Abstract:
A damper box for an orifice air injector, the damper box comprising front and rear faces with respective front and rear openings therein, a pair of sides, a top and a bottom; and a pair of gates pivotally mounted within the damper box and actuatable between open and closed positions.

Description:
[0001]    This invention relates to fossil fuel boilers and more specifically to an improved overfire air injector for fossil fuel fired boilers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Overfire air (OFA) injection is a common technique for reducing NOx emissions from fossil fuel fired boilers equipped with reburn systems. An OFA system typically consists of overfire air injectors installed on the boiler walls; ductwork to route combustion air from the air supply to the injectors; and controls for modulating the overfire air flow rate. In many areas of the country NOx emissions control is a seasonal requirement, so that equipment must be designed with the understanding that it will be out of service for prolonged periods of time. For example, in a typical OFA injector, combustion air must be admitted to the injector when it is out of service in order to maintain the temperature of the injector components below the point at which they will be damaged by exposure to the radiant heat of the furnace. The cooling air flow results in operation of the burners at reduced stoichiometric ratios, and can lead to increased carbon loss and to furnace tubewall corrosion. The increased carbon loss and increased tubewall corrosion lead to increased operating costs and a significant loss of revenue.  
           [0003]    The current solution to reduce the cooling air requirements is to design a water-cooled throat that provides conductive cooling to the OFA injector. This solution can reduce the cooling air flow as compared to a non-water-cooled throat design, but still results in OFA cooling flow rates that are in the range of 5-10% of the total combustion air.  
           [0004]    There remains a need for a more effective way to protect OFA injectors with reduced use of combustion air as cooling air.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0005]    This invention seeks to reduce the cooling air flow to below 5% when the OFA system is out of operation by shielding the OFA injector components from the radiant heat of the furnace. The OFA injector in accordance with an exemplary embodiment of the invention continues to utilize a water-cooled throat, but now includes a housing or damper box on the front end of the injector with actuated gates or doors that may be closed when desired to shield the injector hardware from the high temperature environment of the furnace. The OFA injector may have dual passages to extend the range of operation of the injector, but for some applications, only one passage may be required.  
           [0006]    During normal operation, and when the OFA system is operating, the damper box doors are open. When the OFA system is not in operation, automatic actuators are used to close the doors and thereby shield the OFA injector. It is within the scope of the invention, however, to employ manual actuation if desired. The doors and interior surfaces of the damper box are also covered with refractory or other insulating material to provide additional protection from the high furnace gas temperatures.  
           [0007]    Accordingly, in its broader aspects, the invention relates to a damper box for an orifice air injector, the damper box comprising front and rear faces with respective front and rear openings therein, a pair of sides, a top and a bottom; and a pair of gates pivotally mounted within the damper box and actuatable between open and closed positions.  
           [0008]    In another aspect, the invention relates to a housing for an overfire air injector comprising a rearward portion adapted for connection to a supply duct and a forward portion having an attachment flange; a damper box secured to the attachment flange, the damper box having front and rear faces with respective front and rear openings therein, a pair of sides, a top and a bottom; and a pair of gates pivotally mounted within the damper box adjacent the front opening and actuatable to move the gates between open and closed positions.  
           [0009]    In still another aspect, the invention relates to a method of shielding an overfire air injector in a fossil fuel fired boiler from heat during periods when the overfire air injector is not in use comprising: a) adding a damper box to a front end of a housing enclosing the overfire air injector, the damper box having a front opening and at least one gate actuatable between open and closed positions; and b) closing the front opening by moving the at least one gate to the closed position when the overfire air injector is not in use.  
           [0010]    The invention will now be described in detail in connection with the drawing figures identified below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic depiction of a conventional fossil fuel fired boiler;  
         [0012]    [0012]FIG. 2 is a perspective view of an overfire air injector in accordance with an exemplary embodiment of the invention;  
         [0013]    [0013]FIG. 3 is a perspective view of a damper box for use with the overfire air injector of FIG. 2;  
         [0014]    [0014]FIG. 4 is a lower left perspective view of the overfire air injector shown in FIG. 3;  
         [0015]    [0015]FIG. 5 is a rear perspective view of the overfire air injector of FIG. 2;  
         [0016]    [0016]FIG. 6 is a section view taken along the line  6 - 6  of FIG. 5;  
         [0017]    [0017]FIG. 7 is a top plan view of the overfire air injector shown in FIG. 5;  
         [0018]    [0018]FIG. 8 is a section view of a grand plate surrounding a pivot shaft in the damper box in accordance with the invention;  
         [0019]    [0019]FIG. 9 is a section view taken along the line  9 - 9  of FIG. 6;  
         [0020]    [0020]FIG. 10 is a perspective view of a damper gate of the type shown in FIGS.  1 - 7 ;  
         [0021]    [0021]FIG. 11 is a plan view of the damper gate shown in FIG. 10;  
         [0022]    [0022]FIG. 12 is a side elevation of the damper gate shown in FIG. 10; and  
         [0023]    [0023]FIG. 13 is an end view of the damper gate shown in FIG. 12. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    [0024]FIG. 1 is a schematic depiction of a fossil fuel fired boiler  10  that includes a main combustion zone  12 , a reburning zone  14 , and a burnout zone  16 . The combustion zone  12  is equipped with a plurality of main burners  18  which are supplied with a main fuel, such as coal and air, through a fuel input  20  and an air input  22 , respectively. The main fuel is burned in burners  18  in the presence of air, to form a combustion flue gas  24  that flows in a downstream direction from combustion zone  12  to reburning zone  14 . In some arrangements, about 85% of the total heat input can be supplied by main burners  18 . The reburning fuel, such as natural gas, is injected through reburn fuel input  26  and provides the remaining heat input. Reburn fuel could also be any fossil fuel, i.e., coal, oil, orimulsion or propane gas. In burnout zone  16 , overfire air is injected through an OFA injector  28  to complete combustion, and the flue gas then passes through a series of heat exchangers  30  and out of the boiler via outlet  32 .  
         [0025]    [0025]FIG. 2 illustrates a new OFA injector  28  in more detail, useable in the conventional boiler  10 . The assembly includes an elbow duct  34  that feeds the overfire air into a rectangular spool assembly housing  36 . The housing  36  supports three aspirators  38 ,  40  and  42  on respective top and side walls of the housing. The internal injector hardware is not particularly relevant to this invention and, thus, no detailed description of that hardware is required. In addition, the upstream duct  34  as shown is exemplary only, and would have various cross-sectional shapes.  
         [0026]    The present invention relates to a novel damper box construction to be added to the front face of the rectangular OFA injector housing  36  for protecting the OFA injector hardware when not in use.  
         [0027]    With reference now to FIGS.  3 - 7 , an overfire air injector damper box  44  in accordance with this invention is shown. The damper box  44  includes a flat front face  46 , a flat rear face  48 , sides  50 ,  52 , top  54  and bottom  56 . The sides  50 ,  52 , top  54  and bottom  56  are reinforced by front-to-back webs  58 ,  60  and side-to-side webs  62 ,  64 . Thus, the damper box  44  is a generally square, hollow structure with a front opening  66  and a rear opening  68 . The rear opening  68  is surrounded by a rigid frame structure  70  utilized for mounting the damper box  44  to a similar frame  72  on the front face of the OFA injector  28  by means of holes  74  and bolts or other suitable fasteners.  
         [0028]    Apertures  74  in the frame structure  70  facilitate attachment of the damper box to a wall of the boiler  10 .  
         [0029]    Within the damper box  44  are a pair of doors or gates  76 ,  78  located adjacent the front opening  66  and arranged to swing between open and closed positions vis-a-vis the front opening  66 . Other gate arrangements may be utilized, including the use of a single gate or door where space permits. Since the doors are mirror images of each other, only one need be described in detail. With reference also to FIGS.  8 - 11 , gate  76  is mounted on a hinge shaft  80  that is rotatably supported within the damper box. Specifically, the lower end of shaft  80  is journalled for rotation in a lower gland plate  82  fastened to the underside of the gate bottom  56  via fasteners  84 . Gland plate  82  includes packing  86  that permits the shaft to rotate relative to the plate. Similarly, the upper end of shaft  80  is journalled for rotation in an upper gland plate  88  fastened to the top surface of gate top  54  via fasteners  90 . The gland plate  88  is similar to plate  82  and also includes packing (not shown).  
         [0030]    A pair of split, annular collars  94 ,  96  are located on the shaft  80  under the gate top  54  and above the gate bottom  56 , respectively. Collar  96  is oversized and serves to isolate the shaft and lower gland plate  82  from dust. The lower collar  96  on the damper box bottom  56  is enclosed within a stainless steel cover  98 .  
         [0031]    As best seen in FIGS. 10 and 11, a gate hinge handle shaft  100  is telescopingly received within the shaft  80  with a transverse pin  102  located within a slot  104  in the tubular shaft  100  to insure that shaft  80  will rotate with the handle shaft  100 . A bolt  106  passes through the shafts  80 ,  100  and is secured by nut  108 , just above collar  96  (within the cover  98 ) and serves to lock the shafts  100  to the shaft  80 .  
         [0032]    A gate hinge handle  110  is fastened to the lower end of handle shaft  100  via bolt  112  and nut  114 . It will be appreciated that the handle  110  (and similar handle on the door  78 ) may be operated manually or operatively connected to suitable hydraulic, electrical and/or mechanical controls for automatically moving the doors  76 ,  78  to open the doors.  
         [0033]    With reference also to FIGS.  11 - 14 , the door  76  is constructed of a first plate  116  and a transverse edge plate  118  welded at the hinge end of the door. A corner plate  120  includes mutually perpendicular sides  122 ,  124 , with side  124  welded to the edge plate  118  such that a portion of plate  116 , edge plate  118 , and sides  122 ,  124  of plate  120  surround three sides of the hinge shaft  80 , with plate  116  extending further across the interior face of the door. The back side  126  of plate  116  is reinforced by a rectangular configuration of horizontal stiffening ribs  128 ,  130 ,  132  and vertical stiffening ribs  134 ,  136 . A refractory block  138  is secured to the front side of plate  116  via refractory anchor clips  140 ,  142  and  144 . A second refractory block  146  is secured behind the corner plate  120  about the front of the hinge shaft  80 , and adjacent block  138 .  
         [0034]    Similar refractory blocks are applied to the interior of the damper box as best seen in FIGS. 4, 7 and  10 . Specifically, block  148  (FIG. 7) is applied to the underside of top  54  with the assistance of one or more refractory anchors  150 . Refractory block  152  is applied to the interior side of bottom  56  via one or more anchors  154 . The block  152  is cut out and beveled around the covers  98  as best seen in FIG. 4 to allow for door removal and installation without having to also remove the block  152 . The refractory “blocks” noted above are preferably molded directly onto their respective supporting surfaces but other suitable application techniques may be employed.  
         [0035]    Insulation board panels  156 ,  158  are applied to the interior surfaces of sides  50 ,  52 .  
         [0036]    Refractory blocks  138 ,  148  and  152  have a maximum service temperature of 3200° F., a density of 159 PCf @ 300° F., and a thermal conductivity of 11-43 (BTU-IN/HR—FT 2 ° F.). The refractory material is available under the trade name “Vesuvius Criterion 70 M.” Block  146  has as maximum service temperature of 2300° F., a density of 61 PCf @ 300° F., and a thermal conductivity of 2.4 (BTU-IN/HR—FT 2 ° F.) and is available under the trade name “Vesuvius Litewate 58.” Insulation board panels  156 ,  158  have a maximum service temperature of 2600° F. and a density of 25 PCf @ 3000° F. Other refractory block, insulation board and refractory material with similar insulating properties suitable for this application may be employed.  
         [0037]    By enabling effective heat shielding of the OFA injector hardware when not in use, less than 5% combustion air is required to maintain the injector components at an acceptable temperature.  
         [0038]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.