Abstract:
The invention pertains to a burner for solid fuel, said burner comprising: A stoker ( 41 ) having a fuel outlet ( 29 ) within said burner, and an exhaust ( 31 ), a grate ( 27 ) positioned below said fuel outlet ( 29 ) and between said fuel outlet ( 29 ) and said exhaust ( 31 ), a first blower ( 92 ) connected to a space below said grate ( 27 ), and a second blower ( 68 ) connected to a plurality of injectors ( 61 A,  62 A) provided around said fuel outlet ( 29 ) above said grate ( 27 ) and directed towards said exhaust ( 31 ), said injectors ( 61 A,  62 A) being parallel and unidirectionally directed over said grate ( 27 ), and wherein the injectors ( 61 A,  62 A) are provided along the periphery of said burner, said injectors ( 61 A,  62 A) being configured for injecting secondary air at a speed of 40-80 m/s, thereby ensuring unidirectional parallel flows of secondary air over said grate ( 27 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a burner for burning solid fuel, in particular a burner where primary and secondary air is supplied to the fire. 
       BACKGROUND OF THE INVENTION 
       [0002]    In a burner of this type the solid fuel is typically combusted on a grate where it is deposited by a fuel feeding mechanism, such as for example a screw conveyor including a screw disposed in a metal tube at one end connected to a silo or similar storage of solid fuel, the other end being positioned within the burner above the grate. 
         [0003]    The grate is typically a stepped grate including a number of staggered grates which may be movable in relation to each other in order to displace ashes and other residual combustion products towards the bottom of the burner, from where they can be removed by an ash removal mechanism similar to the fuel feeding mechanism. 
         [0004]    Air is supplied to the burner as primary air, i.e. air supplied to a space below the grate, wherefrom it passes upwardly through the grate and provides oxygen to the combustion process, and as secondary air, i.e. air which is provided to the space above the grate to provide further oxygen to the combustion of the fuel and any further combustion taking place in the flue gas produced by the combustion process. The primary and secondary air is typically supplied by electrical blowers. 
         [0005]    In many cases the burner is combined with a boiler, whereby the flue gas resulting from the combustion process is led through flue gas pipes deposited in a boiler, where the heat of the flue gas is used to heat water circulating through the boiler. The cooled flue gas may then be released into the atmosphere via a chimney, possibly after further cleaning of the flue gas. 
         [0006]    Typically, secondary air is supplied from two opposite directions, whereby the collision of the air streams forces part of the secondary air down through the grate, which hampers the flow of the primary air, thus affecting the oxygen dependent combustion negatively. 
         [0007]    A burner where the secondary air is not supplied from two opposite directions is known from amongst others WO8301671; however, this burner supplies the secondary air as a horizontal rotational flow. Thus, the secondary air may still interfere with the primary air. 
         [0008]    Other examples of burners can be seen in for example U.S. Pat. No. 4,697,530, which discloses an automatic stoker boiler having air openings in an air plenum, which are placed in the opposite side of the stoker as the fuel inlet. Especially the air openings are not placed around the fuel outlet, and WO 02/079693 discloses a burner with a fuel outlet and a grate positioned below said outlet. The burner is equipped with a number of air holes in all the sides for the supply of secondary air, for cooling the sides and cover part. By the disclosed arrangement of air holes in all sides of the burner it is not possible to sustain unidirectional parallel flow of air over the grate. 
         [0009]    US 2007/089733 discloses a wood-burning boiler having a fire chamber and a loading door for fuel supply. The boiler also comprises a grate, under which under fire air is supplied for sustaining the combustion of the fuel. In the rear plate there is provided a plurality of air holes for the supply of secondary over-fire air into the fire chamber. The disclosed arrangement of air holes is therefore not capable of sustaining a unidirectional parallel flow of air over the grate. 
         [0010]    GB 2 193 798 discloses a cylindrical coal burner equipped with a stepped grate, wherein each grate section is equipped with a plurality of venture ports, which are disposed in the side of the cylindrical wall of the burner, and at an angle of approximately 60 degrees relative to the longitudinal portion of the burner. In the end wall there is provided a plurality of air ports for the inlet of air in a direction substantially along the longitudinal extension of the burner. These air ports are not placed above the fuel outlet, and the combined effect of these air ports is to impart a circulatory motion to the coals. 
         [0011]    FR 2 570 472 discloses a cylindrical coal stoker having a fuel outlet and an exhaust, a grate positioned below said outlet and between said fuel outlet and said exhaust. FR 2 570 472 also discloses means for supplying primary air through said grate for the combustion of coal, and means for supplying secondary air from a plenum above said fuel outlet. 
         [0012]    WO 00/75563 discloses a burner for the burning of solid fuels. The burner comprises a fuel outlet and an exhaust, a cartridge with longitudinal apertures for the provision of under-fire air to enhance the combustion of the fuel. Furthermore, adjacent to the fuel outlet there are placed two tube pieces for the provision of air along the longitudinal direction of the burner. Additionally, there are provided air holes in the bottom of the burner for the provision of tertiary air into the combustion chamber in order to create turbulence within the combustion chamber. Moreover the tube pieces are placed so far apart from each other that they will not be able to sustain a unidirectional parallel flow of secondary air over the grate. 
       SUMMARY OF THE INVENTION 
       [0013]    It is a primary object of the present invention to provide a more efficient combustion of solid fuel as compared to the types of burners known in the art. 
         [0014]    This and further objects are achieved by a burner for solid fuel, said burner comprising: a stoker having a fuel outlet within said burner, and an exhaust, a grate positioned below said fuel outlet and between said fuel outlet and said exhaust, a first blower connected to a space below said grate, and a second blower connected to a plurality of injectors provided around said fuel outlet above said grate and directed towards said exhaust, said injectors being parallel and unidirectionally directed over said grate, wherein the injectors are provided along the periphery of said burner, said injectors being configured for injecting secondary air at a speed of 40-80 m/s, thereby ensuring unidirectional parallel flows of secondary air over said grate. 
         [0015]    By placing the air injectors along the periphery of said burner and injecting the secondary air at a speed of 40-80 m/s, it is possible to achieve the effect of ensuring a substantially laminar flow of secondary air over the entire length of the grate, because the secondary air will travel in a laminar flow along the upper periphery of the burner at a sufficient speed to ensure this laminar or substantially laminar flow. This laminar air flow creates a pressure difference between the upper and lower part of the combustion chamber, whereby flue gas from the combustion of the solid fuel is drawn away from the grate. This causes a more efficient combustion, because the secondary air will not collide with the primary airstreams from beneath the grate and therefore not adversely influence the flow of air and thereby oxygen necessary for the combustion of the solid fuel. In fact, the above mentioned pressure difference will enhance the flow of primary air necessary for the combustion. 
         [0016]    According to an embodiment of the invention, there is thus provided a burner where secondary air does not interfere with primary air. 
         [0017]    One embodiment of the invention pertains to a burner for solid fuel, said burner comprising a grate, a first blower connected to a space below said grate, and a second blower connected to a plurality of injectors provided above said grate, wherein said injectors are parallel and unidirectionally directed over said grate. 
         [0018]    Since the injectors are parallel and unidirectionally directed over the grate, the air supplied by the second blower to the injectors is supplied as parallel flows of air over the grate. As the flows are parallel, no collision which could force the secondary air down towards the grate or through the grate where the secondary air could disturb the flow of primary air occurs. Furthermore, the parallel flows of secondary air over the grate cause an ejector effect, whereby the flue gas from the combustion of the solid fuel is drawn away from the solid fuel and the grate, thus facilitating the provision of oxygen, by the primary air to the solid fuel being combusted. 
         [0019]    The burner may comprise a cylindrical enclosure in which the grate is positioned. The cylinder is preferably circular cylindrical and preferably oriented so that the axis of the cylinder is substantially horizontal. The burner and the grate may be made from metal, preferably steel or iron. 
         [0020]    The solid fuel may be any readily combustible solid fuel such as coal, peat, lignite, but is preferably a solid renewable bio fuel such as straw, wood chips or wood pellets. It is further contemplated within the context of the present invention that slurries, i.e. solids in liquid, could be combusted in the burner. 
         [0021]    The grate supports the solid fuel during the combustion thereof while also allowing air from below the grate to pass through the grate to supply the combustion process with fuel. The air may pass between the bars and crossbars of the grate, or alternatively the grate may be defined as a plate with apertures such as holes or slits through which the air may pass. 
         [0022]    The grate is preferably a stepped grate comprising a plurality of staggered grates adapted to be movable in relation to each other for transporting the solid fuel by gravity from the top grate to the bottom grate, where the ashes and other solid residual products from the combustion process may be removed. 
         [0023]    The space is provided below the grate and is bounded by the grate and the walls and floor of the burner so that upon introduction of air into the space, the air may escape the space through the grate. 
         [0024]    The first and second blowers may be centrifugal, axial, or alternatively cross flow fans which may be driven by an electric or alternatively by an internal combustion motor. The fans are preferably provided outside the burner where the first blower is preferably connected to the space below the grate by a first duct, and the second blower is preferably connected to the injectors by a second duct. 
         [0025]    Blowers may be used in tandem where the output of one blower is not enough for delivering the air volume required by the combustion process, the air volume required being dependent on the amount of fuel combusting on the grate. 
         [0026]    In the context of the present invention, the term “parallel” is to be understood as extending or pointing, or being oriented in the same direction. In the context of the present invention, the term “unidirectionally” is to be understood as in one direction only. In the context of the present invention, the term “above” is to be understood as a position above a horizontal plane. In the context of the present invention, the term “over” is to be understood as a position perpendicularly above a bounded surface of a horizontal plane. 
         [0027]    Another embodiment of the present invention pertains to a burner as described above, said burner further comprising a stoker having a fuel outlet within said burner, and an exhaust, wherein said grate is positioned below said fuel outlet and between said fuel outlet and said exhaust, and wherein said injectors are provided around said fuel outlet above said grate and directed towards said exhaust. By positioning the grate below the stoker, the solid fuel is easily deposited on the grate. By positioning the injectors around the fuel outlet, the injectors do not intrude on the space needed for the grate and the combustion process. Moreover, the injectors may help cool the fuel outlet. By providing the burner with an exhaust, and directing the injectors towards the exhaust, the secondary air, injected by the injectors, draws flue gas from the grate and the combusting solid fuel towards the exhaust. 
         [0028]    Preferably said injectors are placed along the periphery of the burner, whereby the flows of secondary air cool the periphery of the burner and prevent ash and other combustion residual products from settling or adhering to the walls. 
         [0029]    In a preferred embodiment the stoker comprises a screw conveyor disposed within a tube, said fuel outlet being defined by one end of said tube, whereby the feeding of particulate as well as granulate, fibrous, pelletized and slurried, i.e. solids in a liquid, fuel is possible. 
         [0030]    Preferably, said exhaust is connected to a plurality of horizontal flue gas pipes extending through a boiler, whereby the heat from the solid fuel is harnessed by a boiler, where it can be used to heat a fluid for example for residential heating or for providing steam to an industrial process. The flue pipes and the boiler are preferably made from metal such as steel. 
         [0031]    According to an embodiment the above mentioned injectors each comprise a tube having a first inner diameter at a first end and a second inner diameter at a second, said second inner diameter being less than said first inner diameter, and a stepped transition from said first inner diameter to said second inner diameter being provided in said tube between said first end and said second end, whereby a simple, cheap and reliable injector is provided, which is easy to clean. The tubes are preferably assembled by welding. The injector could however also be fabricated from a single piece by milling. 
         [0032]    Alternatively, the above mentioned injectors may each comprise a plurality of coaxial tubes provided on within the other each pair of adjacent tubes defining a space between said tubes, thus, yielding an alternative injector causing less turbulence in the burner. 
         [0033]    The above mentioned and further objects are also achieved by a method of combusting solid fuel burning on a grate in a burner comprising the steps of: supplying primary air through said grate from below said grate, and supplying secondary air as a plurality of unidirectional parallel flows above said grate, and along the periphery of said burner, towards an exhaust provided in said burner, wherein said secondary air is injected into the burner at a speed of 40-80 m/s, thereby ensuring a parallel flow of air over said grate and hence that flue gas from said combustion of said fuel is drawn off by said parallel flows of secondary air. 
         [0034]    In a second aspect of the present invention a method of combusting solid fuel is I burning on a grate in a burner is provided, the method comprising the steps of: supplying primary air through said grate from below said grate, and supplying secondary air above said grate, wherein said secondary air is supplied as a plurality of unidirectional parallel flows over said grate, whereby flue gas from said combustion of said fuel is drawn off by said parallel flows of secondary air. 
         [0035]    According to an embodiment of any of the above mentioned methods, a step of heating a fluid by heat exchange with said flue gas may be included in said methods, whereby the heat from the flue gas is transferred to a fluid which can be used for residential heating or to provide steam, while the flue gas is cooled for being safely emitted to the atmosphere. 
         [0036]    According to an embodiment of any of the above mentioned methods, a step of feeding solid fuel to said grate and removing residual combustion products from said burner may be included in any of said methods, whereby a continuous method of combusting solid material with a high combustion efficiency is provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    The invention and its many advantages will be described in more detail below with reference to the accompanying schematic drawings, which for the purpose of illustration show some non-limiting embodiments, and in which 
           [0038]      FIG. 1A  shows a burner according to the first aspect of the present invention, the burner being connected to a boiler, 
           [0039]      FIG. 1B  shows an alternative embodiment of a burner according to the first aspect of the present invention, the burner being connected to a boiler, 
           [0040]      FIG. 2A  shows a view of the injector arrangement in the burner according to the first aspect of the present invention, 
           [0041]      FIG. 2B  shows a view of the injector arrangement in an alternative embodiment of the burner according to the first aspect of the present invention, 
           [0042]      FIG. 3  shows a an injector for use in the burner according to the first aspect of the present invention as shown in  FIGS. 1-2 , and, 
           [0043]      FIG. 4  shows an alternative embodiment of an injector for use in the burner according to the first aspect of the present invention as shown in  FIGS. 1-2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0044]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may however be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. Like elements will thus not be described in detail with respect to the description of each figure. 
         [0045]      FIG. 1A  shows a burner, in its whole designated the reference numeral  2 , according to the first aspect of the present invention. The burner  2  comprises a gas tight cylindrical enclosure, designated the reference numeral  4 , made of welded steel and having a stepped grate, in its whole designated the reference numeral  6 , having a feed plate, designated the reference numeral  7 , and having four descending grates, designated the reference numerals  8 ,  10 ,  12  and  14 , at one end, and an exhaust, designated the reference numeral  16 , at the other end. Fuel is fed to the burner, as indicated by the arrow designated the reference numeral  18 , by a screw conveyor comprising a screw, designated the reference numeral  20 , provided within a tube, designated the reference numeral  22 , which extends through the enclosure  4  and has a fuel outlet, designated the reference numeral  24 , above the grate  8 , onto which solid fuel is deposited after being conveyed by the screw  20  through the tube  22 . By a horizontal reciprocating movement of the grates  8  and  12 , the solid fuel is eventually transported to the bottom, designated the reference numeral  26 , of the enclosure  4 , while being combusted and transformed into ash and combustion residual products, from which the ashes and the residual products are removed by a second screw conveyor (not shown) perpendicular to the screw conveyor defined by screw  20  and tube  22 . 
         [0046]    Primary air is supplied by a first electric blower, designated the reference numeral  28 , which supplies primary air to a space, designated the reference numeral  30 , below the stepped grate  6 , through a primary air duct, designated the reference numeral  32 , as indicated by the arrow designated the reference numeral  34 , from which the primary air is led through holes, one of which is designated the reference numeral  36 , in the grates  8 ,  10 ,  12  and  14 , for supplying the primary air to solid fuel (not shown) burning on the stepped grate  6 , as shown by arrows, one of which is designated the reference numeral  38 . 
         [0047]    A second electric blower, designated the reference numeral  40 , supplies secondary air at a rate of 1200 m 3 /h through a secondary air duct, designated the reference numeral  42 , as indicated by the arrow designated the reference numeral  44 , to an air rail, designated the reference numeral  46 , from which the secondary air is led to injectors, one of which is designated the reference numeral  48 , wherefrom the secondary air is injected as parallel unidirectional air flows, one of which is designated the reference numeral  50 , over the stepped grate  6  and directed towards the exhaust  16 . 
         [0048]    The high speed, typically 55-65 m/s, of the air flows  50  gives rise to an ejector effect, whereby flue gases, as indicated by the curved arrows, one of which is designated the reference numeral  52 , emanating from above the stepped grate  6  from the solid fuel combusting on the stepped grate  6  are drawn away from the stepped grate  6  and entrained by the air flows  50  and moved towards the exhaust  16 . 
         [0049]    After passing the grate  6 , the mixture of flue gasses  52  and air flows  50  becomes turbulent and forms whirls, as is shown by the arrow designated the reference numeral  53 , before being sucked up towards the exhaust  16 . 
         [0050]      FIG. 1  also shows a boiler, in its whole designated the reference numeral  60 , provided above the burner  2 , said boiler comprising a first set of substantially horizontal flue gas pipes, one of which is designated the reference numeral  62 , which are connected to the exhaust  16  of the burner  2 , whereby the mixture of flue gases  52  and the primary and secondary air  38  and  50  is led into the first set of flue gas pipes  62 , as indicated by the arrow designated the reference numeral  54 . 
         [0051]    The boiler includes a boiler tank, designated the reference numeral  64 , through which the first set of flue gas pipes  62  extends. A coupling space, designated the reference numeral  66 , is provided where the mixture of flue gas and primary and secondary air exits the first set of flue gas pipes  62 . A second set of substantially parallel flue gas pipes, one of which is designated the reference numeral  68 , is connected to the coupling space  66 , and the mixture of flue gas and primary and secondary air is routed into the second set of flue gas pipes  68  as shown by the arrow designated the reference numeral  70 . The second set of flue gas pipes  68  extends once more through the boiler tank  64  before connecting to a chimney, designated the reference numeral  72 , allowing the mixture of flue gas and primary and secondary air to be released into the atmosphere as indicated by the arrow designated the reference numeral  74 . 
         [0052]    Water to be heated in the boiler  60  is admitted into the boiler tank  64  at an inlet, designated the reference numeral  76 , as shown by the arrow designated the reference numeral  78 , while heated water is withdrawn from the boiler tank at an outlet, designated the reference numeral  80 , as shown by the arrow designated the reference numeral  82 . 
         [0053]    The burner  2  and the boiler  60  are insulated from the surroundings by 100 mm mineral wool (not shown). 
         [0054]      FIG. 1B  shows an alternative embodiment of the burner according to the first aspect of the present invention, wherein features identical with those of  FIG. 1A  are referenced by the same reference numerals, and wherein features having the same purpose or function but differing in construction are referenced by the same reference numeral, with a superscript roman numeral, as those of  FIG. 1A . 
         [0055]      FIG. 2A  shows a view of the injector arrangement in the burner  2  according to the first aspect of the present invention. The air rail  46  is shaped as an inverted U and surrounds the fuel outlet  24  which is placed above the stepped grate  6 . The injectors  48  are provided on the air rail  46  along the periphery of the enclosure  4 . The second electric blower  40  supplies secondary air to the air rail  46  through the secondary air duct  42 , while the first electric blower  28  supplies primary air through the primary air duct  32  to the space  30  (not shown in  FIG. 2 ) below the stepped grate  6 . 
         [0056]      FIG. 2B  shows a view of the injector arrangement in alternative embodiment of the burner  2 ′ according to the first aspect of the present invention, wherein features identical with those of  FIG. 2A  are referenced by the same reference numerals, and wherein features having the same purpose or function but differing in construction are referenced by the same reference numeral, with a superscript roman numeral, as those of  FIG. 2A . 
         [0057]      FIGS. 3A and 3B , where  3 A is a perspective view, and  3 B is a section view, show a first embodiment of the injector  48  for use in the burner  2  according to the first aspect of the present invention as shown in  FIGS. 1-2 . The injector  48  comprises three metal tubes, designated the reference numerals  90 ,  92 , and  94 , respectively, of different diameters, which are welded together end-to-end to form the injector  48 , having a first end, designated the reference numeral  96 , having a large inner diameter for being mounted to the air rail  46  for receiving secondary air from the air rail  46 , and a second end, designated the reference numeral  98 , having a small inner diameter for injecting a flow of secondary air  50  into the burner  2 . The transition from the large inner diameter of the tube  90  at the first end  96  to the smaller inner diameter of the tube  94  at the second end is achieved by first and second restrictions, designated the reference numerals  100  and  102 , respectively, which are formed where the tubes  90  and  92  join, and where the tubes  92  and  94  join, respectively. Secondary air passing through the injector  48  accelerates due to the difference in inner diameter, i.e. cross sectional area, between the first end  96  and the second end  98 . The inner diameter of the second end  98  is 27 mm, which, depending on the number of injectors, typically 9-10, and the airflow established by the second electric blower  40 , typically 1200 m 3 /h, results in an initial, i.e. as the air leaves the second end  98  of the injector  48 , air speed of 55-65 m/s. 
         [0058]    In  FIG. 3B , the tubes  90  and  92  are shown as having the same wall thickness, while the tube  94  has a thinner wall thickness, which is the preferred embodiment since it simplifies the welding together of the tubes  92  and  94 . The tubes  90 ,  92  and  94  may however also have the same wall thickness. 
         [0059]      FIGS. 4A and 4B , where  4 A is a perspective view, and  4 B is a section view, show an alternative embodiment of an injector, designated the reference numeral  48   I , for use in the burner  2  according to the first aspect of the present invention as shown in  FIGS. 1-2 . The injector  48   I  comprises three metal tubes, designated the reference numerals  90   I ,  92   I , and  94   I , respectively, of different diameters, which are placed one-inside-theother, forming the injector  48   I  having a first end, designated the reference numeral  96   I , for receiving secondary air from the air rail  46 , and a second end, designated the reference numeral  98   I , for injecting a flow of secondary air  50  into the burner  2 . Secondary air is led from the first end  96   I  to the second end  98   I  in the annular spaces, designated the reference numerals  104   I  and  106   I , defined by the tubes  90   I  and  92   I  and the tubes  92   I  and  94   I , respectively, and the circular space, designated the reference numeral  108   I , defined by the tube  94   I , 
         [0060]    The tubes  90   I ,  92   I  and  94   I  are preferably fastened together at the first end  96   I  by radially and angularly spaced struts (not shown) between the respective tubes  90   I  and  92   I , and  92   I  and  94   I . 
         [0061]    By leading the secondary air through the spaces  104   I ,  106   I  and  108   I , the turbulence in the burner may be lessened. Further, by varying the sizes of the spaces  104   I ,  106   I  and  108   I , the injector  48   I  allows different flow profiles for the flows of secondary air. 
       Example 
       [0062]    Tests with a burner and boiler as described in  FIGS. 1-2  with injectors as described in  FIG. 3  have shown that more than 93% of the theoretical energy content of the solid fuel were converted to heat. 
         [0000]    
       
         
               
             
               
               
             
           
               
                   
               
               
                 List of parts with reference to the figures: 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 2. 
                 Burner 
               
               
                 2 | . 
                 Burner (alternative embodiment) 
               
               
                 4. 
                 Enclosure 
               
               
                 6. 
                 Stepped grate 
               
               
                 6 | . 
                 Stepped grate (alternative embodiment) 
               
               
                 7. 
                 Feed plate 
               
               
                 8. 
                 Grate 
               
               
                 8 | . 
                 Grate (alternative embodiment) 
               
               
                 10. 
                 Grate 
               
               
                 10 | . 
                 Grate (alternative embodiment) 
               
               
                 12. 
                 Grate 
               
               
                 12 | . 
                 Grate (alternative embodiment) 
               
               
                 14. 
                 Grate 
               
               
                 14 | . 
                 Grate (alternative embodiment) 
               
               
                 16. 
                 Exhaust 
               
               
                 18. 
                 Arrow indicating feeding of fuel 
               
               
                 20. 
                 Screw 
               
               
                 22. 
                 Tube 
               
               
                 24. 
                 Fuel outlet 
               
               
                 26. 
                 Bottom 
               
               
                 28. 
                 First electric blower 
               
               
                 30. 
                 Space 
               
               
                 32. 
                 Primary air duct 
               
               
                 34. 
                 Arrow indicating primary air 
               
               
                 36. 
                 Hole 
               
               
                 38. 
                 Arrow indicating primary air 
               
               
                 40. 
                 Second electric blower 
               
               
                 42. 
                 Secondary air duct 
               
               
                 44. 
                 Arrow indicating secondary air 
               
               
                 46. 
                 Air rail 
               
               
                 48. 
                 Injector 
               
               
                 48 | . 
                 Injector (second embodiment 
               
               
                 50. 
                 Air flow 
               
               
                 52. 
                 Arrow indicating flue gas 
               
               
                 53. 
                 Arrow indicating turbulence 
               
               
                 54. 
                 Arrow indicating mixture of flue gas and primary and secondary air 
               
               
                 60. 
                 Boiler 
               
               
                 62. 
                 First set of flue gas pipes 
               
               
                 64. 
                 Boiler tank 
               
               
                 66. 
                 Coupling space 
               
               
                 68. 
                 Second set of flue gas pipes 
               
               
                 70. 
                 Arrow indicating mixture of flue gas and primary and secondary air 
               
               
                 72. 
                 Chimney 
               
               
                 74. 
                 Arrow indicating mixture of flue gas and primary and secondary air 
               
               
                 76. 
                 Inlet 
               
               
                 78. 
                 Arrow indicating water flow in 
               
               
                 80. 
                 Outlet 
               
               
                 82. 
                 Arrow indicating water flow out 
               
               
                 90. 
                 Tube 
               
               
                 90 | . 
                 Tube (alternative embodiment) 
               
               
                 92. 
                 Tube 
               
               
                 92 | . 
                 Tube (alternative embodiment) 
               
               
                 94. 
                 Tube 
               
               
                 94 | . 
                 Tube (alternative embodiment) 
               
               
                 96. 
                 First end 
               
               
                 96 | . 
                 First end (alternative embodiment) 
               
               
                 98. 
                 Second end 
               
               
                 98 | . 
                 Second end (alternative embodiment) 
               
               
                 100. 
                 First restriction 
               
               
                 102. 
                 Second restriction 
               
               
                 104 | . 
                 Annular space 
               
               
                 106 | . 
                 Annular space 
               
               
                 108 | . 
                 Circular space