Patent Publication Number: US-2002006591-A1

Title: Method and apparatus for mixing combustion gases

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] This patent application claims priority to U.S. provisional Patent Application Number 60/216,753 filed on Jul. 7, 2000, in the United States Patent and Trademark Office, and to U.S. provisional Patent Application Number 60/222,450 filed on Aug. 8, 2000, in the United States Patent and Trademark Office. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The invention relates in general to combustion burner assemblies of the type used in the production of molten metals, for example copper and aluminum. More particularly, the invention relates to an improved gas mixing device, and a method of mixing combustion gases practiced thereby, adapted for use with a combustion burner assembly.  
       BACKGROUND OF THE INVENTION  
       [0003] The use of shaft furnace burners with the type of shaft furnace disclosed in U.S. Pat. No. 3,199,977 to Phillips et al., for melting metals, to include copper and aluminum, is well known. As known to those of skill in the art, the combustion burner of Phillips et al. is commonly referred to as a shaft furnace burner, and more particularly to a premix burner of the type that burns an oxidizing gas and a fuel gas that have been mixed with one another prior to ignition within a combustion burner. When the furnace burner is operated in a slightly reducing mode, i.e., where less oxygen than required for complete combustion is used in the combustion process, as is typically desirable for the operation of a metal melting shaft furnace, the pre-mixed and substantially homogenous oxidizing gas to fuel gas mixture or ratio utilized produces a flame with a relatively high adiabatic flame temperature.  
       [0004] Although the flame temperature of this type of burner is desirable from the standpoint of melting a metal or metals, the elevated flame temperature also leads to the production of extensive amounts of nitrogen oxide (NO x ) in the off gas, i.e., the exhaust or waste gas emitted from the burner, which is undesirable. Additional examples of premix burners are disclosed in U.S. Pat. No. 3,299,940 to Phillips et al., as well as in U.S. Pat. No. 4,536,152 to Little, Jr. et al., respectively.  
       [0005] Moreover, In a shaft furnace of the type used to melt or refine a metal, it is oftentimes necessary to control the concentration of certain combustion gases, for example oxygen, within the raw molten metal in order to control the mechanical properties of the finished product. The known types of shaft furnace combustion burners normally operate in a combustion region where between 90 to 99 percent of theoretical combustion oxygen is supplied to the burners. This oxygen ratio is controlled strictly by measuring, i.e., sampling, and controlling the products of combustion sampled from the combustion of the premixed oxidizing and fuel gas streams, also known as the combustion gas stream. Deviations on either side of the optimal oxygen range may cause the absorption of excess amounts of oxygen by the molten metal, either from direct exposure to oxygen in lean combustion, or from exposure to the unburned oxygen of the oxidizing gas/oxidizer due to an excessively long combustion flame. A sampled premix burner control method of the type described above is disclosed in U.S. Pat. No. 5,240,494 to Williams et al.  
       [0006] Shaft furnace combustion burner designs that do not premix the oxidizing gas and the fuel gas prior to the injection of the gases into the burner block are know as nozzle mix or non-premix burners. These types of burners operate with a lower adiabatic flame temperature than that of a premix burner, and thus do not attain the melting efficiency of the premix style burners, although they do reduce the amount of nitrogen oxide resulting from the melting process.  
       [0007] What is needed, therefore, but seemingly unavailable in the art is a gas mixing device, as well as a gas mixing method practiced thereby, adapted for use with a combustion burner suitable for use in the melting and production of high-quality metals having an oxygen content in the normal process range, and which also allows for the strict control of the combustion chemistry of the combustion burner for the purpose of reducing the amount of nitrogen oxide resulting from the melting process. Accordingly, what is needed is a combustion gas mixing device and control methodology that permits the combustion burner to act as both a premix burner for attaining the desired oxygen content in the flame and metal molten thereby, as well as a nozzle mix or non-premix burner which allows for the control of the burner&#39;s combustion chemistry, when and as desired.  
       SUMMARY OF THE INVENTION  
       [0008] The present invention provides an improved gas mixing device, as well as a method of mixing combustion gases, for use with a combustion burner that overcome some of the design deficiencies of the prior art. Accordingly, the combustion burner of the present invention comprises a gas mixing device that works in conjunction with a conventional shaft furnace burner. The gas mixing device, as well as the gas mixing and control methods of the invention are suitable for use in the melting and production of high-quality metals requiring an oxygen content in a normal process range, and which also allow for the control of the burner&#39;s combustion chemistry so as to reduce the amount of nitrogen oxide resulting from the metal melting process.  
       [0009] The gas mixing device of the invention therefore operates substantially as a nozzle mix type of burner, with an associated lower flame temperature which results in a lower level of nitrogen oxide production than that of a premix burner, while attaining the performance characteristics of a premix burner. This is achieved by providing a gas mixing device having an elongate gas expansion nozzle adapted to be positioned within a housing provided as a part of a burner assembly. The burner assembly, as known, will have an oxidizing gas supply line in sealed fluid communication with the housing, as well as a fuel gas supply line also in sealed fluid communication with the housing. The gas expansion nozzle has a first end and a spaced second end with a gas expansion chamber defined therein and extending from the first end of the nozzle to the second end thereof. An inlet port is defined at the first end of the gas expansion nozzle, and an outlet port is defined at the second end of the gas expansion nozzle, each of which is in communication with the gas expansion chamber.  
       [0010] The gas mixing device also includes at least one oxidizing gas passageway formed separately of the gas expansion chamber and extending along the gas expansion nozzle, which is provided, and communicates with at least one oxidizing gas outlet at the second end of the expansion nozzle. At least one fuel gas passageway formed separately of the gas expansion chamber is also provided, extending from the first end to the second end of the gas expansion nozzle. The at least one fuel gas passageway is also provided with at least one fuel gas outlet at the second end of the gas expansion nozzle.  
       [0011] The gas mixing device also comprises an elongate gas inlet nozzle spaced from the first end of the gas expansion nozzle. The gas inlet nozzle has a first end, a spaced second end, and defines a gas flow passageway therein extending from the first to the second ends thereof, respectively. A gas inlet port is defined at the first end of the gas inlet nozzle and which is in communication with the gas flow passageway. In like fashion, a gas outlet port is defined at the second end of the gas inlet nozzle which is also in communication with the gas flow passageway.  
       [0012] The gas flow passageway defined within the gas inlet nozzle is formed as a venturi for increasing the velocity of the gas passed therethrough and directed toward the inlet port of the gas expansion nozzle. It is anticipated that the gas outlet port of the gas inlet nozzle will be sized smaller than the inlet port of the gas expansion nozzle so as to minimize, or eliminate, any venturi effect that may occur where the oxidizing gas and the fuel gas enter the inlet port of the gas expansion chamber, as discussed in greater detail below.  
       [0013] As the gas inlet nozzle is spaced from the gas expansion nozzle within the housing, a gap is defined therebetween which functions as a fuel opening for allowing the fuel gas to be passed from a gas manifold extending at least partially about the gas expansion nozzle into the nozzle. The gas flows through this gap into the gas expansion chamber, mixes with the oxidizing gas, and is then passed from the gas expansion chamber as a premixed combustion gas stream into a downstream mixing head, or mixing zone, provided as a part of the burner assembly.  
       [0014] The gas inlet nozzle also includes at least one oxidizing gas passageway formed therein, extending from the first end to the second end thereof, which is positioned with respect to the at least one oxidizing gas passageway of the gas expansion nozzle. An elongate tube or pipe is passed through both of the respective gas inlet nozzle and gas expansion nozzle oxidizing gas passageways, such that the two passageways are sealed to one another for allowing the oxidizing gas to flow therethrough, as well as through the gap between the gas inlet nozzle and the gas expansion nozzle without otherwise being mixed with the fuel gas.  
       [0015] So constructed, the gas inlet nozzle directs at least a portion of the oxidizing gas into the inlet port of the gas expansion nozzle, and also directs at least a portion of the oxidizing gas into the at least one oxidizing gas passageway fashioned within each of the gas inlet nozzle, and the gas expansion nozzle, respectively. The fuel gas supply line, in cooperation with the gas manifold formed about the gas expansion nozzle, ensures that at least a portion of the fuel gas enters the inlet port of the gas expansion nozzle, and also ensures that at least a portion of the fuel gas passes into the at least one fuel gas passageway that extends in the lengthwise direction of the gas expansion nozzle.  
       [0016] Accordingly, oxidizing gas and fuel gas enter into the inlet port of the gas expansion chamber, and are mixed therein to form the premixed combustion gas flow which is passed from the gas expansion nozzle. Simultaneously, separate oxidizing gas and fuel gas streams, respectively, pass through their respective oxidizing and fuel gas passageways and exit through their respective oxidizing gas and fuel gas outlet ports such that the premixed combustion gas stream, as well as the second oxidizing and fuel gas streams, mix with one another in the mixing head of the device. A premix inner combustion flame core is therefore formed, with the second oxidizing gas and fuel gas streams forming a jacket or envelope about the combustion gas flow stream so that when these gases are ignited at a downstream igniter assembly, the premixed combustion gas stream/inner combustion flame core is jacketed by a flame core formed by the second oxidizing and fuel gas streams, once ignited.  
       [0017] The above-described method also includes the steps of sampling the premixed combustion gas stream downstream of the gas expansion chamber, and of selectively adjusting any one, or combination, of the first oxidizing gas or first fuel gas streams passed into the gas expansion chamber, the second oxidizing gas stream, and the second fuel gas stream, respectively, for varying the combustion characteristics and performance of the mixed gases.  
       [0018] The method may also include the steps of matching the oxidizing gas to fuel gas ratio or proportions of the second oxidizing and fuel gas streams, respectively, to the oxidizing gas to fuel gas ratio/proportions of the first oxidizing and fuel gas streams, respectively, of the premixed combustion gas stream.  
       [0019] A novel method for controlling the combustion of a combustion burner assembly also results from the unique design of this invention, which method includes the steps of combining the first stream of the oxidizing gas with the first stream of the fuel gas, respectively, into the premixed combustion gas stream. Thereafter, a sample of the premixed combustion gas stream is obtained through a sampling device extended into the gas stream. A composition analysis is performed of the sampled premixed combustion gas stream, and an ideal oxidizing gas to fuel gas ratio is then calculated using the current combustion burner operating conditions. The calculated ideal ratio is compared to an actual oxidizing gas to fuel gas ratio, as determined by the composition analysis of the premixed combustion gas stream. Thereafter, the oxidizing gas to fuel gas ratio is regulated by adjusting the pressure of the oxidizing gas passed into the gas expansion chamber. This method or control process is continually repeated during the operation of the combustion burner in order to maintain the ideal ratio of oxidizing gas to fuel gas.  
       [0020] A feature of the invention, therefore, is that the combustion of the respective second oxidizing and fuel gas streams can be delayed, having the effect of reducing the temperature of the combustion flame, which in turn reduces the nitrogen oxide content of the combustion burner waste gases passed from the combustion burner assembly, and/or the shaft furnace. The methodology of the invention also includes the steps of checking the overall burner combustion ratio by measuring the composition of the combusted and non-combusted gases at a test burner positioned downstream of the mixing head, and then comparing the results thereof to the composition of the premixed combustion gas stream sampled and measured at the outlet of the gas expansion chamber. Moreover, the oxidizing and fuel gases are introduced into the mixing head at substantially the same pressure for the purpose of minimizing the likelihood of there being a back-pressure condition within the gas expansion nozzle.  
       [0021] It is, therefore, a object of the present invention to provide an improved gas mixing device for use with a combustion burner assembly, as well as an improved method of mixing gases practiced thereby, and also an improved method for controlling the gas mixing operation and the combustion of the gases. It is to this object, as well as the other objects, features, and advantages of the present invention, which will become apparent upon reading the specification, when taken in conjunction with the accompanying drawings, to which the invention is directed. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022]FIG. 1 is a side elevational view in partial cross section of a combustion burner within which the present invention can be used.  
     [0023]FIG. 2 is a side elevational view in cross-section of an enlarged portion of the burner shown in FIG. 1.  
     [0024]FIG. 3 is a side elevational view in cross-section of a preferred embodiment of the gas mixing device of the present invention.  
     [0025]FIG. 4 is a cross-sectional view taken along line  4 - 4  of FIG. 3 illustrating a gas manifold.  
     [0026]FIG. 5 is a cross-sectional view also taken along line  4 - 4  of FIG. 3 of an alternate embodiment of the gas manifold. 
    
    
     DETAILED DESCRIPTION  
     [0027] Referring now in detail to the drawings, in which like reference characters indicate like parts throughout the several views, FIGS. 1 and 2 illustrate a known type of combustion burner assembly adapted for use with a shaft furnace of the type illustrated in U.S. Pat. No. 3,199,977 to Phillips et al., the provisions of which are incorporated herein fully by this reference. In particular, the combustion burner assembly  5  of FIGS. 1 and 2 is a premix type of combustion burner, as disclosed in the aforementioned &#39;977 patent to Phillips et al., and is also disclosed in U.S. Pat. No. 3,299,940 also to Phillips et al., as well as to U.S. Pat. No. 4,536,152, to Little, Jr. et al., the provisions of each of which are incorporated herein fully by this reference.  
     [0028] Referring, therefore, to FIGS. 1 and 2, the combustion burner assembly  5  is illustrated for use with a shaft furnace provided with a furnace wall  7  having a refractory lining  8  and enclosed or encased by a steel shell  9 . The combustion burner assembly includes an igniter assembly  11  comprised of an elongate tubular section  12  extending from and in fluid communication with a burner outlet  13  defined within and extending through the furnace wall. A plurality of sampling ports  15  are defined within the tubular section of the igniter assembly, and are used for withdrawing combustion gas samples for measuring the respective oxidizing and fuel gas components thereof.  
     [0029] The igniter assembly also includes an igniter  16 , which can be any type of desired, and known, igniter. An elongate housing  17  extends upstream of the igniter assembly  11 , and is in sealed fluid communication with an oxidizing gas supply line  19 , and a fuel gas supply line  23 . The oxidizing gas supply line is regulated by a valve  20 , as well as by an orifice plate  21 , as described in greater detail in the aforementioned patents to Phillips et al., as well as to Little, Jr. et al. Additionally, it is anticipated that the combustion burner assembly  5  of FIGS. 1 and 2, as well as the combustion burner assembly  30  of FIGS. 3 through 5, could utilize the control system as described in U.S. Pat. No. 5,240,494 to Williams et al., the provisions of which are incorporated herein fully by this reference.  
     [0030] The fuel gas supply line  23  extends into a gas manifold  24  which extends about the periphery of the housing  17 , and in particular a removable sleeve  25  placed therein. The sleeve  25  defines a mixing chamber or zone  27  within the housing, and is provided with a radially spaced series of fuel gas supply openings or inlets  28  defined therein, such that the fuel gas is passed from the supply line into the fuel gas manifold, and from there through the gas inlets into the mixing zone. The fuel gas stream, denoted by the arrows with the reference character “F,” mixes with the oxidizing gas stream, denoted by the arrows with the reference character “O,” for forming a premixed combustion gas stream, denoted by the reference character “C,” which is passed from the mixing chamber toward the downstream igniter assembly.  
     [0031] A feature of the known type of combustion burner assembly  5  illustrated in FIGS. 1 and 2 is that the mixing of the oxidizing gas and fuel gas can be controlled with some degree of precision for obtaining the desired combustion gas ratios. This results in a combustion gas flame having a relatively high adiabatic flame temperature, but can also result, as known, in the production of undesirable amounts of nitrogen oxide in the off gas or waste gases passed from the combustion burner assembly, as well as the shaft furnace, or other device with which the combustion burner assembly is used. This type of burner assembly construction is contrasted to those types of combustion burners known as nozzle mix burners, or non-premix burners, which operate with a much lower adiabatic flame temperature than that of a premix burner, but which also result in a lesser emission of nitrogen oxide.  
     [0032] The present invention is illustrated in FIGS. 3 through 5. In fashion heretofore unknown in the art, the gas mixing device  30  of FIGS. 3 through 5 provides a combustion burner assembly which can function in many ways similar to a premix type of burner, and yet which is also possessed of the characteristics of a nozzle mix burner for providing a high adiabatic flame temperature, and for also reducing nitrogen oxide emissions when contrasted with the known types of premix burners. This is obtained by providing a elongate gas expansion nozzle  31  which is positioned within, and supported along the longitudinal axis, denoted by the reference character “A,” within the housing  17  of the combustion burner assembly. The gas expansion nozzle has a first end  32  and a spaced second end  34 , and defines a gas expansion chamber therebetween having an inlet port  36  at the first end of the expansion nozzle, and an outlet port  38  at the second end thereof, each of which is in communication with the gas expansion chamber.  
     [0033] As illustrated in FIGS. 3 through 5, at least one oxidizing gas passageway  39  is defined within the gas expansion nozzle separately of the gas expansion chamber, and extends from the first end to the second end thereof. As shown in FIGS. 3 through 5, there is at least a pair of radially spaced oxidizing gas passageways defined within the gas expansion nozzle. The number of oxidizing gas passageways defined within or otherwise formed as a part of the gas mixing device  30  may be as desired, and as determined by the combustion gas requirements of the furnace or other device with which the combustion burner assembly will be used.  
     [0034] Each of the oxidizing gas passageways  39  extends to and is in fluid communication with a respective oxidizing gas outlet  40  formed at the second end of the gas expansion nozzle. These oxidizing gas outlets open into a downstream mixing head or zone  42  which is present externally of the outlet port  38  of the gas expansion nozzle  31 . The premixed combustion gas stream formed within the gas expansion chamber, as discussed in greater detail below, is mixed with separate oxidizing gas and fuel gas streams, respectively, as described in greater detail, below.  
     [0035] Referring to FIG. 3, at least one fuel gas passageway  43  is defined by the gas expansion nozzle in cooperation with the housing  17 , which fuel gas passageway extends from the first end to the second end of the gas expansion nozzle. Similar to the oxidizing gas passageways, each of the fuel gas passageways terminates, i.e. is in communication with, a respective fuel gas outlet  44  opening to the mixing head  42 .  
     [0036] Although the oxidizing gas passageway  39  is shown as being defined within the gas expansion nozzle, and the fuel gas passageway  43  is shown as being defined by the gas expansion nozzle in cooperation with the interior sidewall of the housing  17 , it is anticipated that these gas passageways can be formed in any fashion as desired, and may be defined entirely within the gas expansion nozzle, may be defined by the gas expansion nozzle in association with the housing in which is it placed, or may be fashioned as separate tubes, pipes, or sealed fluid passageways otherwise extending toward the second end of the gas expansion nozzle, and each of which communicates with an oxidizing gas or fuel gas outlet, respectively.  
     [0037] Positioned upstream of the gas expansion nozzle, and spaced from the first end thereof, is an elongate gas inlet nozzle  46 . The gas inlet nozzle has a first end  47 , a spaced second end  48 , and defines a continuous gas flow passageway therein extending from the first to the second ends of the nozzle. A gas inlet port  51  is fashioned at the first end of the nozzle, and a gas outlet port  52  is fashioned at the second end of the nozzle, each of which is in communication with the gas flow passageway.  
     [0038] The gas inlet nozzle also includes at least one oxidizing gas passageway  54  defined therein, and extending from the first end to the second end thereof. As shown in FIG. 3, there are at least a pair of radially spaced oxidizing gas passageways defined within the gas inlet nozzle. As for the gas expansion nozzle  31 , there will be a corresponding, as well as any desired number, of oxidizing gas passageways defined within the gas inlet nozzle, each of which extends into sealed fluid communication with a corresponding oxidizing gas passageway provided as a part of the gas expansion nozzle. Accordingly, and as shown in FIGS. 3 through 5, an elongate tube or pipe  55  is passed through each of the respective oxidizing gas passageways  39 ,  54 , such that each corresponding oxidizing gas passageway is in sealed fluid communication with the other.  
     [0039] As described above, the gas inlet nozzle is spaced from the first end of the gas expansion nozzle by a continuous annular gap  56 , which gap defines a fuel gas supply opening leading into, and extending toward the inlet port of the gas expansion chamber defined within the gas expansion nozzle.  
     [0040] In operation, an oxidizing gas “O” is passed through the oxidizing gas supply line  19  toward, into, and through the gas inlet nozzle  46 . This is accomplished by passing at least a portion of the oxidizing gas through the gas flow passageway  50 , and then through the inlet port  36  of the gas expansion chamber  35 . Another portion of the oxidizing gas, in any desired amount, is passed into the oxidizing gas passageways  54  defined in the gas inlet nozzle, through the elongate tubes  55 , and through the oxidizing gas passageways  39  defined within the gas expansion nozzle. The oxidizing gas passed through the oxidizing gas passageways will exit the gas expansion nozzle through the oxidizing gas outlet ports  40 , and will be passed toward the mixing zone  42 , as described above.  
     [0041] In similar fashion, the fuel gas “F” is passed through the fuel gas supply line  23  and into the annular fuel gas manifold  24  extending about the first end  32  of the gas expansion nozzle  31 , as illustrated in FIGS. 3 through 5. So constructed, at least a portion of the fuel gas is passed through the fuel gas passageway  43  and into the gap  56  for entering the inlet port of the gas expansion chamber, and in so doing, will be mixed with the oxidizing gas being passed from the gas inlet nozzle and into the gas expansion chamber. The oxidizing gas and fuel gas passed into the gas expansion chamber will be thoroughly mixed with one another into a premixed combustion gas stream “C,” as shown in FIG. 3. The premixed combustion gas stream is then passed downstream toward the igniter assembly.  
     [0042] Simultaneous with the passage of at least some of the fuel gas into the inlet port of the gas expansion chamber, a desired amount of the fuel gas is passed through the respective fuel gas passageways  43 , and through the respective fuel gas outlet ports  44  into the gas mixing head or zone  42  formed externally, and downstream, of the outlet port of the gas expansion nozzle. Whereas the oxidizing and fuel gases passed through the gas expansion chamber form the premixed combustion gas stream, the “second” oxidizing gas and fuel gas streams, respectively, passed through their respective oxidizing gas and fuel gas passageways are used in a nozzle-mix fashion for jacketing the premixed combustion gas stream before it is passed into the igniter assembly. The practical effect of this type of combustion gas stream formation is that a two-part flame having an inner flame core comprised of the premixed combustion gas stream is created, with the second oxidizing and fuel gas streams, respectively, combusting and jacketing the inner flame core. This allows for a far greater degree of precision in controlling the gas mixing process, as well as the combustion chemistry, for allowing a relatively high adiabatic flame temperature to be obtained, while also controlling the amount of nitrogen oxide that is emitted from the combustion process.  
     [0043] A gas sampling device  58 , of known construction, is positioned downstream of the outlet port of the gas expansion nozzle such that the gas sampling device measures the respective oxidizing and fuel gases which comprise the premixed combustion gas stream. Positioned further downstream of the gas sampling device, for example approximately 25 inches or so downstream of the mixing head, is a conventional test burner which will sample the combustion gases that have been passed downstream, burn same, and determine the composition of the respective oxidizing and fuel gas ratios therein. Accordingly, using both of the gas sampling and the test burner devices, in association with the described construction of the gas mixing device, a very high degree of control is attainable over the gas mixing process, as well as the combustion process itself, for blending, mixing and maintaining the desired oxidizing gas to fuel gas ratios, all for the purposes of attaining the desired flame temperature and nitrogen oxide measurements.  
     [0044]FIG. 4 is a cross-sectional view of the gas mixing device taken along line  4 - 4  of FIG. 3, illustrating the fuel gas manifold  24  which extends about the exterior circumference of the gas expansion nozzle  31 . The fuel gas passes through the fuel gas supply line  23  such that it is tangentially directed toward the exterior surface of the gas expansion nozzle, and is passed circumferencially about the exterior of the gas expansion nozzle within the gas manifold. In so doing, the fuel gas is evenly distributed or spread about at least the first end of the gas expansion nozzle for uniformly distributing and supplying fuel to the gas expansion chamber through the gap  56 , and from there into the inlet port of the gas expansion chamber.  
     [0045] In FIG. 5, however, the gas supply line  23  is shown intersecting the longitudinal axis A passed through the housing  17  and the gas expansion nozzle  31 , such that a deflector plate  62  is required for directing the fuel gas circumferentially about the exterior surface of the gas expansion nozzle, again for attaining the results of uniformly distributing the fuel gas about the gas expansion nozzle, or at least the first end thereof, and from there passing the fuel gas into the gap  56  and then into the inlet port of the gas expansion chamber.  
     [0046] As shown in FIGS. 4 and 5, by passing the fuel gas flow through the gas manifold and in a direction which is tangent to the exterior surface or periphery of the gas expansion nozzle, a rotational velocity is imparted to the fuel gas therein. This rotational velocity helps to ensure the desired even pressure distribution of the fuel gas about the circumference of the gas manifold, and results in a more uniform entry of the fuel gas into the gap  56  as well as into the fuel gas passageways  43  of the gas mixing device. Additionally, it is anticipated that the rotational velocity of the fuel gas within the manifold may enhance the capability of the gas mixing apparatus to carry pulverized solids therein, as well as other gases or liquids, all as desired. Also, and as one skilled in the art will appreciate, the fuel gas may be passed through the gas inlet nozzle  46  rather than the oxidizing gas, with the oxidizing gas being passed through the gas manifold  24 , and through the gap  56  into the inlet port of the gas expansion chamber, as well as through their separate, and respective, oxidizing and fuel gas passageways, as desired.  
     [0047] The gas expansion nozzle  31 , as well as the gas inlet nozzle  41  may be constructed of any desired material, and preferably of a material which is suitable for use in a high gas flow operation with potentially corrosive gases as well as for use in a heavy duty or severe working environment subject to corrosive forces both internally and externally of the burner assembly. For example, both of the gas expansion nozzle and the gas inlet nozzle may be formed of a ceramic or ceramic coated material, or a metallic material, or of any desired metal or non-metallic material. It is anticipated, however, that the tube  55  passed through the gas inlet nozzle and into the gas expansion nozzle will preferably be formed of a metallic material, and more preferably of a stainless steel for its superior durability and resistance to corrosion. Also, and as shown in FIG. 3, the gas inlet nozzle  41  is sealed to the housing  17 , such that all of the oxidizing gas, or other gas(es) must flow therethrough, and then into either the gas expansion chamber or into the gas mixing zone  42  through the oxidizing gas passageways, as desired.  
     [0048] As described hereinabove, therefore, the gas mixing device of this invention, and primarily the gas expansion nozzle  31  and gas inlet nozzle  46  thereof, replace the interchangeable sleeve  25  of the known combustion assembly  5 . The gas flow passageway  50  of the gas inlet nozzle is formed as a venturi for increasing the velocity of the gases passed therethrough. The gas expansion chamber  35 , however, is formed to be conical, and may be circular in cross-section, for example as shown in FIGS. 4 and 5, although any desired cross-sectional shape of the gas expansion chamber may be used as designed for attaining the desired gas mixing characteristics therein, i.e. for forming the premixed combustion gas stream. Accordingly, the shape of the gas expansion chamber may be, but is not limited to, a conical shape as shown, or may be parabolic, or hyperparabolic, all as desired.  
     [0049] The control methodology practiced by the gas mixing device utilizes a sample of the oxidizing gas and the fuel gas, which may include, for example, the premixed combustion gas stream. The control process thus includes the steps of obtaining a sample of the premixed combustion gas stream through the gas sampling device  58 . A composition analysis of the sampled premixed combustion gas is then performed by burning the premixed sample and analyzing the products of combustion, or by analyzing the ratio of the oxidizing gas to the fuel gas components of the unburned premixed sample directly, or by performing both steps together. Thereafter, an ideal ratio of the oxidizing gas to fuel gas, under the current burner operating conditions, is calculated by using either predetermined data, or chemical calculations, or both, as desired.  
     [0050] The ideal ratio so calculated is then compared to the actual oxidizing gas to fuel gas ratio as measured in the composition analysis of the premixed combustion gas stream. The oxidizing gas to fuel gas ratio is then regulated, as required, by adjusting the pressure of the oxidizing gas supply line, or the fuel gas supply line, or both, again as desired. This control process, as described above, is then repeated throughout the operation of the combustion burner to maintain an ideal oxidizing gas to fuel gas ratio.  
     [0051] Accordingly, during operation of the combustion burner a ceramic water-cooled or heat-resistant metal burner block (not illustrated) provided as a part of the igniter assembly is affixed to the mixing head  42 , and combustion of the oxidizing gas and fuel gas occurs in stages therein. As described above, it is anticipated that during combustion first the premixed combustion gas stream passed through the gas expansion chamber will be combusted, and will burn at the center of a two-part flame. Thereafter, the second oxidizing gas and fuel gas streams passed through their separate oxidizing gas and fuel gas passageways, and through their respective outlet ports and into the mixing head  42 , will be mixed with one another to form an outer core or jacket about the premixed combustion gas stream. The second gas streams are combusted to form the second or outer jacketing portion of the combustion flame. The result of this process is that the delayed burning of the second oxidizing gas and fuel gas streams reduces the flame temperature, which results in a reduced nitrogen oxide content in the burner exhaust or waste gases. The sampled premixed combustion gas stream, taken at the gas sampling device  58  can also be used to indicate the overall combustion chemistry of the flame produced by the combustion for all of the oxidizing gas and fuel gas supplied to the gas mixing device  30 , whether a part of the premixed combustion gas stream or the respective second oxidizing and fuel gas streams passed into the mixing head.  
     [0052] The ratio or proportions of the unmixed oxidizing gas to fuel gas entering the mixing head  42  through the oxidizing gas outlets  40  and the fuel gas outlets  44 , respectively, may be adjusted by selecting an average diameter of the oxidizing gas passageways, and in particular the inner diameter of the tubes  55  passed therethrough, or the fuel gas passageways  43 , or by any other form of resistance that may be placed therein to ensure that the ratio/proportions of the unmixed oxidizing gas to fuel gas are equal to the ratio/proportions of the oxidizing gas to fuel gas in the premixed combustion gas stream, as measured by the gas sampling device  58 . Moreover, the oxidizing gas pressure relative to the fuel gas pressure, at the oxidizing and fuel gas supply lines, respectively, is regulated, in known fashion. The gap  56  formed between the gas inlet nozzle  46  and the gas expansion nozzle  31  and used to pass the fuel gas into the inlet of the gas expansion chamber may also be spaced as desired for regulating the amount of fuel gas allowed to flow into the inlet of the gas expansion chamber.  
     [0053] The overall ratio of the premixed combustion gas stream to the non-mixed or second oxidizing gas and fuel gas streams, respectively, is set at the desired proportion for combustion within the igniter assembly. This ratio will be maintained as long as the oxidizing gas and fuel gas pressure differentials at the oxidizing and fuel gas supply lines, respectively, and at the gap  56  between the nozzles, remains unchanged. The overall burner combustion ratio may be checked by using the test burner  59 , whereupon the composition of the burned or unburned combustion gases may be undertaken and compared to the results of the same quantities of gases measured in the premixed combustion gas stream at the upstream gas sampling device  58 .  
     [0054] It is anticipated that the introduction of the oxidizing gas and the fuel gases into the gas mixing device  30  will occur at substantially the same pressure for each component, in order to mitigate the effects of any back-pressure encountered at the burner outlet, and extending into the interior of the furnace. For example, in the event of a blockage at the burner outlet due to a solidified process metal, the gas pressures will preclude either component, be it the oxidizing gas or the fuel gas, from flowing into their respective supply lines, or into any portion of the gas mixing device normally occupied by the other one of the combustion gas components, which will thus minimize the creation of a dangerous explosive condition within the gas mixing device  30 , and more particularly within the gas expansion nozzle  31 , and/or the mixing head  42 .  
     [0055] To allow for the use of relatively identical oxidizing and fuel gas supply pressures, it is desirable that the diameter of the gas outlet port  52  provided as a part of the gas inlet nozzle  46  be sized smaller than the diameter of the inlet port  36  of the gas expansion nozzle  31 , or that the center line of the gas flow passageway  56  within the gas inlet nozzle be offset laterally with respect to the center line of the gas expansion chamber  35 . By making the diameter of the gas outlet port of the gas inlet nozzle smaller than the inlet port of the gas expansion chamber, or by offsetting the gas flow passageway with respect to the gas expansion chamber, the venturi effect of the gas inlet nozzle in association with the inlet of the gas expansion nozzle will be minimized, if not reduced entirely, while still maintaining the enhanced mixing characteristics created by the device.  
     [0056] An illustrative application of the present gas mixing device would be in the use of a shaft furnace burner used for melting copper or aluminum. The construction of the gas mixing device  30  as described, allows for taking a sample of the premixed combustion gas stream to be analyzed, and the results used to control the combustion process within the furnace, while the secondary oxidizing and fuel gas flow streams passed through the separate oxidizing and fuel gas passageways, respectively, will burn at a lower flame temperature, resulting in a decrease in the quantity of nitrogen oxide produced by the burner.  
     [0057] It is also anticipated, for example, that the gas mixing device  30  of the present invention may fit inside existing combustion burner designs, in place of the cylindrical mixing sleeves  25  (FIGS. 1 and 2) presently used therewith. For example, the gas mixing device may be used in a T-thermal type 300 and 400 series burner, and other similar burners used in existing shaft furnace applications. The ability to substitute the present invention in place of the standard mixing sleeves of the known types of burners allows for the retrofitting of the gas mixing device into many existing shaft furnaces without substantial modification of the furnace piping.  
     [0058] Additionally, it is anticipated that the separate oxidizing gas and fuel gas passageways may be formed such that the combination of the gas expansion nozzle  31  and the gas inlet nozzle  46 , alone, forms a convergent or divergent nozzle allowing a portion of the oxidizing gas and the fuel gas mixture to bypass the nozzle as desired. In this embodiment, the gas mixing device will premix all of the oxidizing gas and fuel gas used in the combustion burner, prior to the introduction of the premixed combustion gas stream into the igniter assembly.  
     [0059] Although several embodiments of the invention have been disclosed in the foregoing specification, it is understood by those skilled in the art that many modifications and other embodiments of the invention will come to mind to which the invention pertains, having the benefit of the teaching presented in the foregoing description and associated drawings. It is thus understood that the invention is not limited to the specific embodiments disclosed hereinabove, and that many modifications and other embodiments are intended to be included within the scope of the invention. Moreover, although specific terms are employed herein, they are used only in a generic and descriptive sense, and not for the purposes of limiting the described invention, and the words “a,” “an,” or “the” can mean one or more, depending upon the context in which the terms are employed.