Abstract:
A method and improved furnace for reducing nitrogen oxide emissions from a furnace having a plurality of primary fuel injectors and a plurality of spaced apart over-fire air injectors positioned above the primary fuel injectors are disclosed. Injection of over-fire air produces zones of cooler combustion gasses containing over-fire air that separate zones of hot combustion gasses containing nitrogen oxides. Reburn fuel injectors inject a reburn fuel into the zones of hot combustion making the effluent combustion gases containing nitrogen oxides partially or totally fuel-rich in order to further reduce nitric oxide.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 62/197,731 filed Jul. 28, 2015. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to methods for reducing nitrogen oxide emissions from coal-fired, oil-fired and gas-fired furnaces. 
       BACKGROUND OF THE INVENTION 
       [0003]    Nitric oxide, formed in combustion of fuels in an enclosed furnace, may be reduced by reburning a second fuel in the effluent combustion flue gas. Conventional reburn practice in the 1980&#39;s was that a cross-section of the furnace be made fuel-rich by injection of a reburn fuel, followed by the injection and mixing of completion air, in the lowered temperature region of the upper furnace. U.S. Pat. No. 5,915,310, issued on Jun. 29, 1999, taught that the reburn fuel stream could be injected into the fuel lean effluent flue gas to form individual pockets or eddies of fuel-richness which would reduce nitric oxide before mixing and dissipating into the overall fuel lean flue gas, without the need for completion air. Both of these techniques require that completion of reburn combustion or dissipation of fuel-rich eddies be completed within the limited volume of the upper burn-out region of the furnace. 
         [0004]    Many furnaces have ports above the burners which inject air, called over-fire air, into the furnace. However, with the use of over-fire air as the prevalent method of nitric oxide reduction in operating furnaces, the lower primary combustion zone of the furnace is operated with lowered excess air (thus reducing nitric oxide formation in that high temperature region) and the staged over-fire air is injected into the upper furnace in order to complete combustion. The result is that with furnaces using over-fire air, the limited volume of the upper furnace is no longer available for the application of conventional or even Fuel Lean Reburn. 
         [0005]    Reburn technology involves injecting a fuel into previously combusted flue gas, after that gas has cooled, for the purpose of reducing unwanted pollution species, in particular nitric oxide (NOx). Natural gas is easily injected and combusted as the reburn fuel of choice at this lower temperature, but other fuels may be used as the reburn fuel. However, application of over-fire air technologies for NOx reduction seemingly has left no room for the application of reburn injection and combustion completion. 
         [0006]    Wherever the reburn zone can be made fuel-rich, any nitric oxide (NO) previously formed in the primary combustion, high temperature adiabatic zone, will act as a strong oxidizer upon mixing into fuel-richness to form C—N and/or H—N or even N 2  type species. Nitric oxide is reduced in this fuel-rich reburn zone but it may reform when the fuel-rich zone mixes with completion air in the upper furnace in order to complete combustion. The final nitric oxide emission and efficiency of this reburn teaching depends upon the relatively lower temperature of the upper furnace volume where this completion of combustion takes place. 
         [0007]    Application of In-furnace NOx reduction patented by Mitsubishi in 1981 and generally all reburn technologies have been limited by the fixed volume available and high furnace exit temperature in the upper furnace of practical boiler designs. If too much reburn fuel is injected at the lowered temperature of the upper furnace, for a given furnace volume, then the completion of combustion will not proceed and the concentration of CO and unburnt hydrocarbons will increase beyond acceptable emission and safety limits. Similarly if the furnace exit temperature is high because of limited furnace volume then the final nitric oxide will be higher upon combustion completion. 
         [0008]    The application of NOx Ports or over-fire air allow the lower furnace, primary flame adiabatic combustion zone to be operated with low excess air and even locally fuel-rich firing, thereby lowering or eliminating the formation of an oxidizer. The over-fire air is then injected into the upper furnace in order to complete combustion at a much lowered temperature, thereby reducing the kinetics of nitric oxide formation, which are very dependent upon both temperature and excess oxygen. 
         [0009]    The prevalent application of over-fire air technology is depicted in  FIGS. 1 and 2  as front and side views of a furnace,  FIG. 1 a   , without over-fire air and a furnace,  FIG. 1 b   , with over-fire air. Anywhere from one to multiple rows and columns of burners  4  may be configured on the front wall of the furnace and also often may be configured on the rear wall or corners of the furnace. The lower, nominally ⅓ region  6  of the furnace volume contains the burners  4  which inject a combustible air/fuel mixture and constitute the primary flame region of the furnace. The region is shaded in  FIG. 1 a    and  FIG. 1 b   . The addition of over-fire air ports  8  are depicted as squares directly above the columns of burners  4  in  FIG. 1   b.    
         [0010]    Normally the flames propagate from the lower burner region into the upper ⅔ of the furnace as the fuel burns-out to complete combustion, as depicted in  FIG. 2 a   . And considering normal design, this volume is predicated on the design of the boiler and the heat balance design of the power generation system. However, the application of over-fire air technology, for NOx reduction, requires all of the upper furnace to be used to complete the combustion, due to the lack of sufficient excess air in lower primary flame region of the furnace. The upper ⅔ and more critically the upper ⅓ of the furnace volume now is required for the complete mixing of the over-fire air to provide adequate excess air before furnace exit, as shown in  FIG. 2 b   . Therefore, there seemingly appears to be no volume available for the injection of reburn fuel and its own mixing and combustion. 
         [0011]    Consequently, there remains a need for a furnace design and method which reduces nitrogen oxide emissions from coal fired and gas fired furnaces. Preferably this new furnace design can be created by retrofitting or modifying existing furnaces. More preferably the new design and method should be able to be implemented without any major modifications of existing furnaces. 
       SUMMARY OF THE INVENTION 
       [0012]    We provide a method and improved furnace for reducing nitrogen oxide emissions from a furnace having primary fuel injectors and spaced apart over-fire air injectors positioned above the primary fuel injectors. Injection of over-fire air produces zones of cooler combustion gasses containing over-fire air that separate zones of hot combustion gasses containing nitrogen oxides. The improved furnace has reburn fuel injectors that inject a reburn fuel into the zones of hot combustion gases making the effluent combustion gases containing nitrogen oxides partially or totally fuel-rich in order to further reduce nitric oxide. 
         [0013]    Our process is such that the reburn fuel reacts, within a purposefully targeted fuel-rich zone, with the nitric oxide (NO) formed in the lower furnace. Thus reducing nitric oxide previously formed. Also, the higher hydrogen content of the reburn fuel, such as natural gas, reacting with nitric oxide in this, now fuel-rich zone, forms not only CO, H 2 O, and N 2 ; but also amine type N—H species or radicals which in turn enhance the nitric oxide reduction. At the same time, the higher hydrogen content of the reburn fuel improves the final burn-out of the carbon in the primary fuel. 
         [0014]    Our method of application involves identifying the zones of hot chimney flow of the high temperature flue gas around the colder and denser over-fire air injection columns. Then the method involves penetrating between existing boiler tubes of the furnace proper to inject the reburn fuel into the high temperature flue gas, which contains nitric oxide formed in the lower furnace. By our method we establish multiple isolated zones of low excess air or fuel-rich, off-stoichiometric combustion of the reburn fuel which reduces nitric oxide, enhances combustion of low hydrogen carbon fuels and mixes or dissipates into the currently operating over-fire air system. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]      FIG. 1 a    is a diagram of a wall in a conventional furnace which has no over-fire air ports. 
           [0016]      FIG. 1 b    is a diagram of a wall in a conventional furnace which has over-fire air ports. 
           [0017]      FIG. 2 a    is a side-view diagram showing the flow of combustion gases in a conventional furnace which has no over-fire air ports. 
           [0018]      FIG. 2 b    is a side-view diagram showing the flow of combustion gases in a conventional furnace which has over-fire air ports. 
           [0019]      FIG. 3  is a diagram showing zones in the flow of combustion gases that we have observed in a furnace which has over-fire air ports. 
           [0020]      FIG. 4  is a diagram similar to  FIG. 3  showing zones in the flow of combustion gases in a present preferred embodiment of our furnace. 
           [0021]      FIG. 5 a    is a diagram of a present preferred reburn fuel injector that can be used in the furnace shown in  FIG. 4 . 
           [0022]      FIG. 5 b    is a diagram of a portion of furnace wall having a slot in the webbing between boiler tubes, into which slot the reburn fuel injector may be placed. 
           [0023]      FIG. 6  is a diagram showing plume penetration of the reburn fuel into the combustion gas flowing through the furnace shown in  FIG. 4 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    It inherently takes the total volume of the upper furnace to fully mix the over-fire air with the combustion effluent from the lower furnace; other-wise, the lower ⅓-primary flame combustion zone might be made even more fuel-rich with increased over-fire air and improved NOx reductions. 
         [0025]    Considering the effect of high temperature upon increasing nitric oxide kinetics, over-fire air injection and mixing is generally accomplished within the colder, upper two-thirds of the furnace. This is accomplished by diverting a part of the normally used wind box combustion air. We have discovered that as the over-fire air mixing takes place, there develop upward flowing parallel zones of buoyant effluent combustion gases which are in the process of mixing but are not yet mixed. The wind box combustion air is preheated to between 500° F. and 900° F., but it is relatively cold in comparison to the combustion effluent which may be hotter than 3000° F. Thus, the injected over-fire air is two or three times more dense than the uprising combustion effluent flue gas. Because of this large density difference the hot and buoyant combustion effluent chimneys around the colder over-fire air and the over-fire air mixes very poorly. This parallel flow of over-fire air is depicted in  FIG. 3 , in an actual 460MW Utility Boiler application. 
         [0026]    These upward flow, buoyant columns of hot gases have been visually observed, empirically measured with a matrix of CO, O 2  and NO measurements and replicated through Computer Furnace Models. These columns of hot gases or spirals, in the case of tangential-fired furnaces, contain the nitric oxide formed in the lower primary combustion region of the furnace. 
         [0027]    We identify and target these parallel columns or spirals of combustion effluent, containing nitric oxide for injection of a reburn fuel. We call this injection Zonal Gas Reburn injection. Zonal Gas Reburn may also be called ZGR or Zonal Off-Stoichiometric Gas Reburn or ZOGR. 
         [0028]    In a present preferred embodiment shown in  FIGS. 4 and 5   b , multiple slot injectors  21  are inserted between the boiler tubes  22  of an actual 460 MW Utility Boiler  20 . These zonal gas reburn injectors are targeted to inject the reburn gas into the combustion effluent containing nitric oxide formed in the primary flame region  12  of the furnace. 
         [0029]    In the case of this 460MW Boiler shown in  FIG. 3 , the reburn fuel injectors  21  are placed in a region between the centerlines  14  of adjacent over-fire air injectors  8  and above those injectors. The reburn fuel injectors in the furnace shown in  FIG. 4  are 14 feet above the over-fire air injectors and over-fire air injectors are 13 feet above the top row of burners. In general the reburn fuel injectors may be placed above or below the over-fire air injectors but in position to mix with the trajectory of the effluent combustion gases, containing the nitric oxide to be reduced. This applies to all type of furnaces whether face-fired, opposed-fired as here, tangentially-fired, turbo-fired or even grate-fired; in every case with over-fire air, the targeted zone will exist before and as the over-fire air mixes. The resulting zonal reburn zone, regardless of furnace type, must extend to the coldest possible region of the upper furnace in order to take full advantage of lowered temperature nitric oxide equilibration, as this reburn zone mixes to complete combustion with the over-fire air. 
         [0030]    The gas reburn fuel injectors  21  may be any type orifice, round, square, oblong, etc., so long as it may fit through a penetration into the furnace or even through the air duct around the primary burners and they inject the gas reburn fuel into the targeted nitric oxide containing effluent. 
         [0031]    We prefer to provide multiple reburn fuel slot injectors to allow for a-priori targeting of the high NOx zones with multiple injectors. We prefer to provide reburn fuel injectors which fit between tubes  22  without requiring expensive tube modifications as shown in  FIGS. 5 a  and 5 b   . The reburn fuel injectors  21  may be horizontally adjustable as they fit through the slots  24  in the webbing  26  between the boiler tubes, as is the one installed as shown in  FIG. 5   b.    
         [0032]    Vertical tilt or up/down adjustment of the slot injector mechanisms shown in  FIG. 5 a    allows for empirical experimentation and adjustments for maximizing nitric oxide reduction and maintaining combustion completion with minimum CO emissions. 
         [0033]    Alternatively, the injectors could be positioned to penetrate the boiler through existing openings. Such openings could be openings for the primary burner air duct. 
         [0034]    The use of high hydrogen containing natural gas or other high hydrogen gas, such as coal or syn-gas improves the burn-out of lower hydrogen primary fuels such as coal or heavy oils. With natural gas reburn injection the moisture content of the effluent combustion gas goes from 6% in the primary furnace to nominally 12% in the Zonal Gas Reburn zone of our process. This increased hydrogen content greatly enhances carbon burn-out; and our adjustable and parallel flow reburn injection method allows the process to take place in the most favorable temperature region of the furnace which will allow for both combustion completion and lowered temperature nitric oxide equilibration. 
         [0035]    In the case of the 460MW boiler shown in  FIG. 3 , two rows of seven Zonal Gas Reburn injectors were installed on the front wall and two rows of seven Zonal Gas Reburn injectors were also installed on the rear wall to allow flexibility when operated at reduced load and with different combinations of coal mills in service. Also one row of four injectors were installed on the boiler sides to allow reduced load following and emission minimization. The measured penetration produced by the gas injected through these ZGR injectors was twenty-eight feet. This is shown proportionately in  FIG. 6  for this seventy-eight foot high furnace volume boiler. 
         [0036]    In the demonstration case shown in  FIG. 6 , the ZGR injector is pointed downward at 20 degrees and the ZGR injection plume mixes for 28 feet before losing its identity within the uprising combustion effluent. Each gas plume spread to 6 feet in diameter at 28 feet penetration and moved upward with the combustion gases at a velocity of 10 feet to 30 feet per half second. At 28 feet the plume eddies could still be seen and are fuel rich. Six feet of the plume had mixed with approximately 60% of the combustion gases such that the plume contained 9 percent fuel (0.6×15% excess air=9% fuel). Then the plume mixed out to form final CO. 
         [0037]    In our method a gaseous fuel can be injected into a furnace which already is using over-fire air for nitric oxide reduction and through our process of targeted Zonal Gas Reburn further nitric oxide reductions can be achieved. 
         [0038]    The gaseous fuel injection can be selectively targeted to zones of buoyant parallel or spiral flow combustion effluent, which chimney around the colder over-fire air flow, to form multiple reburn zones. Our targeted reburn zones rise with the buoyant effluent, which contains nitric oxide formed in the lower furnace, and our fuel-rich Zonal Gas Reburn reduces this nitric oxide. Also, the hydrogen content of the reburn fuel improves the carbon burn-out of solid or liquid primary fuel; while the targeted reburn zone dissipates in the upper furnace, with the over-fire air serving as reburn completion air. Our targeted Zonal Reburn Injection reduces both nitric oxide, CO and carbon emissions. 
         [0039]    Buoyant, vertical or spiral (encountered with tangential firing) zones of effluent, containing nitric oxide from the lower primary fuel combustion region of the furnace can be spatially identified and zonal gas reburn injection can be used to form upward flowing fuel-rich reburn zones which reduce the nitric oxide and then mix with the over-fire air to complete combustion. These zones can be identified by observation of a furnace in which temperatures of the combustion gases are measured at selected locations or by computer modeling. The computer modeling may include flow modeling of high velocity injectors designed to form nitric oxide reburn zones. 
         [0040]    Our process spatially targets multiple zones of a furnace to reduce nitric oxide emissions and at the same time improve combustion and burn-out. 
         [0041]    Our process is compatible with the implementation of over-fire air. Our Zonal Gas Reburn in fact uses the already implemented over-fire air as completion air to complete the combustion of the Zonal Reburn Fuel. 
         [0042]    Reburn gas or higher hydrogen reburn fuel can be injected through boiler wall penetrations or even through the primary burner air supply to target the primary combustion effluent and reduce nitric oxide. Such reburn fuel injectors are purposely targeted to cause the combustion effluent to become fuel-rich thereby reducing its nitric oxide concentration. These injectors can be designed to maximize the reduction of nitric oxide within the targeted effluent. Their design may involve any orifice suitable to provide the design penetration into the targeted nitric oxide containing effluent. The orifice can be round, square or oblong orientation and aspect to provide the design penetration into the targeted nitric oxide containing effluent. 
         [0043]    Slots can be installed in the webbing between boiler tubes, without modification of these tubes for using our method. Through use of these slots significant gas reburn fuel can be injected into targeted furnace effluent flow zones. And further these targeted zones of low excess air combustion effluent are thereby made fuel-rich, so that resulting off-stoichiometric combustion of the reburn gas will reduce nitric oxide in this effluent. 
         [0044]    Slots can be cut between tubes allow the insertion of practical injection nozzles for the purpose of controlled injection and mixing of reburn fuel into targeted effluent arising from the lower furnace. 
         [0045]    Practical injection nozzle assemblies can be fastened to the boiler wall so as to fit through the slots in the webbing. These injection nozzle assemblies allow vertical directional adjustment of injection flow, and the tolerance space around the inserted nozzles allows for their air cooling. 
         [0046]    Our reburn fuel injection assemblies may be installed to provide multiple Zonal Gas Reburn injection points without expensive high pressure boiler tube modifications. 
         [0047]    Multiple reburn fuel injection nozzles can have a high vertical aspect ratio of vertical height to horizontal width from 1.0 to 100 in the direction of the upward flowing effluent, thus providing more targeted gas flow in the upward direction of the effluent and less horizontal spreading into the over-fire air (where there is no nitric oxide). 
         [0048]    Slotted reburn fuel injectors can be positioned to target observed zones of primary combustion effluent, before the effluent mixes with over-fire air, for any type furnace such as, but not limited to: face-fired, opposed-fired, tangential-fired or turbo-fired furnace. 
         [0049]    Although we have shown and described certain present preferred embodiments of our furnace and method for reducing nitrogen oxide emissions it should be distinctly understood that our invention is not so limited and may be variously embodied within the scope of the following claims.