Title of Invention	METHOD AND APPARATUS FOR BACKING-UP OXY-FUEL COMBUSTION WITH AIR-FUEL COMBUSTION
Abstract	A process for maintaining heating of a furnace to an elevated temperature using oxy-fuel combustion, wherein an oxy-fuel flame is introduced into said furnace and an oxidizer is introduced underneath said flame, when oxygen supply for said flame and said oxidizer is eliminated or terminated comprising the steps of replacing said oxy- fuel flame with one of air or oxygen-enriched air and introducing said one of air or oxygen enriched air into said furnace; and replacing said oxidizer with said fuel and introducing said fuel into said furnace to provide combustion and maintain said temperature in said furnace.

Title of Invention

METHOD AND APPARATUS FOR BACKING-UP OXY-FUEL COMBUSTION WITH AIR-FUEL COMBUSTION

Abstract

A process for maintaining heating of a furnace to an elevated temperature using oxy-fuel combustion, wherein an oxy-fuel flame is introduced into said furnace and an oxidizer is introduced underneath said flame, when oxygen supply for said flame and said oxidizer is eliminated or terminated comprising the steps of replacing said oxy- fuel flame with one of air or oxygen-enriched air and introducing said one of air or oxygen enriched air into said furnace; and replacing said oxidizer with said fuel and introducing said fuel into said furnace to provide combustion and maintain said temperature in said furnace.

Full Text	BACKGROUND OF THE INVENTION The present invention pertains to oxy-fuel methods and devices for producing elevated temperatures in industrial melting pumices for such diverse products as metals, glass, ceramic materials and the like. In particular, the present invention pertains to combustion and methods and apparatus for continuation of combustion in the event of curtailed or terminated availability of oxygen for the oxy-fuel process. Use of oxy-feel burners in industrial processes such as glass melting, permits the furnace designer to achieve varying flame momentum, glass melt coverage, and flame radiation characteristics. Examples of such bumers and combustion processes are described in U.S. Patents 5,256,058, 5,346,390, 5,547,368, and 5,575,637 the disclosures of which are incorporated herein by reference. One particularly effective process and apparatus for utilizing oxy-fuel combustion in the manufacture of glass concems staged combustion, which is disclosed in U.S. Patent 5,611,682, the specification of which is incorporated herein by reference. In the beginning of the 1990s, glass manufacturers began converting furnaces from air-fuel combustion to oxy-fuel combustion. Oxygen enrichment of some air-fuel systems has been accomplished where the oxygen concentration is increased up to about 30%. Higher oxygen concentrations in the range of 40-80% are not used because of the increased potential for forming NOx pollutants. It has also been found that using oxy-fuel combustion where oxygen is present in a concentration of between 90-100% results in more favorable economics for the user. Many of the larger oxy-fuel glass furnaces are supplied by oxygen generated on site using well-known cryogenic or vacuum swing adsorption techniques. It is customary and, to date, the only method for backing up the supply of on-site generated oxygen is to keep an inventory of a liquid oxygen at the same site. Thus, when the on-site generation facility is taken off-line either due to a process problem or for routine maintenance, the inventory of liquid oxygen is utilized to supply the oxygen for the oxy-fuel combustion. This method of backing up the on-site generated oxygen requires large insulated tanks for storage of the oxygen in liquid form and vaporizers to enable the liquid oxygen to be converted into gaseous oxygen for use in the oxy-fuel process. It is conventional to utilize trucks to haul Uquid oxygen to the site from a larger air separation facility. Utilizing liquid oxygen back-up with an on-site generated oxygen system permits the user to continue using an oxy-fuel process without interruption. Any oxy-filial combustion system, e.g. one of those disclosed in the above-referenced patents, would benefit from on-site production having a back-up system. Until now, backing up oxy-fuel glass fumaces with an inventory of liquid oxygen has not been considered to be a problem. However, with the conversion of more and more fumaces at multi-pumice sites and the use of oxy-fuel combustion in flat or float glass fumaces which are much larger and use more oxygen, liquid oxygen backup becomes a significant concern to the user because of the high capital cost of storage tanks and vaporizers. In addition to the cost issue, a logistics problem arises relating to the transportation of the liquid oxygen to the site and having enough liquid oxygen available on short notice from a nearby air separation facility used to produce the liquid oxygen. Transportation of liquid oxygen to a user sites in remote locations become even a greater problem fraught with greater difficulties. Normally, when a glass female is converted from air-fuel to oxy-fuel, heat recovery devices such as regenerators and air supply systems are removed. For the user, one of the incentives to convert to oxy-fuel is reduced capital costs due to elimination of the heat recovery devices. Due to the design of oxy-fuel bumers, the fumes cannot be operated by simply substituting air for oxygen in conventional combustion systems in use today. The pressure requirement to provide an equivalent amount of contained oxygen using air in an oxy-fuel bummers would be extremely high, requiring an expensive air supply system. Further, some oxy-fuel bumers would be sonic flow limited if fired at an equivalent firing rate. When using oxy-fuel combustion where the oxygen supply is curtailed or disrupted, the conventional technique is to maintain the fumaces in a condition called "hot hold". Hot hold is a condition where production is stopped and the fumace is kept hot so that the glass does not solidify. Allowing the glass to solidify would severely damage the fumace. Several companies specialize in fumace heat-ups following cold fumace repairs. They use specially designed air-fuel bumers to provide the initial increase in temperature in the fumace. In case of oxygen supply dismption, the same bumers could be used to provide enough heating for hot hold. In this procedure, no special temperature profile for production would be attempted and the maximum temperature achieved by these devices could be about 2200°F. This temperature is not sufficient for production of glass and is the least preferred option to be used by glass manufacturers. The cost of not producing glass is very high to the glass manufacturer, in terms of lost product sales as well as dismption of downstream glass forming lines. Therefore, there is a definite need to provide a method and apparatus for maintaining production in a fumace used for glass manufacturing in the event of a curtailment or disruption in the availability of oxygen. BRIEF SUMMARY OF THE INVENTION The present invention pertains to a method and apparatus to backup an oxy-fuel combustion system with an air-fuel combustion system that can be used with or without oxygen enrichment to maintain production in an industrial fumace such as a glass melting furnace. According to the present invention, a system has been devised which permits operation in an oxy-fuel, and air-fuel, or an oxygen enriched air-fuel mode. The burner according to the present invention has a unique feature relating to operation at very low velocity for the oxy-fuel mode permitting acceptable pressure drops through the bumer when operating in an air-fuel mode. A burner according to the present invention can utilize oxygen enrichment to effect the process. According to the present invention, a conventional bumer block such as described in U.S. Patent 5,611,682 can be used for either oxy-fiiel or air-fuel combustion, allowing the combustion system to be rapidly converted between the two modes. According to the present invention, when a problem with oxygen supply occurs, the oxy-fuel bumers would be turned off, disconnected, and replaced by air-fuel backup bumers that have the same configuration for a connection to the burner block. With the air-fuel backup system, the user would retain the air supply systems from previous air-fiiel systems used in the melting operation or, air blowers would be supplied as part of the back up system. Air fuel bumers according to the present invention should be capable of firing at rates substantially higher than the oxy-fuel bumers. Thus, in one aspect the present invention is a process for maintaining heating of a furnace to an elevated temperature using oxy-fiiel combustion, wherein a flame is introduced into said furnace and an oxidizer is introduced underneath said flame, when oxygen supply for said flame and said oxidizer is eliminated or terminated comprising the steps of replacing the flame with one of air or oxygen-enriched air introduced at a rate to maintain approximately the bumer firing rate when oxygen is the only source of oxidant for combustion, and replacing the oxidizer introduced underneath said flame with said fuel to provide combustion and maintain said temperature in said furnace. In another aspect, the present invention is a combustion system of the type having an oxy-fuel bumer adapted to produce a flame with a pre-combustor mounted on the bumer, the pre-combustor having a first passage with a first end in fluid tight relation to a flame end of the bumer and a second end adapted to direct the flame produced by the bumer for heating in industrial environments in a generally flat fan like configuration and a second separate passage in the pre-combustor disposed beneath and coextensive with the first passage, the second passage terminating in a nozzle end in the second end of the pre-combustor to direct oxidizing fluid underneath and generally parallel to the flame, the improvement comprising; first means to introduce one of air or oxygen enriched air through the bumer into the pre-combustor in place of the flame, and second means to introduce fuel into the second separate passage in the pre-combustor in place of the oxidizing fluid, whereby the combustion system can continue to heat the industrial environment in the event supply of oxygen is interrupted or reduced. In yet another aspect, the present invention contemplates reducing exhaust gas volume in a furnace being heated according to the method and apparatus of the invention by liquid water cooling of the exhaust gases exiting the furnace. In still another aspect the present invention pertains to substitution of air-fuel combustion for oxy-fuel combustion to maintain heating in an industrial environment, the air or oxygen enriched air being introduced into the environment in any manner so there is sufficient volume to effect the required level of heating. In this aspect water cooling of the exhaust gases will be beneficial for lowering exhaust gas volume. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic perspective view of a conventional staged combustion apparatus. Figure 2 is a view taken along line 2-2 of Figure L Figure 3 is a schematic perspective view of an apparatus according to the present invention. Figure 4 is a front view of the bumer block or pre-combustor of the apparatus of Figure 3. Figure 5 is a plot of normalized methane flow rate against normalized oxygen flow rate for conditions from zero production to full production. Figure 6 is a plot of oxygen concentration against normalized oxygen flow rate for the production rates of Figure 5. Figure 7 is a plot of normalized exhaust gas flow rate against normalized oxygen flow rate for several production rates. Figure 8 is a plot of normalized exhaust gas flow rate after dilution with air against oxygen flow rate for production rates between zero and full production. Figure 9 is a plot of normalized exhaust flow rate after dilution with water against normalized oxygen flow rate for furnace production from zero to full production. DETAILED DESCRPTION OF THE INVENTION The present invention pertains to a method and apparatus to back-up an oxy-fuel heating system with an air-fuel heating system. According to the invention, the back-up air-fuel system can be operated with or without oxygen enrichment of the air. The burner according to the invention permits at least two distinct modes of operation, e.g. oxy-fuel or air-fuel. Another feature of the burner is the operation at very low velocity for the oxy-fuel mode, thus permitting acceptable pressure drops through the burner when operating in the air-fuel mode with or without enrichment of the air with oxygen. For the purposes of this invention oxy-fiiel combustion is taken to mean combustion with 80% to 100% by volume oxygen. Oxygen enrichment is taken to mean between 22% and 80% by volume oxygen concentration. According to the invention the same burner block or pre-combustor can be used during either oxy-fuel or air-fuel operation, allowing the combustion system to berapidly converted from one mode to the other. In the case where an operator encounters a problem with the oxygen supply, the oxy-fuel bumers would be turned off, disconnected, and replaced with air-fuel back-up bumers that have the same connection to the burner block. With an air-fuel back-up system, a glass manufacturer would retain its air supply systems present before conversion to oxy-fuel combustion, or blowers would be supplied as part of the back-up system. It is important that the air-fuel back-up bumers are capable of firing at a rate substantially higher than that of the oxy-fuel bumers being backed-up. Higher firing rate for the back-up air-fuel burner is required because of the additional energy losses caused by heating and expelling nitrogen. Furthermore, the air used for combustion in a back-up system will typically not be pre-heated which results in a decrease in fumace efficiency relative to a typical air-fiiel fiimace. A simplified thermodynamic calculation illustrates the need to increase the fuel firing rate when non preheated air is used for combustion. The assumptions for this example are: fuel and oxygen completely react with no excess oxygen and no intermediate products remaining; all gases (e.g. methane, air, or oxygen) enter the fumace at 77°F; and, all gases exhaust the fumace at 2800"^? after complete combustion. Under these conditions, 2.65 times the firing rate is required when firing with air as compared to firing with 100% oxygen in order to maintain the same available heat. Available heat is the energy transferred to the charge and for heat loss from the fumace. Thus, the total oxidant volumetric flow rate will increase dramatically as the oxygen flow rate is reduced. The volume of the oxidant stream is increased by a factor of 4.76 because of the addition of nitrogen and an additional 2,65 times because of the higher firing rate requirement. This means that the flow rate of the oxidant stream is increased by about 12.6 times when air is completely substituted for oxygen. A major concern with using air-fuel combustion in an oxy-fiiel burner assembly is the air supply pressure required to accommodate the higher gas volumes needed. The present invention utilizes a low velocity oxidizer system. Thus, even when firing in an air-fuel mode, pressure drop is low enough to permit the use of relatively inexpensive air blowers, while maintaining burner firing rates equal to or greater than those used with oxy-fuel firing. This, in turn, allows for continuity of production when a user, e.g. a glass melter, is operating in the back-up mode during an emergency loss or curtailment of oxygen supply. Oxy-fuel bumers with oxidant velocities greater than about 90 ft/sec. at any point in the burner, design will be sonic limited at the equivalent-firing rate when air is used asthe oxidant at fiill production. The sonic velocity is defined by the equation a where k is the ratio of specific heats (1.4 for air), R is the gas constant (287 J/kg K), and T is the absolute temperature. For air at 25°C (77°F), the sonic velocity is 346 m/sec. (1135 ft/sec). For an oxy fuel burner with an oxygen velocity of 100 ft/sec, the equivalent flow rate using air will be 12.6 times that amount or 1260 ft/sec. which is greater than the sonic velocity. Therefore, to avoid a sonic limit, the oxy-fiiel bumer must be designed with an oxygen velocity of less than 90 ft/sec. if complete air substitution for oxygen is to be used without changing any part of the bumer assembly, Altematively, this limit can be avoided according to one aspect of the invention where the bumer body is changed when switching between operating modes. The bumer block must be designed so that the superficial velocity is less than the sonic velocity for air-fuel operation. The shape of the flame is also a concern for traditional oxy-fuel burners operating at 2.65 times their rated firing capacity especially with 12.6 times the volumetric flow rate through the oxidant passage(s). The embodiment of the invention disclosed below provides a suitable flame shape for both oxy-fuel and air-fuel operation. Thus according to the invention, concerns relating to oxidant supply pressure, velocity limits, and flame shape are overcome according to the present invention. We have found it is possible to enable a user to switch from oxy-fuel firing to air-fuel firing using the same bumer block while modifying the bumer body of a Cleanfire® HR™ bumer offered to the trade by Air Products and Chemicals Inc. of Allentown Pennsylvania. Referring to Figure 1, a staged combustion apparatus 10 includes an oxy-fuel bumer 12 and a precombustor or bumer block 14. The oxy-fuel bumer 12 includes a central conduit 16 for receiving a fuel such as natural gas, which is indicated by arrow 18. A source of oxygen indicated by arrow 20 is introduced into a passage that is between the fuel conduit 16 and an outer concentric conduit 22. The bumer is described in detail in U.S. Patent 5,611,682, the specification of which has been incorporated herein by reference. The burner 12 is fitted to a bumer block 14 and held in fluid tight relationship thereto at a first end 24 of the precombustor or bumer block 14. The bumer block 14 contains a first or central passage 26, which extends from the first end 24 to a discharge end 28 of the burner block 14. The passage 26 has a width greater than the height and has a diverging shape as shown and as described in the '682 patent. In order to have stage combustion, staging oxygen represented by arrow 30 is introduced into a second passage 32 in the bumer block 14, The passage 32 has a shape complimentary to that of the central passage and has a greater width than height as illustrated and again as described in detail in the '682 patent. Referring to Figure 2, at the first end 24 of the precombustor 14 the oxy-fuel burner 12 has a discharge end with a central fuel conduit 16 surrounded by an oxygen passage 22. The staging oxygen exists a passage 31 that is disposed below the passage for the oxy-fuel flame as shown in Figure 2. Figure 3 shows a combustion apparatus according to the present invention. The combustion apparatus 40 includes a bumer block 14, which is identical to the bumer block 14 of Figure 1. According to the present invention, the bumer 42 is similar to tlie oxy-fuel bumer 12 of Figure 1 with a device 44 to permit introduction of air or oxygen enriched air into the upper passage 50 of bumer 42. The bumer 42 is also adapted to introduce air or oxygen enriched air by passage 48 of bumer 42 into upper passage 50 where oxidant from passages 44 and 48 are mixed, Arrow 46 represents introduction of air or air and oxygen into the device 44 which in tummy introduces the air or oxygen enriched air into the passage 50. Arrow 56 represents introduction of air or air and oxygen into the passage 50. The air or oxygen enriched air moves from passage 50 into the central passage 26 of the bumer block and exits to the furnace. When the bumer is converted over to non or limited oxy-fuel firing, the supply of staging oxygen (indicated by arrow 30 in Figure 1) is replaced by fuel, represented by arrow 54, so that fuel or oxygen enriched fuel exits passage 32 of the bumer block 14. Shown schematically in Figure 4 are the passages 26 and 32 at the front end of the bumer block 14 with passage 26 being used to introduce air or oxygen enriched air into the furnace and passage 32 being used to introduce fuel or oxygen enriched fuel into the fiimace. When the bumer 42 is used in the air-fuel firing mode, the air or oxygen enriched air flows through passage 26 and the fuel or oxygen enriched fuel flows through the passage 32. The bumer block design is such that a stable air-fuel flame is established because of the re-circulation region between the two openings. In addition to the air-fuel firing capability, the device of the present invention permits varying degrees of oxygen enrichment to be accomplished. Use of oxygen enrichment improves flexibility during operation in the backup mode by decreasing the use of oxygen supplied from liquid oxygen storage. It also permits adjustment of the flame length by adding oxygen to the air flow. Supplemental oxygen can be supplied by various methods. For example the air can be enriched with oxygen, oxygen lances could be supplied through either or both the primary passage 26 of pre-combustor 14 or the staging port 32, or separate oxygen lances could be installed at a distance away from the precombustor 14 or staging port 32. Oxygen introduced through the staging port with the natural gas could provide means to create soot for better radiation heat transfer to the fumace charge. Using the method and apparatus of the present invention makes it possible to maintain maximum temperature and temperature distribution needed for glass production. Oxygen enrichment or oxy-fuel firing, preferably should be used on burners with the highest firing rates near the hot spot in the fumace. This will reduce the flow rate of air needed for these burners and reduce the pressure drop. Also, oxygen enrichment increases the peak flame temperature and thereby increases heat transfers in the hot spot. It is well known that a hot spot is required in glass making furnaces to established proper convection cells in the glass men which are required to produce glass of acceptable quality. Other air-fuel technologies can be used to maintain hot hold conditions. This invention is intended to permit the user to continue production. The minimum firing rate provided by the air-fuel backup system is such that at least 20% of the design production rate can be maintained. It is believed that this production rate is sufficient to allow a float glass producer to maintain a continuous glass ribbon in the float bath. Higher velocity oxy-fuel burners could be modified for low velocity operation by adding one or more inlet ports to use the technology disclosed herein. These inlets could be normally closed or used for staging during oxy-fuel operation. Also, one or more additional inlet ports could be added on-the-fly prior to commencing air-fuel backup by drilling a hole in the refractory wall in a location close to the burner port. Another alternative for fumaces using high velocity burners is to replace the burner blocks with blocks having larger openings to reduce the pressure drop. With this method there is the danger of introducing foreign refractory material into the glass melt during the replacement procedure which could cause glass defects. Furthermore, replacement of burner blocks on the fly requires substantial time, possibly too long to avoid interruption of production. Figure 5 shows the methane flow rate required for hot hold conditions (zero production rate), 20%, 50% and full production conditions, assuming, for example, that 35% of the available heat is required for furnace wall heat losses under full production conditions. Hot hold could be achieved at lower firing rates than the plot shows since the overall furnace temperature, would be lowered thereby reducing the wall heat losses. This plot assumes that the heat losses remain the same, independent of production rate or oxygen usage. The methane flow rate is normalized based on the methane flow rate for full production with 100% oxy-fuel, and the oxygen flow rate is normalized based on the oxygen flow rate for full production with 100% oxy-fuel. The normalized oxygen flow rate is 1.0 when all of the oxidant for combustion is supplied by the oxygen source (no air) and zero when all of the oxidant for combustion is supplied by air. Figure 6 is a corresponding plot of oxygen concentration as a function of normalized oxygen flow rate for each of the production rates shown in Figure 5. As indicated by point A in Figure 5, hot hold using only air as the oxidant for combustion (zero normalized oxygen flow rate), the methane flow rate is about the same as required for 100% oxy-fuel at full production (normalized value equal to 1). Hot hold could also be maintained at 35% of the full production oxygen flow rate with 35% of the full production methane flow rate (point B). Referring to Figure 6 (point B), the operating condition represented by point B corresponds to 100% oxy-fuel with no air dilution. Figure 5 shows that the oxygen flow rate and the methane flow rate can each be reduced by half to produce 20% of full production. This means that if production is limited to 20% of the full production rate, the stored oxygen supply can last two times longer. According to Figure 6, this corresponds to 100% oxy-fuel firing. At 50% production, the oxygen flow rate could be reduced to half of the full production flow rate and the methane at about 95% of the fiill production The exhaust gas temperature from an oxy-fuel furnace is higher than a corresponding air-fuel furnace after the heat recovery device. Glass manufacturers therefore must decrease the temperature of the oxy-fuel combustion products by some method before the gases enter the sections of the flue system fabricated with metal. Because of current air pollution regulations, fuel gas treatment for glass furnaces typically include particulate removal devices, such as electrostatic precipitators or bag houses. These devices have a maximum operating temperature significantly lower than the oxy-fuel fumace exhaust temperature, typically around 1000°F. Therefore, exhaust gases must be cooled by cold (ambient) dilution air before these devices. If air is substituted for oxygen for combustion in a fumace designed for oxy-fuel combustion, the exhaust volume will be increased substantially. Figure 7 shows how the exhaust flow rate increases as air is substituted for oxygen for several production rates. The same assumptions regarding inlet and outlet temperatures and heat losses used for the previous figures are used to generate this figure. The exhaust flow rate is normalized with respect to the exhaust flow rate for full production with 100% oxy-fuel. For full production, the exhaust flow rate will be increased by more than nine times if oxygen is completely replaced by air. More than three times the exhaust flow rate can be expected at hot hold conditions where air completely replaces oxygen. As a result of the increased flow of hot exhaust gases, much more dilution air must be provided to decrease the temperature to the same level before the gases enter the metal section of the flue system. Figure 8 shows the result of thermodynamic calculations where fumace exhaust gases at 2800°F are diluted with air at IW to produce a 1000°F gas stream which is a temperature suitable for the metal section of the flue system. The normalized exhaust flow rate after dilution with air is plotted as a function of normalized oxygen flow rate. The exhaust flow is normalized with respect to the 100% oxy-fuel, full production case where exhaust gases at 2800°F are diluted with air at 77°F to produce a 1000°F gas stream. If air is substituted for oxygen under full production conditions and the exhaust gases are diluted with air at 77'^F to produce a logoff stream, the resulting exhaust gas flow rate would be greater than 7,5 times the full production oxy-fuel case. Flue systems are not capable of handling this much of an increase in throughput because of pressure drop limitations. The fumace pressure would increase, possibly leading to structural failure. There are several ways of dealing with the increased flue gas volume: e.g., reduced production, oxygen enrichment for combustion, alternative ways of cooling the flue gases (e.g. with water), using additional flue gas exhaust capability, bypassing the flue gas treatment section, or a combination of two or more of these above methods. A preferred method of resolving the increased volume of flue gases, in accord with the present invention, is to combine water cooling, reduced production, and if necessary oxygen enrichment for combustion. Figure 9 shows the results of a thermodynamic calculation where liquid water at 77°F provides evaporative direct contact cooling. The normalized exhaust flow rate after dilution with water is plotted against the normalized oxygen flow rate. The exhaust flow is normalized with respect to the 100% oxy-fuel, full production case where exhaust gases at 2800°F are diluted with air at 77°F to produce a 1000°F gas stream. The figure shows that the exhaust gas volume can be reduced by 50% for full production oxy-fuel operation when water is substituted for air as the cooling medium in the exhaust stream. For the case of full production using air instead of oxygen for combustion and water is used for cooling the exhaust gases, the exhaust flow rate is 3.6 times the base case full production, full oxy-fuel case. For 50% production using air instead of oxygen as the oxidant, the exhaust stream volume is about 2.5 times greater than the base case full production, full oxy-fuel case. Alternatives to the proposed invention are: Option 1) continued 100% oxy-fuel firing with more oxygen storage, Option 2) hot hold with air-fuel heat up burners, and Option 3) hot hold or some production with high momentum oxy-fuel bumers using air instead of oxygen. The difference between the proposed invention and option 1 is reduced use of oxygen and expense of storage of liquid oxygen. The difference between the invention and option 2 is continued production and expense. The difference between the invention and option 3 is the technical difficulty of supplying air with a high pressure. The benefit of the invention compared to option 1 is lower capital cost (fewer LOX storage tanks). Also, depending on the length of time that the on-site oxygen plant is down, the logistics and availability problems of liquid oxygen are avoided. A benefit of the proposed invention over option 1 is that it can function if there is a problem with the oxygen supply lines or flow control skids. Another benefit of the invention compared to option 2 is higher maximum temperature in the furnace with similar temperature profile needed for glass production. Yet another benefit of the invention compared to option 2 is continued production. The most effective process is where full production is continued using air or oxygen enriched air. Even production at a minimum level to sustain a glass ribbon in the float bath is extremely valuable. Reestablishing the glass ribbon is time consuming and could delay production by one or more days. For example, for a flat glass fiimace, that produces 600 tons/day, and with glass valued at $300/ton, one days production is worth $180,000. A further benefit of the invention compared to option 2 is that the back-up system is in place. Option 2 requires that an outside company must come to the facility and install their equipment. A still further benefit of the invention is that the fumace refractory does not need to be drilled, cut, or otherwise disturbed. The present invention provides the user the ability to use different burners for air-fuel and oxy-fuel operation, a common mounting system for air-fuel and oxy-flier burners, higher maximum fumace temperatures compared to air-fuel heat up burners. The process of the present invention is capable of generating similar temperature distribution in a fumace needed for glass processing, permit higher firing rates at the fumace hot spot by preferentially increasing oxygen concentration, use of separate but closely spaced ports for introduction air and fuel for air-fuel operation, change function of pre-combustor/staging ports for air-fuel and oxy-fuel operation. For oxy-fuel operation, the larger opening is used as a precombustor with oxygen and fuel flow and the smaller opening for oxygen staging. For air-fuel operation, the larger opening is used for flowing air or oxygen enriched air and the smaller port primarily for fuel. It is within the scope of the present invention to have a separate burner block or pre-combustor placed in the fumace wall to introduce air or oxygen enriched air and fuel into the fumace. In this mode the oxy-flier burner would be turned off and the separate burner block would be used to effect combustion according to the teaching of the invention. It is also within the scope of the present invention to introduce air and fuel into the fumace through separate burners or pipes that are independent of the oxy-fuel burners, so long as the air or oxygen enriched air and fuel are introduced in accord with the teachings of the invention. Having thus described our invention what is desired to be secured by Letters Patent of the United States is set forth in the appended claims. BACKGROUND OF THE INVENTION The present invention pertains to oxy-fuel methods and devices for producing elevated temperatures in industrial melting pumices for such diverse products as metals, glass, ceramic materials and the like. In particular, the present invention pertains to combustion and methods and apparatus for continuation of combustion in the event of curtailed or terminated availability of oxygen for the oxy-fuel process. Use of oxy-feel burners in industrial processes such as glass melting, permits the furnace designer to achieve varying flame momentum, glass melt coverage, and flame radiation characteristics. Examples of such bumers and combustion processes are described in U.S. Patents 5,256,058, 5,346,390, 5,547,368, and 5,575,637 the disclosures of which are incorporated herein by reference. One particularly effective process and apparatus for utilizing oxy-fuel combustion in the manufacture of glass concems staged combustion, which is disclosed in U.S. Patent 5,611,682, the specification of which is incorporated herein by reference. In the beginning of the 1990s, glass manufacturers began converting furnaces from air-fuel combustion to oxy-fuel combustion. Oxygen enrichment of some air-fuel systems has been accomplished where the oxygen concentration is increased up to about 30%. Higher oxygen concentrations in the range of 40-80% are not used because of the increased potential for forming NOx pollutants. It has also been found that using oxy-fuel combustion where oxygen is present in a concentration of between 90-100% results in more favorable economics for the user. Many of the larger oxy-fuel glass furnaces are supplied by oxygen generated on site using well-known cryogenic or vacuum swing adsorption techniques. It is customary and, to date, the only method for backing up the supply of on-site generated oxygen is to keep an inventory of a liquid oxygen at the same site. Thus, when the on-site generation facility is taken off-line either due to a process problem or for routine maintenance, the inventory of liquid oxygen is utilized to supply the oxygen for the oxy-fuel combustion. This method of backing up the on-site generated oxygen requires large insulated tanks for storage of the oxygen in liquid form and vaporizers to enable the liquid oxygen to be converted into gaseous oxygen for use in the oxy-fuel process. It is conventional to utilize trucks to haul Uquid oxygen to the site from a larger air separation facility. Utilizing liquid oxygen back-up with an on-site generated oxygen system permits the user to continue using an oxy-fuel process without interruption. Any oxy-filial combustion system, e.g. one of those disclosed in the above-referenced patents, would benefit from on-site production having a back-up system. Until now, backing up oxy-fuel glass fumaces with an inventory of liquid oxygen has not been considered to be a problem. However, with the conversion of more and more fumaces at multi-pumice sites and the use of oxy-fuel combustion in flat or float glass fumaces which are much larger and use more oxygen, liquid oxygen backup becomes a significant concern to the user because of the high capital cost of storage tanks and vaporizers. In addition to the cost issue, a logistics problem arises relating to the transportation of the liquid oxygen to the site and having enough liquid oxygen available on short notice from a nearby air separation facility used to produce the liquid oxygen. Transportation of liquid oxygen to a user sites in remote locations become even a greater problem fraught with greater difficulties. Normally, when a glass female is converted from air-fuel to oxy-fuel, heat recovery devices such as regenerators and air supply systems are removed. For the user, one of the incentives to convert to oxy-fuel is reduced capital costs due to elimination of the heat recovery devices. Due to the design of oxy-fuel bumers, the fumes cannot be operated by simply substituting air for oxygen in conventional combustion systems in use today. The pressure requirement to provide an equivalent amount of contained oxygen using air in an oxy-fuel bummers would be extremely high, requiring an expensive air supply system. Further, some oxy-fuel bumers would be sonic flow limited if fired at an equivalent firing rate. When using oxy-fuel combustion where the oxygen supply is curtailed or disrupted, the conventional technique is to maintain the fumaces in a condition called "hot hold". Hot hold is a condition where production is stopped and the fumace is kept hot so that the glass does not solidify. Allowing the glass to solidify would severely damage the fumace. Several companies specialize in fumace heat-ups following cold fumace repairs. They use specially designed air-fuel bumers to provide the initial increase in temperature in the fumace. In case of oxygen supply dismption, the same bumers could be used to provide enough heating for hot hold. In this procedure, no special temperature profile for production would be attempted and the maximum temperature achieved by these devices could be about 2200°F. This temperature is not sufficient for production of glass and is the least preferred option to be used by glass manufacturers. The cost of not producing glass is very high to the glass manufacturer, in terms of lost product sales as well as dismption of downstream glass forming lines. Therefore, there is a definite need to provide a method and apparatus for maintaining production in a fumace used for glass manufacturing in the event of a curtailment or disruption in the availability of oxygen. BRIEF SUMMARY OF THE INVENTION The present invention pertains to a method and apparatus to backup an oxy-fuel combustion system with an air-fuel combustion system that can be used with or without oxygen enrichment to maintain production in an industrial fumace such as a glass melting furnace. According to the present invention, a system has been devised which permits operation in an oxy-fuel, and air-fuel, or an oxygen enriched air-fuel mode. The burner according to the present invention has a unique feature relating to operation at very low velocity for the oxy-fuel mode permitting acceptable pressure drops through the bumer when operating in an air-fuel mode. A burner according to the present invention can utilize oxygen enrichment to effect the process. According to the present invention, a conventional bumer block such as described in U.S. Patent 5,611,682 can be used for either oxy-fiiel or air-fuel combustion, allowing the combustion system to be rapidly converted between the two modes. According to the present invention, when a problem with oxygen supply occurs, the oxy-fuel bumers would be turned off, disconnected, and replaced by air-fuel backup bumers that have the same configuration for a connection to the burner block. With the air-fuel backup system, the user would retain the air supply systems from previous air-fiiel systems used in the melting operation or, air blowers would be supplied as part of the back up system. Air fuel bumers according to the present invention should be capable of firing at rates substantially higher than the oxy-fuel bumers. Thus, in one aspect the present invention is a process for maintaining heating of a furnace to an elevated temperature using oxy-fiiel combustion, wherein a flame is introduced into said furnace and an oxidizer is introduced underneath said flame, when oxygen supply for said flame and said oxidizer is eliminated or terminated comprising the steps of replacing the flame with one of air or oxygen-enriched air introduced at a rate to maintain approximately the bumer firing rate when oxygen is the only source of oxidant for combustion, and replacing the oxidizer introduced underneath said flame with said fuel to provide combustion and maintain said temperature in said furnace. In another aspect, the present invention is a combustion system of the type having an oxy-fuel bumer adapted to produce a flame with a pre-combustor mounted on the bumer, the pre-combustor having a first passage with a first end in fluid tight relation to a flame end of the bumer and a second end adapted to direct the flame produced by the bumer for heating in industrial environments in a generally flat fan like configuration and a second separate passage in the pre-combustor disposed beneath and coextensive with the first passage, the second passage terminating in a nozzle end in the second end of the pre-combustor to direct oxidizing fluid underneath and generally parallel to the flame, the improvement comprising; first means to introduce one of air or oxygen enriched air through the bumer into the pre-combustor in place of the flame, and second means to introduce fuel into the second separate passage in the pre-combustor in place of the oxidizing fluid, whereby the combustion system can continue to heat the industrial environment in the event supply of oxygen is interrupted or reduced. In yet another aspect, the present invention contemplates reducing exhaust gas volume in a furnace being heated according to the method and apparatus of the invention by liquid water cooling of the exhaust gases exiting the furnace. In still another aspect the present invention pertains to substitution of air-fuel combustion for oxy-fuel combustion to maintain heating in an industrial environment, the air or oxygen enriched air being introduced into the environment in any manner so there is sufficient volume to effect the required level of heating. In this aspect water cooling of the exhaust gases will be beneficial for lowering exhaust gas volume. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic perspective view of a conventional staged combustion apparatus. Figure 2 is a view taken along line 2-2 of Figure L Figure 3 is a schematic perspective view of an apparatus according to the present invention. Figure 4 is a front view of the bumer block or pre-combustor of the apparatus of Figure 3. Figure 5 is a plot of normalized methane flow rate against normalized oxygen flow rate for conditions from zero production to full production. Figure 6 is a plot of oxygen concentration against normalized oxygen flow rate for the production rates of Figure 5. Figure 7 is a plot of normalized exhaust gas flow rate against normalized oxygen flow rate for several production rates. Figure 8 is a plot of normalized exhaust gas flow rate after dilution with air against oxygen flow rate for production rates between zero and full production. Figure 9 is a plot of normalized exhaust flow rate after dilution with water against normalized oxygen flow rate for furnace production from zero to full production. DETAILED DESCRPTION OF THE INVENTION The present invention pertains to a method and apparatus to back-up an oxy-fuel heating system with an air-fuel heating system. According to the invention, the back-up air-fuel system can be operated with or without oxygen enrichment of the air. The burner according to the invention permits at least two distinct modes of operation, e.g. oxy-fuel or air-fuel. Another feature of the burner is the operation at very low velocity for the oxy-fuel mode, thus permitting acceptable pressure drops through the burner when operating in the air-fuel mode with or without enrichment of the air with oxygen. For the purposes of this invention oxy-fiiel combustion is taken to mean combustion with 80% to 100% by volume oxygen. Oxygen enrichment is taken to mean between 22% and 80% by volume oxygen concentration. According to the invention the same burner block or pre-combustor can be used during either oxy-fuel or air-fuel operation, allowing the combustion system to berapidly converted from one mode to the other. In the case where an operator encounters a problem with the oxygen supply, the oxy-fuel bumers would be turned off, disconnected, and replaced with air-fuel back-up bumers that have the same connection to the burner block. With an air-fuel back-up system, a glass manufacturer would retain its air supply systems present before conversion to oxy-fuel combustion, or blowers would be supplied as part of the back-up system. It is important that the air-fuel back-up bumers are capable of firing at a rate substantially higher than that of the oxy-fuel bumers being backed-up. Higher firing rate for the back-up air-fuel burner is required because of the additional energy losses caused by heating and expelling nitrogen. Furthermore, the air used for combustion in a back-up system will typically not be pre-heated which results in a decrease in fumace efficiency relative to a typical air-fiiel fiimace. A simplified thermodynamic calculation illustrates the need to increase the fuel firing rate when non preheated air is used for combustion. The assumptions for this example are: fuel and oxygen completely react with no excess oxygen and no intermediate products remaining; all gases (e.g. methane, air, or oxygen) enter the fumace at 77°F; and, all gases exhaust the fumace at 2800"^? after complete combustion. Under these conditions, 2.65 times the firing rate is required when firing with air as compared to firing with 100% oxygen in order to maintain the same available heat. Available heat is the energy transferred to the charge and for heat loss from the fumace. Thus, the total oxidant volumetric flow rate will increase dramatically as the oxygen flow rate is reduced. The volume of the oxidant stream is increased by a factor of 4.76 because of the addition of nitrogen and an additional 2,65 times because of the higher firing rate requirement. This means that the flow rate of the oxidant stream is increased by about 12.6 times when air is completely substituted for oxygen. A major concern with using air-fuel combustion in an oxy-fiiel burner assembly is the air supply pressure required to accommodate the higher gas volumes needed. The present invention utilizes a low velocity oxidizer system. Thus, even when firing in an air-fuel mode, pressure drop is low enough to permit the use of relatively inexpensive air blowers, while maintaining burner firing rates equal to or greater than those used with oxy-fuel firing. This, in turn, allows for continuity of production when a user, e.g. a glass melter, is operating in the back-up mode during an emergency loss or curtailment of oxygen supply. Oxy-fuel bumers with oxidant velocities greater than about 90 ft/sec. at any point in the burner, design will be sonic limited at the equivalent-firing rate when air is used asthe oxidant at fiill production. The sonic velocity is defined by the equation a where k is the ratio of specific heats (1.4 for air), R is the gas constant (287 J/kg K), and T is the absolute temperature. For air at 25°C (77°F), the sonic velocity is 346 m/sec. (1135 ft/sec). For an oxy fuel burner with an oxygen velocity of 100 ft/sec, the equivalent flow rate using air will be 12.6 times that amount or 1260 ft/sec. which is greater than the sonic velocity. Therefore, to avoid a sonic limit, the oxy-fiiel bumer must be designed with an oxygen velocity of less than 90 ft/sec. if complete air substitution for oxygen is to be used without changing any part of the bumer assembly, Altematively, this limit can be avoided according to one aspect of the invention where the bumer body is changed when switching between operating modes. The bumer block must be designed so that the superficial velocity is less than the sonic velocity for air-fuel operation. The shape of the flame is also a concern for traditional oxy-fuel burners operating at 2.65 times their rated firing capacity especially with 12.6 times the volumetric flow rate through the oxidant passage(s). The embodiment of the invention disclosed below provides a suitable flame shape for both oxy-fuel and air-fuel operation. Thus according to the invention, concerns relating to oxidant supply pressure, velocity limits, and flame shape are overcome according to the present invention. We have found it is possible to enable a user to switch from oxy-fuel firing to air-fuel firing using the same bumer block while modifying the bumer body of a Cleanfire® HR™ bumer offered to the trade by Air Products and Chemicals Inc. of Allentown Pennsylvania. Referring to Figure 1, a staged combustion apparatus 10 includes an oxy-fuel bumer 12 and a precombustor or bumer block 14. The oxy-fuel bumer 12 includes a central conduit 16 for receiving a fuel such as natural gas, which is indicated by arrow 18. A source of oxygen indicated by arrow 20 is introduced into a passage that is between the fuel conduit 16 and an outer concentric conduit 22. The bumer is described in detail in U.S. Patent 5,611,682, the specification of which has been incorporated herein by reference. The burner 12 is fitted to a bumer block 14 and held in fluid tight relationship thereto at a first end 24 of the precombustor or bumer block 14. The bumer block 14 contains a first or central passage 26, which extends from the first end 24 to a discharge end 28 of the burner block 14. The passage 26 has a width greater than the height and has a diverging shape as shown and as described in the '682 patent. In order to have stage combustion, staging oxygen represented by arrow 30 is introduced into a second passage 32 in the bumer block 14, The passage 32 has a shape complimentary to that of the central passage and has a greater width than height as illustrated and again as described in detail in the '682 patent. Referring to Figure 2, at the first end 24 of the precombustor 14 the oxy-fuel burner 12 has a discharge end with a central fuel conduit 16 surrounded by an oxygen passage 22. The staging oxygen exists a passage 31 that is disposed below the passage for the oxy-fuel flame as shown in Figure 2. Figure 3 shows a combustion apparatus according to the present invention. The combustion apparatus 40 includes a bumer block 14, which is identical to the bumer block 14 of Figure 1. According to the present invention, the bumer 42 is similar to tlie oxy-fuel bumer 12 of Figure 1 with a device 44 to permit introduction of air or oxygen enriched air into the upper passage 50 of bumer 42. The bumer 42 is also adapted to introduce air or oxygen enriched air by passage 48 of bumer 42 into upper passage 50 where oxidant from passages 44 and 48 are mixed, Arrow 46 represents introduction of air or air and oxygen into the device 44 which in tummy introduces the air or oxygen enriched air into the passage 50. Arrow 56 represents introduction of air or air and oxygen into the passage 50. The air or oxygen enriched air moves from passage 50 into the central passage 26 of the bumer block and exits to the furnace. When the bumer is converted over to non or limited oxy-fuel firing, the supply of staging oxygen (indicated by arrow 30 in Figure 1) is replaced by fuel, represented by arrow 54, so that fuel or oxygen enriched fuel exits passage 32 of the bumer block 14. Shown schematically in Figure 4 are the passages 26 and 32 at the front end of the bumer block 14 with passage 26 being used to introduce air or oxygen enriched air into the furnace and passage 32 being used to introduce fuel or oxygen enriched fuel into the fiimace. When the bumer 42 is used in the air-fuel firing mode, the air or oxygen enriched air flows through passage 26 and the fuel or oxygen enriched fuel flows through the passage 32. The bumer block design is such that a stable air-fuel flame is established because of the re-circulation region between the two openings. In addition to the air-fuel firing capability, the device of the present invention permits varying degrees of oxygen enrichment to be accomplished. Use of oxygen enrichment improves flexibility during operation in the backup mode by decreasing the use of oxygen supplied from liquid oxygen storage. It also permits adjustment of the flame length by adding oxygen to the air flow. Supplemental oxygen can be supplied by various methods. For example the air can be enriched with oxygen, oxygen lances could be supplied through either or both the primary passage 26 of pre-combustor 14 or the staging port 32, or separate oxygen lances could be installed at a distance away from the precombustor 14 or staging port 32. Oxygen introduced through the staging port with the natural gas could provide means to create soot for better radiation heat transfer to the fumace charge. Using the method and apparatus of the present invention makes it possible to maintain maximum temperature and temperature distribution needed for glass production. Oxygen enrichment or oxy-fuel firing, preferably should be used on burners with the highest firing rates near the hot spot in the fumace. This will reduce the flow rate of air needed for these burners and reduce the pressure drop. Also, oxygen enrichment increases the peak flame temperature and thereby increases heat transfers in the hot spot. It is well known that a hot spot is required in glass making furnaces to established proper convection cells in the glass men which are required to produce glass of acceptable quality. Other air-fuel technologies can be used to maintain hot hold conditions. This invention is intended to permit the user to continue production. The minimum firing rate provided by the air-fuel backup system is such that at least 20% of the design production rate can be maintained. It is believed that this production rate is sufficient to allow a float glass producer to maintain a continuous glass ribbon in the float bath. Higher velocity oxy-fuel burners could be modified for low velocity operation by adding one or more inlet ports to use the technology disclosed herein. These inlets could be normally closed or used for staging during oxy-fuel operation. Also, one or more additional inlet ports could be added on-the-fly prior to commencing air-fuel backup by drilling a hole in the refractory wall in a location close to the burner port. Another alternative for fumaces using high velocity burners is to replace the burner blocks with blocks having larger openings to reduce the pressure drop. With this method there is the danger of introducing foreign refractory material into the glass melt during the replacement procedure which could cause glass defects. Furthermore, replacement of burner blocks on the fly requires substantial time, possibly too long to avoid interruption of production. Figure 5 shows the methane flow rate required for hot hold conditions (zero production rate), 20%, 50% and full production conditions, assuming, for example, that 35% of the available heat is required for furnace wall heat losses under full production conditions. Hot hold could be achieved at lower firing rates than the plot shows since the overall furnace temperature, would be lowered thereby reducing the wall heat losses. This plot assumes that the heat losses remain the same, independent of production rate or oxygen usage. The methane flow rate is normalized based on the methane flow rate for full production with 100% oxy-fuel, and the oxygen flow rate is normalized based on the oxygen flow rate for full production with 100% oxy-fuel. The normalized oxygen flow rate is 1.0 when all of the oxidant for combustion is supplied by the oxygen source (no air) and zero when all of the oxidant for combustion is supplied by air. Figure 6 is a corresponding plot of oxygen concentration as a function of normalized oxygen flow rate for each of the production rates shown in Figure 5. As indicated by point A in Figure 5, hot hold using only air as the oxidant for combustion (zero normalized oxygen flow rate), the methane flow rate is about the same as required for 100% oxy-fuel at full production (normalized value equal to 1). Hot hold could also be maintained at 35% of the full production oxygen flow rate with 35% of the full production methane flow rate (point B). Referring to Figure 6 (point B), the operating condition represented by point B corresponds to 100% oxy-fuel with no air dilution. Figure 5 shows that the oxygen flow rate and the methane flow rate can each be reduced by half to produce 20% of full production. This means that if production is limited to 20% of the full production rate, the stored oxygen supply can last two times longer. According to Figure 6, this corresponds to 100% oxy-fuel firing. At 50% production, the oxygen flow rate could be reduced to half of the full production flow rate and the methane at about 95% of the fiill production The exhaust gas temperature from an oxy-fuel furnace is higher than a corresponding air-fuel furnace after the heat recovery device. Glass manufacturers therefore must decrease the temperature of the oxy-fuel combustion products by some method before the gases enter the sections of the flue system fabricated with metal. Because of current air pollution regulations, fuel gas treatment for glass furnaces typically include particulate removal devices, such as electrostatic precipitators or bag houses. These devices have a maximum operating temperature significantly lower than the oxy-fuel fumace exhaust temperature, typically around 1000°F. Therefore, exhaust gases must be cooled by cold (ambient) dilution air before these devices. If air is substituted for oxygen for combustion in a fumace designed for oxy-fuel combustion, the exhaust volume will be increased substantially. Figure 7 shows how the exhaust flow rate increases as air is substituted for oxygen for several production rates. The same assumptions regarding inlet and outlet temperatures and heat losses used for the previous figures are used to generate this figure. The exhaust flow rate is normalized with respect to the exhaust flow rate for full production with 100% oxy-fuel. For full production, the exhaust flow rate will be increased by more than nine times if oxygen is completely replaced by air. More than three times the exhaust flow rate can be expected at hot hold conditions where air completely replaces oxygen. As a result of the increased flow of hot exhaust gases, much more dilution air must be provided to decrease the temperature to the same level before the gases enter the metal section of the flue system. Figure 8 shows the result of thermodynamic calculations where fumace exhaust gases at 2800°F are diluted with air at IW to produce a 1000°F gas stream which is a temperature suitable for the metal section of the flue system. The normalized exhaust flow rate after dilution with air is plotted as a function of normalized oxygen flow rate. The exhaust flow is normalized with respect to the 100% oxy-fuel, full production case where exhaust gases at 2800°F are diluted with air at 77°F to produce a 1000°F gas stream. If air is substituted for oxygen under full production conditions and the exhaust gases are diluted with air at 77'^F to produce a logoff stream, the resulting exhaust gas flow rate would be greater than 7,5 times the full production oxy-fuel case. Flue systems are not capable of handling this much of an increase in throughput because of pressure drop limitations. The fumace pressure would increase, possibly leading to structural failure. There are several ways of dealing with the increased flue gas volume: e.g., reduced production, oxygen enrichment for combustion, alternative ways of cooling the flue gases (e.g. with water), using additional flue gas exhaust capability, bypassing the flue gas treatment section, or a combination of two or more of these above methods. A preferred method of resolving the increased volume of flue gases, in accord with the present invention, is to combine water cooling, reduced production, and if necessary oxygen enrichment for combustion. Figure 9 shows the results of a thermodynamic calculation where liquid water at 77°F provides evaporative direct contact cooling. The normalized exhaust flow rate after dilution with water is plotted against the normalized oxygen flow rate. The exhaust flow is normalized with respect to the 100% oxy-fuel, full production case where exhaust gases at 2800°F are diluted with air at 77°F to produce a 1000°F gas stream. The figure shows that the exhaust gas volume can be reduced by 50% for full production oxy-fuel operation when water is substituted for air as the cooling medium in the exhaust stream. For the case of full production using air instead of oxygen for combustion and water is used for cooling the exhaust gases, the exhaust flow rate is 3.6 times the base case full production, full oxy-fuel case. For 50% production using air instead of oxygen as the oxidant, the exhaust stream volume is about 2.5 times greater than the base case full production, full oxy-fuel case. Alternatives to the proposed invention are: Option 1) continued 100% oxy-fuel firing with more oxygen storage, Option 2) hot hold with air-fuel heat up burners, and Option 3) hot hold or some production with high momentum oxy-fuel bumers using air instead of oxygen. The difference between the proposed invention and option 1 is reduced use of oxygen and expense of storage of liquid oxygen. The difference between the invention and option 2 is continued production and expense. The difference between the invention and option 3 is the technical difficulty of supplying air with a high pressure. The benefit of the invention compared to option 1 is lower capital cost (fewer LOX storage tanks). Also, depending on the length of time that the on-site oxygen plant is down, the logistics and availability problems of liquid oxygen are avoided. A benefit of the proposed invention over option 1 is that it can function if there is a problem with the oxygen supply lines or flow control skids. Another benefit of the invention compared to option 2 is higher maximum temperature in the furnace with similar temperature profile needed for glass production. Yet another benefit of the invention compared to option 2 is continued production. The most effective process is where full production is continued using air or oxygen enriched air. Even production at a minimum level to sustain a glass ribbon in the float bath is extremely valuable. Reestablishing the glass ribbon is time consuming and could delay production by one or more days. For example, for a flat glass fiimace, that produces 600 tons/day, and with glass valued at $300/ton, one days production is worth $180,000. A further benefit of the invention compared to option 2 is that the back-up system is in place. Option 2 requires that an outside company must come to the facility and install their equipment. A still further benefit of the invention is that the fumace refractory does not need to be drilled, cut, or otherwise disturbed. The present invention provides the user the ability to use different burners for air-fuel and oxy-fuel operation, a common mounting system for air-fuel and oxy-flier burners, higher maximum fumace temperatures compared to air-fuel heat up burners. The process of the present invention is capable of generating similar temperature distribution in a fumace needed for glass processing, permit higher firing rates at the fumace hot spot by preferentially increasing oxygen concentration, use of separate but closely spaced ports for introduction air and fuel for air-fuel operation, change function of pre-combustor/staging ports for air-fuel and oxy-fuel operation. For oxy-fuel operation, the larger opening is used as a precombustor with oxygen and fuel flow and the smaller opening for oxygen staging. For air-fuel operation, the larger opening is used for flowing air or oxygen enriched air and the smaller port primarily for fuel. It is within the scope of the present invention to have a separate burner block or pre-combustor placed in the fumace wall to introduce air or oxygen enriched air and fuel into the fumace. In this mode the oxy-flier burner would be turned off and the separate burner block would be used to effect combustion according to the teaching of the invention. It is also within the scope of the present invention to introduce air and fuel into the fumace through separate burners or pipes that are independent of the oxy-fuel burners, so long as the air or oxygen enriched air and fuel are introduced in accord with the teachings of the invention. Having thus described our invention what is desired to be secured by Letters Patent of the United States is set forth in the appended claims. What is Claimed: 1. A process for maintaining heating of a furnace to an elevated temperature using oxy-fuel combustion, wherein an oxy-fuel flame is introduced into said furnace and an oxidizer is introduced underneath said flame, when oxygen supply for said flame and said oxidizer is eliminated or terminated comprising the steps of: replacing said oxy-fuel flame with one of air or oxygen-enriched air and introducing said one of air or oxygen enriched air into said finance; and replacing said oxidizer with said fuel and introducing said fuel into said furnace to provide combustion and maintain said temperature in said fumace. 2. A method according to claim 1 wherein said furnace is a glass melting fumace with temperature distribution maintained in said fumace by using air fuel combustion except in those bumers adjacent a hot spot in said fiimace where oxygen enriched air combustion is used, 3. A method according to claim 1 including replacing said oxy-fuel flame with air introduced at a flow rate about 12.6 times greater than the flow rate of one of oxy-fuel or oxygen when only oxy-fuel combustion is used. 4. A method according to claim 1 wherein the velocity of said one of air or oxygen enriched air at a discharge end of said burner is less than about 250 ft/sec. 5. A method according to claim 1 including introducing said one of air or oxygen enriched air and fuel through a burner block. 6. A method according to claim 1 including the steps of introducing oxygen with said fuel to enhance radiation heat transfer to a charge being heated in said fumace. 7. A method according to claim 1 including the step of cooling exhaust gases exiting said fumace with liquid water, to decrease the volume of said exhaust gases, as compared to cooling said exhaust gases with air. 8. In a combustion system of the type having an oxy-fuel bumer adapted to produce a flame with a pre-combustor mounted on said burner, said pre- combustor having a first passage with a first end in fluid tight relation to a flame end of said bumer and a second end adapted to direct the flame produced by said bumer for heating in industrial environments in a generally flat fan like configuration and a second separate passage in said bumer block disposed beneath and coextensive with said first passage, said second passage terminating in a nozzle end in said second end of said pre-combustor to direct oxidizing fluid underneath and generally parallel to the flame, the improvement comprising: first means to introduce one of air or oxygen enriched air through said bumer into said pre-combustor in place of said flame; and second means to introduce fuel into said second separate passage in said pre-combustor in place of said oxidizing fluid, whereby said combustion system can continue to heat said industrial environment in the event supply of oxygen is interrupted or reduced. 9. A system according to claim 8, wherein said pre-combustor is between 4 and 18 inches in length. 10 A system according to claim 8, wherein said first passage and said second passage have a width to height ratio of between 5 and 30 at said second end of said pre-combustor. 11. A system according to claim 10, wherein walls defining the width of said first passage and said second passage of said pre-combustor are disposed at an angle between -15to +30 ° on either side of a, central vertical plane through said pre-combustor. 12. A system according to claim 11, wherein said angle is between 0° to +15° on either side of said vertical plane. 13. A system according to claim 8, wherein there is included means to introduce oxygen into said fuel in said pre-combustor. 14. A system according to claim 8, utilized in a heating fumace with means to water cool exhaust gases emerging from said fumace when said burner is in use. 15. A process for maintaining heating of a fumace to an elevated temperature using oxy-fuel combustion, wherein an oxy-fuel flame is introduced into said fumace, when oxygen supply for said flame is eliminated or terminated comprising the steps of: introducing a stream of one of air or oxygen-enriched air into said fumace; and introducing a separate stream of said fuel into said fumace to provide combustion and maintain said temperature in said fumace. 16. A method according to claim 15, wherein said fumace is a glass meshing fumace with temperature distribution maintained in said fumace by using air fuel combustion except in those burners adjacent a hot spot in said fumace where oxygen enriched air combustion is used. 17. A method according to claim 15, including replacing said oxy-fuel flame with air introduced at a flow rate about 12.6 times greater than the flow rate of one of oxy-fuel or oxygen when only oxy-fuel combustion is used. 18. A method according to claim 15, wherein the velocity of said one of air or oxygen enriched air at a discharge end of a burner used to introduced said one of air or oxygen enriched air into said fumace is less than about 250 ft/sec. 19. A method according to claim 15, including the steps of introducing oxygen with said fuel to enhance radiation heat transfer to a charge being heated in said fumace. 20. A method according to claim 15, including the step of cooling exhaust gases exiting said fumace with liquid water to decrease the volume of said exhaust gases, as compared to cooing said exhaust gases with air. 21. A process for maintaining heating of a fiimace substantially as herein described with reference to the accompanying drawings.

Full Text

BACKGROUND OF THE INVENTION
The present invention pertains to oxy-fuel methods and devices for producing elevated temperatures in industrial melting pumices for such diverse products as metals, glass, ceramic materials and the like. In particular, the present invention pertains to combustion and methods and apparatus for continuation of combustion in the event of curtailed or terminated availability of oxygen for the oxy-fuel process.
Use of oxy-feel burners in industrial processes such as glass melting, permits the furnace designer to achieve varying flame momentum, glass melt coverage, and flame radiation characteristics. Examples of such bumers and combustion processes are described in U.S. Patents 5,256,058, 5,346,390, 5,547,368, and 5,575,637 the disclosures of which are incorporated herein by reference.
One particularly effective process and apparatus for utilizing oxy-fuel combustion in the manufacture of glass concems staged combustion, which is

disclosed in U.S. Patent 5,611,682, the specification of which is incorporated herein by reference.
In the beginning of the 1990s, glass manufacturers began converting furnaces from air-fuel combustion to oxy-fuel combustion. Oxygen enrichment of some air-fuel systems has been accomplished where the oxygen concentration is increased up to about 30%. Higher oxygen concentrations in the range of 40-80% are not used because of the increased potential for forming NOx pollutants. It has also been found that using oxy-fuel combustion where oxygen is present in a concentration of between 90-100% results in more favorable economics for the user.
Many of the larger oxy-fuel glass furnaces are supplied by oxygen generated on site using well-known cryogenic or vacuum swing adsorption techniques. It is customary and, to date, the only method for backing up the supply of on-site generated oxygen is to keep an inventory of a liquid oxygen at the same site. Thus, when the on-site generation facility is taken off-line either due to a process problem or for routine maintenance, the inventory of liquid oxygen is utilized to supply the oxygen for the oxy-fuel combustion. This method of backing up the on-site generated oxygen requires large insulated tanks for storage of the oxygen in liquid form and vaporizers to enable the liquid oxygen to be converted into gaseous oxygen for use in the oxy-fuel process. It is conventional to utilize trucks to haul Uquid oxygen to the site from a larger air separation facility. Utilizing liquid oxygen back-up with an on-site generated oxygen system permits the user to continue using an oxy-fuel process without interruption. Any oxy-filial combustion system, e.g. one of those disclosed in the above-referenced patents, would benefit from on-site production having a back-up system.
Until now, backing up oxy-fuel glass fumaces with an inventory of liquid oxygen has not been considered to be a problem. However, with the conversion of more and more fumaces at multi-pumice sites and the use of oxy-fuel combustion in flat or float glass fumaces which are much larger and use more oxygen, liquid oxygen backup becomes a significant concern to the user because of the high capital cost of storage tanks and vaporizers. In addition to the cost issue, a logistics problem arises relating to the transportation of the liquid oxygen to the site and having enough liquid oxygen available on short notice from a nearby air separation facility used to produce the liquid oxygen. Transportation of liquid

oxygen to a user sites in remote locations become even a greater problem fraught with greater difficulties.
Normally, when a glass female is converted from air-fuel to oxy-fuel, heat recovery devices such as regenerators and air supply systems are removed. For the user, one of the incentives to convert to oxy-fuel is reduced capital costs due to elimination of the heat recovery devices. Due to the design of oxy-fuel bumers, the fumes cannot be operated by simply substituting air for oxygen in conventional combustion systems in use today. The pressure requirement to provide an equivalent amount of contained oxygen using air in an oxy-fuel bummers would be extremely high, requiring an expensive air supply system. Further, some oxy-fuel bumers would be sonic flow limited if fired at an equivalent firing rate.
When using oxy-fuel combustion where the oxygen supply is curtailed or disrupted, the conventional technique is to maintain the fumaces in a condition called "hot hold". Hot hold is a condition where production is stopped and the fumace is kept hot so that the glass does not solidify. Allowing the glass to solidify would severely damage the fumace. Several companies specialize in fumace heat-ups following cold fumace repairs. They use specially designed air-fuel bumers to provide the initial increase in temperature in the fumace. In case of oxygen supply dismption, the same bumers could be used to provide enough heating for hot hold. In this procedure, no special temperature profile for production would be attempted and the maximum temperature achieved by these devices could be about 2200°F. This temperature is not sufficient for production of glass and is the least preferred option to be used by glass manufacturers. The cost of not producing glass is very high to the glass manufacturer, in terms of lost product sales as well as dismption of downstream glass forming lines.
Therefore, there is a definite need to provide a method and apparatus for maintaining production in a fumace used for glass manufacturing in the event of a curtailment or disruption in the availability of oxygen.
BRIEF SUMMARY OF THE INVENTION
The present invention pertains to a method and apparatus to backup an oxy-fuel combustion system with an air-fuel combustion system that can be used with or without oxygen enrichment to maintain production in an industrial fumace

such as a glass melting furnace. According to the present invention, a system has been devised which permits operation in an oxy-fuel, and air-fuel, or an oxygen enriched air-fuel mode. The burner according to the present invention has a unique feature relating to operation at very low velocity for the oxy-fuel mode permitting acceptable pressure drops through the bumer when operating in an air-fuel mode. A burner according to the present invention can utilize oxygen enrichment to effect the process.
According to the present invention, a conventional bumer block such as described in U.S. Patent 5,611,682 can be used for either oxy-fiiel or air-fuel combustion, allowing the combustion system to be rapidly converted between the two modes. According to the present invention, when a problem with oxygen supply occurs, the oxy-fuel bumers would be turned off, disconnected, and replaced by air-fuel backup bumers that have the same configuration for a connection to the burner block. With the air-fuel backup system, the user would retain the air supply systems from previous air-fiiel systems used in the melting operation or, air blowers would be supplied as part of the back up system. Air fuel bumers according to the present invention should be capable of firing at rates substantially higher than the oxy-fuel bumers.
Thus, in one aspect the present invention is a process for maintaining heating of a furnace to an elevated temperature using oxy-fiiel combustion, wherein a flame is introduced into said furnace and an oxidizer is introduced underneath said flame, when oxygen supply for said flame and said oxidizer is eliminated or terminated comprising the steps of replacing the flame with one of air or oxygen-enriched air introduced at a rate to maintain approximately the bumer firing rate when oxygen is the only source of oxidant for combustion, and replacing the oxidizer introduced underneath said flame with said fuel to provide combustion and maintain said temperature in said furnace.
In another aspect, the present invention is a combustion system of the type having an oxy-fuel bumer adapted to produce a flame with a pre-combustor mounted on the bumer, the pre-combustor having a first passage with a first end in fluid tight relation to a flame end of the bumer and a second end adapted to direct the flame produced by the bumer for heating in industrial environments in a generally flat fan like configuration and a second separate passage in the pre-combustor disposed beneath and coextensive with the first passage, the second passage

terminating in a nozzle end in the second end of the pre-combustor to direct oxidizing fluid underneath and generally parallel to the flame, the improvement comprising; first means to introduce one of air or oxygen enriched air through the bumer into the pre-combustor in place of the flame, and second means to introduce fuel into the second separate passage in the pre-combustor in place of the oxidizing fluid, whereby the combustion system can continue to heat the industrial environment in the event supply of oxygen is interrupted or reduced.
In yet another aspect, the present invention contemplates reducing exhaust gas volume in a furnace being heated according to the method and apparatus of the invention by liquid water cooling of the exhaust gases exiting the furnace.
In still another aspect the present invention pertains to substitution of air-fuel combustion for oxy-fuel combustion to maintain heating in an industrial environment, the air or oxygen enriched air being introduced into the environment in any manner so there is sufficient volume to effect the required level of heating. In this aspect water cooling of the exhaust gases will be beneficial for lowering exhaust gas volume.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic perspective view of a conventional staged combustion apparatus.
Figure 2 is a view taken along line 2-2 of Figure L
Figure 3 is a schematic perspective view of an apparatus according to the present invention.
Figure 4 is a front view of the bumer block or pre-combustor of the apparatus of Figure 3.
Figure 5 is a plot of normalized methane flow rate against normalized oxygen flow rate for conditions from zero production to full production.
Figure 6 is a plot of oxygen concentration against normalized oxygen flow rate for the production rates of Figure 5.

Figure 7 is a plot of normalized exhaust gas flow rate against normalized oxygen flow rate for several production rates.
Figure 8 is a plot of normalized exhaust gas flow rate after dilution with air against oxygen flow rate for production rates between zero and full production.
Figure 9 is a plot of normalized exhaust flow rate after dilution with water against normalized oxygen flow rate for furnace production from zero to full production.
DETAILED DESCRPTION OF THE INVENTION
The present invention pertains to a method and apparatus to back-up an oxy-fuel heating system with an air-fuel heating system. According to the invention, the back-up air-fuel system can be operated with or without oxygen enrichment of the air. The burner according to the invention permits at least two distinct modes of operation, e.g. oxy-fuel or air-fuel. Another feature of the burner is the operation at very low velocity for the oxy-fuel mode, thus permitting acceptable pressure drops through the burner when operating in the air-fuel mode with or without enrichment of the air with oxygen. For the purposes of this invention oxy-fiiel combustion is taken to mean combustion with 80% to 100% by volume oxygen. Oxygen enrichment is taken to mean between 22% and 80% by volume oxygen concentration.
According to the invention the same burner block or pre-combustor can be used during either oxy-fuel or air-fuel operation, allowing the combustion system to berapidly converted from one mode to the other. In the case where an operator encounters a problem with the oxygen supply, the oxy-fuel bumers would be turned off, disconnected, and replaced with air-fuel back-up bumers that have the same connection to the burner block. With an air-fuel back-up system, a glass manufacturer would retain its air supply systems present before conversion to oxy-fuel combustion, or blowers would be supplied as part of the back-up system. It is important that the air-fuel back-up bumers are capable of firing at a rate substantially higher than that of the oxy-fuel bumers being backed-up.

Higher firing rate for the back-up air-fuel burner is required because of the additional energy losses caused by heating and expelling nitrogen. Furthermore, the air used for combustion in a back-up system will typically not be pre-heated which results in a decrease in fumace efficiency relative to a typical air-fiiel fiimace. A simplified thermodynamic calculation illustrates the need to increase the fuel firing rate when non preheated air is used for combustion. The assumptions for this example are: fuel and oxygen completely react with no excess oxygen and no intermediate products remaining; all gases (e.g. methane, air, or oxygen) enter the fumace at 77°F; and, all gases exhaust the fumace at 2800"^? after complete combustion. Under these conditions, 2.65 times the firing rate is required when firing with air as compared to firing with 100% oxygen in order to maintain the same available heat. Available heat is the energy transferred to the charge and for heat loss from the fumace.
Thus, the total oxidant volumetric flow rate will increase dramatically as the oxygen flow rate is reduced. The volume of the oxidant stream is increased by a factor of 4.76 because of the addition of nitrogen and an additional 2,65 times because of the higher firing rate requirement. This means that the flow rate of the oxidant stream is increased by about 12.6 times when air is completely substituted for oxygen.
A major concern with using air-fuel combustion in an oxy-fiiel burner assembly is the air supply pressure required to accommodate the higher gas volumes needed. The present invention utilizes a low velocity oxidizer system. Thus, even when firing in an air-fuel mode, pressure drop is low enough to permit the use of relatively inexpensive air blowers, while maintaining burner firing rates equal to or greater than those used with oxy-fuel firing. This, in turn, allows for continuity of production when a user, e.g. a glass melter, is operating in the back-up mode during an emergency loss or curtailment of oxygen supply.
Oxy-fuel bumers with oxidant velocities greater than about 90 ft/sec. at
any point in the burner, design will be sonic limited at the equivalent-firing rate
when air is used asthe oxidant at fiill production. The sonic velocity is defined by
the equation a where k is the ratio of specific heats (1.4 for air), R is the gas
constant (287 J/kg K), and T is the absolute temperature. For air at 25°C (77°F), the sonic velocity is 346 m/sec. (1135 ft/sec). For an oxy fuel burner with an oxygen velocity of 100 ft/sec, the equivalent flow rate using air will be 12.6 times that

amount or 1260 ft/sec. which is greater than the sonic velocity. Therefore, to avoid a sonic limit, the oxy-fiiel bumer must be designed with an oxygen velocity of less than 90 ft/sec. if complete air substitution for oxygen is to be used without changing any part of the bumer assembly, Altematively, this limit can be avoided according to one aspect of the invention where the bumer body is changed when switching between operating modes. The bumer block must be designed so that the superficial velocity is less than the sonic velocity for air-fuel operation.
The shape of the flame is also a concern for traditional oxy-fuel burners operating at 2.65 times their rated firing capacity especially with 12.6 times the volumetric flow rate through the oxidant passage(s). The embodiment of the invention disclosed below provides a suitable flame shape for both oxy-fuel and air-fuel operation.
Thus according to the invention, concerns relating to oxidant supply pressure, velocity limits, and flame shape are overcome according to the present invention. We have found it is possible to enable a user to switch from oxy-fuel firing to air-fuel firing using the same bumer block while modifying the bumer body of a Cleanfire® HR™ bumer offered to the trade by Air Products and Chemicals Inc. of Allentown Pennsylvania.
Referring to Figure 1, a staged combustion apparatus 10 includes an oxy-fuel bumer 12 and a precombustor or bumer block 14. The oxy-fuel bumer 12 includes a central conduit 16 for receiving a fuel such as natural gas, which is indicated by arrow 18. A source of oxygen indicated by arrow 20 is introduced into a passage that is between the fuel conduit 16 and an outer concentric conduit 22. The bumer is described in detail in U.S. Patent 5,611,682, the specification of which has been incorporated herein by reference. The burner 12 is fitted to a bumer block 14 and held in fluid tight relationship thereto at a first end 24 of the precombustor or bumer block 14. The bumer block 14 contains a first or central passage 26, which extends from the first end 24 to a discharge end 28 of the burner block 14. The passage 26 has a width greater than the height and has a diverging shape as shown and as described in the '682 patent. In order to have stage combustion, staging oxygen represented by arrow 30 is introduced into a second passage 32 in the bumer block 14, The passage 32 has a shape complimentary to that of the central passage and has a greater width than height as illustrated and again as described in detail in the '682 patent.

Referring to Figure 2, at the first end 24 of the precombustor 14 the oxy-fuel burner 12 has a discharge end with a central fuel conduit 16 surrounded by an oxygen passage 22. The staging oxygen exists a passage 31 that is disposed below the passage for the oxy-fuel flame as shown in Figure 2.
Figure 3 shows a combustion apparatus according to the present invention. The combustion apparatus 40 includes a bumer block 14, which is identical to the bumer block 14 of Figure 1. According to the present invention, the bumer 42 is similar to tlie oxy-fuel bumer 12 of Figure 1 with a device 44 to permit introduction of air or oxygen enriched air into the upper passage 50 of bumer 42. The bumer 42 is also adapted to introduce air or oxygen enriched air by passage 48 of bumer 42 into upper passage 50 where oxidant from passages 44 and 48 are mixed, Arrow 46 represents introduction of air or air and oxygen into the device 44 which in tummy introduces the air or oxygen enriched air into the passage 50. Arrow 56 represents introduction of air or air and oxygen into the passage 50. The air or oxygen enriched air moves from passage 50 into the central passage 26 of the bumer block and exits to the furnace.
When the bumer is converted over to non or limited oxy-fuel firing, the supply of staging oxygen (indicated by arrow 30 in Figure 1) is replaced by fuel, represented by arrow 54, so that fuel or oxygen enriched fuel exits passage 32 of the bumer block 14. Shown schematically in Figure 4 are the passages 26 and 32 at the front end of the bumer block 14 with passage 26 being used to introduce air or oxygen enriched air into the furnace and passage 32 being used to introduce fuel or oxygen enriched fuel into the fiimace. When the bumer 42 is used in the air-fuel firing mode, the air or oxygen enriched air flows through passage 26 and the fuel or oxygen enriched fuel flows through the passage 32. The bumer block design is such that a stable air-fuel flame is established because of the re-circulation region between the two openings.
In addition to the air-fuel firing capability, the device of the present invention permits varying degrees of oxygen enrichment to be accomplished. Use of oxygen enrichment improves flexibility during operation in the backup mode by decreasing the use of oxygen supplied from liquid oxygen storage. It also permits adjustment of the flame length by adding oxygen to the air flow.

Supplemental oxygen can be supplied by various methods. For example the air can be enriched with oxygen, oxygen lances could be supplied through either or both the primary passage 26 of pre-combustor 14 or the staging port 32, or separate oxygen lances could be installed at a distance away from the precombustor 14 or staging port 32. Oxygen introduced through the staging port with the natural gas could provide means to create soot for better radiation heat transfer to the fumace charge.
Using the method and apparatus of the present invention makes it possible to maintain maximum temperature and temperature distribution needed for glass production. Oxygen enrichment or oxy-fuel firing, preferably should be used on burners with the highest firing rates near the hot spot in the fumace. This will reduce the flow rate of air needed for these burners and reduce the pressure drop. Also, oxygen enrichment increases the peak flame temperature and thereby increases heat transfers in the hot spot. It is well known that a hot spot is required in glass making furnaces to established proper convection cells in the glass men which are required to produce glass of acceptable quality.
Other air-fuel technologies can be used to maintain hot hold conditions. This invention is intended to permit the user to continue production. The minimum firing rate provided by the air-fuel backup system is such that at least 20% of the design production rate can be maintained. It is believed that this production rate is sufficient to allow a float glass producer to maintain a continuous glass ribbon in the float bath.
Higher velocity oxy-fuel burners could be modified for low velocity operation by adding one or more inlet ports to use the technology disclosed herein. These inlets could be normally closed or used for staging during oxy-fuel operation. Also, one or more additional inlet ports could be added on-the-fly prior to commencing air-fuel backup by drilling a hole in the refractory wall in a location close to the burner port.
Another alternative for fumaces using high velocity burners is to replace the burner blocks with blocks having larger openings to reduce the pressure drop. With this method there is the danger of introducing foreign refractory material into the glass melt during the replacement procedure which could cause glass

defects. Furthermore, replacement of burner blocks on the fly requires substantial time, possibly too long to avoid interruption of production.
Figure 5 shows the methane flow rate required for hot hold conditions (zero production rate), 20%, 50% and full production conditions, assuming, for example, that 35% of the available heat is required for furnace wall heat losses under full production conditions. Hot hold could be achieved at lower firing rates than the plot shows since the overall furnace temperature, would be lowered thereby reducing the wall heat losses. This plot assumes that the heat losses remain the same, independent of production rate or oxygen usage. The methane flow rate is normalized based on the methane flow rate for full production with 100% oxy-fuel, and the oxygen flow rate is normalized based on the oxygen flow rate for full production with 100% oxy-fuel. The normalized oxygen flow rate is 1.0 when all of the oxidant for combustion is supplied by the oxygen source (no air) and zero when all of the oxidant for combustion is supplied by air.
Figure 6 is a corresponding plot of oxygen concentration as a function of normalized oxygen flow rate for each of the production rates shown in Figure 5.
As indicated by point A in Figure 5, hot hold using only air as the oxidant for combustion (zero normalized oxygen flow rate), the methane flow rate is about the same as required for 100% oxy-fuel at full production (normalized value equal to 1). Hot hold could also be maintained at 35% of the full production oxygen flow rate with 35% of the full production methane flow rate (point B). Referring to Figure 6 (point B), the operating condition represented by point B corresponds to 100% oxy-fuel with no air dilution.
Figure 5 shows that the oxygen flow rate and the methane flow rate can each be reduced by half to produce 20% of full production. This means that if production is limited to 20% of the full production rate, the stored oxygen supply can last two times longer. According to Figure 6, this corresponds to 100% oxy-fuel
firing.
At 50% production, the oxygen flow rate could be reduced to half of the full production flow rate and the methane at about 95% of the fiill production
The exhaust gas temperature from an oxy-fuel furnace is higher than a corresponding air-fuel furnace after the heat recovery device. Glass manufacturers therefore must decrease the temperature of the oxy-fuel combustion products by some method before the gases enter the sections of the flue system fabricated with metal. Because of current air pollution regulations, fuel gas treatment for glass furnaces typically include particulate removal devices, such as electrostatic precipitators or bag houses. These devices have a maximum operating temperature significantly lower than the oxy-fuel fumace exhaust temperature, typically around 1000°F. Therefore, exhaust gases must be cooled by cold (ambient) dilution air before these devices.
If air is substituted for oxygen for combustion in a fumace designed for oxy-fuel combustion, the exhaust volume will be increased substantially. Figure 7 shows how the exhaust flow rate increases as air is substituted for oxygen for several production rates. The same assumptions regarding inlet and outlet temperatures and heat losses used for the previous figures are used to generate this figure. The exhaust flow rate is normalized with respect to the exhaust flow rate for full production with 100% oxy-fuel. For full production, the exhaust flow rate will be increased by more than nine times if oxygen is completely replaced by air. More than three times the exhaust flow rate can be expected at hot hold conditions where air completely replaces oxygen.
As a result of the increased flow of hot exhaust gases, much more dilution air must be provided to decrease the temperature to the same level before the gases enter the metal section of the flue system. Figure 8 shows the result of thermodynamic calculations where fumace exhaust gases at 2800°F are diluted with air at IW to produce a 1000°F gas stream which is a temperature suitable for the metal section of the flue system. The normalized exhaust flow rate after dilution with air is plotted as a function of normalized oxygen flow rate. The exhaust flow is normalized with respect to the 100% oxy-fuel, full production case where exhaust gases at 2800°F are diluted with air at 77°F to produce a 1000°F gas stream. If air is substituted for oxygen under full production conditions and the exhaust gases are diluted with air at 77'^F to produce a logoff stream, the resulting exhaust gas flow rate would be greater than 7,5 times the full production oxy-fuel case. Flue systems are not capable of handling this much of an increase in throughput because of pressure drop limitations. The fumace pressure would increase, possibly leading to structural failure.

There are several ways of dealing with the increased flue gas volume: e.g., reduced production, oxygen enrichment for combustion, alternative ways of cooling the flue gases (e.g. with water), using additional flue gas exhaust capability, bypassing the flue gas treatment section, or a combination of two or more of these above methods. A preferred method of resolving the increased volume of flue gases, in accord with the present invention, is to combine water cooling, reduced production, and if necessary oxygen enrichment for combustion.
Figure 9 shows the results of a thermodynamic calculation where liquid water at 77°F provides evaporative direct contact cooling. The normalized exhaust flow rate after dilution with water is plotted against the normalized oxygen flow rate. The exhaust flow is normalized with respect to the 100% oxy-fuel, full production case where exhaust gases at 2800°F are diluted with air at 77°F to produce a 1000°F gas stream. The figure shows that the exhaust gas volume can be reduced by 50% for full production oxy-fuel operation when water is substituted for air as the cooling medium in the exhaust stream. For the case of full production using air instead of oxygen for combustion and water is used for cooling the exhaust gases, the exhaust flow rate is 3.6 times the base case full production, full oxy-fuel case. For 50% production using air instead of oxygen as the oxidant, the exhaust stream volume is about 2.5 times greater than the base case full production, full oxy-fuel case.

Alternatives to the proposed invention are: Option 1) continued 100% oxy-fuel firing with more oxygen storage, Option 2) hot hold with air-fuel heat up burners, and Option 3) hot hold or some production with high momentum oxy-fuel bumers using air instead of oxygen. The difference between the proposed invention and option 1 is reduced use of oxygen and expense of storage of liquid oxygen. The difference between the invention and option 2 is continued production and expense. The difference between the invention and option 3 is the technical difficulty of supplying air with a high pressure.
The benefit of the invention compared to option 1 is lower capital cost (fewer LOX storage tanks). Also, depending on the length of time that the on-site oxygen plant is down, the logistics and availability problems of liquid oxygen are avoided. A benefit of the proposed invention over option 1 is that it can function if there is a problem with the oxygen supply lines or flow control skids. Another benefit of the invention compared to option 2 is higher maximum temperature in the furnace with similar temperature profile needed for glass production. Yet another

benefit of the invention compared to option 2 is continued production. The most effective process is where full production is continued using air or oxygen enriched air. Even production at a minimum level to sustain a glass ribbon in the float bath is extremely valuable. Reestablishing the glass ribbon is time consuming and could delay production by one or more days. For example, for a flat glass fiimace, that produces 600 tons/day, and with glass valued at $300/ton, one days production is worth $180,000. A further benefit of the invention compared to option 2 is that the back-up system is in place. Option 2 requires that an outside company must come to the facility and install their equipment. A still further benefit of the invention is that the fumace refractory does not need to be drilled, cut, or otherwise disturbed.
The present invention provides the user the ability to use different burners for air-fuel and oxy-fuel operation, a common mounting system for air-fuel and oxy-flier burners, higher maximum fumace temperatures compared to air-fuel heat up burners. The process of the present invention is capable of generating similar temperature distribution in a fumace needed for glass processing, permit higher firing rates at the fumace hot spot by preferentially increasing oxygen concentration, use of separate but closely spaced ports for introduction air and fuel for air-fuel operation, change function of pre-combustor/staging ports for air-fuel and oxy-fuel operation. For oxy-fuel operation, the larger opening is used as a precombustor with oxygen and fuel flow and the smaller opening for oxygen staging. For air-fuel operation, the larger opening is used for flowing air or oxygen enriched air and the smaller port primarily for fuel.
It is within the scope of the present invention to have a separate burner block or pre-combustor placed in the fumace wall to introduce air or oxygen enriched air and fuel into the fumace. In this mode the oxy-flier burner would be turned off and the separate burner block would be used to effect combustion according to the teaching of the invention.
It is also within the scope of the present invention to introduce air and fuel into the fumace through separate burners or pipes that are independent of the oxy-fuel burners, so long as the air or oxygen enriched air and fuel are introduced in accord with the teachings of the invention.
Having thus described our invention what is desired to be secured by Letters Patent of the United States is set forth in the appended claims.

BACKGROUND OF THE INVENTION
The present invention pertains to oxy-fuel methods and devices for producing elevated temperatures in industrial melting pumices for such diverse products as metals, glass, ceramic materials and the like. In particular, the present invention pertains to combustion and methods and apparatus for continuation of combustion in the event of curtailed or terminated availability of oxygen for the oxy-fuel process.
Use of oxy-feel burners in industrial processes such as glass melting, permits the furnace designer to achieve varying flame momentum, glass melt coverage, and flame radiation characteristics. Examples of such bumers and combustion processes are described in U.S. Patents 5,256,058, 5,346,390, 5,547,368, and 5,575,637 the disclosures of which are incorporated herein by reference.
One particularly effective process and apparatus for utilizing oxy-fuel combustion in the manufacture of glass concems staged combustion, which is

disclosed in U.S. Patent 5,611,682, the specification of which is incorporated herein by reference.
In the beginning of the 1990s, glass manufacturers began converting furnaces from air-fuel combustion to oxy-fuel combustion. Oxygen enrichment of some air-fuel systems has been accomplished where the oxygen concentration is increased up to about 30%. Higher oxygen concentrations in the range of 40-80% are not used because of the increased potential for forming NOx pollutants. It has also been found that using oxy-fuel combustion where oxygen is present in a concentration of between 90-100% results in more favorable economics for the user.
Many of the larger oxy-fuel glass furnaces are supplied by oxygen generated on site using well-known cryogenic or vacuum swing adsorption techniques. It is customary and, to date, the only method for backing up the supply of on-site generated oxygen is to keep an inventory of a liquid oxygen at the same site. Thus, when the on-site generation facility is taken off-line either due to a process problem or for routine maintenance, the inventory of liquid oxygen is utilized to supply the oxygen for the oxy-fuel combustion. This method of backing up the on-site generated oxygen requires large insulated tanks for storage of the oxygen in liquid form and vaporizers to enable the liquid oxygen to be converted into gaseous oxygen for use in the oxy-fuel process. It is conventional to utilize trucks to haul Uquid oxygen to the site from a larger air separation facility. Utilizing liquid oxygen back-up with an on-site generated oxygen system permits the user to continue using an oxy-fuel process without interruption. Any oxy-filial combustion system, e.g. one of those disclosed in the above-referenced patents, would benefit from on-site production having a back-up system.
Until now, backing up oxy-fuel glass fumaces with an inventory of liquid oxygen has not been considered to be a problem. However, with the conversion of more and more fumaces at multi-pumice sites and the use of oxy-fuel combustion in flat or float glass fumaces which are much larger and use more oxygen, liquid oxygen backup becomes a significant concern to the user because of the high capital cost of storage tanks and vaporizers. In addition to the cost issue, a logistics problem arises relating to the transportation of the liquid oxygen to the site and having enough liquid oxygen available on short notice from a nearby air separation facility used to produce the liquid oxygen. Transportation of liquid

oxygen to a user sites in remote locations become even a greater problem fraught with greater difficulties.
Normally, when a glass female is converted from air-fuel to oxy-fuel, heat recovery devices such as regenerators and air supply systems are removed. For the user, one of the incentives to convert to oxy-fuel is reduced capital costs due to elimination of the heat recovery devices. Due to the design of oxy-fuel bumers, the fumes cannot be operated by simply substituting air for oxygen in conventional combustion systems in use today. The pressure requirement to provide an equivalent amount of contained oxygen using air in an oxy-fuel bummers would be extremely high, requiring an expensive air supply system. Further, some oxy-fuel bumers would be sonic flow limited if fired at an equivalent firing rate.
When using oxy-fuel combustion where the oxygen supply is curtailed or disrupted, the conventional technique is to maintain the fumaces in a condition called "hot hold". Hot hold is a condition where production is stopped and the fumace is kept hot so that the glass does not solidify. Allowing the glass to solidify would severely damage the fumace. Several companies specialize in fumace heat-ups following cold fumace repairs. They use specially designed air-fuel bumers to provide the initial increase in temperature in the fumace. In case of oxygen supply dismption, the same bumers could be used to provide enough heating for hot hold. In this procedure, no special temperature profile for production would be attempted and the maximum temperature achieved by these devices could be about 2200°F. This temperature is not sufficient for production of glass and is the least preferred option to be used by glass manufacturers. The cost of not producing glass is very high to the glass manufacturer, in terms of lost product sales as well as dismption of downstream glass forming lines.
Therefore, there is a definite need to provide a method and apparatus for maintaining production in a fumace used for glass manufacturing in the event of a curtailment or disruption in the availability of oxygen.
BRIEF SUMMARY OF THE INVENTION
The present invention pertains to a method and apparatus to backup an oxy-fuel combustion system with an air-fuel combustion system that can be used with or without oxygen enrichment to maintain production in an industrial fumace

such as a glass melting furnace. According to the present invention, a system has been devised which permits operation in an oxy-fuel, and air-fuel, or an oxygen enriched air-fuel mode. The burner according to the present invention has a unique feature relating to operation at very low velocity for the oxy-fuel mode permitting acceptable pressure drops through the bumer when operating in an air-fuel mode. A burner according to the present invention can utilize oxygen enrichment to effect the process.
According to the present invention, a conventional bumer block such as described in U.S. Patent 5,611,682 can be used for either oxy-fiiel or air-fuel combustion, allowing the combustion system to be rapidly converted between the two modes. According to the present invention, when a problem with oxygen supply occurs, the oxy-fuel bumers would be turned off, disconnected, and replaced by air-fuel backup bumers that have the same configuration for a connection to the burner block. With the air-fuel backup system, the user would retain the air supply systems from previous air-fiiel systems used in the melting operation or, air blowers would be supplied as part of the back up system. Air fuel bumers according to the present invention should be capable of firing at rates substantially higher than the oxy-fuel bumers.
Thus, in one aspect the present invention is a process for maintaining heating of a furnace to an elevated temperature using oxy-fiiel combustion, wherein a flame is introduced into said furnace and an oxidizer is introduced underneath said flame, when oxygen supply for said flame and said oxidizer is eliminated or terminated comprising the steps of replacing the flame with one of air or oxygen-enriched air introduced at a rate to maintain approximately the bumer firing rate when oxygen is the only source of oxidant for combustion, and replacing the oxidizer introduced underneath said flame with said fuel to provide combustion and maintain said temperature in said furnace.
In another aspect, the present invention is a combustion system of the type having an oxy-fuel bumer adapted to produce a flame with a pre-combustor mounted on the bumer, the pre-combustor having a first passage with a first end in fluid tight relation to a flame end of the bumer and a second end adapted to direct the flame produced by the bumer for heating in industrial environments in a generally flat fan like configuration and a second separate passage in the pre-combustor disposed beneath and coextensive with the first passage, the second passage

terminating in a nozzle end in the second end of the pre-combustor to direct oxidizing fluid underneath and generally parallel to the flame, the improvement comprising; first means to introduce one of air or oxygen enriched air through the bumer into the pre-combustor in place of the flame, and second means to introduce fuel into the second separate passage in the pre-combustor in place of the oxidizing fluid, whereby the combustion system can continue to heat the industrial environment in the event supply of oxygen is interrupted or reduced.
In yet another aspect, the present invention contemplates reducing exhaust gas volume in a furnace being heated according to the method and apparatus of the invention by liquid water cooling of the exhaust gases exiting the furnace.
In still another aspect the present invention pertains to substitution of air-fuel combustion for oxy-fuel combustion to maintain heating in an industrial environment, the air or oxygen enriched air being introduced into the environment in any manner so there is sufficient volume to effect the required level of heating. In this aspect water cooling of the exhaust gases will be beneficial for lowering exhaust gas volume.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic perspective view of a conventional staged combustion apparatus.
Figure 2 is a view taken along line 2-2 of Figure L
Figure 3 is a schematic perspective view of an apparatus according to the present invention.
Figure 4 is a front view of the bumer block or pre-combustor of the apparatus of Figure 3.
Figure 5 is a plot of normalized methane flow rate against normalized oxygen flow rate for conditions from zero production to full production.
Figure 6 is a plot of oxygen concentration against normalized oxygen flow rate for the production rates of Figure 5.

Figure 7 is a plot of normalized exhaust gas flow rate against normalized oxygen flow rate for several production rates.
Figure 8 is a plot of normalized exhaust gas flow rate after dilution with air against oxygen flow rate for production rates between zero and full production.
Figure 9 is a plot of normalized exhaust flow rate after dilution with water against normalized oxygen flow rate for furnace production from zero to full production.
DETAILED DESCRPTION OF THE INVENTION
The present invention pertains to a method and apparatus to back-up an oxy-fuel heating system with an air-fuel heating system. According to the invention, the back-up air-fuel system can be operated with or without oxygen enrichment of the air. The burner according to the invention permits at least two distinct modes of operation, e.g. oxy-fuel or air-fuel. Another feature of the burner is the operation at very low velocity for the oxy-fuel mode, thus permitting acceptable pressure drops through the burner when operating in the air-fuel mode with or without enrichment of the air with oxygen. For the purposes of this invention oxy-fiiel combustion is taken to mean combustion with 80% to 100% by volume oxygen. Oxygen enrichment is taken to mean between 22% and 80% by volume oxygen concentration.
According to the invention the same burner block or pre-combustor can be used during either oxy-fuel or air-fuel operation, allowing the combustion system to berapidly converted from one mode to the other. In the case where an operator encounters a problem with the oxygen supply, the oxy-fuel bumers would be turned off, disconnected, and replaced with air-fuel back-up bumers that have the same connection to the burner block. With an air-fuel back-up system, a glass manufacturer would retain its air supply systems present before conversion to oxy-fuel combustion, or blowers would be supplied as part of the back-up system. It is important that the air-fuel back-up bumers are capable of firing at a rate substantially higher than that of the oxy-fuel bumers being backed-up.

Higher firing rate for the back-up air-fuel burner is required because of the additional energy losses caused by heating and expelling nitrogen. Furthermore, the air used for combustion in a back-up system will typically not be pre-heated which results in a decrease in fumace efficiency relative to a typical air-fiiel fiimace. A simplified thermodynamic calculation illustrates the need to increase the fuel firing rate when non preheated air is used for combustion. The assumptions for this example are: fuel and oxygen completely react with no excess oxygen and no intermediate products remaining; all gases (e.g. methane, air, or oxygen) enter the fumace at 77°F; and, all gases exhaust the fumace at 2800"^? after complete combustion. Under these conditions, 2.65 times the firing rate is required when firing with air as compared to firing with 100% oxygen in order to maintain the same available heat. Available heat is the energy transferred to the charge and for heat loss from the fumace.
Thus, the total oxidant volumetric flow rate will increase dramatically as the oxygen flow rate is reduced. The volume of the oxidant stream is increased by a factor of 4.76 because of the addition of nitrogen and an additional 2,65 times because of the higher firing rate requirement. This means that the flow rate of the oxidant stream is increased by about 12.6 times when air is completely substituted for oxygen.
A major concern with using air-fuel combustion in an oxy-fiiel burner assembly is the air supply pressure required to accommodate the higher gas volumes needed. The present invention utilizes a low velocity oxidizer system. Thus, even when firing in an air-fuel mode, pressure drop is low enough to permit the use of relatively inexpensive air blowers, while maintaining burner firing rates equal to or greater than those used with oxy-fuel firing. This, in turn, allows for continuity of production when a user, e.g. a glass melter, is operating in the back-up mode during an emergency loss or curtailment of oxygen supply.
Oxy-fuel bumers with oxidant velocities greater than about 90 ft/sec. at
any point in the burner, design will be sonic limited at the equivalent-firing rate
when air is used asthe oxidant at fiill production. The sonic velocity is defined by
the equation a where k is the ratio of specific heats (1.4 for air), R is the gas
constant (287 J/kg K), and T is the absolute temperature. For air at 25°C (77°F), the sonic velocity is 346 m/sec. (1135 ft/sec). For an oxy fuel burner with an oxygen velocity of 100 ft/sec, the equivalent flow rate using air will be 12.6 times that

amount or 1260 ft/sec. which is greater than the sonic velocity. Therefore, to avoid a sonic limit, the oxy-fiiel bumer must be designed with an oxygen velocity of less than 90 ft/sec. if complete air substitution for oxygen is to be used without changing any part of the bumer assembly, Altematively, this limit can be avoided according to one aspect of the invention where the bumer body is changed when switching between operating modes. The bumer block must be designed so that the superficial velocity is less than the sonic velocity for air-fuel operation.
The shape of the flame is also a concern for traditional oxy-fuel burners operating at 2.65 times their rated firing capacity especially with 12.6 times the volumetric flow rate through the oxidant passage(s). The embodiment of the invention disclosed below provides a suitable flame shape for both oxy-fuel and air-fuel operation.
Thus according to the invention, concerns relating to oxidant supply pressure, velocity limits, and flame shape are overcome according to the present invention. We have found it is possible to enable a user to switch from oxy-fuel firing to air-fuel firing using the same bumer block while modifying the bumer body of a Cleanfire® HR™ bumer offered to the trade by Air Products and Chemicals Inc. of Allentown Pennsylvania.
Referring to Figure 1, a staged combustion apparatus 10 includes an oxy-fuel bumer 12 and a precombustor or bumer block 14. The oxy-fuel bumer 12 includes a central conduit 16 for receiving a fuel such as natural gas, which is indicated by arrow 18. A source of oxygen indicated by arrow 20 is introduced into a passage that is between the fuel conduit 16 and an outer concentric conduit 22. The bumer is described in detail in U.S. Patent 5,611,682, the specification of which has been incorporated herein by reference. The burner 12 is fitted to a bumer block 14 and held in fluid tight relationship thereto at a first end 24 of the precombustor or bumer block 14. The bumer block 14 contains a first or central passage 26, which extends from the first end 24 to a discharge end 28 of the burner block 14. The passage 26 has a width greater than the height and has a diverging shape as shown and as described in the '682 patent. In order to have stage combustion, staging oxygen represented by arrow 30 is introduced into a second passage 32 in the bumer block 14, The passage 32 has a shape complimentary to that of the central passage and has a greater width than height as illustrated and again as described in detail in the '682 patent.

Referring to Figure 2, at the first end 24 of the precombustor 14 the oxy-fuel burner 12 has a discharge end with a central fuel conduit 16 surrounded by an oxygen passage 22. The staging oxygen exists a passage 31 that is disposed below the passage for the oxy-fuel flame as shown in Figure 2.
Figure 3 shows a combustion apparatus according to the present invention. The combustion apparatus 40 includes a bumer block 14, which is identical to the bumer block 14 of Figure 1. According to the present invention, the bumer 42 is similar to tlie oxy-fuel bumer 12 of Figure 1 with a device 44 to permit introduction of air or oxygen enriched air into the upper passage 50 of bumer 42. The bumer 42 is also adapted to introduce air or oxygen enriched air by passage 48 of bumer 42 into upper passage 50 where oxidant from passages 44 and 48 are mixed, Arrow 46 represents introduction of air or air and oxygen into the device 44 which in tummy introduces the air or oxygen enriched air into the passage 50. Arrow 56 represents introduction of air or air and oxygen into the passage 50. The air or oxygen enriched air moves from passage 50 into the central passage 26 of the bumer block and exits to the furnace.
When the bumer is converted over to non or limited oxy-fuel firing, the supply of staging oxygen (indicated by arrow 30 in Figure 1) is replaced by fuel, represented by arrow 54, so that fuel or oxygen enriched fuel exits passage 32 of the bumer block 14. Shown schematically in Figure 4 are the passages 26 and 32 at the front end of the bumer block 14 with passage 26 being used to introduce air or oxygen enriched air into the furnace and passage 32 being used to introduce fuel or oxygen enriched fuel into the fiimace. When the bumer 42 is used in the air-fuel firing mode, the air or oxygen enriched air flows through passage 26 and the fuel or oxygen enriched fuel flows through the passage 32. The bumer block design is such that a stable air-fuel flame is established because of the re-circulation region between the two openings.
In addition to the air-fuel firing capability, the device of the present invention permits varying degrees of oxygen enrichment to be accomplished. Use of oxygen enrichment improves flexibility during operation in the backup mode by decreasing the use of oxygen supplied from liquid oxygen storage. It also permits adjustment of the flame length by adding oxygen to the air flow.

Supplemental oxygen can be supplied by various methods. For example the air can be enriched with oxygen, oxygen lances could be supplied through either or both the primary passage 26 of pre-combustor 14 or the staging port 32, or separate oxygen lances could be installed at a distance away from the precombustor 14 or staging port 32. Oxygen introduced through the staging port with the natural gas could provide means to create soot for better radiation heat transfer to the fumace charge.
Using the method and apparatus of the present invention makes it possible to maintain maximum temperature and temperature distribution needed for glass production. Oxygen enrichment or oxy-fuel firing, preferably should be used on burners with the highest firing rates near the hot spot in the fumace. This will reduce the flow rate of air needed for these burners and reduce the pressure drop. Also, oxygen enrichment increases the peak flame temperature and thereby increases heat transfers in the hot spot. It is well known that a hot spot is required in glass making furnaces to established proper convection cells in the glass men which are required to produce glass of acceptable quality.
Other air-fuel technologies can be used to maintain hot hold conditions. This invention is intended to permit the user to continue production. The minimum firing rate provided by the air-fuel backup system is such that at least 20% of the design production rate can be maintained. It is believed that this production rate is sufficient to allow a float glass producer to maintain a continuous glass ribbon in the float bath.
Higher velocity oxy-fuel burners could be modified for low velocity operation by adding one or more inlet ports to use the technology disclosed herein. These inlets could be normally closed or used for staging during oxy-fuel operation. Also, one or more additional inlet ports could be added on-the-fly prior to commencing air-fuel backup by drilling a hole in the refractory wall in a location close to the burner port.
Another alternative for fumaces using high velocity burners is to replace the burner blocks with blocks having larger openings to reduce the pressure drop. With this method there is the danger of introducing foreign refractory material into the glass melt during the replacement procedure which could cause glass

defects. Furthermore, replacement of burner blocks on the fly requires substantial time, possibly too long to avoid interruption of production.
Figure 5 shows the methane flow rate required for hot hold conditions (zero production rate), 20%, 50% and full production conditions, assuming, for example, that 35% of the available heat is required for furnace wall heat losses under full production conditions. Hot hold could be achieved at lower firing rates than the plot shows since the overall furnace temperature, would be lowered thereby reducing the wall heat losses. This plot assumes that the heat losses remain the same, independent of production rate or oxygen usage. The methane flow rate is normalized based on the methane flow rate for full production with 100% oxy-fuel, and the oxygen flow rate is normalized based on the oxygen flow rate for full production with 100% oxy-fuel. The normalized oxygen flow rate is 1.0 when all of the oxidant for combustion is supplied by the oxygen source (no air) and zero when all of the oxidant for combustion is supplied by air.
Figure 6 is a corresponding plot of oxygen concentration as a function of normalized oxygen flow rate for each of the production rates shown in Figure 5.
As indicated by point A in Figure 5, hot hold using only air as the oxidant for combustion (zero normalized oxygen flow rate), the methane flow rate is about the same as required for 100% oxy-fuel at full production (normalized value equal to 1). Hot hold could also be maintained at 35% of the full production oxygen flow rate with 35% of the full production methane flow rate (point B). Referring to Figure 6 (point B), the operating condition represented by point B corresponds to 100% oxy-fuel with no air dilution.
Figure 5 shows that the oxygen flow rate and the methane flow rate can each be reduced by half to produce 20% of full production. This means that if production is limited to 20% of the full production rate, the stored oxygen supply can last two times longer. According to Figure 6, this corresponds to 100% oxy-fuel
firing.
At 50% production, the oxygen flow rate could be reduced to half of the full production flow rate and the methane at about 95% of the fiill production
The exhaust gas temperature from an oxy-fuel furnace is higher than a corresponding air-fuel furnace after the heat recovery device. Glass manufacturers therefore must decrease the temperature of the oxy-fuel combustion products by some method before the gases enter the sections of the flue system fabricated with metal. Because of current air pollution regulations, fuel gas treatment for glass furnaces typically include particulate removal devices, such as electrostatic precipitators or bag houses. These devices have a maximum operating temperature significantly lower than the oxy-fuel fumace exhaust temperature, typically around 1000°F. Therefore, exhaust gases must be cooled by cold (ambient) dilution air before these devices.
If air is substituted for oxygen for combustion in a fumace designed for oxy-fuel combustion, the exhaust volume will be increased substantially. Figure 7 shows how the exhaust flow rate increases as air is substituted for oxygen for several production rates. The same assumptions regarding inlet and outlet temperatures and heat losses used for the previous figures are used to generate this figure. The exhaust flow rate is normalized with respect to the exhaust flow rate for full production with 100% oxy-fuel. For full production, the exhaust flow rate will be increased by more than nine times if oxygen is completely replaced by air. More than three times the exhaust flow rate can be expected at hot hold conditions where air completely replaces oxygen.
As a result of the increased flow of hot exhaust gases, much more dilution air must be provided to decrease the temperature to the same level before the gases enter the metal section of the flue system. Figure 8 shows the result of thermodynamic calculations where fumace exhaust gases at 2800°F are diluted with air at IW to produce a 1000°F gas stream which is a temperature suitable for the metal section of the flue system. The normalized exhaust flow rate after dilution with air is plotted as a function of normalized oxygen flow rate. The exhaust flow is normalized with respect to the 100% oxy-fuel, full production case where exhaust gases at 2800°F are diluted with air at 77°F to produce a 1000°F gas stream. If air is substituted for oxygen under full production conditions and the exhaust gases are diluted with air at 77'^F to produce a logoff stream, the resulting exhaust gas flow rate would be greater than 7,5 times the full production oxy-fuel case. Flue systems are not capable of handling this much of an increase in throughput because of pressure drop limitations. The fumace pressure would increase, possibly leading to structural failure.

There are several ways of dealing with the increased flue gas volume: e.g., reduced production, oxygen enrichment for combustion, alternative ways of cooling the flue gases (e.g. with water), using additional flue gas exhaust capability, bypassing the flue gas treatment section, or a combination of two or more of these above methods. A preferred method of resolving the increased volume of flue gases, in accord with the present invention, is to combine water cooling, reduced production, and if necessary oxygen enrichment for combustion.
Figure 9 shows the results of a thermodynamic calculation where liquid water at 77°F provides evaporative direct contact cooling. The normalized exhaust flow rate after dilution with water is plotted against the normalized oxygen flow rate. The exhaust flow is normalized with respect to the 100% oxy-fuel, full production case where exhaust gases at 2800°F are diluted with air at 77°F to produce a 1000°F gas stream. The figure shows that the exhaust gas volume can be reduced by 50% for full production oxy-fuel operation when water is substituted for air as the cooling medium in the exhaust stream. For the case of full production using air instead of oxygen for combustion and water is used for cooling the exhaust gases, the exhaust flow rate is 3.6 times the base case full production, full oxy-fuel case. For 50% production using air instead of oxygen as the oxidant, the exhaust stream volume is about 2.5 times greater than the base case full production, full oxy-fuel case.

Alternatives to the proposed invention are: Option 1) continued 100% oxy-fuel firing with more oxygen storage, Option 2) hot hold with air-fuel heat up burners, and Option 3) hot hold or some production with high momentum oxy-fuel bumers using air instead of oxygen. The difference between the proposed invention and option 1 is reduced use of oxygen and expense of storage of liquid oxygen. The difference between the invention and option 2 is continued production and expense. The difference between the invention and option 3 is the technical difficulty of supplying air with a high pressure.
The benefit of the invention compared to option 1 is lower capital cost (fewer LOX storage tanks). Also, depending on the length of time that the on-site oxygen plant is down, the logistics and availability problems of liquid oxygen are avoided. A benefit of the proposed invention over option 1 is that it can function if there is a problem with the oxygen supply lines or flow control skids. Another benefit of the invention compared to option 2 is higher maximum temperature in the furnace with similar temperature profile needed for glass production. Yet another

benefit of the invention compared to option 2 is continued production. The most effective process is where full production is continued using air or oxygen enriched air. Even production at a minimum level to sustain a glass ribbon in the float bath is extremely valuable. Reestablishing the glass ribbon is time consuming and could delay production by one or more days. For example, for a flat glass fiimace, that produces 600 tons/day, and with glass valued at $300/ton, one days production is worth $180,000. A further benefit of the invention compared to option 2 is that the back-up system is in place. Option 2 requires that an outside company must come to the facility and install their equipment. A still further benefit of the invention is that the fumace refractory does not need to be drilled, cut, or otherwise disturbed.
The present invention provides the user the ability to use different burners for air-fuel and oxy-fuel operation, a common mounting system for air-fuel and oxy-flier burners, higher maximum fumace temperatures compared to air-fuel heat up burners. The process of the present invention is capable of generating similar temperature distribution in a fumace needed for glass processing, permit higher firing rates at the fumace hot spot by preferentially increasing oxygen concentration, use of separate but closely spaced ports for introduction air and fuel for air-fuel operation, change function of pre-combustor/staging ports for air-fuel and oxy-fuel operation. For oxy-fuel operation, the larger opening is used as a precombustor with oxygen and fuel flow and the smaller opening for oxygen staging. For air-fuel operation, the larger opening is used for flowing air or oxygen enriched air and the smaller port primarily for fuel.
It is within the scope of the present invention to have a separate burner block or pre-combustor placed in the fumace wall to introduce air or oxygen enriched air and fuel into the fumace. In this mode the oxy-flier burner would be turned off and the separate burner block would be used to effect combustion according to the teaching of the invention.
It is also within the scope of the present invention to introduce air and fuel into the fumace through separate burners or pipes that are independent of the oxy-fuel burners, so long as the air or oxygen enriched air and fuel are introduced in accord with the teachings of the invention.
Having thus described our invention what is desired to be secured by Letters Patent of the United States is set forth in the appended claims.

What is Claimed:
1. A process for maintaining heating of a furnace to an elevated
temperature using oxy-fuel combustion, wherein an oxy-fuel flame is introduced into
said furnace and an oxidizer is introduced underneath said flame, when oxygen
supply for said flame and said oxidizer is eliminated or terminated comprising the
steps of:
replacing said oxy-fuel flame with one of air or oxygen-enriched air and introducing said one of air or oxygen enriched air into said finance; and
replacing said oxidizer with said fuel and introducing said fuel into said furnace to provide combustion and maintain said temperature in said fumace.
2. A method according to claim 1 wherein said furnace is a glass melting fumace with temperature distribution maintained in said fumace by using air fuel combustion except in those bumers adjacent a hot spot in said fiimace where oxygen enriched air combustion is used,
3. A method according to claim 1 including replacing said oxy-fuel flame with air introduced at a flow rate about 12.6 times greater than the flow rate of one of oxy-fuel or oxygen when only oxy-fuel combustion is used.
4. A method according to claim 1 wherein the velocity of said one of air or oxygen enriched air at a discharge end of said burner is less than about 250
ft/sec.
5. A method according to claim 1 including introducing said one of air or oxygen enriched air and fuel through a burner block.
6. A method according to claim 1 including the steps of introducing oxygen with said fuel to enhance radiation heat transfer to a charge being heated in said fumace.
7. A method according to claim 1 including the step of cooling
exhaust gases exiting said fumace with liquid water, to decrease the volume of said
exhaust gases, as compared to cooling said exhaust gases with air.

8. In a combustion system of the type having an oxy-fuel bumer
adapted to produce a flame with a pre-combustor mounted on said burner, said pre-
combustor having a first passage with a first end in fluid tight relation to a flame end
of said bumer and a second end adapted to direct the flame produced by said bumer
for heating in industrial environments in a generally flat fan like configuration and a
second separate passage in said bumer block disposed beneath and coextensive with
said first passage, said second passage terminating in a nozzle end in said second end
of said pre-combustor to direct oxidizing fluid underneath and generally parallel to
the flame, the improvement comprising:
first means to introduce one of air or oxygen enriched air through said bumer into said pre-combustor in place of said flame; and
second means to introduce fuel into said second separate passage in said pre-combustor in place of said oxidizing fluid, whereby said combustion system can continue to heat said industrial environment in the event supply of oxygen is interrupted or reduced.
9. A system according to claim 8, wherein said pre-combustor is
between 4 and 18 inches in length.
10 A system according to claim 8, wherein said first passage and said second passage have a width to height ratio of between 5 and 30 at said second end of said pre-combustor.
11. A system according to claim 10, wherein walls defining the width of said first passage and said second passage of said pre-combustor are disposed at an angle between -15to +30 ° on either side of a, central vertical plane through said pre-combustor.
12. A system according to claim 11, wherein said angle is between 0° to +15° on either side of said vertical plane.
13. A system according to claim 8, wherein there is included means to introduce oxygen into said fuel in said pre-combustor.

14. A system according to claim 8, utilized in a heating fumace with means to water cool exhaust gases emerging from said fumace when said burner is in use.
15. A process for maintaining heating of a fumace to an elevated temperature using oxy-fuel combustion, wherein an oxy-fuel flame is introduced into said fumace, when oxygen supply for said flame is eliminated or terminated comprising the steps of:
introducing a stream of one of air or oxygen-enriched air into said fumace; and
introducing a separate stream of said fuel into said fumace to provide combustion and maintain said temperature in said fumace.
16. A method according to claim 15, wherein said fumace is a glass meshing fumace with temperature distribution maintained in said fumace by using air fuel combustion except in those burners adjacent a hot spot in said fumace where oxygen enriched air combustion is used.
17. A method according to claim 15, including replacing said oxy-fuel flame with air introduced at a flow rate about 12.6 times greater than the flow rate of one of oxy-fuel or oxygen when only oxy-fuel combustion is used.
18. A method according to claim 15, wherein the velocity of said one of air or oxygen enriched air at a discharge end of a burner used to introduced said one of air or oxygen enriched air into said fumace is less than about 250 ft/sec.
19. A method according to claim 15, including the steps of introducing oxygen with said fuel to enhance radiation heat transfer to a charge being heated in said fumace.
20. A method according to claim 15, including the step of cooling exhaust gases exiting said fumace with liquid water to decrease the volume of said exhaust gases, as compared to cooing said exhaust gases with air.

21. A process for maintaining heating of a fiimace substantially as herein described with reference to the accompanying drawings.

Documents:

883-mas-2000-abstract.pdf

883-mas-2000-claims filed.pdf

883-mas-2000-claims grand.pdf

883-mas-2000-correspondnece-others.pdf

883-mas-2000-correspondnece-po.pdf

883-mas-2000-description(complete) filed.pdf

883-mas-2000-description(complete) grand.pdf

883-mas-2000-drawings.pdf

883-mas-2000-form 1.pdf

883-mas-2000-form 19.pdf

883-mas-2000-form 26.pdf

883-mas-2000-form 3.pdf

883-mas-2000-form 5.pdf

883-mas-2000-other documents.pdf

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Patent Number

211080

Indian Patent Application Number

883/MAS/2000

PG Journal Number

50/2007

Publication Date

14-Dec-2007

Grant Date

16-Oct-2007

Date of Filing

17-Oct-2000

Name of Patentee

M/S. AIR PRODUCTS AND CHEMICALS, INC

Applicant Address

7201 HAMILTON BOULEVARD, ALLENTOWN, PA 18195- 1501, U.S.A.

Inventors:

#	Inventor's Name	Inventor's Address
1	BRYAN CLAIR HOKE, JR.	234 LONGWOOD DRIVE BETHLEHEM, PA 18020, USA
2	ALEKSANDAR GEORGI SLAVEJKOVE	216 MURRAY DRIVE ALLENTOWN, PA 18104, USA
3	MARK DANIEL D'AGOSTINI	1469 WHITE OAK ROAD, ALLENTOWN PA 18104, USA
4	KEVIN ALAN LIEVRE	1115 GREENLEAF CIRCLE, ALLENTOWN, PA 18103, USA.
5	JOSEPH MICHAEL PIETRANTONIO	8797 SUMMIT CIRCLE FOGELSVILLE, PA 18051.

PCT International Classification Number

F23D14/32

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	09/420215	1999-10-18	U.S.A.