Abstract:
An over fire air (OFA) port arrangement for a pulverized coal-fired boiler or furnace has at least one OFA port through each of the sidewalls for injecting OFA to increase residence time for each burner level. Plural OFA ports may be employed, staggered both vertically and horizontally to effectively deliver over fire air to the burner flames at the appropriate time and location to most efficiently reduce the formation of fuel NO x . OFA port configurations for both single-wall and opposed-wall fired furnaces and boilers are provided.

Description:
FIELD AND BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to the field of industrial and utility furnaces and boilers and in particular to new and useful over fire air (OFA) port configurations for a pulverized coal-fired furnace or boiler which effectively reduce NO x  production.  
         [0002]     NO x  is an unintended byproduct from the combustion of fossil fuels, such as coal. Many industrial furnaces and boilers burn pulverized coal as a primary fuel. NO x  emissions have been discovered to have a negative effect on the environment, and so they are now regulated substantially throughout the world.  
         [0003]     Most NO x  in furnaces and boilers burning pulverized coal is formed during combustion from the fossil fuel. This portion of NO x  formation is called fuel NO x . Fuel NO x  is formed by oxidation of fuel-bound nitrogen during devolatilization and char burnout.  
         [0004]     An effective method of reducing NO x  production which has been known for many years is to reduce oxygen availability during the critical step of devolatilization. Oxygen availability can be reduced during devolatilization by removing a portion of the combustion air from the burners and introducing the air elsewhere in the furnace. This method is commonly referred to as air staging.  
         [0005]     Over fire air (OFA) ports are typically used as part of such air staging systems in furnaces and boilers. The use of such OFA ports is disclosed, for example, in U.S. Pat. Nos. 3,048,131, 5,205,226 and 5,809,913. For a better understanding of such OFA systems, the reader is referred to  Steam/its generation and use,  40 th  Ed., Stultz &amp; Kitto, Eds., Copyright © 1992 The Babcock &amp; Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein, and particularly to Chapter 13, pp. 13-6 to 13-11.  
         [0006]     The effectiveness of over fire air in NO x  suppression depends on the quantity of over fire air, the point in the burner flame where the over fire air is reintroduced, and the rate of reintroduction. Increasing the over fire air quantity tends to lower NO x  levels from the burners, but continual increase of over fire air quantity will eventually cause NO x  to increase as well. This results from combustion being displaced to a region of the furnace or boiler beyond the OFA ports.  
         [0007]     The point at which over fire air is introduced into the furnace is critical as well, since the purpose of OFA systems is to enable the chemistry to proceed through a region of lower oxygen concentration in order to suppress NO x  formation as hydrocarbons preferentially scavenge oxygen. Prematurely adding over fire air will negate the benefit as the desired chemistry is disrupted. And, the rate at which OFA is added is also important, so as to avoid creating oxygen-rich regions within the furnace. It is usual to gradually introduce over fire air to the combustion process to complete combustion without locally flooding the flames with oxygen. At the same time the OFA ports must be designed with sufficient jet momentum to penetrate and supply over fire air throughout the furnace enclosure.  
         [0008]      FIGS. 1 and 2  illustrate a common prior art arrangement of burners and OFA ports and the resulting flame paths. The furnace enclosure  10  has three levels of burners  12 ,  14 ,  16 . The enclosure  10  illustrated is typical of opposed-fired boilers; that is, burners  12 ,  14 ,  16  are oriented through both the front and rear walls  30 ,  32  of the enclosure  10 , opposite each other. The uppermost level of openings through each of the front and rear walls  30 ,  32  of the enclosure  10  is comprised of OFA ports  20 .  
         [0009]     In  FIG. 2 , the approximate flame paths  13 ,  15 ,  17  generated by each row of burners  12 ,  14 ,  16  on the front and rear walls  30 ,  32  are displayed. Bottom burners  12  fire horizontally, and so flame paths  13  from the opposed burners  12  collide in about the center of the enclosure  10 . Unburned combustibles and hot gases flow upwardly in a path  13   a  concentrated in the middle of the enclosure  10 . Second level burners  14  are affected by the upward flow  13   a  of gases and combustibles, so that second level flame paths  15  from the opposed burners  14  bend upwardly near the middle of the enclosure  10 . Third level burners  16  are even more affected by the upflow of gases  13   a , and so the third level flame paths  17  from these burners bend upwardly even more quickly than second level flame paths  15 .  
         [0010]     As shown, the OFA air path  22  intersects the second and third level burner flame paths  15 ,  17  and approaches the upwardly flowing gases and combustibles  13   a . This conventional OFA port configuration of  FIGS. 1 and 2 , while useful, provides greatly varying effects when OFA is injected into the enclosure  10 . The effect on reduction of NO x  is not consistent due to differences in residence time between the burners and the OFA ports, and differences in gas flow through the furnace resulting in different interactions of the OFA and flame paths, among other factors.  
         [0011]     The OFA configuration illustrated in  FIGS. 1 and 2 , when used in a 600 MW utility boiler or furnace unit for example, will have a calculated bulk flow residence time from burners to OFA ports of 2.7 seconds for the bottom burners  12 , 1.3 seconds for the second level burners  14  and only 0.6 seconds for the third level burners  16 . Thus, the level  3  burners  16  suffer from insufficient residence time relative to the region of introduction of OFA, which tends to raise the level of NO x  produced. Often, the most efficient method of reducing NO x  emissions in this type of furnace is to disable the third level burners  16 .  
         [0012]     An alternative for increasing residence time for the second and third level burners  14 ,  16  is to increase the distance between the OFA ports  20  and the third level burners  16 . However, this also requires additional space in the upper furnace region of the enclosure  10 . Thus, increasing the OFA port  20  spacing requires a taller furnace enclosure  10 , thereby increasing the costs and making a bigger building.  
         [0013]     An OFA configuration which provides consistent minimum residence time between burner and OFA port but does not require a larger furnace or disabling existing burners is desirable. Further, an OFA port air flow which is better managed for each burner level is also desirable for reducing NO x  emissions.  
       SUMMARY OF THE INVENTION  
       [0014]     It is an object of the present invention to provide a novel arrangement of OFA ports for further reducing NO x  in pulverized coal-fired furnaces and boilers.  
         [0015]     Another object of the invention is to provide an OFA port configuration for improved residence time between the burners and OFA ports.  
         [0016]     Accordingly, a furnace or boiler including the OFA system of the invention is provided in which over fire air ports are provided on the furnace enclosure sidewalls for introducing OFA transverse to the burner flame paths. An OFA port is optimally positioned to inject over fire air at each burner level flame path and provide good residence time. The air flow rate, air jet velocity and momentum are adjusted to produce maximum effectiveness and over fire air penetration into the furnace enclosure at the desired burner level flame path while avoiding increasing NO x  production due to excess oxygen being present in higher burner levels.  
         [0017]     A first OFA port arrangement is provided for an opposed-wall fired furnace or boiler having three burner levels. One OFA port is positioned to inject air at approximately the center of the enclosure where flames from the bottom burners meet. A pair of OFA ports are provided spaced vertically above and horizontally toward the front and rear wall to intersect approximately with the flame path of the second level burners. A second pair of OFA ports are provided a further distance above the first pair and closer to the front and rear walls for injecting OFA to intersect the flame paths of the third level burners.  
         [0018]     An alternate configuration is provided for wide furnace enclosures in which some OFA ports are provided in the sidewall and others are located in the front and/or rear walls. The OFA ports are positioned through the sidewalls and spaced to direct the over fire air to intersect the flame paths from bottom level burners. OFA ports for injecting air into the flame path of the upper level burners, such as the second and third level burners in a three-high burner level arrangement, are located in the front and/or rear walls of the enclosure. Alternately, lower level OFA ports are positioned through the sidewalls and spaced to direct over fire air to intersect the bottom level burner flame path and upper level OFA ports are also positioned through the sidewalls to direct over fire air to intersect the second level burner flame path. OFA ports for injecting over fire air into the burner flame path of the third level burners are located on the front and/or rear walls of the furnace enclosure.  
         [0019]     Another configuration is provided for single-wall fired furnaces and boilers in which burners are only positioned through the front wall. OFA ports are arranged in the furnace sidewalls in a generally diagonal line extending from the lower end of the furnace enclosure adjacent the rear wall toward the upper end of the enclosure adjacent the front wall. A number of OFA ports corresponding to or in excess of the number of burner levels are provided forming the diagonal line arrangement. The OFA ports are positioned to at least inject over fire air across the enclosure and generally into the flame path of each burner level.  
         [0020]     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     In the drawings:  
         [0022]      FIG. 1  is a partial front elevation diagram of burner and OFA ports on a prior art furnace enclosure;  
         [0023]      FIG. 2  is a side elevation diagram of the prior art furnace enclosure of  FIG. 1  illustrating flame paths in the furnace enclosure;  
         [0024]      FIG. 3  is a side elevation diagram of a furnace enclosure having an over fire air port configuration of the invention;  
         [0025]      FIG. 4  is a side elevation diagram of an alternate embodiment of the over fire air port configuration of the invention;  
         [0026]      FIG. 5  is a partial perspective view of yet another embodiment of the over fire air port configuration of the invention;  
         [0027]      FIG. 6  is a partial perspective view of yet another embodiment of the over fire air port configuration of the invention; and  
         [0028]      FIG. 7  is a side elevation diagram of an embodiment of the over fire air port configuration of the invention for a single-wall fired furnace or boiler. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     Referring now to the drawings, wherein which like reference numerals are used to refer to the same or functionally similar elements throughout the several drawings,  FIGS. 3-6  each display a furnace enclosure  10  of an opposed-wall fired furnace including an OFA configuration of the invention. Like the enclosure  10  of  FIGS. 1 and 2 , in each of  FIGS. 3-6 , three burner levels  12 ,  14 ,  16  are located in the front and rear walls  30 ,  32 , respectively. However, as will be appreciated by those skilled in the art, the present invention is applicable to single wall fired and opposed fired furnace enclosures  10  having fewer or a greater number of burner levels.  
         [0030]     In  FIG. 3 , over fire air ports  200 ,  202 ,  204  are located in sidewalls  35 , rather than the front or rear walls  30 ,  32 . The OFA ports  200 ,  202 ,  204  are positioned so that the injected air will generally transversely intersect the burner flame paths  13   a ,  15 ,  17 , respectively. That is, bottom OFA port  200  will inject over fire air for the flames of the bottom burners  12 , middle OFA ports  202  supply OFA for second level burners  14 , and upper OFA ports  204  inject air for the burner flame of third level burners  16 .  
         [0031]     The OFA ports  200 ,  202 ,  204  are spaced vertically and horizontally, so that the bottom OFA port  200  is nearest the furnace lower end and center, while upper OFA ports  204  are closest to front and rear walls  30 ,  32  and nearest the furnace  10  upper end. The OFA ports  200 ,  202 ,  204  are arranged to best supply OFA to the cross-section of the furnace enclosure  10  and burn out combustibles in the burner zone. The quantity of over fire air, the air jet velocity and momentum are selected to ensure that the over fire air is thrust out into the furnace to ensure good mixing of the over fire air supplied via these ports  200 ,  202  and  204  with the burner flame paths  13   a ,  15  and  17 .  
         [0032]     The spacing is designed to deliver OFA to the burner flame paths at a time which minimizes NO x  production. The vertical and horizontal spacing of the OFA ports  200 ,  202 ,  204  prevents undesirable interaction between the over fire air and the flame paths  15 ,  17  of the second and third level burners  14 ,  16 . The staggered arrangement of OFA ports  200 ,  202 ,  204  avoids the problem of known OFA systems in which over fire air is supplied too soon to the flame paths  15 ,  17  of the upper level burners  14 ,  16 . Thus, the transverse OFA supply configuration of the invention provides more efficient fuel NO x  reduction.  
         [0033]     While bottom OFA port  200  is shown elevated above the intersection of the bottom burner flame paths  13 , it may be positioned lower to inject OFA more nearly at the intersection. The flames from the bottom burners  12  are expected to have proceeded through char reactions shortly after the flame paths  13  intersect. Thus, introduction of OFA near that point will not adversely cause more fuel NO x  production.  
         [0034]     The positions of OFA ports  200 ,  202 ,  204  may be adjusted to more accurately direct over fire air into the expected flame paths  13 ,  13   a ,  15  or  17 . At the same time, the OFA port positions are set to provide sufficient residence time between the burners and the over fire air.  
         [0035]     For example,  FIG. 4  illustrates an alternate OFA port configuration with only two levels of OFA ports  200 ,  202 . The OFA ports  200 ,  202  are again staggered horizontally and vertically. However, the bottom OFA ports  200  are arranged substantially symmetrically about a vertical centerline (between the front and rear walls  30 ,  32 ) of the enclosure  10 , and therefore, also about the flow of rising gases and combustibles represented by the burner flame path  13   a . Middle OFA ports  202  are provided above and closer to the front and rear furnace walls  30 ,  32  than are the bottom OFA ports  200 .  
         [0036]     The OFA port arrangement of  FIG. 5  is best suited for use in furnaces  10  where the cross-sectional ratio of width (W) to depth (D) is approaching or exceeding 2, however it may be desirable to apply it to furnaces  10  where the furnace is physically wide (e.g., over 40 feet) regardless of width to depth ratios. OFA ports  200 ,  202  are located through both furnace sidewalls  35  for injecting over fire air transversely at the lower level burners  12 ,  14 . In certain circumstances, only one OFA port  200  may be employed, substantially at the center of each of the sidewalls  35 . If additional OFA ports  202  are employed, they would be arranged symmetrically about the OFA port  200 , and at a somewhat higher elevation as shown and described. Additional OFA ports  208  are positioned near the centerline of the furnace front and rear walls  30 ,  32 , at an elevation above the elevation of the uppermost row of burners  16 . The particular number and placement of these OFA ports  208  can be determined by computational fluid dynamic (CFD) modeling techniques known to those skilled in the art. Generally, as the furnace width W begins to increase, since penetration of the over fire air into the centermost portion is desired, the first OFA ports  208  would be applied at approximately the centerline of the front and rear walls,  30 ,  32 , and as furnace width W increased further (greater W/D ratios) additional OFA ports  208  would be employed, preferably substantially symmetrically on both sides of the centerline of the front and rear walls  30 ,  32 . The front wall OFA ports  208  better direct OFA air into the center of the furnace enclosure  10  when the width begins to increase, than transversely oriented OFA ports alone can. The size of the OFA ports  200 , or  208  are selected to ensure that an adequate quantity of over fire air, the air jet velocity and momentum are provided to ensure that the over fire air is thrust out into the furnace  10  to ensure good mixing of the over fire air supplied via these ports with the burner flame paths  13   a ,  15  and  17 .  
         [0037]     In certain circumstances, it may be desirable to place OFA ports  208  so as to cover a more substantial portion of the width W of the front and rear walls  30 ,  32  even where the furnace 10 W/D ratios are at or close to 1, or even less than 1.  FIG. 6  illustrates such an application to a furnace configuration where the W/D ratio is not much greater than 1, at least one OFA port  200  is employed on each sidewall  35 , and a plurality of OFA ports  208  are employed so as to cover more than just a central portion of the furnace  10  and along furnace width W. The at least one OFA port  200  located on each of the sidewalls  35  is positioned at approximately the same elevation as those OFA ports  208  located on the front and rear walls  30 ,  32 . These side wall OFA ports  200 , in this embodiment, would typically provide approximately 30% of the over fire air, the balance being provided by the plurality of OFA ports  208  located in the front and rear walls  30 ,  32 . Under certain circumstances, the at least one OFA port  200  on each sidewall  35  may be positioned at approximately the same elevation as the elevation of the top row of burners  16 , as schematically and alternately shown in  FIG. 6  by  200 A, or even at a lower elevation approximately corresponding to a center C of the burner zone; i.e. at the elevation of the middle row of burners  14  in a three-level burner arrangement, as schematically and alternately shown in  FIG. 6  by  200 B.  
         [0038]      FIG. 7  displays an alternate configuration of the OFA ports for use with a single-wall fired furnace in which burners  12 ,  14 ,  16  are provided only on the front wall  30  of the furnace enclosure  10 . In this type of furnace, the flame paths are initially affected primarily by the presence of the rear wall  32 . The flame paths  13 ,  15 ,  17  of the bottom, second and third level burners  12 ,  14 ,  16 , respectively are indicated by the lines as shown.  
         [0039]     OFA ports  200 ,  202 ,  204  and  206  are provided through enclosure sidewalls  35  to inject OFA. OFA ports  200 ,  202 , and  204  are arranged to inject over fire air at the flame path of burners  12 ,  14 ,  16 , respectively. OFA port  206  provides additional air nearest to the front wall  30  to ensure complete combustion of the fuel.  
         [0040]     The particular number of OFA ports  200 ,  202 ,  204 ,  206 ,  208  provided at any given level can be changed to best deliver OFA to the selected region. For example, while  FIG. 3  illustrates one bottom OFA port  200  and  FIG. 4  illustrates two, three or more could be used if desired to ensure good combustion and coverage. As noted above, the primary consideration when arranging the OFA ports is to provide OFA to the correct flame path for a given level, thereby ensuring suitable residency time for each burner level.  
         [0041]     The OFA configurations of the invention solve the problem of too rapid air introduction to the second and third level burners without requiring a taller furnace enclosure. The OFA configurations herein provide a more effective system for controlling NO x  without disabling burner levels. These OFA configurations are an inexpensive design which allows tailoring the point of OFA introduction to the flame paths to best control NO x  for a given type of furnace.  
         [0042]     The OFA configurations of the invention also provide better control of air mixing so that flames from the upper level burners are not flooded with air too soon. The transverse orientation of the OFA ports in at least the lower levels permits good injection of the OFA to the bottom level burner flames without interfering with the second and third (or higher) level burner flames. The OFA can be injected in sufficient quantity from the sidewalls to produce good penetration and distribution into the desired flame path, without detriment to the other burner level flame paths. Accordingly, fuel NO x  production remains reduced as the second and third level flames have sufficient time to burn before the introduction of OFA. Thus, air staging is made more effective by the transverse orientation of the OFA ports with respect to the burner levels. It is believed that the present invention will permit the percent of over fire air provided through the sidewalls to be within a range of about 20 to 100% of the total over fire air. The upper end of this represents a situation where all the over fire air is provided via the side wall OFA ports, while the lower end of the range represents a situation where over fire air is introduced by both side wall OFA ports according to the invention, and front and/or rear wall OFA ports.  
         [0043]     While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, those skilled in the art will appreciate that changes may be made in the form of the invention covered by the following claims without departing from such principles. For example, the present invention may be applied to new construction involving industrial or utility steam generators, boilers or furnaces, or to the replacement, repair or modification of existing industrial or utility steam generators, boilers or furnaces. In some embodiments of the invention, certain features of the invention may sometimes be used to advantage without a corresponding use of the other features. For example, the OFA ports may be employed on the sidewalls alone, or in combination with OFA ports on the front, or both of the front and rear furnace walls, depending upon the firing arrangement as described herein. Accordingly, all such changes and embodiments properly fall within the scope and equivalents of the following claims.