Abstract:
In one aspect, a grate segment for a burner, comprises a substantially rigid body ( 30 ) having a dispersed array of cavities ( 38 ) filled with a refractory material. In another aspect, a solid fuel gasifier includes a housing ( 12 ) defining a gasifier chamber ( 14 ), and a solid fuel inlet ( 21 ), grate structure ( 22, 24 ) in the gasifier chamber. The grate structure includes base means, a plurality of agitator segments ( 32 ) positioned relative to the base means for reciprocating movement to agitate a fuel load in the gasifier, and means to support the base means and the agitator segments so that the latter reciprocate in a direction at a substantial inclination to both horizontal and vertical. In use of the gasifier a deep burden of fuel accumulates on the grate structure and the outward stroke of the reciprocation of the agitator elements is in the overall direction of flow of the fuel burden or generally oppositely thereto.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to solid fuel burners and has particular, though not exclusive, application to a solid fuel burner of the type commonly referred to as a gasifier or gasifier combustor.  
         BACKGROUND ART  
         [0002]    A gasifier generally includes a primary combustion chamber into which solid fuel is loaded on to a grate structure on which it is first dried and gasified via controlled primary combustion. The resultant gas is then transferred into a secondary combustion chamber, which may conveniently be a cycloburner, for secondary combustion to produce a high temperature relatively clean flue gas able to be used for a variety of purposes, eg. power generation or heating. There is a small residue of inorganic matter.  
           [0003]    A gasifier of the general type to which the present invention relates is disclosed, for example in U.S. Pat. No. 4,716,842, and the technology generally is of particular interest in waste recycling, especially with an emphasis on so called “green power” generation. Specific solid fuels which may conveniently be gasified in this way include biological waste, agricultural byproducts, wood waste and biomass.  
           [0004]    Problems experienced with gasifiers, as with many other high temperature furnace processes, include sintering or aggregation of solid fines, and the difficulty of achieving thorough mixing of fuel and air/gas streams. It is thought by the inventor that these problems can be minimised over time in the initial gasification process by appropriate design of the grate structure and it is towards this preferred objective that the present invention is especially directed.  
           [0005]    U.S. Pat. No. 5,680,824 discloses a grate structure in which moveable hollow grate plates with coolant passages are reciprocable between staitonary grate plates.  
           [0006]    The invention seeks to provide an improved grate construction for furnaces generally, but especially for solid fuel gasifiers.  
         SUMMARY OF THE INVENTION  
         [0007]    The invention accordingly provides, in a first aspect, a grate segment for a burner, comprising a substantially rigid body having a dispersed array of cavities filled with a refractory material.  
           [0008]    Preferably, the grate segment is one of two or more (more preferably multiple) complementary, generally planar grate segments for forming a grate assembly, in which said cavities are open in a heat-exposed surface of the assembly.  
           [0009]    The refractory material is preferably a ceramic material.  
           [0010]    Advantageously, the dispersed array of cavities is such that the grate segment exhibits a generally honeycomb structure.  
           [0011]    In one application, the solid fuel burner is a gasifier.  
           [0012]    In a second aspect, the invention is directed to a solid fuel gasifier, including:  
           [0013]    a housing defining a gasifier chamber;  
           [0014]    grate structure in said gasifier chamber, including:  
           [0015]    base means;  
           [0016]    a plurality of agitator segments positioned relative to the base means for reciprocating movement to agitate a fuel load in the gasifier; and  
           [0017]    means to support the base means and the agitator segments so that the latter reciprocate in a direction at a substantial inclination to both horizontal and vertical; and  
           [0018]    a solid fuel inlet;  
           [0019]    whereby in use of the gasifier a deep burden of fuel accumulates on the grate structure and the outward stroke of said reciprocation is in the overall direction of flow of the fuel burden or generally oppositely thereto.  
           [0020]    The second aspect of the invention stems from an appreciation by the present inventor that an optimum outcome is achieved by adopting a mobile grate structure which is a compromise between mere horizontal conveyance and the maximum agitation achieved by vertical disturbance of the fuel load, eg. in a fluidised bed burner. By the arrangement of the invention, there is achieved a high degree of agitation of the fuel load simultaneously with a controlled but relatively slow conveyance of the load.  
           [0021]    Preferably, a grate structure according to the second aspect of the invention comprises respective substantially fixed grate sections forming the base means, and moveable grate sections forming the agitator elements. These moveable sections reciprocally slide over the fixed sections. There may be multiple pairs of said fixed grate sections and said moveable grate sections.  
           [0022]    Preferably, means is provided to admit air to the fuel load via passages and/or port means in the fixed grate sections. The air admission means preferably opens into the fuel burden in the end surfaces of the fixed grate sections.  
           [0023]    Preferably, the gasifier has a solid fuel inlet positioned generally behind the grate structure(s) relative to a general direction of projection of the grate structure and overall direction of flow of the fuel, whereby in use of the gasifier a deep burden of fuel accumulates on the grate structure and the outward stroke of said reciprocation is in the overall direction of flow of the fuel burden. Preferably this sustained reciprocating movement is effective to agitate the fuel and enhance its combustion, and to steadily and slowly stroke or convey it along the chamber.  
           [0024]    Preferably, the grate structure is arranged as a plurality of banks of agitator elements, including a first bank with respect to the fuel delivery means that serves as a preheater and drying grate, and a second and optional subsequent banks that provide for gasification of the fuel volatile matter and fixed carbon oxidation.  
           [0025]    Preferably, the preheater grate is arranged so that the agitator segments reciprocate in a direction more steeply inclined to the horizontal than that for the gasification grate(s) and the optional subsequent banks are less steeply inclined than the second grate for gasification.  
           [0026]    In a preferred embodiment of the second aspect of the invention, each grate section is formed from grate segments in accordance with the first aspect of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:  
         [0028]    [0028]FIG. 1 is a diagrammatic vertical longitudinal section of a solid state gasifier incorporating an embodiment of both aspects of the invention;  
         [0029]    [0029]FIG. 2 is an enlargement, with additional detail, of the grate structure forming part of the gasifier illustrated in FIG. 1;  
         [0030]    [0030]FIG. 3 is a further enlargement of part of FIG. 2;  
         [0031]    [0031]FIG. 4 is a plan view of one of the fixed grate segments;  
         [0032]    [0032]FIGS. 5, 6 and  7  are respectively an end elevation, a side elevation and a cross section on the line  7  to  7  of FIG. 4;  
         [0033]    FIGS.  8  to  10  are views corresponding to FIGS.  5  to  7  but for a reciprocating grate segment; and  
         [0034]    [0034]FIG. 11 is a fragmentary isometric view of a modified form of the grate structure. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0035]    The solid fuel gasifier  10  of FIG. 1 (which includes a figure of a man  11  to provide a dimensional context), includes an outer housing  12  about a gasification chamber  14  extending from a fuel delivery auger  9  towards a separately walled cycloburner  16  that defines a secondary combustion chamber  17 . A grate structure  20  includes a preheating grate  22  adjacent the delivery end of auger  9 , and, downstream in the overall direction of flow of the solid fuel, a gasifier grate  24 . The two grates  22 ,  24  each include stepped pairs of fixed  30  and reciprocating  32  grate segments and are linked by a near vertical grate  40  with multiple angled and controllable openings  42  for admission of combustion air from below the grate structure into the fuel load above. It will be seen that the solid fuel inlet  21  into chamber  14  from auger  9  is generally behind the grate structure  20  relative to the general direction of projection of the grate structure and the overall direction of flow of the solid fuel.  
         [0036]    In general, solid fuel delivered via auger  9  accumulates as a deep load or burden  70  on the grate structure while being dried and preheated on grate  22  and gasified above grate  24 . Combustible gas (syngas) is drawn through a transverse slot port  19  into chamber  17 , from one end  15  of which is recovered combusted flue gas or syngas useable for subsequent heating or power generating purposes. Inorganic solid residue or ash that falls over the downstream end of grate  24  is directed by a baffle device  50  into an ash grate  52  arranged on the heated floor  13  of the housing. The ash is gradually agitated along the floor  13 , and moved while remaining carbon is oxidised, for transverse removal and recovery by conveyor  54 .  
         [0037]    An overhead water-cooled refractory lining  56  is suspended from the roof  11  of housing  10  and merges into the wall structure of cycloburner  16 . Lining  56  also defines one edge of slot port  19  through which combustible gases pass from the primary chamber  14  to the secondary chamber  17 . Roof  11  supports an emergency exhaust stack  58 .  
         [0038]    Grates  22 ,  24  are of generally similar construction and essentially differ only in their exact inclination, as will be discussed in due course. Each fixed grate segment  30  is of generally flat plate configuration, is, for example, about 0.3 to 1 metre front to rear, and is formed from a base plate  33  (FIGS.  4 - 5 ) and a top plate  34 . Plates  33 ,  34  may be clamped together by multiple screws  35  applied from below and from the front. The width of the plate is set to match that of chamber  14 . The two plates  34 ,  35  which are cast in metal alloy or a suitable ceramic, preferably a heat resistant/high temperature alloy, so as to form a substantially rigid body, have complementary channels  36   a ,  36   b  that define an open-ended passage  36  within the grate segment that is a square U when viewed in plan. During operation of the gasifier, combustion air is supplied to passage  36  and from there is ejected through the front face of the grate segment via plural nozzles  37  (best seen in FIG. 11). The air delivered via nozzles  37  may be or include recycled gases from the cycloburner or elsewhere.  
         [0039]    Top plate  34  is also punctuated by a dispersed regular array of hexagonal cavities  38  which open in the top surface of the plate, are generally shaped and arranged honeycomb fashion, and are filled with an appropriate high temperature refractory material, eg a suitable ceramic, in the finished grate segment. Cavities  38  are preferably on a 10-100 mm square array, with 5-20 mm alloy wall thickness.  
         [0040]    Air passages  36  communicate with transverse air manifold ducts  39  (FIG. 2) at the rear of each fixed grate segment and these ducts  39  are linked, as diagrammatically depicted in FIG. 2, to an air supply conduit  39   a.    
         [0041]    Each reciprocating grate segment  32  is also of two part construction, in this case comprising a front section  53  of generally right angular form and a main body  54  also having refractory (eg. ceramic) filled hexagonal/honeycomb cavities  58  (FIGS. 3 and 5).  
         [0042]    Water passages  80  may be located about the periphery of the lower surface of each grate segment in order to maintain the segment at a temperature whereby overheating does not occur. However, the particular preferred design of grate segment employed is able to operate at higher than conventional temperatures.  
         [0043]    It will be seen from FIGS. 1 and 2 that, in each grate set  22 ,  24 , the fixed grate segments  30  are staggered in stepped fashion, being aligned at a uniform angle to the horizontal so as to slightly overlap, and that the reciprocating segments  32  are slidable between and atop the fixed segments  30 . The angle of inclination can vary between horizontal and 60° to horizontal, upward or downward, but preferably at least 10°, and is steeper (about 40° versus about 30°) in preheater grate  22  relative to gasifier grate  24 . This is because the typically wet fuel on grate  22  has a greater angle of repose and larger particle size than the fuel on grate  24 .  
         [0044]    In a larger installation, a further grate may be provided downstream of grate  24 , optionally having a still smaller inclination to the horizontal. This third grate zone may enhance post-reduction of the residual ash component from grate  24 . Typically, this third grate would be separated from grate  24  by a second step grate—preferably higher than step grate  40 .  
         [0045]    Grate segments  32  are reciprocated jointly over a stroke extending approximately between the front edges of the adjacent fixed segments: the fully retracted position is depicted by heavy lines in FIG. 1 and the fully extended position by light lines. The outward or forward stroke of the reciprocation is in the overall direction of flow of the fuel burden. The stroke is typically in the range 100-500 mm, and the stroke period is preferably 10 secs or more eg. in the range 15 secs to 10 mins. A suitable arrangement for effecting reciprocation is shown in the drawings. Each set or bank of grate segments has a pair of drive shafts  62  with respective tooth sectors  63 . These engage with racks  64  carried by a respective drive arm  66  which also carries the cooling water. A cross-link  68  is coupled to respective drive arms  66   a  for the other segments so that the grate segments reciprocate in unison.  
         [0046]    The drive gear sectors are arranged to be driven in a first direction which causes the drive arms to move longitudinally relative to the longitudinal axis of the arms so as to extend the moveable grate segments relative to the fixed grate segments. Reversed movement of the drive sectors causes a retraction of the moveable grate segments  32 . Thus, reciprocating movement of the drive sectors causes the moveable grate segments to move back and forth relative to the fixed grate segments  30 .  
         [0047]    In operation, solid fuel is delivered by auger  9  to the combustion chamber  14 . A bed of the fuel forms atop grates  22 ,  24  in the approximate volume indicated by the dot-dash line  70  in FIG. 1. The body of solid material is predried on grate  22 , while gasification or pyrolysis takes place above adjacent grate  24 . Air for combustion (which may be wholly or partly recycled gases from the cycloburner and/or the primary combustion chamber) is delivered into the fuel body both through openings  42  in step grates  40  and via air nozzles  37  in the front faces of fixed grate segments  30 . Sustained reciprocating movement of moveable grate segments  32  is effective to agitate the fuel and enhance its combustion, and to steadily and slowly stroke or convey it along the chamber. There is also a degree of reverse rolling agitation of the material in the fuel burden, further enhancing the gasification reactions. The fines in particular are mixed and agitated but without significant expulsion of airborne particles. The result is an optimum fuel load profile. The height of the fuel pile in the drying zone at grate  22  is regulated by the speed of the grate relative to the fuel feed auger speed, and the height of grate  24 .  
         [0048]    A typical profile of fuel in the various zones can be ascertained from the representation  70  in FIG. 1. As can be seen, the greatest thickness of the fuel pile in any zone is located over the movable grates  32  most adjacent the upstream step grate  40 . It is believed that this fuel pile profile provides good gasification zones. In furnaces, fuel profiles and depths would typically be quite different from that illustrated.  
         [0049]    The openings  42  in step grate  40  may preferably increase in size towards the lower edge of the step grate, or may only extend over a portion, eg. a lower portion, of the step grate. There may be one or more air boxes behind the step grate that preferably taper to a cross-section at the upper side matching the adjacent fixed grate segment  30 . The air box may then supply controlled air through this fixed grate segment.  
         [0050]    [0050]FIG. 11 illustrates a modified form of the grate structure, with like pairs indicated by line reference numerals.  
         [0051]    It is thought that the design of the grates in the preferred construction described above is effective in reducing maintenance relative to that conventionally required with the high temperatures and reducing atmosphere. The ceramic filled honeycomb cavity configuration enables a significantly higher temperature of the fuel mass on the grate surfaces, relative to that achievable with conventional steel surfaced grates, and thereby allows more efficient combustion and gasification.