximately 5 percent excess combustion air with fuel discharged by at least portions of burners defining sides of the burner banks, discharging from at least a portion of at least one of the burners in the lower bank insufficient air to effect a complete combustion of the fuel discharged by such burner, supplying sufficient additional air from at least one of the burners in the upper bank to combust all uncombusted fuel discharged by at least one burner in the lower bank, mixing such additional air with the uncombusted fuel, and combusting the uncombusted fuel with the additional air at a location spaced from the burners.

12. A method according to claim 11 wherein there are provided two burners in each bank, and wherein the step of discharging insufficient air from the burners in the lower bank and additional air from the burners of the upper bank comprises the step of discharging such air from burner portions facing the other burner in the same bank.

13. A method according to claim 12 wherein the step of discharging approximately 5 percent excess air from the side burners comprises the step of discharging the 5 percent excess air from portions of the burners in the banks facing away from the other burner in the same bank.

14. A method according to claim 11 wherein the burners discharge fuel and air in a generally conical pattern, and wherein the step of discharging the additional air comprises the step of discharging such additional air over a downwardly facing section of at least one burner in the upper bank.

15. A method according to claim 14 wherein the step of discharging fuel and insufficient air from the burners in the lower bank comprises the step of substantially evenly discharging fuel and insufficient air from at least one burner in the lower bank in a generally conical pattern, and including the further step of intermixing the additional air with non-combusted fuel.

16. A method according to claim 15 including the step of discharging approximately 5 percent excess air from remaining sections of the burners in the upper bank.