Abstract:
A method and system for enhancing the supply of combustion air delivered to a burner of a furnace to increase productivity and thermal efficiency of the burner while minimizing changes to the nominal combustion ratio of the burner are disclosed. Oxygen and fuel are introduced into an air plenum upstream of the burner assembly in controlled quantities to form a nonflammable premix of pressurized gases in the air plenum formed from the air, fuel, and oxygen. The air plenum has an outlet that communicates the nonflammable premix of pressurized gases to the burner for combustion. The nonflammable premix of pressurized gases has a predetermined percentage level of oxygen and a predetermined percentage level of fuel. The predetermined percentage level of fuel is selected so that the percentage level of fuel is less than that required to create a flammable premix in the air plenum. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that is will not be used to interpret or limit the scope or meaning of the claims. 37 C.F.R. § 1 .72(b).

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This patent application claims priority to U.S. provisional Patent Application No. 60/217,830 filed on Jul. 11, 2000, in the U.S. Patent and Trademark Office. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates in general to air-oxygen-fuel combustion processes. More particularly, the invention relates to an enrichment system for a supply of air, and a method of enriching the supply of air practiced thereby, adapted for use with a combustion burner of a furnace, such as, for example, the type used in the production of molten metals.  
         BACKGROUND OF THE INVENTION  
         [0003]    A majority of combustion processes use air as an oxidizer to combust a fuel such as natural gas, fuel oil, methane, propane, waste oils, and other hydrocarbons and the like. It is known that the performance of many air-fuel combustion processes may be improved by enriching the combustion air with oxygen. Enrichment of the combustion air results in a hotter flame and generally more thermal efficiency since combustion energy is not wasted on heating a large amount of tramp nitrogen. The cost of using oxygen to enrich the combustion air may be offset by the gains in productivity from the enhanced combustion.  
           [0004]    As known, using oxygen to enhance combustion has many benefits, which include increasing productivity and thermal efficiency which are both of interest in many of the high temperature heating and melting processes used in industry. However, the cost of completely replacing the combustion air with high purity oxygen is often cost prohibitive, and may not be otherwise required or desirable. Thus, as a compromise, prior art apparatuses suggest the use of an intermediate oxygen composition comprising a combination of air and high purity oxygen. It has been demonstrated that there is an initial rise in thermal efficiency benefit as the percentage of oxygen within the intermediate oxygen composition increases up to about 60%. Above 60%, however, the efficiency benefits still increase, but at a much lower rate which results in diminishing economic returns.  
           [0005]    Many attempts have been made to develop burners that use both air and oxygen in the combustion process to maximize the benefit to cost ratio while minimizing NOx production. Air-fuel burner manufacturers have designed new-low NOx burners which incorporate many of the known techniques for minimizing NOx formation including fuel or furnace gas recirculation, oxidizer or fuel staging, pulse combination, and controlled premixing of a flammable mixture of fuel, oxygen, and air. However, in many cases there is a reduction in thermal efficiency and productivity. Examples of premix burners are disclosed in U.S. Pat. No. 3,199,977 to Phillips, et al., U.S. Pat. No. 3,299,940 to Phillips, et al., as well as U.S. Pat. No. 4,536,152 to Little, Jr., et al., respectively.  
           [0006]    Furnace combustion burner designs that do not premix the combustion air and the fuel gas prior to the injection of the gases into the burner are known 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 thermal efficiency of premix burners.  
           [0007]    The introduction of oxygen into the combustion air supply of most furnaces and burners generally requires a readjustment of the furnace fuel supply in response to the availability of oxygen in the air supply to avoid a fuel lean condition at the burner. This readjustment is laborious and time consuming in a furnace that typically has multiple burners, and can upset the production process in the furnace if the readjustment is required when the furnace is in production.  
           [0008]    What is needed, therefore, but seemingly unavailable in the art is an enrichment system, as well as method practiced thereby, adapted for use with conventional premix or non-premix burners to enrich the oxygen content of the air supplied to the burner for the purpose of increasing, in a cost-effective manner, the thermal efficiency and productivity of the burner, but which also allows for the maintenance of the combustion ratio of the burner for the purpose of reducing the amount of readjustment required on the burners. Accordingly, what is needed is a enrichment system that enriches the percentage level of oxygen available in the supply of air provided to a burner as well as minimizing the effect on the burner combustion ratio, when and as desired.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides a method and system for enhancing the supply of air delivered to a conventional premix or non-premix combustion burner assembly to increase productivity and thermal efficiency of the burner while minimizing changes to the nominal combustion ratio of the burner. Oxygen and fuel are introduced into an air plenum upstream of the burner assembly in controlled quantities to form a nonflammable premix of pressurized air, fuel, and oxygen in the air plenum. The air plenum has an outlet that communicates the nonflammable premix of pressurized gases to the conventional premix or non-premix burner for combustion. The nonflammable premix of pressurized gases has a predetermined percentage level of oxygen and a predetermined percentage level of fuel. The predetermined percentage level of fuel is selected so that the percentage level of fuel is less than that required to create a flammable premix in the air plenum.  
           [0010]    In use, a pressurized flow of air is directed into an air plenum of a furnace. A supply of pressurized oxygen is passed into the air plenum via a first inlet port and a supply of pressurized fuel is passed into the air plenum via a second inlet port. The pressurized air, oxygen and fuel mix together in the plenum to form the nonflammable premix of gases in which the percentage level of fuel present within the nonflammable premix is not sufficient to support combustion. The premix of pressurized gases exit through an outlet port of the air plenum and which is in communication with the burner, which burner may be of a conventional premix or non-premix design.  
           [0011]    The above-described method may also include the steps of measuring a flow rate of the air supplied to the air plenum and determining, based on the measured flow rate of air, the flow rate of oxygen and fuel required to provide the desired predetermined percentage levels of fuel and oxygen in the nonflammable premix. The method may also include the steps of regulating the supply of oxygen and the supply of fuel entering the air plenum so that the percentage level of oxygen exiting the air plenum is maintained at the predetermined level of oxygen, and so that the percentage level of fuel exiting the air plenum is maintained at the predetermined level of fuel.  
           [0012]    It is, therefore, a object of the present invention to provide a system and method for safely enriching the supply of air to a burner of a furnace to increase the efficiency of the burner, as well as to enrich the supply of air in a timely and cost-effective manner. It is to this object, as well as 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  
       [0013]    [0013]FIG. 1 is a schematic view of a conventional combustion burner assembly connected to a fuel plenum for carrying a pressurized flow of fuel therein, and an air plenum for carrying a pressurized flow of air therein, the fuel plenum and the air plenum in fluid communication with the combustion burner assembly.  
         [0014]    [0014]FIG. 2 is a schematic view of a first embodiment of an enrichment system for mixing oxygen and fuel into the flow of air to form a nonflammable premix of gases in an air plenum.  
         [0015]    [0015]FIG. 3 is a schematic view of a second embodiment of an enrichment system for mixing oxygen, fuel, and a heat absorber into the flow of air to form a nonflammable premix of gases in an air plenum.  
         [0016]    [0016]FIG. 4 is a schematic view of a third embodiment of an enrichment system for mixing oxygen and fuel into the flow of air to form a nonflammable premix of gases in an air plenum, the oxygen and fuel passing into the air plenum via a first and a second critical orifice. 
     
    
     DETAILED DESCRIPTION  
       [0017]    The present invention is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations will be apparent to those skilled in the art. As used in the specification and in the claims, “a,” “an,” and “the” can mean one or more, depending upon the context in which it is used. The preferred embodiment is now described with reference to the figures, in which like reference characters indicate like parts throughout the several figures.  
         [0018]    While the system of the present invention will be described in its preferred form as being utilized to enrich a flow of pressurized combustion air with oxygen and fuel, it will be appreciated that this invention may be practiced with a wide variety of gaseous fuels, including natural gas, CO, hydrogen, propane, hydrogen-rich fuels, refinery waste fuels, other gaseous or vaporized fuels such as, for example, vaporized naptha, and the like. One will also appreciate that the invention may be used with a wide variety of burner constructions and in a wide variety of furnace installations. Additionally, the enrichment system described below is well suited for use with existing furnaces, or may be readily integrated into the design of new furnaces.  
         [0019]    Referring to FIG. 1, a schematic representation of the supply of combustion air and fuel to a typical combustion burner assembly  4  of a furnace  2  is shown. Here, a fuel plenum  20  and an air plenum  10  are in fluid communication with a combustion burner assembly  4 . The combustion burner assembly  4  is of a conventional type which may be, for example, a premix combustion burner assembly or a non-premix combustion burner assembly. Both the fuel plenum  20  and the air plenum  10  are of the conventional type that is used to duct a pressurized flow of a gas. The fuel plenum  20  carries a pressurized flow of fuel from a source of pressurized fuel, and has an outlet port  22  that is adapted to be in fluid communication with the combustion burner assembly. Similarly, the air plenum  10  carries a pressurized flow of combustion air A from a blower  16  or fan to an outlet port  12  that is adapted to be in fluid communication with the combustion burner assembly  4 . The air plenum  10  can have multiple outlet ports  12  if, for example, the air plenum  10  supplies two or more combustion burner assemblies  4 .  
         [0020]    If a premix combustion assembly is used, and as known, the housing of a premix combustion burner assembly generally defines a premixing chamber  6 . The outlet ports  12 ,  22  of the air plenum  10  and the fuel plenum  22  respectively, are in communication with the premixing chamber  6 . Thus, streams of gas exiting from the outlets  12 ,  22  of the respective air and fuel plenums  10 ,  20  pass into the premixing chamber  6  and are mixed together to form a flammable premixed combustion gas stream which is subsequently ignited to provide the flame for the burner assembly.  
         [0021]    Referring now to FIG. 2, a first embodiment of the enrichment system of the present invention is shown. Here, a portion of the air plenum  10  having a plurality of inlet ports  10  is shown. The plurality of inlet ports  30  includes at least a first inlet port  31  and a spaced second inlet port  32 .  
         [0022]    An oxygen supply plenum  50  and a separate fuel supply plenum  70  are connected to the air plenum  10 . The oxygen supply plenum  50  is adapted to be in fluid communication with a supply of pressurized oxygen  52  for supplying pressurized oxygen to the air plenum  10 . The oxygen supply plenum  50  has a distal end  54  that is connected to the first inlet port  31  of the air plenum  30 . In like fashion, the fuel supply plenum  70  is adapted to be in fluid communication with a supply of pressurized fuel  72  for supplying of pressurized fuel to the air plenum  70 . The fuel supply plenum  70  thus has a distal end  74  that is connected to the second inlet port  32  of the air plenum  10 . In the air plenum  10 , the pressurized oxygen and the pressurized fuel, together with the pressurized combustion air previously placed into the air plenum  10 , mix together to form a non-flammable premix of pressurized gases M.  
         [0023]    To ensure that combustible mixtures within the air plenum  10  are not inadvertently formed by pockets of unmixed oxygen and fuel therein, it is preferred that the oxygen and the fuel are introduced into the air plenum at different points along the air plenum  10 . Thus, preferably, the first and the second inlet ports  31 ,  32  are spaced apart at least one air plenum diameter D apart, and more preferably at least four air plenum diameters apart, and still more preferably, at least seven air plenum diameters apart. It is also preferred that the first inlet port  31  be positioned upstream of the second inlet port  32  so that the introduced oxygen is mixed with the combustion air prior to the introduction of the fuel into the air plenum  10 . The inlet ports  30  are therefore spaced proximate to the outlet port  12  of the air plenum  10 , and are thus proximate to the combustion burner assembly  4 .  
         [0024]    Unlike prior art designs, which generally only add oxygen to the air plenum  10  to enhance the thermal efficiency of the burner  4 , in the present invention both fuel and oxygen are added to the air plenum  10  in particular predetermined percentage levels. Thus, the non-flammable premix of pressurized gases exiting the outlet port  12  of the air plenum  10  has a predetermined percentage level of oxygen and a predetermined percentage level of fuel. The predetermined percentage level of oxygen is greater than the percentage of oxygen available in the supply of the combustion air because of the addition of the oxygen into the air plenum  10 . The oxygen enrichment of the gases that are delivered to the combustion burner assembly  4  therefore serves to increase the thermal efficiency of the burner  4 . Further, the addition of the fuel to the non-flammable premix of gases in the air plenum  10  aids in preventing the combustion ratio at the burner  4  from becoming excessively fuel lean. Accordingly, the amount of readjustment required on the burner  4  is minimized while also at the same time the combustion efficiency of the burner  4  is being increased.  
         [0025]    To prevent an explosion and/or combustion hazard, the predetermined percentage level of the fuel is less than that required to create a flammable premix in the air plenum  10 . Thus, the premix of air, oxygen, and fuel supplied to the outlet port  12  of the air plenum  10  is non-flammable. Only upon the addition of additional fuel at the combustion burner assembly  4 , through the separate fuel plenum  20 , will a combustible mixture of gases be formed.  
         [0026]    The first embodiment of the enrichment system may also include an air flow rate sensing device  80  for measuring the flow rate of the air flowing through the air plenum  10  upstream of the first inlet port  31 , a control device  90  for controlling the percentage levels of oxygen and fuel exiting the air plenum  10 , an oxygen regulating device  100  for regulating the supply of pressurized oxygen passed through the first inlet port  31 , a fuel regulating device  120  for regulating the supply of pressurized fuel passed through the second inlet port  32 , an oxygen feedback device  110  for adjusting the oxygen regulating device  100  so that the percentage level of oxygen exiting the air plenum  10  is maintained at the predetermined percentage level of oxygen, and a fuel feedback device  130  for adjusting the fuel regulating device  110  so that the percentage level of fuel exiting the air plenum  10  is maintained at the predetermined percentage level of fuel.  
         [0027]    The air flow rate sensing device  80  generates an air flow rate output signal  82  based on the measured flow rate of the air in the air plenum  10 . The air flow rate sensing device  80  may, for example, comprise a conventional flow rate sensor coupled to the air plenum  10  upstream of the first inlet port  31 . Alternatively, power readings, such as for example, blower amps, from the combustion air blower  16  may be used to determine the flow rate of the air in the air plenum  10 .  
         [0028]    The oxygen regulating device  100  and the fuel regulating device  120  are operably coupled to the respective oxygen supply plenum  50  and the fuel supply plenum  70 . Each regulating device is in fluid communication with a respective one or two inlet ports of the air plenum  10  and is adapted to regulate the flow of the supply of gas in fluid communication with its respective inlet port  30 . In the preferred embodiment, each regulating device  100 ,  120  comprises a regulator defining a passage (not shown) through which a gas traverses, and a flow controlling device (not shown) for adjusting the passage to change the rate of flow of the gas therethrough. The respective oxygen and fuel feedback device  100 ,  120  adjust the flow controlling device of at least one regulator, if necessary, so that the percentage level of oxygen and the percentage level of fuel exiting the air plenum  10  through the outlet gas port  12  is established and maintained at the predetermined percentage levels. The regulator may be electrically actuated or pneumatically actuated, as known in the art. Further, the regulator may be a binary regulator, which is in either a fully open or a fully closed position, or, more preferably, a proportional regulator, in which the passage is opened in different amounts corresponding to the various desired flow rates.  
         [0029]    In this embodiment, the control device  90  is primarily responsive to the air flow output signal  82 . The control device  90  establishes a determined oxygen flow rate from the oxygen supply plenum  50  and a determined fuel flow rate from the fuel supply plenum  70  which, when combined with the flow rate of the pressurized combustion air, will provide the predetermined percentage level of oxygen and the predetermined percentage level of fuel within the nonflammable premix of pressurized gases in the air plenum  10 . The determined oxygen flow rate level and the determined fuel flow rate level are preferably continuously determined and updated to reflect any changes to the flow rate of the combustion air so that the determined oxygen and fuel flow rate levels are sufficient to provide the desired percentage compositional mix of air, oxygen, and fuel in the non-flammable premix of gases. Based on the determination of the respective determined oxygen and fuel flow rates, the control device  90  generates an oxygen meter signal  102  and a fuel meter signal  122 . Preferably, the control device  90  comprises a processor or microprocessor electrically coupled to the air flow rate sensor or blower output that is used as the air flow rate sensing device  80 .  
         [0030]    The control device  90 , in response to the air flow rate output signal  82 , may, as a safety measure, also compare the flow rate of the air to a predetermined air flow rate level and generate the oxygen meter signal  102  and the fuel meter signal  122  only if the measured flow rate of the air is at least equal to the predetermined air flow rate level. This ensures that pressurized oxygen and pressurized fuel are not added to the air plenum  10  if an obstruction exists in the air plenum  10  or the combustion burner assembly  4  downstream of the inlet ports  30  within the air plenum  10 . Thus, a minimum flow rate of air through the air plenum  10  is ensured before pressurized oxygen and pressurized fuel are passed into the air plenum  10 .  
         [0031]    The control device  90  controls the percentage composition of the nonflammable premix of gases exiting the outlet port  12  of the air plenum  10  by correcting the flow rate of oxygen and fuel entering the air plenum  10  through the first and second inlet ports  31 ,  32 . This control process occurs using the oxygen feedback device  110  and the fuel feedback device  130 , which are responsive to the oxygen meter signal  102  and the fuel meter signal  122 , respectively. The oxygen feedback device  110  adjusts the oxygen regulating device  100 , and the fuel feedback device  130  adjusts the fuel regulating device  120  so that the percentage level composition of oxygen and the percentage level composition of fuel in fluid communication at the outlet port  12  of the air plenum  10  is maintained at the predetermined percentage levels.  
         [0032]    Preferably, each feedback device  110 , 130  comprises a driver circuit electrically coupled to the microprocessor and to its respective regulating device, e.g., the flow controlling device. Thus, the oxygen feedback device  110  comprises a first driver circuit  112  coupled to the microprocessor and to the oxygen regulating device  100 , and the fuel feedback device  130  comprises a second driver circuit  132  coupled to the microprocessor and to the fuel regulating device  120 . The first and second driver circuits  112 ,  132  adjust the respective oxygen and fuel regulating device  100 ,  120  based on electrical signal received from the microprocessor, thus varying the percentage level composition of oxygen and fuel within the air plenum  10 .  
         [0033]    For continual feedback, the first embodiment of the enrichment system may also include an oxygen flow rate sensing device  150  for measuring the flow rate of oxygen exiting the oxygen supply plenum  50 , and a fuel flow rate sensing device  170  for measuring the flow rate of fuel exiting the fuel supply plenum  70 . The oxygen flow rate sensing device  150  is preferably disposed adjacent the distal end  54  of the oxygen supply plenum  50  and in fluid communication with the first inlet port  31 . The oxygen flow rate sensing device  150  generates an oxygen flow rate output signal  152  based on the measured oxygen flow rate. Similarly, the fuel flow rate sensing device  170  is preferably disposed adjacent the distal end  74  of the fuel supply plenum  70  and in fluid communication with the second inlet port  32 . The fuel flow rate sensing device  170  generates a fuel flow rate output signal  172  based on the measured fuel flow rate. Each fuel and oxygen flow rate sensing device  150 ,  170  preferably comprises a conventional flow rate sensor. However, any sensor that is capable of determining the flow rate of gas exiting the respective oxygen and fuel supply plenums  50 ,  70  and providing a signal representative of the measured flow rate may by substituted for the respective oxygen flow rate or fuel flow rate sensors.  
         [0034]    If the enrichment system has an oxygen flow rate sensing device  150  and a flow rate sensing device  170 , the microprocessor of the control device  90  is electrically coupled to the oxygen flow rate sensor that is used as the oxygen flow rate sensing device  150  and to the fuel flow rate sensor that is used as the fuel flow rate sensing device  170 .  
         [0035]    The control device  90  compares the output of the oxygen flow rate sensing device  150  to the determined oxygen flow rate and generates an oxygen response signal  104  based upon this comparison. Further, the control device  90  compares the output of the fuel flow rate sensing device  170  to the determined fuel flow rate level and generates a fuel response signal  124  based upon this second comparison. The control device  90  controls the percentage composition of the non-flammable premix of gases exiting the outlet port  12  of the air plenum  10  by correcting the flow rate of oxygen and fuel entering the air plenum  10  through the first and second inlet ports  31 ,  32 . This control process occurs by using the oxygen feedback device  110  and the fuel feedback device  130 , which are responsive to the oxygen response signal  104  and the fuel response signal  124 , respectively. The oxygen feedback device  110  adjusts the oxygen regulating device  100  and the fuel feedback device  130  adjusts the fuel regulating device  120  so that the flow rates of oxygen and fuel entering the air plenum  10  are maintained at the determined oxygen and fuel flow rates, which then allows the percentage level composition of oxygen and the percentage level composition of fuel in fluid communication with the outlet port  12  of the air plenum  10  to be maintained at the predetermined percentage levels.  
         [0036]    A sonic flow rate sensor, such as known in the art, coupled to the distal ends  54 ,  74  of the respective oxygen and fuel supply plenums  50 ,  70  may be a suitable substitute for the described oxygen and/or the fuel flow rate sensors. Each sonic flow rate sensor defines a critical orifice (not shown) which is in fluid communication with the air plenum  10 .  
         [0037]    Alternatively, the distal end  54  of the oxygen supply plenum  50  may define a first critical orifice  56  in fluid communication with the air plenum  10 , and the distal end  74  of the fuel supply plenum  70  may define a second critical orifice  76  which is in fluid communication with the air plenum  10 . The critical orifices  56 ,  76  serve to mix the respective gases within the air plenum  10  due to the pressure wave fronts that propagate from the critical orifices  56 ,  76  proximate the air plenum  10 .  
         [0038]    In use, a pressurized flow of air is directed into the air plenum  10  of the furnace  2 . The supply of pressurized oxygen is passed into the air plenum  10  via the first inlet port  31 , and the supply of pressurized fuel is passed into the air plenum  10  via the second inlet port  32 . The pressurized air, oxygen, and fuel mix together in the air plenum  10  to form the nonflammable premix of gases M in which the percentage level of fuel present within the nonflammable premix is not sufficient to support combustion. The premix of pressurized gases exits thought the outlet port  12  of the air plenum  10  towards the combustion burner assembly  4 .  
         [0039]    The method may also include the steps of measuring a flow rate of the air supplied to the air plenum  10  and determining, based thereon, the flow rate of oxygen and fuel required to provide the desired predetermined percentage levels of fuel and oxygen in the nonflammable premix. The method may also include the steps of regulating the supply of oxygen and the supply of fuel entering the air plenum  10  so that the percentage level of oxygen exiting the air plenum  10  is maintained at the predetermined level of oxygen, and so that the percentage level of fuel exiting the air plenum  10  is maintained at the predetermined level of fuel. Preferably the steps of measuring and regulating occur continuously when the non-flammable premix of pressurized gases is being supplied to the outlet port  12  of the air plenum  10 .  
         [0040]    Before supply of oxygen and fuel to the air plenum  10  is initiated, the method may also comprise the steps of comparing the air flow rate output signal  82  to the predetermined flow rate level of the air, and only supplying pressurized oxygen and pressurized fuel to the air plenum  10  when the air flow rate output signal  82  is at least equal to the predetermined flow rate level of the air.  
         [0041]    The method may also include the steps of monitoring the flow rate of oxygen and the flow rate of fuel entering the air plenum  10 , comparing the measured oxygen flow rate to the determined oxygen flow rate and the measured fuel flow rate to the determined fuel flow rate, and regulating the supply of oxygen and the supply of fuel entering the air plenum  10  so that the flow rate of oxygen supplied to the air plenum  10  is maintained at the determined oxygen flow rate and so that the flow rate of fuel supplied to the air plenum  10  is maintained at the determined fuel flow rate. Preferably, these monitoring, comparing, and regulating steps occur continuously.  
         [0042]    In an example of use, the lower limit of flammability of methane in oxygen or air is approximately 4% at room temperature. Therefore, a mixture of less than approximately 4% methane in air or oxygen will not support combustion. So, by introducing methane into the air plenum  10  at a predetermined 3% level, which is less than that required to create a flammable premix in the air plenum  10 , and by introducing oxygen into the air plenum at a predetermined 6% level, the amount of oxygen available in the resulting nonflammable premix of pressurized gases is increased by approximately 28% and the available nitrogen is decreased by approximately 13%. Additionally, the combustion ratio of the combustion burner assembly  4  will remain generally constant due to the addition of the methane to the non-flammable premix.  
         [0043]    Referring now to FIG. 3, a second embodiment of the present invention is shown. This embodiment further comprises a pressurized heat absorber plenum  180 , a heat absorber regulator  184 , and a heat absorber driver circuit  186 . The heat absorber plenum  180  has a distal end  182  connected to a third inlet port  33  of the air plenum  10  for supplying a pressurized heat absorber, such as, for example, vaporized water, ammonia, and the like, to the air plenum  10 . The third inlet port  33  may be positioned where desired on the air plenum  10 , but is preferably positioned adjacent the second inlet port  32 . The heat absorber regulator  184  is operatively coupled to the heat absorber plenum  180  and is in fluid communication with the third inlet port  33  of the air plenum  10 . The heat absorber driver circuit  186  is operably coupled to the microprocessor and the heat absorber regulator  184  so that the flow rate of heat absorber supplied to the air plenum  10  is maintained at a predetermined heat absorber rate. The addition of heat absorber to the air plenum  10  acts to increase the lower limit of flammability of the non-flammable premix of gases in the air plenum  10 .  
         [0044]    [0044]FIG. 4 illustrates a third embodiment of the enrichment system of the present invention. In this embodiment, the distal end  54  of the oxygen supply plenum  50  defines a first critical orifice  56  of known dimension and the distal end  74  of the fuel supply plenum  70  defines a second critical orifice  76  of known dimension. Critical orifices are known to those skilled in the art, and allow for the sonic or near sonic passage of a predetermined flow rate of a gas from, in this instance, the respective oxygen and fuel supply plenums  50 ,  70  to the air plenum  70 . As noted above, the critical orifices  56 ,  76  also serve to mix the respective gases within the air plenum  10 . Thus, in this embodiment, pressurized oxygen at a predetermined pressure is supplied to the first critical orifice  56  and is passed therethrough and into the air plenum  10  at a predetermined fuel flow rate. Pressurized fuel at a predetermined pressure is supplied to the second critical orifice  76  and is passed therethrough and into the air plenum  10  at a predetermined fuel flow rate. The streams of the combustion air, oxygen, and fuel supplied to the air plenum  10  mix together to form the non-flammable premix stream of pressurized gases which is directed toward, and exits from the outlet port  12  of the air plenum  10 .  
         [0045]    This embodiment of the enrichment system may also include the oxygen flow rate sensing device  150  for measuring the flow rate of oxygen exiting the oxygen supply plenum  50 , the fuel flow rate sensing device  170  for measuring the flow rate of fuel exiting the fuel supply plenum  70 , a control device  90  for controlling the percentage levels of oxygen and fuel exiting the air plenum  10 , the oxygen regulating device  100  for regulating the supply of pressurized oxygen passed through the first inlet port  31 , the fuel regulating device  120  for regulating the supply of pressurized fuel passed through the second inlet port  32 , the oxygen feedback device  110  for adjusting the oxygen regulating device  100  so that the percentage level of oxygen exiting the air plenum  10  is maintained at the predetermined percentage level of oxygen, and the fuel feedback device  130  for adjusting the fuel regulating device  120  so that the percentage level of fuel exiting the air plenum  10  is maintained at the predetermined percentage level of fuel.  
         [0046]    The oxygen flow rate sensing device  150  is in fluid communication with the first critical orifice  56  and the fuel flow rate sensing device  170  is in fluid communication with the second critical orifice  76 . Thus, in this embodiment of the invention, a sonic flow rate sensor of conventional design may be a suitable substitute for the oxygen and fuel flow rate sensors because the critical orifice  56 ,  76  is defined within the sonic flow rate sensor. However, conventional flow rate sensors may be used, as previously discussed.  
         [0047]    Also, in this embodiment the control device  90  is primarily responsive to the oxygen flow rate signal  152  and the fuel flow rate signal  172 . The control device  90  compares the output of the oxygen flow rate sensing device  150  to a predetermined oxygen flow rate level and generates an oxygen response signal  114  based upon the comparison. Further, the control device compares the output of the fuel flow rate sensing device  170  to a predetermined fuel flow rate level and generates a fuel response signal  124  based upon the comparison. The predetermined oxygen and fuel flow rate levels are based on a substantially constant and known flow rate of the combustion air and are sufficient to provide the desired percentage compositional mix of air, oxygen, and fuel in the non-flammable premix of gases. In this embodiment, the control device  90  preferably comprises a processor or microprocessor electrically coupled to the oxygen flow rate sensor and the fuel flow rate sensor that are used as the respective oxygen and fuel flow rate sensing devices  150 ,  170 .  
         [0048]    The control device  90  controls the percentage level composition of the non-flammable premix of gases exiting the outlet port  12  of the air plenum  10  by correcting the flow rate of oxygen and fuel entering the air plenum  10  through the first and second inlet ports  31 ,  32 . This control occurs using the oxygen feedback device  110  and the fuel feedback device  130 , each of which are responsive to the oxygen response signal  114  and the fuel response signal  124 , respectively. The oxygen feedback device  110  adjusts the oxygen regulating device  100  and the fuel feedback device  130  adjusts the fuel regulating device  120  so that the flow rate of oxygen and fuel entering the air plenum  10  is maintained at the desired predetermined oxygen and fuel flow rates and so that the percentage level composition of oxygen and the percentage level composition of fuel in fluid communication with the outlet port  12  of the air plenum  10  are maintained at the predetermined percentage levels.  
         [0049]    The third embodiment of the enrichment system may also include the air flow rate sensing device  80  for measuring the flow rate of the combustion air through the air plenum  10  upstream of the first inlet port  31 . The air flow rate sensing device generates an air flow rate output signal  82  based on the measured flow rate of the air. Thus, as a safety measure, the control device  90  of the third embodiment may be responsive to the air flow rate output signal  82  to compare the flow rate of the air to a predetermined air flow rate level and generate the oxygen response signal  114  and the fuel response signal  124  if the measured flow rate of the air is at least equal to the predetermined air flow rate level. Thus, a minimum flow rate of air through the air plenum  10  is ensured before pressurized oxygen and pressurized fuel are added to the combustion air within the air plenum  10 .  
         [0050]    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.