WO1984001808A1

WO1984001808A1 - Solid fuel burner

Info

Publication number: WO1984001808A1
Application number: PCT/GB1982/000310
Authority: WO
Inventors: Alec Wilfred Smith
Original assignee: Carbonised Fuel Combustion Ltd
Priority date: 1982-10-29
Filing date: 1982-10-29
Publication date: 1984-05-10
Also published as: KR840006398A; EP0139629A1; IT8349235A0; IT1197734B; AU9120382A; JPS59501959A; ZA837751B

Abstract

A burner capable of burning very small grade solid fuel (10) (up to 25% fines of 0.8 mm mesh size) in a controlled manner, on either a domestic or an industrial scale. The fire bottom is a water-jacketed metal plate (26). Primary combustion air passes through the apertures (30) in the water jacket (28) and plate (26). Cratering (34) and uneven burning of the fuel is avoided by a reciprocable member (50) in the throat (20) of a fuel hopper (12) supplying the fuel bed (22). The reciprocation stroke length (54) is controlled, in conjunction with the size of throat and the air supply, to an amount sufficient to compact the burning fuel without pushing it bodily down the fire bed, although an occasional longer stroke (58) is permitted to effect total fuel bed movement.

Description

TITLE

Solid Fuel Burner DESCRIPTION Technical Field

This invention relates to solid fuel burners, which either may be incorporated in boilers or may be designed as pre-burners .for supplying their combustion gases to existing boiler installations.

Background Art

There have been proposed many previous designs of solid fuel burner which employ either grates for the fuel bed or a cooled fire bottom through which primary combustion air is passed into the fuel bed to support combustion. British Pa-tent Specification Number 2092294A discloses a solid fuel burner in which fuel is delivered vertically downwardly from a hopper through a discharge orifice to a fire bottom which comprises a number of cooled inclined overlapping metal plates. The fire bed slides bodily over the cooled fire bottom with no cascading of fresh fuel over the top of the fire bed, and this bodily movement automatically scrapes the clinker that is formed, downwardly over the fire bottom to a clinker collection zone. This solid fuel burner is particularly useful for burning a wide range of particulate solid fuels, but has serious limitations when the fuel contains a appreciable proportion of "fines", that is to say particles of 0.8 mm mesh size. With such fuels the burning tends to localized and uneven, the fire bed tends to become cratered and the carbonmonoxide level in the flue gases tends to rise. This is unfortunate, since there is currently available internationally a vast surplus of such fuel, which at present cannot be burned either on conventional grates or on the cooled fire bottom of British Patent Application Number 2092294A, but which requires the higher technology of fluid zed bed burners. Because such burners are expensive and suitable only for large-scale power generation, the supply of such fuel, largely as a by-product from other fuel manufacturing processes, has outstripped the demand and the international surplus of such fuel continues to grow. It is therefore an object of the present invention to overcome the disadvantages of the prior proposals of solid fuel burners, and provide a burner that is capable of burning particulate solid fuel with a large amount of fines, whether on a domestic or an industrial scale.

The Invention.

The invention provides a solid fuel burner comprising a fuel hopper for delivering particulate solid fuel downwardly through a throat onto a downwardly inclined cooled fire bottom that is apertured to allow primary combustion air to pass through the fuel to support combustion therein as the fuel forms a fire bed that slides bodily down the fire bottom towards a clinker collection zone, characterized in that a reciprocable member is provided in the throat of the hopper for repeatedly compacting the fuel in the fuel bed as it burns.

The location of the reciprocable member has been found to be important. According to the invention the member is provided in the throat of the hopper, and preferably it is aligned with the general plane of the fire bed so that as it extends it compacts the fuel 8

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in the fuel bed, and as it retracts it permits more fuel from the hopper above to fall into the space so created. Advantageously the regular compaction of the fuel in the fuel bed is accompanied by a less frequent stroke of the member, which physically advances the fuel along the fire bottom. This intermittent fuel movement may be the result of an occasional longer stroke of the same member, or alternatively it may be provided by another member immediately beneaththat provided for compaction, the longer stroke or the stroke of the second member extending beyond the throat as far as the fire bed itself.

It has been found that a solid fuel burner according to this invention is capable of supporting the even and controlled combustion of particulate solid fuel that contains up to 25% by weight of fines. Preferably the maximum particle size in the fuel is less than 2.5 mm, and advantageously the fuel contains no more than 15% by weight,preferably no more than 10% by weight, of particles with a size of 8 mm or more.

Commercially available fuels that have been proved to be extremely suitable for a solid fuel burner according to the invention are as follows: Avenue Breeze, which is degraded or comminuted Sunbrite (Trade Mark). Sunbrite is a high temperature hard coke. It* has a high proportion of fines, and is very abrasive, making it unsuitable for many applications.

It burns particularly well in a solid fuel burner according to the invention, and its abrasiveness causes no problems.

Rexco, (Trade Mark) and Coalite, (Trade Mark). These are lower temperature cokes, containing more volatiles than the Avenue Breeze referred to above. In comminuted form they have proved very unsatisfactory for burning on other types of solid fuel burner, but highly satisfactory burning and good efficiency can be obtained in a burner according to the invention. , Coal Fines. These can contain up to 30% volatiles, and by varying the degree of compaction and the depth of the fire bed, efficient burning can be obtained in a solid fuel burner according to this invention without the creation of excessive soot deposits. Processed Coal Tars, optionally blended with comminuted refuse or wood. Heavy coal tar fractions have a high thermal content, and can obtained in pelleted form for use as a fuel. The thermal content of this fuel is such that it is preferable to blend it with a less vigorously burning fuel, such as sawdust or comminuted refuse, thereby to obtain a synthetic fuel that burns particularly well in a solid fuel burner according to this invention.

All of the above fuels will normally be used in their dry state, but it has been found that a quite considerable amount of moisture in the fuel can be tolerated without any serious loss of efficiency. If the fuel is likely to be damp, then the regular pushing of the fuel forward by a longer stroke of the member is found to be of particular advantage. This longer strokeis preferably sufficient to push fu^l into a drying area of the fuel bed, between the throat of the hopper and the fuel combustion area.

The burner of the invention may be provided with automatic ignition in the form of electrical heaters for heating the primary air supplied to at least some of the apertures in the fire bottom to above ignition temperature. It may also be provided with automatic clinker removal. For example the clinker collection zone may lead to a clinker trough into which the clinker falls and from which it is removed automatically by a lead screw.

The burner of the invention enables the simultaneous control of the combustion air (primary and/or secondary) the degree of fuel compaction in the fire bed and the rate of fuel movement down the fire bed. The depth of fire bed is also variable by setting the size of the hopper throat. These different elements_.<*of control, never before contemplated in a solid fuel burner with a simple static cooled fire bottom in place of a conventional grate, have been found sufficient to reduce quite remarkably the unwanted carbon monoxide and particulate stack emissions of the burner even when using the fuels described above that have high fines contents. Best Mode

In the accompanying drawings:

Figure 1 is a partly schematic vertical section through a fuel hopper and fire bottom of a solid fuel burner according to the invention; and

Figure 2 is a vertical section, taken along the line II-II of Figure 1, through the solid fuel burner and an associated boiler. In Figure 2 detail of the boiler is omitted as this forms no part of the invention.

For clarity the particulate solid fuel 10 shown in Figure 1 is omitted from Figure 2 except for a small dotted, line 10a which indicates the top right hand edge of the fuel bed in use. The fuel 10 in Figure 1 is stored in a hopper 12, a front wall 14 and side walls (not shown) of which are vertical, and a rear wall 16 of which is inclined at about 10 to the vertical so as to reduce to a minimum the amount of bridging of the fuel that is encountered as the fuel 10 falls under gravity towards the fire bottom.

At the bottom of the hopper front wall 14 there is provided an adjustable throat plate 18 which defines a throat 20 of the hopper through which fuel must pass to reach the fire bed, indicated generally as 22. tϋϊϊj-

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The fire bed 22 is formed by a layer of fuel lying on a fire bottom provided by inclined plates 24 and 26. The plate 26 is water-cooled, as it has a water jacket 28 formed beneath it for ensuring both the cooling of the plate 26 and a degree of pre-heating of the boiler water. Passages for primary combustion air are formed through the water jacket 28 and plate 26, as indicated generally in Figure 1 as 30. The resulting combustion zone in the fire bed is immediately above a reduction zone as indicated in Figure 1 as 32. It will be seen from Figure 1 that uncontrolled combustion of the fuel in such a fire bed would result in the formation of craters 34. The formation of such craters results in a loss of efficiency of the solid fuel burner, and a marked increase in the stack emission. The present invention avoids the formation of such craters in a manner to be described below.

It will be appreciated from Figure 1 that primary combustion air also passes from a plenum chamber 36 between the overlapping plates 24 and 26, and between the plate 26 and a ceramic or H_0 cooled clinker collection tray 38, in the manner described and claimed in British Patent Application Number 2092294A. Clinker forming on the collection tray 38 eventually falls into a trough 40 from which it is automatically removed by means of a screw 42. Secondary combustion air is supplied from the plenum chamber 36 to a distributor tube 44 either through an upstanding pipe 46 extending through the fuel bed as shown in Figures 1 and 2 or through distribution pipes at the sidesof the fuel bed.

The avoidance of craters, and control of the movement of the burning bed of fuel, is achieved according to the invention by means of actuators 50 and 52 under the control of a ram or torque motor (not shown). In the illustrated embodiment the ram or torque motor imparts a regular oscillating movement to the actuator 50, which reciprocates regularly in the throat20 with a stroke length indicated in Figure 1 as 54. Pins 56 in the actuator 50 are permitted an amount of free movement relative to the actuator 52 such that the actuator 52 is not moved during this regular reciprocable motion of the actuator 50. On a preprogrammed sequence, however, the actuator 50 is caused to perform a longer stroke in excess of the limits of free movement between the actuators 50 and 52, so that the actuator 52 is moved by an amount 58. The extents ofmovement 54 and 58 are chosen so that the^' regular oscillation of the actuator 50 over the distance 54 is sufficient simply to compact the - 1.1

fuel in the fuel bed as it burns, avoiding the creation of craters 34. The intermittent longer movement of the actuator 50 (longer by the distance 58) is sufficient to move the burning bed of fuel bodily down the fire bottom so that on return of the actuators 50 and 52 more fuel is allowed to fall into the throat 20 of the hopper.

The dual length of stroke of the actuatotrs 50 and 52 not only permits the avoidance of craters 34, but also it enables the solid fuel burner to achieve a regular and controlled burning of fuel. The stroke lengths are both capable of adjustment, as are the stroke frequencies and speeds of travel.

Figure 2 shows generally how the solid fuel burner of the invention can be arranged alongside a conventional boiler, so that the combustion gases pass from the burner at 60, beneath a baffle 62, into the boiler 64. The floor 66 of the boiler would typically be of fire bricks or ceramic material.

Figure 2 also shows a water jacketed air inlet 68 by means of which the combustion air is passed to the plenum chamber 36. Clearly control of the air supply would be monitored in use in conjunction with the control of the actuators 50 and 52.

Claims

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1. A solid fuel burner having a hopper (12) for delivering particulate solid fuel downwardly through a throat (2) onto a downwardly inclined cooled fire bottom (26) that is apertured to provide a path for primary combustion air to pass upwardly through a burning bed^' of fuel as the fuel passes over the fire bottom towards a clinker collection zone (38), characterized in that a reciprocable member (5) is provided in the throat of the hopper forcompacting the fuel in the fuel bed as it burns.

2. A solid fuel burner according to claim 1, wherein the reciprocable member- (5) is reciprocable in a plane parallel to the plane of the inclined fire bottom. r

3. A solid fuel burner according to claim 2, wherein the reciprocable member (5) is effective also periodically to move the burning bed of fuel incrementally down the fire bottom (26) towards the clinker collection zone (38).

4. A solid fuel burner according to claim 3, wherein the reciprocable"member (5) has two alternative lengths of stroke one of which (54) is sufficient to compact the fuel in the fuel bed without moving the burning bed of fuel bodily down the fire bottom (26) and the other of which (58) is effective so to move the burning bed of fuel; and means controlling the movement of the reciprocable member (-50) is effective to effect a number of compaction strokes (54) in between successive bed movement strokes (58).

5. A solid fuel burner according to claim 4, wherein the longer bed movement strokes (58) are effective to move an auxiliary reciprocable member (52) so as to assist the bodily movement of the fire bed.

6. A solid fuel burner according to claim 5, wherein the auxiliary reciprocable member (52) is nearer the plane of the fire bottom (26) than is the reciprocable member (50).

7. A solid fuel burner according to claim 1, wherein the throat (20) is defined by an aperture between the fire bottom and an adjustable throat plate (18), so that the depth of the bed of fuel is adjustable.

8. A solid fuel burner according to claim 1, wherein the clinker collection zone (38) leads to a clinker trough (40) from which clinker is removable automatically by means of a lead screw (42).

OMP

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9. A solid fuel burner comprising a fuel hopper having generally vertical side walls and a fuel delivery throat; a fire bottom communicating with the fuel delivery throat, the fire bottom being formed from at least one downwardly inclined metal plate with a water jacket therebeneath in heat exchange relationship with the plate for cooling the plate; a plenum chamber for combustion air; at least one passage for leading said combustion air from the plenum chamber through the water jacket and plate of the fire bottom to a fuel combustion zone above the.fire bottom; reciprocally movable means in the fuel delivery throat of the fuel hopper, movable in the plane of the fire bottom to compact a bed of fuel on the fire bottom; and means controlling the movement of the reciprocally movable means repeatedly to effect said compaction at a rate sufficient to overcome the tendency of fuel burning on the fire bottom to form craters but insufficient to overcome the Inertia of the fuel and move the fuel bodily down the fire bottom.

10. A solid fuel burner according to claim 9, wherein the means controlling the movement of the reciprocally movable means is effective to intersperse the said compaction movements with an occasional movement of greater stroke length that is effective to overcome the inertia of the fuel and move the fuel bodily down the fire bottom.