POWER PLANT WITH COMBUSTION OF A FUEL IN A FLUIDIZED BED

Abstract: Power plant with combustion of a fuel in a fluidized bed, comprising a bed vessel (12), a bottom (14) dividing the bed vessel (12) into an upper combustion chamber (15) and a lower ash discharge chamber (16), openings (22) in the bottom (14) which allow ashes and consumed bed material to pass downwards into the ash discharge chamber (16), feed devices for feeding bed material and fuel into the combustion chamber (15) of the bed vessel (12), a compressor (33) for supplying the bed vessel (12) with air for fluidization and combustion, a discharge device (44) for discharging ashes and consumed bed material from the ash discharge chamber (16), and openings (36) for supplying air to the ash discharge chamber (16) below the bottom (14) of the combustion chamber (15) for cooling of material in the discharge chamber (16). According to the invention the discharge chamber (16) is provided with grates (37,38) with horizontal air channels for spreading cooling air from the central part of the chamber (16) to its outer parts. These grates (37,38) may be formed of U-shaped sections with their openings facing downwards. The channels of a grate (37) may be connected to the combustion chamber (15) by vertical tubes.

Description: The invention relates to a power plant with combustion of a fuel in a fluidized bed according to the precharacterising part of Claim 1. A power plant of this kind is, for example, previously known from the SE-A-450, 163. The combustion material in such plants consists of or con­ tains a sulphur absorbent, such as lime or dolomite, for binding sulphur in the fuel during the combustion. The in­ vention primarily relates to PFBC plants, in which the com­ bustion takes place at a pressure above atmospheric pressure and in which a bed vessel with a combustion chamber is sur­ rounded by a pressure vessel which contains combustion air with a pressure amounting to about 2 MPa. The term "PFBC" is formed by the initial letters in the expression "Pressurized Fluidized Bed Combustion". In a plant of the afore-mentioned kind the bed vessel con­ sists of a container which is divided by a bottom into an upper combustion chamber with a fluidized bed and a lower chamber for the discharge of ashes and consumed bed mate­ rial. The bottom may consist of elongated, parallel air distribution chambers with nozzles for combustion air for fluidization of a bed of particulate bed material above the bottom and for combustion of supplied fuel. The combustion air supplied to the combustion chamber is generated by a compressor. Between the air distribution chambers there are openings through which ashes and bed material may pass from the combustion chamber to the ash discharge chamber below the bed vessel bottom. The ashes and the bed material are cooled by air prior to being discharged via a sluice system. Usually the chamber for discharging ashes and bed material is shaped as a conical or pyramidal hopper with its down­ wardly-directed tip connected to the discharge device via a tube. If in these cases, in order for the cooling air to be utilized as efficiently as possible, the cooling air is in­ troduced at the bottom into the ash discharge chamber, it has been found that the cooling air does not spread uni­ formly over the cross section when flowing upwards through the discharge chamber. This means - for one thing - that the cooling at the outer parts of the ash chamber is insuffi­ cient and - for another - that in the central part of the ash chamber the velocity of the air stream may reach such a level that the material in the ash chamber and between air distribution chambers is fluidized. This fluidization in­ creases the heat transfer to the walls of the air distribu­ tion chambers and may result in such a heating as to jeopar­ dize the strength of the construction parts. Furthermore, the concentration of the ash cooling air results in an unde­ sired and uneven air distribution over the cross section of the combustion chamber, which may disturb the operation. The discharge chamber disclosed in the SE-A-450, 163 is pro­ vided with horizontally extending cooling pipes connected in parallel with one of their ends to an air distribution cham­ ber arranged on one side of the discharge chamber and with their other ends to an air collecting chamber arranged on the other side of the discharge chamber. The cooling air does not get into direct contact with the discharge material surrounding the cooling pipes. The discharge chamber of a combustion plant disclosed in the SE-B-417, 636 consists of a relatively narrow channel ar­ ranged adjacent to the air chamber supplying the fluidized bed with fluidization air. This discharge chamber accommo­ dates a meandering piping system, the pipes being provided with small openings through which the cooling air escapes into the discharge material. The EP-A-0, 063, 173 discloses a combustion plant the coni­ cally shaped discharge chamber of which is provided with blowing pipes arranged in the upper vertical section of the discharge chamber. The pipes are provided with small outlet openings through which the cooling air escapes. The cooling pipes extend across almost the entire width of the discharge chamber. The US-A-2, 812, 592 discloses a furnace for the treatment or drying of particulate material. The material is fed from the top into the furnace vessel which contains a chamber where the material is fluidized and heat-treated. It is then discharged through a hopper-like discharge device where it is cooled by air-conducting pipes and by direct injection of cooling air. The air passing through the pipes fluidizes the material in the heat-treatment chamber. In the top portion of the furnace vessel there is a grating consisting of a plurality of downwardly open U-shaped bars. These bars are provided to separate the up-flowing fluidization air from the fresh material fed in from the top of the furnace ves­ sel. The air collected in these bars is guided to a hollow guide space in the wall of the vessel and discharged there­ from. The invention aims at improving a power plant of the above-­ mentioned kind in such a way that a more uniform distribu­ tion of the cooling air stream in the ash discharge chamber is attained, particularly in those cases where the discharge chamber has a downwardly tapered shape with the cooling air being introduced into the discharge chamber near the bottom thereof. To achieve this aim the invention suggests a power plant ac­ cording to the introductory part of Claim 1, which is char­ acterized by the features of the characterizing part of Claim 1. Further developments of the invention are characterized by the features of the additional claims. According to the invention, the spreading of the cooling air in the ash discharge chamber is improved by locating therein one or more grates having substantially horizontal air chan­ nels with an insignificant flow resistance. The grates are constructed of open sections, for example U-sections, with their openings facing downwards so as to form channels free from material inside these sections. Grates may be lo­ cated at two or more levels. Between these levels there may be arranged vertical pipe connections, through which cooling air may flow up from a channel in a lower grate to a channel located in a higher positioned grate. Cooling air collected in channels in the central part of the discharge chamber flows horizontally outwards and out into the material in the outer parts of the discharge chamber. Furthermore, vertical pipe connections may be arranged between a grate layer and the combustion chamber. Pipes from a grate layer pass be­ tween the air distribution chambers, which form the bed bot­ tom of the bed vessel. These pipes suitably terminate in a nozzle similar to the nozzles of the air distribution cham­ bers and at the same level as these. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 schematically a power plant according to the inven­ tion as applied to a PFBC plant, Figure 2 an alternative embodiment of the lower part of a bed vessel, Figure 3 a section through the ash discharge chamber taken along line A-A in Figure 1, Figure 4 a perspective view of grates with air channels. In the figures, 11 designates a pressure vessel, 12 a bed vessel and 13 a cleaner of cyclone type enclosed within the pressure vessel 11. Only one cyclone 13 is shown, but in re­ ality there is a cleaning plant with a number of parallel groups of series-connected cyclones. The bed vessel 12 in­ cludes a bottom 14 which divides the bed vessel 12 into an upper combustion chamber 15 and a lower ash chamber 16. The bottom 14 consists of a number of parallel air distribution chambers 17 with nozzles 18. Through these chambers 17 the combustion chamber 15 is supplied with combustion air from the space 20 between the pressure vessel 11 and the bed ves­ sel 12. This air fluidizes the particulate material forming the bed 21 and burns fuel supplied to the bed 21. Between the air distribution chambers 17 there are openings 22 through which ashes and consumed bed material may pass to the chamber 16. Fuel and fresh bed material are supplied to the bed vessel 12 through conduits 23 and 24, respectively, from storage means not shown. The combustion chamber 15 con­ tains cooling tubes 25 for cooling the bed and generating steam for a steam turbine (not shown). Combustion gases formed are accumulated in the freeboard 26 above the bed surface 27 and are lead through the conduit 28 to the cleaner 13 where dust is separated from the gases. The dust is discharged through the conduit 30 to a collec­ tion container (not shown). The cleaned gases are lead through the conduit 31 to a turbine 32. The turbine 32 drives a compressor 33 which, via the conduit 34, supplies the space 20 with combustion air. In the lower conical part 35 of the bed vessel 12, which forms the ash chamber 16, there are openings 36 with regu­ lating means 39 for the supply of cooling air from the space 20 to the chamber 16 for cooling of the material present in the chamber 16. In the embodiment according to Figure 1, the chamber 16 accommodates two layers of grates 37 and 38, which are built up of U-shaped sections 40 and 41, respec­ tively, with their openings directed downwards and forming horizontal channels 46 with open bottom portion (see Figure 4). The grate layers 37, 38 are interconnected by vertical tubes 42, which allow vertical transport of cooling gas be­ tween the layers. The tip of the conical part 35 of the bed vessel is connected to an outlet tube 43 equipped with a sluice-type discharge device 44. The cooling air supplied to the lower part of the conical ash chamber 16 encounters the downwardly-flowing bed mate­ rial and the ashes and flows through the bottom 14 up into the combustion chamber 15, where it is utilized for the com­ bustion. The cooling air on its way upwards through the material takes that path that offers the smallest flow resistance. This results in the air flow concentrating in the centre of the chamber 16, thereby posing the risk of an undesired flu­ idization of the material in the centre of the chamber 16. Furthermore, the material close to the wall of the conical part 35 will not be sufficiently cooled. Since part of the cooling air is captured in the downwardly open U-shaped sec­ tions 40, 41 of the grates 37, 38, where they can flow hori­ zontally through the lateral direction in the material-free channels 46 with no significant flow resistance, a more uni­ form distribution of the cooling air flow over the entire cross section is attained. The air in the channels 46 of the U-shaped sections 40, 41 finds its way out into the material in the chamber 16 along the sections, as indicated by the arrows 45. As shown in Figure 3, the grate 37, 38 form open­ ings 47 through which ashes and bed material can pass. In the embodiment according to Figure 2, the chamber 16 is only provided with one grate 37 built up of sections 40. To these sections 40 there are connected vertical tubes 50, passing up between the air distribution chambers 17 and opening out with their nozzles 51 at the same level as the air nozzles 18. At the bottom of the air distribution cham­ bers 17 there are outflow openings 52 for a minor part of the combustion air. The air flow is indicated by arrows 53. Since the flow resistance in the tubes 50 is smaller than the flow resistance in the material layer between the grate 37 and the bottom 14, the cooling air captured by the sec­ tions 40 will to a considerable extent flow to the com­ bustion chamber 15 via the tubes 50. In this way, also in the case of a considerable flow of cooling air, it is possi­ ble to provide such a low flow within the layer between the grate 37 and the bottom 14 that the risk of fluidization in the layer and between the air distribution chambers 17 can be eliminated. It will be recognized that the two or more layers of air distribution channels 46 need not be in alignment in the vertical direction but may be staggered relative to each other or may have different patterns of channels. In such a case the vertical connection pipes 42 and possible tubes 50 originating from lower layers of air channels can be ar­ ranged slightly oblique to the vertical direction.

Claims: 1. Power plant with combustion of a fuel in a fluidized bed, comprising a bed vessel (12), a bottom (14) dividing the bed vessel (12) into an upper combustion chamber (15) and a lower ash discharge chamber (16), openings (22) in the bottom (14) which allow ashes and con­ sumed bed material to pass downwards into the ash discharge chamber (16), feed devices for feeding bed material and fuel into the com­ bustion chamber (15) of the bed vessel (12), a compressor (33) for supplying the bed vessel (12) with air for fluidization and combustion, a discharge device (44) for discharging ashes and consumed bed material from the ash discharge chamber (16), and openings (36) for supplying air to the ash discharge chamber (16) below the bottom (14) of the combustion chamber (15) for cooling of material in the discharge chamber (16), characterized in that-the ash discharge cham­ ber (16) below the bottom (14) of the combustion chamber (15) is provided with air collecting and distributing de­ vices (37, 38) arranged above said air supplying openings (36) and that said collecting and distributing means (37, 38) include substantially horizontal downwardly open chan­ nels for collecting upwardly flowing cooling air and distributing said collected air over the area of said ash discharge chamber (16). 2. Power plant according to Claim 1, characteriz­ ed in that said channels (46) are formed with preferably U-shaped cross section. 3. Power plant according to Claim 1 or 2, charac­ terized in that the sections (40, 41) of the air distribution channels (46) form one or more layers of air distribution channels (46), located at different levels. 4. Power plant according to any of the preceding Claims, characterized in that the air distribution channels (46) in each layer form a horizontal grate (37, 38) with openings (47) for the passage of the material. 5. Power plant according to any of Claims 3 or 4, cha­ racterized in that the air channels (46) of dif­ ferent layers are interconnected by vertical or slightly oblique connecting pipes (42). 6. Power plant according to any of the preceding Claims, characterized in that vertical or slightly oblique tubes (50) are connected to the channels (46) of the sections (40, 41), said tubes (50) opening out into the bot­ tom (14) of the combustion chamber (15), preferably at substantially the same level as the air nozzles (18) of the bottom (14). 7. Power plant according to any of the preceding Claims, characterized in that the bottom of the air distribution chambers (17) is provided with one or more air outflow openings or nozzles (52).

Inventors: JONSSON, ARNE
Applicants: ASEA STAL AKTIEBOLAG
Application year: 1987
Main classification: F23C11
IPC codes: F27B5, F23C10, B01J8