Abstract:
An apparatus includes a furnace structure defining a combustion zone, and a tube that discharges fuel-oxidant premix into the combustion zone through an outlet from the tube. A reactant supply system provides the tube with unmixed fuel and oxidant for forming the premix. A flashback control means inhibits or extinguishes flashback in the tube.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of provisional U.S. Patent Application No. 61/418,096, filed Nov. 30, 2010, which is incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This technology relates to a furnace with a premix burner in which flashback may occur. 
       BACKGROUND 
       [0003]    As shown in  FIG. 1 , a prior art furnace includes a premix burner  10  mounted on a furnace wall  12  in a position to fire into an adjacent process chamber  15 . The burner  10  has a rear portion  16  defining an oxidant plenum  17  and a fuel plenum  19 . The oxidant plenum  17  receives a stream of combustion air from a blower system  20 . The fuel plenum  19  receives a stream of fuel from a fuel source  22 . 
         [0004]    Mixer tubes  30  are located within the oxidant plenum  17 . The mixer tubes  30  are preferably arranged in a circular array centered on a longitudinal axis  31 . Each mixer tube  30  has an open inner end  32  that receives a stream of combustion air directly from within the oxidant plenum  17 . Each mixer tube  30  also receives streams of fuel from fuel injector conduits  34  that extend from the fuel plenum  19  into the open inner end  32 . These streams of fuel and combustion air flow through the mixer tubes  30  to form a combustible mixture known as premix. 
         [0005]    An outer portion  40  of the burner  10  defines a stabilized combustion chamber  41  with an outlet port  45 . The premix is ignited in the combustion chamber  41  upon emerging from open outer ends  46  of the mixer tubes  30 . Ignition is initially accomplished by use of an igniter before the combustion chamber  41  reaches the auto-ignition temperature of the premix. Combustion continues as the premix is injected from the outlet port  45  into the furnace process chamber  15 . 
         [0006]    As further shown in  FIG. 1 , the burner  10  is coupled with a reactant supply system  50 . This includes the blower system  20  and a duct  52  through which the blower system  20  draws air from the ambient atmosphere. Another duct  54  extends from the blower system  20  to the oxidant plenum  17  at the burner  10 . A fuel line  56  communicates the fuel source  22  with the fuel plenum  19  at the burner  10 . Other parts of the reactant supply system  50  include a controller  60  and valves  62  that are operated by the controller  60 . 
         [0007]    The controller  60  has hardware and/or software that is configured for operation of the burner  10 , and may comprise any suitable programmable logic controller or other control device, or combination of control devices, that is programmed or otherwise configured to perform as described and claimed. As the controller  60  carries out those instructions, it operates the valves  62  to initiate, regulate, and terminate flows of reactant streams that provide the premix at the outlets  46  of the mixer tubes  30 . The controller  60  is preferably configured to operate the valves  62  such that the fuel and combustion air are delivered to the burner  10  in amounts that form premix having a lean fuel-to-oxidant ratio. The fuel-lean composition of the premix helps to avoid the production of NO x . 
         [0008]    Flashback can occur in a premix of fuel and oxidant when the flame speed exceeds the velocity of the reactants. Specifically, flashback can occur in a boundary layer of premix at the inner wall surface of a mixer tube  30 . It is believed that a flame propagates/flashes back upstream through the premix in the lower velocity portions of the boundary layer that are spaced away from the wall at a distance greater than the quenching distance for the given fuel/oxidant mixture. The quenching distance is the distance from a wall where combustion is prevented by heat loss and chemical radical absorption by the wall. Accordingly, the likelihood of flashback is increased when the thickness of the boundary layer is greater than the quenching distance. 
       SUMMARY 
       [0009]    An apparatus includes a furnace structure defining a combustion zone, and a tube that discharges fuel-oxidant premix into the combustion zone through an outlet from the tube. A reactant supply system provides the tube with unmixed fuel and oxidant for forming the premix. A flashback control means inhibits or extinguishes flashback in the tube. In one embodiment, the flashback control means includes means for electrically charging the tube. In another embodiment, the flashback control means includes means for cooling the tube. 
         [0010]    In other embodiments, the flashback control means responds to flashback in the tube by changing the flame speed of the fuel provided to the tube to form premix. Some embodiments include one or more inner tubes within an outer tube. Those embodiments provide stratified flammability that increases radially inward of the outer tube. 
         [0011]    In another embodiment, the flashback control means injects non-flammable fluid across the tube to form a curtain that blocks flashback of the premix. An additional embodiment controls flashback by adding diluent fluid to the premix fuel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a partial view of a prior art furnace equipped with a premix burner and a reactant supply and control system. 
       
    
    
       [0013]    Each of  FIGS. 2-11  is a partial view of a modified version of the furnace of  FIG. 1 , showing a respective embodiment of means for controlling premix flashback. 
       DETAILED DESCRIPTION 
       [0014]    The embodiments shown in  FIGS. 2-11  are illustrated with reference to mixer tubes and other parts of furnaces which, in addition to having the premix burner  10  and the parts described above with reference to  FIG. 1 , are further equipped with flashback control means. All or part of each of the multiple flashback control means can be used in combination with all or part of any one or more of the others. 
         [0015]    In the example of  FIG. 2 , flashback control is accomplished by electrically charging the mixer tube  30 . Specifically, the mixer tube  30  is included in an electrical circuit  102  with a voltage source  104  and a switch  106 . A refractory or other electrically insulating material  108  is interposed where needed between the mixer tube  30  and the other structural parts  110  of the premix burner  10 . The controller  60  ( FIG. 1 ) operates the switch  106  to apply a voltage to the mixer tube  30  to create an electric field. This inhibits flashback by increasing the boundary layer quenching distance through which combustion is prevented by chemical radical absorption in the metal material at the inner surface  112  of the mixer tube  30 . 
         [0016]    In the example of  FIG. 3 , flashback control is accomplished by cooling the mixer tube  30  to increase the boundary layer quenching distance through which combustion is prevented by heat loss at the inner surface  112  of the mixer tube  30 . In this embodiment, the controller  60  operates a pump  120  to drive a flow of cooling fluid from a source  122  through a coil  124  of heat exchange tubing on the outer surface  126  of the mixer tube  30 . The alternative embodiment of  FIG. 4  includes a sleeve  130  through which the cooling fluid flows in contact with the outer surface  126  of the mixer tube  30 . The alternative embodiment of  FIG. 4A  cools the mixer tube  30  with thermoelectric elements  140  in a modification of the circuit  102  of  FIG. 2 . 
         [0017]    In the example of  FIG. 5 , the flashback control means introduces non-flammable fluid into the mixer tube  30  at an intermediate inlet location  150 . The intermediate inlet location  150  is between the outlet  46  and the open inner end  32  at which the mixer tube  30  receives unmixed fuel and oxidant, but is preferably closer to the outlet  46  than to the inner end  32 . In this embodiment, the mixer tube  30  has a porous media fluid inlet structure  152  through which the non-flammable fluid enters the mixer tube  30 . The non-flammable fluid inhibits flashback from the outlet  46  by forming a non-flammable boundary layer along the inner surface of the mixer tube  30  between the outlet  46  and the intermediate location  150 . 
         [0018]    The inlet structure  152  receives the non-flammable fluid from a surrounding annular duct  154 . As shown schematically in  FIG. 5 , the controller  60  operates a valve  158  to provide the duct  154  with the non-flammable fluid, which can comprise liquid water, water in the form of vapor or steam, air from the blower system  20 , recirculated flue gas, an inert gas, carbon dioxide, or any other suitable non-flammable fluids or mixtures of such fluids. 
         [0019]    The porous inlet structure  152  is formed of solid material that is permeable to fluids and, although sintered metal is preferred, may comprise any such material or materials that can withstand the high temperatures of combustion in the burner  10 . It has a skeletal structure called the matrix or frame that has voids. A porous medium has distinct advantages over a solid with manufactured holes, which are known as through holes. A porous medium can have a pore size and density that is not attainable through machining or fabrication techniques. Introducing a flow along the inner wall surface of the mixer tube  30  is effective through the use of a porous medium because the permeability of the porous section will allow the non-flammable fluid to gradually displace the bulk mixer flow radially inward from the wall surface to form a more uniform boundary layer composition that is more resistant to flashback. With proper porous medium selection the flow passing through the medium will have a low exit velocity at the wall of the mixer tube  30  which reduces its interaction with the bulk mixer flow. 
         [0020]    The embodiment of  FIG. 5  further includes a sensor  160  to detect flashback. The sensor  160  may comprise any suitable device known in the art, such as a flame detector, a UV sensor, an acoustic sensor, a temperature sensor, and the like, as described in the provisional application. The controller  60  can respond to the sensor  160  by operating the valve  158  to initiate a flow of the non-flammable fluid into the mixer tube when flashback has been detected, to regulate the flow as needed to control flashback, and to terminate the flow when flashback has been extinguished. 
         [0021]    In the embodiment of  FIG. 5 , the porous medium inlet structure  152  is formed of sintered metal. It is shaped as a cylinder, and is welded in place at its opposite ends to form a longitudinal section of the mixer tube  30  at the intermediate location  150 . In the embodiment of  FIG. 5A , the porous medium inlet structure  152  is formed of ceramic material. The ceramic material fills the annular duct  154 , which is welded between adjacent sections  162  and  164  of the mixer tube  30  to provide the structural strength needed to support the tube  30  at the intermediate location  150 . 
         [0022]    As shown in  FIG. 6 , the furnace can be further equipped with multiple sources of differing fuels for forming the premix in the mixer tube  30 . The controller  60  in this example is configured to respond to flashback by combining or switching between the fuel source  22  and one or more additional sources  170  of fuels, each of which has a different flame speed. Specifically, under given conditions of temperature, pressure, and fuel-to-oxidant ratio, most fuels have respective flame speeds that differ from each other. High flame speed fuels include hydrogen at 170 cm/sec, and acetylene at 144 cm/sec, in fuel-air premix at 25 degrees C., 1 atm pressure, and an equivalence ratio of 1.0. Others include ethylene, propylene, and ethylene oxide with flame speeds of 68 cm/sec, 70.2 cm/sec, and 88.8 cm/sec, respectively, in fuel-air premix under the same conditions of 25 degrees C., 1 atm pressure, and an equivalence ratio of 1.0. Low flame speed fuels under that set of conditions include methane at 43.4 cm/sec, ethane at 44.5 cm/sec, propane at 45.6 cm/sec, and butane at 44.8 cm/sec. When flashback is detected, the controller  60  can respond by shifting from one or a combination of high speed fuels to one or a combination of lower speed fuels, or by varying a combination of high and/or low speed fuels, to reduce the overall flame speed of the premix fuel. The use of a premix fuel with a lower overall flame speed would preferably occur only momentarily, with a resumption of the original premix fuel when flashback has been extinguished. 
         [0023]      FIGS. 7 and 8  show an alternative structural arrangement for one or more mixer tubes and the accompanying fuel injector conduits at the open inner ends  32  of the mixer tubes  30 . This arrangement includes a pair of inner mixer tubes  180  and  182  nested concentrically within an outer mixer tube  30 . The open inner ends  184  and  186  of the first and second inner tubes  180  and  182  have respective fuel injector conduits  188  and  190 . The open inner end  32  of the outer tube  30  does not have a respective fuel injector conduit. The burner  10  in this embodiment is thus configured for the outer tube  30  to receive only oxidant in the form of combustion air from the oxidant plenum  17  ( FIG. 1 ). 
         [0024]    In operation of the embodiment of  FIGS. 7 and 8 , combustion air flows from the plenum  17  into the outer tube  30 , through an annular flow space  191  radially between the outer tube  30  and the first inner tube  180 , and into a cylindrical mixing section  193  of the outer tube  30 . The mixing section  193  reaches longitudinally from the outlets  194  and  196  of the inner tubes  180  and  182  to the outlet  46  of the outer tube  30 . 
         [0025]    The controller  60  is configured for the inner tubes  180  and  182  to receive combustion air from the plenum  17  and fuel from the injector conduits  188  and  190  at respective ratios that differ from each other. Specifically, the first inner tube  180  receives unmixed fuel and oxidant at a first ratio, and the second inner tube  182  receives unmixed fuel and oxidant at a second ratio that is more fuel rich that the first ratio. Premix then flows from the first inner tube  180  into the mixing section  193  at the first ratio, and from the second inner tube  182  into the mixing section  193  at the second, relatively fuel rich ratio. This inhibits flashback upstream from the outlet  46  by providing stratified ratios of fuel-to-oxidant that increase radially inward of the outer tube  30 . 
         [0026]    In a variation of the embodiment of  FIGS. 7 and 8 , the outer tube  30  has fuel injector conduits  198  as shown in  FIG. 9 . The controller  60  in this embodiment is configured for the outer tube  30  to receive combustion air from the plenum  17  and fuel from the conduits  198  at a third ratio. The third ratio is more fuel lean than the first ratio at the first inner tube  180 , and the premix formed in the annular flow space  191  is preferably non-flammable. The controller  60  may also be configured to provide the tubes  30 ,  180  and  182  with fuels having differing flame speeds for flashback control, as described above, as an alternative to differing ratios for flashback control, or in combination with differing ratios. 
         [0027]    The embodiment of  FIG. 10  includes a nonflammable fluid source  200 , a fluid line  202 , a valve  204  in the fluid line  202 , and an injector  206  configured to inject a curtain  209  of the non-flammable fluid across the outlet  46  of a mixer tube  30 . The non-flammable fluid of  FIG. 10  may be the same as the non-flammable fluid described above with reference to  FIG. 5 , and the controller  60  may operate the valve  204  in response to a flashback sensor  212  in the same manner as described above with reference to  FIG. 5 . 
         [0028]      FIG. 11  is a view similar to  FIG. 1 , but shows a flashback control means comprising a source  230  of diluent fluid and a diluent fluid line  232  communicating the source  230  with the fuel plenum  19  at the burner  10 . The diluent fluid may comprise any of the non-flammable fluids described above. 
         [0029]    Each mixer tube  30  in the embodiment of  FIG. 11  is equipped with a flashback sensor  234 . The controller  60  can respond to one or more of the sensors  234  by operating a valve  236  to initiate a flow of the diluent fluid into the fuel plenum  19  when flashback has been detected. This dilutes the premix to a fuel-to-oxidant ratio at which the flashback will be extinguished. The controller  60  can further regulate the flow of diluent through the valve  236  as needed to control flashback with reference to the sensors  234 , and to terminate the flow of diluent through the valve  236  when flashback has been extinguished. The controller  60  can also interrupt the flow of fuel through the valve  62  until the detected flashback is extinguished. 
         [0030]    This written description sets forth the best mode of carrying out the invention, and describes the invention so as to enable a person of ordinary skill in the art to make and use the invention, by presenting examples of the elements recited in the claims. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have elements that do not differ from the literal language of the claims, or if they have elements with insubstantial differences from the literal language of the claims.