A PROCEDURE AND A DEVICE FOR CONTROLLING THE FLOW OF COMBUSTION AIR IN A PFBC PLANT

Abstract: A procedure and a device for controlling the flow of combustion air in a PFBC plant which, besides combustion chamber (1) and pressure tank (2), includes a dual shaft gas turbine (3, 5, 6, 7) comprising a low-pressure compressor (3) and a low-pressure turbine (7) connected by a first shaft and a high-pressure compressor (5) and a high-pressure turbine (6) connected by a second shaft, wherein air is drawn into the low-pressure compressor (3), conducted further to the high-pressure compressor (5), and on into the pressure tank (2) and combustion chamber (1), after which the return flow in the form of combustion air containing particles is conducted to the high-pressure turbine (6) and onto the low-pressure turbine (7) from whence the return air is discharged to the surroundings, the combustion air containing particles being prevented from finding its way radially in towards the bearings of the turbines (6, 7) by barrier air being pressed into the middle of the high-pressure turbine (6).

Description: A procedure and a device for controlling the flow of combustion air in a PFBC plant. TECHNICALHELD: The present invention relates to a procedure and a device for preventing soot and dust particles from causing operating disturbances in a PFBC plant (pressurised fluidised bed combustion). BACKGROUNDART: PFBC plants have been described in a large number of publications and patent specifications, such as in EP 0124842 "Power plant having a combustion chamber with fluidised bed". A PFBC plant includes compressor and turbine assemblies arranged so that a low-pressure compressor and a low-pressure turbine are connected together via a first shaft and a high-pressure compressor and a high-pressure turbine are connected via a second shaft. The system supplying the combustion chamber with compressed air consists of a dual-shaft gas turbine where, as already mentioned, the low-pressure compressor (LK) and low-pressure turbine (LT) are connected by a fixed shaft and the high-pressure compressor (HK) and high pressure turbine (HT) are connected by a fixed shaft. The high- pressure assembly drives a generator via a transmission. The passage of the air through the compressors and to the fluidised bed, and the equivalent passage of the waste gases through the turbines and out to the surroundings pertain to known technology and will therefore only be related briefly. The air is drawn in to the low-pressure compressor and is conducted via an intermediate cooler to the high- pressure compressor. From there the air is conducted to the pressure tank and is thus pressurised to a certain over-pressure in relation to the combustion chamber. The waste gases formed at the combustion are now conducted from the combustion chamber via cyclones to the high- pressure turbine and then on to the low-pressure turbine from whence, possibly after cleaning and cooling, the gases are discharged into the surroundings. The passage of air and gas through the compressor and turbine assemblies as described here obviously need not be associated with a fluid bed plant. Patent specification SE 8602003-9 (453 114) "Method of operating a turbine assembly" describes how such a turbine assembly can be used together with other types of combustion chambers for driving an electric generator connected to the common shaft of the high-pressure compressor and the high-pressure turbine, for instance. It can be read from the construction of this plant that the effective member for controlling the output of the generator consists of a guide vane ring connected to the inlet of the low-pressure turbine and having adjustable guide vanes, which the gas flow from the high-pressure turbine and to the low-pressure turbine must pass. Patent specification SE 9102651-8 (469 039) "A method of air-flow control in a PFBC plant and device for performing the method", reveals an almost identical design for a PFBC plant to that shown in the present application. The application concerns pressure tanks and combustion chambers in a PFBC plant which is pressurised by means of pressure- flow control so that the right amount of air can be supplied for every load case. However, drawbacks arise when the plant is shut down in that the air containing particles from the combustion finds its way into the bearings of the turbines. This causes considerable problems when the plant is to be started up again. PURPOSE OF THE INVENTION: The purpose of the present invention is to provide a procedure and a device which solves the problems described above of sooty air in a PFBC and particularly sooty air in the bearings of the turbine. SUMMARY OF THE INVENTION: The present invention comprises a procedure and a device for controlling the flow of the combustion air in a PFBC plant wherein a barrier air flow presses combustion air out of the turbine bearings both during operation and during standstill. BRIEF DESCRIBED OF THE DRAWINGS: The invention will now be described in further detail with reference to the accompanying drawings. The same component in one figure has the same designation as in the other figure of the drawings. Figure 1 shows the main parts of a PFBC plant relevant to the invention. Figure 2 shows a section through part of a turbine according to the present invention. DESCRIPTION OF THE INVENTION: Figure 1 shows those parts of a PFBC plant that constitute the most important components in an air flow system, substantially as they appear in said patent specifications. The figure shows a combustion chamber 1 within a pressure tank 2. Air is drawn in to a low-pressure compressor 3 and passes an intermediate cooler 4 on its way to a high- pressure compressor 5. From here the air is conducted to the pressure tank 2. Gas from the combustion chamber is conducted via cyclones, not shown, to a high-pressure turbine 6 and passes a low-pressure turbine 7 on its way out to the surroundings. The figure also shows a storage tank 8 with bed substance for adjusting the height "h" of the bed in accordance with changes in the load. Fuel is supplied at the inlet 9. An electric machine 10 is connected via a transmission box to the high- pressure shaft, which machine can run as motor upon start-up and as generator during normal operation. The power extraction from tubes in the bed is not shown in the figure. A fan 11 is also connected to a heater 12 so that the fan presses surrounding air through the heater so that it is heated to a suitable working temperature, after which it is conducted into the middle of the high-pressure turbine HT 6 and possibly also into the low-pressure turbine LT 7. The heater 12 thus achieves a quicker starting process by heating up the parts of the turbine with surrounding air prior to startup. A first barrier air valve 13 is placed in a barrier air conduit between the heater 12 and an inlet 14 which is shown schematically in Figure 1 as the point at which the barrier air may be divided between the high- pressure turbine HT 6 and the low-pressure turbine LT 7. A second barrier air valve 15 is situated in a barrier air conduit between the outlet side of the high-pressure compressor HK 5 and the inlet 14. During normal operation of the plant the first barrier air valve 13 is closed and the second barrier air valve 15 is open, which means that compressed surrounding air, in the form of barrier air, is pressed in between the high-pressure turbine HT 6 and the low-pressure turbine LT 7. When the plant is to be shut down the second barrier air valve 15 is closed and the first barrier air valve 13 is opened so that surrounding air, barrier air, is pressed by the fan 11 through the heater 12 and in to the high-pressure turbine HT 6 and possibly also the low-pressure turbine LT 7. Figure 1 also shows a conventional intercept valve 16 by means of which the air flow to the pressure tank 2 and the gas flow from the combustion chamber 1 can be influenced. During operation, the air flow from the high-pressure compressor HK 5 is divided into a barrier portion and a pressure tank portion which flows through the intercept valve 16 into the pressure tank 2 and on through the combustion chamber 1. Figure 1 also indicates an air flow control with a guide vane ring 17 having adjustable guide vanes on the input side of the low-pressure turbine 7, i.e. between the high-pressure turbine HT 6 and the low- pressure turbine LT 7. Figure 2 shows the turbine arrangement with the first step 18 of the high-pressure turbine with its turbine blade and the second step 19 with its turbine blade. The first step 18 of the high-pressure turbine and the second step 19 of the high-pressure turbine are rotatably journalled in a turbine housing 20 about a common axis of rotation with a common turbine hub 21. A guide vane ring 23 with its ring 22, secured against turning, is fitted in the turbine housing 20 between the first step 18 of the high-pressure turbine with its turbine blade and the second step 19 of the high-pressure turbine with its turbine blade. The turbine hub 21 is arranged having one or more barrier air channels 24 extending axially into the hub and opening radially outwardly between the first step 18 and the second step 19 of the high-pressure turbine immediately opposite the ring 22 of the guide vane ring. The barrier air thus pressed into the channel(s) 24 is distributed on both sides of the ring and is pressed outwards into the area of the sooty air from the combustion chamber 1. The turbine hub 21 is also provided in the area for the channel orifice with a seal 25 of labyrinth type, for instance. An alternative embodiment is for the axially oriented barrier air channel(s) 24 to open into a dish-shaped or annular opening which distributes the barrier air uniformly around the entire turbine hub. A combination of these two embodiments is also feasible within the scope of the appended claims.

Claims: CLAIMS 1. A procedure for controlling the flow of combustion air in a PFBC plant which, besides combustion chamber (1) and pressure tank (2), includes a dual shaft gas turbine (3, 5, 6, 7) comprising a low- pressure compressor (3) and a low-pressure turbine (7) connected by a first shaft and a high-pressure compressor (5) and a high-pressure turbine (6) connected by a second shaft, wherein air is drawn into the low-pressure compressor (3), conducted further to the high-pressure compressor (5), on into the pressure tank (2) and combustion chamber (1), after which the return flow in the form of combustion air containing particles is conducted to the high-pressure turbine (6) and on to the low-pressure turbine (7) from whence the return air is discharged to the surroundings, characterized in that the combustion air containing particles is prevented from finding its way radially in towards the bearings of the turbines (6, 7) by barrier air being pressed into the middle of the high-pressure turbine (6). 2. A procedure as claimed in claim 1, characterized in that when the plant is stationary the barrier air consists of the surrounding air which is pressed into the high-pressure turbine by a fan (11). 3. A procedure as claimed in claim 2, characterized in that the surrounding air is heated by a heater (12) before being pressed into the high-pressure turbine (6). 4. A procedure as claimed in claim 1, characterized in that when the plant is in operation the barrier air consists of compressed surrounding air from the outlet of the high-pressure compressor (5) which is pressed into the high-pressure turbine (6). 5. A procedure as claimed in any of the preceding claims, characterized in that the barrier air is pressed in between the first step (18) of the high-pressure turbine (6) and the second step (19), immediately opposite the ring (22) of the guide vane ring (17), to be distributed equally on both sides of the ring (22). 6. A barrier air device in a PFBC plant which, besides combustion chamber (1) and pressure tank (2), includes a dual shaft gas turbine (3, 5, 6, 7) comprising a low-pressure compressor (3) and a low- pressure turbine (7) connected by a first shaft and a high-pressure compressor (5) and a high-pressure turbine (6) connected by a second shaft, wherein air is arranged to be drawn into the low-pressure compressor (3), conducted further to the high-pressure compressor (5), and into the pressure tank (2) and combustion chamber (1), after which the return flow is arranged to be conducted to the high-pressure turbine (6) and on to the low-pressure turbine (7) from whence the return air is discharged to the surroundings, characterized in that the barrier air is arranged to be pressed in between the first step (18) and second step (19) of the high-pressure turbine. 7. A device as claimed in claim 6, characterized in that the barrier air is arranged to be pressed in between the first step (18) and second step (19) of the high-pressure turbine (6) through a partially radially extending barrier air channel (24) situated in the hub (21) of the turbine. 8. A device as claimed in either of the claims 6 or 7, characterized in that when the plant is stationary the barrier air consists of the surrounding air which is pressed in between the turbine steps (18, 19) by a fan (11). 9. A device as claimed in claim 8, characterized in that the barrier air is arranged to be heated by a heater (12). 10. A device as claimed in either of the claims 6 or 7, characterized in that when the plant is in operation the barrier air consists of compressed surrounding air from the outlet of the high-pressure compressor (5) which is arranged to be pressed in between the turbine steps (18, 19).

Inventors: KNUDSEN KNUD, LINDER ULF, KNUDSEN, KNUD, LINDER, ULF
Applicants: ABB CARBON AB, KNUDSEN KNUD, LINDER ULF, KNUDSEN, KNUD, LINDER, ULF
Application year: 1997
Main classification: F01D11
IPC codes: F01D11, F01K23, F02C3, F02C9