WO1997039279A1

WO1997039279A1 - A method of combustion and a combustion plant

Info

Publication number: WO1997039279A1
Application number: PCT/SE1997/000596
Authority: WO
Inventors: Christopher Adams; Jim Anderson; Mats Andersson; Roine Brännström; John Weatherby
Original assignee: Abb Carbon Ab
Priority date: 1996-04-12
Filing date: 1997-04-10
Publication date: 1997-10-23
Also published as: SE9601391D0; CN1168459A; SE9601391L

Abstract

A fuel is combusted in a combustion plant having a combustion chamber (1) enclosing a bubbling fluidized bed (4) in which the combustion of a fuel is arranged to be performed while forming combustion gases. Furthermore, the plant comprises members (5) for feeding of a oxygen-containing gas from beneath to the bed (4), a tube arrangement (22) provided in the combustion chamber (1) and arranged to receive heat generated at the combustion, and a purification device (9, 10, 11) for purifying said combustion gases. The purification device comprises a separating member (9) arranged to separate particulate solid material from said combustion gases and a device (23) arranged to recirculate the separated material to the combustion chamber (1). Furthermore, means (41, 51, 47) are arranged to regulate the quantity of separated solid material supplied to the combustion chamber (1) in such a manner that the heat transfer to said tube arrangement (22) is controlled.

Description

A method of combustion and a combustion plant

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention refers to a method of combustion of a fuel in a combustion chamber enclosing a bubbling fluidized bed, comprising the steps of: feeding an oxygen-containing gas to the bed from beneath; transferring the heat generated by the combustion to a tube arrangement provided in the combustion chamber for heating of water and/or superheating of steam; receiving of the energy generated by the combustion by said tube arrangement; collecting combustion gases formed during the combustion in a space located above the bed in the combustion chamber; separating solid material from said combustion gases; and recirculating the solid material separated to the combustion chamber. Moreover, the invention refers to a combustion, plant comprising: a combustion chamber which is provided to enclose a bubbling fluidized bed and in which a combustion of a fuel is intended to be performed while forming combustion gases; means for feeding an oxygen-containing gas to the bed from beneath; a tube arrangement provided in the combustion chamber and arranged to receive the heat generated during the combustion for heating of water and/or superheating of steam; and a purification device for purifying said combustion gases, said purification device comprising, a separating member, arranged to separate particulate material from said combustion gases, and a device arranged to recirculate the material separated to the combustion chamber. It is known to combust different fuels in a bed of particulate, incombustible material, which bed is supplied with combustion air from beneath through nozzles in such a manner that the bed becomes fluidized. One differs between different types of such combustion in a fluidized bed, which operate according to different principles and under different conditions. Firstly, one differs between an atmospheric bed and a pressurized bed. In comparison with an atmospheric bed a pressurized, fluidized bed is characterized by a small plant size in relation to the effect produced, by a high efficiency, and in that the combustion occurs under advantageous conditions from an environmental and economical point of view. A pressurized bed may have a larger height than an atmospheric bed since one may operate with greater pressure drops. Among the atmospheric beds so called circulating beds are frequently used, in which the bed material is permitted to circulate through a separating device in order to be recirculated to the bed. In such a way possibly unburnt fuel may be recirculated, which improves the efficiency of the combustion, and also absorbent material not used for absorption of in the first place sulphur, which decreases the discharge of contaminates from the combustion. However, such circulating beds operate with relatively high fluidizing velocities, in typical cases in the order of 5-12 m/s. By fluidizing velocity is meant the velocity that the gas would have had if it would have flowed through the combustion chamber without the presence of particles. This causes problems with erosion on for instance the steam tube arrangement provided in bed in such a way that the lifetime thereof significantly decreases. Furthermore, one may discern the so called bubbling beds in which the fluidizing velocity is relatively low, in typical cases between 0,5 and 2 m/s. Such a bed is relatively well defined in a vertical direction and there is formed a space, a so called freeboard, in the combustion chamber above the bed. In this freeboard a relatively small amount of dust particles are present in comparison with a circulating bed but there is essentially no pressure drop across the freeboard.

In recent time one have tried to provide a certain circulation also in pressurized beds by supplying the combustion gases leaving the combustion chamber to a cyclone for separation of solid material, which is recirculated to the combustion chamber. In order to obtain completely the desired effect concerning the degree of utilisation of the absorbent and the combustion efficiency by the recirculation, the solid material should be supplied at the bottom of the fluidized bed. This means that one has to overcome the pressure drop which is present in the bed and in the cyclone, in typical cases about 0,5 bars.

In order to overcome this pressure drop it has been suggested to provide a dosing device, for example of a cell feeding type, at the end of a recirculating pipe provided preferably vertically and connecting the cyclone to a combustion chamber. The dosing device may comprise a rotatable shutter provided on the pipe and having a weight which in normal cases keeps the shutter in a closed position. When the amount of material in the pipe is sufficient the weight thereof will overcome the weight of the shutter which means that the shutter is opened and the material is discharged. Such a device leads to an intermittent recirculation of solid material. However, such devices do not function in the way intended in the environment of a fluidized bed due to the movements occurring in the bed and the forces caused by these movements. Furthermore, such devices are rapidly destroyed due to the aggressive, erosive and corrosive environment.

An other solution is a L-valve located in the bed and having a vertical portion in which a column of material is built up. In order to provide a flow of material through the channel such a device requires that gas is injected in the lower portion of the L-valve and in order to provide stability it is necessary to continuously measure the height of the column of material, which is very difficult, if not impossible, in the actual environment.

SE-B-460 148 suggests another way of overcoming this pressure drop. SE-E-460 148 discloses a combustion plant having a combustion chamber enclosing a pressurized fluidized bed for the combustion of a fuel while forming combustion gases. Furthermore, the plant comprises a purification of said combustion gases in several stages. In a first stage particulate material is separated by means of a cyclone from the combustion gases and supplied to a collection chamber beneath the cyclone. Via a horizont l recirculating channel the collected dust particles are fed back to the combustion chamber in order to improve the use of unburnt fuel and absorbent material. The recirculation is accomplished by means of an air driven ejector blowing the material into the combustion chamber. However, such an air injection is very expensive. The gain of the absorbent utilization and the combustion efficiency is lost in the effect for the compressor providing primary air to the ejector. In addition this method leads to erosion.

Such pressurized, fluidized beds frequently comprise regulating equipment by which the height of the bed may be regulated by the supply and discharge, respectively, of bed material for controlling the load of the combustion plant. A circulation of solid material being separated from the combustion gases may result in the recirculated fine part forming as much as 10-40% of the mass of the bed, which leads to a powerful influence of the heat transfer coefficient to the tubes present in the bed. During load changes, the supply and discharge of bed material will change the quantity of fine material in the bed and thus the heat transfer coefficient to the tubes in the bed, which means that the regulation dynamic of the plant is deteriorated and that the possible range of regulation decreases. By fine material is meant the so called fine part being particles having a largest diameter of about 300-400 μm and an average particle diameter of about 50-150 μm. Tests in PFBC-plants have shown that it may take 10-20 hours before the fine part in the bed has been readjusted to a stationary value and a desired load may be obtained. During this time the range of regulation is deteriorated from the normal of 20-100% of the load to 60-100% of the load, which of course reduces the value of the plant.

Pressurized, fluidized beds are well suitable for combustion of a fuel having a varying composition, for example distribution of size, composition and energy content. For instance, it is desired in such beds to be able to combust pit coke, anthracite, brown coal, pet coke, oil shale, gas, oil, lignite and biofuel of different types, such as chips, olive stones, coconut shale etc. Also one and the same fuel may have properties which vary from one day to another in such a manner that the maximum output power of the plant is influenced. Furthermore, the heat transfer coefficient between the bed and the tubes provided therein is influenced by a great number of parameters such as for example the size distribution, the shape, the heat capacity and the density of the bed material, and the fluidizing velocity, the local oxygen-content in the bed, depositions on the tubes etc. Therefore, it is desirable to be able to control the heat transfer to the tubes present in the bed in order to be able to optimize the thermal efficiency of the plant and in order to be able to achieve full power in all situations with a large variation of the fuel. EP-B-176 293 discloses another combustion plant having a combustion chamber which encloses a fluidized bed and in which combustion of a fuel is intended to be performed while forming combustion gases. The bed is of a bubbling type but the combustion chamber operates at atmospheric pressure. Furthermore, the plant comprises a cyclone for separating particulate material from the combustion gases and provided above the combustion chamber. The particulate material separated is conducted via a pipe back into the bed by letting the material simply fall freely through the pipe. This is possible since the bed disclosed in this document has a relatively low height, about 1 m. Thereby, also the pressure drop is relatively small.

A large pressure drop in an atmospheric bed leads to large energy losses. Therefore, such a bed must be kept on a low level.

SE-A-8304558-3 discloses a power plant for combustion of a fuel in a pressurized, fluidized bed. In order to regulate the heat of the bed, the plant comprises a silo for storing bed material. The silo is connected to the bed by a first pipe for discharging bed material from the combustion chamber and a second pipe for recirculate bed material to the combustion chamber.

SUMMARY OF THE INVENTION

The object of the present invention is to optimize the effect and the efficiency of a combustion plant, by influencing the heat transfer coefficient between the bed and the tubes present in the bed.

This object is obtained by the method initially defined and characterized in ^•'"hat the heat transfer to said tube arrangement is co-.crolled by r- julating the quantity of separated solid material supplied to the bed. Since a circulation of solid material being separated from the combustion gases results in a great recirculated fine part and a powerful influence of the heat transfer coefficient between the bed and the tubes present therein, possibilities to control this heat transfer is provided in accordance with the invention. By regulating the flow of the recirculating fine part, the quantity of the fine part in the combustion chamber and the bed may be controlled and in this way one will be able to influence the heat transfer from the bed to the tube arrangement. This may be done in a stationary state i.e. the quantity of recirculating fine part is regulated in order to be able to remain at a constant load when the fuel or the heat transfer vary. However, the regulation in accordance with the invention also results in advantages during dynamic conditions, i.e. when for example it is desirable to perform a rapid load change at a constant temperature. By rapidly increase the quantity of recirculated separated material from the separating member a fast effect increase may be obtained. At full load and at the highest possible bed height the effect produced by the tube arrangement may be too low for example due to a temporary deterioration in the quality of the fuel. By increasing the recirculation of separated material, and thus the fine part of the bed, in this situation, full effect may be obtained.

According to an embodiment of the invention, the regulation of the quantity of separated solid material supplied to the combustion chamber is performed in such a manner that, the efficiency of the plant is maximized and/or full output power is obtained.

According to a further embodiment of the invention the quantity of separated solid material supplied to the combustion chamber is regulated in such a manner that solid separated material is discharged at a load decrease. This means that at a load decrease less separated solid material is recirculated than at a stationary state. Thereby, this recirculated material will not to the same extent as previously result in a higher effect of the plant, for instance heating of a steam tube arrangement in the combustion chamber. Thereby, the possibilities to regulate the plant are significantly improved. According to a further embodiment of the invention, the quantity of separated solid material supplied to the combustion chamber is regulated in such a manner that, at a load increase, separated solid material discharged is recirculated.

According to a further embodiment of the invention, the quantity of separated solid material recirculated to the combustion chamber is regulated by the discharge of separated solid material to a collecting member located outside the combustion chamber. Furthermore, the separated material present in the collecting member may according to a preferred embodiment be recirculated therefrom to the combustion chamber at load increase of the plant. In such a manner the possibilities to regulate the plant are improved by the utilization of the recirculated solid material being separated from the combustion gases. In particular, a very rapid load change may be obtained by such an intermediate storage of separated solid material, since the recirculation from the collecting member may provide an additional supply to the fine part. Such an additional supply may in certain cases be necessary in order to reach full load.

According to a further embodiment of the invention, the separated material is recirculated to the combustion chamber through a channel. Advantageously, the channel has an orifice below said tube arrangement. The fuel to a bubbling fluidized bed is normally supplied in the bottom area of the bed. In addition, since the oxygen cont nt is t e highest in this area most of the combustion will be performed in this area. Thus, a temperature gradient is formed in the combustion chamber, which in typical cases may be about 50°C, i.e. the temperature is higher in the lower portion of the combustion chamber than in the upper portion. Simultaneously, the combustion is limited to an absolutely highest temperature in any point in order to avoid sintering. This highest temperature varies depending on the fuel etc., and may in typical cases amount to about 900°C. By recirculation of relatively cold separated material to the bed, this temperature gradient may be reduced and one may maintain a higher temperature of the combustioned gases leaving the combustion chamber and supplied to a gas turbine. Thereby, the efficiency of the gas turbine may be increased. Thus, the regulation method according to the invention enables the increase of the total efficiency of the plant by a combined adaption of the heat transfer to the steam tube arrangement and of the temperature of the combustion gases.

According to a further embodiment the discharge is performed by suction of separated solid material from the channel to the collecting member. The recirculation from the collecting member to the combustion chamber is suitably performed by pneumatic supply.

According to another embodiment, the bed is pressurized and the discharge is performed by transporting separated solid material from the channel to the collecting member located outside the pressurized area.

According to a further embodiment of the invention, the regulation of the quantity of separated solid material supplied to the combustion chamber is performed by changing the flow area of a passage for the separated material, which is provided in the lower portion of the channel. According to a further embodiment of the invention the separated material is supplied to the channel in such a manner that a column of material is formed therein. The height of the column of material exceeds the height of the bed and the column of material recirculates the separated material to the combustion chamber in a continuous flow merely due to its weight. Thus, merely the weight of the column of material will provide a continuous and uniform recirculation of separated solid particulate material to the combustion chamber. Since the recirculation device according to the invention comprises passive means not requiring any compressor or other driving member for overcoming the pressure difference and discharging material from the column of material, the cost of the recirculation device is very favourable, with respect to manufacture as well as operation. Furthermore, the erosion problems occurring by ejector feeding of the material are avoided. Thus, since the recirculation channel according to the invention does not have any moveable constructional elements the reliability thereof is very high.

The object defined above is also obtained by the combustion plant initially defined and characterized by means arranged to regulate the quantity of separated solid material supplied to the combustion chamber in such a manner that the heat transfer to said tube arrangement is controlled.

Preferred embodiments of the combustion plant are defined in the claims 15-34.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained by means of different embodiments, defined by way of example, and with reference to the drawings attached. Fig 1 discloses schematically a PFBC-power plant according to a first embodiment having a combined gas and steam cycle (the latter not disclosed) .

Fig 2-5 discloses different embodiments of a combustion chamber of the power plant according to the invention.

Fig 6-12 discloses different embodiments of a recirculation channel of the plant in Fig 1.

DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS

The invention will now be explained with a reference to a so called PFBC-power plant (pressurized, fluidized bed combustion) . However, it should be noted that the invention also is applicable to other types of plants, in particular combustion plants without power production. A PFBC-power plant, i.e. a plant for the combustion of particulate fuel in a pressurized, fluidized bed, is schematically disclosed in Fig 1. The plant comprises a combustion chamber 1 being housed in a pressure vessel 2, having a volume in the order of 10 m and which may be pressurized to for example between 7 and 30 bars (abs). Compressed oxygen-containing gas, in the example disclosed air, is supplied to the pressure vessel 2 at 3 for pressurizing the combustion chamber 1 and for fluidizing a bed 4 in the combustion chamber 1. The compressed air is supplied to the combustion chamber 1 via schematically indicated fluidizing nozzles 5 being provided in the bottom of the combustion chamber 1 for fluidizing the bed 4 enclosed in the combustion chamber 1. The air_, is supplied in such a manner that a fluidizing velocity of about 0.5 - 2.0 m/s is obtained. The bed 4 is of a bubbling type and may have a height h being about 2-6 m. It comprises a non-combustible, particulate bed material, particulate absorbent and a particulate fuel. The particle size of the bed material not circulating, the absorbent and the fuel is between about 0.5 and 7 mm. The bed material comprises for example ashes and/or sand and the absorbent a lime containing material, for example dolomite or lime stone for absorption of the sulphur or possible other undesired substances released during the combustion. The fuel is supplied in such a quantity that it forms about 1% of the bed. By fuel is meant all fuels which may burn such as for example pit coal, brown coal, coke, peat, biofuel, oil shale, pet coke, waste, oils, hydrogen gas and other gases, etc. The bed material, the absorbent and the fuel are supplied to the bed 4, via a conduit 6 schematically disclosed. The fuel is combusted in the fluidizing air supplied to the bed 4 while forming combustion gases. These are collected in a space 7 located above the bubbling bed 4, a so called freeboard, and are then conducted via a channel 8 to a separating member 9, in the example disclosed a cyclone. From there the combustion gases are conducted further to further purification devices, which are disclosed schematically at 10 and which for example may comprise cyclones provided in several stages. Thereafter, the combustion gases are conducted further via for example a high temperature filter 11 to a gas turbine 12 which in the example disclosed comprises a high pressure stage 13 and a low pressure stage 14. The high pressure turbine 13 is provided on the same shaft as the high pressure compressor 15 and a generator 16 which in this manner is driven by the high pressure turbine for producing electrical energy. The high pressure compressor 15 delivers compressed air to the combustion chamber 1 via the conduit 17,

The combustion gases expanded in the high pressure turbine 13 are conducted to a low pressure turbine 14. The combustion gases leaving the low pressure turbine 14 still comprise energy which may be recovered in an economizer 18. The low pressure turbine 14 is provided on the same shaft as the low pressure compressor 19 which is supplied with air from the atmosphere via a filter 20. The low pressure compressor 19 is thus driven by the low pressure turbine 14 and provides from its outlet the high pressure compressor 15 with air which has been compressed in a first stage. Between the low pressure compressor 19 and the high pressure compressor 15 an intermediate cooler 21 is provided for lowering the temperature of the air supplied to the inlet of the high pressure compressor 15.

Furthermore, the power plant comprises a steam turbine side, which is not disclosed, but indicated by the arrangement in a form of a tube set 22, which is submerged in the fluidized bed 4. In the tube set 22 water is circulated evaporated and superheated by heat-exchange between the tubes and the bed material for receiving the heat produced by the combustion performed in the bed 4.

In the cyclone 9 provided in connection to the combustion chamber and also called zero step cyclone, solid particulate material is separated from the combustion gases. These solid particulate material comprises on one hand bed material and ashes but also unburnt fuel and absorbent. It is therefore desirable to recirculate this unused material to the bed 4 for, if possible, combust unburnt fuel and utilize unused absorbent. This recirculation is performed by a recirculation device comprising a channel 23. The channel 23 shall be configured in such a manner that a column 24 of material is formed in the channel 23 during the operation of the plant. The column 24 of material so formed shall have a height h' exceeding the height h of the bed 4. Due to this height difference the gravity will influence the material in the column 24 of material in such a manner that this is fed continuously downwardly into the combustion chamber 1 and in the examples disclosed downwardly into the bed 4 under the tube set 22. This height difference may be provided by a variety of different embodiments of the channel 23. The channel 23 may have an arbitrary cross-section, for instance circular, oval elliptic, rectangular, polygonal etc. In Fig 1 the recirculation device comprises an inclined wall 25 in the lowest portion of the channel 23. Thus, the orifice of the channel 23 is formed by the lowest edge of the inclined wall 25 and an edge of the channel 23 thereabove. The inclined wall may have an angle v of inclination in relation to the vertical axis which amounts to about 20 to 90°, i.e. in the extreme case is perpendicular to said vertical axis. A preferred angle v of inclination is between about 21 and 39°. The inclined wall 25 prevents the gas flowing upwardly from the nozzles 5 from entering the channel and functions as sliding surface for the material flowing downwardly. In such a manner a column of material of the downwardly flowing material is formed. In order to reduce the recirculation velocity the opening area of the orifice may be less than the cross-section area of the channel 23. It should be noted that the orifice in the example disclosed in Fig 1 is completely located in an essentially vertical plane. Since merely small quantities of the combustion air flowing upwardly thus may enter the channel 23 no fluidizing of the material present in the channel 23 will take place.

Fig 2-5 disclose other embodiments of the recirculation channel 23 and the separating member 9. It should be noted that elements having a corresponding function have been provided with the same reference signs in the different embodiments.

The recirculation device disclosed in Fig 2 comprises a relatively soft curve 26 in the lower part of the channel 23. The orifice is also in this example formed by cutting the channel 23 in an essentially vertical plane. A lower tangential plane of the curve 26 at the end of the channel is inclined in relation to a ve tical axis by the angle v which may have the same value as in the example disclosed in Fig 1. The curve 26 disclosed will prevent gas flowing upwardly from entering the channel 23 and function as a sliding surface for the material flowing downwardly. In order to reduce the recirculation velocity of the material the channel 23 may have a smaller cross-section area at the curve 26 than upstream thereof. In addition, the cyclone 9 disclosed in Fig 2 is completely enclosed in the combustion chamber 1.

The recirculation device disclosed in Fig 3 comprises a channel 23 which extends outside the combustion chamber 1 and in a direction which forms an angle v to a vertical axis. The channel 23 extends through a passage in the wall of the combustion chamber 1, which passage forms the orifice of the channel 23. The angle v may for example be between 10 and 50°, preferably between 21 and 39°. By means of such a sloping recirculation channel 23 the quantity of gas flowing upwardly in the channel is reduced, resulting in the formation of a column 24 of material extending upwardly above the bed 4. The weight of this column 24 of material ensures an equal and continuous discharge of the solid material separated. In order to reduce the recirculation velocity of the solid material flowing downwardly, alsc in this case the cross-section area at said passage, i.e. in the proximity of the orifice of the channel 23, may be less than at a higher position of the channel 23. The cyclone 9 is in this example located completely outside the combustion chamber 1 and is connected therewith via the schematically disclosed pipe conduit 8. Although the orifice of the channel 23 in Fig 3 is located at the same height as ,the tube set 22, it should be noted that the orifice disclosed in Fig 3 may be located below or above the level of the tube set 22.

Fig 4 discloses another variant of a recirculation device having a channel 23 extending essentially vertically. In this case the recirculation device comprises a portion 27 of the channel 23 sloping downwardly, which reduces the quantity of gas flowing upwardly in the channel 23 and functions as a sliding surface for the solid particulate material flowing downwardly. The portion 27 forms a passage having a flow area which has such a dimension that a column 24 of material is formed and having a height h' exceeding the height h of the bed 4.

Fig 5 discloses another variant of a recirculation device comprising a channel 23 having a passage formed by a funnel- shaped extension in which a cone is located. The construction of this passage is explains more closely in connection with the description of Fig 12.

Fig 6-12 disclose further variants of the recirculation device according to the invention. In Fig 6 this device comprises a channel 23 similar to the one in Fig 1 but the lower plate 25 extends essentially perpendicular to a vertical axis. This embodiment is especially simple from a manufacturing point of view. There will be formed an accumulation 29 of material flowing downwardly in the corner formed by the plate 25 and the channel 23. This accumulation will function as a sliding surface for the material flowing downwardly. The channel 23 disclosed in Fig 7 comprises a portion 27 similar to the one in Fig 4 but the lower part of the portion 27 sloping with the angle v is prolonged in the direction of the outflowing material in relation to the upper part of the sloping portion 27. In such a manner the orifice of the channel 23 will have an angle a , of inclination in relation to a vertical axis. By this embodiment the quantity of gas flowing upwardly and into the channel 23 is reduced. The channel disclosed in Fig 8 is similar to the one disclosed in Fig 1 but the sloping plate 25 is shortened in such a manner that seen from oeneath the plate does not cover the whole cross-section area of the channel 23. Thus, the orifice of the channel 23 forms an angle b to a vertical axis. By such an embodiment most of the gas flowing upwardly will certainly be prevented from entering the channel 23 but a part thereof is permitted to mix up with the column 24 of material. This may be desirable in certain applications when one wishes a gas mixture in the material separated. In Fig 9 the channel 23 comprises a plate 30 being fixed in the channel 23 in such a manner that an essentially peripheral opening is formed between the plate 30 and the channel 23. The plate 30 may be fixed by means of a number of barlike rods schematically disclosed at 31. It should be noted that the plate 30 also may be provided sloping with an angle v in relation to a vertical axis. The recirculation device disclosed in Fig 10 comprises a downwardly completely open channel 23 having an orifice precisely above a bottom plate 32 of the combustion chamber 1. In the portion 33 of the bottom plate 32 being located below the channel 23 there are no fluidizing nozzles 5 which otherwise are provided over essentially the whole surface of the bottom plate 32. In such a manner no gas flowing upwardly from the nozzles 5 may enter the channel 23 and causing a fluidization of the material present therein. Thereby, a column 24 of material may be built up and an uniform and continuous discharge of material to the lower part of the bed is obtained. The material so discharged will thereafter be brought upwardly in the bed due to the gas flowing upwardly from the nozzles 5. Fig 11 discloses a recirculation device similar to the one in Fig 10 but the portion 33 provided in the bottom plate 32 and having no fluidizing nozzles 5 is raised in relation to the other surface of the bottom plate 32. The recirculation device disclosed in Fig 12 comprises the channel 23 having a funnel-shaped conical extension 34 being open downwardly. In this extension 34 a cone is provided by means of one or more attachment plates 36. The extension 34 and the cone 35 form a cone angle v in relation to the vertical axis. This angle v is, as these in the preceding example, between 20 and 90°, preferably between 21 and 39°.

In order to improve the possibility to regulate the power plant according to the invention, said plant comprises equipment for regulating the quantity of separated solid material recirculated to the bed 4 of the combustion chamber 1. With reference to Fig 1 such an equipment, according to a first embodiment, is disclosed. The channel 23 is provided with an obliquely upwardly directed pipe portion 40. A pipe conduit 41 has an orifice in the pipe portion 40 and has a smaller cross-section area than the pipe portion 40 and is provided essentially concentrically in the pipe portion 40. The pipe conduit 41 extends upwardly to a collecting member comprising a cyclone 42 and a collecting receptacle 43 connected to the cyclone 42. The collecting receptacle 43 may be heat insulated and comprise a heating member 43A for instance an electrical circuit, for heating and/or warming the material present therein. The cyclone 42 is via a conduit 44 connected to the atmosphere. The conduit 44 may be provided with cooling members 45 and with a valve 46 provided outside the pressure vessel 2 and controlled by means of a control device 47 schematically disclosed. Downstream of the valve 46 there is a throttling 48. Furthermore, there is a conduit 49 connecting the inner space of the pipe portion 40 to the high pressure prevailing in the pressure vessel 2. The conduit 49 comprises a regulating valve 50 controlled by means of the control device 47. During the discharge of separated material from the column 24 of material, the regulating valve 50. is firstly opened. Thereby, gas is fed to the pipe portion 40, and the valve 46 is opened and a flow of gas will be sucked via the conduit 41 tangentially into the cyclone 42 and from there further out to the atmosphere via the conduit 44 and the throttling 48. "hereafte- separated material will be sucked in the cond 41 an arrive to the cyclone 42 in which the separated material is separated and falling down into the collecting receptacle 43. In this way, a part of the separated material in the column 24 of material will be supplied to the collecting receptacle 43 and the quantity of material recirculated to the bed 4 is thus reduced. Advantageously, the throttling 48 is configured in such a manner that the flow therethrough is critical, so that a constant velocity of the flow through the conduit 41 and 44 is obtained independent of the pressure level in the plant. In such a manner the flow of separated material discharged through the conduit may be controlled exactly.

Furthermore, from the collecting receptacle 43 a recirculation conduit 41 extends back to the combustion chamber 1. Thereby, the recirculating conduit 51 has an orifice at essentially the same height as the location of the pipe portion 40. The conduit 51 has a curve immediately outside the inlet to the combustion chamber 1, which forms a so called L-valve 52. Furthermore, there is a conduit 53 arranged to supply pressurized gas from the pressure vessel 2 into the L-valve 52. The conduit 53 comprises a regulating valve 54 being controlled by the control device 47. The material discharged through the pipe conduit 41 and transported to the collecting receptacle 43 will fall down into the latter and into the recirculating conduit 51. However, the material will be stopped at the curve formed by the L-valve 52. When the regulating valve 54 is open gas will be supplied to the L-valve 52 and the material present in the channel 51 is automatically fed back into the bed 4. In such a manner, it is possible to continuously _' feed material present in the collecting receptacle 43 into the bed 4 at, for instance, a load increase. In particularly, at start-up of the plant it may be preferred to preheat the separated material to be recirculated to the combustion chamber 1 by means of the heating member 43A. Finally, it should be mentioned that a discharge pipe 55 may extend from the L-valve 52 out of the pressure vessel 2 and to a container device 56. The pipe 55 may be provided with a cooling device 57 and comprises a regulating valve 58 controlled by means of the control device 47. When the regulating valve 58 is open material present in the collecting receptacle 43 will be discharged therefrom and thus be removed from the power plant. This final discharge will be described more closely in connection with Fig 4.

Fig 4 discloses the equipment according to another embodiment of the invention for regulating the quantity of separated solid material to be recirculated to the combustion chamber depending on the load state of the plant. In the channel 23, a pipe 59 is introduced from beneath. The pipe 59 may have a cross-section area which is less than the one of the channel 23. The pipe 59 comprises a cooling device 60 for receiving the heat of the material discharged from the channel 23 and a regulating valve 61 arranged to be controlled by means of a control device 62. Downstream of the valve 61 a container device is provided. This device comprises a first container 63, a second container 64 and a final container 65. Between the containers 63 and 64, a valve 66 is provided and between the containers 64 and the final container 65, a valve 67 is provided. Furthermore, between the valve 66 and the container 64, there is a conduit leading to the atmosphere and having a valve 68 and a throttling 69. The valves 66, 67 and 68 are controlled as well as by means of the control device 62. The arrangement disclose functions a a type of a sluice, ^"f the valve 61 is open and the valve 5 and 67 are close, material from the column 24 of material present in the cnannel 23 will be transported through the pipe 59 downwardly into the container 63 due to the gravitation. Thereafter, the valve 61 is closed and the valve 66 open, so that the material present in the container 63 will be transported downwardly into the container 64. In order to protect the valve 67 from erosion, the container 64 is depressurized by opening of the valve 68. Finally, the valve 66 is closed and the valve 67 open so that the material present in the container 64 is transported downwardly into the final container 65. Thereafter, the process may be repeated.

It should be noted that the discharge equipment disclosed in Fig 1 also may be provided in connection with the recirculating channel disclosed in Fig 4. In the same way the discharge equipment disclosed in Fig 4 may be provided in connection with the recirculating channel disclosed in Fig 1. In the same way, the embodiments disclosed in Fig 2, 3 and 6-12 may be provided with at least one of the discharge equipment disclosed in Fig 1 and 2.

The recirculating device disclosed in Fig 5 comprises a passage provided in a lower portion of the channel 23 and having a conical extension 34 similar to the one in Fig 12 and a cone 35 displaceably provided in the extension 34. The cone 35 is adjustable via a drive bar 71 by means of a drive member 70 provided outside the combustion chamber 1 and the pressure vessel 2. The drive bar 71 is sealingly disposed through the wall cf the pressure vessel by means of a sealing member 72. The drive bar 71 extends through the bottom wall of the combustion chamber 1 via a passage 73 arranged to permit air supply to the combustion chamber 1 in the purpose of keeping the passage 73 clean so that the reliability of the adjustability of the cone 35 may be ensured. By displacing the cone 35 it is thus possible to adjust the flow area of the passage of the channel 23 "and thereby one may regulate the flow of separated material recirculated to the combustion chamber 1. It should be noted that the constructive configuration of such a passage having a variable flow area may be performed in many different ways, and the embodiment disclosed in Fig 5 is an example thereof. The present invention is not in any way limited to the embodiments disclosed above but may be varied and modified within a scope of the following claims.

In certain applications of the present invention, it might be advantageous to provide two or more separating members 9 in a parallel configuration with each other. Each separating member 9 is in this case preferably provided with a recirculation channel 23. Such a parallel configuration may for example be necessary in order to achieve an appropriate separation efficiency.

Claims

1. A method of combustion of a fuel in a combustion chamber enclosing a bubbling fluidized bed, comprising the steps of:

- feeding an oxygen-containing gas to the bed from beneath;

- transferring the heat generated by the combustion to a tube arrangement provided in the combustion chamber for heating of water and/or superheating of steam; - receiving the energy generated by the combustion by said tube arrangement;

- collecting combustion gases formed during the combustion in a space located above the bed in the combustion chamber;

- separating solid material from said combustion gases; and - recirculating the solid material separated to the combustion chamber, characterized in that the heat transfer to said tube arrangement is controlled by regulating the quantity a separated solid material supplied to the combustion chamber.

2. A method according to claim 1, characterized in that the regulation of the quantity of separated solid material supplied to the combustion chamber is performed by in such a manner that the efficiency of the plant is maximized and/or that full output effect is obtained.

3. A method according to any one of claims 1 and 2, characterized in that the quantity of separated solid material supplied to the combustion chamber is regulated in such a manner that separated solid material is discharged at a load decrease.

4. A method according to claim 3, characterized in that a quantity of separated solid material supplied to the combustion chamber is regulated in such a manner that separated solid material discharged is recirculated at a load increase.

5. A method according to any one of the preceding claims, characterized in that a quantity of separated solid material recirculated to the combustion chamber is regulated by discharging separated solid material to a collecting member located outside the combustion chamber.

6. A method according to claim 5, characterized in that the separated material present in the collecting member is recirculated therefrom to the combustion chamber.

7. A method according to any one of the preceding claims, characterized in that the separated material is recirculated to the combustion chamber through a channel.

8. A method according to claim 7, characterized in that the channel has an orifice beneath said tube arrangement.

9. A method according to any one of claims 7 and 8, characterized in that the discharge is performed by transporting separated solid material from the channel to the collecting member.

10. A method according to claim 9, characterized in that the recirculation from the collecting member to the combustion chamber is performed by pneumatic supply.

11. A method according to claim 5 and 7, characterized in that the bed is pressurized and that the discharge is performed by transporting separated solid material from the channel to the collecting member provided outside the pressurized space.

12. A method according to any one of claims 7-11, characterized in that the regulation of the quantity of the separated solid material supplied to the combustion chamber is performed by changing the flow area of a passage for the separated material, which is provided in a lower portion of the channel.

13. A method according to any one of claims 7-12, characterized in that the separated material is supplied to the channel in such a manner that a column of material is formed therein, so that the height of the column of material exceeds the height of the bed and the column of material merely due to its weight recirculates the separated material in a continuous flow to the combustion chamber.

14. A combustion plant comprising:

- a combustion chamber (1) which is provided to enclose a bubbling fluidized bed (4) and in which a combustion of a fuel is intended to be performed while forming combustion gases;

- means (5) for feeding an oxygen-containing gas to the bed (4) from beneath;

- a tube arrangement (22) provided in the combustion chamber (1) and arranged to receive the heat generated during the combustion for heating of water and/or superheating of steam; and

- a purification device (9, 10, 11) for purifying said combustion gases, said purification device comprising a separating member (9), arranged to separate particulate material from said combustion gases, and a device ('23) arranged to recirculate the material separated to the combustion chamber (1), characterized by means (41, 51, 59, 35, 47, 62, 70) arranged to regulate the quantity of separated solid material supplied to the combustion chamber (1) in such a manner that the heat transfer to said tube arrangement (22) is controlled.

15. A combustion plant according to claim 14, characterized in that the regulating means (41, 51, 59, 35, 47, 62, 70) are arranged to regulate the quantity of separated solid material supplied to the combustion chamber (1) in such a manner that the efficiency of the plant is maximized and/or that full output effect is obtained

16. A combustion plant according to any one of claims 14 and 15, characterized in that the regulating means (41, 51, 59, 35, 47, 62, 70) are arranged to regulate the quantity of separated solid material supplied to the combustion chamber (1) in such a manner that separated solid material is discharged at a load decrease.

17. A combustion plant according to claim 16, characterized in that regulating means (41, 51, 59, 35, 47, 62, 70) are arranged to regulate the quantity of separated solid material supplied to the combustion chamber (1) in such a manner that separated solid material discharged is recirculated to the combustion chamber (1) at a load increase.

18. A combustion plant according to any one of claims 14- 17, characterized in that the regulating means comprise a first connection (41, 55, 59) provided between the recirculating device (23) and a collecting member (43, 56, 65) located outside the combustion chamber (1) and 'are arranged to enabling the discharge of separated solid ir. terial from the recirculating device (23) to the collecting member in order to reduce the quantity of solid separated material to be recirculated to the combustion chamber (1) .

19. A combustion plant according to claim 18, characterized in that the heating member (43a) , preferably a electrical heat circuit, is arranged to heat the material present in the collecting member (43) .

20. A combustion plant according to any one of claims 18 and 19, characterized in that the regulating means comprise second connection (51) provided between the collecting member (43) and the combustion chamber (1) and are arranged to enabling recirculation of separated material present in the collecting member (43) to the combustion chamber (1) .

21. A combustion plant according to any one of claims 14- 20, characterized in that the combustion chamber (1) and the separating member (9) are enclosed in a pressure vessel (2) and that means (5, 15, 19) is arranged to maintain a pressure above atmospheric pressure, preferably between 7-30 bars (abs), in the pressure vessel (2).

22. A combustion plant according to claim 21, characterized in that the collecting member {43) is provided in the pressure vessel (2) and via a conduit member (44) connected to the atmosphere outside the pressure vessel (2) in order to transport the separated material from the channel (23) to the collecting member (43) via the first connection (41) by means of a suction effect.

23. A combustion plant according to claim 20 and 22, characterized in that the second connection (51) via a conduit (53) is connected to a pressure gas source (2) arranged to supply transport gas for pneumatic recirculation of separated material present in the collecting member (43) to the combustion chamber (1) .

24. A combustion plant according to any one of claims 21-23, characterized in that the collecting member comprises a container device (56, 63, 64, 65) provided outside the pressure vessel (2) .

25. A combustion plant according to claims 20 and 24, characterized in that the container device (56) via a second conduit member (55) is connected to the second connection (51).

26. A combustion plant according to any one of claims 14-25, characterized in that the recirculating device comprises a channel (23) extending between the separating member (9) and the combustion chamber (1) .

27. A combustion plant according to claim 26, characterized in that the first connection (59) comprises a pipe extending between the container device (63, 64, 65) and the channel (23).

28. A combustion plant according to claim 27, characterized in that the cross-section area of the pipe (59) is less than the cross-section area of the channel (23) and that the pipe (59) extends into the channel (23) from beneath.

29. A combustion plant according to any one of claim 26-28, characterized in that the recirculating device comprises a passage (23, 25, 26, 27, 35) provided in a lower portion of the channel (23) and arranged in such a manner that a column (24) of material is formed in the channel (23) having a height (h' ) exceeding the height (h) of the bed (4) in the combustion chamber (1) during the operation of the combustion plant.

30. A combustion plant according to claim 29, characterized in that the regulating means comprise a member (70, 35) arranged to regulate the flow area of said passage.

31. A combustion plant according to claim 32, characterized in that the said regulating member comprises a cone (35) provided at the orifice (34) of the channel (23) and displaceable towards and away from the orifice.

32. A combustion plant according to any one of claims 29-31, characterized in that said passage (23, 25, 26, 27, 35) permits the weight of the column (24) of material to discharge the material therefrom in a continuous flow.

33. A combustion plant according to claim 29-32, characterized in that said passage (23, 25, 26, 27, 35) is arranged to prevent the gas from beneath from entering the channel (23) .

34. A combustion plant according to any one of claims 14- 33, characterized in that the recirculating device (23) has an orifice in the bed (4), preferably below said tube arrangement (22) .