Abstract:
A burner system that utilizes a plurality of single stage rapid mix burner cells which are organized into one or more self-stabilizing matrices. The assemblage combines the advantageous operating characteristics of nozzle mix and premixed type burners and achieves extremely low NO x , CO, and hydrocarbon emissions.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority from U.S. provisional patent application Ser. No. 60/962,304, filed on Jul. 27, 2007. 
     
    
     FIELD OF INVENTION 
       [0002]    This invention relates generally to combustion apparatus, and more specifically relates to a burner system that utilizes a plurality of single stage Rapid Mix Burner cells, which are organized into one or more self-stabilizing matrices. The assemblage combines the advantageous operating characteristics of nozzle mix and premixed type burners and achieves extremely low NO x , CO, and hydrocarbon emissions. 
       BACKGROUND OF INVENTION 
       [0003]    In the gas burner invention of my U.S. Pat. No. 5,407,347 (the disclosure of which is hereby incorporated by reference) extremely low NO x , CO, and hydrocarbon emissions are achieved, while maintaining the desirable features of a nozzle mix burner. This is accomplished by injecting the fuel gas, such as natural gas, in a position that would be typical for a nozzle mix burner, while generating such rapid mixing that, effectively, premixed conditions are created upstream of the ignition point. 
         [0004]    In such burner apparatus (hereinafter referred to as “the basic rapid mix burner”, or “single stage RMB”, or simply “RMB”) an outer shell is provided which includes a windbox and a constricted tubular section in fluid communication therewith. A generally cylindrical body is mounted in the shell, coaxially with and spaced inwardly from the tubular section so that an annular flow channel or throat is defined between the body and the inner wall of the tubular section. Oxidant gases are flowed under pressure from the windbox to the throat, and exit from a downstream outlet end. A divergent quarl is adjoined to the outlet end of the throat and defines a combustion zone for the burner. A plurality of curved axial swirl vanes are mounted in the annular flow channel to impart swirl to the oxidant gases flowing downstream in the throat. Fuel gas injector means are provided in the annular flow channel proximate or contiguous to the swirl vanes for injecting the fuel gas into the flow of oxidant gases at a point upstream of the outlet end. The fuel gas injection means comprise a plurality of spaced gas injectors, each being defined by a gas ejection hole and means to feed the gas thereto. The ratio of the number of gas ejection holes to the projected (i.e. transverse cross-sectional) area of the annular flow channel which is fed fuel gas by the injector means is at least 200/ft 2    
         [0005]    One or more turbulence enhancing means may optionally be mounted in the throat at least one of the upstream or downstream sides of the swirl vanes. These serve to induce fine scale turbulence into the flow to promote microscale mixing of the oxidant and fuel gases prior to combustion at the quarl. The gas injectors can be located at the leading or trailing edges of the swirl vanes, and inject the fuel gas in the direction of the tangential component of the flow imparted by the swirl vanes. The gas injectors can also be disposed on a plurality of hollow concentric rings which are mounted in the throat downstream of the swirl vanes. The injectors can similarly comprise openings disposed in opposed concentric bands on the walls which define the inner and outer radii of the annular flow channel. The gas injectors can also be located at the surfaces of the swirl vanes, with the vanes being hollow structures fed by a suitable manifold. Preferably the geometry of the burner is such that the product of the swirl number S and the quarl outlet to inlet diameter ratio C/B is in the range of 1.0 to 3.0. 
         [0006]      FIG. 1  is a prior art showing which is actually FIG. 17 of my aforementioned U.S. Pat. No. 5,407,347 patent, and is included here to illustrate a typical prior art single stage RMB which may be utilized in the present invention.  FIG. 1  is a plan view of such a device  110 . The view is somewhat simplified and schematic in nature. Windbox  112  is fed combustion air and flue gas in the direction  114  (by pressurizing means not shown). The entirety of the windbox is not shown, but is rather broken away at its upper end. The arrangement of apparatus  110  enables a more compact device than certain other embodiments of the prior art apparatus. Specifically, it will be seen that a constricted tubular section  115  is provided, which is in direct communication with the interior of wind box  112  through the open end defined by diverging flange  116 . The flange  116 , while shown to diverge linearly, can also be dish-shaped to assist in air flow. Combustion air proceeds through the annular space  118  defined between tubular member  115  and the generally cylindrical body  120  mounted coaxially within said member. Cylindrical body  120  consists of a central tube  122  within which is received an oil gun  124  terminating in a nozzle  126 . Oil is provided to gun  124  by port  125 . A combustible gas feed may alternatively or additionally be used. More generally, cylindrical body  120  functions as a bluff body and may take the form of or include an oil gun (which gives the burner both gas and oil firing capabilities); or can simply be an open or closed end tube in the case of a gas only burner. Tube  122  in turn is surrounded by a spaced tubular member  130 . The spacing between tube  130  and tube  122  defines an annular space  132  a possible function of which is discussed in my earlier patent. Tube  130  is, in turn, surrounded toward its forward end by a further cylinder  134 , which is closed at each end and defines within same an annular fuel receiving manifold  136 . Fuel for manifold  136  is fed via a connector  138 . The gas injector means in the device  110  comprises a series of prism-like hollow members  139  which are mounted transversely to cylinder  134  and intersect and are open to the interior manifold space  136  within same. The members  139  are provided on their lateral faces with openings  140 . The members  139  are directly in contact with and contiguous with the leading edges of swirl vanes  142 , so that the gas is injected directly at such leading edge. As discussed previously, a diverging quarl  144  is provided at the outlet end of burner throat  118 . 
         [0007]    As is known in the art, an igniter is only used for start-up. Once the main flame is established, the igniter is removed and the flame is self-stabilizing. The ignition point is determined by the temperature and mass flow rate of the internal recirculation gases, which in turn is determined by the burner geometry and the amount of external FGR that is used. 
         [0008]    Pursuant to another aspect of my U.S. Pat. No. 5,407,347, a method is provided for injection of gaseous fuel in a forced draft burner of the type which includes an annular throat of outer diameter B, having an inlet connected to receive a forced flow of air and recirculated flue gases, and an outlet adjoined to a divergent quarl. The gaseous fuel is injected at an axial coordinate which is spaced less than B in the upstream direction from the axial coordinate at which the quarl divergence begins; and sufficient mixing of the gaseous fuel with the air and recirculated flue gases is provided that these components are well-mixed down to a molecular scale at the axial coordinate of ignition. This procedure results in extremely low NOx, CO, and hydrocarbon emissions from the burner. 
         [0009]    In a further aspect of my U.S. Pat. No. 5,407,347 invention, the swirl vanes, which are mounted with their leading edges parallel to the axial flow of fuel and oxidant gases, and then slowly curve to the final desired angle, have a constant radius of curvature along the curved portion of the vane, whereby the curved portion is a section of a cylinder. This shape simplifies manufacturing using conventional metal fabricating techniques. 
         [0010]    Also to be noted is that in accordance with the invention in my further U.S. Pat. No. 5,470,224 (the disclosure of which is also incorporated herein by reference), a two stage rapid mix burner design (herein referred to at times as the “D-RMB”) provides apparatus and method which both significantly reduces the burner size of a rapid mix burner, and/or the burner pressure drop, while maintaining the rapid mix feature and stability of the basic rapid mix design. The two stage rapid mix burner design can also be easily altered to fit in non-circular geometries, such as a corner or tangential fired boiler. 
         [0011]    The D-RMB burner uses a circular basic rapid mix burner located internally inside a larger burner which can be non-circular. The inner burner provides the flow of hot gases which stabilizes the outer burner. In effect, the combustion gases produced in the inner burner replace the strong internal recirculation flow generated by the basic RMB as an ignition source for the outer burner flow. The inner burner uses the same type swirler, burner and quarl geometry as the basic RMB burner described in my previous applications and consequently has the desired stability and NO x , CO and HC performance. The outer portion of the burner uses a rapid mix injection grid and consequently also has the desired NO x , CO and HC performance. Since the flame stability is provided by the inner burner, swirl vanes or a divergent quarl for the outer portion of the burner are not required. It may also be noted that in FIGS. 11 and 12 of the U.S. Pat. No. 5,470,224 patent a D-RMB is shown having a rectangular outer burner portion. This geometry corresponds to corner (or tangentially fired boilers), which make up a significant fraction of the large industrial and utility boiler market. FIG. 13 also shows a view of 6 burners, as they would appear in one corner of a typical corner fired boiler application. The inner burner is conceptually the same as the annular two stage burner described e.g. in FIGS. 7 through 10 of the patent. The quarl of the inner burner has the same outside diameter as the smaller dimension of the rectangular boundary comprising the outer burner. 
         [0012]    These two versions of the rapid mix burners (RMB and D-RMB) have been in commercial use for a number of years, with approximately 200 burners installed thus far, and have been meeting NO x  emissions of 0.011 lb/MMBtu (9 ppm @ 3% O 2 ) and CO emissions of less than 0.04 lb/MMBtu (50 ppm @ 3% O 2 ). However, in certain regions of the country, including California&#39;s Central Valley, some boilers and furnaces are required to meet NO x  emissions less than 0.007 lb/MMBtu (6 ppm @ 3% O 2 , while maintaining CO emissions of 0.04 lb/MMBtu, or lower). The existing versions of the RMB and D-RMB tend to generate high CO emissions (well above 0.04 lb/MMBtu) as the NO x  level approaches 0.007 lb/MMBtu, and in some cases the flames will become unstable. The CO emissions are typically created at the outer edges of the flame, as relatively cold furnace gases are entrained into the flame quenching combustion reactions leaving unburned combustion products at the furnace exit. As more and more of the combustion reactions are quenched, the flame eventually no longer generates enough heat to self stabilize. 
       SUMMARY OF INVENTION 
       [0013]    Now in accordance with the present invention a burner system is disclosed that utilizes a plurality of single stage Rapid Mix Burner cells which are organized into one or more self-stabilizing matrices. Any number of RMB cells, two or more, may be organized into such a matrix. The assemblage combines the advantageous operating characteristics of nozzle mix and premixed type burners and achieves extremely low NO x , CO, and hydrocarbon emissions. This invention is referred to herein as a Cell Rapid Mix Burner or “C-RMB”. The C-RMB is so effective that it may be operated at NO x  levels below 0.007 lb/MMBtu while maintaining a stable flame with CO emission below 0.04 lb/MMBtu. The C-RMB concept is applicable to any package or field erected boiler, so long as the front wall meets certain dimensional requirements and the front wall is refractory. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]    The invention is diagrammatically illustrated, by way of example, in the drawings appended hereto, in which: 
           [0015]      FIG. 1  is a prior art showing (FIG. 17 of my aforementioned U.S. Pat. No. 5,407,347 patent), and illustrates a typical prior art single stage RMB which may be utilized in the present invention: 
           [0016]      FIG. 2  is a longitudinal cross-sectional view, schematic in nature, of a two element C-RMB in accordance with the invention; 
           [0017]      FIG. 3  is a front schematic end view of the apparatus of  FIG. 2 ; 
           [0018]      FIG. 4  is a further schematic longitudinal cross-sectional view, of a four element C-RMB in accordance with the invention; and 
           [0019]      FIG. 5  is a front schematic end view of the apparatus of  FIG. 4 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0020]    Typical preferred embodiments of the invention are shown in  FIGS. 2 ,  3 ,  4 , and  5 . The single stage RMBs used in the two cell arrangement of  FIGS. 2  and  3 , and in the four cell arrangement of  FIGS. 4 and 5 , are substantially those of  FIG. 1 , and accordingly corresponding components to the prior art showing of  FIG. 1  are identified by the same reference numerals. 
         [0021]    The invention is used in combination with a boiler or furnace having a refractory bound wall opening for receiving one or more burners. The burner system  102  ( FIGS. 2 and 3 ) is mounted at the wall opening  108  of front wall refractory  112  and includes a plurality of rapid mix burner cells (here two in number) which are configured into a self-stabilizing matrix of side-by-side cells. Air and recirculated flue gases are provided under pressure by conventional fan means (not shown) via a conduit  90  to a burner windbox  92  from which as shown by arrows  94  they proceed into annular space or throat  118  as in  FIG. 1 . As in  FIG. 1 , tube  130  is, in turn, surrounded toward its forward end by a further cylinder  134 , which is closed at each end and defines within same an annular fuel receiving manifold  136 . Fuel for manifold  136  is fed via an inlet  138 . The gas injector means in the cells  104  and  106  again comprises a series of hollow members  139  which are mounted transversely to cylinder  134  and intersect and are open to the interior manifold space  136  within same. The members  139  are provided with openings  140 . The members  139  are directly in contact with and contiguous with the leading edges of swirl vanes  142 , so that the gas is injected directly at such leading edge. The central tube  122  in each burner cell is provided with combustible gas via an inlet  125  and is open at its forward end to provide center firing for the burner. The individual burner cells  104  and  106  each terminate in a divergent refractory quarl  143  and  144 , and are positioned in the matrix so that the centerlines of adjacent cells are no more than 1.6 burner throat diameters apart (dimension S) both vertically and horizontally. A common refractory section  110  is formed into the front wall  112  of the burner system and borders the matrix. The flame from each cell exits into the common refractory section, promoting flame-to-flame self stabilization and increasing the flame  114  residence time in an environment in which the entrainment of cold furnace gases into the flame is limited. The total axial spacing resulting from the refractory quarl at the distal end of each burner cell, plus the said common refractory section, should be at least 20″ before the flame is exposed to externally recirculated furnace gases. Preferably as well each burner cell includes means for swirling the air flow from each cell in the same direction. The rotating burner flow brings combustion products from the outer portion of the flame, that may have mixed with relatively cold furnace gases, into the reacting zone between the flames from each burner cell, thereby causing the gases rotated between the flames to be reheated and further oxidation of unburned fuel to occur. 
         [0022]    Reiterating, the individual burner cells that make up a matrix are thus placed so that the burner centerlines are no more than 1.6 burner throat diameters apart both vertically and horizontally (S/D&lt;1.6 in  FIG. 2 ). The flame from each C-RMB cell exits into a common refractory section formed into the front wall, promoting flame-to-flame self stabilization and increasing the flame residence time in an environment in which the entrainment of cold furnace gases into the flame is limited. And as indicated, the total axial extending length of each burner cell refractory plus the common refractory section provides a total refractory length of at least 20″ before the flame is exposed to externally recirculated furnace gases (dimension L in  FIG. 2 ). 
         [0023]    The air flow from each burner cell is swirled in the same direction. The rotating burner flow brings combustion products from the outer portion of the flame, that may have mixed with relatively cold furnace gases, into the reacting zone between the flames from each burner cell. This causes the gases rotated between the flames to be reheated and further oxidation of unburned fuel to occur. The swirl direction can be either clockwise or counterclockwise but is the same for all burner cells. 
         [0024]    Each cell of the C-RMB system is thus seen to consist of a single stage RMB having a structure in accordance with my U.S. Pat. No. 5,407,347, typically with center fire gas and with the burner refractory angle and dimensions modified from the standard single stage RMB design criteria to meet the S/D&lt;1.6 and L&gt;20″C-RMB requirements. 
         [0025]    The burner systems  202  shown in  FIGS. 4 and 5  is based on four single stage cell RMBs, arranged in a rectangle or square, where each of the individual cell burners  204 ,  206 ,  208 , and  210  correspond to the burners in  FIGS. 2 and 3  and operate in similar fashion. 
         [0026]    The C-RMB system of the invention is further illustrated and its unexpected advantages shown by the following Example: 
       Example 
       [0027]    A C-RMB system in accordance with the invention was tested in a 30,000 lb/hr water tube boiler.  FIGS. 2 and 3  schematically illustrate the C-RMB used for the testing. The C-RMB system thus consisted of a two cell RMB matrix burner, similar to the C-RMB arrangement shown in  FIGS. 2 and 3 . Data from the testing is shown in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Two cell C-RMB testing in a 30,000 lb/hr Boiler 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 FUEL FLOW 
                 9 
                 13.6 
                 18.4 
                 23.4 
                 27.8 
               
               
                 KSCFH 
               
               
                 % FGR 
                 39.8 
                 43.4 
                 43.4 
                 42.2 
                 40.8 
               
               
                 STACK O 2  % 
                 4.7 
                 4.7 
                 4.7 
                 4.4 
                 4 
               
               
                 STACK NOX 
                 4 
                 4 
                 4 
                 4 
                 4 
               
               
                 PPM @ 3% O 2   
               
               
                 STACK CO 
                 30 
                 30 
                 38 
                 30 
                 25 
               
               
                 PPM @ 3% O 2   
               
               
                 NO X   
                 .0054 
                 .0054 
                 0.0054 
                 0.0053 
                 0.0052 
               
               
                 LB/MMBTU 
               
               
                 CO LB/MMBTU 
                 .0264 
                 .0264 
                 .0336 
                 0.026 
                 0.02 
               
               
                   
               
             
          
         
       
     
         [0028]    The data shows NO x  emissions well below the target value of 0.007 lb/MMBtu can be achieved with CO emissions below the target value of 0.04 lb/MMBtu, using the C-RMB concept. 
         [0029]    While the present invention has been particular set forth in terms of specific embodiments thereof, it will be understood in view of the present disclosure, that numerous variations on the invention are now enabled to those skilled in the art, which variations yet reside within the scope of the present teaching. Accordingly, the invention is to be broadly construed and limited only by the scope and spirit of the disclosure and of the claims now appended hereto.