WO1988007649A1

WO1988007649A1 - Solid fuel furnaces

Info

Publication number: WO1988007649A1
Application number: PCT/GB1988/000252
Authority: WO
Inventors: Mark Edward Crowther; Stephen Arumugam Thambirajah Pathmarajah
Original assignee: Ekman, Carl, Oscar, Alexander
Priority date: 1987-03-31
Filing date: 1988-03-31
Publication date: 1988-10-06
Also published as: GB8707674D0; EP0360801A1; AU1492888A; JPH02504303A; ZA882319B

Abstract

A furnace for burning solid fuel in pieces has a horizontal mechanically operated grate (7) for supporting and advancing a burning fuel bed. Primary and secondary air channels (11) and (12) are formed in the grate to pass airflows up through the fuel bed. Fuel (29) is delivered directly onto a fuel-receiving surface (26) of the grate (7) from a rotary feeder drum (30), which forms an air seal, down a steeply inclined chute (50). Additional air is blown in under pressure to the space above the primary air channels (11) to cause turbulence and assist in burning volatile components given off by the fuel.

Description

SOLID FUEL FURNACES

This invention relates to furnaces for burning solid fuel.

According to the invention, there is provided a furnace for burning solid fuel supplied in pieces, comprising a substantially horizontal mechanical grate for supporting within an enclosure a fire bed of burning fuel and for advancing the fire bed from a fuel receiving end to an ash disposal end, an enclosed fuel delivery shaft for delivering raw fuel directly onto the fuel receiving end of the grate, a substantially airtight fuel metering and feeding device for delivering fuel at a controlled rate into the inlet shaft, a set of primary air channels extending upwards through the grate adjacent but downstream of the fuel receiving end of the grate, a set of secondary air channels extending upwards through the grate downstream of the primary air channels, and means for delivering a stream of further air to the upper part of the enclosure in the region over the primary air channels. Typically, the enclosure will be subject to an induced draught fan in an outlet flue leading from the enclosure. Primary and secondary air streams will be thereby drawn up through the respective channels in the grate. Preferably, the further air stream is delivered into the enclosure under pressure to ensure turbulent mixing with volatile and gaseous components given off by the fuel as it is ignited and begins to burn over the primary air channels.

Advantageously, the upstream ends of the primary air channels are defined by wall surfaces of the grate bars which over the greater part of the height of the grate bars are inclined upwardly and in the downstream direction while the uppermost parts of these walls are cut back so as to be for example vertical. With this arrangement, any tendency for the high speed primary air flowing through the primary air channel] s to create eddies where the air streams emerge from the grate prevent such eddies comjng into contact with the fuel immediately above the said walls, which would otherwise cause high temperatures to be created at these positions with rapid erosion of the grate bar material. Similarly, the upstream edges of the secondary air channels are protected by bleeding air flows from the primary air channels into the upstream parts of the secondary air channels through bleed channels extending through the division between the primary and secondary air channels. Preferably, these bleed channels are inclined upwardly in the downstream direction. As a result, air floWs upwards at similar velocities adjacent both the upstream and the downstream faces of the division between the primary and secondary air channels and the tendency for eddies to form is reduced.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which :-

Figure 1 is a vertical longitudinal sectional view through a coal burner mounted in a boiler, and

Figure 2 is a plan view of part of the grate of the coal burner shown in Figure 1, on an enlarged scale.

Figure 1 shows part of a boiler 1 having an outer cylindrical shell 2, a front wall 3 and a flame tube 4 extending from a front opening 5 in the front wall 3 to further tubing (not shown) forming two more passes within the water space in the boiler and leading to an outlet flue which includes an induced draft fan (not shown).

Mounted in the front opening 5 is a coal burner 6 having a substantially horizontal mechanical grate 7 located in the upstream end of the flame tube 4. The grate 7 comprises a set of grate bars 8 (Figures 1 and 2) which are slidably supported on the framework 9 of the coal burner 6 and on partitions 10 and 11 in the flame tube 4. As can be seen in Figure 2, each grate bar 8 is formed from two grate bar elements 8a and 8h secured together by rivets 8c. Each grate bar thus defines a primary air channel 11a of width 4mm in this embodiment and a secondary air channel 12a, here of width 3mm, each of the channels 11a and 12a being in the form of a slot defined between the two grate bar elements 8a and 8b. Further primary and secondary air channels lib and 12b of the same widths as the channels 11a and 12a respectively are defined between adjacent grate bars 8. As can be seen in Figure 1, the primary air channels 11 communicate with an undergrate space 13 defined by the upper side of the partition 10 and communicating with the atmosphere through the front opening 5 of the flame tube 4. The secondary air channels 12 communicate with a further under grate space 14 defined between the underside of the partition 10 and. the partition 11 and forming a diffusion space into which a limited stream of secondary air is admitted through a slot 15 of. adjustable width formed between the front lower edge of the partition 10 and a damper plate 16 whose vertical position can be adjusted and thereafter clamped by means of a nut or hand wheel 17.

The grate bars 8 are divided into two sets with the grate bars of one set alternating with the grate bars of the other set. The two sets of grate bars are reciprocated by a grate bar driving mechanism 18 comprising an electric motor 19, a gearbox 20, a set of face cams 21 on the output shaft of the gearbox 20 and cam followers 22 cooperating with the cams 21, located by short connecting rods 23 and connected to the respective set of grate bars by long connecting rods 24. The cycle of operation of the grate mechanism is such that first one set of grate bars is withdrawn towards the front of the burner to the position shown in Figure 2, then the other set of grate bars is withdrawn towards the front of the coal burner and thereafter both sets are advanced together in the downstream direction. This action both advances the burning fuel bed on the top of the grate and also promotes some internal movement in the fire bed particularly as a result of the sloping surface 25 formed by the top surface of the grate bars in the region of the primary and secondary air channels 11 and 12.

The upstream portion of the grate bars defines a fuel receiving surface 26 onto which solid fuel, such as coal in pieces, is deposited from a coal-metering feeder 27 comprising a hopper 28 containing a supply of coal 29, a rotary drum 30 driven by an electric motor 31 through a gearbox 32 and an enclosed delivery shaft 33 forming a steeply inclined chute surface for conveying coal directly from the rotary drum 30 onto the coal receiving surface 26 of the grate. The drum 30 has a pocket which holds a predetermined quantity of fuel, for example 3Kgs which is filled with coal 29 when the drum is in the position shown in Figure 1 and which deposits this coal when the drum has rotated through about 180°. In all angular positions, the drum effectively closes the upper end of the fuel inlet shaft 33.

The lower portion of the chute surface of the inlet shaft 33 is formed of refractory material, here in the form of a lower refractory block 34 which is fixed to the framework 9 and an upper refractory block 35 carried by an inspection door 36 which is normally closed but can be swung down about its hinges 37 to give easy access to the interior of the coal burner. The lower block 34 forms a reaction surface which effectively pushes the fuel on the upstream part of the grate along the grate when the grate bars are being withdrawn towards the front of the coal burner.

A discharge nozzle 38 is mounted to point in the downstream direction in the upper part of the flame tube 4 and is supplied through a pipe 39 with air (from a centrifugal fan, not shown) under sufficient pressure to ensure turbulent mixing in the upper part of the flame tube.

In operation, in starting up the coal burner from cold, the drum 30 is rotated by its motor, a sufficient number of times to deposit a pile of coal on the coal receiving surface 26. This coal is then ignited by one or more electrical ignitors 40 positioned between the hopper 29 and the front wall 3 of the boiler, pointing downwards onto the coal at the bottom end of the fuel inlet shaft. At the same time, the induced draught fan is started to pull combustion products along the flame tube 4. When ignition has been detected, the grate driving mechanism 18 is started and further fuel is added by the fuel feed device 27. As the burning fuel bed builds up with the air supply to the line 39 supplying the nozzle 38, fuel travelling from the coal receiving surface 26 into the region of the primary air channels 11 meets the high intensity primary air streams flowing up through these primary air channels and is subjected to rapid combustion. The volatile and gaseous components evolved from the coal in the initial part of this combustion are intermitently mixed with air from the nozzle 38 and under the effect of the high temperature generated by the burning fuel under the effect of the primary air streams are rapidly burnt. Combustion continues as the fuel is moved along the top surface 25 of the grate until it passes over the division 41 between the primary and secondary air channels and then moves along over the secondary air channels 12 where combustion of the fuel is completed at lower intensity with the more restricted lower speed air streams supplied through the secondary air channels 12 from the inlet slot 15 via the diffusion space 14.

Finally, the burnt out fuel falls as ash from the downstream ends of the grate bars 8 into an ash receiving space 42 formed either by an ash bin or part of an ash removal system.

The rate of heat output of the coal burner is determined by the rate at which the fuel feeder 27 delivers fuel to the burner i.e. by the speed of operation of the motor 31. The speed of operation of the motor 19 determines the speed of operation of the grate and thus also the residence time of the fuel on the grate. Thus, it may take for example about 30 minutes for a 4 cm piece of coal to burn out completely. The speed of operation of the grate can accordingly be chosen to ensure sufficient time for the fuel to burn out completely.

The motors 19 and 31 (together with the ignitors 40, the fan supplying the line 39 and the induced draught fan) are all under the control of a control system (not shown) which also responds to the demand of the installation.

Since the major part of the air for combustion of the fuel is drawn up through the primary air channels 11, there is a tendency for this relatively high speed stream to cause eddies at the upstream and downstream ends of the primary air channels. As shown in Figure 1, the upstream end walls 43 are curved so as to impose a substantial component. of velocity on this air in the downstream direction. Further, the uppermost part of these end walls is effectively cut away at 44 so as to be substantially vertical. This provides a space within which such eddies can occur without causing excessively rapid combustion of the adjacent fuel with consequent damage to the surface of the grate at this point.

Similarly, to avoid eddies causing high temperatures on top of the division 41 between the primary and secondary channels and at the upstream end of the secondary channels 12, bleed channels 45 are formed in the divisions 41 so as to permit air streams to be bled from the primary air channels 11 with an upward component so as to emerge upwardly through the upstream ends of the secondary air channels 12 and thereby stifle the tendencies for combustion-increasing eddies to occur.

The fuel inlet shaft may be cooled by providing a small number, e.g. two of small air inlets 50 below the rotary drum 30.

Claims

CLAIMS.

1. A furnace for burning solid fuel supplied in pieces, comprising a substantially horizontal mechanical grate for supporting within an enclosure a fire bed of burning fuel and for advancing the fire bed from a fuel receiving end to an ash disposal end, an enclosed fuel delivery shaft for delivering raw fuel directly onto the fuel receiving end of the grate, a substantially airtight fuel metering and feeding device for delivering fuel at a controlled rate into the inlet shaft; a set of primary air channels extending upwards through the grate adjacent but downstream of the fuel receiving end of the grate and a set of secondary air channels extending upwards through the grate downstream of the primary air channels characterised by means for blowing a stream of further air under pressure into the upper part of the enclosure in the region over the primary air channels and in that the fuel delivery shaft opens without restriction into the combustion space above the grate.

2. A furnace according to Claim 1 characterised in that the upstream ends of the primary air channels are defined by wall surfaces of the grate bars which over the greater part of the height of the grate bars are inclined upwardly and in the downstream direction while the uppermost parts of these walls are cut back so as to be for example vertical.

3. A furnace according to Claims 1 or 2 characterised in that the upstream edges of the secondary air channels are protected by bleeding air flows from the primary air channels into the upstream parts of the secondary air channels through bleed channels extending through the division between the primary and secondary air channels.

4. A furnace according to Claim 3, characterised in that the bleed channels are inclined upwardly in the downstream direction.