Abstract:
A method and apparatus for preheating a furnace during a warm-up phase of furnace operation. The furnace has main burners with a tubular fuel supply surrounded by a main combustion air duct defining an annular space between the supply and the duct that extends in an axial direction of the main burner. A pilot nozzle in the annular space extends in an axial direction of the burner towards an interior of the furnace and discharges readily ignitable fluid fuel jets through orifices in the nozzle toward the interior of the furnace. Combustion air from the duct is directed past the nozzle and is mixed with the fuel discharged from the orifices to form an ignitable mixture that is ignited to form the furnace heating pilot flame downstream of the nozzle. The flame is stabilized and anchored to the pilot nozzle by recirculating portions of the flame and its constituents from the furnace interior back towards the nozzle by protecting the air passing through the primary ignition zone from being directly affected by air flowing through the main combustion air conduit, diverging the fuel jets relative to the axial direction by an angle between about 20° to 80°, and giving the fuel jets a tangential directional component relative to the axial direction to spin the flame about the axis of the pilot.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Provisional Patent Application No. 60/967,915 filed on Sep. 6, 2007, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to warming up large utility-type furnaces, such as are used, for example, to generate steam for major electrical power generating plants, particularly but not limited to coal-fired plants, prior to the start-up of such furnaces to commence their production phase of operation. 
         [0003]    Such large furnaces can have the size of a large building, and they often employ dozens of spaced-apart burners to provide the needed heat for generating large amounts of electrical power. These furnaces are fired with all types of fuels, including, but not limited to, oil, gas, coal, bio-mass, etc. The furnaces require an initial heat-up to bring their interior to the required operating temperature at which all burners can be fired fully without causing flameouts, generating large amounts of smoke and other pollutants, potentially damaging portions of the furnace due to excessive heat differentials, and the like. 
         [0004]    Operators of such furnaces desire that the warm-up period is as short as possible because during warm-up phase, expensive fuel is consumed without generating any power. In the past, so-called warm-up guns were preferred over smaller pilot burners, especially for coal-fired furnaces, to generate sufficient amounts of heat over a relatively short period of time so that the production phase of the furnace can commence as soon as possible. 
         [0005]    While natural gas and oil can be quickly ignited and do not require long warm-up periods, coal-fired furnaces encounter the problem of having to heat the furnace sufficiently so that the large mass of coal consumed by the furnace during its production phase can be ignited and will burn cleanly and completely without emitting excessive pollutants. Conventional burner pilot lights that are also used for the main burner ignition and flame stabilization either have too low a heat output to accomplish the required warm-up over a desired period of time, or require an additional air supply besides the air passing through the main burners, which makes their installation more complicated and expensive. 
         [0006]    It is not normally feasible to increase the size of the pilot burners to enhance their heat output because pilots are arranged in the relatively small annular passage between the main burner and the surrounding combustion air duct. This limits the size of pilots, for most industrial installations, to no more than about four to five inches in diameter. With such size limitations, the maximum heat output of pilots not using additional air supply is typically limited to around 3 to 7 million BTUs per hour. 
         [0007]    Increasing the fuel flow rate through the pilots beyond that range results first in unstable operation sensitive to the regime of air flow through the main burner. The operation becomes unsatisfactory because the resulting high velocity fuel jets can snuff out the pilot flame. 
         [0008]    Further, the high capacity pilots need to be located at the main burner discharge end to generate the flame in the furnace interior and prevent the flame from burning the main burner. At the same time the pilots need to be protected from the main burner flame. In order to meet both these requirements, such installations often require complicated mechanisms to subsequently retract the pilot rearwardly out of the heat and away from fuel particles into the combustion air duct, which are costly, require much maintenance, and are subject to early failures. 
         [0009]    Separate warm-up guns not tasked with the main burner ignition were therefore widely employed for warming up furnaces. Although such guns are capable of generating large amounts of heat and, therefore, can significantly shorten the warm-up period for even coal-fired utility-type furnaces, they require their own combustion air supplies as well as relatively complicated installations including their own piping, fans, motors, controls, gun retracting mechanism and the like, all of which make separate warm-up guns expensive to install and maintain. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention improves the manner in which large furnaces, such as are used for commercial power generation, and particularly coal-fired furnaces, are preheated during the initial start-up phase of furnace operation when the interior furnace temperature must be raised sufficiently to commence the production firing of the furnace. This is accomplished in accordance with the present invention by placing a pilot burner (hereafter typically “pilot”) with a much higher heat output in the same limited annular passage between the fuel, e.g. coal supply conduit, and the surrounding combustion air supply duct, where prior art burners have been commonly placed in the past. The pilot of the present invention is operated with combustion air for the main burner and eliminates the need for a separate air supply for the pilot. The pilot flame is ignited and stabilized by injecting a portion of gaseous fuel delivered to the pilot in a spinning pattern that creates intense recirculation and mixing of the discharged pilot fuel with appropriate amounts of air passing through the main burner around the pilot. 
         [0011]    Such pilots can provide a heat output in the range between about 4 to 50 million BTU per hour, which is much higher than the heat output that could be achieved with prior art pilots operating without the additional air, and assures a rapid heat-up of the furnace and a relative quick start-up of its production phase. Substantial amounts of fuel otherwise used by the pilot without producing useable steam or electricity are thereby saved. 
         [0012]    The present invention provides both a method and an apparatus for preheating furnaces, particularly large utility-type furnaces that have many burners which often operate with difficult-to-ignite coal during the warm-up phase of furnace operation. Generally speaking, this involves a main production burner that includes a first conduit for directing a fuel, for example coal, into an interior of a furnace. An air duct surrounds the coal conduit to define an annular combustion air passage into the furnace where the coal and combustion air are mixed and ignited during the production phase of furnace operation. 
         [0013]    A pilot nozzle is positioned in the air passage of the main burner so that a downstream end of the nozzle is proximate the downstream end of the burner. The nozzle is surrounded by a tubular hood which has an open downstream end proximate the downstream end of the nozzle and an upstream end. Air flowing through the main burner passage for air is prevented from directly entering the hood by placing a flow inhibitor, such as a plate, over the upstream end of the hood while permitting air to enter the hood via a gap formed between the hood and the plate. 
         [0014]    A relatively lesser portion of a pressurized fluid fuel, e.g. a gas, is flowed through igniter orifices in the nozzle located inside the tubular hood at a rate commensurate with the amount that can be burned within the limited space inside the tubular hood. The fuel gas jets inside the hood are directed at angles that facilitate entrainment of air through the hood. A major portion of the pilot gas is discharged from main pilot orifices—a plurality of spaced-apart pilot orifices in a downstream end portion of the nozzle immediately adjacent to the hood. The fuel jets from the main pilot orifices are oriented so that the emitted fuel jets angularly diverge in the downstream direction and have a tangential flow component relative to the longitudinal axis of the pilot nozzle. As fuel jets discharging from the main pilot orifices pass in the vicinity of the hood downstream end, they also facilitate the flow of air through the hood. 
         [0015]    Fuel emitted by the igniter orifices inside the tubular hood is mixed with air passing through the hood and is ignited to generate an igniter flame that propagates past the downstream end of the hood. The igniter flame in turn ignites the mixture of fuel from the main pilot orifices and air passing through the main burner to generate a pilot flame that extends into and heats the furnace interior. Portions of the pilot flame and its constituent gases recirculate from the furnace interior rearwardly towards the nozzle while the flame as a whole spins relative to the nozzle axis to maintain a stable pilot flame. 
         [0016]    One reason why placing high heat output pilot burners inside the combustion air duct of the main burner has heretofore been unsuccessful was that the volume of air flowing through the duct may vary substantially so that the pilot fuel flowing with a fixed rate often fails to ignite, or to maintain the flame, due to unfavorable fuel-to-air ratios, unless the pilot has its own air supply and controls. This is overcome by the present invention because the amount of air entering the hood is substantially proportional to the fuel delivered inside the hood through the igniter orifices and only to a small degree affected by the amount of flow through the duct as its upstream flow inhibitor effectively shields the pilot fuel from the effect of the high velocity air flowing through the duct. The hood forms a small combustion chamber where a relatively minor amount of the pilot fuel is initially ignited to form the igniter flame which propagates in a downstream direction past the downstream end of the hood, where the major portion of the pilot fuel is discharged via the appropriately positioned and oriented pilot orifices. 
         [0017]    The hood, including the earlier mentioned flow diverter, also effectively shields sensitive components like the spark electrode inside the hood from the heat of the main pilot flame and the furnace, which allows operating the burner without having to retract the pilot into the burner. 
         [0018]    In addition, to maintain a pilot flame, it must be stable and remain anchored to the nozzle. High heat output pilots require high fuel velocities through the burner orifices of as much as 500-1500 ft./sec. Such high fuel jet velocities lead to undesirable flame instabilities which are significantly reduced or entirely eliminated in accordance with the present invention by imparting a spin to the pilot flame downstream of the nozzle that facilitates establishing a recirculating flow downstream of the nozzle. To attain such a spin, the axes of the pilot orifices are tangentially offset relative to the pilot axis as described in more detail below. The tangential flow component of the jets provides the spinning results obtained with common prior art burners by placing relatively large spinners around the nozzles that cannot be applied here due to the earlier mentioned space limitations. 
         [0019]    Another important advantage of the present invention is that the amount of air entering the interior of the hood automatically adjusts itself to the amount of fuel emitted by the igniter orifices inside the hood because as the volume of emitted fuel varies, its speed varies correspondingly, which in turn lowers or raises the fuel pressure inside the hood inversely to the velocity of the fuel emitted from the igniter orifices. With the lowered pressure, more air from the air duct is aspirated into the hood interior so that an approximate stoichiometric balance between the fuel and the air in the hood is maintained. This assures an uninterrupted igniter flame to maintain the main pilot flame even in the event of a temporary flameout. The amount of air drawn into the hood is correspondingly lowered as less fuel is emitted from the igniter orifices of the nozzle and the pressure inside the hood rises correspondingly. 
         [0020]    Thus, the pilot burner of the present invention is relatively inexpensive because it has no moving parts and needs no internal or external controls. 
         [0021]    A further advantage attained with the present invention is that the pilot burner is shielded from the high temperature and abrasive/corrosive/contaminating influences of the gases, dust and particles on the furnace interior because the pilot is located inside the air duct, which reduces maintenance costs and prolongs the life of the burner. Still further, since the pilot burner of the present invention requires no external controls, separate air supply lines and the like, it can be made relatively larger in the limited space available in the air ducts of industrial burners. This in turn makes it possible to increase the heat output of the burner and thereby shorten the warm-up period for the furnace, all of which reduces operating costs for the furnace warm-up and pilot burner maintenance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  schematically illustrates a large, e.g. utility-type, furnace arrangement for driving a steam turbine as used in large electric power generating plants; 
           [0023]      FIG. 2  is a schematic, cross-sectional view through a burner, including a high heat-output pilot constructed in accordance with the present invention for installation in the furnace shown in  FIG. 1 ; 
           [0024]      FIGS. 3A and 3B  are sectional views of the pilot of the present invention; 
           [0025]      FIG. 4  is a schematic front elevational view of the main burner and the pilot shown in  FIG. 1 ; 
           [0026]      FIG. 5  schematically illustrates the formation of a pilot flame recirculation zone in accordance with the present invention; 
           [0027]      FIG. 6  is an end view of an air flow restrictor plate of the pilot shown in  FIGS. 3A and 3B ; 
           [0028]      FIG. 7  is an end view of an air flow straightener that prevents combustion air from flowing directly into a hood surrounding the pilot; and 
           [0029]      FIGS. 8A and 8B  are end and side elevational views, respectively, of the nozzle of the pilot shown in  FIGS. 3A and 3B . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]      FIG. 1  schematically illustrates a large power generation installation, as is commonly used, for example, by public utility companies for generating electricity for the public. The installation has at least one large, utility-type furnace  2  and many, typically dozens, of main production burners  4  which extend through at least one wall  6  of the furnace into its interior  8 . Such furnaces can be and are fired with all kinds of fuels, with oil, coal and natural gas being the most common. The present invention has particular (but not sole) applicability to firing the furnaces with coal which is typically ground to fine powder or dust. As is well known, the heat generated by the fuel on the interior of the furnace generates steam  10  that can be used to drive a turbine  12  which may be connected, for example, to an electric generator (not shown). Exhaust gas from the furnace is released to the atmosphere through a stack  14 , typically (but in many of the areas of the world not necessarily) after having been appropriately cleaned and/or scrubbed to limit atmospheric pollution. 
         [0031]      FIG. 2  schematically shows the use of the present invention with a main production burner  4  mounted on and operatively extending through one of the furnace walls  6  and constructed to burn coal, typically finely ground or pulverized coal. It has a coal supply source  18  and a coal supply conduit  20  in which powdered, pulverized or the like coal flows in a downstream direction to a discharge end  22 , which may include a spinner or diverter  24  for discharging the coal via an outwardly flared burner throat  26  in furnace wall  6  into the interior  8  of the furnace. Main burner  4  further has a combustion air supply duct  32  which concentrically surrounds coal supply conduit  20  to form an annular combustion air passage  34  between the coal supply conduit and combustion air duct. During operation, combustion air needed to burn the coal (or other fuel) is discharged from the downstream end  22  of the burner into the furnace interior. The main burner may include a supplemental fuel supply tube  28  which runs coaxially through (the horizontal portion of) the main burner and has a fuel discharge end cap  30  that can be used to provide additional heat from firing oil or gas, for example during peak demand periods for electricity when more heat output is needed. 
         [0032]    The construction and operation of such main burners is well known to those of ordinary skill in the art and, therefore, is not further described herein. 
         [0033]    Burner installation  4  includes a pilot burner  36  constructed in accordance with the present invention to initiate combustion in the furnace interior and, during a start-up phase of operation of the furnace, to warm up the furnace interior until main burners  16  can be fired after the furnace interior has reached the required temperature for maintaining a stable and complete combustion of the coal (or other fuel). The pilot has a feed tube  38  through which a fluid fuel, such as natural gas for example, is supplied from an appropriate source (not shown) to a pilot nozzle  40 . The nozzle is surrounded by a tubular shield or hood  42 , the ends of which are open, and an igniter, e.g. an electrical spark igniter  44 , is provided for igniting the fuel, as is further described below. 
         [0034]      FIGS. 3A , B and  4  show the pilot burner of the present invention in greater detail. Nozzle  40  includes and is attached to a downstream end of feed tube  38 , has a discharge (or downstream) end  50 , and has a plurality of pilot fuel discharge orifices  52  from which pilot fuel jets flow. The pilot fuel jets are discharged at an oblique angle relative to the longitudinal axis of the pilot burner, and they are additionally tangential to the axis of the pilot as is further described below. 
         [0035]    Tubular hood  42  has open upstream and downstream ends  66 ,  68 , respectively. A flow straightener and conditioner  70  (shown also in  FIG. 7 ) is positioned inside the upstream end of the hood and extends some distance into the hood. A fuel feed tube  38  and an igniter support pipe  76 , respectively, extend into the hood  42 . The flow straightener includes a plurality of ribs  80  placed between the hood  42  and the fuel feed pipe  38  parallel to the igniter burner axis  96 . The ribs define multiple flow straightening passages  82  that extend in an axial direction of the pilot. Air flowing between the ribs  80  becomes better oriented in the axial direction of the pilot, a feature which is particularly useful in instances when air flowing through the passage  34  is at an oblique angle relative to the pilot axis. 
         [0036]    Pilot  36  is further fitted with a damper plate  84  (also shown in  FIG. 6 ) which is spaced apart from the upstream end  66  of hood  42 . The damper includes a tubular hub  86  that surrounds pilot fuel feed tube  38  and is slidably movable therealong. Opposite hub  86  is a U-shaped cutout  88  through which igniter support pipe  76  extends. 
         [0037]    The axial position of damper plate  84  relative to the upstream end of the hood can be adjusted by moving the plate along fuel supply tube  38  of the pilot burner to vary the width of a gap  90  between the upstream end of the hood and the damper plate to accommodate specific characteristics of the fuel and provide a range of air flows through the burner  32 . 
         [0038]    The downstream end of igniter support pipe  76  ends at a bluff body  92  ( FIGS. 3A , B) attached to the inside of the tubular hood  42 . An electronic igniter  94  is placed inside the support tube  76  end about flush with the bluff body  92 . On the side facing the flow, the bluff body  92  is shaped with a slope  93  that eliminates stagnation areas to the flow upstream of the igniter  94 . Suitable hardware and wiring (not shown) for the electronic igniter extends through the igniter pipe  76  to an igniter control (not shown). 
         [0039]    In a presently preferred embodiment of the invention, pilot nozzle  40  is configured as a cap attached to the downstream end of fuel feed tube  38  and has a multiplicity of fuel discharge orifices  52  arranged in a plurality of, e.g. two, rows  52 A,  52 B that are spaced apart in the axial direction of the nozzle, as illustrated in  FIG. 3B . Each orifice diverges in a downstream direction relative to the pilot burner axis  96  by an angle α (shown in  FIG. 8B ) in a range between about 20° to 80°, preferably in a range between about 35° to 75° and in the presently preferred embodiment at an angle of about 60°. 
         [0040]    In addition, each orifice  52  is arranged so that its center line  98  is offset relative to a radius line  100  with its origin at the center  96  of the nozzle so that each orifice is also tangential relative to this center, as is illustrated in  FIG. 8A . This causes the fuel flow and flame in the wake of the nozzle  50  to spin in a manner analogous to a conventional spinner and anchors the flame to the pilot in spite of the high velocity fuel jets emitted from the orifices. 
         [0041]    In a presently preferred embodiment, the pilot nozzle  40  additionally includes relatively small-diameter center holes  102 . In use, gas flows through the center holes which cools the nozzle center. 
         [0042]    Referring to  FIG. 4 , pilot nozzle  40  and igniter  44  are offset relative to the axis of tubular hood  42  so that the pilot nozzle is adjacent one side of hood  42 , to thereby define an enlarged space  104  between the periphery of the pilot nozzle and the opposite wall of the hood where an initial igniter flame is generated, as is further described below. Arrows  106  in  FIG. 4  illustrate the tangential positioning and orientation of fuel jets  53  (shown in  FIG. 5 ). 
         [0043]    Turning to the operation of pilot  36  for starting up a cool furnace, combustion air flows through annular passage  34  of burner  32  in a downstream direction past tubular hood  42  and then into the furnace interior  8 . The gas for the pilot is flowed through feed tube  38  to orifices  46  and pilot nozzle  40 . Sizing of the orifices  46  is such that a relatively minor portion of the fuel exits through igniter orifices  46  in the feed tube  38  which are oriented to direct resulting fuel jets into the enlarged space  104  inside the hood and in the vicinity of igniter  44 . At the same time, air from annular passage  34  of the main burner enters the interior of hood  42  via gap  90  between the upstream end of the hood and damper plate  84 . Flow straightener  70  straightens out the incoming air so that it flows generally in the direction of the pilot axis and becomes mixed with fuel from igniter orifices  46 . The resulting mixture is ignited by spark igniter  94  to form an igniter flame  47  in the enlarged space  104  which propagates in a downstream direction past downstream end  68  of the hood, as is illustrated in  FIG. 5 . 
         [0044]    The bulk of the fuel for preheating the furnace is ejected through orifices  52  in nozzle  40  as gas jets  53  which diverge outwardly in the downstream direction so that the ejected fuel becomes mixed with combustion air that flows through the annular passage  34  of the main burner. This mixture is ignited by the igniter flame  47  exiting from the downstream end of the hood which maintains the main pilot flame  54 . 
         [0045]    The amount of combustion air typically flowing through the annular passage  34  depends on the operational needs of the regime and is substantially independent of the pilot burner operation. The rate at which fuel is needed for the pilot also may be changed for operational reasons. To maintain the igniter flame  47 , the amount of air fed to the burner must reflect the amount of fuel ejected by the igniter orifices to maintain an overall flammable mixture inside the hood  42  on the downstream part of bluff body  92 . 
         [0046]    To properly control the flow of air into hood  42 , damper plate  84  blocks combustion air flowing through annular passage  34  directly into the hood. Instead, combustion air must first flow from the annular passage in a radial direction (relative to hood  42 ) through gap  90  and is then redirected past flow straightener  70  into the interior of the hood, thus minimizing the effects of air flow velocity through the passage  34  onto the amount of air flow entering the hood  42 . The axial position of damper plate  84  relative to the upstream hood end can be adjusted by moving the plate, including its flange  86 , along feed tube  38  to set the proper width for gap  90  to permit a sufficient air flow into the hood while preventing variations in the combustion air flow in the annular passage from materially affecting the air flow rate through the hood. 
         [0047]    In use, the position of the damper plate is not normally changed. The air intake via gap  90  into the hood is nevertheless automatically varied as a function of the gas flow rate through igniter orifices  46  because as the gas velocity through the igniter orifices increases or decreases, the pressure inside the hood changes inversely to the pressure changes. An increase in the gas velocity through the igniter orifices lowers the pressure in the hood, which causes an increase in the air flow rate through gap  90  into the hood and vice versa. This air flow variation occurs automatically and requires no controls of any type. 
         [0048]    Accordingly, the pilot burner of the present invention is self-regulating and maintains the igniter and pilot flames  47 ,  54  regardless of changes in the combustion air flow rate while stabilizing the pilot flame  54  and anchoring it to the end of the pilot burner. This assures a continuing, uninterrupted, self-regulating operation of the pilot burner to fully heat up the furnace as quickly as possible. 
         [0049]    It is typically preferred to maintain the igniter flame  47  inside hood  42  for the duration of the pilot burner operation so that in the event the main flame generated by the pilot becomes extinguished, it is immediately reignited by the pilot flame. 
         [0050]      FIG. 5  schematically illustrates the main pilot flame  54  generated downstream of the pilot burner  36  and its interaction with pilot flame  47  extending from downstream end  68  of the hood. As was earlier described, fuel jets  53  emanating from orifices  52  of pilot nozzle  40  are directed outwardly and away from pilot axis  96  into the furnace interior. To achieve the required heat input, the gaseous fuel jets  53  have velocities which typically range between 500 to 1500 ft./sec. These high velocities also help mix fuel jets with sufficient air to efficiently burn large quantities of fuel gas delivered through the pilot. 
         [0051]    In order to assure reliable flame propagation from the flame  47  through the high velocity fuel jets  53 , flammable mixtures in substantial parts of the flow immediately adjacent to the nozzle  40  have to be achieved and maintained over the duration necessary to ignite the fuel. This is accomplished by placing orifices  52  about the circumference of the nozzle  40  in two or more staggered rows axially spaced from each other and by the tangential positioning of the orifices spinning off fuel emitted from pilot orifices  52 . In each row, the orifices are typically spaced by about one to three times the diameter of the orifices. In a presently preferred embodiment, the spacing between the orifices is approximately twice the nozzle diameter. 
         [0052]    Propagation of the flame through gas jets  53  is not sufficient for the flame  54  stabilization. Flow recirculation  58  enhanced by the spinning of fuel emitted from pilot orifices  52  caused by the tangential positioning of the orifices makes the pilot operation efficient and reliable. 
         [0053]    As is well known to those skilled in the art, a tangential component imparted to fuel jets to form a forward-directed spiral motion facilitates the formation of gaseous recirculation patterns. The greater the spiral effect, the better the recirculation. The recirculation component of the gas is a function of the so-called “swirl number” S according to the following formula: 
         [0000]    
       
         
           
             S 
             = 
             
               
                 
                   ∑ 
                   
                       
                   
                 
                  
                 
                     
                 
                  
                 
                   axial 
                    
                   
                     
                         
                     
                      
                     
                         
                     
                   
                    
                   flux 
                    
                   
                       
                   
                    
                   of 
                    
                   
                       
                   
                    
                   angular 
                    
                   
                       
                   
                    
                   momentum 
                 
               
               
                 axial 
                  
                 
                     
                 
                  
                 thrust 
                 × 
                 R 
               
             
           
         
       
       
         wherein the axial thrust is the axial force exerted by the combustion air and gas flows entering the recirculation zone, 
         R is the radial distance (from the center of the pilot nozzle) of pilot orifices  52 , and the angular momentum is the rotational force at R generated by the gas jets  53 . 
       
     
         [0056]    For certain fuels, such as oil, for example, pilot nozzle  40  can extend past the downstream end of main burner  4  into burner throat  26 . However, for coal-fired burners, the pilot is recessed into the annular space  34  between coal supply conduit  20  and combustion air conduit  32  to keep the pilot away from the heat, smoke, dust, particulates and the like that are typically present on the interior of coal-fired furnaces, but which are kept out of annular passage  34  and therefore also away from the pilot nozzle by the flow of combustion air. 
         [0057]    The combustion of fuel from pilot  36  is continued until the furnace interior has reached the desired temperature, at which time the production fuel, e.g. coal, can be ignited and stably combusted without generating large amounts of pollutants as would occur if combustion were commenced before the required furnace temperature has been reached.