METHOD FOR IGNITING A COMBUSTION CHAMBER WITH A FLUIDIZED BED AND A POWER PLANT FOR CARRYING OUT THE METHOD

Abstract: Method for igniting a combustion chamber with a flui­ dized bed upon start-up and a power plant for carrying out the method. According to the invention bed material, which is heated to the ignition temperature of a start-up fuel, is transferred from a storage container (40) to the combustion chamber (8), and fuel is supplied to the bed (14). The airflow through the nozzles (13) of the combustion chamber bottom (12) are reduced during said transfer of bed material and are increased again when the fuel is supplied. The power plant according to the invention is equipped with means for tem­ porarily by-passing the nozzles (13) of the combustion cham­ ber during the transfer of the hot bed material to the combustion chamber. For preheating and storing the bed material to be supplied to the combustion chamber, an iso­ lated container (40) is provided, which is separated from the combustion chamber and positioned at a level above the bottom of the combustion chamber. In a plant with a com­ bustion chamber (8) enclosed in a pressure vessel (10), a connection with a valve (55) for by-passing the air flow may be provided between the space (11) in the pressure vessel and the freeboard (18) of the combustion chamber (8) for reducing the air flow through the bed. Further, control means (38, 80) are provided for carrying out the necessary opera­ tions of valves, and the like, during the start-up operation.

Description: The invention relates to a method for igniting a combustion chamber with a fluidized bed according to the precharacterising part of claim 1. The invention also relates to a power plant for carrying out the method. Numerous methods have been proposed and applied for starting up the combustion in a fluidized bed. Common to most of them is that the entire bed or a limited part of the bed is heated by hot gas which is allowed to pass through and fluidize the bed. This gas is heated in special start-up combustion chambers which are fired, for example, by gaseous or liquid substances. Alternatively, the heating of the bed material is performed by combustion of gas directly in the bed, in the lower part thereof. As the bed temperature rises, an increasingly large part of the supplied heat escapes with gases leaving the bed. Towards the end of the heating period, a stationary state is achieved, in which the temperature of the bed is approximately equal to that of the heating gas and consequently all supplied heat escapes with the gases which leave the freeboard of the combustion chamber. SE-A-7809559-3 with publication number 423 928 (corresponding to US-A-4 378 206) discloses a combustion plant with a pressurized combustion chamber, in which the start-up bed material is heated to the ignition temperature with the aid of hot gas generated in a number of ignition combustion chambers and introduced into the bed through a number of nozzles. In a document DE-810 25 923, National Coal Board, Leatherhead (England). Coal Utilization Research Lab. "Pressurized Fluidized-Bed Combustion. Vol. III. Test 2 Results, by A.G. Roberts, K.K. Pillai, P. Raven and P. Wood a combustor arrangement is described comprising a storage container for heated bed material which is positioned at a level above the bottom of the combustion chamber. The storage container is connected with a combustion chamber by means of a steeply inclined conduit that leads from the storage container to a point considerably below the bottom of the combustion chamber and from there upwards into the bottom of the combustion chamber. This connection conduit is provided with a shut-off device and serves to transfer hot bed material in both directions, from the combustion chamber to the storage container and vice versa. An attempt to use the heated bed material in the storage container to restart the combustion chamber has failed because of the intensive cooling of the bed material by the conveying gas needed for the transport of the bed material. The invention aims to develop a method of the above-mentioned kind and a power plant for carrying out the method, which allows a very rapid start-up of the combustion chamber and which causes only little losses of heat during the start-up period. In order to achieve this aim the invention suggests a method according to the introductory part of claim 1, which is characterized by the features of the characterizing part of claim 1. Further developments of this method are characterized by the features of the claims 2 to 12. A power plant for carrying out the method according to the invention is characterized by the features of claim 11. A further development of this power plant is characterized by the features of claims 14. According to the invention, ignition in the combustion chamber is performed through the combustion of a fuel in the fluidized bed, which is supplied with air for fluidization of the bed material and for combustion of supplied fuel with the aid of the bed material, which is stored in a separate storage vessel at a temperature which equals or exceeds the self-ignition or spontaneous ignition temperature of the fuel. It is desired to transfer the hot bed material to the combustion chamber as rapidly as possible, and an ignition fuel is supplied to the heated bed material. To enable this rapid transfer, the heated bed material, which is stored in said isolated container separated from the combustion cham ber, is transferred into the combustion chamber via said substantially vertical or steeply inclined conduit, which is equipped with a valve between the container and the combustion chamber. An ignition and start-up fuel is supplied to the heated bed material in connection with, or immediately after, the transfer of the heated bed material to the combustion chamber. If the temperature of the transferred bed material reaches the corresponding operating temperature of the combustion chamber during normal operation, it is possible to use as ignition and start-up fuel the same kind of fuel as used during normal operation. However, it is suitable to use a special start-up fuel with a low self-ignition temperature. The temperature of the start-up bed material in the storage vessel may then be lower and a greater cooling of the bed material can he allowed in connection with its transfer to the combustion chamber. The temperature affects the choice of start-up fuel. A more reliable ignition and start-up of the combustion chamber can be obtained if a special and suitably chosen start-up fuel is used. The bed material is heated to the normal operation temperature, usually to 750-950° C, with start-up fuel. During this heating period, a successive change-over to normal fuel is suitably made. According to one alternative of the method according to the invention, the air flow through the nozzles of the combustion chamber bottom is temporarily reduced, whereupon a bed material which is heated to the ignition temperature of the fuel is introduced into the combustion chamber. Cooling of the bed material in connection with its transfer to the combustion chamber is thus reduced. Thereafter, the air flow through the bottom nozzles is again increased so that the hot bed material is fluidized. At the same time a fuel, suitably a special start-up fuel having a low self-ignition temperature, is added. The bed material is heated to normal operating temperature, often to about 750-950°C, and at a suitable time during this heating period, the start-up fuel is replaced, suitably successively, by the normal fuel for the plant. In a pressurized fluidized bed with a combustion chamber enclosed in a pressure vessel, the reduction of the flow through the bottom of the combustion chamber can be achieved by opening a valve connecting the pressure vessel space and the free-board of the combustion chamber, so that the pressure difference between these spaces is reduced, and part of the air is by-passed. According to another alternative of the method according to the invention, the start-up fuel is mixed with heated bed material during the transfer to the combustion chamber. In this case, start-up fuel is supplied to combustion air which constitutes activating gas for a so-called L-valve in the conduit for transfer of bed material to the combustion chamber. An advantage of this method is that the bed material is not cooled before fuel is supplied. The power plant according to the invention contains at least one but suitably a plurality of isolated storage containers for bed material with a heating device for heating and/or sustaining the temperature of the bed material. From the container to the combustion chamber there is a conduit which permits rapid transfer of the hot bed material to the combustion chamber. Further, in a preferred embodiment, means are provided for temporarily reducing the air flow through the bottom of the combustion chamber and means are provided for supplying fuel to the bed material when the latter is fluidized by increasing the air flow. In a plant with a pressurized fluidized bed in a combustion chamber in a pressure vessel comprising compressed combustion air, there is suitably provided at least one valve allowing to establish a connection between the pressure vessel space and the freeboard. Further, control means are pro vided for coordinating opening of this valve, transfer of heated bed material from the containers, closing of the valve, and start-up of the supply of ignition fuel. The invention will be described in greater detail with reference to the accompanying drawings which illustrate in Figure 1 schematically an embodiment of a power plant according to the invention, Figure 2 a second embodiment of the storage container for heated bed material and the connection of the container to the combustion chamber. The power plant according to the invention comprises a low pressure compressor 1, a high pressure compressor 2, a high pressure turbine 3, a low pressure turbine 4 and a power turbine 5 which drives a generator 6. Figure 1 shows a conventional three-shaft embodiment having the low and high pressure parts and the power turbine with its generator on their respective shafts. This embodiment is only one of several possible arrangements and constitutes merely an example. Turbines 3, 4 and 5 receive their energy from a combustion chamber plant 7 with a combustion chamber 8 in a pressure vessel 10, i.e. a so-called PFBC plant. Compressors 1 and 2 feed the space 11 with combustion air. At full power the pressure may be 0.5-3.0 MPa. The bottom 12 of combustion chamber 8 is provided with nozzles 13, through which the combustion chamber 8 is supplied with air for fluidization of the bed 14 and combustion of fuel supplied to the bed 14. The bed material is granular and may consist of sulphur absorbent, for example limestone or dolomite. The height of the bed varies with the load. The lowest bed surface is indicated at 15 and the highest at 16. The bed height can be varied by transferring bed material from the combustion chamber 8 to storage containers 40 and by returning it to the combustion chamber 8 in accordance with the method and equipment described in EP-A-84104821.8. The fuel is fed into the bed 14 by means of a fuel supply system 17. Hot combustion gases produced in bed 14 accumulate in the free-board 18 of combustion chamber 8, leave the combustion chamber through conduit 20, are cleaned from fly ash in cyclones 21, 22 and 23, and are then led via conduit 24 to the high pressure turbine 3. Figure 1 only shows one group of series-connected cyclones; in reality there are several such groups connected in parallel. Ashes and other dust from the bed which are separated in the cyclones 21, 22 and 23 are fed out from the bed in an known manner, for example through an ash discharge device 25 of the kind described in greater detail in SE-A-8205748-0 (corresponding to EP-A-83306073.4). This ash discharge device may be positioned in one or more air plenums or ducts 26 below the combustion chamber bottom 12. At the inlet 19 between duct 26 and space 11 there may be provided a valve 27, by means of which the air flow can be influenced. In case of a plurality of air ducts, the distribution of the air flow between the chambers can be influenced. Ash discharge device 25 is suitably located in the air flow in duct 26 thereby simultaneously acting as an ash cooler. From ash discharge device 25 the ash is led to a collection container 28, where the ash is separated from the transport gas. This gas is cleaned in filter 29 before leaving container 28. The fuel supply system comprises a first container 30 for normal fuel, for example crushed coal, which is used for normal operation of the plant, and a second container 31 for ignition fuel, for example coconut shell, sawdust, wood chips, or the like, having a low self-ignition temperature. Further, the fuel supply system comprises rotary vane feed ers 32, 33 for feeding out fuel from containers 30 and 31 to fuel conveying pipe 34. Transport gas is obtained from compressor 35, which suitably takes its air from space 11. The rotary feeders 32, 33 are driven by motors 36 and 37, respectively, the numbers of revolution of which are controlled by means of a control device 38, which communicates with transducers (not shown) in the plant. The fuel is introduced into bed 14 via a number of nozzles (not shown). In the pressure vessel adjacent combustion chamber 8 there is at least one container 40 surrounded by heat insulation 41 for storing hot bed material. Container 40 is provided with a heating means 42, suitably an electrical resistor element, for keeping the bed material at or heating it to at least the self-ignition temperature of the fuel, which may be a special start-up fuel. Container 40 communicates with combustion chamber 8 by means of a first conduit 43 provided with a mechanical valve 44a in the embodiment according to Figure 1, or an L-valve 44b in the embodiment according to Figure 2, for transferring hot bed material to the combustion chamber. A second conduit 45 is provided for transferring bed material from combustion chamber 8 to container 40. In the embodiment with an L-valve according to Figure 2, there is provided a conduit 39 with a valve 49, through which the L-valve can be supplied with air from space 11 for fluidizing the material in the L-valve 44b and for removing the blocking function of this valve. A booster compressor 70 may possibly be provided in conduit 39. Conduit 43 should have such conveying capacity that the bed material in container 40 may be very rapidly, preferably in less than 30 seconds, transferred to the combustion chamber. Via conduit 46 with valve 47, which is operated by operating device 48, container 40 can be put into connection with a space having lower pressure than the fluidized bed 14. In this way the pressure in container 40 may be reduced so that bed material from combustion chamber 8 will be transferred to container 40 through conduit 45. Conduit 46 is suitably connected with space 11 via a throttle means 50, thus causing a permanent small air flow through conduit 46. In the event of a leaking valve 47, this air flow prevents the outflow of hot gas from combustion chamber 8 through valve 47. A similar storage container and the method of transferring bed material between container and combustion chamber, but only for control of the bed level, are disclosed in EP-A-84104821.8. Special start-up fuel can be supplied to the activating air in conduit 39 from a separate startup fuel container 71 (Figure 2). Between container 71 and conduit 39 there is a rotary feeder 72 or other means for controlling the fuel flow. Drive motors 73 and 74, respectively, of booster compressor 70 and rotary feeder 72 are connected via control conductors 76, 77, 78 to a common control device 80. By using an L-valve 44b in the steeply inclined conduit 43, as shown in Figure 2, no mechanical movable valve parts are needed in the supply conduit for the hot bed material. Further, a very great bed material flow with a very insignificant amount of gas for activation of the valve can be achieved. For the supply of about 500 kilos of bed material, only about 1 kilo transport gas is needed. In a conveyor tube with a diameter of about 150 mm, a flow of 10-20 kilos of bed material per second may be achieved. The plant includes a steam turbine 51 which drives a generator 52. Steam for this turbine 51 is generated in a tube coil 53 in combustion chamber 8. This tube coil is completely immerged in the fluidized bed 14 at full combustion chamber power and maximum bed height. Tube coil 53 is supplied with feed water through a feed water pump 54 from a feed water tank (not shown). Freeboard 18 of combustion chamber 8 can be put into communication with space 11 in pressure vessel 10 through a valve 55 (by-pass valve) with an operating device 56, whereby the pressure difference between these two spaces can be reduced. By opening valve 55 the air flow through nozzles 13 of bottom 12 is reduced or almost completely terminated. Low pressure compressor 1 and high pressure compressor 2 are connectable to starter motors 57 and 58, respectively, by means of couplings 59 and 60, respectively. Upon start-up of the plant, starter motors 57 and 58 are connected to compressors 1 and 2 and air is pumped into pressure vessel space 11. A certain air flow is obtained through the combustion chamber. The flow resistance in valve 19, air duct 26, nozzles 13 and possible bed material in bed 14 causes a pressure drop so that a pressure difference arises between vessel space 11 and freeboard 18. When a chosen suitable pressure has been obtained in space 11, valve 55 is opened to reduce the pressure difference between space 11 and freeboard 18 to a certain level. The flow resistance in valve 55 determines the remaining pressure difference and the continued air flow through bottom 12. This latter flow should be low, and therefore valve 55 should have a large area of passage to provide little resistance. The valve area should be larger than the total passage area of the nozzles 13, suitably it should be many times larger. The greater part of the air flow to combustion chamber 8 passes through valve 55. Accordingly valve 55 has to be dimensioned suitably so that 70 - 90% of the air flow passes through valve 55 while only a small part passes through nozzles 13. Bed material 61, which is heated to a suitable ignition temperature, 600-900° C, is present in container or containers 40. Valve 44a (Figure 1) in the conduit is opened or L-valve 44b (Figure 2) is supplied with transport air, and bed mate rial 61 travels down into combustion chamber 8 through conduit 43. This conduit 43 should have a large area so as to achieve quick feeding of bed material into the combustion chamber. The aim is to bring about a minimum bed height for stable fluidization and combustion in about 30-60 seconds. To achieve such fast feeding, it may be necessary to use a plurality of containers 40. The necessary bed material temperature depends, among other things, on how fast the feeding of bed material can be achieved, on the cooling air flow through nozzles 13 during the feeding, and on the ignition temperature of the fuel used during start-up of the combustion chamber. When a minimum bed height has been achieved, valve 55 is shut, the pressure difference between space 11 and freeboard 18 is increased, and the entire air flow passes through the nozzles 13 and fluidizes bed 14. This fluidization is attained almost instantaneously. At the same time the fuel feed is started. The temperature of bed 14 must exceed the self-ignition temperature of the fed-in fuel. At very high bed temperature it is possible to use the ordinary fuel, for example crushed coal, when igniting the combustion chamber. To reduce the stresses caused by sudden changes in temperature, a certain preheating of the combustion chamber prior to ignition is desirable as well as a relatively low bed material temperature. It may therefore be suitable to use a special start-up fuel with a low ignition temperature. Crushed nutshells, especially from coconuts, sawdust, or wood chips constitute suitable start-up fuels. Also liquid and gaseous ignition fuels may be used. To enable several starting attempts in the event that one or several start attempts should fail, the plant is suitably equipped with a plurality of containers 40. One or more of them can be utilized for each starting attempt. As shown in Figure 1, one container 40 has not been used and is thus filled with bed material 61 also after start-up of combustion chamber 8. Figure 2 illustrates an extinguished combustion chamber 8, which is emptied of bed material.

Claims: Method of starting up a combustor with combustion of a fuel in a fluidized bed (14) of particulate bed material, preferably in a power plant, with air for fluidization and fuel combustion being supplied at the bottom (12) of the combustion chamber (8) and with heated bed material (61) being delivered from an isolated storage container (40), separated from the combustion chamber (8) and positioned at a level above the bottom (12) of the combustion chamber (8), via a vertical or steeply inclined conduit (43) equipped with a shut-off device (44a, 44b), characterized in that the bed material in the storage container (40) is heated up to and/or sustained at at least the self-ignition temperature of a start-up fuel, that the lowermost point which the heated bed material passes on its way from the storage container (40) to the combustion chamber is positioned above the bottom of the combustion chamber and that the start-up fuel is supplied to said bed material in connection with or immediately after the transfer of said bed material to the combustion chamber (8). Method according to claim 1, characterized in that the supply of air flow through the nozzles (13) at the bottom (12) of the combustion chamber (8) is reduced during the transfer of the heated bed material. Method according to claim 1 or 2, characteriz ed in that at the end of the transfer of the heated bed material the air flow through the nozzles (13) at the bottom (12) of the combustion chamber is increased so that the bed material is fluidized, and that the start-up fuel being ignitable at the prevailing temperature of the bed material is supplied to the bed (14). Method according to claim 3, characterized in that in a power plant with a pressurized fluidized bed (14) in a combustion chamber (8) enclosed in a pressure vessel (10), the reduction of the air flow through the nozzles (13) of the combustion chamber bottom (12) is brought about by opening a connection between the pressure vessel (10) and the freeboard (18) of the combustion chamber (8). Method according to claim 3, characterized in that in a power plant with a pressurized fluidized bed (14) in a combustion chamber (8) enclosed in a pressure vessel (10), the reduction of the air flow through the nozzles (13) of the combustion chamber bottom (12) is brought about by a throttle means (27) upstream of the combustion chamber bottom (12). Method according to any of claim 1 - 5, charac terized in that the start-up fuel is of a special, easily inflammable kind. Method according to claim 6, characterized in that the start-up fuel consists of an inflammable fluid or gas. Method according to any of claim 1 - 5, charac terized in that the fuel used for normal operation is also used as start-up fuel. Method according to claim 6, characterized in that the start-up fuel consists of crushed nutshells from, for example, coconuts, wood chips, or sawdust. Method according to any of the preceding claims, cha racterized in that heated bed material is supplied with start-up fuel and combustion air during the transfer of said heated bed material from the storage container (40) to the combustion chamber (8). Method according to claim 9, characterized in that start-up fuel is mixed into combustion air serving as activating gas for an L-valve (44b) arranged in the steeply inclined conduit (43). Method according to any of the preceding claims, cha racterized in that the storage container (40) is supplied with bed material from the combustion chamber via a separate-conduduit (45). Power plant for carrying out the method according to any of the preceding claims with at least one isolated storage container (40), separated from the combustion chamber for storage of heated bed material (61), this storage container being positioned at a level above the bottom (12) of the combustion chamber (8), with a vertical or steeply inclined conduit (43) connecting the storage container with the combustion chamber and equipped with a shut-off device (44a, 44b), and with means for supplying fuel to the bed (14) of transferred bed material either in connection with, or immediately after, the transfer of said bed material to the combustion chamber (8) characterized in that there are means (42) for heating and/or sustaining the temperature of the bed material (61) in the storage container, and that the lowermost point of said vertical or steeply inclined conduit (43) between said storage container and the combustion chamber (8) for transfer of the hot bed material (61) from the container (40) to the combustion chamber (8) is positioned above the bottom of the combustion chamber. Power plant according to claim 13 with a pressurized fluidized bed (14) in a combustion chamber (8) enclosed in a pressure vessel (10) with combustion air under pressure, characterized in that said storage container is located inside the pressure vessel (10), and that an openable connection (55) is arranged between the freeboard (18) of the combustion chamber (8) and the surrounding space (11) in the pressure vessel (10), said openable connection being held open during the transfer of heated bed material from the container (40) to the combustion chamber (8) so that the air flow through the bed is reduced for the purpose of reducing the cooling of bed material (61) transferred to the combustion chamber (8).

Inventors: BRAENNSTROEM, ROINE, DIPL.-ING., Brännström, Roine, Dipl.-Ing.
Applicants: ASEA STAL AKTIEBOLAG
Application year: 1985
Main classification: F23C11
IPC codes: F23C10