AU552679B2

AU552679B2 - Particle entrainment combustion

Info

Publication number: AU552679B2
Application number: AU14763/83A
Authority: AU
Inventors: Bruce Rixson Little
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-04-29
Filing date: 1983-04-29
Publication date: 1986-06-12
Anticipated expiration: 2003-04-29
Also published as: JPS59500681A; AU1476383A; FI834845A0; EP0107683A4; FI834845A; EP0107683A1; WO1983003887A1

Description

PARTICLE ENTRAINMENT COMBUSTION

TECHNICAL FIELD

This invention relates to combustion entailing entrain ent of particles in a gas flowing upwardly from a fluidized particulate bed. The combustion principles of the invention have application in the incineration of waste, and/or in the generation of heat, or in a combination of these or other purposes. The inventive method is especially well adapted to the combustion of gaseous fuel and may have further application in the rapid heating of fluidizable solids using a gaseous fuel. ^*

BACKGROUND ART

Combustion entailing entrainment of particles in an upward gaseous flow from a fluidized particulate bed has generally been conceived as a modification of conventional fluidized bed technology. For example, Australian patent specification 504,408 to Dorr-Oliver Incorporated discloses what is termed a fluid bed waste incinerator in which fluidizing air admitted from a windbox through the usual constriction plate is of such superficial velocity within the bed as to elutriate particles from the whole surface of the fluidized bed. The patentee describes the result as the production, within the cylindrical reaction chamber, of a dilute fluidized bed in which there is an imperceptible gradation in density from the denser regions of the bed adjacent the constriction plate to the less dense region at the top of the chamber.

Particles entrained in the exhaust gases are recovered in a pair of cyclone separators and^' returned via respective dip-legs to the denser region of the bed. Fuel is lance-injected into this denser region and combustion is said to take place throughout the "dilute bed",. viz. throughout the chamber. In a similar vein, Australian patent specification 500,206 to Metallgesellschaft A.G discloses an arrangement in which, the whole bed chamber is filled with particles so that there is "no step in density between a dense phase and an over-lying dust-containing space but the concentration of solids in the reactor decreases continuously in an upward direction". In this case,- the dilute particulate phase is generated across the whole bed by a primary gas flow through the usual constriction plate and several secondary gas flows injected through^' the sides of a conical lower portion of the bed chamber. Solids entrained in the exhaust gases are separated externally, of the fluidized bed chamber and returned to the bed.

OMH United States patent 4,154,581 to Nack et al discloses a more complex fluidized bed system for power or steam generation which entails formation in a cylindrical main chamber of two vertically over-lapping particulate zones: an entrained bed of smaller particles and a dense fluidized bed of larger particles. The smaller particles in the exhaust gas stream are recovered externally of the main chamber in a cyclone separator, for storage in a reservoir prior to return to the main chamber. The only air supply to the beds is through the usual constriction plate extending across the bottom of the main chamber.

Other prior fluidized bed combustion systems have incorporated recycling arrangements to deal with or take advantage of the inevitable entrainment of fines, and of some larger particles arising, for example, from gas bursts at the fluidized bed surface. For example. United States patent 3,716,003 to Battcock describes the provision of a vortex combustion chamber at the top of and within the main chamber. In the vortex combustion chamber, fine entrained particles are burnt while relatively larger particles are removed and re-introduced to the surface of the fluidized bed via a dip-leg. British patent specification 2,009,905 to Northern Engineering

Industries Limited discloses a fluidized bed boiler in which partly burned fuel and bed material are separated from the flue gas outside the combustion chamber and returned to the bed. Australian patent specification 512,86.7 to A. Ahlstrom Osakeyhtio is concerned with a fluidized bed reactor, especially for

OMPI sludge wastes, in which entrained fines are recovered from the flue gases and mixed as a pre-treatment with the sludge for return to the bed.

Conventional fluidized bed combustion systems, and the modified systems detailed above, suffer from several disadvantages. Firstly, the constriction plate and its associated plenum equipment, which underly the whole of the bed, are expensive items. The_. late must be resistant to very high temperatures, usually necessitating a high-grade alloy in its formation. Its apertures will typically be fitted with devices to prevent backflow of bed material, and it may be associated with a relatively complex underside cooling facility. In general, the constriction plate together with its associated air plenum chamber are intended to distribute the fluidizing air as evenly as possible to the particulate bed above it, and accordingly their construction must achieve a high standard of performance. All of these requirements cause the constriction plate and its associated plenum chamber to have a high initial cost and high maintenance costs.

A second disadvantage of conventional fluidized bed combustion systems is the difficulty encountered in the burning of gaseous fuel within the bed. Typically, the gaseous fuel will burn predominantly above the particle bed, causing undesirable minor explosions in this space and large pressure fluctuations in the air supply system and the flue gas duct work. Furthermore, as the gaseous fuel is usually being burnt as a means of raising the temperature of the bed particles, the heat transfer from the burning gas to the bed particles is poor because the combustion is not taking place within the bed. There is also a serious. risk that a flame front will burn back through apertures in the constriction plate, causing an explosion in the underlying windbox. Because of these problems, it is conventional practice to avoid burning gaseous fuel within the bed and to instead burn the fuel in an external gas burner. The exhaust gases from such a burner then provides hot fluidizing air for the bed, but this approach clearly entails substantial additional costs.

DISCLOSURE OF INVENTION

It is an objective of the invention to_. provide a novel method of, and apparatus for, combustion utilizing a fluidized bed, which may be put into practice at materially lower cost than conventional systems and which permits the provision of economic small scale fluidixed bed combustors.

It is a further objective to provide a fluidized bed combustion system which permits gaseous fuel to be burnt without difficulty within the bed.

This invention devolves from the realization that the key to a lower cost fluidized bed combustion system resides in providing for entrainment of particles in an upward flow from the bed, bμt that it is not sufficient to merely superimpose entrainment upon an otherwise conventional bed facility. In accordance with the invention, the entrainment zone is confined to a limited portion of the bed and the rest of the bed is fluidized, principally for the purpose of replenishing the entrainment zone. The invention accordingly provides, in a first aspect, a method of combustion comprising maintaining a flow of gas into, through and upwardly from a particulate bed, and admitting a fuel to the bed and/or to said flow, characterized in that said flow of gas comprises a primary flow to a limited zone of the bed, of sufficient velocity in said limited zone to entrain particles thereof and carry them upwardly from the bed, and a secondary flow arranged to fluidize the bed, thereby to promote particle replenishment of said limited zone and so sustain the bed in said limited zone.

When it is indicated herein that the bed is fluidized by the secondary flow, it will be appreciated that this does not imply strictly that the whole of the bed is fluidized. In practice, it is preferably that the whole of the bed is fluidized in order to preclude dead spots and fouling through agglomeration.

Advantageously, the secondary flow is into the bed below and/or about the limited entrainment zone of the bed. Preferably, the primary flow impinges on the limited zone of the bed in a downwardly inclined direction, thereby preventing backflow of bed material along the duct when the plant is shut down.. Preferably, at least the major proportion of the entrained particles is separated from the upward flow

OM?l at a location displaced from the bed, and the separated particles returned to the bed. This may be effected within the substantially closed chamber containing the bed, and may arise under natural gravitational forces, or may -be effected in an external chamber such as a cyclone separator. The lateral walls of the chamber may be downwardly and inwardly inclined relative to the bed whereby to further promote particle replenishment of the limited entrainment zone of the bed.

The secondary flow is, in terms of mass flow rate, at least equal to, and preferably between 1.5 and 4 times, said primary flow.

In a second aspect of the invention, there is provided combustion apparatus comprising: a housing defining a combustion chamber, which includes a portion adapted, to retain a particulate bed; means to maintain a flow of gas into, through and upwardly from the bed; and means to admit a fuel to the bed and/or to said flow; characterized in that said flow maintaining means is arranged whereby said flow of gas comprises: a primary flow to a limited zone of the bed, of sufficient velocity in said limited zone to entrain particles thereof and carry them upwardly from the bed, and a secondary flow arranged to fluidize the bed, thereby to promote particle replenishment of said limited zone and so sustain the bed in said limited zone. Preferably the flow maintaining means comprises multiple gas ports to admit the secondary gas flow into the bed below and/or about said limited entrainment zone of the bed. These multiple gas ports may conveniently comprise an array of downwardly open ports^' in the bed retaining portion of the chamber.

Preferably, the apparatus further includes means for separating from the upward gas flow at least the major proportion of still entrained particles at a location displaced from the bed. ^' The separation means may be disposed in an upper part of the chamber or may be disposed externally of the chamber. In either case, separated particles may be returned to the bed.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a somewhat schematic cross-sectional view of a first embodiment of particle entrainment combustion apparatus in accordance with the invention;

Figure .2 is a similar view of the apparatus, but shows particle movement when the apparatus is in use; and

Figure 3 is a schematic cross-sectional view of a second embodiment of particle entrainment apparatus according to the invention. MODES FOR CARRYING OUT THE INVENTION

The particle entrainment combustion apparatus 10 depicted in Figure 1 includes a vertically upright, somewhat elongate, housing 12 enclosing a combustion 5 chamber 13, gas supply means 14 including a downwardly inclined duct 16 and a toroidal manifold 18, a fuel admission tube 20 which opens co-axially into duct 16, and an upper exhaust flue 22. Not shown are components which would be present according to the ^■ io particular application of the apparatus, for example heat exchange coils and/or fins where heat recovery was a required purpose, or means for introducing and removing bed material from the chamber where the apparatus was applied to rapid heating of fluidizable

15 solids.

The upper part of casing 12 may be cylindrical, in which case the lower part is preferably in the shape of an inverted cone. Alternatively, the upper part of the casing may be of rectangular or square

20 cross-section and the lower part is then preferably in the shape of an inverted pyramid. In situ, the lower part contains a particulate bed 11 of suitable refractory material such as silica.

Gas in the preferred form of atmospheric air is

25 supplied to duct 16 and manifold 18 by way of a blower 24 and associated windbox or plenum chamber 26, subject to control by respective valves 28, 29. Duct 16 is connected directly to windbox 26, manifold 18 by duct 19. Duct 16 opens at its lower end, forming a

30 mouth 17 from the duct, within the bed at a location above manifold 18 but a little to one side of the central axis of casing 12. Manifold 18 is provided on its underside with an annular array of downwardly directed apertures 30 which open into the bed at a finite distance from the base of casing 12. These apertures open downwardly to prevent backflow of bed particles when there is no air flow through manifold 18.

In a practical installation, adjustable valves in windbox 26, such as those indicated at 28, 29, may not be required: the flows may be set, perhaps by prior determination, perhaps during commissioning, by means of a sharp-edged orifice or like restrictor devices at the intake end of, e.g. ducts 16 and 19. In use of the combustion apparatus, a primary air flow, including fuel mixed from tube 20, is directed along duct 16 to create a sufficient velocity in a limited zone 32 of the bed, (defined very approximately by broken line 33 in Figure 2) adjacent mouth 17, to entrain particles of the zone and carry them upwardly from the bed 11. The resultant upwardly moving flow of combustion gases and entrained particles is depicted at 34 in Figure 2. Simultaneously, a secondary air flow is introduced to bed 11 through apertures 30 from manifold 18 below and about zone 32, so as to fluidize the bed and thereby to promote particle replenishment of zone 32. In this way the bed within zone 32 is sustained and particles may continuously be entrained to form upward flow 34. It will be appreciated that entrainment zone 32 has no precise boundary, but that its extent will depend. inter alia, upon the rate and velocity of the primary air flow, on the nature of the bed particles, and on the ratio of the primary and secondary air flows. Upward flow 34 spreads out and occupies a substantial proportion of chamber 13: the combustion gases travel in the direction generally indicated by arrows 35 to the outer rim of an inertial separator 36 and thence to exhaust via .flue 22. A substantial proportion of the entrained particles fall back to the bed, as indicated by arrows 37, while those still in train at the top of chamber 13 are recovered in separator 36, and returned to the side of the bed by dip leg 39.

Replenishment of entrainment zone 32 is further promoted by the preferred conical or pyramidal configuration of the lower part of casing 12, which provides a lateral internal wall or walls, downwardly and inwardly inclined relative to the bed. It will be observed from Figure 2 that the surface 12a of the bed itself acquires a concave shape, being lowest at its centre, where surface 12a lies just above the entrainment zone and from which particles are continuously entrained into flow 34.

To assist and promote complete combustion tertiary air admission means may be provided. This comprises multiple nozzles 40 directed into chamber 13 just above the bed from a toroidal plenum 42 to which air is supplied from windbox 26 via duct 44, subject to valve 46. The air jets from nozzles 40 tend to induce the secondary air rising from the bed into the main upward flow 34. To initiate and maintain combustion, an appropriate fuel, which may of course be waste, must be supplied to chamber 13. In the illustrated case, the fuel is gaseous or liquid and is mixed with the primary air in duct 16 from co-axial fuel supply tube 20. It will be appreciated that combustion takes place both in the bed and throughout most of the chamber 13 above the bed. . Entrained and disengaging particles in chamber 13 above the bed continuously transfer heat to and from the combustion gases, thus preventing any undesirable high temperatures or temperature variations in this region. In addition, the said particles enhance the transfer of heat to any heat recovery coils or fins which may be present. Figures 1 and 2 also show, in broken lines, two other possible fuel admission ports comprising a liquid or solid fuel port.20a and a solid fuel conveyor 20b. Any desired combination or plurality of these arrangements could of course be used. It is found that for most successful operation, the secondary air flow from manifold 18 should be, in terms of mass flow rate, about twice the primary flow from duct 16. In practice, acceptable results will be obtained provided the secondary flow is at least equal to the primary flow, and good results where the secondary flow is, in terms of mass flow rate, between 1.5 and 4 times the primary flow. If the secondary flow is less than the primary flow, it is found that the replenishment of entrainment zone 32 is. adversely affected and that, depending upon the nature of the material of the bed, some combustion may occur in the absence of bed particles. This will cause higher local temperatures in the upper part of chamber 13 and less than effective transfer of heat to any heat exchange coils in chamber 13. The flow through 5 nozzles 40 is typically a minor proportion, e.g. about 10%, of the total gas flow into the chamber.

It is desirable that the shape and cross-section of chamber 13 be chosen, in conjunction with the air flow rates so that, under normal operating conditions, 0 the superficial air velocity in the upper part of chamber 13 is no greater than the transport velocity for the smallest retained bed particles, but so that the superficial air velocity in the lower, part of the chamber in the region of the entrainment zone is 5 greater than the transport velocity of the largest bed particles. The upper limit to this superficial air velocity is governed by the mechanical attrition and hence the wastage of the bed particles which can be tolerated. 0 In the modified embodiment shown in Figure 3, in which like components of the apparatus are indicated by like reference numerals, the toroidal manifold for admitting the secondary air flow is replaced by a sealed tray 18' which forms the base of casing 12 so 5that an annular chamber is formed from which air is admitted through-apertures 30' about the casing. Inclined duct 16 is replaced by a rising duct 16' which projects through the base of tray 18' so that its' mouth 17' underlies rather than lies aside the Qentrainment zone 32'. Duct 16' would need to have provision to prevent backflow of bed particles where there is no gas flow. The side wall of casing 12 is formed as a hollow jacket through which cooling air is continuously circulated from windbox 26'. In this case, tertiary air nozzles 40' are fed by the air jacket. Inertial separator 36 is replaced by a cyclone separator schematically indicated at 36'.

It has been found that the illustrated and described particle entrainment combustion system enables substantial capital cost savings to be made in comparison with conventional fluidized bed combustors, including those entailing elutriation of particles from the bed. Firstly, the inventive system eliminates the need for the complex and expensive constriction plate and its associated cooling and plenum equipment, because the requirement for accurately distributing the fluidizing air through the bed no longer applies. Manifold 18, with its downwardly open ports 30, is a much simpler and less expensive item than conventional contriction plates. Secondly, the prior limitation upon combustion air superficial velocity, to be much less than entrainment velocity for the bed particles employed, no longer applies. This allows a reduction in the bed cross-sectional area for a given combustion duty and Consequent capital cost savings. Thirdly, the combustion system of the invention provides for efficient combustion throughout substantially the entire combustion chamber, in contrast to the conventional fluid bed combustor where the combustion process is essentially completed within the particle bed. This improvement provides the required residence time to complete combustion in a smaller volume combustion chamber than is possible with a conventional fluidized bed system.

A significant disadvantage of a conventional fluidized bed combustion system is the need for adjustment of the fluidizing air flow rate as the bed temperature changes. Both the specific volume and the viscosity of air increase with increasing temperature, and in a conventional fluidized bed combustion system the air flow rate is progressively decreased during start up from ambient to operating temperature in order to allow for these physical changes in the fluidizing air, and to prevent particle entrainment conditions being reached. The need for air flow rate adjustments during start up is eliminated in combustion according to the invention because particle entrainment is a desired feature and is indeed required for efficient combustion. The air flow rate control system is thus greatly simplified, thereby achieving significant capital cost economies. The disadvantages of prior fluidized bed combustion systems in relation to the burning of gaseous fuel have already been mentioned. The present invention avoids these disadvantages by allowing the gaseous fuel and air to pass from a single supply duct through the entrainment zone of the particle bed in a continuous turbulent stream rather than in bubble form. Because the gas/air mixture is introduced at high speed along one broad duct, there is no possibility that a flame front can burn back into the duct. If the air flow fails, it is feasible to

"V

OMH. provide for automatic shut down of the fuel supply. Moreover, entraining bed particles with the burning gases provides excellent heat transfer to the particles and prevents undesirable high temperatures in the combustion space above the bed.

A typical fluidized bed combustion chamber with a 600m deep bed of silica sand particles would incur an air pressure drop across the particulate bed of approximately 12Kpa. In the combustion system of this invention, and assuming that a similar particle bed was employed, the air pressure drop across the bed would be approximately 6Kpa. This air pressure drop reduction is derived from the effect of the entrainment zone on the particle bed surface in Figure 2. The air pressure drop reduction allows capital cost savings and operating cost savings associated with the size and power of the air blower.

PI

Claims

1. A method of combustion comprising maintaining a flow of gas into, through and upwardly from a particulate bed, and admitting a fuel to the bed and/or to said flow, characterized in that said flow of gas comprises a primary flow to a limited zone of the bed, of sufficient velocity in said limited zone to entrain particles thereof and carry them upwardly from the bed, and a secondary flow arranged to fluidize the bed, thereby to promote particle replenishment of said limited zone and so sustain the bed in said limited zone.

2. A method according to claim 1 further characterized in that secondary flow is into the bed below and/or about said limited zone of the bed.

3. A method according to claim 1 or 2 further characterized in that said primary flow impinges on said limited zone of the bed in a downwardly inclined direction.

4. A method according to claim 1, 2 or 3 f rther characterized in that the fuel comprises a gaseous constituent of said primary flow of gas.

5. A method according to any preceding claim further characterized by causing at least the major proportion of said entrained particles to be separated from said upward flow, above the bed,, and returned to the bed.

OMPI

6. A method according to any preceding claim further characterized in that said combustion is effected in a substantially closed chamber containing the bed, the lateral wall(s) of the chamber being downwardly and inwardly inclined relative to the bed whereby to further promote said particle replenishment of said limited zone of the bed.

7. A method according to any preceding claim further characterized by admitting a tertiary flow of gas to said primary flow, at a location above the bed, to enhance complete combustion of the fuel.

8. A method according to any preceding claim further characterized in that the gas of both the primary and secondary flows includes a major proportion of air.

9. A method according to any preceding claim further characterized in that said secondary flow is, in terms of mass flow rate, at least equal to said primary flow.

10. A method according to claim 9 further characterized in that said secondary flow is, in terms of mass flow rate, between 1.5 and 4 times said primary flow.

11. Combustion apparatus comprising: a housing defining a combustion chamber, which includes a portion adapted to retain a particulate bed; means to maintain a flow of gas into, through and upwardly from the bed; and means to admit a fuel to the bed and/or to said flow; characterized in that said flow maintaining means is arranged whereby said flow of gas comprises: a primary flow to a limited zone of the bed, of sufficient velocity in said limited zone to entrain particles thereof and carry them upwardly from the bed, and a secondary flow arranged to fluidize the bed, thereby to promote particle replenishment of said limited zone and so sustain the bed in said limited zone.

12. Combustion apparatus according to claim 11 further characterized in that said flow maintaining means comprises multiple gas ports to admit the secondary gas flow into the bed below and/or about said limited zone of the bed.

13. Combustion apparatus according to claim 12 further characterized in that said multiple gas ports comprise an array of downwardly open ports in the bed-retaining portion of the chamber-.

14. Combustion apparatus according to claim 11, 12 or 13 further characterized in that said flow maintaining means is arranged to impinge the primary flow on said limited zone of the bed in a downwardly inclined direction.

r oM i

15. Combustion apparatus according to any one of claims 11 to 14 further characterized in that the flow maintaining means includes a downwardly inclined duct which opens into the bed during use of the apparatus.

16. Combustion apparatus according to any one of claims 11 to 15 further characterized by means to mix said fuel as a gaseous constituent of the primary flow of gas.

17. Combustion apparatus according to any one of claims 11 to 16 further characterized in that the flow of gas and the chamber are such that at least the major proportion of said entrained particles are separated from said upward flow above the bed, and returned to the bed.

18. Combustion apparatus according to claim 17 further characterized in that separation means is disposed in an upper part of said chamber wherein still entrained particles are separated from said upward flow and returned to the bed.

19. Combustion apparatus according to claim 17 further characterized in that said combustion chamber has lateral wall(s) downwardly and inwardly inclined relative to the bed whereby to further promote said particle replenishment of said limited zone of the bed. . -

20. Combustion apparatus according to any one of claims 11 to 19 further characterized by means connected to admit a tertiary flow of gas to said primary flow, at a location above the bed, to enhance complete combustion of the fuel.

OMP¹