Abstract:
Described are a burner and its method of operation that are useful in providing heat within an enclosure such as the forehearth associated with a glassmelting furnace.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to heating molten glass in the forehearth of a glass furnace. 
       BACKGROUND OF THE INVENTION 
       [0002]    The production of articles of glass customarily involves feeding solid starting materials such as soda ash and silica, and/or recycled pieces of glass, into a glassmelting furnace in which the starting materials are melted to produce molten glass which flows out of the glassmelting furnace, through one or more passages known as forehearths, to apparatus in which the molten glass is formed into the desired products. 
         [0003]    It is desirable to provide heat to the molten glass flowing through the forehearth, in order to maintain the molten glass in a sufficiently flowable condition and to compensate for any heat losses at the surface of the molten glass so as to promote temperature uniformity throughout the molten glass. The present invention provides apparatus and methodology to do this, in a way that improves efficiency and avoids drawbacks such as losses of material or deterioration of the apparatus that is used to heat the molten glass. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    One aspect of the present invention is a method of heating molten glass adjacent to a side wall of a glass furnace forehearth in which the molten glass is flowing, or other high temperature stream, comprising 
         [0005]    into a burner that extends into the forehearth above the molten glass from a side wall of the forehearth, wherein the burner comprises 
         [0006]    an oxidant conduit having an inlet outside the forehearth and an outlet within the forehearth, and a fuel conduit having an inlet outside the forehearth and an outlet within the forehearth, wherein said outlets open in a direction which is upstream or downstream relative to the direction in which said molten glass is flowing, 
         [0007]    each conduit having a first segment extending from the side wall into the forehearth and a second segment extending from the end of the first segment to the conduit outlet, wherein each second segment is at an angle relative to the first segment, so that the axes of the conduit outlets form an angle relative to the side wall of 5 to 30 degrees projected in the horizontal plane and form an angle relative to the horizontal plane of 0 degrees to 5 degrees upward, 
         [0008]    refractory material encasing the conduits that protects the conduits from degradation by the forehearth conditions and protects gaseous fuel in the fuel conduit from thermal decomposition, and 
         [0009]    an opening in the refractory material aligned with the fuel conduit outlet and the oxidant conduit outlet, wherein the fuel conduit outlet and the oxidant conduit outlet are recessed relative to the outer surface of the opening in the refractory material, and the opening in the refractory material is straight or diverges outwardly from the oxidant conduit outlet to the outer surface of the refractory material, 
         [0010]    wherein the fuel and oxidant conduit outlets are recessed from the refractory opening a distance of 1 to 6 times the inner diameter of the oxidant conduit outlet, and 
         [0011]    wherein the distance from the axis of the fuel conduit at the fuel conduit outlet to the side wall from which the burner extends is 4 to 10 times the inner diameter of the outlet of the oxidant conduit, 
         [0012]    feeding fuel into the fuel conduit inlet at a rate so that the fuel emerges from the fuel conduit outlet at a velocity of 40 to 300 fps and feeding oxidant into the oxidant conduit inlet at a rate so that the oxidant emerges from the oxidant conduit outlet at a velocity of 3 to 100 fps, and the ratio of the oxidant velocity to the fuel velocity at said outlets is 1:1 to 1:20, 
         [0013]    and combusting oxidant and fuel that emerge from said outlets at the opening in the refractory material to produce a flame and heat of combustion in close proximity to the sidewall of a glass furnace forehearth. 
         [0014]    In a preferred embodiment, one of said conduits is within the other of said conduits. 
         [0015]    Another aspect of the invention is a method of heating molten glass adjacent to a side wall of a glass furnace forehearth in which the molten glass is flowing, or other high temperature stream, comprising 
         [0016]    into a burner that extends into the forehearth above the molten glass from a side wall of the forehearth, wherein the burner comprises 
         [0017]    first and second oxidant conduits each having an inlet outside the forehearth and an outlet within the forehearth, and first and second fuel conduits having an inlet outside the forehearth and an outlet within the forehearth, wherein the outlets of said first oxidant conduit and said first fuel conduit open in a selected direction which is upstream or downstream relative to the direction in which said molten glass is flowing and the outlets of said second oxidant conduit and said second fuel conduit open in a direction which is opposite to said selected direction, 
         [0018]    each conduit having a first segment extending from the side wall into the forehearth and a second segment extending from the end of the first segment to the conduit outlet, wherein each second segment is at an angle relative to the first segment, so that the axes of the conduit outlets form an angle relative to the side wall of 5 to 30 degrees projected in the horizontal plane and form an angle relative to the horizontal plane of 0 degrees to 5 degrees upward, 
         [0019]    refractory material encasing the conduits that protects the conduits from degradation by the forehearth conditions and protects gaseous fuel in the fuel conduit from thermal decomposition, and 
         [0020]    a first opening in the refractory material aligned with the first fuel conduit outlet and the first oxidant conduit outlet, wherein the first fuel conduit outlet and the first oxidant conduit outlet are recessed relative to the outer surface of the first opening in the refractory material, and a second opening in the refractory material aligned with the second fuel conduit outlet and the second oxidant conduit outlet, wherein the second fuel conduit outlet and the second oxidant conduit outlet are recessed relative to the outer surface of the second opening in the refractory material, and the first and second openings in the refractory material are straight or diverge outwardly from the respective oxidant conduit outlets to the outer surface of the refractory material, 
         [0021]    wherein the first and second fuel and oxidant conduit outlets are recessed from the respective first and second refractory openings a distance of 1 to 6 times the inner diameter of the respective oxidant conduit outlets, and 
         [0022]    wherein the distance from the axis of the first fuel conduit at its outlet to the side wall from which the burner extends is 4 to 10 times the inner diameter of the outlet of the first oxidant conduit and the distance from the axis of the second fuel conduit at its outlet to the side wall from which the burner extends is 4 to 10 times the inner diameter of the outlet of the second oxidant conduit, 
         [0023]    feeding fuel into the first and second fuel conduit inlets at a rate so that the fuel emerges from each fuel conduit outlet at a velocity of 40 to 300 fps and feeding oxidant into the first and second oxidant conduit inlets at a rate so that the oxidant emerges from each oxidant conduit outlet at a velocity of 3 to 100 fps, and the ratio of the oxidant velocity to the fuel velocity at said first oxidant outlet and said first fuel outlet, and the ratio of the oxidant velocity to the fuel velocity at said first oxidant outlet and said first fuel outlet, are each 1:1 to 1:20, 
         [0024]    and combusting oxidant and fuel that emerge from said outlets at the first and second openings in the refractory material to produce flames and heat of combustion in close proximity to the sidewall of said glass furnace forehearth. 
         [0025]    In a preferred embodiment, one of said first oxidant conduit and said first fuel conduit is within the other and one of said second oxidant conduit and said second fuel conduit is within the other. 
         [0026]    Yet another aspect of the invention is a method of heating molten glass adjacent to a side wall of a glass furnace forehearth in which the molten glass is flowing, or other high temperature stream, comprising 
         [0027]    into a burner that extends into the forehearth above the molten glass from a side wall of the forehearth, wherein the burner comprises 
         [0028]    an oxidant conduit having an inlet outside the forehearth and first and second outlets within the forehearth, and a fuel conduit having an inlet outside the forehearth and a first and second outlets within the forehearth, wherein said first oxidant conduit outlet and said first fuel conduit outlet open in a selected direction which is upstream or downstream relative to the direction in which said molten glass is flowing and said second oxidant conduit outlet and said second fuel conduit outlet open in a direction which is opposite to said selected direction, 
         [0029]    each conduit having a first segment extending from the side wall into the forehearth and second and third segments extending from the end of the first segment to the first and second conduit outlets respectively, wherein each second segment and each third segment are at an angle relative to the first segment, so that the axes of the conduit outlets form an angle relative to the side wall of 5 to 30 degrees projected in the horizontal plane and form an angle relative to the horizontal plane of 0 degrees to 5 degrees upward, 
         [0030]    refractory material encasing the conduits that protects the conduits from degradation by the forehearth conditions and protects gaseous fuel in the fuel conduit from thermal decomposition, and 
         [0031]    a first opening in the refractory material aligned with the first fuel conduit outlet and the first oxidant conduit outlet, wherein the first fuel conduit outlet and the first oxidant conduit outlet are recessed relative to the outer surface of the first opening in the refractory material, and a second opening in the refractory material aligned with the second fuel conduit outlet and the second oxidant conduit outlet, wherein the second fuel conduit outlet and the second oxidant conduit outlet are recessed relative to the outer surface of the second opening in the refractory material, and the first and second openings in the refractory material are straight or diverge outwardly from the respective oxidant conduit outlets to the outer surface of the refractory material, 
         [0032]    wherein the first and second fuel and oxidant conduit outlets are recessed from the respective first and second refractory openings a distance of 1 to 6 times the inner diameter of the respective oxidant conduit outlets, and 
         [0033]    wherein the distance from the axis of the first fuel conduit at its outlet to the side wall from which the burner extends is 4 to 10 times the inner diameter of the outlet of the first oxidant conduit and the distance from the axis of the second fuel conduit at its outlet to the side wall from which the burner extends is 4 to 10 times the inner diameter of the outlet of the second oxidant conduit, 
         [0034]    feeding fuel into the fuel conduit inlet at a rate so that the fuel emerges from each fuel conduit outlet at a velocity of 40 to 300 fps and feeding oxidant into the oxidant conduit inlet at a rate so that the oxidant emerges from each oxidant conduit outlet at a velocity of 3 to 100 fps, and the ratio of the oxidant velocity to the fuel velocity at said first oxidant outlet and said first fuel outlet, and the ratio of the oxidant velocity to the fuel velocity at said first oxidant outlet and said first fuel outlet, are each 1:1 to 1:20, 
         [0035]    and combusting oxidant and fuel that emerge from said outlets at the first and second openings in the refractory material to produce flames and heat of combustion in close proximity to the sidewall of said glass furnace forehearth. 
         [0036]    In a preferred embodiment, one of said conduits is within the other of said conduits. 
         [0037]    In other preferred embodiments, the amount of oxygen fed from its oxidant outlet is insufficient to completely combust with the fuel that is fed from the adjacent fuel outlet, and auxiliary oxygen is fed into the forehearth to combust with the fuel. 
         [0038]    A further aspect of this invention is carrying out the above feeding and combusting operations in a forehearth with a plurality of burners as described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]      FIG. 1  is a perspective view of a portion of a forehearth including a burner in accordance with the present invention. 
           [0040]      FIG. 2  is a top cross-sectional view of a portion of the forehearth portion shown in  FIG. 1 , seen in the plane formed by line 2′-2″ that appears in  FIG. 1 . 
           [0041]      FIG. 3  is a cutaway view of a portion of the burner that is shown in  FIG. 2 . 
           [0042]      FIG. 4  is a perspective view of a portion of a forehearth showing another embodiment of the present invention. 
           [0043]      FIG. 5  is a top cross-sectional view of a portion of the forehearth portion shown in  FIG. 4 , seen in the plane formed by the line 5′-5″ that appears in  FIG. 4 . 
           [0044]      FIG. 6  is a top cross-sectional view of a portion of the forehearth showing another embodiment of the present invention. 
           [0045]      FIG. 7  is a top cross-sectional view of a portion of the forehearth showing another embodiment of the present invention. 
           [0046]      FIG. 8  is a top cross-sectional view of a portion of the forehearth showing another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0047]      FIG. 1  depicts one representative embodiment of the practice of the present invention. This embodiment is illustrative, so the present invention is not to be limited to the embodiment in the Figures of this application. 
         [0048]    As seen in  FIG. 1 , a forehearth  1  includes side walls  2  and  3  and bottom  4  which together define a channel through which molten glass  5  is held and flows as indicated by arrow D. The molten glass may be flowing in the direction from D2 toward D1, in which case the flames  13  are oriented to extend downstream relative to the direction in which the molten glass is flowing, or the molten glass may be flowing in the direction from D1 toward D2, in which case the flames  13  are oriented upstream relative to the direction in which the molten glass is flowing. Burner  11  extends from side wall  2  into the interior of the forehearth  1 , above molten glass  5 . Burner  11  includes opening  12  from which, during operation of burner  11 , flame  13  extends within forehearth  1 . 
         [0049]      FIG. 2  illustrates the side wall  2  and burner  11  of  FIG. 1 , as seen from directly above burner  11 , in a cross-sectional view in a horizontal plane taken along line 2′-2″ which appears in  FIG. 1 . As seen in  FIG. 2 , burner  11  extends through a suitable opening in side wall  2 . Preferably there is as little free space as possible between burner  11  and side wall  2 , where burner  11  passes through side wall  2 , in order to minimize or prevent passage of gases out of the forehearth or into the forehearth from the surrounding atmosphere. 
         [0050]    Oxidant conduit  21  extends from oxidant conduit inlet  23  which is outside the forehearth, through first segment  25  and second segment  27 , and terminates at oxidant conduit outlet  29 . Fuel conduit  22  extends from fuel conduit inlet  24  which is outside the forehearth, through first segment  26  and second segment  28 , and terminates at fuel conduit outlet  30 . First segment  25  is angled with respect to second segment  27 , and first segment  26  is angled with respect to second segment  28 , so that each conduit extends into the forehearth in a direction generally across the direction of flow of the molten glass, then turns toward the respective outlets, and terminates in the outlets that are generally aligned to open in a direction that is upstream or downstream relative to the direction of flow of the molten glass. The respective first and second segments of each conduit can be defined by a bend in the conduit (which form a gradually curving connection between the segments), or by a joint between the two segments of conduit (which form a relatively sharp angle). More specifically, the axis  31  of each of the conduits at the respective openings forms an angle of 5 degrees to 30 degrees relative to the side wall  2 , when projected in a horizontal plane. This angle is shown as angle A in  FIG. 2 . The axis  31  also forms an angle of zero degrees (i.e. horizontal) to 5 degrees above horizontal, relative to the horizontal plane. 
         [0051]    In  FIGS. 2 and 3 , the fuel conduit is inside the oxidant conduit. This construction is preferred, although the conduits can instead be arranged so that the oxidant conduit is inside the fuel conduit. Alternatively, the fuel conduit and the oxidant conduit can be next to each other rather than one inside the other, as shown in  FIG. 7  in which the reference numerals have the same meanings as with respect to  FIGS. 2 and 3 . 
         [0052]    Burner  11  extends into the forehearth a distance such that the distance from the side wall  2  from which burner  11  extends, to the axis  31  of the fuel conduit outlet, at the fuel conduit outlet  30 , is 4 to 10 times the inner diameter of the oxidant conduit outlet  29 . Preferably, this distance is 4 to 6, and more preferably 4 to 5, times the inner diameter of the oxidant conduit outlet  29 . 
         [0053]    Burner  11  includes refractory material that encases the oxidant conduit and the fuel conduit, to protect the conduits from degradation by the conditions within the forehearth of high temperature and potentially corrosive atmosphere. Preferably there are two layers of different refractory material, shown in  FIG. 3  as layers  35  and  36 . Layer  35  denotes a layer of material that provides protection against high temperature and a corrosive, highly-alkaline atmosphere. Examples of suitable material for this function include AZS (alumina-zirconia-silica), and alumina-silica, or chrome-alumina. Layer  36  denotes a layer of material that provides protection against high temperatures and that also has low thermal conductivity, preferably thermal conductivity at 1500 degrees Fahrenheit that is no greater than a value of 2 BTU/square foot/hour/degreeFahrenheit/inch of thickness. Examples of suitable material for this function include “Delta t Crete 25” from Mt. Savage Refractory Co. If desired, layers  35  and  36  can be comprised of the same material, provided that the desired properties of protection and low thermal conductivity are provided. 
         [0054]    The oxidant conduits and the fuel conduits can be made of any metal or ceramic material that can retain its structural integrity when used (encased in the refractory material) at the temperatures which are encountered within the forehearth. Examples of materials having the desired properties include stainless steel grades  304 ,  316 ,  310 , and  321 , and Inconel and Incoloy. Other examples are known in this field and can readily be identified. 
         [0055]    Opening  12  extends from the interior of the burner  11 , through the refractory material, to provide access from the outlets of the fuel conduit and the oxidant conduit to the interior of the forehearth. Opening  12  can be straight, by which is meant that the sides of opening  12  can be parallel to each other and to the axis of the fuel conduit opening, or opening  12  can diverge outwardly.  FIG. 3  shows an embodiment in which opening  12  diverges outwardly from the oxidant conduit outlet  29  and the fuel conduit outlet  30  toward the outer surface  37  of the burner  11 . Thus, the angle formed by opening  12  is preferably zero degrees (straight) or greater than zero up to 11 degrees (diverging) relative to the axis of the fuel conduit opening. 
         [0056]    The oxidant conduit outlet  29  and the fuel conduit outlet  30  are recessed relative to the outer surface  37  of the burner  11 . Preferably, each of the oxidant conduit outlet and the fuel conduit outlet are recessed, relative to outer surface  37 , a distance of 1.5 to 6 times the inner diameter of the oxidant conduit outlet  29 . This recessing helps protect against degradation of the conduit outlets and helps prevent formation of soot and other undesired byproducts at the openings  12  and  30 . The outlet ends can be flush with each other but are not required to be flush with each other. 
         [0057]    The burner  11  is preferably situated relative to the molten glass such that the distance from the upper surface of the molten glass to the fuel conduit outlet is at least 5 times the inner diameter of the oxidant conduit outlet, and preferably 10 to 30 and more preferably 20 to 30 times the inner diameter of the oxidant conduit outlet. 
         [0058]    For operation of this embodiment of the present invention, the oxidant conduit inlet  23  is connected to a source of oxidant, associated with controls to permit the operator to control whether or not oxidant flows through the oxidant conduit and to control the rate of flow of the oxidant in the oxidant conduit. In addition, the fuel conduit inlet  24  is connected to a source of fuel, associated with controls to permit the operator to control whether or not fuel flows through the fuel conduit and to control the rate of flow of the fuel in the fuel conduit. 
         [0059]    Suitable oxidants include, oxygen-enriched air having an oxygen content of at least 30 vol.% oxygen, and commercial-grade oxygen having an oxygen content of at least 88 vol.%. 
         [0060]    Suitable fuels include any combustible gaseous, atomized liquid or solid (in finely divided form) material. Preferred examples include gaseous hydrocarbons and gaseous mixtures of hydrocarbons, including methane and natural gas, as well as atomized fuel oil and hydrocarbon liquids and mixtures that are liquid at room temperature and gaseous at temperatures above 1900 degrees Fahrenheit. 
         [0061]    In operation, the fuel is fed into the fuel conduit so that the fuel emerges from the fuel conduit outlet at a velocity of 40 to 300 feet per second (fps), and the oxidant is fed into the oxidant conduit so that the oxidant emerges from the oxidant conduit at a velocity of 3 to 100 fps. The ratio of the oxidant velocity to the fuel velocity (at the respective outlets) should be in the range of 1:1 to 1:20. 
         [0062]    The fuel and oxidant fed through burner  11  at the rates described herein are combusted as they emerge from their respective outlets. This combustion produces a flame whose base is at opening  30  and which extends from opening  30  in a direction that is up or downstream, preferably upstream, relative to the flow of the molten glass. The flame provides heat to the upper surface of the molten glass, preferentially along the edge of the molten glass closest to the side wall. The flame may not contact the side wall that is closest to the burner from which the flame extends. The axis of the flame is not directed downwards toward the molten glass surface. The flame should not contact the molten glass, but the gas fed through the burner may contact the molten glass at a velocity that is controlled, preferably to less than 33 feet per second at the molten glass surface, to lessen any alkali volatilization. 
         [0063]      FIGS. 4 and 5  depict another embodiment of the practice of the present invention. 
         [0064]    As seen in  FIG. 4 , the forehearth  1 , side walls  2  and  3 , and bottom  4 , and the channel through which molten glass  5  is held and flows, are as described above with respect to  FIG. 1 . Flames  13  produced by burner  111  are oriented to extend both downstream and upstream relative to the direction in which the molten glass is flowing. Burner  111  extends from side wall  2  into the interior of the forehearth  1 , above molten glass  5 . Burner  111  includes openings  112  and  112 A from which, during operation of burner  111 , flames  13  extend within forehearth  1 . 
         [0065]      FIG. 5  illustrates the side wall  2  and burner  111  of  FIG. 4 , as seen from directly above burner  11 , in a cross-sectional view in a horizontal plane taken along line 4′-4″ which appears in  FIG. 4 . As seen in  FIG. 5 , burner  111  extends through a suitable opening in side wall  2 . Preferably there is as little free space as possible between burner  111  and side wall  2 , where burner  111  passes through side wall  2 , in order to minimize or prevent passage of gases out of the forehearth or into the forehearth from the surrounding atmosphere. 
         [0066]    First oxidant conduit  121  extends from first oxidant conduit inlet  123  which is outside the forehearth, through first segment  125  and second segment  127 , and terminates at first oxidant conduit outlet  129 . First fuel conduit  122  extends from first fuel conduit inlet1  124  which is outside the forehearth, through first segment  126  and second segment  128 , and terminates at first fuel conduit outlet  130 . First segment  125  is angled with respect to second segment  127 , and first segment  126  is angled with respect to second segment  128 , so that each conduit extends into the forehearth in a direction generally across the direction of flow of the molten glass, then turns toward the respective outlets, and terminates in the outlets that are generally aligned to open in a direction that is upstream or downstream relative to the direction of flow of the molten glass. 
         [0067]    Second oxidant conduit  121 A extends from second oxidant conduit inlet  123 A which is outside the forehearth, through first segment  125 A and second segment  127 A, and terminates at second oxidant conduit outlet  129 A. Second fuel conduit  122 A extends from second fuel conduit inlet  124 A which is outside the forehearth, through first segment  126 A and second segment  128 A, and terminates at second fuel conduit outlet  130 A. First segment  125 A is angled with respect to second segment  127 A, and first segment  126 A is angled with respect to second segment  128 A, so that each conduit extends into the forehearth in a direction generally across the direction of flow of the molten glass, then turns toward the respective outlets, and terminates in the outlets that are generally aligned to open in a direction that is upstream or downstream relative to the direction of flow of the molten glass. 
         [0068]    The respective first and second segments of each conduit can be defined by a bend in the conduit (which form a gradually curving connection between the segments), or by a joint between the two segments of conduit (which form a relatively sharp angle). More specifically, the axes  131  and  131 A of each of the conduits at the respective openings forms an angle of 5 degrees to 30 degrees relative to the side wall  2 , when projected in a horizontal plane. These angles are shown as angles A and A* in  FIG. 5 . The axes  131  and  131 A also form an angle of zero degrees (i.e. horizontal) to 5 degrees above horizontal, relative to the horizontal plane. 
         [0069]    In  FIG. 5 , the fuel conduits are inside the oxidant conduits. This construction is preferred, although the conduits can instead be arranged so that each oxidant conduit is inside a fuel conduit. Alternatively, the fuel conduits and the oxidant conduits can be next to each other rather than one inside the other, as shown in  FIG. 7  except that there would be two pairs of conduits rather than the one pair shown in  FIG. 7 . 
         [0070]    Burner  111  extends into the forehearth a distance such that the distance from the side wall  2  from which burner  111  extends, to the axes  131  and  131 A of the first and second fuel conduit outlets, at the fuel conduit outlets  130  and  130 A, is 4 to 10 times the inner diameter of the oxidant conduit outlet  129 . Preferably, this distance is 4 to 6, and more preferably 4 to 5, times the inner diameter of the oxidant conduit outlet  129 . 
         [0071]    Burner  111  includes refractory material that encases the oxidant conduit and the fuel conduit, to protect the conduits from degradation by the conditions within the forehearth of high temperature and potentially corrosive atmosphere. Preferably there are two layers of different refractory material, as described herein with respect to burner  11  and as shown in  FIG. 3  as layers  35  and  36 . One layer of refractory material on burner  111  would be material that provides protection against high temperature and a corrosive, highly-alkaline atmosphere. Examples of suitable material for this function include AZS (alumina-zirconia-silica), and alumina-silica, or chrome-alumina. The other layer on burner  111  would be material that provides protection against high temperatures and that also has low thermal conductivity, preferably thermal conductivity at 1500 degrees Fahrenheit that is no greater than a value of 2 BTU/square foot/hour/degreeFahrenheit/inch of thickness. Examples of suitable material for this function include “Delta t Crete 25” from Mt. Savage Refractory Co. If desired, the layers of refractory material on burner  111  can be comprised of the same material, provided that the desired properties of protection and low thermal conductivity are provided. 
         [0072]    The conduits  121 ,  121 A,  122  and  122 A can be made of any metal or ceramic material that can retain its structural integrity when used (encased in the refractory material) at the temperatures which are encountered within the forehearth. Examples of materials having the desired properties include stainless steel grades  304 ,  316 ,  310 , and  321 , and Inconel and Incoloy. Other examples are known in this field and can readily be identified. 
         [0073]    Openings  112  and  112 A extend from the interior of the burner  111 , through the refractory material, to provide access from the outlets of the fuel conduits and the oxidant conduits to the interior of the forehearth. Openings  112  and  112 A can be straight, by which is meant that the sides of openings  112  and/or  112 A can be parallel to each other and to the axis of the fuel conduit opening, or openings  112  and/or  112 A can diverge outwardly.  FIG. 5  shows an embodiment in which the opening  112  diverges outwardly from the oxidant conduit outlet  129  and the fuel conduit outlet  130  toward the outer surface  137  of the burner  111 , and the opening  112 A diverges outwardly from the oxidant conduit outlet  129 A and the fuel conduit outlet  130 A toward the outer surface  137  of the burner  111 . Thus, the angles formed by openings  112  and  112 A are preferably zero degrees (straight) or greater than zero up to 11 degrees (diverging) relative to the axes of the respective fuel conduit openings. 
         [0074]    The oxidant conduit outlets  129  and  129 A, and the fuel conduit outlets  130  and  130 A, are recessed relative to the outer surface  137  of the burner  111 . Preferably, each of the oxidant conduit outlets and the fuel conduit outlets are recessed, relative to outer surface  137 , a distance of 1.5 to 6 times the inner diameter of the oxidant conduit outlet  129 . This recessing helps protect against degradation of the conduit outlets and helps prevent formation of soot and other undesired byproducts at the openings  112  and  130 . The ends of the fuel and oxidant outlets can be flush with each other but are not required to be flush with each other. 
         [0075]    The burner  111  is preferably situated relative to the molten glass such that the distance from the upper surface of the molten glass to the fuel conduit outlet  130  is at least 5 times the inner diameter of the oxidant conduit outlet  129 , and preferably 10 to 30 and more preferably 20 to 30 times the inner diameter of the oxidant conduit outlet  129 . 
         [0076]    For operation of this embodiment of the present invention, the oxidant conduit inlets  123  and  123 A are connected to a source of oxidant, associated with controls to permit the operator to control whether or not oxidant flows through the oxidant conduits and to control the rate of flow of the oxidant in the oxidant conduits. In addition, the fuel conduit inlets  124  and  124 A are connected to a source of fuel, associated with controls to permit the operator to control whether or not fuel flows through the fuel conduits and to control the rate of flow of the fuel in the fuel conduits. 
         [0077]    Suitable oxidants and fuels include those described herein above. 
         [0078]    In operation, the fuel is fed into the fuel conduits so that the fuel emerges from each of the fuel conduit outlets at a velocity of 40 to 300 feet per second (fps), and the oxidant is fed into the oxidant conduits so that the oxidant emerges from each of the oxidant conduit outlets at a velocity of 3 to 100 fps. The ratio of the oxidant velocity to the fuel velocity (at each of the respective outlets) should be in the range of 1:1 to 1:20. 
         [0079]    The fuel and oxidant fed through burner  111  at the rates described herein are combusted as they emerge from their respective outlets. This combustion produces flames whose bases are at openings  130  and  130 A and which extend from openings  130  and  130 A in directions downstream and upstream relative to the flow of the molten glass. The flames provide heat to the upper surface of the molten glass, preferentially along the edge of the molten glass closest to the side wall. The flame may not contact the side wall that is closest to the burner from which the flame extends. The axis of the flame is not directed downwards toward the molten glass surface. The flame should not contact the molten glass, but the gas fed through the burner may contact the molten glass at a velocity that is controlled, preferably to less than 33 feet per second at the molten glass surface, to lessen any alkali volatilization. 
         [0080]    Another embodiment of the practice of the present invention is described with reference to  FIGS. 4 and 6 . 
         [0081]    The embodiment of  FIG. 6 , referred to as burner  211 , can be positioned in a forehearth  1  in the manner as shown in  FIG. 4 . The forehearth  1 , side walls  2  and  3 , and bottom  4 , and the channel through which molten glass  5  is held and flows, are as described above with respect to  FIG. 1 . Flames  13  produced by burner  211  are oriented to extend both downstream and upstream relative to the direction in which the molten glass is flowing. Burner  211  extends from side wall  2  into the interior of the forehearth  1 , above molten glass  5 . Burner  211  includes openings  112  and  112 A from which, during operation of burner  211 , flames  13  extend within forehearth  1 . 
         [0082]      FIG. 6  illustrates the side wall  2  and burner  211  of  FIG. 1 , as seen from directly above burner  11 , in a cross-sectional view in a horizontal plane taken along line 6′-6″ which appears in  FIG. 4 . As seen in  FIG. 6 , burner  211  extends through a suitable opening in side wall  2 . Preferably there is as little free space as possible between burner  211  and side wall  2 , where burner  211  passes through side wall  2 , in order to minimize or prevent passage of gases out of the forehearth or into the forehearth from the surrounding atmosphere. 
         [0083]    Oxidant conduit  221  extends from oxidant conduit inlet  223  which is outside the forehearth, through first segment  225 , and splits into second segments  227  and  227 A, which terminate at first oxidant conduit outlet  229  and second oxidant conduit outlet  229 A. Fuel conduit  222  extends from fuel conduit inlet  224  which is outside the forehearth, through first segment  226 , and splits into second segments  228  and  228 A, which terminate at first fuel conduit outlet  230  and second fuel conduit outlet  230 A. First segment  225  is angled with respect to second segments  227  and  227 A, and first segment  226  is angled with respect to second segments  228  and  228 A, so that each conduit extends into the forehearth in a direction generally across the direction of flow of the molten glass, then turns toward the respective outlets, and terminates in the outlets that are generally aligned to open in a direction that is upstream or downstream relative to the direction of flow of the molten glass. 
         [0084]    The respective second segments of each conduit can be defined relative to the respective first segments by a bend in the conduit (which form a gradually curving connection between the segments), or by a joint between the segments of conduit (which form a relatively sharp angle). More specifically, the axes  231  and  231 A of each of the conduits at the respective openings form an angle of 5 degrees to 30 degrees relative to the side wall  2 , when projected in a horizontal plane. These angles are shown as angles A and A* in  FIG. 6 . The axes  231  and  231 A also form an angle of zero degrees (i.e. horizontal) to 5 degrees above horizontal, relative to the horizontal plane. 
         [0085]    In  FIG. 6 , the fuel conduit is inside the oxidant conduit. This construction is preferred, although the conduits can instead be arranged so that the oxidant conduit is inside the fuel conduit. Alternatively, the fuel conduit and the oxidant conduit can be next to each other rather than one inside the other, as shown in  FIG. 8  in which the reference numerals have the same meanings as with respect to  FIG. 6 . 
         [0086]    Burner  211  extends into the forehearth a distance such that the distance from the side wall  2  from which burner  211  extends, to the axes  231  and  231 A of the first and second fuel conduit outlets, at the fuel conduit outlets  230  and  230 A, is 4 to 10 times the inner diameter of the first oxidant conduit outlet  229 . Preferably, this distance is 4 to 6, and more preferably 4 to 5, times the inner diameter of the first oxidant conduit outlet  229 . 
         [0087]    Burner  211  includes refractory material that encases the oxidant conduit and the fuel conduit, to protect the conduits from degradation by the conditions within the forehearth of high temperature and potentially corrosive atmosphere. Preferably there are two layers of different refractory material, as described herein with respect to burner  11  and as shown in  FIG. 3  as layers  35  and  36 . One layer of refractory material on burner  211  would be material that provides protection against high temperature and a corrosive, highly-alkaline atmosphere. Examples of suitable material for this function include AZS (alumina-zirconia-silica), and alumina-silica, or chrome-alumina. The other layer on burner  211  would be material that provides protection against high temperatures and that also has low thermal conductivity, preferably thermal conductivity at 1500 degrees Fahrenheit that is no greater than a value of 2 BTU/square foot/hour/degreeFahrenheit/inch of thickness. Examples of suitable material for this function include “Delta t Crete 25” from Mt. Savage Refractory Co. If desired, the layers of refractory material on burner  211  can be comprised of the same material, provided that the desired properties of protection and low thermal conductivity are provided. 
         [0088]    The conduits  221  and  222  can be made of any metal or ceramic material that can retain its structural integrity when used (encased in the refractory material) at the temperatures which are encountered within the forehearth. Examples of materials having the desired properties include stainless steel grades  304 ,  316 ,  310 , and  321 , and Inconel and Incoloy. Other examples are known in this field and can readily be identified. 
         [0089]    Openings  212  and  212 A extend from the interior of the burner  111 , through the refractory material, to provide access from the outlets of the fuel conduits and the oxidant conduits to the interior of the forehearth. Openings  212  and  212 A can be straight, by which is meant that the sides of openings  212  and/or  212 A can be parallel to each other and to the axis of the fuel conduit opening, or openings  212  and/or  212 A can diverge outwardly.  FIG. 6  shows an embodiment in which the opening  212  diverges outwardly from the oxidant conduit outlet  229  and the fuel conduit outlet  230  toward the outer surface  237  of the burner  211 , and the opening  212 A diverges outwardly from the oxidant conduit outlet  229 A and the fuel conduit outlet  230 A toward the outer surface  237  of the burner  211 . Thus, the angles formed by the openings  212  and  212 A are preferably zero degrees (straight) or greater than zero up to 11 degrees (diverging) relative to the axes of the respective fuel conduit openings. 
         [0090]    The oxidant conduit outlets  229  and  229 A, and the fuel conduit outlets  230  and  230 A, are recessed relative to the outer surface  237  of the burner  211 . Preferably, each of the oxidant conduit outlets and the fuel conduit outlets are recessed, relative to outer surface  237 , a distance of 1.5 to 6 times the inner diameter of the oxidant conduit outlet  229 . This recessing helps protect against degradation of the conduit outlets and helps prevent formation of soot and other undesired byproducts at the openings  212  and  230 . The ends of the fuel and oxidant outlets can be flush with each other but are not required to be flush with each other. 
         [0091]    The burner  211  is preferably situated relative to the molten glass such that the distance from the upper surface of the molten glass to the fuel conduit outlet  230  is at least 5 times the inner diameter of the oxidant conduit outlet  229 , and preferably 10 to 30 and more preferably 20 to 30 times the inner diameter of the oxidant conduit outlet  229 . 
         [0092]    For operation of this embodiment of the present invention, the oxidant conduit inlet  223  is connected to a source of oxidant, associated with controls to permit the operator to control whether or not oxidant flows through the oxidant conduit and to control the rate of flow of the oxidant in the oxidant conduit. In addition, the fuel conduit inlet  224  is connected to a source of fuel, associated with controls to permit the operator to control whether or not fuel flows through the fuel conduit and to control the rate of flow of the fuel in the fuel conduit. 
         [0093]    Suitable oxidants and fuels include those described herein above. 
         [0094]    In operation, the fuel is fed into the fuel conduit so that the fuel emerges from each of the fuel conduit outlets at a velocity of 40 to 300 feet per second (fps), and the oxidant is fed into the oxidant conduit so that the oxidant emerges from each of the oxidant conduit outlets at a velocity of 3 to 100 fps. The ratio of the oxidant velocity to the fuel velocity (at each of the respective outlets) should be in the range of 1:1 to 1:20. 
         [0095]    The fuel and oxidant fed through burner  211  at the rates described herein are combusted as they emerge from their respective outlets. This combustion produces flames whose bases are at openings  230  and  230 A and which extend from openings  230  and  230 A in directions downstream and upstream relative to the flow of the molten glass. The flames provide heat to the upper surface of the molten glass, preferentially along the edge of the molten glass closest to the side wall. The flame may not contact the side wall that is closest to the burner from which the flame extends. The axis of the flame is not directed downwards toward the molten glass surface. The flame should not contact the molten glass, but the gas fed through the burner may contact the molten glass at a velocity that is controlled, preferably to less than 33 feet per second at the molten glass surface, to lessen any alkali volatilization. 
         [0096]    In any of the foregoing embodiments, the amounts of oxidant and fuel fed into the burners should be sufficient so that the combustion produces sufficient heat to maintain the glass molten and flowable along the sidewalls of the forehearth. The amounts of fuel to combust can readily be determined taking into account the heating value and heat of combustion of the fuel being use, the size of the forehearth, and the number of burners being employed. 
         [0097]    In one embodiment, the relative amounts of the fuel and the oxygen in the oxidant fed, that emerge from the respective outlets described above, should correspond to a range from 20% stoichiometric excess of oxygen to a 20% excess of fuel. Of course, lesser excess, such as up to 10% or up to 5% excess, are preferable as representing less wasted reagent. 
         [0098]    In another embodiment, the amount of oxygen in the oxidant fed from an oxidant outlet is 50% to 90% of the stoichiometric requirement based on the fuel fed from the adjacent outlet, and auxiliary oxidant is fed into the forehearth from an additional port  16  (shown in  FIG. 1 ) on burner  11  and combusts with fuel that is fed from one or more fuel outlets. This provides staging of the combustion, which lessens the formation of oxides of nitrogen (“NOx”) that are considered to be pollutants. The auxiliary oxidant reaches port  16  from a source outside the forehearth, through its own conduit or through a section of conduit that branches off from oxidant conduit  21 ,  121 ,  121 A,  221  or  221 A as the case may be. 
         [0099]    In practice, it is advantageous to have more than one burner  11 ,  111  and/or  211  (or any combination thereof) in a forehearth, each operated as described herein. Burners can be situated through one side wall, or preferably through both side walls as shown in  FIGS. 1 and 4 . Burners on a side wall are preferably spaced apart so that the distance from the fuel conduit outlet of a given burner to the fuel conduit outlet of the next adjacent burner in the downstream direction is 1 to 6, preferably 2 to 4, and more preferably 1.5 to 3, times the length of the flame produced from the given burner. Burners on opposing side walls can be positioned so that they are directly opposite each other, or staggered so that each burner does not have a burner directly opposite it. 
         [0100]    The burners described herein, and the manner of operation described herein, have been found to provide numerous advantages. Controlling the velocities of each stream, and controlling the ratio of the velocities, has been found to reduce or eliminate the overheating of the conduits, to reduce or eliminate deterioration of the refractory material on the outer surfaces of the burner, and to reduce or eliminate the formation of soot and other byproducts at the outlets. The range of velocities described herein avoids excessive cooling of the refractory material in the vicinity of the conduit outlets (which would be the result of higher velocities of the fuel and/or the oxidant), and the lessened cooling leads to lessened condensation of material such as corrosive alkali from the forehearth atmosphere onto the refractory material. The angle of the outlets, and the associated angle of the axis of the flame, lessens the exposure of fuel and oxygen conduits to radiation from the furnace which could cause cracking of the fuel species in the fuel, mitigates the formation of hot spots on the side walls from the radiation and contact of the flame which could lead to deterioration of the side walls, and lessens the exposure of the molten glass surface to the flame which could cause excessive volatilization of material from the molten glass (which could in turn lead to loss of glassmaking material, increased emissions from the forehearth into the atmosphere that is vented from above the molten glass, and/or increased corrosive attack on the exposed surfaces of the forehearth including the crown or roof that may be over the molten glass.