Self-sustaining premixed pilot burner for liquid fuels

Abstract

A burner for liquid fuel comprises a mixture chamber for producing a liquid fuel air mixture. The mixture chamber has a heating element, an air inlet for receiving air, the air inlet being configured so as to facilitate air flow over at least a part of the heating element, and a liquid fuel inlet. An atomizer is mounted in a path of flow of the liquid fuel air mixture formed by the mixture chamber. A combustion chamber for combusting the liquid fuel air mixture is provided. The combustion chamber has a flame holder, an ignition source located proximal the flame holder, and a combustion zone located downstream of the flame holder.

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'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'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 160OF 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 2,000F. 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.

Claims

1. 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;

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.

2. The burner for liquid fuel as claimed in claim 1 further comprising a means for preheating the air before its introduction to the air receiving means.

3. The burner for liquid fuel as claimed in claim 2 wherein the means for preheating the air comprises an external jacket formed about at least a portion of the combustion chamber.

4. The burner for liquid fuel as claimed in claim 2 wherein the means for preheating the air is a preheat coil located within the combustion chamber.

5. The burner for liquid fuel as claimed in claim 1 wherein the heating element means comprises a glow plug connected by electrical leads to a power source.

6. The burner for liquid fuel as claimed in claim 1 further comprising a catch tray for located between the heating element means and the for catching liquid fuel droplets.

7. The burner for liquid fuel as claimed in claim 1 wherein the atomizer has an exit opening configured so as to produce a cone shaped liquid fuel air mixture stream.

8. A burner for burning a low-volatility liquid fuel, the burner comprising:

an inlet air means through which burner air flows;

an inlet fuel means through which burner fuel flows;

a fuel air mixture preparation chamber downstream of the said inlet air means and said inlet fuel means and in which a fuel air mixture is mixed;

a combustion housing means for combusting a fuel air mixture that is in heat exchange relationship with the said burner air flow and in fluid connection with the said fuel air mixture preparation chamber.

9. The burner of claim 8 wherein the combustion housing means further comprises a flame holder and ignition source.

10. The burner of claim 8 wherein the fuel air mixture preparation chamber means further comprises a heating element to preheat the said burner air during start-up of the burner.

11. The burner of claim 8 wherein the fuel air mixture preparation chamber means further comprises a liquid fuel atomizer to enhance the vaporization of fuel in the fuel air mixture.

12. 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.