GB2218788A

GB2218788A - Solid fuel burner

Info

Publication number: GB2218788A
Application number: GB8803108A
Authority: GB
Inventors: Eric Peel
Original assignee: K B P BOILERS Ltd
Current assignee: K B P BOILERS Ltd
Priority date: 1988-02-11
Filing date: 1988-02-11
Publication date: 1989-11-22
Also published as: GB8803108D0

Abstract

The solid fuel burner includes a combustion chamber (20) having a floor (21) occupied by a fuel bed, a primary air supply (15) under the combustion chamber floor, and a secondary air supply (12) within the combustion chamber and above the fuel bed, for improving combustion of the fuel. Means (10) for adjusting the amount of air from each supply may be provided, and the secondary air supply (12) may be positioned over part of the fuel bed just outside the burning zone. The secondary air supply (12) may be operated in conjunction with a fuel feeder to minimise smoke generation when new fuel is added to the fuel bed, and exhaust gases may be routed through a chimney (5) including a fines arrester (51) for trapping fine particles. Virtually smokeless operation can be achieved without special fuels. The burner may be applied to water heating systems. <IMAGE>

Description

SOLID FUEL BURNER The present invention relates to a solid fuel burner, particularly but not exclusively to a coal burner for use in water heating systems.

A problem of conventional solid fuel burners is that appreciable amounts of smoke and fine particles ("fines") are emitted when burning ordinary grades of coal. This means that in practice many burners are restricted to use with relatively expensive smokeless solid fuels or high-grade coal, particularly in socalled "smokeless zones Accordingly, it would be highly advantageous if a solid fuel burner could be produced which was capable of burning ordinary grades of coal and like fuels without emitting appreciable amounts of smoke and fines.

According to a first aspect of the present invention, there is provided a solid fuel burner comprising a combustion chamber, having a floor which in use is occupied by a fuel bed; primary air supply means located under the combustion chamber floor, operable to supply air to the fuel bed from underneath; and secondary air supply means located inside the combustion chamber and above the fuel bed, operable to supply air to the fuel bed from above in such a way as to effect substantially complete combustion in the combustion chamber.

In a first embodiment of the invention, air supply control means are provided for selectively controlling the air supply from the primary and secondary air supply means.

In a second embodiment of the invention, the secondary air supply means is adapted to supply air to a region of the fuel bed outside a burning zone of the combustion chamber, so as to cause combustion of coal gas emitted from this region.

Conveniently, in either case, the burner has a fuel feeder operable to feed new fuel into the combustion chamber periodically. In-a first embodiment, the fuel feeder is controlled automatically in co-operation with the air supply control means, in such a manner that the supply of air from the secondary air supply means is increased shortly before operation of the fuel feeder and maintained at an increased level for a short time period following operation of the fuel feeder. Operation of the burner in this manner minimises the generation of smoke.

Preferably, in both embodiments, an air impeller is provided for drawing air for the primary and secondary air supply means from the outside atmosphere. Usually, the burner will be provided with a chimney for emitting exhaust gases from the combustion chamber, and the air drawn from the atmosphere is preferably ducted from the impeller through the chimney to the primary and secondary air supply means, so as to be heated by the exhaust gases before being introduced into the combustion chamber.

Conveniently, in both embodiments, the secondary air supply means comprises an air chamber or housing having a sloping-sided upper portion and a substantially flat lower portion. The sloping sides of the upper portion will, in use, collect fines and return them to the fuel bed, whereas the lower portion will act as a fuel leveller for maintaining the upper surface of the fuel bed at a substantially uniform height.

In the first embodiment, the secondary air supply means further comprises a number of adjustable ports which can be positioned so as to introduce air into the combustion chamber at any suitable angle.

In this case, the air supply control means may comprise an electromagnetically operated valve for receiving an air supply from the impeller and distributing the air to the primary and secondary air supply means. This valve and the fuel leveller are preferably operated under control of an electric or electronic timer.

In the second embodiment of the invention, the secondary air supply means further comprises a tube having formed therein a row of tapered holes, which act as jets for blowing air onto the fuel bed just outside the burning zone.

In either case, a control panel may further be provided, for programming the timer and/or operating the air supply control means and fuel feeder directly.

The chimney preferably incorporates a fines arrester, for collecting fines emitted from the combustion chamber so as to prevent these from being emitted into the outside atmosphere. The fines arrester may comprise a catchment plate positioned over the top of the chimney, a funnel located underneath the catchment plate and around the chimney, and a tube connected to the funnel for supplying collected fines to an ash pan.

In a preferred form of the invention, the burner forms part of a water heating system in which a number of heating tubes, for carrying exhaust gases through a water tank, are positioned above the combustion chamber. In this case, exhaust gases from the combustion chamber pass through the water heating tubes before passing into the chimney, so as to heat up water in the tank.

An automatic flue cleaner is preferably provided for cleaning the heating tubes, and this may consist of, for each tube, a number of short cylindrical sections fitted inside the tube and connected by a rod. The rod is reciprocable by an automatic driving means so as to produce an up-and-down motion of the cylindrical sections which cleans the interior wall of the tube. A flue cleaner constructed in this way also acts to retard the exhaust gases so enabling a greater heat transfer to the water in the tank.

According to a second aspect of the present invention, there is provided a method of operating a solid fuel burner, the burner having a combustion chamber containing a fuel bed, primary air supply means for supplying air to the fuel bed from underneath, and secondary air supply means for supplying air to the fuel bed from above; the method comprising the steps of, in sequence: (i) increasing the air supply from the secondary air supply means; (ii) adding a quantity of fuel to the combustion chamber; (iii) maintaining the increased air supply from the secondary air supply means for a predetermined period; and (iv) returning the air supply from the secondary air supply to a normal level.

This method is most usefully applied to the first embodiment of the invention; the second embodiment enables fuel to be added to the fuel bed without any requirement for special operating procedures.

Conveniently, in step (i) the air supply from the primary air supply means is reduced at the same time as the supply from the secondary air supply means is increased.

Preferred embodiments of the invention, applied to a water heating apparatus, will now be described with reference to the accompanying drawings in which: Figure 1 is a sectional side view of a water heating apparatus incorporating a solid fuel burner in accordance with a first embodiment of the invention; Figure 2 is a sectional view of the apparatus along a line A-A' in Fig.1; Figure 3 is a sectional view of the apparatus along a line B-B' in Fig.1; Figure 4 is a sectional view of the apparatus along a line C-C' in Fig.1; Figure 5 is an enlarged detail of Figure 1; Figure 6 is a sectional view along a line D-D' in Fig.5; Figures 7(a) and (b) illustrated examples of a fines collector; Figures 8(a) and (b) illustrate a flue cleaner for cleaning water heating tubes of the apparatus; ; Figure 9 is a sectional side view of a water heating apparatus incorporating a solid fuel burner in accordance with a second embodiment of the invention, and Figure 10 is a sectional view of the apparatus along a line A-A' in Figure 9.

The principal parts of the water heating apparatus incorporating a solid fuel burner in accordance with the first embodiment of the present invention are shown in Figures 1 and 2. A combustion chamber 20 has a floor 21 on which, in use, a bed of combusting solid fuel (e.g. coal) rests. Additional fuel is supplied from a fuel store 22 using a feeder (not shown) which pushes the fuel into the combustion chamber. Heating tubes 17 carry away exhaust gases from the combustion chamber through a water tank thereby transferring heat to the water. The exhaust gases pass up a chimney cavity 5, through a fines arrester (e.g. as shown in Figure 7(a) or (b)) to the atmosphere.

A fan 1 (Fig.2) draws air from the outside atmosphere and feeds this air into an outer chamber 2 formed around a chimney. The air is guided through baffle plates 3 (Fig.2) and tubes 4 (Fig.1) provided within the chimney cavity 5, so as to be heated up by the hot exhaust gases in the chimney, and ducting 6 carries the heated air from the top of the chimney to a base chamber 24 below the combustion chamber 20. An electromagnetic valve 10 controls the flow of air to primary ducting 9 leading to Venturis 15 and to secondary ducting 8 leading to an air chamber 11 and ports 12. The air is then emitted from the Venturis 15 below the fuel bed, and the ports 12 above the fuel bed, for maintaining combustion.

The present invention deals with the problem that when an amount of additional fuel (e.g. ordinarygrade coal) is introduced into the combustion chamber 20 by the fuel feeder, air from underneath the fuel bed blown through the Venturis 15 will not initially be sufficient to ensure complete combustion, resulting in considerable amounts of smoke being generated.

In an example method of operation in this embodiment, five seconds or so before additional fuel is to be delivered to the combustion chamber, the air supply through ducting chamber 9 to the Venturis 15 is reduced, and the air supply through ducting chamber 8 leading to the ports 12 is increased, by operation of the electromagnetic valve 10. The increased air supply to the ports 12 is maintained for ten seconds or so to ensure that as little smoke as possible is generated during this initial phase. The air supplies through ducting chambers 8 and 9 are then restored to normal.

This operation can be completely controlled electronically, with the help of an electronic timer and under settings determined by a control panel, with the valve 10 and fuel feeder being operated automatically in synchronism. A flue cleaner (described below) and means for removal of cold ash and fines (not illustrated) can also be operated electronically so as to allow completely automatic operation of the apparatus.

As shown in Figures 1 and 2, the apparatus is constructed in two halves: a lower portion 100 containing the combustion chamber 20 and base chamber 24, and an upper portion 101 containing the water tank 18 and heating tubes 17. The apparatus is constructed in this way for ease of manufacture and assembly, and to facilitate repairs, and the two halves are joined together by spigots 102 locating in similarly-shaped recesses, as illustrated in Figure 2. Figure 1 also shows a top plate 103 which can be removed for access when servicing the apparatus.

Fig.3 illustrates the array of heating tubes 17 extending through the water tank 18. As can be seen from the Figure, the tubes are arranged in rectangular fashion in five rows of four tubes each. This rectangular arrangement enables the tank to be formed with a rectangular cross-section using flat plates, which is easier to build than a tank of circular crosssection. Figure 3 also illustrates the fuel store 22 and the chimney 5 together with the outer ring-section chamber 2 formed around the chimney for heating up the drawn-in air. The chimney is illustrated as having a circular cross-section, but a chimney of square crosssection would also be possible.

Figure 4 is a view looking downwards from the same plane as Figure 3. Two rows of Venturis 15 open out onto the combustion chamber floor 21, so that air can be supplied to the fuel bed from below. Venturis are employed in preference to ordinary ports for increasing the blowing pressure of the incoming air.

Each Venturi may be protected from damage caused by the fuel bed, by means of a cap having a large number of small holes through which the air passes.

Figures 5 and 6 are close-up views of the combustion chamber and of the means for supplying air thereto. As mentioned above, in this embodiment the ducting 7 for the incoming air leads to an electromagnetic valve 10 which divides the air in any desired proportions to ducting chambers 8 and 9.

Chamber 9 leads to the Venturis 15 whereas chamber 8 leads via ducting 81 to the air chamber 11 from which the air is supplied via ports 12. Figure 6 shows that a row of four ports 12 are provided. As already mentioned, these ports can be adjusted so as to direct air at any desired angle for maximum combustion efficiency. The ducting 81 and ports 12 are surrounded by cooling water as shown in Figure 6, and this cooling water is preferably linked into the tank water system so as to make use of the heat transferred thereto.

As shown in Figure 5, the air chamber 11 and ports 12 are located within a housing 14 having an angled upper surface and a substantially flat lower surface. The lower surface acts as a leveller; that is, it physically obstructs the fuel bed so as to prevent it from becoming too high and irregular. The upper surface also helps to smooth out the fuel bed surface, because exhaust particles which settle out from the exhaust gases strike the upper surface and slide downwards back onto the fuel bed.

A graphite plate 13 is also mounted to the housing 14. A part of this plate protruding into the combustion chamber becomes red hot during use and is then effective for burning any surplus gas, helping to reduce further the amount of smoke in the combustion chamber emissions before these emissions reach the heating tubes 17. The plate 13 could alternatively be of a ceramics material.

Plate 16 also acts as a fuel bed leveller, by smoothing off incoming fuel from the fuel store 22.

Reference numeral 23 indicates guide bars for the fuel feeder (see Figure 6), which is not illustrated but which may consist simply of a pusher for urging fuel from the fuel store into the combustion chamber.

Figure 7 illustrates alternative examples of a fines arrester for mounting to the chimney S. The purpose of the fines arrester is to trap fine particles ("fines") in the exhaust gases passing up the chimney so as to reduce still further the already low smoke emission from the apparatus. Figure 7(a) shows an example suitable for a circular cross-section chimney, and Figure 7(b) an example suitable for a squaresection chimney. In either case, a catchment plate 51 is positioned a short distance above the top of the chimney 5. The emissions from the chimney strike the plate and whereas the gases simply flow around and up to escape through an upper chimney 54, fines are stopped and fall down onto a conical funnel 52 (Figure 7(a)) or an inclined rectangular plate 57 (Figure 7(b)) for disposal through a collection pipe 53 to an ash pan or the like. Figure 7(a) illustrates an adjustment mechanism 55 which can be provided in either case to vary the height of the catchment plate. Adjustment, e.g. by trial and error, may be necessary in order to maximise the effectiveness of the fines arrester, especially when the heat output from the combustion chamber is varied for any reason.

The example shown in Figure 7(b) is somewhat simpler to build, and this makes a square-section chimney preferable to a circular-section one.

It will be appreciated that the secondary air supply from the ports 12, the graphite plate 13, and the fines arrester, together ensure that the final emissions from the burner are almost smoke-free. The result is that "smokeless" running may be achieved without the necessity for relatively expensive smokeless fuel or high-grade coal.

Figures 8(a) and (b) illustrate a flue cleaner for cleaning exhaust deposits from the heating tubes 17. The flue cleaner comprises an assembly of a series of cylindrical segments 60 joined by a rod 61, the segments being mounted to the rod by fixings 62. As shown in Figure 8(b) each segment is generally hollow with a transverse bar at each end through which the rod 61 penetrates. Such an assembly is permanently disposed in each heating tube. The segments and intervening lengths of rod are each a few centimetres long and hence if the flue cleaner is periodically reciprocated up and down by this amount, then the whole inner wall area of the heating tube is scoured by the segments 60, cleaning off any deposits which then fall down the tube back onto the fuel bed. Automatic driving means for the flue cleaners are housed in a chamber 104 above the heating tubes.The driving means may be controlled by an electronic timer to operate for a few minutes once a day.

The flue cleaner need not be of the form illustrated and could be of any known type, e.g.

helical-form.

The flue cleaner in each heating tube also acts as a retarder, by impeding the upward flow of exhaust gases. Consequently the hot exhaust gases spend longer in the heating tubes before proceeding to the chimney, and transfer more of their heat energy to the water in the tank.

Figures 9 and 10 illustrate a water heating apparatus in accordance with a second embodiment of the present invention. This second embodiment is similar to the first as described above, except that the secondary air supply means is modified so as to enable complete combustion without the need for special operating procedures.

In the second embodiment, the ducting 6 leads to base chamber 24 as before but simply opens out into this chamber at an outlet 109. There is no arrangement corresponding to the ducting chambers 8,9 and electromagnetic valve 10 of the first embodiment. Part of the air from outlet 109 passes up a fabrication 110, which leads the air through an outer part of the water tank 18 to a tube 111 mounted within the combustion chamber 20. The tube 111, which provides the secondary air supply means in this embodiment, is positioned along the side of the combustion chamber over an area of the fuel bed outside the normal burning zone, that is outside the area covered by the Venturis 15.

As before, this secondary air supply means has a housing: a lower plate of the housing 14 extends back from the tube 111 towards the fuel store, to act as a leveller, and an angled upper surface collects exhaust particles and returns these to the fuel bed.

In the region below the tube 111, the fuel does not normally burn due to lack of oxygen; instead it dries and bakes in the heat of the combustion chamber, generating coal gas which would normally pass through the heating tubes and up the chimney leading to undesirable emissions. A row of tapered holes 112 is formed in the tube 111, the holes being angled some twelve degrees or so from the horizontal, and air escapes from these holes in the form of downward pointing jets. The air escaping from the jets, which is pre-heated in the tube by the heat of the combustion chamber and which incidentally has a cooling effect on the tube, preventing warping, is forced into the coal gas and pushes a mixture of coal gas and oxygen into the burning zone where it ignites. The result in practice is that flames are produced from the tapered holes, rather as in a gas burner.

This "gas burner" effect increases the temperature within the combustion chamber, leading to more complete combustion of the fuel, and eliminates the coal gas which would otherwise be emitted from the chimney. The emission of coal gas from the coal before it reaches the burning zone has the effect of turning the coal partially or wholly into coke, further promoting complete combustion of the fuel. Unlike the previous embodiment, it is found that a steady supply of air from the secondary air supply means (the volume of which is simply determined by the dimensions of the fabrication 110, tube 111 and tapered holes 112) is adequate to allow smokeless operation at all times, even following the addition of new fuel to the fuel bed. Consequently no special operating procedure for the air supply is required when operating the feeder.

The present invention thus provides a boiler unit capable of virtually smoke-free operation using ordinary fuel. It is thought that the boiler unit will be of particular application to high-capacity water heating systems, such as heating systems for market gardeners' greenhouses.

Claims

CLAIMS:

1. A solid fuel burner comprising a combustion chamber, having a floor which in use is occupied by a fuel bed; primary air supply means located under the combustion chamber floor, operable to supply air to the fuel bed from underneath; and secondary air supply means located inside the combustion chamber and above the fuel bed, operable to supply air to the fuel bed from above in such a way as to effect substantially complete combustion in the combustion chamber.

2. A solid fuel burner as claimed in claim 1, wherein air supply control means are provided for selectively controlling the air supply from the primary and secondary air supply means.

3. A solid fuel burner as claimed in claim 1 or 2, wherein the secondary air supply means is adapted to supply air to a region of the fuel bed outside a burning zone of the combustion chamber, so as to cause combustion of coal gas emitted from this region.

4. A solid fuel burner as claimed in claim 1, 2 or 3, wherein the burner has a fuel feeder operable to feed new fuel into the combustion chamber periodically.

5. A solid fuel burner as claimed in claim 4 as appended to claim 2, further comprising means for controlling the fuel feeder automatically in cooperation with the air supply control cleans, in such a manner that the supply of air from the secondary air supply means is increased shortly before operation of the fuel feeder and maintained at an increased level for a short time period following operation of the fuel feeder.

6. A solid fuel burner as claimed in any preceding claim, further comprising an air impeller for drawing air for the primary and secondary air supply means from the outside atmosphere.

7. A solid fuel burner as claimed in claim 6, wherein the burner is provided with a chimney for emitting exhaust gases from the combustion chamber, and the air drawn from the atmosphere is ducted from the impeller through the chimney to the primary and secondary air supply means, so as to be heated by the exhaust gases before being introduced into the combustion chamber.

8. A solid fuel burner as claimed in any preceding claim, wherein the secondary air supply means comprises an air chamber or housing having a slopingsided upper portion and a substantially flat lower portion.

9. A solid fuel burner as claimed in any preceding claim, wherein the secondary air supply means is provided with a number of adjustable ports capable of being positioned so as to introduce air into the combustion chamber at any suitable angle.

10. A solid fuel burner as claimed in claim 2, or any of claims 3 to 9 in combination with claim 2, wherein the air supply control means comprises an electromagnetically operated valve for distributing air to the primary and secondary air supply means.

11. A solid fuel burner as claimed in claim 10, wherein the electromagnetically operated valve is operated under control of an electric or electronic timer.

12. A solid fuel burner as claimed in claim 3, or any of claims 4 to 11 in combination with claim 3, wherein the secondary air supply means further comprises a tube having formed therein a row of tapered holes, which act as jets for blowing air onto the fuel bed just outside the burning zone.

13. A solid fuel burner as claimed in claim 7, or any of claims 8 to 12 in combination with claim 7, wherein the chimney incorporates means for collecting fines emitted from the combustion chamber so as to prevent these from being emitted into the outside atmosphere

14. A solid fuel burner as claimed in claim 13, wherein said collecting means comprises a catchment plate positioned over the top of the chimney, a funnel located underneath the catchment plate and around the chimney, and a tube connected to the funnel for supplying collected fines to an ash pan.

15. A solid fuel burner as claimed in any preceding claim, wherein the burner forms part of a water heating system in which a number of heating tubes, for carrying exhaust gases through a water tank, are positioned above the combustion chamber.

16. A solid fuel burner as claimed in claim 15, further comprising an automatic flue cleaner provided for cleaning the heating tubes.

17. A solid fuel burner as claimed in claim 16, wherein the automatic flue cleaner comprises, for each heating tube, a number of cylindrical sections fitted inside the tube and connected by a rod, the rod being reciprocable to produce an up-and-down motion of the cylindrical sections which cleans the interior wall of the heating tube.

18. A solid fuel burner, substantially in accordance with any of the embodiments hereinbefore described with reference to the accompanying drawings.

19. A method of operating a solid fuel burner as claimed in any preceding claim, the method comprising the steps of, in sequence: (i) increasing the air supply from the secondary air supply means; (ii) adding a quantity of fuel to the combustion chamber; (iii) maintaining the increased air supply from the secondary air supply means for a predetermined period; and (iv) returning the air supply from the secondary air supply to a normal level.

20. A method as claimed in claim 19, wherein in step (i) the air supply from the primary air supply means is reduced at the same time as the supply from the secondary air supply means is increased.

21. A method of operating a solid fuel burner as claimed in any one of claims 1 to 18, the method being substantially as hereinbefore described with reference to the accompanying drawings.