WO1999040370A1

WO1999040370A1 - A combustion chamber and a method for controlling the combustion in a combustion chamber

Info

Publication number: WO1999040370A1
Application number: PCT/SE1999/000167
Authority: WO
Inventors: Mats C. Andersson
Original assignee: Abb Ab
Priority date: 1998-02-10
Filing date: 1999-02-10
Publication date: 1999-08-12
Also published as: SE9800380D0; SE9800380L

Abstract

The present invention relates to a combustion chamber and a method for combustion in a fluidised bed. The combustion chamber (1) is arranged to house a fluidised bed and comprises a plurality of heat exchanger tubes (20). The combustion chamber is arranged for sub-stoichiometric combustion in a first zone (21) in the bed (4) and arranged for stoichiometric or over-stoichiometric combustion in a second zone (22). The combustion chamber is arranged to allow the mixing of partially non-combusted gases from the first zone (21) and gas with a surplus of oxygen from the second zone (22) in a space above the bed (4).

Description

A combustion chamber and a method of controlling the combustion in a combustion chamber

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention concerns a combustion chamber, arranged to include a fluidised bed and comprising an array of steam tubes, and arranged for substoichiometric combustion with regard to oxygen within a first zone in the bed and for stoichiometric or superstoichiometric combustion with regard to oxygen within a second zone in the bed. The invention also concerns a method of controlling the combustion in a combustion chamber, which comprises a fluidised bed and an array of steam tubes, which method comprises supplying a fuel and an oxygen-containing medium to the fluidised bed, and that at least one of the supply of the fuel and the supply of the oxygen-containing medium to a first zone in the bed is controlled such that the combustion in this zone is substoichiometric with regard to oxygen and to a second zone in the bed is controlled such that the combustion in this zone is stoichiometric or superstoichiometric with regard to oxygen.

Combustion chambers and methods of the above-mentioned kind are previously known within the technical field which includes the combustion of for example coal in pressurised, fluidised beds, such as is done in PFBC-plants (PFBC = Pressurised Fluidised Bed Combustion) . In such plants, the fumes generated in the combustion chamber are used usually to run a low pressure gas turbine and a high pressure gas turbine and compressors connected to these turbines, which compressors pump a pressurised medium, which usually is air, to the combustion chamber for pressurising and oxygenating the bed arranged in the combustion chamber. Said array of steam tubes is arranged in the fluidised bed as is connected to a steam turbine. Water which is led into the array of steam tubes is heated, vaporised and overheated before it is finally led to the steam turbine in order to run the same. The array of steam tubes fills essentially the whole cross- sectional area of the bed. The pressurised air which is supplied to the bed is led to the same via a container located under the bed, a so called wind-box and a number of nozzles connected to the wind-box, which nozzles are distributed over the cross-sectional area of the bed.

From the Swedish document SE 458 955 a combustion chamber is known, which is arranged for substoichiometric combustion with regard to oxygen within a first zone, or chamber, in the bed and for superstoichiometric combustion with regard tσ oxygen within a second zone, or chamber, in the bed. The first zone thereby functions as a gasifier, from which partly unburned gas is led to a secondary combustion chamber where it is allowed to burn completely and at a later stage is mixed with the combustion gases from the second zone, in order to increase the temperature of the gas which is supplied to a subsequent gas turbine. The two zones are, above the bed and partly in the bed, separated from each other by a partition wall such that in principal two chambers are formed. A disadvantage with this device is the complicated arrangement of the secondary combustion chamber and the separate conduit system which is required to and from the same. The main purpose with this device is to solve the problem of reaching essentially higher gas temperatures (1200-1300°C) at the inlet to the gas turbine.

It is advantageous if the fumes which leave the combustion chamber via a gas outlet in its upper part have a temperature which is so high that the gas has a suitable working temperature for the subsequent gas turbine when it reaches the same . The temperature may however not be higher then the softening temperature of the ashes. If a higher temperature is wanted, a special purification of the fumes is necessary before the temperature can be risen. Therefore, the temperature of the fumes in the area of the gas outlet should be about the range 800-900°C. It has, in particular for a partial load operation of a PFBC-plant with which the combustion chamber is connected, become apparent that it is difficult to reach such a temperature of the fumes in said area. The reasons for this are well described in SE 470 222. Inter alia, a part of the array of steam tubes which during partial load operation is uncovered above the bed itself will cool the fumes leaving the bed. According to SE 470 222 it is thereby possible to still reach a nominal working temperature of the fumes in that burning of a complement fuel is arranged in the fumes downstream of uppermost tubes in the bed. This burning is accomplished in that a fuel is injected, well mixed with the fumes and ignited. Special devices are however necessary for arranging said burning of a complement fuel .

SUMMARY OF THE INVENTION

A purpose with the present invention is to provide a combustion chamber which makes it possible to, at a given amount of burned fuel and added oxygen, control the temperature of the fume which leaves the combustion chamber such that a suitable working temperature is obtained in the gas for running a gas turbine connected to the combustion chamber .

This purpose is achieved with the initially defined combustion chamber, which is characterised in that it is arranged to allow mixing of partly unburned gas from the first zone with gas with an excess of oxygen from the second zone in a space above the bed. Thereby, unburned gases from the first zone will be transported towards the gas outlet of the combustion chamber and fumes with an excess of oxygen will be transported towards said outlet from the second zone. These two gases will be mixed with each other in an area above the fluidised bed or in the area of the gas outlet and thereby a combustion of the previously unburned gas from the first zone will take place because of the oxygen supplied together with the fumes from the second zone. The gas combustion at this late stage leads to an increase in the temperature of the gas leaving the combustion chamber compared to the case where essentially all gas combustion would have taken place in the bed itself or in its closest surroundings .

According to a preferred embodiment of the combustion chamber, the array of steam tubes is arranged essentially in the second zone but not in the first zone. Thereby it is avoided that the steam tubes arranged in the bed are exposed to the corrosive environment which is obtained in the zone where the combustion takes place under substoichiometric conditions with regard to oxygen, that is in the first zone.

According to a still further embodiment, the combustion chamber is arranged to include one single bed.

According to still another embodiment, the combustion chamber does not comprise any partition wall or other separating device, wherein said first and second zone is defined by controlling the combustion.

According to a further preferred embodiment, the combustion chamber comprises means which allows for separate supplied amount of oxygen to the first and second zones, respectively. Thereby, the degree of substoichiometric combustion in the first zone and superstoichiometric combustion in the second zone, respectively, may be separately controlled and optimal conditions may be set for different load conditions of the combustion in the combustion chamber.

According to a further preferred embodiment, said means comprises a first device for the injection of an oxygen- containing medium into the first zone and a second device for the injection of an oxygen-containing medium into the second zone, wherein said first and second device constitute separate containers, via which the oxygen-containing medium via inlet nozzles is supplied to the bed. The term container hereby includes the so-called wind-boxes which conventionally are used for the supply of an oxygen- containing gas, such as air, to the fluidised bed. Such an arrangement offers a very simple solution to the problem with separate supply of an oxygen-containing medium, such as an oxygen-containing gas, to the respective zone.

According to a further preferred embodiment, the combustion chamber comprises a member arranged to inject a supplement medium which includes at least one of water, steam, a combustible gas or an oil into said first device. In case the oxygen-containing medium is air, the injection of water or steam which is brought into contact with the air will lead to that the amount of oxygen available for the combustion is reduced and an increase of the substoichiometric condition in the first zone is obtained. By the injection of a combustible gas or oil so that it is brought into contact with for example air which is supplied to the first zone a combustion of the gas or oil will take place, which also has as a result that the amount of oxygen which is available for the combustion in the first zone is reduced and the substoichiometric condition in this zone is consequently increased. It should be noted that the air or gas/gas mixture which is brought into contact with said supplement medium has a considerable temperature at the place where the injection is done, which makes the described result possible.

According to a further preferred embodiment, the combustion chamber comprises means for separate control of the amount of fuel which is supplied to the first and to the second zone, respectively. Thereby an additional possibility to separately control the degree of substoichiometric combustion in the first zone and superstoichiometric combustion in the second zone is obtained.

According to a further preferred embodiment, the combustion chamber comprises a gas outlet and the first zone is located essentially directly under the gas outlet . Such an embodiment is in particular advantageous when the second zone is essentially symmetrically arranged on opposite sides of or around the first zone . Thereby the mixing of the gases from the two zones is done closer to the gas outlet and the mixing of the gases may be more complete, possibly with the help of a possible member especially arranged for the mixing in the area of the gas outlet .

A further purpose with the invention is to provide a method by means of which it is possible to, at a given amount of supplied oxygen and supplied fuel to the bed, control the temperature of the fumes leaving the combustion chamber such that a suitable working temperature is obtained in the gas for running a gas turbine connected to the combustion chamber.

This purpose is obtained with a method as initially defined which is characterised in that incompletely burned gas from the first zone and gas with an excess of oxygen from the second zone are mixed in a space above the bed. Thereby a flow of at least partially unburned gas from the first zone to a gas outlet of the combustion chamber is created and a flow of oxygen containing fumes from the second zone towards the gas outlet is created, these gases being mixed above the bed or in the area of the gas outlet itself and thereby a combustion of the previously unburned gases is obtained, which leads to the effect that the fumes which leave the combustion chamber from the gas outlet obtain a higher temperature than would otherwise have been the case.

According to a preferred embodiment of the method, the combustion chamber is arranged to include one single bed, wherein said first and second zone is defined by said control of the combustion.

According to a preferred embodiment of the method, the oxygen-containing medium is essentially a gas. It can partly or completely constitute air and at least a part of the oxygen-containing medium which is to be supplied to the first zone may previously be brought into contact with a supplement medium which includes at least one of water, steam, a combustible gas or an oil. Thereby at least a part of the available oxygen is bound in compounds where it does not lead to any further combustion in the bed, whereby the degree of substoichiometry in the first zone is increased further. By controlling the amount and kind of supply medium the degree of substoichiometry in the first zone may thus be controlled in a complementary way.

According to a further embodiment, the oxygen-containing medium is brought into contact with the supplement medium in that the latter is injected into a container, via which the oxygen-containing medium is supplied to the first zone in the bed. Advantageously said container comprises a wind-box of a conventional kind, but arranged separately for the supply to the first zone. This solution has the advantage to offer a very simple way in which the supplement medium is supplied to the oxygen-containing medium, which preferably is air but which could be any other kind of oxygen- containing gas mixture.

Of already previously mentioned reasons, the supply of the oxygen-containing medium to the first zone is done essentially directly under a gas outlet arranged in the upper part of the combustion chamber.

Further advantages with and features of the combustion chamber and the method according to the invention are clear from the remaining dependent claims and from the following description.

SHORT DESCRIPTION OF THE DRAWINGS

The present invention will below be described by means of non-limiting examples with reference to the appendant drawings, wherein:

Fig 1 is a schematic cross-sectional view of an embodiment of the combustion chamber according to the invention. Fig 2 is a cross-sectional view from above according to II-II in Fig 1 of the bed of the combustion chamber, showing the extension of the respective zones . Fig 3 is a cross-sectional view according to III-III in

Fig 1, and Fig 4 is a schematic general view of a PFBC-plant which comprises the combustion chamber according to the invention. DETAILED DESCRIPTION OF AN EMBODIMENT

The invention is possible to use in a plant for the combustion of a particle formed fuel in a pressurised, fluidised bed, in particular in a bubbling kind of bed. It may be arranged for use in for example a so-called PFBC- power plant, and consequently it will hereinafter be described as applied to such a plant and the general construction and function of an embodiment of such a plant will now be explained with reference to Fig 4.

The plant comprises a combustion chamber 1, which is included in a pressure vessel 2, which may have a volume of about the range 10⁴ m² and which may be pressurised to for example about 16-18 bar. Compressed air 3 for pressurising the combustion chamber and for fluidising a bed 4 in the combustion chamber is supplied to the pressure vessel 2. The compressed air is supplied to the combustion chamber via schematically indicated fluidising nozzles 5 at the bottom of the combustion chamber for fluidising the bed included in the combustion chamber. The bed 4 is constituted by bed material, particle formed absorbent and a particle formed fuel, preferably crushed coal, which is burnt in the fluidising air supplied to the bed. The combustion gases, also called fumes, from the bed are supplied to a purification device 7 illustrated by a block, which device for example may be constituted of a set of cyclones arranged to separate particles from the fumes. The fumes continue to a gas turbine 9 which comprises a high pressure stage 10 and a low pressure stage 11. The high pressure turbine 10 is arranged on the same shaft as a high pressure compressor 12 as well as a generator 13, which in this manner is driven by the high pressure turbine in order to deliver electric energy to an electricity supply network connected thereto. 10

The high pressure compressor 12 delivers compressed air to the combustion chamber 1 via the conduit 14.

The gas expanded in the high pressure turbine 10 is supplied to a low pressure turbine 11. The exhaust which leaves the low pressure turbine still contains energy, which an economizer 16 is arranged to take advantage of. On the shaft of the low pressure turbine also a low pressure compressor 17 is arranged, which is supplied with atmospheric air via a filter 18. The low pressure compressor is thus driven by the low pressure turbine and supplies, from its outlet, the high pressure compressor 12 with air which has been compressed at a first stage. Between the low pressure compressor and the high pressure compressor an intermediate cooler 19 is connected in order to lower the air temperature of the air which is supplied to the inlet of the high pressure compressor 12.

Furthermore, the plant comprises a steam turbine side, which is not shown here, but which is indicated by an array of steam tubes 20 submerged into the fluidised bed 4, in which tubes water is circulated and vaporised and overheated by heat exchange between the tubes and the bed material for absorbing a part of the heat which is produced by the combustion in the bed.

In Fig 1 an embodiment of a combustion chamber according to the invention is shown, suitable to constitute the combustion chamber 1 schematically shown in Fig 4. The combustion chamber thus comprises a fluidised bed 4, fluidising nozzles 5 and an array of steam tubes 20. The steam tubes 20 are hereby in a conventional manner arranged in a disc or plate-like formation, wherein each disc or plate of steam tubes 20 is called a tube plate. It should be noted that the tube plates are connected to each other via not in more detail shown tube branches which make it 11

possible for steam or liquid to be transported from one tube plate to another.

The bed 4 is divided into essentially two zones, a first zone 21 and a second zone 22, here divided into two subzones separated by the first zone 21, which is also clear from Fig 2 and 3. The first zone 21 is thereby arranged for essentially substoichiometric combustion in the bed 4, while the second zone 22 is arranged for essentially stoichiometric or superstoichiometric combustion with regard to oxygen in the bed. In order to achieve these different combustion conditions in the respective zones 21 and 22 the combustion chamber comprises separate containers 23-25, which may be constituted of so-called wind-boxes or sparge- tubes and via which the fluidising air is arranged to be guided to the fluidising nozzles 5 which emanate from the respective zone 21, 22. The flow of the fluidising air in the respective containers 21-25 via which it is guided to the fluidising nozzles 5 of the first and second zones 21, 22, respectively, is arranged to be individually controlled by means of separate flow control means for the respective containers 24-25. Said flow control means is in Fig 2 exemplified by means of schematically shown valves 26-28, by means of which the flow of the fluidising air into each respective container may be controlled individually. In order to further control the amount of the, for the combustion in the bed 4 available, oxygen which via the separate container 23 and its fluidising nozzles 5 reaches the first zone 21, the combustion chamber further comprises a member 29 arranged to inject a supplement medium which comprises at least one of water, steam, a combustible gas or an oil into the separate container 23. Due to the supply of such a supplement medium, the amount of oxygen available for combustion which reaches the first zone may be further reduced and the degree of substoichiometric combustion in said zone 21 may be further increased. The amount of 12

supplied oxygen may thereby also be varied without essentially changing the flow of the fluidising air. That it, the amount of the supplied oxygen may be changed without changing the fluidising properties of the bed.

In order to further increase the possibilities of individually controlling the degree of substoichiometric and superstoichiometric combustion, respectively, in the respective zones 21, 22 the combustion chamber comprises means for separate control of the amount of fuel which is supplied to the first and the second zones 21, 22 respectively. This means 30 comprises separate fuel supply members for the respective zones 21, 22, which fuel supply members 30 allow individual control of the amount of supplied fuel to the respective zones 21, 22.

As is clear from Fig 2 and 3, the array of steam tubes 20 is only arranged in the second zone 22 and not in the first zone 21. Thereby it is avoided that the array of steam tubes 20 is exposed to the locally strongly corrosive environment which is formed in a first zone 21 due to the substoichiometric combustion with regard to oxygen. Steam tubes 20 which are arranged in the border area between the first and second zones 21, 22 may if necessary be arranged to withstand corrosion and erosion better than the remaining steam tubes. For example, they may be made of a corrosion resistant material .

The combustion chamber further comprises a centrally arranged gas outlet 31, and the first zone 21 is arranged essentially directly under this gas outlet .

The first and second zones 21, 22 are consequently arranged at the side of each other and separated along a border area with an essentially vertical extension. In this embodiment the combustion chamber has an essentially rectangular cross- 13

section seen from above, with a length of about 7.5 m and a width of about 4m in an embodiment for producing about 100MW electricity. The tube plate free area, which essentially corresponds to the first zone, extends over the whole width of the combustion chamber, is rectangular and measures 1 x 4m. It may, as well as the first zone 21, however have a larger width, for example 2m. In order to in some manner compensate for the absence of tube plates in the first zone 21, the combustion chamber may for example be brought to work with a somewhat higher bed hight than what is otherwise the case .

It is obvious that a number of varieties and alternative embodiments of the combustion chamber according to the invention and of the method which has been described above will be clear to the person skilled within the field without this person thereby leaving the scope of the invention^ For example, it is obvious that the principle of the invention is applicable also to other combustion chambers than those which have an essentially rectangular cross-section. It is also possible that the first zone 21 is divided into several subzones separated from each other. In such a manner, an alternate arrangement of zones with substoichiometric and superstoichiometric combustion, respectively, is obtained.

The combustion chamber comprises essentially no partition wall or other dividing device. Preferably, the combustion chamber comprises no such dividing wall or other separating device at all. The combustion chamber thus preferably comprises one single bed.

The different zones are thereby not formed by the fact that the bed is divided with the help of such separating devices.

Instead, the zones are defined by controlling the combustion. This is done by separate supply of oxygen and/or 14

wall by separate supply of fuel to different parts of the bed, corresponding to the different zones.

According to an advantageous embodiment of the invention, the size of the zones may be modified. This may, for example, be done in that the supply of oxygen may be controlled more locally. For example, it is possible to arrange a plurality of separate wind-boxes in a border area between the zones. Thereby the size of the area with an excess and a shortage of oxygen, respectively, may be controlled. Thereby, the size of the area with a shortage of oxygen may be controlled, that is the size of said first zone may be varied.

It is also possible to arrange some kind of mixing device in the so-called freeboard above the bed or in the area of the gas outlet, with the help of which device an improved mixing of the gases which reach this point from the respective zones is obtained.

In case the combustion in the second zone is completely or very close to completely stoichiometric, and thus no extra oxygen or only a small amount of extra oxygen is transported together with the fumes leaving this zone in the direction towards the outlet 31, the invention comprises the possibility of, in an upper area of the combustion chamber 1, arranging means for injecting an oxygen-containing medium with the purpose of still obtaining a complete or almost complete combustion of the gases which rise from the first zone 21 in this upper area of the combustion chamber, and to thereby obtain an increased temperature in the gas which leaves the combustion chamber 1 relative to prior, conventional technology.

Claims

15Claims

1. A combustion chamber, arranged to include a fluidised bed (4) and comprising an array of steam tubes (20) , and arranged for substoichiometric combustion with regard to oxygen within a first zone (21) in the bed (4) and arranged for stoichiometric or superstoichiometric combustion with regard to oxygen within a second zone (22) in the bed (4) , characterised in that it is arranged to allow mixing of partly unburned gas from the first zone (21) and gas with an excess of oxygen from the second zone (22) in a space above the bed (4) .

2. A combustion chamber according to claim 1, characterised in that the array of steam tubes (20) is arranged essentially in the second zone (22) but not in the first zone (21) .

3. A combustion chamber according to claim 1 or 2, characterised in that the combustion chamber is arranged to include one single bed (4) .

4. A combustion chamber according to claim 3 , characterised in that the combustion chamber does not comprise any partition wall or other separating device, wherein said first (21) and second (22) zone is defined by controlling the combustion.

5. A combustion chamber according to anyone of the claims

1-4, characterised in that it comprises means (23-28) which allows for separate supplied amount of oxygen to the first and second zones (21,22) , respectively.

6. A combustion chamber according to claim 5 , characterised in that said means (23-28) comprises a first 16

device (23) for the injection of an oxygen-containing medium into the first zone (21) and a second device (24,25) for the injection of an oxygen-containing medium into the second zone (22) .

7. A combustion chamber according to claim 6, characterised in that said first (23) and second (24,25) device constitute separate containers, via which the oxygen- containing medium via inlet nozzles (5) is supplied to the bed.

8. A combustion chamber according to claim 6 or 7, characterised in that it comprises a member (29) arranged to inject a supplement medium which includes at least one of water, steam, a combustible gas or an oil into said first device (23) .

9. A combustion chamber according to anyone of claims 6-8, characterised in that the oxygen-containing medium is a gas and completely or partly consists of air.

10. A combustion chamber according to anyone of claims 1-9, characterised in that it comprises means (30) for separate control of the amount of fuel which is supplied to the first and to the second zone (21,22), respectively.

11. A combustion chamber according to anyone of claims 1- 10, characterised in that it comprises a gas outlet (31) and that the first zone (21) is located essentially directly under the gas outlet (31) .

12. A method of controlling the combustion in a combustion chamber (1) , which comprises a fluidised bed (4) and an array of steam tubes (20) , which method comprises supplying a fuel and an oxygen-containing medium to the fluidised bed (4) , and that at least one of the supply of the fuel and the 17

supply of the oxygen-containing medium to a first zone (21) in the bed is controlled such that the combustion in this zone is substoichiometric with regard to oxygen and to a second zone (22) in the bed is controlled such that the combustion in this zone is stoichiometric or superstoichiometric with regard to oxygen, characterised in that incompletely burned gas from the first zone (21) and gas with an excess of oxygen from the second zone (22) are mixed in a space above the bed (4) .

13. A method according to claim 12, characterised -in that the array of steam tubes (20) is arranged essentially in the second zone (22) but not in the first zone (21) .

14. A method according to claim 12 or 13, characterised in that the combustion chamber is arranged to include one single bed (4), wherein said first (21) and second (22)^~zone is defined by said control of the combustion.

15. A method according to anyone of claims 12-14, characterised in that the oxygen-containing medium is essentially a gas .

16. A method according to anyone of claims 12-15, characterised in that the oxygen-containing medium essentially comprises air.

17. A method according to claim 16, characterised in that at least a part of the oxygen-containing medium which is to be supplied to the first zone (21) previously is brought into contact with a supplement medium which includes at least one of water, steam, a combustible gas or an oil.

18. A method according to claim 17, characterised in that the oxygen-containing medium is brought into contact with the supplement medium in that the latter is injected into a 18

container (23) , via which the oxygen-containing medium is supplied to the first zone (21) in the bed (4) .

19. A method according to anyone of claims 12-18, characterised in that the supply of the oxygen-containing medium to the first zone (21) is done essentially directly under a gas outlet (31) arranged in the upper part of the combustion chamber (1) .