Patent Publication Number: US-2004058290-A1

Title: Self-sustaining premixed pilot burner for liquid fuels

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims the benefit of U.S. Provisional Application No. 60/301,546 filed Jun. 28, 2001, which is incorporated herein in its entirety. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The invention relates generally to burners that can be used with low-volatility liquid fuels. The invention can be used with pilot burners which are generally used to light off main or primary burners which use low-volatility liquid fuel. Additionally, the invention can also be used with the primary burners. Yet additionally, the invention also relates to low BTU output burners that can be used to combust soot, which is generated by internal combustion diesel engines.  
       BACKGROUND OF THE INVENTION  
       [0003] Pilot burners are used to light off the main flame in industrial burners. They are especially essential in lighting the main flame in burners, which use heavy liquid fuels such as diesel or higher-grade oils. Various kinds of pilot burners are used in such burners.  
       [0004] One of the essential requirements of a pilot burner is that the pilot flame should light easily. Often the pilot flame is lit using a spark from a spark plug. In some cases, the pilot flame is lit by contacting the pilot fuel with a hot surface such as a glow element. Therefore an easily ignitable fuel is generally used to provide the pilot flame in a pilot burner. This is especially true for burners where the main flame is provided by the combustion of a heavy liquid fuel such as diesel or higher-grade oils. Thus natural gas or propane is very often used to provide the pilot flame in pilot burners, which are used with such burners.  
       [0005] The use of natural gas or propane to provide the pilot flame in a burner, which operates primarily on a heavy liquid fuel, introduces complexity into the burner system. The natural gas has to be piped to the system using a separate natural gas train. The natural gas train components add to the cost and complexity of the system. The additional parts increase the chances for breakdowns resulting in system shutdowns and additional maintenance.  
       [0006] Similarly, the use of propane in the pilot burner necessitates the use of a propane storage and feed system. The propane system adds to the cost of the system. Additionally the storage of propane increases the hazards associated with the system.  
       [0007] Therefore, it would be advantageous to provide a pilot burner which is easy to light off and which operates on the same heavy liquid fuel as the main burner. Such a pilot burner would greatly reduce the complexity of the burner system. Further, such a system would greatly reduce the costs associated with the necessity of providing natural gas or propane to fuel the pilot burner. Yet further, such as system would greatly reduce the hazards associated with storing a gaseous fuel such a natural gas or propane on the user&#39;s premises.  
       [0008] The invention can also be used in a device for burning off solid particles, in particular soot particles, in the exhaust gas of internal combustion engines, which use diesel as an operating fuel.  
       [0009] Burn-off devices of this kind are used in particular in motor vehicles having diesel engines, for the direct disposal of the soot filtered out of the exhaust gas by electrostatic soot traps. In such a device, the soot is delivered to the combustion chamber of the burn-off device along with a secondary flow of exhaust gas that amounts to less than lo of the total exhaust gas. In the burn-off device, the soot is burned at a flame temperature between 550° C. and 1000° C. At this high temperature, essentially complete combustion of the soot and other combustibles takes place. Therefore, the combustion products are free of toxic substances and suitable for discharge to atmosphere. The combustion products are then expelled to the atmosphere via the engine exhaust system.  
       [0010] To generate the burn-off flame, a pilot burner is mounted on the combustion chamber of the burn-off device. Several embodiments of pilot burners that are suitable for such applications are described in the prior art. For example, U.S. Pat. No. 4,858,432 (Knauer) describes a pilot burner for an apparatus for burning off solid particles in the exhaust gas of internal combustion engines. In this pilot burner, the liquid fuel is injected over a shield, which covers a glow plug. The liquid fuel is vaporized when it contacts the hot surface of the shield. The vaporized fuel is then passed into a combustion chamber wherein it is mixed with preheated air. The mixture is then passed over a glow element, which ignites the fuel and helps to maintain the combustion of the fuel. The hot combustion gases are passed out the combustion chamber. They then pass into a secondary combustion chamber wherein soot particles from the engine exhaust are also introduced. The hot gases ignite the soot particles, which combust to form typical products of combustion such as carbon dioxide and water. The products of combustion are then passed out of the combustion chamber to the atmosphere. Implementation of this type of pilot burner has durability issues. Under low fuel flow conditions and under continuous operation, the liquid fuel in direct contact with the hot surface of the shield may reach a temperature that may promote decomposition of a portion of the fuel. This typically results in the formation of carbon or soot that can build up on the shield. The soot eventually plugs the fuel flow passages or channels and obstructs the flow of the liquid fuel. This results in unreliable re-start characteristics and application durability issues.  
       [0011] U.S. Pat. No. 4,716,728 (Dettling) describes another embodiment of a pilot burner, which is integrated into the soot burning apparatus. In this device the liquid fuel is passed over a glow plug, which evaporates it into a gaseous state. The evaporated liquid fuel is then passed over a second glow plug, which further raises its temperature to near it flash point limit. The heated evaporated fuel is then mixed in a combustion zone with air to initiate combustion. The fuel and air combine to form products of combustion, which are passed from the combustion zone into a soot burning zone. In the soot burning zone, soot is introduced and is mixed with the hot products of combustion. The soot gets heated to above its ignition temperature. The heated soot combines with the oxygen in the hot products of combustion. Further combustion takes place wherein the soot is burnt to produce relatively harmless carbon-dioxide.  
       [0012] It should be noted that in these and other embodiments of pilot burners, which are described in the prior art, the liquid fuel is sprayed or otherwise contacted with a hot surface to effect evaporation. However, when liquid fuel contacts a hot surface, some of the liquid fuel gets overheated. The overheating causes the liquid fuel to crack and deposit carbon on the internal surfaces of the pilot burner. Thus pilot burners, which use liquid fuel are very susceptible to fouling due to deposited carbon from the liquid fuel. This is especially true with very low flow burners and burners that operate under near continuous duty.  
       [0013] Therefore, it would be advantageous to provide a pilot burner for a soot burning apparatus used with internal combustion engines wherein carbonization of the liquid fuel is reduced or minimized. Such a pilot burner would greatly reduce the down time of internal combustion engines for the purpose of cleaning the deposited carbon from the burner system. Further such a pilot burner will greatly reduce the maintenance required for cleaning the deposited carbon from the burner system.  
       SUMMARY OF THE INVENTION  
       [0014] According to one aspect of the invention, there is provided a burner for liquid fuel, the burner comprising: a mixture chamber for producing a liquid fuel air mixture, the mixture chamber having a heating element means, an air receiving means for receiving air, the air receiving means configured so as to facilitate air flow over at least a part of the heating element, and a liquid fuel receiving means; an atomizer mounted in a path of flow of the liquid fuel air mixture formed by the mixture chamber; and a combustion chamber for combusting the liquid fuel air mixture, the combustion chamber having a flame holder, an ignition source located proximal the flame holder, and a combustion zone located downstream of the flame holder.  
       [0015] According to another aspect of the invention, there is provided a method for operating a self-sustaining liquid fueled burner comprising following the steps: initiating air flow to the burner through the inlet air means; initiating the electrical energy to the heating element to preheat the air flow to the burner; initiating the fuel flow to the burner through the inlet fuel means to make a fuel air mixture, initiating the electric energy to the ignition source to ignite the fuel air mixture; monitoring the air preheat temperature prior to the heating element and once temperature greater than the lower volatility limit of the fuel is achieved turn off the energy to the heating element; monitoring the combustion chamber temperature and turning off the ignition source when the combustion chamber temperature reaches 1200 F.  
       [0016] This invention relates to a liquid fuel using pilot burner, which can be used as a pilot for lighting off the main flame in a liquid fuel fired burner or initiating the reaction in a process reactor. The invention can be applied to primary combustors and to a combination of a pilot burner and a primary combustor. More specifically, the invention relates to a self sustaining burner that is configured to allow the burner&#39;s process air to be preheated prior to being mixed with the fuel and prior to this fuel air mixture contacting the surface burner element or flame holder. Preheating of the process air is achieved by one of two methods, directly by a heating element during start-up and by flame heat recuperation during the self-sustaining operation. The preheated air is used to enhance the atomization of the liquid fuel and to partially vaporize of the liquid fuel. A heating element or spark source is used to ignite the partially vaporized liquid fuel air mixture and the flame is established on the surface burner element or flame holder.  
       [0017] The invention is also for integrating a recuperation heat exchanger surface to the combustion chamber of the burner. This may be achieved by adding an external shell around the combustion chamber to create a flow passage through which the burner air is introduced into the burner. Once the burner is ignited, heat from the flame may be transferred to the inlet burner air raising its temperature to above the lower flash point and below the auto ignition point of the specific fuel being used. This recuperative heat raises the temperature of the flame, which in turn allows for additional air to be added to the burner to maintain appropriate combustion chamber temperatures. The effect of adding the additional air is to produce a lean flame without reducing the flame temperature. The increased oxygen content in the combustion chamber enhances combustion and ensures a soot free flame. The invention includes other configurations such as coiled tubing or the use of heat transfer devices such as heat pipes that can be used to promote the transfer of heat energy from the flame to the inlet process air, and therefore, the invention is not limited to the simplest implementation, which is illustrated herein as an external air-heating jacket.  
       [0018] A second aspect of this invention is the use of a glow plug or heating element downstream of the recuperative heat exchanger section and in heat exchange relationship to the inlet burner air. This aspect allows for the use of electrical energy to initially preheat the burner air during burner start-up. This air preheating is important to promote the initial vaporization of a portion of the liquid fuel such that a combustible air-fuel mixture is provided to the ignition device above the flame holder or surface burner element. By indirectly providing the heat to vaporize the fuel through the heated airflow, this invention eliminates the potential of carbon or soot formation within the atomizer and fuel-air mixing chambers. This aspect directly addresses the shortcomings of the prior art configurations. This heating element can controlled based on the air preheat temperature exiting the recuperative heat exchanger section.  
       [0019] Once recuperative heat is sufficient to raise the air temperature to within its desired range, the heating element may be de-energized. An alternate configuration of the heating element can be used with this invention. The heating element can be selected from a group of self-regulating elements that automatically shut off once the temperature of the element achieves a pre-established range. This can be achieved by designing the heating element resistance to increase exponentially above a defined temperature range or by incorporating a temperature sensitive switch within the heating element. Both of configuration of the heating element will eliminate the need for the preheat temperature sensor.  
       [0020] A third aspect of this invention is the use of an atomizer with preheated air. The preheated air enhances the atomization process of the liquid fuel through two mechanisms. First, the hot air has a lower density, and therefore the actual flow rate per mass of air increased. Secondly, the hot air causes vaporization of the liquid fuel as the atomization process is occurring, which further atomizes the fuel creating smaller droplets which combust easily.  
       [0021] The use of a surface burner element as the flame holder provides a mixture of fine pore structures and coarse pore structures. The coarse pores allow easy passage of the vapor state fuel air mixture at low-pressure drops. The fine pore structures provide a high surface tension region that attracts and holds the liquid portion of the fuel air mixture. The surface element promotes both combustion within the mesh and radiant heat and promotes the formation of flame-lets that are held close to the surface. The heat from the flame is partially transferred to the mesh, which supports the continued vaporization of the liquid fuel and its subsequent combustion. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022]FIG. 1 is a cross-section of a longitudinal view of an embodiment of a pilot burner according to the present invention;  
     [0023]FIG. 2 is a cross-section of a longitudinal view of another embodiment of a pilot burner according to the present invention;  
     [0024]FIG. 3 is a cross-section of a longitudinal view of the embodiment of the pilot burner shown in FIG. 2 further modified for the combustion of soot particles from the exhaust of an internal combustion diesel engine; and  
     [0025]FIG. 4 is a representation of a liquid-fuel fired burner which uses the pilot burner assembly FIG. 2 for igniting and maintaining the main flame of the burner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0026] Referring now to FIG. 1 of the drawings, the pilot burner comprises a mixture preparation chamber  10  and a combustion chamber  102 . The mixture preparation chamber (MPC)  10  is configured as a hollow, horizontal cylindrical housing  11 , which is open at its first end  12  and closed at its second end  14 . The first end  12  has an adaptation  16  wherein a glow plug adapter  24  is attached to the first end  12 . The adaptation could be internal threads, which engage mating threads on glow plug adapter  24 . However, other means of attaching end  12  to glow plug adapter  24  could also be used.  
     [0027] A glow plug  20  is attached to glow plug adapter  24  such that glow plug  20  extends at least partially into the housing  11  of the mixture preparation chamber  10 . Electrical leads  22  are provided in the glow plug  20  for attachment to a source of electricity to enable glow plug  20  to reach its normal operating temperature within the range of 800° F. to 1600° F. It should be noted that the glow plug  20  is mounted along the longitudinal axis of housing  11 . The relative dimensions of the diameter of housing  11  and the diameter of glow plug  20  are such that an annular flow space  26  is formed between the glow plug  20  and the inner surface  28  of the housing  11 . Further, the glow plug  20  and glow plug adapter  24  closes the housing  11  at the first end  12  to prevent leakage.  
     [0028] The glow plug  20  can be any of a variety of heating elements with or without shields, and the term glow plug is intended to refer to any and all of these components throughout this disclosure. The glow plug  20  is a standard component which is readily available from suppliers such as Wellman Thermal Systems, Inc., USA.  
     [0029] Air inlet ports  17  are provided circumferentially near the end  12  of the housing  11 . The air inlet ports  17  are located to allow for fluid communication with annular flow space  26  between the glow plug  20  and the inner surface  28  of the housing  11 . The air inlet ports  17  are used to introduce preheated air in to the combustion chamber  102  as will be described later.  
     [0030] At the second end  14  of the housing  11 , an opening  18  is provided for the gas-tight entry of a fuel tube  30 . As shown in FIG. 1, the fuel tube  30  has an opening  32  at one end for the entry of the fuel and a threaded end  34  for connection of the fuel tube  30  to an atomizer  50  at the other end. The threaded end  34  on fuel tube  30  is configured to engage the threads on the fuel flow passage of the atomizer  50 .  
     [0031] Between the heated end  23  of glow plug  20  and the end wall  14  of the mixture preparation chamber  10 , there is positioned an adapter  56  which provides a seating for the atomizer  50 . The adapter  24  is an annular piece of metal whose outer diameter generally matches the inner diameter of housing  11  of the mixture preparation chamber  10 . The adapter  24  slides into the housing  11  and is received snugly inside the housing  11 . The inner diameter of the adapter  56  is threaded and engages a matched threaded tube  57  through which the hot air is introduced into atomizer  50 . The threaded tube  57  engages threads on the air-flow passage of the atomizer  50 .  
     [0032] The atomizer  50  is a standard component that is readily available from suppliers such as Lechler Inc, USA. As shown in FIG. 1, the atomizer  50  has an inverted “T” shape. On one side, the atomizer  50  has a threaded air inlet flow passage  54  for the introduction of air into the atomizer  50 . On its other side, the atomizer  50  has a threaded fuel-inlet flow passage  58  for the introduction of fuel into atomizer  50 . The fuel and air mix within the atomizer  50 , and a fine spray of atomized fuel, is carried on a stream of air through a vertical ejection port  52 . The air-inlet passage  54  of the atomizer  50  is connected to the air flowing from the mixture preparation chamber  10  by the threaded tube  57 . The fuel-inlet passage  58  of the atomizer  50  is connected to the fuel flowing into chamber  10  by t tube  30 . An opening  19  accommodates the atomizing orifice  52  of atomizer  50 .  
     [0033] A housing  100  is connected to the opening  19  of combustion chamber  102 . In FIG. 1, the housing  100  is a substantially vertical tube open at both ends. The lower end of housing  100  is connected in a gas tight manner around the opening  19  of the mixture preparation chamber  11 . The lower end of housing  100  is bounded by atomizer  50 .  
     [0034] The upper end of housing  100  terminates in an opening  104  through which the products of combustion are exhausted from the combustion chamber  102 . Depending on the application of the pilot burner, the opening  104  could communicate to the atmosphere, or to the main burner chamber, or to a soot burning combustion chamber.  
     [0035] A flame holder  60  is located above the atomizer  50 . The flame holder could be any suitable matrix such as a fiber mesh, a metal screen, or steel-wool, whose function is to evenly distribute the fuel air mixture ensuring even combustion, to support a stable flame formation, and to temporarily adsorb liquid components of the fuel air mixture. The flame holder  60  can also support radiant surface combustion of the air fuel mixture.  
     [0036] A spark-plug  70  is located above flame holder  60  to ignite the fuel air mixture. The spark plug  70  is inserted into the combustion chamber  102  through an opening  106  in the wall of the housing  100 .  
     [0037] The hot products of combustion are removed from the combustion chamber  102  through the exhaust opening  104 , which could be connected to the atmosphere or to a main chamber of the main burner or the soot-burning chamber.  
     [0038] As shown in FIG. 1, a preheater  80  is provided for preheating the air. The air enters at air inlet ports  17  of the housing  11  of the mixture preparation chamber  10 . The preheater  80  comprises a jacket  82  which is formed around the housing  100  of the combustion chamber  102  and the housing  11  of the mixture preparation chamber  10 . The jacket  82  is cylindrical in cross-section and is sized so that an annular space  81  is created for the flow of air between the wall forming the housing  100  of the combustion chamber  102  and the internal cylindrical surface of the jacket  82 . The upper end of the jacket  82  is sealingly connected in to an upper closure piece  83 . The lower horizontal end of the jacket  82  is sealingly connected to a lower closure piece  84 .  
     [0039] The upper closure piece  83  has an outer diameter  87  substantially equal to the diameter of jacket  82 , and is attached, for example by welding, brazing, etc., to the upper end of the jacket  82 . The upper closure piece  83  has an inner diameter  86  which is substantially equal to the outer diameter of the housing  100  of the combustion chamber  102 . The inner diameter  86  of upper closure  83  is connected to the outside of the housing  100  of the combustion chamber  102 .  
     [0040] The lower closure piece  84  has an outer diameter  89  substantially equal to the diameter of jacket  82 . This outer diameter  89  is attached to the end of jacket  82 , as shown in FIG. 1. The lower closure  84  has an inner diameter  88  substantially equal to the outer diameter of the housing  11  of the chamber  10 .  
     [0041] The housing  100  of the combustion chamber  102  functions as a heat transfer surface to transfer heat from the hot products of combustion within combustion chamber  102  to the relatively colder air flowing in the annular space  81 . The extent of heat transfer area is selected to provide an air preheat temperature of 160 to 600° F. The actual preheat temperature would depend on the flash point and auto-ignition points of the liquid fuel used in the burner. For example, diesel fuel has a flashpoint of  160 OF and an auto-ignition point of about 600° F. Thus, if diesel is used as a fuel in the burner, the preheat temperature of the air would be maintained above 160° F. to cause the fuel in the chamber  10  to vaporize when it contacts the preheated air. However, the preheat temperature of the air would also be maintained at less than 600° F. so as not to cause ignition of the fuel when it is mixed with the preheated air  44  in the chamber  10 . The combination of preheated air  44  and atomized fuel from the fuel orifice  36  contacting the surface element  60  facilitates a soot free, lean combustion, self-sustaining pilot burner. The actual dimensions of the jacket  82  would be selected based upon factors such as the preheat required for the particular fuel used in the burner as well as the rate of heat transfer through the housing  100  between the hot products of combustion in combustion chamber  102  and the air in the annular space  81 .  
     [0042] As shown in FIG. 1, an opening  98  is provided in the jacket  82  of the preheater  80  to accommodate an air inlet nozzle  90 . The air inlet nozzle  90  has an open inlet end  92  through which air introduced into jacket  82 . An opening  94  is also provided in the jacket  82  for receiving a spark-plug adapter  72 . A spark plug  70  is located in the adapter  72  and extends through the annular space  81  and into an opening  106  formed in the combustion chamber housing  100 . An opening  96  is also provided in jacket  82  for the introduction of the fuel tube  30  which passes through the annular space  81  and is received in the opening  18  formed in the chamber  11 .  
     [0043] The operation of the pilot burner will now be described.  
     [0044] To start the operation of the pilot burner  5 , the leads  22  of the glow plug  20  are connected to a source of electricity. This activates the electrical heating element (not shown) within glow plug  20 . After a period of time (usually a few minutes or less), the glow plug  20  will have reached its normal operating condition.  
     [0045] Ambient air  40  is now introduced through the inlet nozzle  90  and the opening  92 , where it enters the jacket  82  of the preheater  80  and flows downwardly (see arrow  42 ) through annular space  81 . Since the pilot burner  5  is still cold, there is no combustion taking place in combustion chamber  102 . Thus, there is no heat to transfer to the air, which is still at ambient temperature when it reaches the air inlet ports  17 . The location of air inlet ports  17  is chosen so that the air is distributed over at least a portion of the hot surface of glow plug  20 . The air  40  absorbs heat from the glow plug  20  and is heated to a temperature between the flash-point temperature and the auto-ignition temperature of the fuel that is used in the pilot burner  5 . The preheated air is shown in FIG. 1 as  44 .  
     [0046] The preheated air  44  flows through the tube  57  into the air inlet flow passage  54  of the atomizer  50 , and then out of atomizing orifice  52  under the flame holder  60 . The preheated air  44  then flows through the flame holder  60  into the combustion chamber  102  and exits through the outlet or opening  104 . After the heated air  44  has flowed through the various components of the pilot burner  5  for a period of time and an operating temperature is reached, fuel  46  is introduced into the system through the opening  32  of fuel tube  30 . The fuel  46  flows through fuel tube  30  to the fuel inlet flow passage.  
     [0047] The heated air  44  and the fuel  46  mix within the atomizer  50  to provide a finely atomized fuel-air stream  48  formed by the low vapor pressure portion of the liquid fuel  46  evaporating within the hot air  44 . The stream  48  exits from the atomizing orifice  52  and passes through the flame holder  60 . The liquid components of the fuel-air stream  48  adsorb on the surfaces of the flame holder  60 , while the vapor components travel through the flame holder  60 .  
     [0048] The angle of the stream  48  exiting the orifice  52 , and the distance between the orifice  52  and lower surface of flame holder  60 , are selected such that the stream  48  covers a most of the flame holder  60 . Typically, a full cone shaped stream  48  is desired, but other shapes such as hollow cones and oval shaped cones can be used. Other configurations of atomizers  50  which minimize or eliminates the need for pressurized air can also be used instead of the atomizer  50  shown in FIG. 1. In fact, any atomizer  50  can be used as long as the output from the atomizer  50  is an atomized air-fuel stream  48  which flows to contact the lower surface of flame holder  60 .  
     [0049] The vapor portion of the stream  48  passes through flame holder  60  and enters the lower portion  105  of the combustion chamber  102 , between the flame holder  60  and the firing tip  107  of the spark plug  70 . An electric current is passed through spark plug  70  to create a spark at the firing tip  107  which ignites the stream  48  to create flame. The flame is held by the flame holder  60  and heats the flame holder  60  so that it is able to transfer heat to the incoming stream  48  and initiate its ignition without the assistance of a spark from spark plug  70 . The products of combustion  110  flow upward through the combustion chamber  102  and exit through the opening  104 .  
     [0050] The combustion process raises the operating temperature within combustion chamber  102  to around  1 , 600  to  2 , 000 F. As the hot products of combustion  110  flow through combustion chamber  102 , they contact the housing  100  and lose some of their heat to the air  42  which is flowing through the annular space  81  outside housing  100 . The heat transfer raises the temperature of the air  42  and decreases the temperature of the products of combustion  110  exiting the burner through the opening  104 .  
     [0051] As cold air  40  continues to pass over the housing  100 , it gradually increases in temperature until it reaches its target temperature before entering the chamber  10  through the air inlet ports  17 . A temperature sensor  140 , located in the flow path of preheated air  42  before it enters the inlet ports  17 , senses the temperature of the preheated air  42 . When the air preheat target temperature is reached, the temperature sensor  140  activates control circuitry (not shown) to switch off the flow of electricity to the leads  22  of the glow plug  20 . Thus, excessive preheating of the air is avoided to reduce the chances of auto-ignition or premature combustion of the fuel-air mixture.  
     [0052] The flow of air  40  and fuel  46  into the pilot burner are continued for the duration of the operation of the pilot burner. Thus a self-sustaining flame is provided on flame-holder  60  by the preheating of air  40  to a temperature greater than the flash-point temperature of fuel  46 , mixing the preheated air  44  and fuel  46  using atomizer  50  and passing the fuel-air stream mixture  48  over the hot flame-holder  60 . As the temperature of the air  44  increases, the flame temperature above flame holder  60  increases, which allows for the amount of air  40  to be increased to lean out the combustion increasing the oxygen content in the combustion chamber. This increased amount of air enhances combustion and ensures the elimination of the soot formation in the product of combustion  110 .  
     [0053] The temperature of the products of combustion  110 , which are exhausted through the exhaust opening  104  of the combustion chamber  102 , is high enough such that it can provide the thermal energy for initiating the ignition of a fuel-air mixture within the main chamber of a liquid fuel burner or can initiate reaction within a downstream reactor. Thus the hot products of combustion  110  can act as a pilot flame to initiate and maintain the main flame in a liquid-fuel burner.  
     [0054] Alternately, the hot products of combustion  110  are hot enough such that they can initiate the combustion of soot particles, which may come into contact with it. Such an application can be found in diesel engines, wherein the soot from the exhaust of the engine is trapped and burnt in a combustion chamber using a small pilot flame-producing device. This application of the invention is described further in FIG. 3 below.  
     [0055] The use of preheated air to vaporize the liquid fuel has several advantages over the prior art implementations of the invention wherein the liquid fuel is vaporized by injection over the hot surface of a glow-plug. Pilot burners of the prior art are therefore susceptible to coking due to the cracking of the fuel by contact with excessive hot surfaces. This causes un-necessary equipment shut-downs and maintenance requirements as well as loss of production in steam-generating equipment which use liquid-fuel fired burners as the source of energy.  
     [0056] Another embodiment of the pilot burner shown in FIG. 1 is shown in FIG. 2 wherein the pilot burner assembly  5  is constructed in a straight-line configuration rather than the L-shaped configuration of FIG. 1. In the embodiment shown in FIG. 2, a housing  11  of the chamber  10  is integrated into a housing  100  of the combustion chamber  102 . Thus, the housing  100  is extended under the atomizer  50  to form the chamber  10  while eliminating the housing  11  shown in FIG. 1. The pilot burner  5  of FIG. 2 also is simpler to construct than the pilot burner  5  of FIG. 1.  
     [0057] Another difference in the construction is the location of an adapter  56 , which directly engages the atomizer  50  around the atomizing orifice  52 . Thus the short tube  57  shown in FIG. 1 for holding the atomizer  50  in the chamber  10  is eliminated.  
     [0058] The major components of the pilot burner  5  of FIG. 2 function similarly to the major components of the pilot burner  5  in FIG. 1 and are therefore given the same reference numbers. An additional component that is incorporated into the pilot burner  5  of FIG. 2 is a perforated catch tray  120  that is located in between the atomizer  50  and the glow plug  20 . The function of catch tray  120  is to catch any fuel drops that may inadvertently fall from a threaded fuel nipple  34 , especially during startup. A plurality of through perforations  122  are provided in catch tray  120  to allow the preheated air  44  to pass through the catch tray  120 . The preheated air  44  evaporates any liquid fuel that may have been trapped by the catch tray  120 . Thus the liquid fuel is prevented from contacting the hot surface of the glow-plug  20  and being carbonized. Therefore, nuisance shutdowns and unnecessary maintenance is reduced through the use of the catch tray  120 .  
     [0059] The catch tray  120  is also used to evenly flow the heated air  44  out of chamber  10  and to direct it into the air-inlet passage  54  of the atomizer  50 .  
     [0060] The operation of the pilot burner  5  of FIG. 2 is substantially identical to the operation of the pilot burner  5  of FIG. 1. The only additional step is that the preheated air  44  flows through the perforations  122  of catch tray  120  before reaching the air-inlet passage  54  of the atomizer  50 . [ 061 ] An embodiment of the pilot burner  5  shown in FIG. 2 that is adapted for the burning of soot particles is shown in FIG. 3 of the drawings. The pilot burner  5  of FIG. 3 is substantially identical in construction and operation to the pilot burner of FIG. 2 except for the provision of means to inject a fluidized air stream containing soot particles into the combustion chamber  102 .  
     [0061] In FIG. 3, this means to inject the soot-particles containing a fluidized air stream is a straight injection tube  140 , which is inserted vertically into the combustion chamber  102 . The injection tube  140  has a fluidized air inlet opening  142  at its upper end and a fluidized air outlet opening  144  at its lower end. The soot, which is trapped from the exhaust of an internal combustion engine, is fluidized using a small portion of the engine exhaust gas. A fluidized soot stream  132  is introduced into inlet opening  142  of the tube  140 . The soot stream  132  flows downwardly in the tube  140  and absorbs heat from the hot products of combustion  110  flowing over the outer surface of the tube  140 . The soot stream  132  therefore is heated to a temperature which is selected to be below the auto-ignition temperature of carbon to prevent premature combustion of the carbon in the tube  140 . Thus the danger of flashback due to premature combustion of the carbon in tube  140  is reduced.  
     [0062] The heated soot stream  132  is shown in FIG. 3 by reference number  134 . The heated soot stream  144  exits through the outlet opening  144  into the combustion chamber  102 . Upon contact with the hot products of combustion in the combustion chamber  102 , the soot particles in the heated soot stream  144  are heated to a temperature greater than the auto-ignition temperature of carbon. The soot particles therefore combust and are converted to carbon-dioxide which is carried away in the hot products of combustion  110  as it passes through the exhaust opening  104  of the combustion chamber  102 .  
     [0063] The tube  140  is arranged for counter-flow between the soot stream  132  and the hot products of combustion  110  so that heat-transfer between the two gas streams can take place using a minimum heat-transfer area for the given heat-duty. Other arrangements can be used instead of the tube  140 . For example, the tube  140  could be configured as a helical coil that is inserted into the combustion chamber  110  to further provide a more compact arrangement while maximizing the heat transfer.  
     [0064] All other aspects of the operation of the pilot burner  5  of FIG. 3 with respect to the combustion of liquid fuel  46  follows the description given for the pilot burner  5  of FIG. 2.  
     [0065] The overall heat transfer efficiency of the pilot burner  5  could be further enhanced by providing other means to recover heat from the hot products of combustion. For example, the hot products of combustion could be passed through other heat-transfer devices such as water-heaters, space-heaters, etc. to further recover the residual heat in the hot products of combustion.  
     [0066] An embodiment of a main burner, which utilizes the pilot burner shown in FIG. 2 for igniting and maintaining the main flame, is shown in FIG. 4. The main burner comprises a combustion chamber  200  and an atomization chamber  202  which form the upper zone and the lower zones respectively of a cylindrical tube  228 . The two zones are separated by a flame holder  224 . The lower end of tube  228  is closed in a gas tight manner by an oversized end-wall  240 . Thus the atomization chamber  202  is bounded by the end-wall  240 , the flame-holder  224 , and a portion of the tube  228 .  
     [0067] A cylindrical jacket  226  is provided around the tube  228 . The jacket  226  is closed at its lower end by the end wall  240  and at its upper end by an end-wall  242 . The end-wall  242  is attached in a gas tight manner to an outer surface of the tube  228  at its inside diameter and at its outside diameter to the jacket  226 . Thus an annular flow volume  225  is defined by the outer surface of the tube  228 , the inner surface of the jacket  226 , the end wall  240 , and the end-wall  242 .  
     [0068] A main fuel atomizer  206  is located in the atomization chamber  202 , and is held in place by an adapter  244  in a manner similar to that described previously for the pilot burner  5  in FIG. 2. A main fuel supply tube  208  is inserted in a gas tight manner through the jacket  226  and the tube  228  and is attached to the fuel-inlet passage of the atomizer. The atomizer  206  is a scaled up version of the atomizer that is used in pilot assembly  5  in order to accommodate the higher flow-rate of the main fuel stream in the main burner.  
     [0069] Primary air inlet ports  229  are located in the atomization chamber  202  under the atomizer adapter  244  for introduction of the primary air for atomization of the liquid fuel. As will be described, a small portion of the total air that is required for complete combustion of the liquid fuel is introduced into the atomization chamber  202  through primary air inlet ports  229 . The air enters the atomizer  206  and atomizes the fuel to produce an air-spray containing droplets of fuel. The fuel containing air-stream is blown against the lower side of the flame-holder  224 .  
     [0070] Also located in atomization chamber  202  above atomizer adapter  244  are secondary air inlet ports  226 . The secondary air inlet ports  226  introduce a larger quantity of air than the primary air inlet ports  229 . This air can be cold or preheated as shown in FIG. 4. The purpose of the secondary air is to vaporize the low-boiling fraction of the liquid fuel and to provide additional oxygen for the combustion process to take place.  
     [0071] The pilot burner assembly  5  is located above the flame holder  224  in the combustion chamber  200 . The exhaust gas outlet  104  of the pilot burner assembly  5  is inserted above the flame holder  224  through suitable gas tight openings  222  and  223  in the jacket  226  and the tube  228  respectively. The hot exhaust gases  110  from the pilot burner  5  are directed so that they ignite the fuel-air mixture flowing through the flame-holder  224 . All of the primary and secondary air is introduced into the annular flow space  225  through the main air inlet  246  connected to the jacket  226 . Since a large excess of combustion air is used, the combustion of the liquid fuel takes place at a relatively lower temperature than in conventional liquid fuel fired burners. Thus relatively lower quantities of thermal NOx are generated in the burner of the present invention compared to conventional liquid fuel-fired burners.  
     [0072] During operation, the pilot burner  5  is first lit as described previously with respect to FIG. 2. The hot exhaust gas  110  produced by the pilot burner  5  is directed into the combustion chamber  200 . Heat from the hot exhaust gas  110  is transferred to the flame-holder  224  and the tube  228 . When the flame-holder  224  and the tube  228  are sufficiently heated, the main air  212  is introduced into annular volume  225  through the main air inlet  246  in the jacket  226 . The main air  212  flows through the annular volume  225  and is preheated. As it passes through the annular volume  225 , a portion  218  of the main air is diverted into the atomization chamber  202  above the atomizer adapter  244  through secondary air inlet ports  226 . The final portion of the main air  212 , shown in FIG. 4 as primary air  220 , passes into atomization zone  202  through primary air inlet ports  229 .  
     [0073] It is not necessary that secondary air  218  be supplied by main air  212 . It may be advantageous to use separate sources of air for the primary and secondary air requirements of the burner, especially where a high pressure is required for the primary air in order to provide the motive force for atomization of the liquid fuel in the atomizer. In such cases, a lower pressure source could be used for the secondary air resulting in savings of energy required for compression of the air to a high pressure for carrying out the atomization.  
     [0074] It is also not necessary that the secondary air be heated. Air at ambient temperatures could be used for the secondary air and only the primary air could be heated as shown in FIG. 4. This may be required to maintain flame temperature within certain limits to produce lower quantities of pollutants such as thermally produced NOx.  
     [0075] While not shown in FIG. 4, it is well known to provide means to individually control the proportions and amounts of air that are used as primary and secondary air for burner-tuning and flame optimization purposes. Such means could include manually or automatically controlled flow-control dampers or other such devices.  
     [0076] After the air  212  has been sufficiently heated, fuel  210  is introduced to the atomizer  206  through the fuel supply tube  208 . The air  220  causes the fuel  210  to atomize to produce an air spray  232  containing droplets of fuel. The angle  204  of the air spray  232  is selected to cover the complete lower surface of flame-holder  224 . Heat is transferred from the hot air to the fuel within air spray  232  to vaporize the low boiling fraction of the fuel to produce an easily combustible mixture, which ignites on contact with the hot flow-passage surfaces within flame-holder  224 . The heat of combustion further maintains the flame-holder  224  at a high temperature and vaporizes the high boiling fraction of the fuel. The partially combusted fuel-air mixture contains a mixture of liquid fuel and products of combustion and is shown in FIG. 4 as  234 .  
     [0077] As the partially combusted fuel-air mixture  234  passes across the pilot burner  5 , further combustion takes place to produce a relatively clean combustion product gas  238 , which flows out of combustion chamber  200  through exhaust gas outlet  230 .