POWER PLANT WITH A FLUIDIZABLE BED FOR FUEL COMBUSTION

Abstract: Power plant with a fluidizable bed for fuel combustion enclosed within a bed vessel (11). It comprises one or more cleaners (12) for separating dust form the combustion gases leaving the bed (15). The gas cleaner (12) is built up as a panel or unit with a number of parallel-connected centrifugal cleaners (40, 60) each comprising a first cylindrical inlet part (44) with guide vanes (43) which impart a rotating movement to the gas flowing through, and a second cylindrical part (45) sliding into the first part (44). Together these two cylindrical parts form an annular gap (46) with an outlet which com­ municates with a common space (52) and/or with a separator (60) arranged in series with the parallel separators (40). The panel or unit (12) is directly connected to an outlet opening (27) from the bed vessel (11) and forms part of a bed vessel wall.

Description: The invention relates to a power plant according to the precharacterising part of claim 1, as known from the document US-A-3 137 133. The invention is particularly intended for cleaning gases which leave a bed vessel in a plant with combustion in a fluidized bed, in particular a bed in which the combustion takes place at a pressure considerably exceeding the atmospheric pressure. Such a plant, also called a "PFBC plant" according to the initial letters of the term P ressurized F luidized B ed C ombustion, is capable of operating at a pressure of up to about 2 MPA (20 bar) and delivering propellant gas to a gas turbine. The separation of solid material from the combustion gases has to be made, for one reason, in view of the environment and, for another reason, in view of the erosion in the gas turbine. In a PFBC plant, in which the combustion gases are used for driving a gas turbine, the separation must take place at a high temperature, which involves particular problems. When cleaning combustion gases in a PFBC plant the main objectives are: 1. Sufficiently good cleaning in view of the gas turbine and the surrounding environment. 2. With regard to the pressure, keeping as small as possible the pressure vessel surrounding the combustion chamber and as a result of this a small volume of the cleaning plant in the space between the combustion chamber and the pressure vessel. 3. As small a pressure drop as possible to gain a high gas turbine effect. 4. As small a loss flow as possible for ash conveyance etc. 5. The lowest possible cost. 6. A sufficiently long life of the plant in spite of the fact that the cleaning plant is subjected to high thermal stresses because of high temperature of the gas which is to be cleaned. Because of the large volume, the high cost and the insufficient life, cleaning plants with filters are not used in PFBC plants. Instead, up to now, in PFBC plants there have been used cleaning plants of cyclone type the positive properties of which are: Sufficiently good separation for propellant gas to the turbine, no loss flow, moderate costs, a long life. A negative property is a high pressure drop. The invention aims at developing a power plant of the above-mentioned kind with a cleaning plant which is superior to a cyclone type cleaning plant. To achieve this aim the invention suggests a power plant according to the introductory part of Claim 1, which is characterized 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, there are employed centrifugal separators having a straight flow pipe with guide valves at the inlet which impart a rotary movement to the through-flowing gas. The separator comprises a first cylindrical part and a second cylindrical part extending into the first part and forming a peripheral gap together with the first part. Through this gap the dust-laden flow nearest to the cylindrical wall is separated from the main flow of the combustion gas. The main flow of clean gas is led to a turbine. A number of parallel-connected centrifugal separators are built together into a panel which is connected to openings in a bed vessel wall. A sufficiently good separation for the gas turbine as well as the environment is obtained. The volume is small. The direct connection of a panel unit to a bed vessel wall entails a very insignificant volume requirement, which makes it possible to reduce the volume of a pressure vessel surrounding the bed vessel and the cleaner. The pressure drop is small. The cost is low. Centrifugal cleaners can be manufactured of a heat-resistant material giving a satisfactory life. The disadvantage of the above type of dust separators, namely the approximately 10 per cent loss flow in each stage in the case of a satisfactory degree of separation, can be substantially eliminated by bringing together the loss flows from a group of centrifugal separators and allowing them to pass through a centrifugal separator connected in series with the first-mentioned group. In this subsequent separator the dust in the loss flow is separated. The cleaning gas from the this separator is joined with the main flow of gas from tip other centrifugal separators. With a loss flow of 10 per cent in both the parallel connected group of centrifugal separators and in the subsequent centrifugal separator for the loss flow from the group, the final loss flow will be 10 per cent of 10 per cent, that is, only 1 per cent. To bring about the necessary flow in the series-connected centrifugal cleaner for the loss flow, the main flow has to be throttled somewhat, but in spite of this a low total pressure drop care be obtained. For economic reasons it may be suitable to construct a cleaning group in such a way that all the centrifugal separators may have an identical design. If the loss flow is 10 per cent, a cleaning group may consist of 10 parallel-connected units and one unit for the loss flow in series with these units or 10 times x parallel-connected units and x units connected in series with these. Several centrifugal separators may be connected in series for both the main flow and the loss flow. As previously mentioned, in a power plant according to the invention, the necessary volume for the cleaning plant is small, making up only a few per cent of the volume needed for a conventional cleaning plant with cyclones. In this way, the size of a surrounding pressure vessel and hence the cost of the plant can be reduced. The pressure drop is only about 1 per cent of the pressure drop in a cleaning plant with cyclones. The loss flow of combustion gas is acceptable and of just the right magnitude for carrying off solid dust. Because the cleaning plant is compact and is directly connected to the wall of the bed vessel, the heat losses are smaller than in a cleaning plant with cyclones. The very short through-flow time for gases, about 10 ms, reduces the problems with chemical reactions between the gas and the material in the cleaners. 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 PFBC power plant to which the invention is applied, Figure 2 a schematic section of a panel forming part of a wall in a bed vessel and comprising a number of centrifugal type separator units, Figure 3 schematically an alternative embodiment of such a panel, Figure 4 an embodiment of a centrifugal separator in the panel according to Figure 2. In the figures, 10 designates a pressure vessel surrounding a bed vessel 11 and a number of gas cleaner units 12, connected to the bed vessel 11, for separating dust from combustion gases leaving the bed vessel 11. The bed vessel 11 is provided with a bottom 13 with air nozzles 14 and supports a bed 15 of fluidizable particulate material, in which a fuel supplied through a fuel pipe 16 from a fuel storage, not shown, is burnt. Tubes 17 are provided in the bed portion of the bed vessel 11 for carrying off heat from the bed 15 and heat water or generate steam for operation of a steam turbine (not shown). Air for fluidization of the bed 15 and combustion of the fuel is supplied to the bed vessel 11 from the space 18 between the pressure vessel 10 and the bed vessel 11. Combustion gases are collected in the freeboard 20 above the bed 15, pass through the gas cleaners 12, in which accompanying solid particles, consisting of consumed normal bed material and ashes from the fuel, are separated. From the gas cleaners 12 the gas is brought via the conduits 21 to the collecting pipe 22 and through this to a gas turbine 23. The gas turbine 23 drives a generator 24, which delivers current to a network (not shown), and a compressor 25, which delivers compressed combustion air through the conduit 26 to the space 18 inside the pressure vessel 10. In one or more of the walls of the bed vessel 11 there are openings 27. The gas cleaners 12 are formed as panels or units which are connected to these openings 27 and form part of the bed vessel wall. The gas cleaners 12 can suitably be connected to the bed vessel wall by bolt joints 42. Material separated in the gas cleaners 12 is carried of via the conduits 32a, 32b, 32, the cooler 33 in the shaft 34 and the conduit 35 to the cyclone 36, in which the solid material is separated from transport gas. This transport gas is cleaned in a filter in the container 36. Dust is discharged via the valve 37. The cooler 33 preheats the combustion air, whereby heat is extracted from the dust and the transport gas. Figure 2 and 3 schematically show sections of two different embodiments of a cleaner 12. In the embodiment according to Figure 2 the cleaner 12 comprises a number of centrifugal separators 40 arranged in parallel, the upstream sides of which are connected to a sheet 41 which is provided with openings adapted to the separators 40. The sheet 41 is larger than the opening 27 and is connected at its periphery to the bed vessel wall, for example with a number of bolts 42 (Figure 1). Each separator comprises a first cylindrical part 44 having guide vanes 43 at its inlet portion, and a second cylindrical part 45 extending into the first part 44 and forming together with that part an annular gap 46. The separators 40 are connected at their ends to the sheet 41 and to a sheet 47 arranged in parallel with sheet 41, and together with side sheets 50, 51 they form a space 52. The gap 46 opens into or communicates with the space 52. The separators 40 are connected to a gas collecting channel 53 by means of tubes 54 with a throttle means 55 which brings about a certain pressure drop. The tubes 54 are connected to the wall 57 in the channel 53. Gas flowing into a separator 40 is put in rotation by the guide vanes 43, whereby solid particles are collected at the wall in the part 44 and together with part of the gas, for example 10 per cent thereof, flow into the gap 46 and further to the space 52. The rest of the gas, about 90 per cent, flows through the tubes 54 to the gas collecting channel 53. The space 52 communicates through the channel 56 with the inlet side of a dust separator 60, in which the loss gas and the separated dust from the first cleaning stage are separated. The dust separator 60 may have exactly the same design and size as the separators 40. The components included have been given the same designations as those in the separators 40. Dust and about 10 per cent of the gas are diverted through the gap 46 of the separator 60 and carried away through the conduit 32, the cooler 33 and the conduit 35 to the container 36 (Figure 1). The main part of the gas quantity, about 90 per cent, is conducted through the cylindrical part 45 to the gas collecting channel 53. The gas loss is of the order of magnitude of 1 per cent. The throttle means 55 is adapted such that the pressure drop in the tubes 54 is approximately the same as in the dust separator 60 in order to obtain the same flow conditions in the separator 60 as in the separators 40. In the embodiment according to Figure 3, two centrifugal separator 40a and 40b are arranged in series and two spaces 52a and 52b are provided for collecting loss gas and dust. These spaces are formed by the walls 41, 47a, 50a, 51a and 47a, 47b, 50b, 51b, respectively. The space 52a communicates with the separator 60a through the channel 56a, and the space 56b communicates with the separator 60b via the channel 56b. Separated dust and part of the gas are carried off through the conduits 32 and 32b, respectively. The other gas is passed to the collecting channel 53 and is passed together with the main stream of cleaned gas to the turbine 23. The centrifugal separators 40 in the cleaner unit 12, made in the form of a panel, may be designed as shown in Figure 4. The first cylindrical part of the separator may consist of a sleeve 70 with flanges 71, 72 which are each provided with a projecting centering ring 73, 74, by means of which the sleeve 70 is connected to openings in the sheet walls 41 and 47, respectively. The sleeves are fixed to the walls by welds 75 and 76, respectively. The second part 45 of the separator consists of a sleeve 80 sliding into the sleeve 70, the inner part 80a of a the sleeve 80 together with the sleeve 70 forming the annular gap 46. The outer part 80b is formed as a guide means centering the sleeve 80 in the sleeve 70. In the wall of the sleeve 70 there is/are one or more opening(s) 82, through which the downstream part of the gap 46 communicates with the surrounding space 52. In a larger plant there may be provided a considerable number of cleaning units and, in total, thousands of individual centrifugal separators may be included in these cleaner units.

Claims: 1. Power plant with a fluidizable bed for fuel combustion comprising a bed vessel (11) housing said fluidizable bed and comprising a cleaning plant (12) including a number of parallel-connected separate cleaners which are connected to a wall of the bed vessel (11) and which are designed as centrifugal separators (40) each consisting of a first cylindrical inlet part (44) with guide vanes (43) for imparting a rotating movement to the passing gas flow, and a second cylindrical part (45), which extends into the first cylindrical part (44) and together with that part forms an annular gap (46) the outlet end of which is connected to a space (52) for separated dust, the space (52) being common to several centrifugal separators, characterized in that the guide vanes (43) in the first cylindrical inlet part (44) of the centrifugal separators (40) are located opposite to the portion of the first cylindrical inlet part (44) which forms said annular gap (46) with said second cylindrical part (45), and that the centrifugal separators or groups thereof form a panel or a unit (12) which constitutes a replaceable part of the wall which immediately confines the freeboard space of the bed vessel (11), with the gas capable of immediately entering from the freeboard into the inlet opening of the centrifugal separators. 2. Power plant according to claim 1, characterized in that the the centrifugal separators (40) are designed such that the gas flow passes through the separators in a straight forward way without any mentionable flow diversion. 3. Power plant according to claim 1 or 2, characterized in that two or more separators (40) are arranged in series with each other in the panel (12). 4. Power plant according to any of the preceding claims, characterized in that the gap (46) in a number of parallel-connected separators (40) is connected to a common centrifugal separator (60) which forms a second cleaning stage for the flow coming from the gaps (46) in the separators (40). 5. Power plant according to any of claims 1 to 3, characterized in that the space (52), which is common to a number of parallel-connected separators (40) and communicates with said gap (46), is connected to a centrifugal separator (60) connected in series with the separators (40), said centrifugal separator (60) forming a second cleaning stage for the flow coming through the gaps (46). 6. Power plant according to any of the preceding claims, characterized in that a plurality of panels (12) form parts of one or more bed vessel walls. 7. Power plant according to any of claims 4 to 6, characterized in that the centrifugal separators (40, 60) in both the first and the second cleaning stage are connected, on their downstream sides, to a common outlet channel (53), and that between the centrifugal separators (40) of the first stage and the common outlet channel (53) there is a throttle means (55), which brings about a pressure drop corresponding to the pressure drop in the centrifugal separator (60) of the second stage.

Inventors: VIND, TYGE
Applicants: ASEA STAL AKTIEBOLAG
Application year: 1987
Main classification: F23C11
IPC codes: B01D45, B04C3, F23J15, F23C10, B01J8