GB2034445A - Fluidised bed combustion apparatus - Google Patents

Abstract

An incinerator comprises two fluidised bed chambers (1 and 45) material to be incinerated is introduced into one of the chambers (at 42) and the exhaust from that chamber (1) is used for fluidising the other chamber (45). Incineration in the first chamber is generally conducted in the presence of inadequate oxygen, and further oxygen is added for the second chamber (at 59). <IMAGE>

Description

SPECIFICATION Fluidised bed apparatus and process There have recently been many proposals for disposing of combustible refuse by fluidised bed incinerators, that is to say incinerators in which the refuse is burnt in a fluidised bed of hot inert refractory particulate material.

A difficulty with fluidised bed incinerators is that there is a substantial risk, particularly with large volume incinerators, of dust particles being carried from the fluidised bed by the gases that are used for fluidising and that are formed on combustion. An effective system for removing these dust particles from the exhaust gases is necessary if effluent problems are to be avoided.

Another difficulty with known incinerators is that there is a risk that incompletely burnt gases can escape from the incinerator and again this can cause pollution problems.

A new fluidised bed apparatus comprises a first fluidised bed chamber, a second fluidised bed chamber, each fluidised bed chamber having at its base a fluidised bed base on which a fluidised bed can be formed, means for supplying gas for fluidising the first fluidised bed to and up through the base of the first fluidised bed chamber, an inlet for supplying material to be incinerated into this gas or direct into the first chamber, and means for passing substantially all the exhaust gases from the first fluidised bed to and up through the base of the second fluidised bed chamber for fluidising the second fluidised bed, and means for heating the first bed.

Combustible material is burnt in the first fluidised bed and the second bed may trap particles in the exhaust gases and/or cause a chemical reaction in the exhaust gases. For instance the gases may be desulphurised if lime is included in the second bed or the gases (or particles entrained in them) may be oxidised by being burnt.

The process is of value for incineration of refuse or other combustible solid or liquid or gaseous fuel, with the gases preferably being fully burnt before being vented to the atmosphere, or for the production of gaseous fuel from a solid or liquid fuel, the combustion being conducted in the presence of insufficient oxygen for complete combustion, a lean gas thus being produced. The hot lean gas leaving the apparatus, which is thus serving as a substoichiometric combustor, may be used in, for instance, a lime or cement kiln.

Irrespective of whether the final exhaust is to be a lean gas or fully combusted combustion in the first fluidised bed is preferably conducted in the presence of insufficient oxygen for complete combustion, the exhaust being formed thus comprising incompletely burnt fas and, often, dust particles that may be combustible, and this exhaust may be burnt in the second fluidised bed in the presence of additional oxygen.

Combustion will generally be sufficiently exothermic to maintain the desired bed temperatures and so it will generally only be necessary to supply substantial quantities of heat at the start of the process, to initiate combustion, although it is generally preferred to preheat the fluidising gases to some extent throughout the process, for example by simple heat exchange between the exhaust gases.

When the treatment is insufficiently exothermic, or is endothermic, substantial quantities of heat may be needed throughout the treatment. Gas or oil burners may be located in or around the first bed, and sometimes the second bed also, or in the air stream used forfluidising the first bed.

The first bed generally comprises inert particulate material for instance refractory material such as crushed fire brick. The particle size of the inert material may be 300 microns to 5 mm, usually about 2mm.

The material to be burnt may be fed onto or into the first bed while this is at the treatment temperature. As a result material is subjected to high thermal shock and this can have the very beneficial result of causing disintegration, especially of compacted solids, into small pieces when it is a solid material. Typical solid includes refuse, sewage or coal dust. The material may also be introduced in liquid form, for instance as a liquid suspension of solid combustible material, e.g. sewage sludge, or a combustible liquid, e.g. waste organic solvent. When the material to be burnt is in solid or liquid form it is introduced into the apparatus through an inlet that leads into the chamber, generally close to the base, for instance, into the bed or just above the first bed.The material to be burnt may alternatively be gaseous, in which event it may be introduced either through such an inlet or with the fluidising gas.

Oxygen that is to be supplied to the second bed may be introduced in the form of, for instance, air.

Some oxygen may be supplied through the base of the second fluidised bed, so as to facilitate fluidisation, but generally the oxygen is supplied above the base of the second bed.

By supplying some or all of the oxygen containing gas above the base the volume of this gas can be controlled wholly independently of the control necessary to maintain optimum fluidisation. Thus the supply of air or other oxygen containing gas can be increased or decreased without having regard to the need to maintain fluidisation but merely to obtain optimum combustion of the fluidising gases.

The oxygen containing gas that is introduced above the base may be introduced from the sides of the chamber or from above and may be introduced into the bed or above the bed. Preferably, however, it is introduced from above to one or more positions centrally above the bed. Thus the apparatus preferably includes a duct that has a downwardly facing opening positioned above the fluidised bed and that leads from means outside the fluidised bed chamber for supplying oxygen containing gas to the chamber.

By providing a downwardly flowing gas stream in this manner one tends to reduce the upward velocity of the gases within the chamber, and thereby reduces the risk of entrainment of solid particles from the fluidised bed in the gases leaving the chamber.

The second fluidised bed is generally formed substantially entirely of inert particulate material, such as discussed above, but may also contain some solids from the exhaust gases from the first bed. Means are provided in the apparatus for feeding inert particulate material to the first and second beds. There may be an inlet for inert particulate material to each chamber but instead of or in addition to providing such an inlet to the first chamber the means for feeding inert material to that chamber may comprise means for transferring solids from the second bed to the first bed. Thus inert material may be fed initially to the second bed and subsequently transferred to the first bed.

During operation incombustible material will accumulate, especially in the first bed, and the bed will need to be replaced from time to time. This can be done batchwise manually or automatically or can be done continuously and automatically. A continuous automatic system is descrbied, for instance, in British Patent Specification No. 1,299,125. Preferably however whenever it is desired to be able to transfer solids from a bed, e.g. from the first bed out of the apparatus or from the second bed towards the first bed, the apparatus includes means for tipping the base of that bed downwardly, and thereby emptying solids in that bed from the bottom of the chamber containing it. Preferably both the first and second beds are provided with means for tipping their solids downwardly.

An advantage of tipping a bed base downwardly is that the weight of the bed will facilitate tipping. The tipping means normally include means for holding the bed horizontally in sealing engagement with the bed portion of the fluidised bed chamber and means for lowering the bed base out of sealing engagement andfortipping it The means for holding the bed in sealing engagement may be by, for instance, releasable clips or abutments beneath the bed but preferably the bed is merely held in position by a torsion or toggle mechanism which can also be adapted to provide the desired hinging mechanism.Preferably the bed is supported in sealing engagement with the bed portion ofthefluidised bed chamber by an arm that is pivotally connected to the centre of the lower side of the bed and that can be hinged, by a torsion or toggle mechanism, to force the bed into sealing engagement or to lowerthe bed and tip it.

Beneath the first bed there is preferably a sealed plenum chamber into which the solids may be tipped and through which the gases that are to fluidise the solids on the bed are passed. The plenum chamber will generally have an outlet in its base that can either be opened occasionally to permit batchwise removal of solids or through which solids can be removed continuously, for instance by a screw conveyor. The resultant mixture of solids can then be screened or treated in conventional manner, e.g.

to separate desired inert particulate refractory mater ial from the remainder and the desired material can be recycled.

Each fluidised bed base will comprise a solid support for the material to be fluidised and a plurality of openings through the support for the fluidising gas.

Preferably each opening is provided with means for preventing solids in the bed passing down into that opening.

The optimum degree of fluidisation for any particular treatment can be found by experiment, but a "boiling" or "slugging" bed is often best.

Preferably the openings through the support include means for causing the gas to emerge from the top of each opening with a substantially horizontal motion. These means may be a baffle over each opening but conveniently the means for causing this horizontal motion and for preventing solids entering the openings are the same. Thus as described in patent specification No. 1,488,347 a valve may be provided in each opening, each valve being biased to the closed position but openable by upwards gas pressure, each valve having a stem which passes through the opening and permits only limited freedom for upward movement, a cap fitted on the top of the stem, which may rest on the top of the base to close the opening to downward passage of solids on the base, and a duct leading along the stem for conducting gases along the stem and for discharging them beneath the cap.

The lower part of a fluidised bed chamber can conveniently be termed the bed portion. The bed portion of each chamber in the invention preferably has a smaller cross sectional diameter than the higher parts of that chamber and most preferably each chamber comprises the bed portion, an expansion portion of larger cross section and a diffuser portion joining the bed and the expansion portions.

The combination of the diffuser portion, in which the walls slope outwardly at an angle giving optimum diffusion effect, and the larger cross section area of the expansion portion results in higher pressure and lower gas flow velocities through the expansion por tion, thereby reducing the risk of entrainment of solids through the expansion portion. This is particularly important in the invention because of the very high degree of entrainment that might otherwise be obtainable due to the large volumes of gas generated by combustion in the bed. Preferably the height of the bed and diffuser portions are each from 1.3 to 5 times, and most preferably 1.8 to 2.5, e.g. about 2 times the height of the fluidised bed. Thus typically the bed might be 30 cm deep and the bed portion, including the bed, 45 to 100 cm deep. These dimensions are very suitable when the bed diameter is about 70 cm. The height of the expansion portion is preferably at least 10 times the depth of the bed and usually at least three times the combined height of the bed and diffuser portions.

Each chamber and the various portions of it can have any desired cross section, but generally it is circular.

Preferably a single bed fits within each bed portion and preferably has the same cross sectional area as the bed portion. However if desired a plurality of fluid bed bases may be provided in the bed portion, generally all in the same horizontal plane. They may fit against one another or may fit within a framework provided in the bed portion.

The first and second chambers may be connected by, for instance, ducting but since the system should be sealed (to prevent the escape of exhaust gases from the first bed) the chambers are best mounted one above the other in the same vessel. Thus the apparatus may comprise a tower with the second fluidised bed chamber positioned above the first fluidised bed chambers It may include additional chambers above the second.

The invention is illustrated in the accompanying drawings in which: Figure 1 is a diagrammatic representation of apparatus according to the invention.

Figure 2 is an enlargement of part of this apparatus.

Figure 3 is an enlargement of a cross section of part of a fluidised bed base suitable for use in the apparatus, and Figure 4 illustrates a modification of part of Figure 1.

The apparatus shown in Figures 1 and 2 comprises a first fluidised bed chamber 1 and a second fluidised bed chamber45. Thefluidised bed chamber 1 has a bed portion 2, a diffuser portion 3 and an expansion portion 4. A fluidised bed base 5 is fitted at the bottom of the bed portion 2, and thus at the bottom of the chamber. Afan 6 is used to drive air around an oil injector or gas burner 7 and into a primary combustion chamber 8, thereby producing heated air, and into a plenum chamber 9.

There is an inlet 40 by which compressed solids can be forced by piston 41 on to the top of the bed in the bed chamber 1. Preferably the duct forming the inlet 40 is so constructed that the compressed solids in the duct form a substantially gas tight seal to prevent the escape of gases from the chamber. Since the inert particulate refractory material for the bed can be provided by tipping of a higher bed (see below) it may not be necessary to provide in the chamber 1 a separate inletforthe refractory solids.

However if there is such an inlet it may be as shown at 42, with a plate 43 to prevent escape of gas through it during normal operation, and the inlet may be charged from a hopper 44.

The base 5 is in sealing engagement with the walls of the bed portion, so that the pressurised gas in the plenum chamber can substantially only pass through the fluidising apertures in the base. As shown in Figure 2 it is held in that position by an arm 16 pivoted to it at a central position buy a ball joint 17 and hinged at 18 by a torsion spring mechanism fitted outside the plenum chamber and fluidised bed chamber and operated by lever 19. Upon raising the lever 19 the torsion spring mechanism passes its dead centre position and readily drops into the lowered position 20 shown in Figure 2, thereby discharging the solids on the base 5 into the plenum chamber 9. Upon depressing the lever 19 the base, free of the solids on it, can then be raised back into position and locked in sealing engagement against the bottom of the bed portion 2.

The chamber 1 is coaxial with the second fluid bed chamber 45 consisting of a bed portion 46 integral with the expansion chamber 4 of the first chamber and containing bed base 70, a diffusion portion 47 and an expansion portion 48. There is an inlet 49 for inert particulate refractory material, provided with a flap 50 for closing and a hopper 51 for charging it, and an exhaust outlet 52 leading to a cyclone 53 from which gases escape by outlet 54 while solids are collected in a water tramp 55. Water may be sprayed in the cyclone to assist collection of the solids, in conventional manner. There is an induced draught fan 56 in the outlet 54 to control the rate of exiting of exhaust gases.

A heat exchanger coil 57 is provided in the chamber45to remove heat from the exhaust gases.

The gases leaving the outlet 54 may also pass through a heat exchanger. The use of heat exchangers is valuable in all combustion processes but is of particular importance when the material to be burnt is a fuel such as coal dust and the apparatus is being used for heat or power generation.

Each fluidised bed base may be of conventional construction but preferably includes means for preventing downward passage of solids and for causing horizontal movement of the fluidised gas as illustrated in Figure 3. Thus the base, for instance base 5, may have a plurality of apertures 30 in it and at each of these there is fitted a valve comprising a stem 31, a cap 32 and a stop 33. The stem 31 is hollow and leads gas from beneath the base to a series of openings 34 at the top of the stem beneath the cap. When the gas pressure is sufficient it causes the stem and cap to rise so that gas then escapes beneath the skirt 35 of the cap with a substantially horizontal direction.

In an example the lower bed may have a diameter of 70 cm and a depth, before fluidising of 40 cm while the upper bed may have a diameter of 90 cm and a depth before fluidising of 20 cm. The lower bed may have 7 openings 30 equally spaced over it and, in order to reduce the velocity through the upper bed to near incipient fluidisation and hence improve its filtration characteristics it may have more openings 30, e.g. 20 openings.

Solids to be burnt are introduced in compacted form through inlet 40 onto the bed in the base of chamber 1 which is preheated and fluidised by heated gas from chamber 9. When the material being burnt has moderate or high calorific value it is generally only necessary to use the oil injector or other burner during start up although preheating of the fluidising air, for instance by the coil 57, may be desirable. Preheating of the second bed may be needed at start up, suitable means for this being an ignition burner fitted above the bed.

The refractory particles forming the bed on base 70 are fluidised by the exhaust gases from expansion chamber 4. The rate of supply of air or other oxygen containing gas into that chamber through the plenum chamber 9 is less than is required for combustion of the combustible solids introduced at 40, the chamber 1 thus operating in a starved air or sub-stoichiometric state. Air or other oxygen containing gas is introduced into the chamber45 through duct 58 that terminates in a downwardly facing opening 59 just above the bed, thereby forming a blanket of oxygen rich gas overthe bed to promote combustion not only over the bed but also in the bed.It is particularly advantageous that the base 70 should be provided with valves such as those shown in Figure 2 in order to provide horizontal motion to the fluidising gases as this creates a cyclic motion within the bed which helps carry the oxygen down into the bed. The rate of supply of oxygen through the duct 58 can be controlled by the fan 60.

Instead of or in addition to supplying all the oxygen through the duct 58 some or all of it may be used for fluidising. Thus as shown in Figure 4, there may be an annular manifold 71 beneath the base 70 to which an oxygen containing gas is supplied from a duct 72. When there is an independent gas supply as shown in Figure 4 it may be convenient to have the bed fixed in conventional manner but preferably there is no separate gas supply to the bed and the bed is capable of being tipped by mechanism comprising a pivot point 63, an arm 64 and a torsion spring hinge 65, all generally as described in Figure 1.

It is readily possible to select the flow rate of air, and thus the fluidisation, through the first bed (e.g.

by variation in the speed of the blower 6) to give a chosen volume of exhaust gases of chosen calorific valueforfluidising the upper bed. The upper bed will act as a filter which will virtually eliminate any carry-over of solid particles. Moreover, limestone for example may be added to this upper bed as a desulphurising agent when a clean gas rises into the chamber 45. Air, oxygen-enriched air or any other oxygen carrying gas may be introduced into chamber 45 via the downcomer 59 from blowing fan 60. Fan 56, the induced draft fan, is so adjusted that it is capable of handling the whole volume of combustion products through the cyclone and associated ducting, 52,53 and 54 whilst holding only minimum negative pressure at exit 52 from chamber 45. It can now be seen that any volume of gas, as may be necessary to ensure total combustion and eliminate any pollutant, may be introduced above the bed into chamber 45, via 59, without affecting correct fluidisation of the upper bed.

Claims (13)

1. Afluidised bed apparatus comprises a first fluidised bed chamber, a second fluidised bed chamber, each fluidised bed chamber having at its base a fluidised bed base in which a fluidised bed can be formed, means for supplying gas for fluidising the first fluidised bed to and up through the base ofthefirstfluidised bed chamber, an inlet for supplying material to be incinerated into this gas or direct into the first chamber, and means for passing substantially all the exhaust gases from the first fluidised bed to and up through the base of the second fluidised bed chamber for fluidising the second fluidised bed, and means for heating the first bed.

2. Apparatus according to claim 1 including an inlet for adding oxygen to the exhaust gases from the first bed just before or after they pass through the second fluidised bed base.

3. Apparatus according to claim 2 in which the inlet is positioned to discharge oxygen downwardly onto the second fluidised bed.

4. Apparatus according to any preceding claim comprising a tower with the second fluidised bed chamber positioned above the first fluidised bed chamber.

5. Apparatus according to claim 4 in which each fluidised bed chamber is substantially cylindrical and comprises a base portion, an expansion portion of larger diameter and a diffuser portion between the base and expansion portions and in which the walls slope outwardly.

6. Apparatus according to any preceding claim including means for feeding inert particulate material to the first and second fluidised bed chambers.

7. Apparatus according to claim 6 in which the means for feeding inert particulate material to the first fluidised bed chamber comprises means for transferring solids from the second bed to the first bed.

8. Apparatus according to any preceding claim including means for transferring solids from the first bed out of the apparatus.

9. Apparatus according to claim 7 or claim 8 in which the means for transferring solids from a fluidised bed comprises means fortipping the base of that bed downwardly and thereby emptying solids in the bed from the bottom of the chamber containing the bed.

10. Apparatus according to claim 1 substantially as herein described.

11. A method of burning combustible material in apparatus according to claim 1 comprising burning the material in the first fluidised bed and fluidising the second bed with the exhaust gases.

12. A method of burning combustible material in apparatus according to claim 1 comprising burning the material in the first fluidised bed in the presence of insufficient oxygen for complete combustion, thereby forming an exhaust of incompletely burnt gas and fluidising the second bed with this exhaust.

13. A method according to claim 12 in which the exhaust is burnt with additional oxygen in the second fluidised bed.