GB2032590A - A fluidised bed grate - Google Patents

Abstract

A fluidised bed grate comprising at least three parallel horizontal stringers (1) disposed side-by-side with an elongate gap between each pair of adjacent stringers (1). An inverted trough (5) extends along and above each gap with a slot (8) in each side wall of the trough (5). A plenum chamber is formed below the stringers (1). <IMAGE>

Description

SPECIFICATION A fluidised bed grate This invention relates to a fluidised bed grate.

The technique of buring coal or other solid fuel, including waste material, using the fluidised bed combustion process is essentially based on providing a means whereby the coal or other solid fuel rests on a horizontal bed or grate in a uniform layer across the total area of the bed or grate.

Such a layer when at rest is typically several inches thick. The particle size of coal or other solid fuel can vary from 1/32" or indeed less to 2" or so and a bed can contain a mixture of particle sizes. It is also customary for the coal or other solid fuel to be a small percentage of the total bed layer, typically 10%. The remainder of the bed layer can be composed of sand, crushed refractory and fine ash or other similar inert material.

All is blown upwards through suitable apertures in the grate in sufficient volume and pressure so as to cause the whole of the solid material in the bed layer to be just suspended in the air flow stream immediately above the bed or grate level. In this condition the thickness of the original static bed layer will be expanded upwards by a factor of 2 or 3 and all the solid particles will in fact be continuously moving in a tubulent manner. In this condition the bed is said to Ce fluidised. If the coal or other solid fuel is ignited in this fluidised condition the inert matter in the bed becomes incandescent and the total bed assumes a near uniform temperature throughout.

The operating temperature of fluidised beds as described above can vary widely depending on the nature of heat exchange surface in or adjacent to the bed, typical operating temperatures are between 9500C and 12000C.

The means of providing proper air distribution through the bed or grate to achieve uniform fluidisation is of considerable importance, since in addition to the question of air distribution such means have to be resistant to the high temperature environment and resistant to the possibility of allowing fine ash, or other fine particles to fall back through these apertures into the air supply system when the fluidisation process is stopped or to becoming blocked by fine ash or small particles when the fluidisation process is stopped.

The majority of fluidised bed air distribution systems are either by a series of fixed holes or slots in the bed or grate or by a series of small stand pipes rising from the bed or grate. It will be appreciated that the holes, slots or stand pipes are numerous relative to the total bed area and have to extend in a uniform pattern across the whole bed area.

Whilst the holes or slots are relatively inexpensive to provide they have a disadvantage in that protective shields or cowls are necessary to prevent fall back and/or clogging on the part of fine ash or other fine solids. Such shields or cowls are difficult to design consistent with their functions and not impeding proper air distribution.

Also such shields or cowls can be subject to overheating during the combustion process.

The use of stand pipes minimises the problem of fall back or clogging, however it is often necessary to provide a shield or cowl to further minimise this problem at the top extremity of such stand pipes. By their nature stand pipes can be subject to overheating and their use significantly increases the manufacturing cost of fluid bed equipment.

Further the provision of holes, slots or stand pipes does not provide for any adjustment or trimming of air flow characteristics after manufacture or during normal operations of the fluidized bed.

It is an object of this invention to provide a fluidised bed grate in which at least one of the described disadvantages of the known grate is alleviated.

According to this invention there is provided a fluidised bed grate comprising at least three generally horizontal members disposed side-byside with an elongate gap between each pair of adjacent members, an inverted trough extending along and above each gap with a slot between each side wall of the trough and the corresponding horizontal member or in each side wall of the trough, and a plenum chamber formed below the generally horizontal members.

Preferably, an upwardly projecting wall extends along the upper edge of each member, the walls being covered by the troughs.

In one embodiment of this invention, lifting means are provided for lifting the troughs in relation to the members.

Shifting the troughs varies the air flow through the slots.

Preferably, said lifting means comprise cams secured to cam shafts extending below and parallel to the gaps, and vertically extending rods secured at their upper ends to the horizontal walls of the troughs and at their bottom ends to respective cam followers which engage respective cams.

Preferably, spacers are disposed in the elongate gap between each pair of adjacent members and means are provided for urging the generally horizontal members together.

Preferably, the said means comprise bolts extending through the horizontal members and the gaps so as to be transverse to the gaps.

The widths of the gaps may be changed by changing the spacers.

In another embodiment of this invention the spacers are in the form of face cams mounted on the bolts, the adjacent sides of the members acting as cam followers.

In this embodiment rotation of the bolts varies the widths of the gaps and therefore the air flow.

The slots are preferably covered with elongate meshes.

Embodiments of this invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a partial plan view of a fluidised bed grate in accordance with this invention; Figure 2 is a sectional elevation on line Y-Y of Figure 1; Figures 3, 4 and 6 are partial sectional elevations illustrating different modifications; and Figure 5 shows the embodiment of Figure 4 set to a different position from that shown in Figure 4.

Figures 1 and 2 indicates a novel design of a fluid bed or grate which incorporates an improved air flow distribution system, an improved means of preventin'g fall back or clogging by the fine ash or other fine particles, less risk of overheating during the fluidised bed combustion process and most importantly a means of adjusting the airflow after manufacture or during the normal operations of the fluidised bed.

Item 1 is a series of steel or cast iron stringers that extend on a side-by-side basis across the total bed area.

These stringers are separated at intervals by "C".

washers, item 2, which are located in their respective positions by a series of long bolts, item 3, these being located through drilled holes in the stringers at right angles to the length of the stringers. With the bolts item 3 tightened the stringers are properly aligned and a solid bed is formed with a gap item 4 between each stringer, the width of the gap being dictated by the thickness of the "C" washer, item 2.

Along each edge of the stringer there is welded a vertical plate, item 5 of the proportions shown on Figure 2, the two vertical plates of adjacent stringers forming a continuous extension of the gap item 4 between the stringers, item 1.

Located above the vertical plate gap, item 5 is an inverted trough section, item 6 which runs for the full length of the vertical plate gap item 5. This inverted trough section has a series of extended flanges along its length on either side which allows it to be clamped hand down on the stringer top faces by the simple clamp device item 7. The inverted trough section has a series of long slots along each side and such that they are adjacent to the top of the stringer face as indicated at 8.

With the stringers, item I firmly held by the long bolts, item 3, as described above a steel or cast iron boundary rim is clamped above and below the bed by a series of clamps, item 9 at suitable intervals around the four sides of the bed assembly.

Fluidising air is supplied via a plenum chamber arrangement under the bed and rises through the slots 4 and through the vertical plates 5 into the inverted trough section 6 and through the long slots at 8 in the bed material to be fluidised, which will normally rest as a layer across the whole surface of the bed area.

From the above and with reference to Figures 1 and 2 it can be seen that the air distribution slot item 4 cannot be subject to fall back or clogging from fine bed particles. It can also be seen that any accumulations of fine bed particles in the vicinity of the long slots 8 will rapidly be cleaned when fluidising air is supplied. The height of the inverted trough section, item 6, is limited so that the risk of overheating is minimised.

Further and importantly the width of the slots 4 and its consequent effect on fluidising air supply to the bed can be easily adjusted after manufacture by the use of different thickness C washers item 2 with appropriate adjustment to the long boit tightening, item 3, and the clamping of the inverted trough items 6 and 7, and the clamping of the boundary rims, items 9 and 10.

The above describes the bed in its simplest form. A further and desirable requirement is the capability of adjustment to the fluidising air supply to the bed whilst the bed is in operation as a combustion process.

Reference to Figure 3 will show a modified inverted trough section which is not fixed to the stringer, item 1, but supported relative to the stringer by a series of rods along its length, item 12, these rods being secured to a common base plate, item 14. Item 1 3 are guide fins welded to the rods so as to properly locate the inverted trough at all times. Adjustment of the cam 11 will cause a variation in the vertical position of the inverted trough 6 with a consequent variation in the air distribution slot or passage at 8. it should be noted that the adjustment cam or other such height adjusting mechanism is in the cool air supply stream beneath the bed and these unaffected by operational heat.

A further important advantage of the adjustable mechanism described above and as shown in Figure 3 is that the air supply can be completely cut off from the bed by fully lowering the inverted trough 6 and closing the slots at 8. This in turn will avoid any possibility of clogging the air supply aperture 8 with fine ash when the bed is not fluidising.

The adjustable mechanism shown in Figure 3 is capable of being adapted by particular design to a system whereby the air supply pressure in the plenum chamber and consequently underneath the inverted trough 6 will regulate the vertical position of the inverted trough 6 and consequently the air supply rate through the aperture at 8.

Reference to Figures 4 and 5 will show an alternative method of providing adjustment to the fluidising air supply in this basic design of bed.

In this case the "C" washer item 2 are replaced by cam collars and these are secured to the long bolts item 3 by pins, or some other means. In this case the long bolts, item 3, do not serve to tighten and pull the stringers, item 3, together as described above but simply to rotate and increase or decrease the air supply gap 4 by the action of the cam collars 2. The stringers are forced into proper and parallel contact with the cam collars by substantial compression springs located at one side of the bed acting along the axis of the long bolts 3. Obviously these compression springs will be located, well clean of the combustion zone and therefore unaffected by heat. To accommodate the relative movement of the stringers the inverted trough, item 6 will be clamped to one stringer only and resting on the other stringer. The position oF the inverted trough will be such as to allow variation in the vertical gap plate width, item 5, without altering the air supply quantity within the trough, item 6, to each of the supply slots, item 8.

In the embodiment shown in Figure 6, the slots 8 are covered by flexible steel meshes 1 5 which will breakup large bubbles of air.

Claims (9)

1. A fluidised bed grate comprising at least three generally horizontal members disposed sideby-side with an elongate gap between each pair of adjacent members, an inverted trough extending along and above each gap with a slot between each side wall of the trough and the corresponding horizontal member or in each side wall of the trough, and a plenum chamber formed below the generally horizontal members.

2. A fluidised bed grate as claimed in claim 1 wherein an upwardly projecting wall extends along the upper edge of each member, the walls being covered by the troughs.

3. A fluidised bed grate as claimed in claim 1 or 2, wherein lifting means are provided for lifting the troughs in relation to the members.

4. A fluidised bed grate as claimed in claim 3, wherein said lifting means comprise cams secured to cam shafts extending below and parallel to the gaps, and vertically extending rods secured at their upper ends to the horizontal walls of the troughs and at their bottom ends to respective cam followers which engage respective cams.

5. Afluidised bed grate as claimed in claim 1 to 4 wherein spacers are disposed in the elongate gap between each pair of adjacent members and means are provided for urging the generally horizontal members together.

6. Afluidised bed grate as claimed in claim 5, wherein the said means comprise bolts extending through the horizontal members and the gaps so as to be transverse to the gaps.

7. A fluidised bed grate as claimed in claim 6, wherein the spacers are in the form of face cams mounted on the bolts, the adjacent sides of the members acting as cam followers.

8. A fluidised bed grate as claimed in claims 1 to 7, wherein the slots are covered with elongate meshes.

9. A fluidised bed grate substantially as hereinbefore described with reference to the accompanying drawings.