Abstract:
In an industrial glass furnace which contains recuperators, regenerators, electric boost or other devices for providing heat to glass batch material an oxy-fuel burner mounted in the roof of the furnace provides additional heat to melt the batch material. A method of mounting and using such a roof-mounted oxy-fuel burner including the operating parameters to maximize heat transfer while minimizing the disturbance of the batch material is disclosed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to the use of at least one oxygen-fuel burner in the roof of a glass melting furnace to boost production capacity or maintain current production capacity with either reduction of electro-boost or as a result of deterioration of existing heat recovery equipment such as recuperators or regenerators. The process involves the replacement of a portion of existing or previously existing air-fuel or electrical energy capacity with oxy-fuel energy. With the exception of end-fired regenerative furnaces and electric furnaces the process involves the blocking of regenerative ports or isolation of recuperative burners. In particular the design selection, angling and positioning of the burners over the raw batch materials entering the furnace improves the rate of melting, increase product yield, better energy efficiency and improve glass quality. Accurate control of the stoichiometric ratio of combustion in the burner, rich lean interaction of burners and furnace zonal fuel/oxygen staging are used to optimise heat transfer while minimizing oxides of nitrogen and sulfur dioxide emissions.  
           [0002]    Regenerative, recuperative, electric and direct fired furnaces have been commonly involved in the manufacture of glass and related frit products.  
           [0003]    Air-fuel regenerative furnaces fall into two categories: cross-fired and end-fired. Cross-fired regenerative furnaces have multiple ports typically three to eight on each side of the furnace that connect to either a common or compartmentalized regenerator to preheat the combustion air. The regenerators which come in various shapes and sizes reverse every 15-30 minutes dependent on furnace operation. During each reversal cycle combustion air from a fan passing through one passage in the reversal valve enters the base of the regenerator on one side of the furnace and is preheated prior to entering the ports which connect to the furnace. Fuel in the form of oil and/or gas is injected either, under, over, through or side of port to produce a flame which is combusted in the glass melting furnace. The hot products of combustion exit the furnace through the opposing side port, down through the regenerator checker bricks releasing heat and then exiting to the exhaust stack through a second passageway in the reversal valve. As the air-side regenerator cools, the exhaust regenerator heats until the reversal valve reverses and combustion air enters the previously hot exhaust regenerator.  
           [0004]    The glass is melted partly due to the radiation of the air-fuel flame but mainly from re-radiation from the roof and walls which are heated by the products of combustion. To obtain higher furnace glass production capacity, many furnaces use electric boost by means of electrodes immersed in the glass. This is costly and can cause damage to the glass contact tank walls. Through time, regenerators can start to block due to thermal/structural damage and/or carry-over of raw glass forming materials, also known as batch materials or batch, or condensation of volatile species released from the glass batch. As the regenerators start to block or fail, the preheat temperature of the air in the furnace will reduce. Because of the increased pressure drop, the exhaust side will limit the removal of exhaust gases and therefore limit energy input into the furnace thus reducing furnace glass production.  
           [0005]    To recover production capacity lost to preceding regenerator issues or to increase production in a non-encumbered furnace, oxygen has been used by four means: general air enrichment with oxygen, specific oxygen lancing under the port flames, installation of an oxy-fuel burner between first port and charging end wall, and water-cooled oxy-fuel burners installed through the port. The capacity increases from these technologies is limited by access, process requirements or refractory temperature limits.  
           [0006]    The End-Fired Regenerative furnace is similar in operation to a cross-fired furnace, however, has only two ports in the end wall which connect to individual regenerators. Regenerator deterioration is the same mechanism as in cross-fired furnaces and similarly electric and oxygen boost is utilized.  
           [0007]    To recover production capacity lost to the aforementioned regenerator issues or to increase production, oxygen has been used by three means: general air enrichment with oxygen, specific oxygen lancing under the port and installation of oxy-fuel burners through the furnace side walls down tank. These technologies are typically limited on capacity because of location and concerns for overheating of the furnace.  
           [0008]    The recuperative furnace utilizes at least one recuperator type heat exchanger. Unlike the regenerator, the recuperator is continuous with hot concurrent flow heat exchanger where exhaust gases preheat combustion air which is ducted to individual air fuel burners along the sides of the furnace. Recuperative furnaces can also use electric boost. As with regenerative furnaces, recuperators can start to lose their efficiency and ability to preheat the air. They can become blocked or develop holes.  
           [0009]    To recover production capacity lost from the aforementioned recuperator issues or to increase production, oxygen has been used by three means: general air enrichment with oxygen, specific oxygen lancing under the air fuel burners and installation of oxy-fuel burners either through the furnace side or end walls. These technologies are typically limited on capacity because of location restrictions and concerns for overheating of the furnace.  
           [0010]    Direct fired furnaces do not utilize preheated air and are therefore less efficient than the preceding examples of furnace design. To improve thermal efficiency or increase production capacity, side wall oxy-fuel burners have replaced air fuel burners.  
           [0011]    Electric furnaces or furnaces which utilize electricity for majority of melting are typically costly to operate and are subject to a shorter campaign life than the typical fossil fuel furnaces. Once designed it is difficult to increase the production capacity. This invention relates to hot top and warm top electric furnaces and is not applied to cold top furnaces  
           [0012]    U.S. Pat. No. 5,139,558 to Lauwers discloses the use of a high-momentum roof-mounted auxiliary oxygen fired burner in a glass melting furnace which is directed to the interface of the melted and solid glass forming ingredients whereby the solid glass forming ingredients are mechanically prevented from escaping the melting zone.  
           [0013]    U.S. Pat. No. 3,337,324 to Cable discloses a process for melting batch material in a glass furnace using a burner positioned to fire substantially down over the feed end of a water-cooled furnace.  
           [0014]    Co-pending U.S. patent application Ser. No. 08/992,136 discloses the use of roof-mounted burners as the primary source of heat in a glass melting furnace having no regenerators or recuperators.  
         SUMMARY OF THE INVENTION  
         [0015]    Briefly, according to this present invention, glass melting furnaces of all designs can be boosted using at least one roof-mounted oxygen fuel burners positioned over the raw batch materials as the materials enter the furnace to improve the rate of melting and improve glass quality and/or glass product yield. Because of the increased rate and yield of the glass melting generated by the design and positioning of these burners, depending on furnace condition and type, at least one or more of the following can be achieved: increased glass production, improved glass quality, reduction in electric boost, recovery of production lost due to inefficient heat recovery (i.e., blocked regenerators), reduction of oxygen use by replacing oxygen enrichment of the furnace atmosphere, reduction of oxygen use by replacing oxygen lancing, reduction of oxygen use by replacing conventional oxy-fuel burners positioned through the walls of a glass furnace, increased furnace campaign life, improved energy efficiency, reduction in emissions of oxides of nitrogen and oxides of sulfur, reduction in fossil fuel usage, reduction in cullet and increased product glass yield.  
           [0016]    This invention may be applied to the following types of furnaces. In hot top electric furnace applications of this invention, at least one oxygen-fuel burner will be mounted in the roof of the furnace. In cross-fired regenerative furnace applications of this invention may necessitate at least one pair of the opposing ports to be fully or partially blocked or isolated. In end-fired regenerative furnace applications of this invention, at least one oxygen-fuel burner will be mounted in the roof of the furnace and the combustion air flow reduced by a portion of the original design maximum flow. In all recuperative furnace applications of this invention, at least one oxygen-fuel burner will be mounted in the roof of the furnace. In multi-burner furnaces, burners adjacent to the roof mounted burners should be removed and the air supply isolated. In single burner or single port applications the combustion air flow will be reduced by a portion of the original design maximum flow.  
           [0017]    In all direct fired furnace applications of this invention, at least one oxygen-fuel burner will be mounted in the roof of the furnace. In multi-burner furnaces, burners adjacent to the roof mounted burners should be removed and the air supply discontinued. In single burner or single port applications, the combustion air flow will be reduced by a portion of the original design maximum flow.  
           [0018]    In all the above cases the scope of the invention is effectively the same: glass melting which was previously performed by air-fuel or oxy-fuel including but not exclusive of furnaces that utilize electric boost or conventional oxygen boosting methods, is replaced by roof-mounted oxy-fuel burners positioned over the raw batch materials entering the furnace to improve the rate of melting and/or improve glass quality and/or glass product yield. Because of the ability to position these burners at specific locations, increased heat transfer to the unmelted raw batch materials is achieved.  
           [0019]    In all cases at least one roof-mounted oxy-fuel burner is positioned over the raw batch materials entering the furnace to improve the rate of melting and improvement in quality is utilized and in all multi-port and multi-burner air fuel applications at least one pair of ports or pair of burners are isolated. In all single port and single burner applications the air and fuel are reduced to a portion below the maximum design. The more efficient roof mounted burners provide energy to replace the conventional energy removed from the process and the additional energy to achieve the desired process requirements. The positioning of the burners over the raw batch entering the furnace improves the rate of melting. The stoichiometric ratios and control of the roof-mounted burners and remaining air-fuel burners is critical to minimizing the emission of nitrous oxide and sulfur dioxide. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    Further features and other objects and advantages of this invention will become clear from the following detailed description made with reference to the drawings in which:  
         [0021]    [0021]FIG. 1 is a cross sectional longitudinal view of a glass meting furnace in accordance with the present invention.  
         [0022]    [0022]FIG. 2A is a cross-sectional plan view of a cross-fired regenerative embodiment of the glass melting furnace of FIG. 1 along line  2 - 2 .  
         [0023]    [0023]FIG. 2B is a cross-sectional plan view of an end-fired regenerative embodiment of the glass melting furnace of FIG. 1 along line  2 - 2 .  
         [0024]    [0024]FIG. 2C is a cross-sectional plan view of a cross-fired recuperative embodiment of the glass melting furnace of FIG. 1 along line  2 - 2 .  
         [0025]    [0025]FIG. 2D is a cross-sectional plan view of an end-fired recuperative embodiment of the glass melting furnace of FIG. 1 along lines  2 - 2 .  
         [0026]    [0026]FIG. 2E is a cross-sectional plan view of a unit melter embodiment of the glass melting furnace of FIG. 1 along lines  2 - 2 .  
         [0027]    [0027]FIG. 3 is a cross sectional view of the glass melting furnace of FIG. 1 along line  3 - 3  illustrating two oxygen-fuel burners adjacent the upstream end wall of the furnace.  
         [0028]    [0028]FIG. 4 is an alternate cross sectional view of the glass melting furnace of FIG. 1 along line  3 - 3  illustrating one oxygen-fuel burner adjacent the upstream end wall of the furnace.  
         [0029]    [0029]FIG. 5 is a cross sectional view of an oxygen fuel burner and a schematic representation of a burner flame from the oxygen burner. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    Referring to the figures, there is shown a glass melting furnace  10  for providing molten glass to a glass forehearth or refiner  12  wherein the molten glass is further refined and subsequently fed to one or more glass-forming machines such as containers, fiberizers, float baths and the like (not shown). In considering the figures, it will be appreciated that for purposes of clarity certain details of construction are not provided in view of such details being conventional and well known by someone skilled in the art once the invention is disclosed and explained. Specific items excluded are the regenerator ports, air-fuel burners and exhausts since these are different for each type of furnace.  
         [0031]    The glass melting furnace  10  typically includes an elongated channel having an upstream end wall  14  and a downstream end wall  16 , side walls  18  a floor  20  and a roof  22  all made from appropriate refractory materials such as alumina, silica, alumina-silica, zircon, zirconia-alumina-silica chromeoxide and the like. The roof  22  is shown generally as having an arcuate shape transverse to the longitudinal axis of the channel, however, the roof may be of most any suitable design. The roof  22  of the typical glass melting furnace  10  is positioned between about 3-15 feet above the surface of the raw glass-forming material. As well known in the art, the glass melting furnace  10  may optionally include one or more bubblers  24  and/or electrical boost electrodes (not shown). The bubblers and/or electrical boost electrodes increase the temperature of the bulk glass and increase the molten glass circulation under the batch cover.  
         [0032]    The glass melting furnace  10  includes two successive zones, a melting zone  27  and a downstream fining zone  28 . The melting zone  27  is considered the upstream zone of the glass melting furnace  10  wherein raw glass-forming material is charged into the furnace using a charging device  32  of a type well known in the art. The raw glass-forming material  30  may be a mixture of raw materials typically used in the manufacture of glass. It will be appreciated that the composition of the raw glass-forming material (or batch)  30  is dependent on the type of glass being produced. Normally, the material comprises, inter alia, silica containing materials including scrap glass commonly referred to as cullet. Other glass-forming materials including feldspar, limestone, dolomite, soda ash, potash, borax and alumina may also be used. To alter the properties of the glass, a minor amount of arsenic, antimony, sulfates, carbon and/or fluorides may also be added. Moreover, color forming metal oxides may be added to obtain the desired color.  
         [0033]    The raw glass-forming material  30  forms a batch layer of solid particles on the surface of the molten glass in the melting zone  27  of the glass melting furnace  10 . The floating solid batch particles of raw glass-forming material  30  are melted principally by at least one oxygen-fuel burner  34  having a controlled impinging flame shape and length mounted within the roof  22  of the glass melting furnace  10 . It will be appreciated that it has been found that the installation and proper control of at least one oxygen-fuel burner  34  in the roof  22  of the glass melting furnace  10  over the raw glass-forming material  30  in accordance with the present invention increases the melting rate of the solid raw glass-forming material and, at the same time, maintains the operating temperature of the surrounding refractory material within acceptable operating limits.  
         [0034]    As used herein, the phrase “at least one oxygen-fuel burner” means one or more oxygen fuel burners. Furthermore, as used herein the phrase “principally by at least one oxygen-fuel burner” refers to the condition wherein the additional or recovered glass production capacity and replaced air fuel and or electric/oxygen boost energy for melting of the raw glass-forming material is from at least one oxygen-fuel burner. In one particular embodiment, as shown in FIGS. 1 and 2A the glass melting furnace  10  includes three oxygen-fuel burners  34 . A single oxygen-fuel burner  34  is positioned upstream of two adjacently positioned downstream oxygen fuel burners. However, it will be appreciated that any number of oxygen-fuel burners  34  may be positioned at almost any suitable location in the roof  22  of the furnace  10  over the batch to melt the raw glass-forming material  30 . For example, two oxygen-fuel burners  34  may be positioned in a side-by-side relation as depicted in FIG. 3 or a single oxygen-fuel burner may be used as depicted in FIG. 4. Nonetheless, in accordance with the present invention, the angular orientation of each oxygen-fuel burner  34  in the roof  22  of the glass melting furnace must be such that the flame  36  produced is directed substantially perpendicular to the glass batch surface to produce a flame which impinges on the glass surface to form an impingement area  26 . In a preferred embodiment, the oxygen-fuel burners  34  are positioned substantially perpendicular to the batch material at an angle of about 90 degrees relative to the raw glass-forming material  30 . The angle may deviate from the perpendicular in the direction of the downstream end-wall by as much as 30 degrees, but preferably less than 10 degrees. It has been found that the glass production rate and the quality of glass produced may be improved by melting the raw glass-forming material  30  with at least one downwardly firing oxygen-fuel burner  34  having a controlled impinging flame shape and length in accordance with the present invention.  
         [0035]    The at least one oxygen-fuel burner requires fuel and an oxidant. The fuel can be either gaseous or liquid or combinations of both. Gaseous fuels include natural gas (methane), towns gas, producer gas, LPG, propane, butane and blends of the aforementioned gases. Liquid fuels include heavy, medium and light fuel oils, kerosene and diesel. Liquid fuels require to be atomized and/or vaporized. The atomization can be either by mechanical means or a secondary atomizing mediums which include air, steam, oxygen, any of the aforementioned gaseous fuels and in some cases an inert gas. Vaporization relies on the heat of the surrounding products of combustion gases to evaporate the oil. The oxidant can be either 100% pure oxygen or a blend of oxygen and inert gas with an oxygen concentration 40-100%.  
         [0036]    Referring to FIG. 5, the at least one oxygen-fuel burner  34  within the roof  22  of the glass melting furnace  10  has at least one fuel conduit  40  for providing fuel and at least one oxygen conduit  42  for providing oxygen flow. The oxygen-fuel burner  34  may have a capacity ranging from about 1-15 MM Btu/hr depending upon the glass melting furnace  10  size and desired pull rate. The oxygen-fuel burner  34  is designed to use a higher percentage of oxygen than is present in air and thus the temperature above the area of impingement of the flame  36  from the oxygen-fuel burner  34  is substantially higher than in a conventional glass melting furnace utilizing air-fuel burners. Notwithstanding, as well known to one skilled in the art the temperature of the flame  36  imparted by an oxygen-fuel burner  34  is dependent on the quality of the fuel and the oxygen/fuel ratio. In a preferred embodiment, the oxygen concentration of the oxygen-fuel burner  34  is typically at a level of about 95-125 percent of the stoichiometric amount of oxygen required to combust the fuel. The fuel to oxygen ratio can be varied, however, to produce a range of operating conditions in the glass melting furnace  10  to effect one or more desired properties, including, for example, redox level, glass color, the level of gaseous bubbles known as seeds in the trade and other glass properties.  
         [0037]    The oxygen-fuel burner  34  extends downwardly from a burner block  38  located in the roof  22  of the glass melting furnace  10 . Each primary burner block  38  includes an opening having an inside diameter (id) which is at least as great as the external diameter of the largest conduit  42  or  40  dependent on configuration. The inside diameter (id) of the opening of the burner block  38  may range between about 2-8 inches. The end of the oxygen-fuel burner  34  primary combustion zone is located from the end of the burner block  38  a distance (LBb) between about 0-18 inches. The secondary and in some cases tertiary combustion zone is external to the burner block  38 . It will be appreciated that the opening of the burner block  38  between the end of the oxygen-fuel burner  34  and the end of the burner block in some instances acts to focus the burner flame and prevent the burner flame from spreading outwardly but moreover protects the conduits of the burner. The burner block  38  is made of a refractory material as well known in the art and may be of most any suitable outside shape such as rectangular and the like.  
         [0038]    The bottom surface of the burner block  38  may be flush with the inside surface of the roof  22  or the bottom surface may project below the inside surface of the roof to a maximum distance of 2 inches to protect the burner block  38  from wear. Furthermore, as shown in FIG. 5, the fuel conduits  40  and oxygen conduits  42  of the oxygen-fuel burner  34  extend downwards within the burner block  38  and terminate at either substantially the same vertical height or totally different vertical heights from the exit of burner block  38 .  
         [0039]    Dependent on height of burner block  38  from raw batch and desired operating conditions of the burner, the greater the fraction of fuel staging and oxygen staging internal and external to the burner block  38  will vary. Additional oxygen injectors  60  are positioned to delay complete combustion until after the flame has impinged on the raw batch. The location of these additional injectors  60  is dependent on number and position of roof mounted burners however can be located in any point of the roof and walls.  
         [0040]    In accordance with the present invention, the downwardly directed impinging flame  36  produced by the at least one oxygen-fuel burner  34  is precisely controlled to give a flame length greater than or equal to the distance from the exit of burner block  38  to the surface of the raw glass-forming ingredients  30  and the surface of the molten glass and away from the surrounding refractory thereby reducing the risk of overheating the roof  22  and side walls  18  of the glass melting furnace  10 . The impinging flame  36  may be controlled by such control devices as are conventional and standard in chemical processing. For example, valves, thermocouples, thermistors coupled with suitable servo circuits, heater controllers and the like are readily available and conventionally used for controlling the quantity and velocity of the fuel and oxygen from the oxygen-fuel burner  34 .  
         [0041]    The impinging flame  36  is precisely controlled by controlling both the relative velocity and the maximum and minimum velocities of the fuel and of the oxygen streams and the internal and external staging from the at least one oxygen-fuel burner  34 .  
         [0042]    The maximum and minimum velocity of the fuel and oxygen flow impinging on the surface of the raw glass-forming material  30  must be controlled to prevent the disturbance of the batch material and entrainment of or the displacement of glass batch material against the side walls  18  and roof  22  while maintaining optimum convective heat transfer to the surface of the raw glass-forming material. It will be appreciated that the displacement of glass batch material against the side walls  18  and roof  22  will adversely effect the refractory material and possibly shorten the operating life of the glass melting furnace  10 .  
         [0043]    In order to determine the proper maximum velocity for the fuel and oxygen flow a burner was vertically mounted and fired downwards into a bed of glass sand across which grooves had been made. While the burner was adjusted to different heights from the sand and burner retractions into the block (LBb) the firing rates at which sand movement was discerned was noted. The data from these experiments was compared against simulations run on a commercially available computational fluid dynamics code thus yielding a maximum velocity across the surface above which sand would be disturbed in the aforementioned experiments.  
                                                                     TABLE 1                           Maximum. Burner Firing Rates (MMBtu/Hour)                Height (feet)            L(Bb)   5 ft   6 ft   7 ft   8 ft                    13   3.9   4.4   5.4   6.2       11.5   4.9   5.0   6.2   6.8       9   5.5   6.1   6.4   7.1       6.5   6.4   7.2   7.4   8.1       4   6.9   8.8   8.3   9.1                  
 
         [0044]    From these experiments the maximum surface velocity was ascertained by comparison with the CFD models to be approximately 21 m/s. Due to variations in batch material, batch glazing and batch particle cohesion the exact maximum may differ from the above calculated maximum, therefore, it should be possible for one skilled in the art to vary the maximum velocity up to approximately 25 m/sec. To minimize disturbance and entrainment of the batch material, however, the maximum velocity should be kept below 30 m/sec.  
         [0045]    The maximum and minimum velocity of the fuel and of the oxygen of the oxygen-fuel burner  34  are also controlled to harness the maximum energy from the impinging flame  36  without damaging the surrounding refractory material. The maximum energy from the impinging flame  36  is achieved by minimizing the amount of heat released to the glass melting furnace  10  combustion space and maximizing the heat transfer to the raw-glass forming material  30 . The operational maximum and minimum velocity range for the oxygen-fuel burner  34  to generate an acceptable heat transfer rate to the raw glass-forming material  30  without damaging the refractory material furnace walls and superstructure is a function of the design and location of the oxygen-fuel burner, burner block opening geometry, the velocities of the fuel and oxygen from the oxygen-fuel burner  34 , burner staging, interaction of adjacent oxygen-fuel burners, fuel burners and furnace exhaust.  
         [0046]    The second region, the stagnation region  56 , is the region where the flame  36  penetrates the thermal boundary layer and impinges upon the surface of the raw glass-forming material  30 . Within this region  56 , the flame  36  penetrates the thermal boundary layer and impinges on the surface of the raw glass-forming material building a sharp pressure gradient at the surface that accelerates the horizontal flow of the deflected flame causing the flame to spread outwardly radially along the impinged surface. The end of the stagnation region  56  is defined as the location on the surface of the raw glass-forming material where the pressure gradient generated by the impinging flame  36  drops to zero. Within the stagnation region  56 , by carefully controlling the momentum of flame  36 , the thermal boundary layer that naturally exists at the surface of the raw glass-forming material  30  is penetrated and eliminated and thus its strong heat resistive features are attenuated. Accordingly, the heat generated by the impinging flame  36  penetrates more easily into the partially melted raw glass-forming material  30 . Furthermore, within the stagnation region  56  the flame  36  luminosity significantly increases which enhances the radiation heat transfer into the relatively colder raw glass-forming material  30 .  
         [0047]    At the radial limits of the stagnation region  56  the wall jet region  58  begins. In this region, the flame  36  flows essentially parallel to the impinged surface and the thermal boundary layer grows along the impingement surface and outward from the stagnation region  56 , thus the thermal boundary layer starts to build up restoring the surface resistance to the heat flow into the raw glass-forming material surface.  
         [0048]    The controlled flame heat generation in the free-jet region  54  is the result of the design of the oxygen-fuel burner  34 , inside diameter of the opening (id) of the burner block  38  and both the relative velocities and maximum and minimum velocities of the oxygen and fuel streams. By selectively controlling the design of the oxygen-fuel burner  34 , the burner block  38  geometrical design and the velocities of the oxygen and fuel streams a reduced shear stress between the oxygen and gas streams is produced providing controlled partial combustion and reduced thermal radiation emissions. It will be appreciated that by optimizing burner design and operation of the oxygen-fuel burner  34 , the flame heat generated in the free jet region  54  and the heat transfer resistance at the raw glass surface in the stagnation region  56  are minimized thereby maximizing the heat generated in the stagnation region.  
         [0049]    The heat generated in the free-jet region  54  is the result of the following processes. First, the controlled partial combustion in the free-jet region  54  permits controlled combustion at the surface of the raw glass-forming material  30  thereby bringing the combustion process proximate to the surface of the raw glass-forming material. Bringing the combustion process proximate the surface of the raw glass-forming material  30  generates an elevated temperature gradient at the surface of the raw glass-forming material thereby improving the convection heat transfer. Second, the controlled partial combustion in the free-jet region  54  generates an acceptable temperature for the chemical dissociation of the combustion gases and the products of combustion. These dissociated species, once impinged on the relatively colder surface of the raw glass-forming material  30 , partially recombine, exothermically, generating significant heat at the surface of the raw glass-forming material. The heat from the exothermic reactions further augments the convective heat transfer process. The minimization of the heat resistance at the stagnation region  56  of the surface of the raw glass-forming material  30  is the result of the following factors.  
         [0050]    First, the thermal boundary layer is eliminated through the controlled flame  36  momentum and the turbulence generated by the carefully controlled combustion characteristics at the surface of the raw glass-forming material  30 . Second, the localized surface heat generation allows for the conversion of the low thermal conductive raw glass-forming material  30  into a significantly better conductive molten glass material. This conversion allows for the heat generated at the surface to penetrate more efficiently into the raw glass-forming material depth.  
         [0051]    In the cross-fired regenerative furnace FIG. 2A with regenerators  81  the preferred embodiment of the present invention utilizes at least one crown mounted burner  34  positioned over the raw batch materials entering the furnace to improve the rate of melting and improvement in quality to recover or boost production capacity or reduce electric boost capacity. Crown-mounted burner  34  impinges the surface of the batch material  30  in impingement area  26 . In all cross-fired regenerative furnace applications of this invention at least one pair of the opposing ports  71  will be fully or partially blocked or isolated. This will typically be the first port and perhaps the second port dependent on the amount of boost required. Additional roof-mounted burners can be located down the glass tank provided the crown mounted burners are positioned over the unmelted batch materials. The energy deliberated from the crown mounted burners replaces energy removed from the previously firing ports, the removed conventional electric or oxy-boost.  
         [0052]    In the end-fired regenerative furnace of FIG. 2B with regenerators  81  the preferred embodiment of the present invention utilizes at least one crown mounted burners  34  positioned over the raw batch materials entering the furnace to improve the rate of melting and improvement in quality to recover or boost production capacity or reduce electric boost capacity. In all end-fired regenerative furnace applications of this invention the combustion air and conventional fuel requirements will be reduced from the previous design and replaced with energy from the at least one crown mounted burners  34  positioned over the raw batch materials and impinging the batch materials in impingement area  26 . Additional roof-mounted burners can be located down the glass tank provided the crown mounted burners are positioned over the unmelted batch materials. The energy deliberated from the crown mounted burners replaces energy reduced from the firing port, the removed conventional electric or oxy-boost.  
         [0053]    In the cross-fired recuperative furnace of FIG. 2C with recuperator  82  the preferred embodiment of the present invention utilizes at least one roof mounted burner  34  positioned over the raw batch materials entering the furnace to improve the rate of melting and improvement in quality to recover or boost production capacity or reduce electric boost capacity. In all cross-fired recuperative furnace applications of this invention at least one pair of the opposing burners  73  will be fully or partially blocked or isolated using a block  74 . This will typically be the first zone of burners and perhaps the second zone dependent on the amount of boost required. Additional roof-mounted burners can be located down the glass tank provided the crown mounted burners are positioned over the unmelted batch materials. The energy deliberated from the crown mounted burners replaces energy removed from the previously firing ports, the removed conventional electric or oxy-boost.  
         [0054]    In the end-fired recuperative furnace of FIG. 2D with recuperator  82  the preferred embodiment of the present invention utilizes at least one crown mounted burner  34  positioned over the raw batch materials entering the furnace to improve the rate of melting and improvement in quality to recover or boost production capacity or reduce electric boost capacity. In all end-fired recuperative furnace applications of this invention the combustion air and conventional fuel requirements will be reduced from the previous design and replaced with energy from the at least one crown mounted burner  34  positioned over the raw batch materials. Additional roof-mounted burners can be located down the glass tank provided the crown mounted burners are positioned over the unmelted batch materials. The energy deliberated from the crown mounted burners replaces energy reduced from the firing port, the removed conventional electric or oxy-boost.  
         [0055]    In the direct-fired furnace of FIG. 2E The preferred embodiment of the present invention utilizes at least one crown mounted burners positioned over the raw batch materials entering the furnace to improve the rate of melting and improvement in quality to recover or boost production capacity or reduce electric boost capacity. In all direct fired furnace applications of this invention the combustion air and conventional fuel requirements will be reduced from the previous design and replaced with energy from the at least one crown mounted burners positioned over the raw batch materials. In multi-air-fuel-burner  73  applications at least one burner  74  will be isolated. Additional roof-mounted burners can be located down the glass tank provided the crown mounted burners are positioned over the unmelted batch materials. The energy deliberated from the crown mounted burners replaces energy reduced from the firing port, the removed conventional electric or oxy-boost.  
         [0056]    In an electric hot top furnace the preferred embodiment of the present invention utilizes at least one crown mounted burners positioned over the raw batch materials entering the furnace to improve the rate of melting and improvement in quality to recover or boost production capacity or reduce electric boost capacity. Additional roof-mounted burners can be located down the glass tank provided the crown mounted burners are positioned over the unmelted batch materials. The energy deliberated from the crown mounted burners replaces energy reduced from the firing port, the removed conventional electric or oxy-boost.  
         [0057]    In all cases nitrogen oxides and sulfur dioxide can be reduced by the careful selection of stoichiometric ratio of the different roof-mounted burners and remaining air fuel burners. Referring to FIG. 2A as an example in the cross-fired furnace application., the burners  34  mounted in the AL or AR positions are operated with excess stoichiometric oxygen to create a fuel lean (oxidizing) zone in the furnace. By operating either burner  34  at position BC and or burners at the second port  71  with less than stoichiometric oxygen or air creates a fuel rich (reducing) zone in the furnace. The remaining ports are operated with excess stoichiometric oxygen to create a fuel lean (oxidizing) zone in the furnace. This rich-lean-rich configuration effectively stages the combustion zones of the furnace to optimize heat transfer and minimize nitrogen oxide formation by creating a carbon monoxide screen.  
         [0058]    The at least one roof mounted oxygen-fuel burner  34  may be either placed in a new air-fuel glass melter furnace  10  or retrofitted into an existing air-fuel glass melter furnace to increase the glass quality relative to an air-fuel only fired furnace. It will be appreciated that the present invention facilitates a substantial pull rate increase, reduction in glass melting furnace  10  wall temperature and improved glass quality as compared to the same air-fuel furnace that is not retrofitted with at least one roof mounted oxygen-fuel burner as described herein. Furthermore, as will be readily appreciated by one skilled in the art, the use of at least one oxygen-fuel burner as opposed to an all air-fuel system appreciably reduces NO x  emissions.  
         [0059]    One demonstration of this invention was the oxygen boosting, 100% oxygen conversion, re-conversion to oxygen boost and finally conventional air fuel firing of an existing hot 3-port cross-fired regenerative furnace. The furnace was initially firing all air fuel. Port#1 firing was replaced with at least one roof mounted oxygen fuel burner. The furnace fired conventionally air fuel regenerative on remaining two ports. Port#2 firing was replaced with at least one roof mounted air fuel burner and the furnace fired conventionally air fuel regenerative on the third port. Port#3 firing was replaced with energy in the already installed roof-mounted oxy-fuel burners. The furnace capacity was increased from 55 to 85 tons per day with reduced energy input from 23.5 mmBTU/hr to 18 mmBTU/hr. The furnace was re-converted to air fuel firing in incremental stages. This example demonstrates the ability to selectively boost an existing air fuel furnace. The process does not require water-cooled burners.  
         [0060]    The patents and documents described herein are hereby incorporated by reference.  
         [0061]    Although the invention has been described in detail with reference to certain specific embodiments, those skilled in the art will recognize that there are other embodiments within the spirit and scope of the claims.