GB2477562A - Grate mechanism - Google Patents

Abstract

A grate mechanism 20, suitable for use in a gasifier, comprises a multiplicity of grate bars 10, 18 arranged side-by-side in a common plane, alternate gate bars 10 being movable lengthwise so as to move relative to adjacent gate bars 18, and wherein each grate bar defines a multiplicity of recesses 14 along both sides of the bar, recesses on one side of the bar being arranged alternately with recesses on the opposite side of the bar along the length of the bar. Preferably, the alternate grate bars are connected to a linking mechanism whereby the alternate grate bars can be moved simultaneously. The linking mechanism can ensure that lengthwise movement of alternate grate bars does not subject the grate bars to tension. The upper surfaces of all the grate bars may be provided with upwardly projecting teeth ( 12, Fig. 1). A processing plant incorporating the gate mechanism is also claimed.

Description

Grate Mechanism The present invention relates to a grate mechanism, particularly but not exclusively a grate mechanism for use in a gasification plant; to a processing plant incorporating a grate mechanism; and to an element of the grate mechanism.

Biomass can be converted to a gas stream by the process of gasification, which may also be referred to as pyrolysis. The biomass might for example be straw, bagasse, or other agricultural wastes, waste paper, or wood, and the process may be applied to biomass grown specifically for the purpose, or to waste materials. In particular waste wood is a suitable material for this process. The gasification process involves heating the biomass to an elevated temperature, possibly in the presence of a restricted quantity of cxygen, to break down organic materials and to generate carbon monoxide and hydrogen. Other products of the process are charcoal, particulate carbon, and tar. As described in GB 2 I55 $69 A (0-Gen [5K Ltd), a fluidic vortex gas/liquid contactor may be used to remove tar, after filtering the hot gases produced in the gasification process. The gasification process is analogous to a combustion process, although with insufficient oxygen, and may be carried out in a down-flow gasification reactor. The material being subjected to gasification is supported on a grate, and a down-flow of gases through the gasifier is created by an external draft fan; this has the benefit that much of the tar created during gasification is raised to a sufficiently high-temperature within the gasifier that it is cracked into significantly smaller molecules.

In such a gasifier, and indeed in other gasifiers and other combustion processes, it is desirable if the material undergoing gasification or ccrabustion is supported on a grate and that the grate provides a substantially constant restriction to gas flow. It is commonly the case that solid material that has undergone the gasification or combustion process is arranged to fall through the grate, and is then processed appropriately. There the solid material is charcoal or ash, this may be achieved with a static grate, or alternatively by occasionally shaking the grate. However problems can arise particularly where waste material is being processed, as the solid material may include inert material such as bricks or stones, or metal objects, that were present in the waste material and were not separated from the wood during initial processing. Such materials may be too large to fall through the gaps in the grate, and can have the effect of blocking the grate. This prevents continuous operation of the gasifier.

ccording to the present invention there is provided a grate mechanism comprising a multiplicity of grate bars arranged side-by-side in substantially a common plane, alternate grate bars being movable lengthwise so as to move relative to adjacent grate bars, and wherein each grate bar defines a multiplicity of recesses along both sides of the bar, recesses on one side of the bar being arranged alternately with recesses on the opposite side of the bar along the length of the bar.

Preferably alternate grate bars are connected to a linking mechanism whereby the alternate grate bars can be moved lengthwise simultaneously. The lengthwise movement of the bars by a distance equal to the length of a recess has the effect that a recess on one bar moves into a position where it is next to a recess on the adjacent bar, so effectively doubling the width of the gap between these adjacent portions of the bars.

In one embodiment all the qrate bars are of substantially identical shape, at least along their central portions, and may be made of cast iron, or of steel. The movable grate bars may be connected to a frame of steel or of stainless steel, and the bars may be linked to the frame by an arrangement such as a pin extending through a slot, so that the bars are always moved under compression.

Preferably the upper surfaces of all the grate bars are provided with upwardly-projecting teeth, for example with an asymmetrical sawtooth shape. As the bars are moved, such teeth ensure that the solid material resting on the bars is disturbed. And preferably the sides of the grate bars taper downwardly, at least at the positions of the recesses, so that the width of the recesses increases in the downward direction.

In a further modification each grate bar may be coated with a ceramic oxide, or with plasma-sprayed titanium, to improve wear and high-temperature resistance.

In addition, the present inventicn provides a grate bar for use in such a grate mechanism; and also provides a processing plant incorporating such a grate mechanism.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a side view of a grate bar of the invention; Figure 2 shows a plan view of the grate bar of figure 1; Figures 3a and 3b show sectional views of the grate bar, on the lines A-A and B-B of figure 2 respectively; Figure 4 shows a side view of a grate assembly that incorporates grate bars as shown in figure 1 (being a view on arrow 4 of figure 6); Figure 5 shows a perspective exploded view of the grate assembly of Figure 4; Figure 6 shows a plan view of the grate assembly of Figure 4, in the direction of arrow 6; and Figure 7 shows a plan view of the grate assembly of Figures 4 and 5, after moving the grate bars.

Referring to figures 1 and 2, a grate bar 10 is, in this example, 0.84 m long, of width 23 mm and of height mm, and is of cast iron. Along the upper surface are teeth 12, generally of a sawtooth shape, each of height 5 mm and total length 30 mm, and the steepest side of each tooth 12 being inclined at an angle of 115° to the longitudinal axis of the grate bar 10. There are six recesses 14 along the central section of the grate bar 10, three recesses 14 on one side alternating with three recesses 14 on the opposite side, and each recess being of overall length 100 mm, each recess 14 being of depth 11 mm at the top of the grate bar 10 and being of gradually increasing depth towards the bottom of the grate bar 10. Hence, as shown in figures 3a and 3b the grate bar 10 tapers downwardly, and the recess 14 becomes deeper, at an angle of 87° to the vertical.

Referring in particular to figure 2, the grate bar has a section at one end (the left-hand end as shown) of length about 90 mm where there are no recesses, and has a section at the other end of length about 150 mm where there are no recesses. The grate bar 10 described above is used as a sliding grate bar in a grate assembly (described below) . The grate assembly 20 also includes static grate bars 18 (shown in figure 5) that are identical to the sliding grate bars 10 except that the sections where there are no recesses are shorter: the section at one end is only 80 mm instead of 90 mm, and the section at the other end is only 40 mm instead of 150 mm; and that there are small projecting lugs 19 on the underside, at each end (shown in figure 5) Referring now to figures 4 to 6, a grate assembly 20 includes a rectangular frame 22 consisting of stainless steel side rails 23 linked at each end by support rails 24, and also linked at intermediate positions by five strengthening ribs 25, these components being welded together. The side rails 23 project 50 mm above the plane defined by the support rails 24. The overall length of this frame 22 is 720 mm. Resting on the frame 22 are nine slidable grate bars 10 spaced apart by 27 mm wide gaps, and within each gap is a fixed grate bar 1Sf such that there are 2 mm wide gaps between adjacent grate bars 10 and 18. The fixed grate bars 18 are held on the support rails 24, by the lugs 19 locating in corresponding recesses 26 in the support rails 24.

The slidable grate bars 10 are connected together at their ends by a front end plate 30 and a rear end plate 32, both of stainless steel, the slidable grate bars 10 locating at the ends in castellated plates 34 that are fixed onto the end plates 30 and 32. The castellated plates 34 ensure that the slidable grate bars 10 are held at the correct spacing, but there is a gap (e.g. 3 mm) between each end of the slidable grate bar 10 and the face of the end plate 30 or 32. The end plates 30 and 32 are linked together by two stainless steel slide rods 36 that extend through guide apertures in the support rails 24 and extend through larger apertures through the strengthening ribs 25.

A stainless steel boss 38 is mounted on the outer face of the front end plate 30. A pneumatic piston (not shown) may be connected to the steel boss 38 to move the front end plate 30 and consequently the slidable grate bars 10 axially. If the boss 38 and the front end plate are moved in the direction of arrow P, then the front end plate 30 pushes on the slidable grate bars 10; if the boss 38 and the front end plate 30 are moved in the direction of arrow Q, the force is transmitted through the slide rods 36 to pull on the rear end plate 32, and the rear end plate 32 pushes on the slidable grate bars 10. This arrangement ensures that the slidable grate bars 10 are not put under tension.

With the grate assembly 20 in the position shown in figure 6, the recesses 14 on the right-hand side of the slidable grate bars 10 (as shown) align with the recesses 14 on the left-hand side of the static grate bars 18.

Similarly the recesses 14 on the left-hand side of the slidable grate bars 10 align with the recesses 14 on the right-hand side of the static grate bars 18. The maximum width of the gaps between adjacent grate bars 10 and 18 is therefore about 24 mm i.e. twice the depth of a recess 14 plus the spacing between the adjacent grate bars 10 and 18.

Referring now to figure 7, the grate assembly 20 is shown after the steel boss 38 and so the slidable grate bars 10 have been moved forwards (in the direction of arrow P of figure 6) through a distance of 100 mm.

Consequently the recesses 14 in all the bars 10 and 18 are aligned with each other, so that each recess 14 on a grate bar 10 or 18 faces a non-recessed portion of the adjacent grate bar 10 or 18. The maximum width of the gaps between adjacent grate bars 10 and 18 is therefore about 13 mm i.e. the depth of a recess 14 plus the spacing between the adjacent grate bars 10 and 18.

In use, in one example, the grate assembly 20 is installed just above the base of a generally cylindrical gasifier, with chopped pieces of wood resting on the grate bars 10 and 18. After ignition, gasification occurs in a low oxygen environment with a downwards gas flow drawn by a fan through the grate assembly 20, converting the wood to charcoal and gas. In the rest position the grate assembly 20 is in the closed position shown in figure 7, with the gaps being at their least value, but at intervals the grate assembly 20 is moved into the open position shown in figures 4 and 6, and is then returned to the closed position. The teeth 12 on the upper surface of the grate bars 10 and 18 ensure that the material that rests on the grate bars 10 and 18 is disturbed as the slidable grate bars 10 are moved; and the changing width of the gaps enables charcoal pieces of different sizes to fall through to the space below the grate assembly 20 (from where it can be removed for subsequent treatment) . The gasifier can therefore operate substantially continuously. Even if the chopped pieces of wood contain pieces of metal, such as nails, these fall through the changing-width gaps and do not block the grate assembly 20. Although the two positions of the grate assembly 20 may be referred to as open' and closed', the effective available area for gas flow is unchanged; the difference is only in the width of the gaps through which solid material can fall.

Since, as shown in figure 6, when the grate assembly is in the open position the strengthening ribs 25 are aligned with the positions on all the grate bars that are between successive recesses 14, the strengthening ribs 25 do not significantly obstruct the apertures defined by the pairs of opposed recesses 14.

It will be appreciated that the above description is of one embodiment of the invention, and that grate mechanisms may differ from that described above while remaining within the scope of the invention. For example the grate bars may differ as to their length and width, and as to the number and the shape and size of the recesses.

Claims (8)

1. Claims 1. A grate mechanism comprising a multiplicity of grate bars arranged side-by-side in substantially a common plane, alternate grate bars being movable lengthwise so as to move relative to adjacent grate bars, and wherein each grate bar defines a multiplicity of recesses along both sides of the bar, recesses on one side of the bar being arranged alternately with recesses on the opposite side of the bar along the length of the bar.
2. 2. A grate mechanism as claimed in claim 1 wherein alternate grate bars are connected to a linking mechanism whereby the alternate grate bars can be moved lengthwise simultaneously.
3. 3. A grate mechanism as claimed in claim 2 wherein the linking mechanism ensures that lengthwise movement of alternate grate bars does not subject the grate bars to tension.
4. 4. A grate mechanism as claimed in any one of the preceding claims wherein aiLl the grate bars are of substantially identical shape, at least along their central portions.
5. 5. A grate mechanism as claimed in any one of the preceding claims wherein the upper surfaces of all the grate bars are provided with upwardly-projecting teeth.
6. 6. A grate mechanism substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
7. 7. A grate bar for use in a grate mechanism as claimed in any one of the preceding claims.
8. 8. 7 processing plant incorporating a grate mechanism as claimed in any one of claims 1 to 6.