WO1997000721A1

WO1997000721A1 - A method in as well as a power plant with combustion of particle-like fuel

Info

Publication number: WO1997000721A1
Application number: PCT/SE1996/000803
Authority: WO
Inventors: John Weatherby
Original assignee: Abb Carbon Ab
Priority date: 1995-06-21
Filing date: 1996-06-19
Publication date: 1997-01-09
Also published as: SE9502247L; SE9502247D0

Abstract

A power plant having a combustor (1) with a bed (4) fluidized through air supplied in the bottom region thereof and members (22, 23) adapted to feed a particle-like fuel into the bed at defined locations in the bottom region thereof for combustion of the fuel in the bed comprises means (24, 25, 27) for adjustably supplying additional air separated from the fluidizing air to the bed at said locations.

Description

Applicant: ABB CARBON AB

A METHOD IN AS WELL AS A POWER PLANT WITH COMBUSTION OF PARTICLE-LIKE FUEL

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a power plant having a combustor and a bed located therein, to the bottom region of which air is supplied for fluidizing the bed, particle¬ like fuel being fed into the bed at defined locations in the bottom region thereof for combustion of the fuel in the bed as well as a method in combustion of particle-like fuel according to the preamble of the appended claim 1.

Firing of particle-like fuel in a fiuidized bed requires a very good distribution of the fuel fed into the bed already in the very region of feeding it thereinto, i.e. in the bottom region of the bed, so as to avoid the problems discussed below.

It is namely extremely important that all the fuel supplied to the bed is also combusted in the bed, i.e. that there is in every part of the bed sufficient air, in the practice oxygen, so as to make it possible for all the fuel present there to be combusted, since otherwise uncombusted fuel will reach the region above the bed and be combusted there in the so called free board or in tubes or cyclones in the path of the flue gases downstream said bed, so that the problem with an unacceptable high gas temperature may arise there, through which it may be necessary to run the power plant at a lower load than otherwise would be possible. There is also an imminent risk of fires further in the downstream direction of the plant as a consequence of fuel still in the flue gases there. Furthermore, local shortages of oxygen leads to a lowered desulphurization of the fuel there and accordingly an increased production of SO_x being harmful to the environment. An increased supply of fluidiz¬ ing air to the bed is at the same time no acceptable solution to this problem, especially since this gives rise to another large problem, since the production of N0_X increases remarkably in combustion during a too large excess of O2.

The problem mentioned above could to a certain degree be solved by arranging a very great number of points for feeding in fuel, so that a uniform distribution in the fiuidized bed of fuel and the combustion air fluidizing the bed is obtained. Such a construction will then be very costly and creates other difficulties with the complicated network of distribution conduits to the different nozzles and problems should the different conduits become plugged.

Another possible solution consists in arranging any form of distributor of the fuel in the bed after feeding it there¬ into, which is known through for example the patent GB 1 326 651, in which a distributor in the form of a rotating wheel provided with blades is used. However, such movable details are strongly influenced by degradating powers, such as for example the high temperature, which in combination with the fact that the availability for maintenance of possible devices is strongly reduced or disturbs the operations of the power plant, means that this is a bad so- lution of the problem. It has also earlier been proposed to arrange a spreading device in such a bed for distributing the gas and fuel in the bed between the nozzles for feeding in the fuel and tubes for steam generation.

However, none of these already known measures manage to solve the problem mentioned above in a satisfactory manner. It gets particularly difficult to avoid locally generated so called plumes at the locations of the feeding of fuel into the bed, i.e. a region or volume extending from the nozzle for feeding the fuel and upwardly in the bed, and in which a shortage or deficit of oxygen prevails and thereby is not all the fuel supplied combusted, when a fast load change in the power plant takes place, since a reduction the average 02-level in the bed will then easily be ob- tained, so that in such a case there will be produced too much S0_X locally.

Another disadvantage of the prior art plants is that they require the use of a particle-like fuel having a low content of volatile substances, which is strongly limiting for the plants. Volatile substances present in the fuel will namely in contact with air rapidly take care of a great part of the oxygen existing therein, so that there will be a shortage of oxygen at the locations where the fuel is fed in. Also if a fuel having a low content of volatile substances is used the problem with a tendency to a deficit of oxygen will still be there in the prior art devices with the problems this will bring about at the locations for feeding the fuel into the bed, should not un- acceptably much fluidizing air be supplied to the bed, which then will cause a tremendous production of NO_x in other parts of the bed.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method as well as a power plant of the type defined in the intro¬ duction, which find a remedy to the inconveniences men¬ tioned above of such methods and plants already known. This object is according to the invention obtained by pro¬ viding such a power plant with means for adjustably supply¬ ing additional air separated from the fluidizing air to the bed at said locations as well as a method according to the independent method patent claim.

Through such an adjustable supply of additional air to the bed at the locations for feeding fuel thereinto, it will be possible to locally increase the amount of oxygen available exactly where it is needed, so that all the fuel may be combusted completely also in the plume-like regions other¬ wise produced while obtaining a desired degree of desulphu- rization, at the same time as no too large production of N0_X takes place in other regions of the bed. Said adjusta- bility makes it possible to achieve an adaption of the amount of additional air supplied to the conditions instan¬ taneously prevailing, accordingly to a load change of the power plant possibly going on or a certain type of fuel when the fuel type is arranged.

Thuε, the invention makes it possible to use fuels having different contents of volatile substances and when a fuel having a high content of volatile substances is used an ad¬ justment towards supply of a greater flow of additional air to the locations for feeding in fuel takes place, so that there will be a sufficient amount of oxygen left in the re¬ gions over these locations for being enough for the combus¬ tion of the fuel there after the volatile substances have taken care of that part of the oxygen they will demand. It is namely well possible that fuels of different types, such as of different carbon types, may have the same thermal value and be fed into the bed with the same feed rate, but they may be combusted differently fast with a factor 3. The faster the combustion takes place the more air is of course required locally so as to avoid the problem mentioned above. According to a preferred embodiment of the invention said power plant comprises a control unit arranged to control said means, and said means comprise at least one conduit for conducting said additional air to said locations as well as at least one regulating member arranged to be controlled by the control unit and regulate the flow of said additional air in said conduit. A supply of exactly the amount of additional air required on every occasion at the locations for feeding in fuel with regard to possible load changes and the fuel type used is in this way obtained in an advantageous way.

According to a further preferred embodiment of the inven- tion the device comprises members arranged to record data concerning the nature of the fuel fed to said locations and important for the combustion of the fuel, and the control unit is arranged to control said adjusting means on the basis of said data. The supply of said additional air may in this way reliably be adapted to important nature data, such as the contents of volatile substances, of the fuel fed into the bed for obtaining an optimum combustion of the fuel without any oxygen deficit and exaggerated formation of SO_x. The definition of said members is here intended also to comprise members recording these nature data through a purely manual adjustment thereof through an operator.

According to yet another preferred embodiment of the invention the power plant comprises members adapted to record changes of the power tapped from the power plant, and the control unit is adapted to control said adjusting means on the basis of this recordal. It is in this way ensured that the supply of said additional air will be adapted to changes of the power tapped from the power plant, which will have a tendency to change the availabil¬ ity of oxygen in the bed.

According to another preferred embodiment of the invention the power plant comprises an additional conduit for supply of said additional air besides a conduit for feeding fuel into the bed at said locations. This embodiment may be used for all types of fuel supply, but it is especially intended for the case in which the fuel is fed into the bed in the form of a liquid paste, since it will in such a case not be possible to supply said additional air directly together with the fuel.

According to another embodiment of the invention, which is applied to a power plant having members for feeding in fuel adapted to feed the fuel into the bed through feeding air conveying the fuel, said regulating means are arranged to regulate the supply of said additional air through changing the air/fuel-ratio in the fuel feeding air. In such a so called "dry-feeding in" of fuel said regulation may accord¬ ingly be obtained in a very simple way by providing the ar¬ rangement for feeding in fuel already there with a means for regulating the air/fuel-ratio in the conduit for feeding in fuel.

Further advantages and advantageous characteristics of the invention will appear from the following description as well as the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description of methods and power plants according to preferred embodiments of the invention cited as examples.

In the drawings: Fig 1 schematically illustrates a PFBC-power plant having a combined gas and steam cycle (the latter is not shown), in which a device according to the invention may be applied,

Fig 2 is a schematic, partially sectioned view of the pressure vessel with the combustor arranged therein in the power plant according to Fig 1, which illustrates a first preferred embodiment of the method according to the inven- tion and the device according to the invention,

Fig 3 is a simplified, sectioned view from above of the pressure vessel and the combustor according to Fig 2, and

Fig 4 is a view corresponding to Fig 2, which illustrates a second preferred embodiment of the method according to the invention and the device according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention is not in any way restricted to the use in a PFBC-power plant, i.e. a plant for combustion of particle¬ like fuel in a pressurized fiuidized bed, but it has special advantages when used in such a power plant, which is the reason for describing it applied to such a plant, and the overall construction and function of a possible type of such a power plant will now be explained with reference to Fig 1.

The plant comprises a combustor 1, which is received in a pressure vessel 2, which may have a volume in the order of 10^ m3 and which may be pressurized to for example about 16 bars. Compressed air 3 for pressurizing the combustor and for fluidizing a bed 4 in the combustor is supplied to the pressure vessel. The compressed air is supplied to the combustor through fluidizing nozzles 5 schematically indicated at the bottom of the combustor for fluidizing the bed enclosed in the combustor. The bed is made of bed material, granule-shaped absorbent and a particle-like fuel, preferably crushed carbon, which is combusted in the fluidizing air supplied to the bed. The combustion gases, hereinafter called flue gases, in the bed are led to a pu¬ rification plant 7 illustrated through a box, which for ex¬ ample may be constituted by a set of cyclones arranged to separate particles from the flue gases. The flue gases are then proceeding to a gas turbine 9 comprising a high pressure step 10 and a low pressure step 11. The high pressure turbine 10 is arranged on the same axle as a high pressure compressor 12 and a generator 13, which in this way is driven by the high pressure turbine for output of useful energy. The high pressure compressor 12 delivers compressed air to the combustor 1 through the conduit 14.

The gas expanded in the high pressure turbine 10 is led to a low pressure turbine 11. The gases leaving the low pressure turbine still contains energy, which is utilized by an economizer 16. There is also a low pressure compres¬ sor 17 arranged on the axle of the low pressure turbine and atmospheric air is supplied thereto through a filter 18. Thus, the low pressure compressor is driven by the low pressure turbine and provides from the outlet thereof the high pressure compressor 12 with air compressed in a first step. An intercooler 19 is arranged between the low pres¬ sure compressor and the high pressure compressor for lowering the air temperature of the air supplied to the in¬ let of the high pressure compressor 12.

The power plant has also a steam turbine side, which is not shown here, but it is indicated through a set of tubes 20 immersed in the fiuidized bed 4, in which water is circu¬ lated and evaporated and superheated through heat exchange between the tubes and the bed material for absorbing the heat generated through the combustion carried out in the bed.

The main principles of the function of a power plant of this type will appear well from the description above.

It is illustrated schematically in Fig 2 how fuel may be fed into the bottom region of the bed through fuel feeding conduits 22 emerging at different locations 21. The con¬ duits 22 derive from a common main conduit 23 for supply of fuel from a fuel supply. It is pointed out that the number of conduits 22, the proportions of these with respect to each other, the fluidizing nozzles and other components of the power plant are only completely arbitrarily chosen in the Figures, which only have the purpose to illustrate the idea of the invention.

The fuel is preferably, but not at all necessarily, consti- tuted by crushed carbon, which may be supplied as a liquid paste, i.e. preferably mixed up with water, or in a dry form through air transport. However, the embodiment of the invention shown in Figs 2 and 3 is especially applied to supply of fuel in the form of a liquid paste. The power plant has further a main conduit 24 for supply of air to the different locations 21 for feeding in of fuel through conduits 25 branched therefrom. The conduit 24 takes preferably the air from the air present in the pressure vessel 2 by emerging thereinto at 26. The conduit 24 is provided with a regulating member in the form of a valve 27, by which the flow of air, i.e. the amount of supplied air/time unit, to the locations 21 for feeding in of fuel may be regulated. The plant further comprises a control unit indicated by a box 28 adapted to control the valve 27. It is also indicated through lines 29 and 30 that members are arranged to co-operate with the control unit and these are arranged to record data concerning the nature of the fuel fed to said locations important for the combustion of the fuel and changes of the power tapped from the power plant, respectively, said control unit being adapted to control the valve 27 on the basis of this information from the members 29 and 30. Even if the fuel fed into the bed will be spread comparatively uniformly in the bed, it may not be neglected that a high concentration of fuel will be there exactly at the locations 21 for feeding it into the bed. This means that the air supplied to the bed through the fluidizing nozzles 5 will exactly there have a tendency to be rapidly depleted with respect to oxygen, which the locally increased combustion of the fuel requires. Quite an amount of oxygen will also at the locations for feeding in the fuel be used for the reaction with the volatile sub¬ stances contained in the fuel, and the size of this part depends on the contents of volatile substances in the fuel. For example in the case of crushed carbon, the fuel con¬ sists mainly of C, H, N, 0, S and ashes and in the case of a supply of the fuel in the form of a liquid paste of H2O, of which primarily H2O, H, N and 0, but also C and S, may be considered as volatile substances. The contents of volatile substances may for example when different carbon types are used vary between for example 20 and 50 % by weight without changing the thermal value of the carbon for that sake.

By the supply of said additional air through the conduits 25 to the locations for feeding in fuel and the regulation of this supply in dependence upon the nature of the fuel supplied and changes of the power tapped from the power plant, it may be ensured that there will not be any deficit of oxygen locally at the locations for feeding in the fuel and in the region thereabove, which would lead to a too large production of S0_X as well as an uncomplete combustion of the fuel, or a somewhat too high excess of oxygen, which would lead to an unacceptable, increased NO_x-production. Lime is preferably supplied to the bed together with the fuel for the desulphurization reaction, and the desulphuri- zation reaction may be written: CaC03 + SO2 + h® ^→ CaSθ4 + CO2, and oxygen is accordingly consumed in said desulphu¬ rization, which accordingly may not take place to a desired extent when there is a deficit of oxygen and unacceptable amounts of SO2 will then come into the open air through the chimney of the power plant.

It is pointed out that the fluidizing air is always sup¬ plied so that there will always be an average 02~σontent in the flue gases in the free space 31 over the bed 4 within a certain interval, but it may without the supply according to the invention of additional air to the points for feeding in fuel well be so that the contents of O2 over the bed over the locations for feeding in fuel may be lower than this interval, while it is over this interval at another location. However, this lower value may well mean a still lower value at the location for feeding in fuel, which may be so low that it is unacceptable. A rapid increase of the load of such a power plant may in absence of the supply of the additional air according to the invention well result in an average 02-content in the free space 31 being lower than said interval, which indicates that the deficit of oxygen may prevail at the locations 21 for feeding in fuel.

A second embodiment of the invention is illustrated in Fig 4, in which the members for feeding in fuel are arranged to feed fuel into the bed through feeding air conveying the fuel, so called dry feeding in of fuel. Such a dry feeding in is necessary for example when crushed carbon having a low thermal value is used, since the efficiency in the combustion thereof will be much too low if water is sup¬ plied and it is converted into a paste. Parts of this embodiment having substantially the same function as in the embodiment according to Figs 2 and 3 are provided with the same reference numerals provided with a ' . The conduits 22 and 25 for supply of fuel and additional air are here combined through conduits 32, which emerge into the bottom region of the bed at locations for feeding in fuel. The conduit 24' with air from the pressure vessel to the conduits 32 for feeding in fuel comprises besides the valve 27' a fan member 33 for feeding the air, said control unit 28 being connected to the fan member 33 as well as the valve 27' for regulation thereof so as to regulate the air/fuel-ratio in the air provided with fuel fed into the bed at said locations 21. Additionally, in the conduit 23' for supply of fuel to the gas flow a valve 34 is arranged, through which the amount fuel supplied per time unit may be regulated in a conventional way, in which signals related to the position of this valve are intended to be transmit¬ ted to the control unit 28' . The function of the part of the power plant illustrated in Fig 4 is evident in the light of the description thereof and the discussions above about the power plant part shown in Figs 2 and 3.

The invention is of course not in any way restricted to the preferred embodiments described above, but may possibili- ties to modifications thereof will be apparent to a man skilled in the art without departing from the basic idea of the invention.

The number of locations for feeding in fuel may for example be totally different than what is shown in the Figures.

It may also be mentioned that it is not at all necessary that said additional air is taken from the space surround¬ ing the combustor, and it will also be well possible to make extra openings in the bottom of the bed with addi- tional air supply nozzles at locations for feeding in fuel, should it be possible to provide an adjustability thereof.

With respect to the definition "air" in this disclosure, it has to be given a very broad sense and comprises not only the normal gas mixture existing in the free atmosphere, but all types of gas mixtures which at least have a portion of oxygen. Thus, for example air having a constituent not existing in normal air and air from which a constituent ex- isting in the normal air has been removed are also com¬ prised.

Claims

1. A method for combustion of particle-like fuel in a power plant having a combustor (1) and a bed (4) located therein, to the bottom region of which air is supplied for fluidiz¬ ing the bed, particle-like fuel being fed into the bed at defined locations (21) in the bottom region thereof for combustion of the fuel in the bed, characterized in that additional air separated from the fluidizing air is adjust¬ ably supplied to the bed at said locations.

2. A power plant having a combustor (1) with a bed (4) fiuidized through air supplied thereto in the bottom region thereof and members arranged to feed particle-like fuel into the bed at defined locations (21) in the bottom region thereof for combustion of the fuel in the bed, character¬ ized in that it comprises means (24, 25, 27, 24', 27', 33) for adjustable supply of additional air separated from the fluidizing air to the bed at said locations.

3. A power plant according to claim 2, characterized in that it comprises a control unit (28, 28') adapted to control said means.

4. A power plant according to claim 3, characterized in that said means comprise at least one conduit (24, 25, 24', 32) for conducting said additional air to said locations (21) as well as at least one regulating member (27, 27', 33) adapted to be controlled by the control unit and regulate the flow of said additional air in said conduit.

5. A power plant according to claim 4, characterized in that it comprises at least one said regulating member in the form of a valve member (27, 27') adapted to be con- trolled through said control unit (28, 28') to locally change the flow cross-section of said conduit.

6. A power plant according to claim 4 or 5, characterized in that it comprises at least one said regulating member in the form of a member (33) adapted to feed said additional air into said conduit and to be controlled through said control unit to change the feeding power thereof.

7. A power plant according to any of claims 3-6, character¬ ized in that it comprises members (29, 29') arranged to re¬ cord data concerning the nature of the fuel fed to said lo¬ cations (21) and important for the combustion of the fuel, and that the control unit (28, 28') is arranged to control said adjusting means on the basis of said data.

8. A power plant according to any of claims 3-7, character¬ ized in that it comprises members (30, 30') adapted to record changes of the power tapped from the power plant, and that the control unit (28, 28') is adapted to control said adjusting means on the basis of this recordal.

9. A power plant according to any of claims 3-8, character¬ ized in that said adjusting means are adapted to increase the flow of said additional air to said locations (21) on an increase of the power tapped from said power plant.

10. A power plant according to any of claims 3-9, charac¬ terized in that said adjusting means are arranged to increase the flow of said additional air to said locations (21) on an increase of the proportion of volatile sub¬ stances in the fuel fed into the bed and conversely.

11. A power plant according to any of claims 2-10, pharac- terized in that it comprises an additional conduit (25) for supplying said additional air besides a conduit (22) for feeding the fuel into the bed at said locations (21).

12. A power plant according to claim 11, characterized in that the members for feeding in the fuel are adapted to feed the fuel into the bed in the form of a liquid paste.

13. A power plant according to any of claims 2-10, in which said members for feeding in the fuel are adapted to feed the fuel into the bed through feeding air conveying the fuel, characterized in that said regulating means (27', 33) are arranged to regulate the supply of said additional air through changing the air/fuel-ratio in the fuel feeding air.

14. A power plant according to any of claims 2-13, charac¬ terized in that said combustor (1) is arranged in a pressure vessel (2) in which an air overpressure is in¬ tended to be generated for supply of the fluidizing air with overpressure.