Abstract:
An apparatus is provided including a furnace structure defining a reaction chamber in which oxidant and fuel are reacted to form combustion products and having an anchor surface having an opening through which the oxidant and fuel are introduced into the reaction chamber. The reaction chamber is configured to recirculate the combustion products back toward the anchor surface. The apparatus also includes an anchor outlet configured to create a layer of combustion products directed along the anchor surface under the influence of the recirculated combustion products. The apparatus further includes a primary outlet configured to direct primary fuel to flow into the reaction chamber through the opening and the layer of combustion products so as to ignite the primary fuel.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is directed to the field of premix combustion systems, particularly those of the type with reduced emissions.  
         BACKGROUND  
         [0002]    Premix is obtained through the mixing of fuel and oxidant prior to introducing the premix to the source of ignition stabilization. As the fuel and oxidant content of premix approaches uniformity, i.e. homogeneity, the rate of combustion is primarily determined by the reaction rate of the premix. Premix combustion results in a short flame with rapid energy release, permitting smaller combustion chambers and/or increased burner capacity compared to diffusion combustion.  
           [0003]    Premix combustion differs from diffusion combustion in that in diffusion combustion fuel and oxidant are separate until they mix at the flame front. In diffusion combustion, the fuel and oxidant content is less uniform, i.e. less homogeneous, than in premix combustion. In diffusion combustion, the rate of combustion is primarily determined by the mixing rate of the fuel and the oxidant. Because mixing occurs on a time scale that is approximately 10 4  times longer than reaction rates, diffusion combustion results in a flame that is longer than a premix combustion flame.  
           [0004]    “Lean” premix indicates a fuel/oxidant mixture containing more oxidant than the amount required to completely combust the fuel. This can be compared to a “rich” premix that indicates a fuel/oxidant mixture containing less oxidant than what is required to completely combust the fuel. A “lean” premix can be indicated mathematically by stating the mixture has an equivalence ratio of less than one. The equivalence ratio (Φ)) is a normalized way of measuring the proportion of an actual fuel and oxidant ratio compared to the fuel and oxidant ratio required for stoichiometric combustion. This is expressed as:  
       Φ   =         (       Q   Fuel     /     Q   Oxidant       )        Actual         (       Q   Fuel     /     Q   Oxidant       )                   Stoiciometric                             
 
           [0005]    where Q represents an amount, or flow rate, of fuel or oxidant.  
           [0006]    It is sometimes desirable to control the emissions of nitrogen oxide compounds (NO x ) and carbon monoxide (CO). It has been observed that lean premix having an equivalence ratio of 0.55&lt;Φ&lt;0.65 (when using natural gas as fuel and air as oxidant) produces very low levels of NO x  and CO. As Φ approaches 1.0, NO x  increases as flame temperature increases. At stoichiometric combustion (Φ=1.0) fuel and air are in optimum proportion, providing maximum peak flame temperatures. As such, the short, intense premix flame can be hot enough to causing nitrogen and oxygen to react, creating NO x .  
           [0007]    While it is sometimes desirable to lower the equivalence ratio to reduce NO x , this can increase CO production. However, as a flame temperature drops in response to a lower equivalence ratio, the flame can become less stable and the flame approaches the lower stability limit. An unstable flame can result in areas of incomplete combustion within the flame, which can produce elevated levels of CO. If the equivalence ratio is lowered further, a lower flammable limit (LFL) is reached, where the heat of combustion is absorbed by the thermal ballast and the combustion reaction cannot be sustained.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides an apparatus including a furnace structure in which premix is reacted to form combustion products including an anchor surface having openings through which the premix is introduced into the furnace structure, an array of at least three premix jets, each configured to introduce a corresponding flow of premix into the furnace structure through a corresponding one of the openings, and a supplementary outlet configured to provide anchor combustion products that ignite each corresponding flow of premix introduced into the furnace structure near the openings. Additionally, the present invention provides that the supplementary outlet is further configured to provide anchor combustion products such that each flow of premix is ignited and the ignition distance from the point of ignition of each flow of premix is equidistant from each corresponding one of the openings.  
           [0009]    The present invention also provides an apparatus including a furnace structure defining a reaction chamber in which oxidant and fuel are reacted to form combustion products and having an anchor surface having an opening through which the oxidant and fuel are introduced into the reaction chamber. The reaction chamber is configured to recirculate the combustion products back toward the anchor surface. The apparatus also includes an anchor outlet configured to create a layer of combustion products directed along the anchor surface under the influence of the recirculated combustion products. The apparatus further includes a primary outlet configured to direct primary fuel to flow into the reaction chamber through the opening and the layer of combustion products so as to ignite the primary fuel.  
           [0010]    The present invention also provides a method including providing a furnace structure defining a reaction chamber having an anchor surface with an opening through which a primary outlet introduces oxidant and fuel into the reaction chamber. The reaction chamber is configured to recirculate combustion products back toward the anchor surface. The method also includes directing anchor fuel to flow from an anchor fuel inlet into the reaction chamber to create an anchor layer that is directed along the anchor surface by the recirculated combustion products. The method further includes directing the primary fuel to flow from the primary outlet into the reaction chamber through the anchor layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic view of an apparatus comprising a first embodiment of the invention.  
         [0012]    [0012]FIG. 2 is a schematic view of an apparatus comprising a second embodiment of the invention.  
         [0013]    [0013]FIG. 3 is a schematic view of the apparatus shown in FIG. 2 along the line  4 - 4 .  
         [0014]    [0014]FIG. 4 is an enlarged schematic view of part of the apparatus shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0015]    An apparatus  10  comprising a first embodiment of the invention is shown in FIG. 1. The apparatus  10  is a reduced NO x  emission burner for process heating.  
         [0016]    The apparatus  10  includes a furnace structure  12  that defines a reaction chamber  14 . The reaction chamber  14  is generally cylindrical and is centered on an axis  15 , and is tapered radially inward, i.e. narrows, as it approaches an exit  16  from the reaction chamber  14 . The tapering is the result of a choke configuration  18  of the furnace structure  12  centered on the axis  15  and disposed from the rear to the front of the reaction chamber  14 . The reaction chamber  14  is configured so that combustion of fuel and oxidant can occur inside the reaction chamber  14 . An anchor surface  20  is located at the rear end of the reaction chamber  14 , opposite the exit  16 . The anchor surface  20  faces forward into the reaction chamber  14  towards the exit  16 .  
         [0017]    An anchor outlet  22  is located on the periphery of the anchor surface  20 . The anchor outlet  22  directs anchor fuel from an anchor fuel structure  24  into the reaction chamber  14 . The anchor fuel structure  24  is an inwardly oriented combination premix/diffusion type burner. An anchor fuel line  26  supplies anchor fuel to the anchor fuel structure  24 .  
         [0018]    A mixing structure  60  is located opposite the exit  16 . The mixing structure  60  has a primary outlet  62  that communicates the mixing structure  60  with the reaction chamber  14 . The primary outlet  62  directs primary fuel to flow from the mixing structure  60  into the reaction chamber  14  through an opening  64  in the anchor surface  20 . A primary fuel line  66  supplies primary fuel to the mixing structure  60 . An oxidant line  68  supplies oxidant to the mixing structure  60 . The mixing structure  60  is configured to premix primary fuel with the oxidant supplied by the oxidant line  68 .  
         [0019]    During operation of the apparatus  10 , primary fuel and oxidant are each supplied to the mixing structure  60  via the primary fuel line  66  and the oxidant line  68 , respectively. The primary fuel and oxidant are mixed in the mixing structure  60  prior to entering the reaction chamber  14  so as to form a premix. The premix is introduced to the reaction chamber  14  through the primary outlet  62 . The premix is directed to flow into the reaction chamber  14  through the opening  64  in the anchor surface  20 .  
         [0020]    An igniter, not shown, initiates combustion of the premix in the reaction chamber  14 . The igniter may comprise any suitable device known in the art. The combustion products of the premix are recirculated by the configuration of the reaction chamber  14 . More specifically, the tapered choke configuration  18  encourages the recirculation of some of the combustion products. Some of the combustion products recirculate in the reaction chamber  14  and some of the combustion products leave the reaction chamber through the exit  16 , as shown by the flow paths  69 .  
         [0021]    Coinciding with the supply of premix to the reaction chamber  14 , anchor fuel is supplied to the anchor fuel structure  24  via the anchor fuel line  26 . The anchor fuel structure  24  delivers anchor fuel to the reaction chamber  14  through the anchor outlet  22 . The anchor outlet  22  directs the anchor fuel to flow into the reaction chamber  14  across the anchor surface  20 .  
         [0022]    The anchor fuel is also ignited in the reaction chamber  14 . As the recirculating combustion products move toward the rear wall of the reaction chamber  14 , i.e., the anchor surface  20 , they impinge on the anchor fuel in the reaction chamber  14  and anchor it to the anchor surface  20 . Thus, the anchor layer  70  is anchored to the anchor surface  20  under the influence of the recirculating combustion products. In this embodiment, the anchor layer  70  is a radial inward-directed, transverse combustion flow extending from the periphery of the anchor surface  20  over the opening  64 , and thus also over the primary outlet  62 .  
         [0023]    As noted above, the primary outlet  62  is oriented such that primary fuel exiting the primary outlet  62  passes through the opening  64  in the anchor surface  20 . Because the anchor layer  70  extends over the primary outlet  62 , the primary fuel also passes through the anchor layer  70  upon entering the reaction chamber  14 .  
         [0024]    The anchor layer  70  of combustion products imparts thermal energy to the premix as the premix flows through the anchor layer  70 . That is, as the premix flows through the anchor layer  70 , the combustion products from the anchor layer  70 , being the first gases the premix flow encounters, are the first gases to be entrained into the flow of primary fuel. As the premix is directed to flow through the anchor layer  70  the ignition distance of the premix is shortened and is anchored to the primary outlet  62 . The ignition distance is the distance from the opening to a point where a substantial portion of the premix has begun to chemically react. This ignition anchoring supplies combustion products adjacent to the primary outlet  60 . The combustion products can contain radical species and heated gases.  
         [0025]    Anchoring the combustion products to the anchor surface  20  in the anchor layer  70  near the primary outlet  60  allows rapid premix ignition. This can be compared to delayed premix ignition that occurs when combustion products are not anchored to the anchor surface  20 . Thus, ignition anchoring can provide more rapid completion of combustion to minimize CO production. Also, ignition anchoring can reduce the lower stability limit so that it is closer to the lower flammability limit (LFL). A decreased lower stability limit can result in a decreased equivalence ratio. Ignition anchoring can also decrease the level of cold, non-combusted premix entering the reaction chamber  14 .  
         [0026]    An apparatus  400  comprising a second embodiment of the invention is shown in FIG. 2. The apparatus  400  has many parts that are substantially the same as corresponding parts of the apparatus  10 . This is indicated by the use of the same reference numbers for such corresponding parts in FIGS. 1 and 2. However, the second embodiment of the invention includes an anchor outlet  402 . The anchor outlet  402  differs from the embodiment of FIG. 1 in that it is centered on the axis  15  and directs anchor fuel to flow radially outward across the anchor surface  20 . Additionally, a mixing structure  404  in an anchor structure  410  is included.  
         [0027]    The anchor outlet  402  communicates the anchor structure  410  with the reaction chamber  14  through an opening  424  in the anchor surface  20  and is angled, widening from the opening  424  toward the primary fuel outlets  62 . The anchor structure  410  is a radial flame burner. An anchor fuel line  422  supplies the anchor structure  410  with anchor fuel. The mixer structure  404  is a spin plate that induces a swirl. In an alternate embodiment, the anchor outlet  402  is not angled outward, instead, the anchor outlet  402  forms a widening curve from the opening  424  toward the primary fuel outlets  62 . The widening curve is configured to direct the anchor fuel towards the primary fuel outlets  62 .  
         [0028]    The mixing structure  60  communicates with the reaction chamber  14  through the primary outlet  62  that is coextensive with the opening  64  in the anchor surface  20 . The primary outlet  60  is one of an array of three primary outlets, two of which are shown in FIG. 2. A view along line  4 - 4  shows the arrangement of the three primary outlets in the array in relation to the anchor outlet  402  in FIG. 3. Primary outlets are located in a circular array extending from the anchor outlet  402  and spaced from the axis  15 . The array is arranged so that each primary outlet  60  is equidistant, or substantially so, from the anchor outlet  402 .  
         [0029]    During operation, primary fuel is supplied to the mixing structure  60  from the primary fuel line  66  and oxidant is supplied to the mixing structure  60  from the oxidant line  68 . The mixing structure  60  mixes the primary fuel and oxidant to form premix. Anchor fuel is supplied to the anchor fuel structure  410  through the anchor fuel line  422 .  
         [0030]    The primary outlet  62  directs premix to flow into the reaction chamber  14  through the opening  64 . Simultaneously, the anchor outlet  402  directs anchor fuel to flow into the reaction chamber  14  through the opening  424 . In this embodiment, the amount of anchor fuel can be as little as 2% to 5% of the total fuel that is supplied to the reaction chamber  14 . Total fuel is the combination of fuel supplied to the reaction chamber in the form of anchor fuel and premix. The mixer structure  404  spins the anchor fuel causing the anchor fuel to spread out, generally in a disk shape overlaying the anchor surface  20 .  
         [0031]    An igniter, not shown, initiates combustion of the fuel in the reaction chamber  14 . The igniter may comprise any suitable device known in the art. The combustion products are recirculated by the configuration of the reaction chamber  14 . More specifically, the tapered choke configuration  18  recirculates some of the combustion products, as shown by the flow paths  69 . Some of the combustion products recirculate in the reaction chamber  14  and some of the combustion products leave the reaction chamber through the exit  16 .  
         [0032]    The configuration of the anchor outlet  402  creates a stabilizing recirculating flow pattern during operation of the apparatus  400 . Additionally, the angled anchor surface  20  around the anchor outlet  402  also directs the combustion products from the anchor outlet  402  toward the primary outlet  62 . The recirculating flow pattern created by the anchor outlet  402  can complement or supplant the recirculation pattern flow paths  69  in the reaction chamber  14 . Similar to the recirculating combustion products, the stabilizing recirculating flow pattern in this embodiment also influences the combustion products of the anchor fuel to anchor the combustion of premix entering the reaction chamber  14  to the primary outlet  62 . That is, the anchor combustion products are directed toward the primary outlet  62 .  
         [0033]    The anchor fuel combustion flow spreads, adjacent to the anchor surface  20 , radially outward from the anchor outlet  402  in a disk shaped thin layer. The anchor layer  470  overlays the anchor surface  20  and extends to each primary outlet  62 . The anchor structure  410  can also act as a pilot for the premix entering the reaction chamber  14 .  
         [0034]    Because the anchor fuel combustion flow extends to the primary outlet  62 , the primary fuel passes through the anchor fuel combustion products upon entering the reaction chamber  14 . The anchor layer  470  of combustion products imparts thermal energy to the premix. That is, as the premix flows through the anchor layer  470 , the combustion products, being the first gases the premix flow encounters, are the first gases to be entrained into the flow of premix. This results in the ignition distance of the premix being shortened and anchored to the primary outlet  62 .  
         [0035]    [0035]FIG. 4 is an enlarged partial view of the anchor fuel structure  410  of the second embodiment, shown in FIG. 2. The anchor structure  410  shown in FIG. 4 injects an amount of supplemental fuel from a diffusion outlet  450  into the reaction chamber  14 . The injected supplementary fuel depresses the lower stability limit of the oxidant/fuel mixture in the reaction chamber  14  so that it approaches the lower flammable limit (LFL).  
         [0036]    As will be appreciated, the invention is capable of other and different embodiments and its several embodiments are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive.