Abstract:
A round burner capable of being operated with reduced CO and NO x  emissions includes a venturi tube positioned to direct a flow of air through the burner and into a combustion zone in a combustion chamber through an entrance in a wall of the combustion chamber. The venturi tube has inlet and outlet ends and a throat. The outlet end of the venturi tube has a larger internal diameter than either the inlet end or the throat and the same is positioned adjacent the entrance to the combustion chamber. The inlet end of the venturi tube is also positioned further from the entrance than the outlet end of the venturi tube. The burner may include a duct system that includes an inlet disposed in fluid communication with the combustion zone and an outlet disposed in fluid communication with the venturi tube adjacent the throat thereof. The system is arranged and adapted to recirculate a stream of flue gas from a location within said combustion chamber adjacent the combustion zone by induction into the venturi tube at a low pressure location adjacent the throat of the venturi tube so that the recirculated stream of internal flue gas is inducted into and intermixed with the flow of air at the throat of the venturi tube. Alternatively or cumulatively, the burner may include a fuel gas injector arrangement having an injector nozzle extending through the wall at a location adjacent the combustion zone. The injector nozzle is in fluid communication with the combustion chamber and is positioned to direct a flow of fuel gas into the combustion chamber at a location in the wall radially beyond the inner edge of the entrance. Also disclosed is a method for operating the burner to reduce CO and NO x  emissions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based at least in part on the disclosure of provisional application Ser. No. 60/089,570 filed on Jun. 17, 1998 and priority under 35 U.S.C. §119(e) is claimed from such provisional application. The entirety of the disclosure of said provisional application is hereby incorporated herein be reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention of the present application relates to burners for large scale industrial applications. Such burners may be adapted for burning gaseous fuels including natural gas. Such burners may also be adapted for burning fuel oil. And in many cases the burners may be adapted for burning both gaseous fuels and fuel oil either alternatively or at the same time. In particular the invention relates to industrial burners which burn fuel gas and/or oil and are specially constructed and engineered for emitting low levels of nitrogen oxide (NO x ) and carbon monoxide (CO) air pollution. The invention also relates to the methodology for operating such burners. More particularly the invention relates to a burner and the methodology for operating the same whereby substantial reductions of CO and NO x  emissions are achieved relative to existing burners. 
     2. The Prior Art Background 
     Many designs exist for delivering fuel and air to a furnace combustion chamber or firebox. Virtually all modern prior art designs are intended to enhance combustion efficiency. In addition, tube metal temperatures and other furnace component limitations must be taken into consideration in designing furnace burners. More recently governmental regulations and social pressures require designers to take into consideration the reduction of CO and NO x  emissions. 
     One of the best of the more recently developed industrial burners is the Todd Variflame No Internal FGR Injection and No External Gas Injection Burner which uses an array of internal poker tubes for delivering fuel and air to a furnace firebox. This system is the subject matter of U.S. Pat. No. 5,860,803 to Schindler et al. which issued on Jan. 19, 1999 (the “&#39;803 patent”). The entirety of the disclosure of the &#39;803 patent is hereby incorporated herein by reference. 
     In spite of the efforts of many prior art workers in the field, a perfect solution to the CO and NO x  emissions problem remains elusive. Some have tried to reduce NO x  emissions by recirculating flue gas into the firebox. However, when flue gas is recirculated from a downstream location, the costs associated with providing and forcing such recirculation are substantial. 
     SUMMARY OF THE INVENTION 
     The present invention provides a device and methodology for efficiently and economically reducing the amount CO and NO x  emission from a combustion chamber without substantially effecting thermal efficiency and/or reaction parameters of the same. In particular the invention provides a novel burner design and novel operating methodology which utilizes internal flue gas recirculation and/or external fuel injection in a venturi tube burner system. More particularly, the invention provides a venturi tube burner system which provides swirled primary and straight line secondary combustion air in the venturi tube and straight line tertiary air outside the venturi tube to provide novel effects in the burner flame formed under the above conditions. Preferably the burner includes internal flue gas recirculation and/or external fuel injection. 
     As a result of extensive research and development conducted by the present inventors, an improved burner design has been developed whereby it is possible to achieve substantial reductions in CO and NO x  emissions without substantial loss of burner efficiency. Thus, in accordance with one aspect of the present invention, a novel round burner is provided which comprises a venturi tube positioned to direct a flow of air through the burner and into a combustion zone in a combustion chamber through an entrance in a wall of the combustion chamber. The venturi tube has inlet and outlet ends and a throat located between the inlet and outlet ends. The outlet end has a larger internal diameter than either the inlet end or the throat. The outlet end of the venturi tube is positioned adjacent the entrance to the combustion chamber and the inlet end of the venturi tube is positioned further from the entrance than the outlet end. 
     The novel burner of the invention also provides a duct system that includes at least one inlet disposed in fluid communication with the combustion zone, and at least one outlet disposed in fluid communication with the throat of the venturi tube. The duct system is arranged and adapted to recirculate flue gas from a location within said combustion chamber adjacent said combustion zone and into said venturi tube at a location adjacent said throat, whereby the recirculated flue gas is inducted into and intermixed with said flow of air at said throat of the venturi tube. Thus, NO x  emission reduction may be achieved without the expense of an external flue gas recirculation system. 
     In another aspect of the invention, the invention provides a round burner which comprises a venturi tube positioned to direct a flow of air through the burner and into a combustion zone in a combustion chamber through an entrance in a wall of the combustion chamber. The novel burner of this aspect of the invention includes a fuel gas injector arrangement including at least one injector nozzle extending through the wall of the combustion chamber at a location adjacent said combustion zone. Such injector nozzle is in fluid communication with the combustion chamber. The injector nozzle is positioned to direct a flow of fuel gas into said combustion chamber at a location in the wall radially outward of and beyond the inner edge of the entrance. 
     In yet another aspect of the invention, the novel burner may include both the duct system for recirculated flue gas and the fuel gas injector arrangement described above. 
     In its more specific aspects, the burner of the present invention may include a first fuel gas nozzle that is located in the venturi tube and which is positioned to introduce a supply of fuel gas into the air flowing through the venturi tube. The burner may also include a swirler positioned so that at least a primary portion of the air flow passes therethrough. Ideally the arrangement of the outlet end of the venturi tube and the swirler may be such that a secondary portion of the air flow does not pass through the swirler. Even more ideally, an annular gap may be provided between the outer periphery at the outlet end of the venturi tube and an inner edge of said entrance. Such gap may be positioned to direct a tertiary air flow around the periphery of the venturi tube and through the entrance into said combustion chamber. 
     Preferably, at least one first fuel gas nozzle may be positioned centrally of the venturi tube adjacent a longitudinal axis thereof and at a location to introduce fuel gas into said primary portion of the flow of air. At least one fuel gas poker nozzle may also be included at a position to introduce fuel gas into said secondary portion of the flow of air. 
     The burner of the invention may be equipped to burn either fuel gas or oil. 
     The invention also provides a method for operating a venturi tube equipped round burner of the sort described above. In accordance with this aspect of the invention, the method comprises directing a flow of air through said venturi tube and into a combustion zone in said combustion chamber through said entrance and recirculating flue gas from a location in said combustion chamber adjacent said combustion zone and into the venturi tube at a location adjacent the throat of the venturi tube, whereby said recirculated flue gas is inducted into and intermixed with the combustion air flow at the low pressure throat of the venturi tube. 
     In another aspect of the invention, the method may comprise directing a flow of air through the venturi tube and into a combustion zone in a combustion chamber through an entrance in a wall of the combustion chamber and injecting a flow of fuel gas into said combustion chamber at a location radially outward and beyond the inner edge of the entrance and adjacent to said combustion zone. Furthermore, the novel method may include both the recirculation of flue gas and external fuel gas injection as described above. 
     In a more specific sense, the method may include a step of introducing a first supply of fuel gas into said flow of air. The method also may include a step of passing at least a primary portion of said flow of air through a swirler. Even more specifically, the method may be such that a secondary portion of said flow of air does not pass through the swirler. 
     In another important preferred aspect of the invention, the method may include a step of causing a tertiary air stream to flow around the periphery of the venturi tube, through a gap provided between the large end of the venturi tube and an inner edge of the entrance to the combustion chamber, and on into the combustion zone. 
     In another preferred aspect of the invention, the method for operating a venturi equipped round burner may include a step of introducing a first supply of fuel gas into the primary portion of the flow of air, and introducing a second separate supply of fuel gas into said secondary portion of the flow of air. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional elevational view of an embodiment of a combustion chamber burner of the invention and its associated elements taken essentially along the vertical centerline of the combustion chamber windbox; 
     FIG. 2 is a front end elevational view of the burner of FIG. 1; 
     FIG. 3 is a graph illustrating the number of scanner signals and the air pressure drop data obtained in a combustion chamber burner of the invention as the ratio of swirled air flow to straight air flow is changed; 
     FIG. 4 is a graph illustrating the amount of the relative available internal flue gas recirculation flow measured when the ratio of primary and tertiary air flows is changed; 
     FIG. 5 is a graph illustrating the improved performance of the burner of the invention in terms of achieving reduction of CO and NO x  emissions as the ratio between excess air factors in the secondary and tertiary air flows is varied; 
     FIG. 6 is a graph illustrating the improved performance of the burner of the invention in terms of achieving reduction of CO and NO x  emissions as the injector fuel gas flow rate is varied with respect to the total fuel gas flow rate; and 
     FIG. 7 is a graph illustrating the improved performance of the burner of the invention in terms of achieving reduction of NO x  emissions when internal flue gas is recirculated. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A burner assembly which embodies the features, concepts and principles of the invention is illustrated in FIG. 1 where it is identified by the reference numeral  10 . As is conventional and well known to those of ordinary skill in the relevant art, the burner  10  may be surrounded by a windbox  12  which provides combustion air to the burner at a pressure sufficient to cause it to flow into the combustion zone  14  in a combustion chamber or firebox  16  through an entrance  18  in a wall  20  of the combustion chamber  16 . As is also well known to those of ordinary skill in the art, an entrance, such as the entrance  18 , may preferably be in the form of a generally circular opening which extends through the wall  20  of combustion chamber  16 . 
     The burner  10  is equipped with an elongated venturi tube  22  having an inlet end  25  that is spaced from entrance  18  and a outlet end  26  that is positioned adjacent to and in alignment with entrance  18 . The venturi tube  22  also has a throat  24  disposed between inlet end  25  and outlet end  26 . As would be well known to the routineer in the burner art, the venturi tube  22  may generally be circular in cross-sectional configuration, and the outlet end  26  thereof should preferably and generally be larger in diameter than either the inlet end  25  or the throat  24 . 
     As illustrated in FIG. 1, outlet end  26  of venturi tube  22  is preferably positioned within and surrounded by entrance  18 . Additionally, the outer periphery  28  of outlet end  26  is smaller in diameter than the annular inner edge surface  30  of entrance  18 . Thus, an annular gap  32  is presented between the outer periphery  28  of the outlet end  26  of the venturi tube  22  and the inner edge surface  30 . An annular shroud  33  is positioned within entrance  18  and is mounted on edge surface  30  so as to provide a mouth  35  for the gap  32 . 
     The burner assembly  10  is also provided with a swirler  34  which is positioned centrally within the outlet end  28  of the venturi tube  22 . As can be clearly seen in FIG. 1, the outer diameter of the swirler  34  is smaller than the internal diameter of the venturi tube  22  at the outlet end  28  of the latter. This provides an annular space  36  which surrounds the swirler  34  within the venturi tube  22 . 
     The burner assembly  10  of the invention also may preferably be provided with a conventional ignitor  38  and one or more central fuel gas nozzles  40 . Only a single nozzle is shown in FIG. 1; however, one of ordinary skill in the burner art would understand that the burner  10  may include a plurality of central fuel gas nozzles spaced evenly around the longitudinal axis of the venturi tube  22 . The determinative factor in choosing the number of central fuel gas nozzles to use is simply to make sure that the central or primary gas flow is evenly distributed in the combustion air. The nozzle or nozzles  40 , as the case may be, provide fuel gas to the air flowing through the center of the venturi tube  22 . The burner assembly  10  may also preferably be equipped with a conventional steam operated fuel oil atomizer unit  42  so that the burner  10  is adapted to burn fuel oil as well as gaseous fuels including natural gas. 
     In accordance with the concepts and principles of the invention, the burner assembly includes at least one fuel gas poker  44  for delivering fuel gas to the air traveling through the venturi tube  22  on its way to the combustion zone  14 . Although only a single poker  44  is shown in FIG. 1, the burner assembly  10  may preferably include three or more fuel gas pokers  44  spaced evenly around the inside of the venturi tube  22 . Conventionally the burner may include six to eight pokers  44  as illustrated in FIG. 2; however, if the invention of the &#39;803 patent is employed, the burner  10  may need only three pokers  44 . The pokers  44  may each include an elongated tube  45  and a nozzle  47 , and the same may conventionally be linked together by a fuel gas manifold  46  as shown in FIG. 2 . The principal design consideration in selecting the correct number of pokers for any given installation is that the fuel gas be distributed evenly around the entire circumference of the venturi tube  22 . 
     Desirably burner assembly  10  of the invention may include one or more ducts  48  for internal recirculating flue gas  49  from a point within the combustion chamber  16  adjacent combustion zone  14  to the air flowing through venturi tube  22  at the low pressure zone  72  in throat  24  thereof. A single duct  48  is shown in FIG. 1 for illustrative purposes. However, burner assembly  10  preferably may include four ducts  48  spaced 90 degrees apart around the periphery of the venturi tube  22  as best shown in FIG.  2 . Again, the principal design consideration in selecting the correct number of ducts  48  for a given application is simply that the recirculated flue gas be distributed evenly around the entire circumference of the venturi tube. Ducts  48  may each be provided with an outlet  50  which is connected to the venturi tube at a point adjacent to the low pressure zone  72  at the throat  24  of the venturi tube  22  so that recirculated flue gas  49  is inducted into the venturi tube  22 . Each duct  48  also preferably has an inlet  52  which is in fluid communication with the interior of the combustion chamber via an opening  54  in wall  20 . Thus, flue gas  49  from adjacent the combustion zone  14  in chamber  16  may be inducted into the air flowing through the venturi tube  22  and intermixed therewith at throat  24 . 
     As is illustrated in FIG. 1, the burner  10  of the invention may also be provided with at least one external fuel gas injector  56 . The injector  56  may preferably include an elongated tube  58  and a nozzle  60 . The nozzle  60  protrudes through an opening  62  which extends through wall  20  such that the nozzle  60  is positioned in outwardly spaced relationship relative to entrance  18 . That is to say, opening  62  is positioned outwardly beyond the inner edge surface  30  of entrance  18  and therefore the nozzle  60  is positioned to direct a flow of fuel gas into said combustion chamber  16  at a location adjacent to and externally of the combustion air flowing into combustion zone  14 . 
     A single fuel gas injector  56  is shown in FIG. 1 for illustrative purposes. However, as shown in FIG. 2, the burner assembly  10  may preferably include four to eight fuel gas injectors  56  spaced 45 degrees apart around the periphery of the venturi tube  22 . Again, the principal design consideration in selecting the correct number of fuel gas injectors  56  for a given application is that the fuel gas be distributed evenly around the entire periphery of the combustion zone  14 . The injectors  56  are provided with a manifold  64  which distributes fuel gas thereto. 
     In operation, combustion air enters the burner  10  from windbox  12  and is divided into three separate and distinct portions. The flow path of primary air is designated by the arrow  66 , the flow path of secondary air is designated by the arrow  68  and the flow path of tertiary air is designated by the arrow  70 . As dictated by the shape and size of the venturi tube  22 , the shape and configuration of the swirler  34  and the shape and size of the entrance  18 , primary air  66  moves to the center of the venturi tube  22  where it is mixed with fuel gas from the centrally located fuel nozzle  40  and caused to flow through the swirler  34  which rotates the primary air/central fuel gas mixture in a manner well known to the routineer in the burner art. Thus, primary air  66  and central fuel gas from nozzle  40  are thoroughly mixed and agitated as the same are directed into the center core of the combustion zone  14 . 
     Secondary air  68  moves in a generally straight line through the venturi tube  22  and passes into the combustion zone. As the secondary air  68  passes around the swirler  34 , it is in the shape of an annular envelope that surrounds the swirler  34  and the swirled primary air  66 . As can be seen viewing FIG. 1, the fuel gas pokers  44  are positioned radially outwardly relative to the swirler  34  and such that the fuel gas from the poker nozzles  47  is intermixed with the secondary air  68 . Thus, straight line secondary air  68  and the fuel gas from poker nozzles  47  are directed in a straight line into the combustion zone  14  at a position which is radially outward of the center of the latter. 
     Tertiary air  70  moves in a straight line around the periphery of the venturi tube  22  and is guided by the mouth  35  so that it passes through the gap  32  between the outlet end  26  of the venturi tube  22  and the inner edge surface  30  of the entrance  18 . The tertiary air  70  is in the shape of an annulus which surrounds the venturi tube  22  and the secondary air  68  as it is introduced into the combustion zone  14 . 
     Fuel gas from the injectors  56  is introduced into the combustion chamber  16  at a position which is radially outward relative to the center of the combustion zone  14  and to the primary, secondary and tertiary air flows  66 ,  68  and  70 . 
     Generally speaking, the outlet end of the venturi tube  22  may preferably be about 6 to about 40 inches in diameter. The shape of the venturi tube  22  is not necessarily critical to the operation of the burner  10 . That is to say, the shape of the venturi tube is in some measure dictated by the desired air flow rate characteristics. However, it has been determined experimentally that the venturi tube  22  may preferably be shaped such that the ratio of the diameter of the throat  24  to the diameter of the outlet end  26  may preferably be in the range of from about 1:1.2 to about 1:1.6. It has also been determined experimentally that the ratio of the total cross-sectional area of the annular gap  32  to the total cross-sectional area of the outlet end  26  of the venturi tube  22  may preferably, but not necessarily, be in the range of from about 1:6 to about 1:8. It is also preferred, but not necessarily required, that the swirler  34  be positioned at a distance from the outlet end  26  which is within the range of from about 0.4 to about 0.6 times the internal diameter of outlet end  26 . 
     The difference between the forward velocity of the swirled primary air stream  66  and the forward velocity of the straight line secondary air stream  68  is associated with the physical design of the burner. Conceptually, all of the primary air stream  66  passes through the swirler  34 . On the other hand, the secondary stream  68  passes around the swirler  34  and theoretically none of it passes through the swirler  34 . Clearly none of the tertiary air flow  70  passes through the swirler  34 . The swirler  34  imposes a degree of aerodynamic resistance on the primary stream  66  passing therethrough. Thus, the velocities of the straight line streams  68  and  70  are greater than the velocity of the primary stream  66 . As can be seen from FIG. 3, when the ratio of swirled primary air flow to straight line air flow (secondary+tertiary) is greater than about 0.2, air resistance increase rapidly. On the other hand, when the ratio of swirled primary air flow to straight line air flow is less than about 0.08, flame stability problems occur. From these parameters, the preferred relative air flow velocities may be determined. Thus, in actual operation, it is preferred that the ratio of the forward velocity of the primary swirled air stream  66  to the forward velocities of the straight line air streams  68  and  70  should be in the range of from about 1:1.1 to about 1:1.5. 
     As set forth above, the preferred lower limit of the tertiary air flow velocity is about 1.1 times the primary air velocity. In accordance with FIG. 4, an increase in the velocity of the tertiary air velocity is accompanied by a decrease in the amount of recirculated flue gas  49  which can be induced into the combustion air by the venturi effect at low pressure zone  72  in venturi tube  22 . There is a comparatively small influence on the amount of flue gas recirculated by induction when the ratio of the velocities of the tertiary and primary air streams is 1.5 or less. However, when this ratio exceeds 1.5, the recirculated flue gas rate drops off quickly. This phenomena also supports the preference for a primary air velocity to tertiary air velocity ratio of 1.5 or less. In accordance with the invention, the recirculated internal flue gas rate should preferably be within the range of from about 4% to about 8%, inclusive, based on the total amount of combustion air supplied to the burner. The effectiveness of such recirculation is apparent from FIG.  7 . 
     The center core of the burner flame is located in the central part of the combustion zone  14 . This part of the flame, which is fed primarily by the primary air flow and the fuel from the central fuel nozzles  40 , is responsible for stability and vibration of the entire flame. In addition, the core of the flame plays a role as a flame pilot whenever the heat load is reduced to a minimum. It is well known to the routineer in the burner art that the most stable flame occurs when the conditions in the burner are stoichiometric. From a practical viewpoint, however, flames are sufficiently stable whenever the amount of air is at least 70% of the amount that is theoretically sufficient to burn all of the fuel and no greater than 110% of such amount. Thus, the fuel/air ratio in the primary air stream should be maintained such that the available oxygen ranges from about 70% to about 110% of theoretical at the time the primary air stream enters the combustion zone. 
     As can be seen from FIG. 5, however, there is an effective reduction in emitted NO x  without a corresponding increase in emitted CO when the ratio of the excess air factor in the secondary stream  68  to the excess air factor in the tertiary air stream  70  is in the range of from about 1.3:1 to about 2.7:1. When this ratio is less than about 1.3:1, NO x  reduction is negligible. When this ratio is above about 2.7, CO emission becomes unacceptable. Coupled with the foregoing information, one must take into consideration the fact that the state of the art knowledge is that the local excess air factor should preferably never be more than 2.0 to prevent local cooling of the flame, and should preferably never be less than about 0.7 to avoid the unacceptable concentrations of incompletely combusted products in the flue gas. Based on these considerations, and in accordance with the concepts and principles of the present invention, it has been determined that the excess air factor provided by the primary stream  66  should preferably be in the range of from about 0.7 to about 1.1, that the excess air factor provided by the secondary stream  68  should preferably be in the range of from about 0.7 to about 2, and that the excess air factor provided by the tertiary stream  70  should preferably be in the range of from about 0.5 to about 0.7. 
     With reference to the foregoing considerations the preferred relative primary fuel gas flow can be determined. Thus, the primary fuel gas flow is a multiplication product of the relative primary air flow and the primary excess air factor, which is (0.08-0.20)×(0.7-1.1)=(0.056-0.22). It is known that in order to avoid stability and vibration problems when the heat load is reduced, such reduction should be accompanied by an increase in the proportion of the fuel gas fed to the core of the flame. Usually, under full load conditions, the amount of fuel fed to the core of the flame should be about 6% of the total fuel flow rate. Tests have shown that the amount of fuel gas fed to the center of the flame should be increased at a rate which is about the fourth degree root of the burner turndown. Thus, to accommodate a standard turndown of 12.5:1, the fuel fed to the core of the flame should amount to 6 −4 ×12.5=19.6% of the total fuel rate. So the amount of the total fuel in the primary air stream  66  should preferably range from about 6% to about 19%. These numbers are comparatively close to the numbers calculated above. 
     With reference to FIG. 6, it can be seen that a desirable degree of NO x  reduction is achieved without an unacceptable increase in CO emissions when the ratio of the fuel gas rate from the injector nozzles  60  ranges from about 65% to about 85% of the total fuel rate. Thus, under full load, the secondary fuel gas flow from the poker nozzles  47  should preferably range from about 9% to about 29% of the total fuel gas flow. Under partial loads, the secondary fuel gas flow from the poker nozzles  47  should preferably be a little less than about 5% of the total fuel gas flow. So the overall secondary fuel gas flow rate from the poker nozzles  47  should preferably range from about 5% to about 29% of the total fuel gas flow. 
     In sum, and in accordance with the concepts and principles of the present invention, it has been determined that the flow rate of the primary fuel gas from nozzles  40  should preferably be in the range of from about 6% to about 19% of the total fuel supplied to the burner, that the flow rate of the secondary fuel fed from poker nozzles  47  should preferably be in the range of from about 5% to about 29% of the total fuel supplied to the burner, and that the flow rate of the tertiary fuel supplied from nozzles  60  should preferably be in the range of from about 52% to about 89% of the total fuel supplied to the burner. 
     It has also been determined in accordance with the principles and concepts of the invention, that the ratio of recirculated internal flue gas  49  to total combustion air flow ( 66 ,  68  and  70 ) should preferably be in the range of from about 0.04:1 to about 0.08:1. This factor is determined by a balance between flame stability and emission reduction and is controlled by the various flow rates of the combustion air as discussed above.