FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a ^' device for extracting heat energy from flue gases produced through a combustion of fuel in a combustor of a power plant in a part of the power plant separated from said combustor, which comprises a conduit for conveying the flue gases along heat exchange surfaces inside which a cooling medium is intended to be circulated in an arrangement for transferring heat from the flue gases to said medium, as well as a method according to the preamble of the appended claim 1.

Devices of this type are arranged in nearly all types of power plants in which flue gases are produced by combustion of a fuel, such as power plants having fluidized beds for combustion of particle-like fuel, oil-fired boilers, soda pans and so on, and the aim thereof is to extract as much as possible of the heat energy contained in said flue gases before letting them out into the open air. Said heat energy extraction from the flue gases takes place in a part of the power plant located downstream the combustor or combustion chamber as seen in the path of the flue gases, and energy has often already been extracted from the flue gases after the combustion, for instance by driving a gas turbine, before they have reached the device of the type described above. "Power plant" is defined, as appears from above, in this disclosure as comprising not only plants for generat-

CORRECTED

ing electricity but also all types of plants for generating heat.

In order to illuminate the invention but not in any way re- strict it such a device applied in a PFBC-power plant, i.e. a plant for combustion of particle-like fuel in a pressur¬ ized fluidized bed, will be described hereinafter, although the invention is directed to all conceivable power plants in which flue gases are produced. A device of this type is in such a power plant arranged in the path of the flue gases after a gas turbine, in which the flue gases have been expanded and cooled down to a considerably lower temperature than they had after the combustion in the combustor, and before a chimney, so as to take care of the heat energy that these flue gases still contain after leaving the low pressure turbine step of the power plant. Such a device is, not only in this application, given the name economizer. Such devices already known have one or more sets of tubes projecting into said conduit and in which a cooling medium, usually water, is circulated so as to be heated by the flue gases sweeping along the heat exchange surfaces of said set of tubes, and said conduit is made wider in the room in which the sets of tubes are arranged. The different tubes have an amount of flanges ar- ranged comparatively close to each other so as to provide as large heat exchange surfaces as possible for the flue gases and for making the flue gases to be cooled down as much as possible. However, after some time of operation particles remaining in the flue gases, i.e. combustion remainders ( ashes) , will plug the space between the flanges, which leads to a remarkably reduced heat transfer between the flue gases and said cooling medium. Accord¬ ingly, a cleaning equipment is arranged for this sake, by means of which it will be possible to try to rinse the flanges from flue dust at regular time intervals, but such an equipment is both costly as such and is not always that

efficient and lowers the power that may be delivered by the plant by requiring steam that could produce electricity in¬ stead.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method and a device of the type defined in the introduction, which find remedy to the inconveniences mentioned above of such methods and devices already known and makes it possible to considerably improve the heat exchange between the flue gases and said medium so as to take care of as much as possible of the heat energy remaining therein.

This object is according to the invention obtained by providing such a device with means adapted to provide the room in the flue gas conduit surrounding the heat exchange surfaces with a particle-like material for transferring heat energy from the flue gases passing to said heat exchange surfaces through said material, as well as a method according to the appended claim 1.

By the fact that said means provides the room surrounding the heat exchange surfaces with a particle-like material and this is used as an heat exchange medium between the flue gases and the heat exchange surfaces, the heat trans¬ fer between the flue gases and the heat exchange surfaces will be improved considerably, so that it is no longer necessary to arrange said comparatively costly flanges so as to obtain as large heat exchange surfaces as possible, and thereby the problems with plugging of the spaces between such flanges are solved. Furthermore, the particle¬ like material will through the movement it is given by the flue gases passing "sweep" these heat exchange surfaces clean, i.e. usually the outer walls of tubes, which contribute to an improved heat transfer between the flue

gases and these surfaces and also means that the problems with plugging of the spaces between adjacent flanges would be reduced remarkably in the case said flanges had still been there.

According to a preferred embodiment of the invention said means are arranged to provide said room with a particle¬ like material containing lime for desulphurization of the flue gases passing it, so that this may take place before they are decharged into the open air.

According to another preferred embodiment of the invention the device . comprises members for supply of the particle¬ like material to said room with an elevated temperature with respect to the temperature of the surrounding of the power plant, which improves the heat transfer between the flue gases and the heat exchange surfaces through the particle-like material. Through yet another preferred embodiment of the invention said members may be arranged to supply the particle-like material to said conduit down¬ stream said room and cause transport thereof to said room while being heated by meeting flue gases, whereby it will be possible to use a pre-heated particle-like material and in that way increase the heat exploitation of the device further by cooling the flue gases also through heat trans¬ fer to this particle-like material after they have passed said heat exchange surfaces and this heat is then trans¬ ferred to said heat exchange surfaces by the particle-like material.

According to another preferred embodiment of the invention the power plant comprises a bed in which said combustion is intended to take place and said members are arranged to feed said particle-like material to said conduit in the form of bed ashes from said bed. It will in this way be possible to take advantage of the heat still included in

the bed ashes fed out of the combustor bed through the heat transfer to said heat exchange surfaces.

According to a further preferred embodiment of the inven- tion said means comprise at least a bottom arranged in said conduit and provided with nozzles for letting said flue gases through, said bottom having arranged thereon a bed of particle-like material at least partially surrounding said heat exchange surfaces. Said room surrounding the heat exchange surfaces is hereby in a very advantageous way provided with particle-like material, wherein said flue gases going through said nozzles will preferably fluidize the bed for increasing the heat transferring capacity.

According to a preferred further development of the embodi¬ ment last mentioned the device comprises members for regulating the height of the bed for adjusting the degree of coverage of said heat exchange surfaces through the bed material and thereby adaption of the power output of the device to varying conditions.

According to a further development of the embodiment last mentioned the device has a plurality of said arrangements with heat exchange surfaces arranged in said conduit one after another and members for individually regulating the height of the different beds. It is thereby possible to regulate the different power outputs in the different arrangements when a plurality of such arrangements are arranged one after the other, which is often the case in known economizers, so that no types of other special arrangements, such as by-pass valves, for obtaining such an individual regulation is required as was necessary until now.

According to yet another preferred embodiment of the invention the conduit extends substantially vertically in

that part thereof in which said room is located, and it comprises members for supply of said particle-like material above said room for falling downwardly and passing said heat exchange surfaces. Thanks to the so called falling bed obtained in this way it will be possible to obtain heat exchange between the flue gases and said heat exchange surfaces through the particle-like material without any provision of any bottom in the conduit for this material.

Further advantages and advantageous characteristics of the invention 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 two preferred embodiments of the invention cited as examples. In the drawings:

Fig 1 is a schematic, partially sectioned view of a pres¬ sure vessel with a combustor arranged therein in a PFBC- power plant and a device according to a first preferred em¬ bodiment of the invention, and

Fig 2 is a view corresponding to Fig 1 of a device accord¬ ing to a second preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention is as already mentioned not in any way restricted to use in a PFBC-power plant, but it will nevertheless hereinafter be described as applied in such a plant for explaining the basic idea of the invention. Thus, a part of such a power plant is schematically shown in Fig

1, which comprises a combustor 1, which is received in a vessel 2, which may have a volume in the order of 10^ m^ and which may be pressurized to for example about 16 bars. Compressed air 3 for pressurizing the combustor 1 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 in the bottom of the combustor for fluidizing the bed enclosed in the combustor. The bed is made of bed material, granual-shaped absorbent and particle-like fuel, preferably crushed carbon, which is combusted in the fluidizing air supplied to the bed. The combustion gases, hereinafter called flue gases, from the bed are then led through purification means not shown, such as cyclones, further to a gas turbine 24 schematically indicated, in which the flue gases are expanded for extracting energy therefrom, in which the flue gases usually first pass a high pressure turbine and then a low pressure turbine and then through a conduit 6 arrive to a so called economizer 7, in which heat energy is extracted from the flue gases while cooling them before they are led out into the open air through a chimney not shown. The flue gases formed in the combustion in the combustor have usually a temperature of 800-850°C, but the flue gases arriving to the economizer 7 in the conduit 6 have usually a temperature of 300-350°C. A PFBC-power plant of this type is described for example in the Swedish patent application 9501097-1 of the applicant.

The economizer 7 is constituted by several, most often two or three and in this case three, part economizers 8, 9, 10, which have a set of tubes each projecting into a room 12 formed by a widened portion 11 of the conduit 6, and even if the drawings do not show this they cover substantially the entire cross-section of the room. The sets of tubes 8, 9, 10 have heat exchange surfaces with respect to the surroundings and a cooling medium, preferably water,

circulating inside thereof, said medium having a tempera¬ ture above about 130°C when it leaves said room 12, since otherwise sulphur will condensate on the "cold" tubes.

A bottom 13 covering the cross-section of the conduit 6 is arranged under each set of tubes, and this is provided with nozzles 14 for passing said flue gases and fluidizing the bed 23 of particle-like material 15 resting on the respec¬ tive bottom 13. Said particle-like material 15 will absorb heat from the flue gases passing therethrough and effi¬ ciently transfer this heat to the heat exchange surfaces 16 of the sets of tubes and thereby to the cooling medium . circulating therein. Thanks to the movement of the parti¬ cle-like material achieved by the flue gases, this will also "sweep" the heat exchange surfaces of the sets of tubes clean, so that these will not be plugged.

The particle-like material is in the present case formed by bed ashes from the bed 4 present in the combustor 1, the device having members 17, indicated through a conduit branching to each bed and arranged to feed bed ashes to the respective bed. The device also comprises members indicated through tube fragments 18 for discharging bed material from the respective bed, said members 18 being individually regulated, so that an individual regulation of the height of the respective bed may be achieved for adjusting differ¬ ent power outputs of the different part economizers 8, 9, 10. It is hereby not absolutely necessary to have a bypass- valve for the power regulation of the economizer 7.

By utilizing the bed ashes as particle-like material for the heat transfer medium in the economizer 7, the heat that this has when it is discharged from the bed 4 in the combustor 1 may be used for heating the sets of tubes 8-10, and the bed ashes discharged from the bed 4 has typically a temperature of about 200-250°C. The bed ashes supplied to

the respective bed 14 is preferably provided with lime for desulphurization of flue gases passing through the beds 14, so that the amount of S0 _X emitted into the open air later on is reduced.

The device also comprises a filter 19 arranged downstream the very economizer 7, which is intended to remove possible particles carried along with the flue gases downstream the economizer from the flue gases before they are let out in the open air.

Through a device of the type according to the invention heat may in a remarkably more efficient way than in .such devices already known and described in the introduction be extracted from the flue gases before they are let out in the open air, which is primarily obtained by a dramatic in¬ crease in the heat transfer capacity, which may be several times higher, of the device according to the invention thanks to the fact that particle-like material surrounding said heat exchange surfaces 16 is used as heat transfer me¬ dium.

The device according to a second preferred embodiment of the invention is schematically illustrated in Fig 2, in which parts corresponding to parts in the first embodiment are provided with the same reference numerals. In this embodiment there is no real bed, but a so called "falling bed" of particle-like material 15 is formed, which is supplied to the conduit 6 from a container 20 through members indicated by a conduit 21 in the path of the flue gases downstream said room 12 with the sets of tubes 8, 9 and 10, the conduit 6 in said room 12 being substantially vertically arranged. Thus, the small particles 15 will fall along the sets of tubes while heating the flue gases and in this way absorb heat from the flue gases and transfer this to the heat exchange surfaces of the sets of tubes and

thereby to the cooling medium circulating therein. Thanks to the fact that said members are arranged to supply the particle-like material 21 upstream said room 12, it may be supplied with a comparatively low temperature, preferably ambient temperature, since it will be heated by the flue gases before these are subjected to a further cooling on the other side of the room 12 and the energy output from the flue gases is thereby further improved. It is then necessary that the particles are not too small, but a sufficient relation prevails between the particle size/flue gas velocity, so that the fall velocity of the particles is higher than the so called terminal velocity, i.e. they are moving downwards. On the other hand, the particles shall of course not be so great that they fall too fast through the room 12. This may however also be regulated through suit¬ able dimensioning the cross-section surface of said room 12 for adapting the flow rate of the gases to the particle size. The device has also members 22 arranged to discharge particle-like material arrived thereto and feed it back to the container 20 for refeeding downstream said room 12, in the path of the flue gases upstream said room 12, any cooling device and/or cleaning device may possibly be arranged between the discharging member 22 and the container 20.

Thus, the particle-like material is in this case not any bed ashes from the combustor of the power plant, but it is any other suitable particle-like material, such as sand or ashes, preferably including lime for desulphurization.

The invention is not in any way restricted to the preferred embodiments described above, but an amount of possibilities to modifications thereof would be apparent for a man skilled in the art without departing from the basic idea of the invention.

It would in the case of the presence of a "fixed" bed, i.e. a bed resting on a bed bottom, also in the case of several sets of tubes arranged one after another be possible to arrange one single bottom and thereby bed, although such an arrangement would not be just as flexible with respect to regulation of the different power outputs from the different sets of tubes by means of bed-height regulation.

The particle-like material could be another than the materials mentioned above, so long as it has the desired good heat exchange properties and no negative tendencies with respect to adhering to the sets of tubes.

The particle-like material may of course also in the embodiment shown in Fig 2 be supplied to the conduit 6 in a pre-heated state, and it could then possibly be supplied to the conduit 6 directly to said room 12.

The definition of said means in the device main claim also is intended to comprise the case of the supply of no material to a bed, but this is kept substantially unchange .

That said room is provided with a particle-like material does not mean that the entire room has to be filled with said material, which is not either the case for example when heat exchange surfaces (sets of tubes) are partially covered by the bed material of a bed formed thereby.