A METHOD IN A PFBC-POWER PLANT AND A TOPPING COMBUSTOR DEVICE OF SUCH A PLANT

Abstract: A topping combustor (7) of a PFBC-power plant comprises a first room (22) arranged to conduct flue gases from the combustor of the power plant through the topping combustor. Means (8) are arranged for supplying fuel gases to the topping combustor as well as at least a burner (2) is arranged to cause a combustion of the fuel gases in the topping combustor for mixing the gases resulting therefrom with said flue gases for increasing the temperature thereof. The topping combustor further comprises at least one second room (27) located beside the first room and by that separated from said flow. Said means are arranged to supply the fuel gases to the second room, and the burner (28) is arranged to cause a combustion of these fuel gases in a first zone (29) of the topping combustor located within the second room. The application also relates to a method for increasing the temperature of flue gases from the combustor of a PFBC-power plant, using the topping combustor described above.

Description: A method in a PFBC-power plant and a topping combustor device of such a plant FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a topping combustor device for increasing the temperature of flue gases deriving from a pri¬ mary combustor of a PFBC-power plant to a temperature making them suited as propelling gases for a gas turbine of the plant, said topping combustor comprising a first room with an inlet and an outlet for conducting a flow of said flue gases through the topping combustor, said device comprising means for supply¬ ing fuel gases to the topping combustor as well as at least a burner arranged to cause a combustion of the fuel gases in the topping combustor for mixing the gases resulting therefrom with said flue gases for increasing the temperature thereof, as well as a method for obtaining such a temperature increase of said flue gases in such a plant according to the preamble of the ap¬ pended claim 1. A device of this kind is already known through for instance SE published patent application 458 955. During the combustion in the primary combustor of such a power plant flue gases, i.e. combustion gases, having a temperature between about 850 and 950"C result. Would these flue gases be directly supplied to the turbine(s) belonging to the PFBC-power plant, these would receive a propelling gas having approximately this temperature. A higher temperature is not obtainable due to limitations in the chemistry of the bed. If the temperature is raised further an agglomeration of the bed takes place. As a consequence of the remarkable increase of the turbine power with increasing temperature of the propelling gas a substantially higher pro¬ pelling gas temperature, in the region of 1200-1500°C, is de- sired for obtaining a high power output in the gas turbine part of the plant and by that a higher total efficiency of the plant. Such a topping combustion is arranged for this reason, and it has accordingly as an object to raise the temperature of the flue gases deriving from the primary combustor of the plant and by that of the propelling gas for the gas turbine to a de¬ sired level. When this temperature increase in the topping combustor is pro- duced, there is a risk of increasing the emissions of N0 X of the plant to unacceptable levels. Prior art devices of this type have an emission of N0 X being too high at least under cer¬ tain operation conditions, especially under low load conditions of the plant, when the 02~content in the flue gases will be comparatively high resulting in the formation of high contents of N0 X in the combustion of the fuel gas in the presence of oxygen by a reaction between the oxygen and the N-containing constituents of the fuel at very high temperatures (>2000 C C) prevailing in such a combustion. SUMMARY OF THE INVENTION The object of the present invention is to provide a device and a method of the type discussed above, which find a remedy to the inconveniences mentioned above of already known such de¬ vices and methods without for this sake being complicated. This object is in accordance with the invention obtained by the fact that the topping combustor in such a device comprises at least one second room located beside the first room and by that separated from said flow, that said means are arranged to sup¬ ply the fuel gases to the second room, and that the burner is arranged to cause a combustion of these fuel gases in a first zone of the topping combustor located within this second room. Thanks to the fact that the fuel gases are combusted in a room separated from the flow of the flue gases from the primary com- bustor the content of the oxygen in the flue gas flow will not have any influence at all upon the combustion carried out in said first zone, which means that it will be possible to keep the oxygen content of the fuel gases, which consists of a mix- ture of fuel and air, at a constant and low level under all op¬ eration conditions for producing so little N0 X as possible on high temperature combustion of the fuel gases. Another advan¬ tage of carrying out the combustion of the fuel gases outside the flue gas flow is that the combustion under formation of flames will take place outside said first room, so that no com¬ plicated flame stabilising is required, which would be required should the combustion under formation of flames be generated in the flue gas flow. According to a first preferred embodiment of the invention said means are arranged to supply said fuel gases to the second room at such a mixing ratio of the fuel and the air therein that the combustion in the first zone will be sub-stoichiometric so as to make the gases resulting from the combustion contain uncom- busted fuel for reaction thereof with oxygen contained in the flue gas flow on said mixing of these gases with that flow in a second zone located in the first room. By the fact that a sub- stoichiometeric combustion of the fuel may always be ensured in said second room, i.e. a combustion under 02~deficit, small amounts of NO x will be produced in spite of the very high tem- peratures (>2000 β C) during flame combustion in the first zone. Combustion at such high temperatures gives namely rise to a very strong NO x -production, when θ is present in excess. Such advantageous sub-stoicheometeric combustion will always be pos¬ sible to assure independent of the load of the power plant thanks to the location of said first zone outside the flue gas flow. The incomplete combustion of the fuel will lead to a re¬ action of the uncombusted fuel with the oxygen being present in the flue gas flow in said first room in the form of an exotherm oxidation, which takes place without any flame generation and thus at a considerably lower temperature than a combustion with flame, so that very low contents of N0 X are produced also in this second zone. According to another preferred embodiment of the invention said burner is of the type with a supply of fuel gases into a space with walls converging in the feed direction for vortex forma¬ tion and air supply into the region beyond the end of said walls as seen in said direction, i. e. a burner of the EV-type patented by ABB. Thanks to the utilisation of such a burner in the second room a very perfect mixing of the air and the fuel is obtained in the combustion carried out in said first zone. This prolongs the stay time of the fuel gases in the first zone, which has turned out to result in a considerably lower conversion ratio of N-constituents of the fuel into N0 X than for a shorter stay time. According to a third preferred embodiment of the invention this second room is directed to direct a flow of said gases result¬ ing from said combustion of the fuel gases into the first room transversally to the flue gas flow through the first room. Thanks to the direction of said flow transversally to the flue gas flow into said room the hot gases in the flow from the sec¬ ond room will be mixed very intimately with the flue gas flow and ensure that the combustible gas molecules in the flow from the second room come into contact with the residue oxygen of the flue gas flow for a reaction in the form of an exotherm oxidation therewith in said second zone. According to a fourth preferred embodiment of the invention said second room is arranged to direct said flow into the first room substantially perpendicularly to the flue gas flow therein. A directing of said flow in such an angle with respect to the flue gas flow leads to a very good mixing of the τwo flows with the advantages meantioned above as a consequence. According to a fifth preferred embodiment of the invention the device comprises means arranged to supply said gases resulting from the combustion in the first zone to the first room at places distributed substantially uniformally therearound. A further improved and uniformally mixing of the gases of the two flows is achieved in this way. According to a sixth preferred embodiment of the invention the device comprises a plurality of second rooms having a burner each, and these rooms are arranged substantially uniformally distributed around a tube-like first room. The previous embodi- ment may in this way be realised in a simple and advantageous way. According to a seventh preferred embodiment of the invention said means for supplying the fuel gases to the second room are arranged to supply these gases with an overpressure exceeding the pressure of the flue gas flow arriving to said first room. Thanks to this pressure difference between the second room and the first room the gases resulting from the combustion in the first zone may be efficiently mixed with the flue gas flow in the second zone by the kinetic energy they have. According to an eight preferred embodiment of the invention the device comprises means for supplying air into a third zone of the first room located downstreams of a zone in which said mix- ing is intended to take place for mixing said air with the gas flow mixture for burnout of the combustible medium contained therein. Thanks to the supply of this air into the third zone combustible medium remaining in the gas flow mixture may be ul¬ timately combusted and the NO x -content in the gas flow may be reduced further by reaction of O2 with N0 X to less harmful com¬ pounds before the gas flow is supplied to the gas turbine as propelling gas. According to a still further preferred embodiment of the inven- tion said air supply means comprise channel means arranged to conduct the air outside and along walls delimiting the first and second zone of the topping combustor for heating thereof before it enters into the first room in the third zone as well as for cooling of these walls. Heating of the air through in¬ teraction with said walls is in this way achieved, which im¬ proves the result of the ultimate combustion in said third zone and provide necessary cooling of said walls. According to a still further preferred embodiment of the inven¬ tion the outlet of the topping combustor forms a direct inlet to the gas turbine, which makes the construction of the topping combustor with gas turbine simple and heat losses of the pro¬ pelling gas between the topping combustor and the gas turbine may also be avoided without any problems. According to a method according to the invention of the type mentioned above, the fuel gases are brought to be combusted in a first zone in a second room of the topping combustor, which is separated from a first room thereof, through which said flue gases are intended to flow through the topping combustor. The advantages of such a method are the same as those mentioned above for the independent device claim. Further advantages and advantageous characteristics of the in¬ vention 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 de¬ scription of a specific embodiment of the invention cited as an example. In the drawings: Fig. 1 illustrates schematically a PFBC-power plant having a combined gas and steam cycle (the latter is not shown), in which a topping combustor according to the invention is arranged, Fig. 2 is a schematic, partially sectioned view of a device ac¬ cording to a preferred embodiment of the invention, and Fig. 3 is a front view of the device according to Fig. 2 with some parts broken away. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION In order to illuminate how a PFBC-power plant, i. e. a plant for combustion of a particle-like fuel in a pressurised flui- dised bed, in which a device according to the invention may be included, may be realised, such a plant will now be briefly de¬ scribed with reference to Fig. 1. The plant comprises a combustor 1, which is received in a pres¬ sure vessel 2, which may have a volume in the order of 10 4 m^ and which may be pressurised to for example about 16 bars. Com¬ pressed air 3 for pressurising the combustor 1 and for fluidis- ing a bed in the combustor is applied to the pressure vessel. The compressed air is applied to the combustor through fluidis- ing nozzles not shown at the bottom of the combustor for flui- dising the bed enclosed in the combustor. The bed is made of bed material, granule-shaped absorbent and a particle-like fuel which is combusted in the fluidising air supplied to the bed. The flue gases 4 from the bed are led to a purification appara¬ tus 5, which in the example consists of a high temperature fil¬ ter intended for high pressures. The flue gases passes an in¬ tercept valve 6 and enters after that a topping combustor 7, the construction of which forms the object of the present in¬ vention and will be described further on with reference to Fig. 2 and 3. A fuel gas is also led to the topping combustor 7 through a conduit 8 from a gasifier 9 of known type through a filter 10. The fuel gas is combusted in the topping combustor in connection with a supply of compressed air through the con¬ duit 11 from a high pressure compressor 12 and is mixed with the flue gases from the combustor 1 for increasing the tempera- ture thereof, so that the gases leaving the topping combustor have a temperature (1200-1500°C) making them well suited as propelling gases for driving a high pressure turbine 13. The temperature of said flue gases has been increased from about 850-950°C to 1200-1500°C through the topping combustor. The high pressure turbine and the high pressure compressor are arranged on the same axle as a generator 14, from which useful energy may be taken. The high pressure compressor 12 delivers also compressed air to the PFBC-combustor 1 through the conduit 11 in a way which will appear clearer from the description with reference to Fig. 2. An intercept valve 15 is arranged between the high pressure compressor and the combustor 1. It also de¬ livers air through the conduit 16 for gasifying in the gasifyer 9. The fuel could also be natural gas and the gasifier may be omitted. The gas expanded in the high pressure turbine 13 is led to a low pressure turbine 17. The exhaust gases leaving the low pressure turbine still contain energy, which an economiser 18 may take care of. A low pressure compressor 19 is also arranged on the axle of the low pressure turbine and atmospheric air is supplied therethrough through a filter 20. Thus, the low pres¬ sure compressor is driven by the low pressure turbine and pro- vides the high pressure compressor 12 with air compressed in a first step from the outlet thereof. An intercooler 21 is ar¬ ranged between the low pressure compressor and the high pres¬ sure compressor for lowering the air temperature of the air supplied to the inlet of the high pressure compressor 12. A preferred embodiment of the topping combustor according to the invention will now be described with reference to Fig. 2. The topping combustor 7 comprises a first tube-like room 22 having an inlet 23 for the flue gases 24 deriving from the pri- mary combustor 1 of the PFBC-plant and an outlet 25 located at the opposite end and which forms a direct inlet 26 to the high pressure gas turbine 13 extending with its axle perpendicularly to the longitudinal axis of said first room. The topping combustor comprises several, in the present case four, second rooms 27 uniformly distributed around the first room on the same level in the flow direction of the flue gases 24, and these second rooms are accordingly directed radially with respect to the first room and arranged separated from the flue gas flow to the topping combustor. Each second room 27 is provided with a burner 28, to which pressurised fuel gases from the gasifier 9 are arranged to be supplied through the conduit 8. The fuel therein is preferably carbon or oil. The burner 28 is of the EV-type, i.e. the fuel gases are supplied in a room having walls diverging in the supplying direction for vortex formation, and air for the combustion is supplied in the region in said direction behind the end of these walls so as to achieve an almost perfect mixing thereof and said vortex are collapsed in a first zone 29 in the second room for combustion of the fuel gases. How the air is supplied will be described further on, but it is supplied with an oxygen deficit with re¬ spect to the amount of fuel supplied, so that the combustion taking place in the first zone 29 under flame formation and at a very high temperature (> 2000°C) will be sub-stoicheometeric and the NO x -formation will by that be kept at a low level. The good intermixing of the fuel gases and the air in the first zone 29 also leads to a longer stay time of the gases resulting of the combustion in the second room, which has turned out to have a very favourable influence on the NO x -formation, i.e. re¬ duce it. Thus, substantially all the oxygen supplied will be used for the very combustion and very small amounts will be at disposal for N0 X -formation. Thanks to the overpressure of the fuel gas supplied to the second room 27 with respect to the flue gas flow in the first room, the gases resulting from the combustion in said first zone 29 will be led into the flue gas flow 24 in the first room with such a high kinetic energy that an efficient intermixing takes place. The efficiency of this intermixing is also posi¬ tively influenced by the fact that the gas flow from the second room emerges in an angle with, preferably perpendicularly, into the gas flow in the first room, and that gas flows from second rooms 27 are supplied to the flue gas flow in the first room at places distributed substantially uniformly therearound. Thus, the flue gases 24 deriving from the combustor 1 will in a second zone 30 located in the first room of the topping combus- tor be mixed with the gases deriving from the combustion in the first zone 29, whereby an exother oxidation without any flame formation and at a temperature at about 1200-1300°C takes place between fuel being still uncombusted and the residue oxygen in the flue gases 24. The low temperatures in this reaction leads to a low formation of N0 X . The high pressure compressor 12 is arranged to conduct com¬ pressed air into the first room 22 of the topping combustor into a third zone 31 located downstreams as seen in the flue gas flow direction of the second zone 30 through channel means comprising two channel portions extending parallel, a first 32 of which is arranged to conduct air at a distance from the in¬ ner wall 30 of the topping combustor past the third zone and to the region of the first zone and emerge there into a second channel portion 34 arranged to conduct the air back between the first channel portion and said wall 33 for emerging into the third zone. Thanks to this conducting of the air along the in¬ ner wall 33 it will be preheated before it reaches the third zone 31. A portion of the air will at the transition from the first to the second channel portion be led into the region of the burner 28 in the first zone 29 so that the high pressure compressor 12, accordingly, will provide both the first and the third zone with air. An ultimate combustion of combustible me¬ dium still contained in the gas flow will take place in the third zone and the additional oxygen introduced will then react with and further reduce the NO x -content by forming other less harmful compounds. Thanks to this ultimate combustion a maximum part of work invested in the plant may be recovered and util¬ ised for driving the gas turbine 13. The air flow out of the topping combustor and by that the size of the gas turbine at a determined fuel energy fed may be minimized through this almost total combustion. The overall length of the portion of the first room containing the second and third zone exceeds in practise preferably 1,5 times the diameter of the tube forming the first room. The air flowing in the channel portion 34 serves to cool the walls in the rooms 22 and 27. Walls surrounding the topping combustor 7 have for the sake of clearness been omitted in Fig. 2, although these walls will in practise be substantially circumferential. The walls 35 of the topping combustor are surrounded by outer walls 36 and the com- pressed air to be delivered by the high pressure compressor to the combustor 1 of the plant is led to the latter past the top¬ ping combustor inside said outer walls along certain parts through a double shell 37, whereby an advantageous cooling of the walls of the topping combustor is obtained. An intercept valve 6 is arranged at the inlet to the topping combustor 7, and intercept valves 15 are arranged between said outer 36 and inner 36 walls of the topping combustor in the conduit 38 between the high pressure compressor and the combus- tor 1. The intercept valve 6 may be controlled to close the in¬ let to the topping combustor and isolate it from the large vol¬ umes in the combustor 1 upon a stop of the operation of the plant, so that an overspeed and by that a damage of the rotor 12, 13, 14 is avoided. Furthermore the combustor 1 may be iso- lated from the high pressure compressor through the valves 15 for efficiently retarding the combustion in the combustor 1. It is shown in Fig. 3 how the topping combustor device illus¬ trated in Fig. 2 may look like in practise. The simple but nevertheless appropriate design of the intercept valve arrange¬ ment 39 in direct connection with the topping combustor is es¬ pecially noticeable. It also appears that the openings 40 for outlet of air in the third zone are closely arranged around the entire topping combustor tube. The invention is of course not in any way restricted to the preferred embodiment described above, but many modifications thereof would be apparent to a man skilled in the art without departing from the basic idea of the invention.

Claims: Claims 1. A method for increasing the temperature of flue gases deriv¬ ing from a primary combustor (1) of a PFBC-power plant to a temperature making them suited as propelling gases for a gas turbine (13) of the plant, in which the flue gases are led into a topping combustor (7), fuel gases are supplied to the topping combustor for combustion and the gases resulting therefrom are mixed with said flue gases for increasing the temperature thereof, characterized in that the combustion of the flue gases is car¬ ried out in a first zone (29) in a second room (27) of th top¬ ping combustor, which is separated from a first room (22) thereof, through which said flue gases are intended to flow through the topping combustor. 2. A method according to claim 1, characterized in that the fuel gases are fed to the second room (27) with such a mixing ratio of the fuel and air included therein that the combustion in the first zone (29) gets sub- stoichiometric, and that uncombusted fuel contained in said re¬ sulting gases is brought to react with oxygen included in the flue gas flow in a second zone (30) located in the first room after mixing of these gases with the flue gas flow through the first room (22). 3. A method according to claim 1 or 2, characterized in that the gases resulting from the combustion in the first zone (29) are led in a flow directed transversally to said flue gas flow into the latter in the first room (22). 4. A method according to any of claims 1-3, characterized in that the gases resulting from the combustion in the first zone (29) are fed to the first room (22) at loca- tions distributed substantially uniformly therearound. 5. A method according to any of claims 1-4, characterized in that air is fed to the first room (22) down¬ streams of a second zone (30) with respect to the flue gas flow direction, in which said mixing is intended to take place, and is mixed with the gas flow mixture there for combustion of com- bustible medium remaining therein. 6. A topping combustor device for increasing the temperature of flue gases deriving from a primary combustor (1) of a PFBC- power plant to a temperature making them suited as propelling gases for a gas turbine (13) of the plant, said topping combus¬ tor comprising a first room (22) with an inlet and an outlet for conducting a flow of said flue gases through the topping combustor, said device comprising means (8, 9) for supplying fuel gases to the topping combustor as well as at least a burner (28) arranged to cause a combustion of the fuel gases in the topping combustor for mixing the gases resulting therefrom with said flue gases for increasing the temperature thereof, characterized in that the topping combustor comprises at least one second room (27) located beside the first room (22) and by that separated from said flow, that said means are arranged to supply the fuel gases to the second room, and that the burner (28) is arranged to cause a combustion of these fuel gases in a first zone (29) of the topping combustor located within this second room. 7. A device according to claim 6, characterized in that said means are arranged to supply said fuel gases to the second room (27) at such a mixing ratio of the fuel and the air therein that the combustion in the first zone (29) will be sub-stoichiometric so as to make the gases resulting from the combustion contain uncombusted fuel for re¬ action thereof with oxygen contained in the flue gas flow on said mixing of these gases with that flow in a second zone (30) located in the first room (22). 8. A device according to claim 6 or 7, character!zed in that said burner (28) is of the type with a supply of fuel gases into a space with walls converging in the feed direction for vortex formation and air supply into the re¬ gion beyond the end of said walls as seen in said direction. 9. A device according to any of claims 6-8, characterized in that the second room (27) is directed to di¬ rect a flow of said gases resulting from said combustion of the fuel gases into the first room (22) transversally to the flue gas flow through the first room. 10. A device according to claim 9, characterized in that the second room (27) is arranged to di¬ rect said flow into the first room (22) substantially perpen- dicularly to the flue gas flow (24) therein. 11. A device according to claim 9 or 10, characterized in that the second room (27) is arranged to direct said flow substantially in the direction towards the centre of the flue gas flow in the first room (22). 12. A device according to any of claims 6-11, characterized in that it comprises means arranged to supply said gases resulting from the combustion in the first zone (29) to the first room (22) at places distributed substantially uni¬ formly therearound. 13. A device according to claim 12, charac erized in that comprises a plurality of second rooms (27) having a burner (28) each, and that these rooms are ar¬ ranged substantially uniformly distributed around a tube-like first room (22). 14. A device according to any of claims 6-13, characterized in that said means (8, 9) for supplying the fuel gases to the second room (27) are arranged to supply these gases with an overpressure exceeding the pressure of the flue gas flow arriving to said first room (22) . 15. A device according to any of claims 6-14, characterized in that the device further comprises means (12, 32, 34) for supplying air into a third zone (31) of the first room (22) located downstreams of a zone (30) in which said mix¬ ing is intended to take place for mixing said air with the gas flow mixture for burnout of the combustible medium contained therein. 16. A device according to claim 15, characterized in that said air supply means comprise channel means (32, 34) arranged to conduct the air outside and along walls (33) delimiting the first and second zone of the topping combustor for heating thereof before it enters into the first room in the third zone (31) as well as for cooling of these walls. 17. A device according to claim 16, characterized in that the channel means comprises two channel portions extending parallely, a first (32) of which is arranged to conduct air at a distance from said walls past the third zone (31) and to the region of the first zone (29) and emerge there into a second (34) arranged to conduct the air back be¬ tween the first one and said walls for emerging into the third zone. 18. A device according to any of claims 15-17, characterized in that it comprises means (32, 34) for supplying air both for the fuel gases for the first zone (29) and to the third zone (31) from a high pressure compressor (12) of the plant driven by the gas turbine (13) . 19. A device according to any of claims 6-18, characterized in that the walls (35) of the topping combustor are surrounded by outer walls (36), and that the compressed air to be delivered to the primary combustor (1) of the plant by a high pressure compressor (12) driven by the gas turbine is led to said combustor past the topping combustor inside said outer walls. 20. A device according to any of claims 6-19, characterized in that it comprises an intercept valve (6) ar¬ ranged in the first room (22) upstreams of the place where the second room (27) emerges into the first room and arranged to be controlled to shut off the flue gas flow through the topping combustor (7) at tripping of the plant. 21. A device according to any of claims 6-20, characterized in that the outlet (25) of the topping combustor (7) forms a direct inlet (26) to the gas turbine (13). 22. A device according to claim 21, characterized in that the first room (22) is tube-like with re¬ spect to its extension between the inlet (23) and the outlet (25) of the topping combustor, and that the gas turbine (13) is arranged at the outlet end of the tube with the axis thereof substantially perpendicular to the longitudinal axis of the tube.

Inventors: BRAENNSTROEM ROINE, DOEBBELING KLAUS, LOEVGREN ANDERS, SATTELMAYER THOMAS, VEENHUIZEN DIRK, WINKLER DIETER, BRAENNSTROEM, ROINE, DOEBBELING, KLAUS, LOEVGREN, ANDERS, SATTELMAYER, THOMAS, VEENHUIZEN, DIRK, WINKLER, DIETER
Applicants: ABB CARBON AB, BRAENNSTROEM ROINE, DOEBBELING KLAUS, LOEVGREN ANDERS, SATTELMAYER THOMAS, VEENHUIZEN DIRK, WINKLER DIETER, BRAENNSTROEM, ROINE, DOEBBELING, KLAUS, LOEVGREN, ANDERS, SATTELMAYER, THOMAS, VEENHUIZEN, DIRK, WINKLER, DIETER
Application year: 1996
Main classification: F23C6
IPC codes: F01K23, F02C3, F23C6, F23C7, F23C10, F23C99