AU2005225032B2 - Grate structure for solid fuel burners - Google Patents

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Grate structure for solid fuel burners The following statement is a full description of this invention, including the best method of performing it known to us: SGRATE STRUCTURE FOR SOLID FUEL BURNERS

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0 oO Field of the Invention SThis invention relates generally to solid fuel burners and has particular, O though not exclusive, application to a solid fuel burner of the type commonly referred to as a gasifier or gasifier combustor.

SBackground Art 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.

A gasifier of the general type to which the present invention relates is disclosed, for example in US patent 4716842, 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.

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.

Untied States patent 5,680,824 discloses a grate structure in which moveable hollow grate plates with coolant passages are reciprocable between

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2 staitonary grate plates.

The invention seeks to provide an improved grate construction for furnaces generally, but especially for solid fuel gasifiers.

Summary of the Invention 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.

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.

The refractory material is preferably a ceramic material.

Advantageously, the dispersed array of cavities is such that the grate segment exhibits a generally honeycomb structure.

In one application, the solid fuel burner is a gasifier.

Brief Description of the Drawings The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic vertical longitudinal section of a solid state gasifier incorporating an embodiment of both aspects of the invention; Figure 2 is an enlargement, with additional detail, of the grate structure forming part of the gasifier illustrated in Figure 1; 3 0 0 Figure 3 is a further enlargement of part of Figure 2; oo00 Figure 4 is a plan view of one of the fixed grate segments; SFigures 5, 6 and 7 are respectively an end elevation, a side elevation and a 0 cross section on the line 7 to 7 of Figure 4; Iu 5 the next pages is page c-I 0 Figures 8 to 10 are views corresponding to Figures 5 to 7 but for a oo reciprocating grate segment; and Figure 11 is a fragmentary isometric view of a modified form of the grate Sstructure.

cI 5 Description of Preferred Embodiments C The solid fuel gasifier 10 of Figure 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.

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.

6 o An overhead water-cooled refractory lining 56 is suspended from the roof oO 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.

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 (Figures 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 36a, 36b 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 Figure 11). The air delivered via nozzles 37 may be or include recycled gases from the cycloburner or elsewhere.

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 10-100mm square array, with 5-20mm alloy wall thickness.

Air passages 36 communicate with transverse air manifold ducts 39 (Figure 2) at the rear of each fixed grate segment and these ducts 39 are linked, as diagrammatically depicted in Figure 2, to an air supply conduit 39a.

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 S7 0 54 also having refractory (eg. ceramic) filled hexagonal/honeycomb cavities 58 0 (Figure 3 and 00 Water passages 80 may be located about the periphery of the lower Ssurface of each grate segment in order to maintain the segment at a temperature 5 whereby overheating does not occur. However, the particular preferred design of Sgrate segment employed is able to operate at higher than conventional 8temperatures.

It will be seen from Figures 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 100, and is steeper (about 400 versus about 300) 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.

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 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 Figure 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- 500mm, and the stroke period is preferably 10 secs or more eg. in the range 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 S8

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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 o respective drive arms 66a for the other segments so that the grate segments reciprocate in unison.

The drive gear sectors are arranged to be driven in a first direction which cI causes the drive arms to move longitudinally relative to the longitudinal axis of the Sarms 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 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 Figure 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.

A typical profile of fuel in the various zones can be ascertained from the representation 70 in Figure 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 09 Ustep grate 40. It is believed that this fuel pile profile provides good gasification 0 zones. In furnaces, fuel profiles and depths would typically be quite different from 00 that illustrated.

CThe 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 Sgrate that preferably taper to a cross-section at the upper side matching the Sadjacent fixed grate segment 30. The air box may then supply controlled air through this fixed grate segment.

Figure 11 illustrates a modified form of the grate structure, with like pairs indicated by line reference numerals.

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.