A METHOD AND A DEVICE FOR GENERATING ADDITIONAL ENERGY IN A POWER PLANT

Abstract: A device for generating additional energy in a power plant having a combustor (1) with a bed (4), to which air and fuel are intended to be supplied for combustion of the fuel therein while forming hot flue gases over the bed, and a gas turbine driven by these flue gases for generating energy, comprises means (21) arranged to supply additional fuel in the path (22) of said flue gases between said bed and the gas turbine for utilising excess oxygen available in the flue gases for combustion of this fuel there and members (23) arranged to supply a cooling medium to the flue gases in connection with said supply of additional fuel to the flue gas path for regulating the temperature of the flue gases to a desired level.

Description: A method and a device for generating additional energy jiya power plant FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a method and a device for generating additional energy in a power plant having a com¬ bustor with a bed, to which air and fuel are intended to be supplied for combustion of the fuel therein while forming hot flue gases over the bed, as well as a gas turbine driven by these flue gases for generating energy. The invention comprises such generating of additional energy in all types of power plants having these characteristics, and heat energy will in practise nearly always be tapped from the bed by heat exchange with for instance a set a tubes immersed therein for generating steam and this is by that used as an excellent heat conductor, but the invention is not in any way restricted to any necessity that this has to take place. The bed may be of different nature, but in order to eliminate but not in any way restrict the invention the case of such a generating of additional energy in a power plant having a fluidised bed and the set of tubes im¬ mersed therein for generating steam to a steam turbine will be described hereinafter. It would sometimes be of a great advantage to be able to generate additional energy in such a power plant besides the energy generated through the regular operation of the plant. It would for example be desirable in an existing such power plant to be able to increase the maximum power which may be delivered by the plant when needed by such generation of ad- ditional energy. Furthermore, it would be possible to util¬ ise such a generation of additional energy, so as to achieve a faster load increase of the power plant, should there be a need of rapidly increasing the load thereof starting from a partial load operation. However, there are difficulties to introduce such generation of additional energy in such power plants, since this will lead to different types of problems, such as pluggings, too high temperatures locally, and so on. Furthermore, some part of the power plant usually sets a limit for generating a higher power through that part than the power plant will manage during full load operation. In the case discussed here, the steam turbine side may for in¬ stance be limiting for how much power the power plant may generate. SUMMARY OF THE INVENTION The object of the present invention is to provide a method as well as a device of the type defined in the introduction, which make it possible to generate additional energy in such a power plant so as to meet the desires mentioned above. This object is according to the invention obtained by pro¬ viding a method and a device according to the appended inde- pendent claims, in which such a device comprises means ar¬ ranged to supply additional fuel in the path of said flue gases between said bed and the gas turbine for utilising ex¬ cess oxygen available in the flue gases for combustion of this fuel there and members arranged to supply a cooling me- dium to the flue gases in connection with said supply of ad¬ ditional fuel to the flue gas path for regulating the tem¬ perature of the flue gases to a desired level. Thus, the additional energy aimed at may be generated by utilising the excess of oxygen present in the hot flue gases deriving from the bed for combustion of additional fuel sup- plied in said flue gas path and combusted there through con¬ tact with the hot flue gases in the presence of the oxygen present therein. By the supply of said cooling medium the temperature of the flue gases may at the same time be kept on an acceptable level, primarily for the gas turbine. The additional energy generated in this way will result in an increased generation of power of the gas turbine, which means that in power plants in which this will not reach its maximum capacity at regular full load operation an increased power may be delivered by the gas turbine and the maximum total power of the plant may by that be increased. As an ex¬ ample it may be mentioned that in the case discussed above of a power plant having a fluidised bed, the oxygen content in the flue gases leaving said bed will be at least at a certain value, since it is necessary to have such a high oxygen content there for avoiding that locally in the bed, such as close to the point for feeding fuel thereinto, a shortage of oxygen will be created with the disadvantages this will lead to in the form of a power reduction, in- creased production of S0 X and the like. It will on the other hand be sufficient to have the lower content of oxygen in the flue gas paths so as to avoid different problems such as corrosion therein, which means that there is a quantity oxy¬ gen that may be used for combustion in the flue gas path. A load increase required from a certain partial load to an¬ other partial load or from a partial load to the full load may also be accelerated through the device according to the invention. If the volume flow to the gas turbine of a power plant equipped with the device according to the invention is kept constant, which often is the case, the fact that the gas (usually water steam) produced through the combustion of the additional fuel in said flue gas paths will occupy a part of the flue gas flow, means that the net flow of air to the pressure vessel will increase, since the flow of the air to the bed is somewhat reduced, while the flow into the pressure vessel will not be changed with respect to the ab¬ sence of the combustion of the additional fuel. Since a load raise in a PFBC-power plant is primarily restricted by how fast the pressure in the pressure vessel surrounding the combustor may be raised, the invention results in this way in a possibility to a faster load increase when applied on this type of power plant. If there instead is a possibility to increase the volume flow to the gas turbine it will as an alternative be possible to let the pressurising of the pres- sure vessel take place just as fast as without the inven¬ tion, but it would then be possible to by means of the in¬ vention directly deliver a higher power thanks to the in¬ creased volume flow. A fast load increase has until now normally been started by lowering the power, since the gas turbine compressor will directly demand more power so as to be able to press an increased air flow into the pressure vessel, but this is thanks to the invention accordingly not necessary any longer. According to a preferred embodiment of the invention said means and members are arranged to supply said additional fuel and the cooling medium, respectively, to the combustor in the free space over the bed for combustion of the addi¬ tional fuel there. It is particularly advantageous to util- ise the free space, the so called free board, over the fuel bed, as a space for the combustion according to the inven¬ tion, since this has a comparatively large volume, so that a comparatively large amount of additional fuel may be sup¬ plied to one and the same place at the same time as it is easy to supply the fuel and the cooling medium there. Addi¬ tionally, there is no risk for damaging any sensitive equip¬ ment there as a consequence of any locally increased tem¬ perature. According to another preferred embodiment of the invention said means and members for supply of the additional fuel and the cooling medium are arranged to supply these together in a mixture, which simplifies the design of a device according to the invention further. A mixture of oil and water may be sprayed into said flow gas path. According to another preferred embodiment of the invention the device comprises an arrangement for controlling the sup¬ ply rate to the flue gas path of the additional fuel and the cooling medium and a ratio of these two rates, which makes it possible to achieve an additional energy generation by the device according to the different needs and conditions existing. The device comprises according to a preferred em¬ bodiment of the invention a member arranged to measure the temperature of the flue gases in said flue gas path down- stream the location for supply of the additional fuel and the cooling medium, and the control arrangement is arranged to carry out said control in dependence of the temperature information received from this member. It will in this way be possible to ensure that the flue gases get exactly the temperature desired, in which a not too high temperature of the flue gases is allowed to be obtained, since this would lead to damages in the flue gas paths at the same time as the temperature may not be too low, since this will lower the efficiency of the gas turbine. It is here pointed out that the invention is particularly advantageous in a power plant, in which the flue gases leav¬ ing the bed have already a temperature being close to what the flue gas paths to the gas turbine may withstand, since it will in such a case be of a totally decisive importance for the possibility to any additional energy generation, that the combustion carried out therein will be followed by a direct cooling for maintaining the temperature of the flue gases at substantially this level. The invention is never- theless of course applicable to and very advantageous also in power plants allowing a certain temperature raise of the flue gases, for instance by the fact that a high temperature filter is arranged comparatively far upstream in the flue gas path. According to a still further preferred embodiment of the in¬ vention the device comprises a member arranged to measure the oxygen content of the flue gases in said flue gas path downstream the location for supply of the additional fuel and the cooling medium, and the control arrangement is ar- ranged to carry out said control in dependence of data about the oxygen content in the flue gases received from this mem¬ ber. It will by that be possible to in an optimum way util¬ ise the oxygen present in the flue gases coming out of the bed, since it may be ensured that the oxygen content after the combustion of the additional fuel is kept over but as close as possible to the lower limit set for avoiding corro¬ sion in the flue gas path. According to a method according to the invention of the type defined in the introduction, additional fuel is supplied in said flue gas path between said bed and the gas turbine for utilising excess oxygen available in the flue gases for com¬ bustion of this fuel therein, and a cooling medium is sup¬ plied to the flue gases in connection with said supply of fuel to the flue gas path for regulating the temperature of the flue gases to a desired level. The advantages of such a method are the same as discussed above for the independent device claim. Further advantages as well as advantageous characteristics of the invention appear from the following description and the other dependent claims. BRIEF DESCRIPTION OF THE DRAWING-- With reference to the appended drawings, below follows a de¬ scription of a preferred embodiment of the invention cited as an example. In the drawings: Fig. 1 illustrates schematically a PFBC-power plant hav- ing a combined gas- and steam cycle (the latter is not shown), in which a device according to the in¬ vention may be applied, and Fig. 2 is a schematic partially sectioned view of the pressure vessel with the combustor arranged therein of a power plant according to Fig. 1, which illustrates a preferred embodiment of the method according to the invention and the device according to the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION The invention is as already mentioned not in any way re- stricted to the use in a PFBC-power plant, i.e. a plant for combustion of particle-like fuel in a pressurised fluidised bed, but it has great advantages in the use in such a power plant, which is the reason for now describing it applied in such a plant and the general construction and the function of an embodiment 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^ m^ and which may be pressurised to for example about 16 bars. Compressed air 3 for pressurising the combustor 1 and for fluidising a bed 4 in the combustor is applied to the pressure vessel. The compressed air is applied to the com¬ bustor through fluidising nozzles 5 schematically indicated at the bottom of the combustor for fluidising the bed en- closed in the combustor. The bed is made of bed material, granule-shaped absorbent and particle-like fuel, preferably crushed carbon, which is combusted in the fluidising air led to the bed. The combustion gases, hereafter called flue gases, from the bed are led to a purification plant 7 illus- trated through a box, which for instance may be constituted by a set of cyclones arranged to separate particles from the flue gases. The flue gases continues to a gas turbine 9 com¬ prising 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 as well as a generator 13, which in this way is driven by the high pressure turbine for taking useful energy therefrom. 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 exhaust gases leaving the low pressure turbine will still contain energy, which an econo- miser 16 may take care of. A low pressure compressor 17 is also arranged on the axle of the low pressure turbine and atmospheric air is supplied thereto through a filter 18. Ac¬ cordingly, the low pressure compressor is driven by the low pressure turbine and provides the high pressure compressor 12 with air compressed in a first step from its outlet. An intercooler 19 is arranged between the low pressure compres¬ sor and the high pressure compressor, so as to lower the air temperature of the air supplied to the inlet 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 20 of tubes immersed in the fluidised bed 4, in which water is circu¬ lated and evaporated and superheated through heat exchange between the tubes and the bed material for absorbing heat generated by the combustion carried out in the bed. The main principles of the function of a power plant of this type would well appear from the description above. The power output, i.e. the load of a power plant of this type, may be controlled by regulating the level of the bed as well as the flow of the fuel and air supplied to the bed. However, in certain situations it may be desired to obtain a faster change of the load of the plant than the control pa¬ rameters enable, and it may also be desired to tap more power from the plant than it will manage to generate through the regular function thereof. The invention shows solutions to this problem, in which it when meeting the latter desire requires that the gas turbine of the plant is not operating at the power limit thereof in the regular full load state of the plant. A device according to a preferred embodiment of the inven¬ tion will now be explained with reference to primarily Fig. 2, in which the pressure vessel 2 and the combustor 1 ar- ranged therein in the plant are schematically shown. During the combustion of the fuel supplied to the bed 4 hot flue gases having a temperature of 800-850 β C are created over the bed, which corresponds to the temperature which may be han¬ dled by the gas paths to the gas turbine 9 without different problems, should not the plant have a high temperature fil¬ ter. In order to avoid that any air deficit will occur in different parts of the bed where the combustion of the fuel supplied thereto is running faster, so much air has to be supplied to the bed through the fluidising nozzles 5, that the average content of oxygen in said flue gases will have a certain value. On the other hand it is only necessary that the oxygen content in the flue gases reaches a lower value for avoiding corrosion and other problems in the flue gas path to the gas turbine. Thus, there is an amount of oxygen available for a combustion in said flue gas paths. The de- vice according to the invention has a means 21 schematically indicated and arranged to spray an additional fuel into the free space 22, the so called free board, over the bed 4. This fuel may be constituted by natural gas, oil, carbon or the like. The device comprises further a member 23 arranged to spray the cooling medium, preferably water in liquid state or in steam phase into the free space 22 for cooling the flue gases, so that the temperature thereof may be regu¬ lated to a level desired especially for the flue gas paths and the gas turbine. The water may then advantageously be supplied in the form of liquid droplets, so that also the evaporation heat thereof may be used for an efficient cool¬ ing. It is pointed out that the additional fuel and the cooling medium may be supplied in the way that is considered to be the most advantageous one in every given situation, and the illustration in Fig. 2 is only made so as to explain the invention. Thus, the additional fuel and cooling medium may advantageously be supplied in the form of a mixture, for example oil droplets mixed up with water droplets. The device comprises also members 24 and 25 schematically indicated and arranged to measure the temperature and the O2~content, respectively, in the flue gases downstream the location for supply of the additional fuel and the cooling medium and send an information about these parameters to the control arrangement schematically indicated, which is ar¬ ranged to control the supply rate of the additional fuel and the cooling medium to the free space 22 and the ratio be¬ tween these rates in dependence of the magnitudes of the pa¬ rameters measured. Supply rate is defined as amount sup- plied/time unit. It is preferably also possible to provide possibilities for hand-operated influence of the control ar- rangement or directly of said means 21 and the member 23. It would also be possible to let the control arrangement work according to any given control programme, which for instance starts the supply of the additional fuel and cooling medium as soon as it is called for a load increase. The function of the device according to the invention is as follows. When the power plant operates at full load and it is desired to be able to tap additional power from the plant said means 21 and the members 23 may be controlled to supply additional fuel and the cooling medium to the free space 22 for cooled so called "free board firing" there by utilising the excess of oxygen present in the flue gases, if the gas turbine is not limiting for the plant. The mass flow to the gas turbine is in this way increased, which at a constant volume flow leads to an increased pressure and by that a higher power delivered by the gas turbine. It would also be well possible to increase the maximum total power which may be tapped from the power plant in this way by in the order of 10%. When the plant is driven at a comparatively low load, the bed 4 will only cover a part of the set 20 of the tubes, which means that the gases leaving the bed have a lower temperature than desired, maybe 600-650°C and the de¬ vice according to the invention may in this case be used for raising the temperature of the flue gases by possibly only spraying additional fuel in through the means 21 without any cooling for obtaining a high efficiency of the plant at par¬ tial load, even though it is not this principle on which the invention is based. When after that a rapid raise of the load is desired, said means 21 and the member 23 may supply the additional fuel and the cooling medium to the free space 22 so as to immediately utilise oxygen present therein for an additional combustion by which an accelerating load in¬ crease may be obtained as mentioned in the introduction either through keeping the mass flow to the gas turbine con¬ stant, which enables a faster pressurising of the pressure vessel and by that load increase, or the mass flow to the gas turbine is increased, but the pressurising of the pres¬ sure vessel is allowed to take place just as fast as without any additional combustion, which means that it will be pos¬ sible to directly have a higher power, so that the load in¬ crease does not have to be started by a lowering of the ad¬ ditional power directly required by the gas turbine compres¬ sor so as to be able to press an increased flow of air into the pressure vessel. Thus, the invention meets in a very advantageous way the de¬ sires of generation of additional energy sometimes appearing in such a power plant. This may take place by very simple means and to a low cost, since the oxygen already there in the flue gas paths is utilised for the combustion. Thus, no extra pressurising of air is required, which would cost power and reduce the efficiency of the additional combus¬ tion. The device according to the invention will thereto be cheap since it will not require any expensive compressors or the like. Furthermore, there is no sensitivity for the bal¬ ance between the gas turbine and the steam turbine any longer, but the boiler may advantageously be made a little bit "too large". It is pointed out that the fact that the excess of oxygen in the flue gases deriving from the bed is used for additional combustion of the additional fuel does not of course exclude an additional supply of air or oxygen in another way, and the invention is intended to cover such a combination of oxygen consumption for combustion of the additional fuel. The invention is of course not in any way restricted to the preferred embodiment described above, but many possibilities to modifications thereof would be apparent to a man skilled in the art, without departing from the basic idea of the in¬ vention.

Claims: Claims 1. A method for generating additional energy in a power plant having a combustor (1) with a bed (4), to which air and fuel are supplied for combustion of fuel therein while forming hot flue gases over the bed, as well as a gas tur¬ bine (9) driven by the flue gases for generating energy, characterized in that additional fuel is supplied in the path of said flue gases between said bed and the gas turbine for utilising excess oxygen available in the flue gases for combustion of this fuel there, and that a cooling medium is supplied to the flue gases in connection with said supply of additional fuel to the flue gas path for regulating the tem¬ perature of the flue gases to a desired level. 2. A method according to claim 1, characterized in that the additional fuel and the cooling medium are supplied to said flue gas path in the free space (22) above the bed (4) in the combustor (1) . 3. A method according to claim 1 or 2, characterized in that it is applied to a power plant in which said gas turbine (9) has not reached its capacity limit when the plant is running on full load in the absence of the supply of said additional fuel, and that the additional fuel and the cooling medium are supplied to said flue gas path when the plant is running at full load so as to increase the power delivered by the gas turbine and by that the maximum total power of the plant. 4. A method according to any of claims 1-3, characterized in that the additional fuel and the cooling medium are supplied to said flue gas path when said plant is running at partial load and this load is to be increased for accelerating the load increase of the plant. 5. A method according to any of claims 1-4, characterized in that the temperature of the flue gases is measured down¬ stream the location for supply of the additional fuel and cooling medium, and that the supply rate to the flue gas path of the additional fuel and the cooling medium as well as the ratio thereof is controlled in dependence of the mag¬ nitude of the temperature measured. 6. A method according to any of claims 1-5, characterized in that the oxygen content of the flue gases is measured down¬ stream the location for the supply of the additional fuel and the cooling medium, and that the supply rate to the flue gas path of the additional fuel and the cooling medium as well as the ratio thereof is controlled in dependence of the magnitude of the oxygen content measured. 7. A device for generating additional energy in a power plant having a combustor (1) with a bed (4), to which air and fuel are intended to be supplied for combustion of the fuel therein while producing hot flue gases over the bed, as well as a gas turbine (9) driven by these flue gases for generating energy, characterized in that it comprises means (21) arranged to supply additional fuel in the path (22) of said flue gases between said bed and the gas turbine for utilising excess oxygen available in the flue gases for com¬ bustion of this fuel there and members (23) arranged to sup¬ ply a cooling medium to the flue gases in connection with said supply of additional fuel to the flue gas path for regulating the temperature of the flue gases to a desired level. 8. A device according to claim 7, characterized in that said means (21) and members (23) are arranged to supply said ad¬ ditional fuel and cooling medium, respectively, to the com- bustor in the free space (22) over the bed (4) for combus¬ tion of the additional fuel there. 9. A device according to claim 7 or 8, characterized in that it is intended to be arranged in a power plant in which said gas turbine (9) in absence of the device has not reached its maximum capacity when the plant is running on full load, and that said means (21) and members (23) are arranged to supply said additional fuel and cooling medium, respectively, to said flue gas path when the plant is running on full load so as to increase the power delivered by the gas turbine and by that the maximum total power of the plant. 10. A device according to any of the claims 7-9, character¬ ized in that said means (21) and members (23) are arranged to supply said additional fuel and cooling medium, respec- tively, to said flue gas path when said plant is running on partial load and the load thereof is to be increased for ac¬ celerating the load increase of the plant. 11. A device according to any of claims 7-10, characterized in that it comprises an arrangement (26) for controlling the supply rate to the flow gas path of said additional fuel and cooling medium and the ratio of these to rates. 12. A device according to claim 11, characterized in that it comprises a member (24) arranged to measure the temperature of the flue gases in said flue gas path downstream a loca¬ tion for supply of said additional fuel and the cooling me¬ dium, and that the control arrangement (26) is arranged to carry out said control in dependence of a temperature infor- mation received from this member. 13. A device according to claim 11 or 12, characterized in that it comprises a member (25) arranged to measure the oxy¬ gen content of the flue gases in said flue gas path down- stream the location for supply of said additional fuel and cooling medium, and that the control arrangement (26) is ar- ranged to carry out said control in dependence of data about the oxygen content in the flue gases received from this mem¬ ber. 14. A device according to any of claims 7-13, characterized in that said means (21) and members (23) for supply of said additional fuel and the cooling medium are arranged to sup¬ ply these together in a mixture. 15. A device according to any of claims 7-14, characterized in that said members (23) are arranged to supply water as cooling medium. 16. A device according to any of claims 7-15, characterized in that it is intended to be arranged in a power plant hav¬ ing a bed (4) fluidised by supply of air to the bottom re¬ gion thereof, in which a set (20) of tubes having a circu¬ lating medium for energy absorption is immersed. 17. A device according to any of claims 7-16, characterized in that it is intended to be arranged in a power plant hav¬ ing the combustor (1) arranged in a pressure vessel (2) in which an air overpressure is intended to be generated through one or several compressors (12, 17) driven by the gas turbine (9).

Inventors: SANDELIN CARL JOHAN, SANDELIN, CARL, JOHAN
Applicants: ABB CARBON AB, SANDELIN CARL JOHAN, SANDELIN, CARL, JOHAN
Application year: 1996
Main classification: F02C3
IPC codes: F01K23, F02C3, F02K3, F23C10, F23L7