GB2076687A - Fluidised bed combustion apparatus - Google Patents

Abstract

Fluidised bed combustion is initiated by fluidising a central region of the bed with a gas-air mixture, igniting the mixture to heat the central region and increasing the flow of air to fluidise the whole bed. Suitable apparatus comprises an air supply to the fluidised bed by feeding air to a plenum chamber (4) beneath said fluidised bed, the plenum chamber being divided into compartments which communicate with the separate regions of the fluidised bed, air flow restrictor means constituted by orifice plates (9) being provided in the flow passages (5) leading to the outer compartments of the plenum chamber (4) so that depending upon the rate at which air is delivered to a common manifold (6) the air supply is sufficient to fluidise the central region of the bed or the whole bed. <IMAGE>

Description

SPECIFICATION Fluidised bed combustion apparatus This invention relates to fluidised bed combustion apparatus and particularly to boilers embodying fluidised bed combustion.

In recent years many attempts have been made to burn both conventional and unconventional fuels in so-called fluidised beds. These consist generally of a contained quantity of refractory particulate material, for example, sand, which may be fluidised by passing a gas into it. The bed may be fluidised by passing air into it and it may be heated either by the introduction of a solid fuel which combusts in the bed and/or by the introduction of a liquid or gaseous fuel into the bed. For example a mixture of air and natural gas may be introduced which simultaneously functions to fluidise the bed and to combust in it.

The heat generated by fluidised bed combustion may be used in a variety of known ways, e.g. to heat a working fluid in thermal exchange with the bed, either for process heating purposes or for example for producing hot water or steam. Alternatively the fluidised bed combustion may be used to incinerate solid materials fed into the bed and this provides a convenient way of disposing of difficult waste products such as dangerous chemical substances, toxic wastes and insanitary wastes such as produced by hospitals.

There are two separate areas of difficulty which are widely appreciated in the operation of fluidised bed combustion apparatus. The first is that of the start-up procedure and the second that of controlling the fluidised bed combustion appropriately once continuous steady state conditions have been reached. The present invention seeks to provide improvements in both these aspects of fluidised bed combustion operation.

According to one aspect of the invention, there is provided a method of initiating a continuous fluidised bed combustion process which comprises providing a fluidised bed, introducing into a central region of the fluidised bed sufficient gas and air mixture to effect fluidisation thereof, the gas and air mixture being one which can be ignited, igniting the gas and air mixture to heat only the central region of the bed and thereafter increasing the flow of air to the remainder of the bed sufficiently to fluidise the whole bed.

This may be very conveniently effected by dividing a plenum chamber below a perforated base plate into a central region and one or more outer regions and arranging a branched feed of air to the compartments of the so divided plenum chamber, restrictor means constituted by fixed controlled air flow orifice plates being provided in those branches other than that leading to the central area. In this way, a common blower fan may be used to feed air into the bed, the fan itself or the output of the fan being controlled initially to blow only so hard that the unrestricted flow to the central area is just sufficient to fluidise only the central area of the fluidised bed; thereafter the flow is increased to fluidise the whole of the bed.

According to a further aspect of the invention, there is provided fluidised bed apparatus including a plenum chamber from which air is to be fed upwards into a combustion chamber, the plenum chamber being divided into a central compartment and at least one outer compartment, and means whereby a branched supply of air can be fed to the compartments of said plenum chamber, the or each air flow passage leading to said at least one outer compartment of the plenum chamber being provided with restrictor means constituted by a fixed controlled air flow orifice plate so that, depending upon the rate of flow of air through a common air supply manifold from which the branched supply of air passes, the main flow of air can be through the central compartment of the plenum chamber so that only the central part of the bed is fluidised or, alternatively, the flow of air can take place through all the compartments simultaneously so that the whole bed is fluidised.

The or each controlled air flow orifice plate may be provided with a centrally disposed single orifice of circular shape or, alternatively, with a plurality of equally spaced orifices. The fluidised bed may be of circular or substantially circular shape in which case the plenum chamber may be divided into a central compartment with parallel side walls extending across the width of the chamber and a pair of lateral compartments of segmental shape.

On the other hand, if the bed is of circular or substantially circular shape the plenum chamber may be divided into a central compartment of circular shape and a peripheral compartment of annular shape around said central compartment.

The floor of the combustion chamber will preferably be constituted by a perforated plate which has set into its perforations respective sparge pipes closed at their upper ends but having holes in their side walls for the flow of air into the combustion chamber. In this case, gas will preferably be fed to the combustion chamber through a plurality of vertically extending tubes which extend upwardly into respective sparge pipes which extend into the combustion chamber from the central compartment of the plenum chamber. The bed when fluidised will preferably have a depth of at least 30 centimetres. The apparatus may be in the form of a water heating boiler and in this case the boiler will preferably be a vertical tube boiler with the boiler tubes surrounding the fluidised bed.

In a particularly preferred mode of operation, the fluidised bed may be fed with gas and air for initial start-up as described and thereafter with particulate solid combustible material, for example coal. Once combustion has been established, it is then possible to cut off the gas supply entirely and run simply on the combustion of the combustible particulate fuel fed to the top of the bed.

As noted above in order to fluidise the bed at all it is necessary to blow air (and at least during start-up gas) into the bed to fluidise it and it is common practice, to ensure an adequate gas stream flow through the entire apparatus, to pull the flue gases away from the area above the fluidised bed and force them up an appropriate chimney stack. The commonest way of achieving this is to provide a so-called induced draft fan which is arranged to suck out the combustion products, and push them up a chimney, usually in collaboration with some form of centrifugal separator to remove particulates.

Heretofore, in arrangements of this nature the combustion conditions have been controlled by measuring the pressure e.g. at the top of the combustion chamber and using the measured pressure to control the so-called forced draft fan i.e. the fan which blows air into the fluidised bed.

The speed at which the forced draft fan was then run in turn was used to control the feed of fuel into the bed, either by ganging it together with a fuel injection pump in the case of gas, or by coupling it to e.g. a screw conveyor in the case of particulate solid fuels.

According to a still further aspect of the invention, in steady state conditions the feed of combustible fuel to the fluidised bed is controlled directly by the difference between a preselected bed temperature and the actual temperature measured within the body of the fluidised bed. If the temperature rises too high, fuel feed is reduced or even cut off while if the temperature drops below the set point, fuel feed may be increased until the temperature rises sufficiently.

At the same time, the forced draft and induced draught fans are set to operate substantially independently of the fluidised bed conditions, with the exception that appropriate means are provided in known fashion to cutout both fans should for some reason the bed temperature rise above a predetermined maximum allowable figure.

Means may be provided in known fashion to ensure that the induced draft fan always comes into operation a short while before the forced draft fan in order to clear any uncombusted but combustible gas mixture from the combustion chamber before the bed is refluidised and combustion recommences. If this were not done, there is the possibility of an explosive mixture building up over a quiescent bed which would be ignited as soon as the bed was fluidised and already hot material projected into the space above it.

The present invention is particularly applicable to the construction of heating boilers, particularly for heating water to produce steam in quantities of 4000 to 20,000 Ibs per hour. Such fluidised bed apparatus may make use of a variety of boiler types, but we have found the most effective type to be a vertical tube boiler with the boiler tubes surrounding the fluidised bed. We have further found that the fluidised bed (when fluidised) should have a depth of at least 30 centimetres and preferably at least 45 centimetres. It may consist of sand or other known fluidised bed material.In order that the invention may be fully understood and readily carried into effect, the same will now be described, by way of example only, with reference to the accompanying drawings, of which: Fig. lisa perspective view of fluidised bed apparatus embodying the invention, Fig. 2 is a side view thereof, Fig. 3 is a rear view thereof, Fig. 4 is a plan view, Fig. 5 is a further perspective view partly broken away to show various component parts, Fig. 6 is a diagrammatic view showing the control functions, and Fig. 7 is a sectional view, drawn to a somewhat larger scale than Figs. 1 to 6, which illustrates a detail of the floor of the fluidised bed.

Referring to the drawings, the boiler consists basically of a central combustion chamber 1 in the form of a substantially vertical upright cylinder.

The floor of the combustion chamber consists of a plate 2 which is perforated and which has set in each perforation a short stainless steel so-called sparge pipe 3 which is closed at its upper end but which has near its upper end a number of holes in its wide walls. Located below the base plate is plenum chamber 4 which is divided into three separate chambers fed by respective ducts 5, the three ducts emerging from a common manifold 6 which is fed with air from a forced draught blower 7 via an adjustable valve 8. Each of the two outer ducts has set within it an orifice plate 9 which is absent from the centre duct. As indicated in Fig. 5, each orifice plate 9 is provided with a plurality of equally spaced orifices of a size and number such that at a particular air delivery pressure they allow a controlled air flow.The orifice plate nearer to the forced draught blower 7 has a greater number of orifices than the orifice plate remote from the blower to allow for the drop in pressure along the manifold. (In the apparatus illustrated the orifices in the orifice plate nearer the blower are thirty two in number whilst the orifices in the orifice plate remote from the blower are twenty two in number.

All the orifices are 9/1 6" in diameter).

Surrounding part of the centre duct is a box 10 in which a gas distribution manifold 11 is arranged. This is connected externally to a source of gas and the outlets from the manifold consist of a plurality of vertically extending tubes 12 which extend from the manifold which is located in the central plenum chamber upwardly into respective vertically extending sparge pipes 3, terminating a little way below the top of said pipes.

The upper part of the combustion chamber is surrounded by a water jacket 13 which in operation contains the water to be heated to produce steam. A water inlet pipe is shown at 1 4 and a steam outlet is shown at 1 5. A plurality of so-called thermic syphon tubes 1 6 are provided and extend through the combustion chamber as shown. The lower ends of said tubes communicate with the water jacket, as shown, above the level of the fluidised bed; their upper ends extend through the top of the combustion chamber to communicate with the part of the water jacket overlying the combustion chamber.At the top oi the combustion chamber fluid gases are extracted and they pass down and around water filled pipes 18 and then up past a further set of water filled pipes 1 9 giving up heat all the time as they do so to the water in the boiler. The flue gas is then passed into a helical duct 20 where the particulates are forced to the exterior of the duct by centrifugal action and are then removed laterally at the top position of the first turn of the duct and fed to a centrifugal separator 21 which deposits them in an ash bin 22. The main flue gases are taken from the middle of the first spiral section by means of a centrifugal fan 23 and blown at very high speed up a conventional smoke stack 24.

Located in the floor of the combustion chamber are a plurality of thermocouples 25 set in blind tubes extending from the plenum chamber and these act as bed temperature sensors. An inspection door 26 is provided and an ignition burner 27 is set angled down at the side of the inspection door. Air and gas may be fed to the ignition burner in order to play a gas flame on the surface of the fluidised or unfluidised bed.

In addition, particulate fuel may be fed to the bed by means of a particulate fuel auger 28 which is rotated by a motor 29 and which extends from a fuel bin 30 up to a feed tube 31 down which fuel raised by the auger falls into the fluidised bed during operation.

The sequence of operation of the fluidised bed boiler shown in the drawings is conveniently explained with reference to Fig. 6 which shows various control elements and a central controller 32.

First of all a sensor detects whether the boiler has adequate but not too much water in it and provided that that condition is fulfilled, the induced draught fan 23 is actuated. Shortly thereafter the forced draught blower 7 is actuated but the valve 8 is controlled so that only the central portion of the bed is fluidised. The ignitor 27 is actuated to direct a gas flame down onto the surface of the bed and thereafter gas is caused to flow through manifold 11 and is entrained with the air already being pumped into the bed by the forced draught blower 7. Because of the ignitor flame this gas/air mixture emerging from the top of the sparge tubes 3 in the centre section of the bed ignites and commences to heat up the centre action of the bed.Either before, simultaneously with or after the temperature of the bed has reached a certain figure, for example 4000C, particulate fuel may be fed to the bed. At a suitable time also e.g. when the bed temperature reaches 4000C, the flow of air from the forced draught fan 7 may be increased to render the whole of the bed fluidised. The fluidisation of the whole of the bed causes the remainder of the bed to heat up, under the influence of the combustion of the gas fed into the manifold and of the particulate fuel fed in from above. When the temperature of the bed rises to near the operating temperature, for example 8000 C, the flow of gas is then cut off automatically.

The ignitor 27 is allowed to continue to operate for a little while after the flow of gas is cut off since the bed temperature at this stage may drop a little and the gas flow to the manifold is then resumed. This may happen several times before the whole of the bed is hot enough to maintain combustion over the preselected gas cut-off point but once that has been established for a suitable amount of time the igniter 27 is cut off. All this time the water in the boiler has been heated up and generally by the time the bed reaches the desired operating temperature the boiler is producing steam.

Once those steady state conditions have been reached, the boiler may simply be left to run for as long as desired, the amount of solid fuel being fed to the fluidised bed being controlled in direct dependence upon the deviation of the sensed bed temperature from a desired bed temperature.

It will be understood that by virtue of the automatic air flow restrictor means constituted by the orifice plates in the air flow passages to the outer compartments of the plenum chamber it is a very simple matter selectively as required to cause the whole bed to be fluidised when the apparatus is required to operate at full load or to cause the outer parts of the bed to be "slumped" purely as a result of varying the rate of flow of air through the common air supply manifold. Consequently, the temperature in the outer parts of the bed can very quickly be increased from minimum to maximum or reduced from maximum to minimum to accomplish a smooth load change.

A particular advantage of the boiler just described is the ease with which it may be shut down and started up again. For example, by simply turning off the forced draught blower 7 the fluidised bed collapses to a quiescent flat unfluidised bed which gradually loses heat.

However, for some considerable time the bed can be refluidised and put back into operation simply by turning on the fan again, the igniter and the gas feed being automatically brought back into operation if the temperature has dropped below the desired gas cut-off point. Flexibility of operation in this way is particularly desirable where the boiler is used to supply a process heat on a discontinuous basis and/or where considerations of energy saving makes it desirable to tailor the output of the boiler more accurately to the needs of the user.

Various modifications may be made. For example, the fluidised bed need not necessarily be divided into central and outer parts in the manner described. Instead of the central compartment of the plenum chamber extending across the width of the chamber it could be a central compartment of circular shape with a peripheral compartment of annular shape around it.

The or each orifice plate need not necessarily be provided with a plurality of equally spaced orifices. On the contrary, the or each orifice plate could have a single centrally disposed hole or orifice.

The characteristics of the orifice plates are such that they suit the particular heat output requirements of the particular boiler. The exact hole size or the number of orifices required in each orifice plate for a particular heat output requirement can be determined by trial and experiment. It is a very simple matter to fit appropriately different orifice plates in the air flow passages to the outer compartments of the plenum chamber of an otherwise identical fluidised bed apparatus when different heat output is required. It has been found that the fitment of alternative flow restrictor means in the air flow passages to said outer compartments does not give satisfactory operation. The use of orifice plates in the manner described apart from the fact that they can so easily be designed to suit different heat output requirements, gives smooth air flow through the air flow passages concerned and results in excellent operation of the apparatus.

Claims (14)

1. A method of initiating a continuous fluidised bed combustion process which comprises providing a fluidised bed, introducing into a central region of the fluidised bed sufficient gas and air mixture to effect fluidisation thereof, the gas and air mixture being one which can be ignited, igniting the gas and air mixture to heat the central region of the bed and thereafter increasing the flow of air to the remainder of the bed sufficiently to fluidise the whole bed.

2. A method according to claim 1 wherein the fluidised bed overlies a perforated plate covering a plenum chamber, the plenum chamber being divided into a central compartment and at least one outer compartment and wherein a branched supply of air is fed to the compartments of the so divided plenum chamber, restrictor means constituted by fixed controlled orifice plates being provided in the or each air flow passage leading to said at least one outer compartment of the plenum chamber, the air supply being provided first at a rate sufficient to fluidise the bed only in the central region and then at a rate sufficient to fluidise the whole bed.

3. A method according to claim 1 or 2, wherein the fluidised bed is fed with gas and air for initial start-up and thereafter in addition with particulate solid combustible material.

4. A method according to claim 1,2 and 3 wherein, at and above a first predetermined bed temperature, combustible particulate fuel is fed to the bed, and the air flow is increased when a second predetermined bed temperature is reached, which may be above, equal to or below said first predetermined bed temperature.

5. A method of starting up a fluidised bed combustion apparatus, substantially as herein before described.

6. A method of operating a fluidised bed combustion apparatus, wherein during combustion after start-up, in steady state conditions, the feed of combustible fuel to the fluidised bed is controlled directly in accordance with the difference between a preselected bed temperature and the actual temperature measured within the fluidised bed.

7. A method according to claim 6, wherein forced draft and induced draft fans are set to operate substantially independently of the fluidised bed conditions during steady state running.

8. Fluidised bed apparatus including a plenum chamber from which air is to be fed upwards into a combustion chamber, the plenum chamber being divided into a central compartment and at least one outer compartment, and means whereby a branched supply of air can be fed to the compartments of said plenum chamber, the or each air flow passage leading to said at least one outer compartment of the plenum chamber being provided with restrictor means constituted by a fixed controlled airflow orifice plate so that, depending upon the rate of flow of air through a common air supply manifold from which the branched supply of air passes, the main flow of air can be through the central compartment of the plenum chamber so that only the central part of the bed is fluidised or, alternatively, the flow of air can take place through all the compartments simultaneously so that the whole bed is fluidised.

9. Fluidised bed apparatus according to claim 8, in which the or each controlled air flow orifice plate is provided with a centrally disposed single orifice of circular shape.

10. Fluidised bed apparatus according to claim 8, in which the or each controlled air flow orifice plate is provided with a plurality of equally spaced orifices.

11. Fluidised bed apparatus according to any one of claims 8 to 10, in which the bed is of circular or substantially circular shape and the plenum chamber is divided into a central compartment with parallel side walls extending across the width of the chamber and a pair of lateral compartments of segmental shape.

12. Fluidised bed apparatus according to any one of claims 8 to 10, in which the bed is of circular or substantially circular shape and the plenum chamber is divided into a central compartment of circular shape and a peripheral compartment of annular shape around said central compartment.

13. Fluidised bed apparatus according to any one of claims 8 to 12, in which the floor of the combustion chamber is constituted by a perforated plate which has set into its perforations respective sparge pipes closed at their upper ends but having holes in their side walls for the flow of air into the combustion chamber.

14. Fluidised bed apparatus according to claim 13, in which gas can be fed to the combustion chamber through a plurality of vertically extending tubes which extend upwardly into respective sparge pipes which extend into the combustion chamber from the central compartment of the plenum chamber.

1 5. Fluidised bed apparatus according to any one of claims 8 to 14, in which the bed when.

fluidised has a depth of at least 30 centimetres.

1 6. Apparatus according to any one of claims 8 to 1 5, in the form of a water heating boiler.

1 7. Apparatus according to claim 16, and in the form of a vertical tube boiler with the boiler tubes surrounding the fluidised bed.

1 8. A method of operating and controlling combustion in fluidised bed combustion apparatus substantially as hereinbefore described.

1 9. Water heating fluidised bed combustion apparatus substantially as hereinbefore described and illustrated with reference to the accompanying drawings.