Patent Publication Number: US-6702571-B2

Title: Flex-flame burner and self-optimizing combustion system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to combustion systems having means for automatic, real-time control of the combustion process which can be applied with significant advantage to a wide range of furnaces, boilers and combustors. This invention also relates to a burner for said combustion system which, in addition to means for adjusting the firing rate and air/fuel ratio, also comprises means for adjusting flame size and shape and the degree of mixing of fuel and oxidant. 
     2. Description of Prior Art 
     For many years, efforts in the area of combustion have been focused on improving burner efficiency and lowering emissions from the combustion process. These efforts have provided significant advances in burner technology while increasing efficiency and lowering emissions. However, these effort have provided diminishing returns to combustion system operators. Currently, the greatest potential for furnace combustion improvement rests with taking a more global approach in which burners are considered as part of an interactive, real-time furnace control system. Such systems would be able to monitor, control, regulate, set or adjust the combustion process including flame characteristics and emissions over a wide turndown range and with fuel switching providing maximum thermal efficiency and minimum emissions production over substantially all furnace operating conditions, including transient operation. 
     Conventional combustion systems in use today comprise burners which are adjustable primarily with respect to firing rate and air/fuel ratio. As a result, these burners are tuned to a compromise setting so as to provide reasonable values of emissions and heat transfer over a wide range of firing rates. However, other than changes in flame characteristics resulting from changes in firing rate and/or oxidant/fuel ratio, these systems do not provide flame shape control or oxidant/fuel mixing control. In addition, burners used by conventional combustion systems are frequently exposed to high temperatures resulting in high maintenance and shortened service life. Accordingly, there is a need for a “smart” combustion system which can provide interactive and flexible control of the combustion process in furnaces and other combustion chambers, very effective heat transfer to a load with emissions control over high turndown ratios, with multiple fuels, and during both steady-state and transient operation. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is one object of this invention to provide an interactive, real-time furnace control system which, in addition to providing control over firing rate and oxidant/fuel ratio, also provides flame shape control and oxidant/fuel mixing control. 
     It is another object of this invention to provide a flexible combustion system which provides very effective heat transfer to a load over high turndown ratios, with multiple fuels, and during both steady-state and transient operation. 
     It is yet another object of this invention to provide a combustion system with a burner providing controlled localized flame stoichiometry over a wide turndown range using multiple fuels. 
     These and other objects of this invention are addressed by a combustion system comprising a burner body having a primary combustion oxidant (first fluid) inlet end forming at least one primary combustion oxidant (first fluid) inlet and a combustion oxidant (first fluid) outlet end forming at least one combustion oxidant (first fluid) outlet, a fuel (second fluid) inlet distal from the combustion oxidant (first fluid) outlet and at least one fuel (second fluid) outlet proximate the combustion oxidant (first fluid) outlet, and internal adjustment means for adjusting the flow cross-sectional area for a first fluid, typically oxidant, and/or a second fluid, typically fuel, disposed within the burner body. However, it will be apparent to those skilled in the art that the first fluid may be a fuel and the second fluid an oxidant. The combustion system further comprises interactive flame sensing and control means for providing interactive, real-time control over the combustion process, including control over flame size and shape and air/fuel mixing at constant fuel input. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein: 
     FIG. 1 is a diagram showing benefits which are derivable from the combustion system of this invention; 
     FIG. 2 is a diagram showing the broad concept of the burner employed in the combustion system in accordance with this invention; 
     FIG. 3 is a general diagram showing the self-optimizing combustion system of this invention; 
     FIG. 4 is a cross-sectional lateral view of a flexible-flame burner with oxidant staging in accordance with one embodiment of this invention; 
     FIG. 5 is a cross-sectional lateral view of a flexible-flame burner with fuel staging in accordance with one embodiment of this invention; and 
     FIG. 6 is a cross-sectional lateral view of a flexible-flame burner with both fuel and oxidant staging in accordance with one embodiment of this invention. 
    
    
     DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS 
     The invention claimed herein is a self-optimizing combustion system which provides interactive and flexible control of the combustion process in furnaces and other combustion chambers. The flexibility to provide controlled heat transfer to a load over high turndown ratios, with multiple fuels, and during both steady-state and transient operation is provided by combining two components, a flexible-flame burner in accordance with embodiments shown in FIGS. 4,  5  and  6  and a real-time flame sensing and control system as shown in FIG.  3 . 
     The flexible-flame burner of the combustion system of this invention can be adjusted for firing rate and for oxidant/fuel ratio as well as for flame shape and degree of oxidant/fuel mixing. FIG. 1 shows the results of operation of a flexible-flame burner in accordance with one embodiment of this invention. As shown, operating the flexible flame burner at constant fire and constant excess air produced as much as twice as much NO x  at air flow cross-sectional area “A” compared with air flow cross-sectional area setting “C”. This result was repeated at several excess air levels and several firing rates. 
     The capability of the combustion system of this invention for flame control is shown in Table 1 below. The flexible-flame burner was fired using natural gas at an optimized baseline condition to establish a baseline performance. The burner was then operated at a lower excess air level resulting in increases in NO x  emissions, lengthening of the flame and a change in color of the flame from blue to yellow. Adjustment of the air flow cross-sectional area for the burner restored the NO x  and flame characteristics to the optimized baseline conditions. In a second test, the fuel was changed from natural gas to propane, again resulting in deterioration of the flame shape. Again, adjustment of the air flow cross-sectional area restored the optimized baseline conditions. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Low Excess Air 
                 Switch to Propane 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Optimized 
                 No 
                 Flex- 
                 No 
                 Flex- 
               
               
                   
                 Baseline 
                 Adjustment 
                 flame 
                 Adjustment 
                 flame 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 NO x ,vppm 
                 38 
                 53.5 
                 31.5 
                 — 
                 — 
               
               
                 Flame 
                 2.5 
                 3.75 
                 2.75 
                 3.25 
                 2.5 
               
               
                 Length 
               
               
                 Flame 
                 Blue 
                 Yellow 
                 Blue 
                 Yellow 
                 Blue 
               
               
                 Appearance 
               
               
                   
               
            
           
         
       
     
     As previously indicated, in addition to firing rate and oxidant/fuel ratio, the flex-flame burner of this invention allows the degree of oxidant/fuel mixing and the outlet velocity of the fuel and oxidant to be changed while maintaining a constant firing rate or while changing the firing rate. This is accomplished by internally adjusting fuel and/or oxidant flow cross-sectional areas within the burner. By changing the flow cross-sectional areas, oxidant and fuel velocities are changed and mixing patterns are adjusted. Flow cross-sectional areas are adjusted by a set of internal flow blocking devices, as will be described in more detail below, appropriately sized to more and less partially block the flow cross-sectional area passages for fuel and/or oxidant. This technique advantageously enables velocity adjustments to be made with no need for contact between hot metal surfaces. 
     In accordance with one embodiment of this invention, flow cross-sectional area size adjustments are made for both primary and secondary oxidant. This enables the oxidant velocity to be altered at will. Flame length, degree of mixing, flame color and amount of NO x  formed are all adjusted by changing the cross-sectional flow areas. The same effect is achievable by the flex-flame burner of this invention where fuel flow areas are altered for multi-fuel burners or where several oxidants, such as air and oxygen are used for primary and secondary firing. 
     As shown in FIG. 2, conceptually, the flex-flame burner of this invention comprises a fuel inlet/fuel flow control section  10 , an oxidant inlet/oxidant flow control section  11  disposed downstream of the fuel inlet/fuel flow control section  10 , and a flame shape adjustment area  12  disposed downstream of the oxidant inlet/oxidant flow control section  11  comprising a secondary oxidant flow control means and/or a secondary fuel flow control means. 
     FIG. 3 is a diagram showing a self-optimizing combustion system in accordance with one embodiment of this invention for providing complete, real-time control of the combustion process. The system comprises a non-intrusive optical system comprising optical sensor  20  which is adapted to observe the flame  21  produced by flex-flame burner  22  and provide information for automatic, real-time control of the combustion process. Optical sensor  20  is operatively connected to a signal processor  23  which, in turn, is operatively connected to the furnace controller  24 . Furnace controller  24  is operatively connected to burner controller  25  which controls actuators  26  operatively connected to flex-flame burner  22 . Actuators  26  operate to move internal adjustment means disposed within flex-flame burner  22  for adjusting the flow cross-sectional areas within the burner. Burner controller  25  is also operatively connected to means for controlling the fuel and oxidant flow into the burner, such as flow control valves. As can be seen, in addition to receiving input from the optical sensor  20 , furnace controller  24  also receives input from furnace sensors, indicated by arrows  27 , for input into burner controller  25 . 
     Measurements by optical sensor  20  can be made in the ultraviolet, visible and/or infrared regions. In accordance with one preferred embodiment, the optical sensors are chemiluminescent optical diagnostic sensors. The chemiluminescent emission from flames may be interpreted as a signature chemical reaction and heat release from which flame geometry can be determined. Chemiluminescent emission line-of-sight measurements can provide information regarding flame topography, stability, behavior and even pollutants. Capabilities of the self-optimizing combustion system of this invention include 1) measurement of the flame shape, including length, height and width; 2) measurement of mixing by observing luminosity of flame regions; 3) detection of emissions in the flame, including CO and NO x ; 4) thermal conditions; 5) data processing and integral, feed-back control algorithms to provide monitoring and control; and 6) rapid response to adjust the combustion process to furnace instabilities, fuel changes, firing rate change (turndown), and non-steady state process heating. 
     FIG. 4 is a diagram showing an exemplary flex-flame burner for a combustion system in accordance with one embodiment of this invention. Although shown as an air-staged flex-flame burner with no swirl, flex-flame burners using air staging, fuel staging, multiple fuels, internal recirculation, external recirculation, oxidant or fuel preheat and swirl are also deemed to be within the scope of this invention and no limitation of the scope of the invention to the configuration shown in FIG. 4 is to be inferred or otherwise considered to exist. 
     As shown, flex-flame burner  40  comprises burner body  41  having a combustion oxidant (first fluid) inlet end  42  and a combustion oxidant (first fluid) outlet end  43 . Combustion oxidant outlet end  43  forms at least one combustion oxidant (first fluid) outlet  48  and combustion oxidant inlet end  42  forms at least one combustion oxidant (first fluid) inlet  49 . In this example, fuel conduit  44  is disposed within burner body  41 , forming a fluid flow region  45  between burner body  41  and fuel conduit  44 . In accordance with one preferred embodiment of this invention, fuel conduit  44  is concentrically disposed within burner body  41 . Fuel conduit  44 , also referred to herein as inner conduit  44 , has a fuel (second fluid) inlet  46  distal from combustion oxidant outlet end  43  of burner body  41  and a fuel (second fluid) outlet  47  proximate oxidant outlet end  43  of burner body  41 . It will be apparent to those skilled in the art that the roles of fuel conduit  44  and burner body  41  with respect to fluids flowing therethrough can be exchanged, whereby oxidant (first fluid) flows through inner conduit  44  and fuel (second fluid) flows through fluid flow region  45 , and such embodiments are deemed to be within the scope of this invention. Multiple fuel conduits are also deemed to be within the scope of this invention. 
     In accordance with the embodiment shown in FIG. 4, to adjust the flow cross-sectional areas of the combustion oxidant, flex-flame burner  40  comprises internal adjustment means disposed within burner body  41 . Said internal adjustment means comprises a first flow blocking means disposed within burner body  41  suitable for partially blocking the at least one combustion oxidant outlet  48 . The first flow blocking means preferably comprises at least one bluff body  50  sized to fit into the at least one combustion oxidant outlet  48 . The at least one bluff body  50  is longitudinally adjustable within said burner body  41  by adjustment means such as rod  51  connected at one end to the at least one bluff body  50 . In accordance with one embodiment of this invention, the at least one bluff body  50  is a needle-type structure as shown in FIG.  4 . It will be apparent to those skilled in the art that other structures having other shapes may be used as well. In accordance with one embodiment of this invention as shown in FIG. 4, combustion oxidant outlet end  43  forms a plurality of combustion oxidant outlets  48  and a corresponding bluff body  50  is provided for each such outlet. 
     In accordance with one embodiment of this invention, the internal adjustment means comprises internal pressure adjustment means for adjusting the internal pressure in burner body  41 . The internal pressure adjustment means preferably comprises a second flow blocking means disposed within burner body  41  suitable for partially blocking flow of said combustion oxidant within said burner body  41  upstream of combustion oxidant outlet  48 . As shown in FIG. 4, the internal pressure adjustment means comprises an interior wall  60  disposed in fluid flow region  45  upstream of combustion oxidant outlet  48  extending from an outer surface of fuel conduit  44  to an inner surface of burner body  41  and forming at least one opening  61  for enabling flow of combustion oxidant from the primary combustion oxidant inlet  49  to the at least one combustion oxidant outlet  48 . This allows for variable velocity at constant pressure, or variable flow rate at constant pressure, or constant velocity proportional to flow rate. At least one pressure altering blocking means suitable for altering flow or pressure drop of the combustion oxidant through the at least one opening  61  is disposed within burner body  41 . In accordance with one embodiment of this invention, the at least one pressure altering blocking means comprises at least one bluff body  62  sized to fit into the at least one opening  61 , which at least one bluff body  62  is longitudinally adjustable within burner body  41 . Longitudinal adjustment of the at least one bluff body  62  in accordance with one embodiment of this invention is achieved by means of a rod  63  connected at one end to the at least one bluff body  62 . In accordance with one preferred embodiment of this invention, interior wall  60  forms a plurality of openings  61  and a corresponding bluff body  62  is provided for each opening  61 . It will be appreciated by those skilled in the art that other devices can be used for parts  60 ,  61 ,  62  and  63  to achieve the same pressure alteration. Such other devices are considered to be within the scope of this invention. 
     As shown in FIG. 4, in accordance with one embodiment of this invention, flex-flame burner  40  further comprises second stage oxidant means for introducing secondary combustion oxidant into burner body  41  between the internal adjustment means and combustion oxidant outlet  48 . The second stage oxidant means comprises a secondary oxidant plenum  65  disposed around at least a portion of the outside of burner body  41 . Burner body  41  forms a secondary oxidant opening  66  providing fluid communication between fluid flow region  45  and secondary oxidant plenum  65  whereby oxidant from fluid flow region  45  can flow from fluid flow region  45  into secondary oxidant plenum  65 . Secondary oxidant plenum forms at least one secondary oxidant outlet  68  through which secondary oxidant is introduced into a furnace or process heater. Secondary oxidant may flow through dedicated channels  69  in refractory block  70 . To control the flow of secondary oxidant into secondary oxidant plenum  65 , secondary oxidant bluff body  67  is provided. Secondary oxidant bluff body  67  is sized to fit into secondary oxidant opening  66 . The bluff body  67  is similar to bluff body  48 . 
     FIG. 5 is a diagram of a flex-flame burner in accordance with one embodiment of this invention wherein fuel is introduced in stages into the burner. In accordance with this embodiment, inner conduit or fuel conduit  44  forms a second stage fuel outlet  82  which is in fluid communication with second stage fuel plenum  84  by means of second stage fuel conduit  81  extending between fuel conduit  44  and second stage fuel plenum  84 . Second stage fuel flow is adjusted by second stage fuel bluff body  80  proximate second stage fuel inlet  85  of plenum  84  and sized to fit into second stage fuel inlet  85 . Second stage fuel flows through dedicated channels  83  formed by refractory block  70 . 
     FIG. 6 is a diagram of a flex-flame burner in accordance with one embodiment of this invention having both first and second fluid staging. This embodiment essentially constitutes a combination of the embodiments of FIGS. 4 and 5. 
     The flex-flame burner of this invention, as can be seen, is a relatively simple, highly adjustable burner which includes features for providing a wide range of operating conditions. This advanced burner design allows for multi-fuel capability, high turndown ratio (10:1 or greater), automatic oxidant-fuel ratio adjustment, automatic flame shape adjustment at a constant firing rate, automatic flame velocity control over a wide range of turndown ratios, flame velocity adjustment with impact on oxidant inlet pressure, automatic oxidant or fuel staging adjustment between primary and secondary oxidant or fuel introduction, and automatic mixing pattern control through the addition of desired degrees of swirl. 
     While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.