METHOD AND DEVICE FOR FEEDING FUEL INTO A FLUIDIZED BED

Abstract: To a fluidized combustor bed (1) there is passed particulate fuel via a fuel flow distributor (8) which comprises a vessel (9) for receiving a particulate fuel from a fuel storage (10). The fuel is fed into the vessel (9) of the fuel flow distributor (8) from the fuel storage (10), whereupon the fuel is fluidized in a bed (14) above a bed bottom (15) in the vessel of the fuel flow distributor. The fuel flow distributor (8) comprises at least two feed pipes (7) with associated inlets (18) located adjacent the surface (19) of the bed (14) in the fuel flow distributor, where fluidization gas (16) which leaves the vessel (9) via a feed pipe (7) feeds out fuel to at least one nozzle (6) in the bed (1) of the combustor (2), extra gas (20) being supplied to the feed pipes (7) at the inlets thereof, such that the flow of fuel through an individual feed pipe (7) can be controlled by controlling the gas flow (20) supplied to the inlets of the respective feed pipe.

Description: Method and device for feeding fuel into a fluidized bed TECHNICAL FIELD The present invention relates to a fuel flow distributor by means of which dry particulate fuel is fed into a fluidized bed with the aid of nozzles in the bed and wherein the fuel feed to the bed may be controlled in that the feeding from each nozzle into the bed is to a certain extent individually controlled. BACKGROUND ART Into a fluidized bed in a combustor, for example an FBC or a PFBC power plant, there is fed particulate fuel in dry state via a large number of feed nozzles. The fuel may also be fed in by adding liquid, whereby a paste is obtained. Paste feeding is not discussed in this specification. The large number of nozzles is justified by the need to dis¬ tribute the fuel as efficiently as possible in the bed to thereby achieve a complete combustion of the fuel. Usually, the fuel is fed from a storage container to the nozzles by means of mechanical and/or pneumatic conveyors, which entails a number of parallel rotary vane feeders or other devices for controlling the fuel flow to the nozzles. Rotary vane feeders or other feeding devices are expensive, so the aim is to reduce the need of such devices. This may mean that, for example, two parallel rotary vane feeders may be arranged to feed two separate systems of supply conduits with a network of feed nozzles associated with each such system. A natural disadvantage with such a system with two feeding systems which by means of, for example, rotary vane feeders feed fuel more or less independently of each other is the difficulty of being able to adjust the quantity of fuel individually for each nozzle in the fluidized bed. It may appear to be contrary to what has been said above regarding the importance of achieving an even distribution of a fuel fed into the bed to desire an individual control of each nozzle. The main principle, of course, is to attain the good distri¬ bution of the fuel, but various factors may contribute to the desire to control the flow of fuel fed into the bed indivi¬ dually per nozzle. Such factors may be the vicinity to com- bustor walls and heat-absorbing tube surfaces, differences in mixing and fluidization patterns at or near the nozzle, etc. US-A-4,655,147 shows a method of achieving a simplified pneumatic feeding system of particulate fuel to a fluidized bed. This is achieved by arranging a fuel distributor down¬ stream of a flow-controlling feeding device, the aim of this fuel distributor, placed near the combustor, being to reduce the number of supply conduits between the flow-controlling feeding device and nozzles in the bed. Also a device according to this description suffers from the disadvantage that the flow of fuel cannot be controlled separately from each nozzle. It is also known to feed the particulate fuel to the fuel flow distributor in the form of a separate vessel, wherein the fuel is fluidized to a bed by supplying fluidizing air via a per¬ forated bed bottom, on which the bed of fuel rests. Above the surface of the fuel bed, there is a freeboard. At a bed sur¬ face a number of discharge pipes from the fuel flow distri¬ butor are arranged. These discharge pipes lead to the flui- dized bed in a combustor, where these pipes serve as feed pipes to the bed of the combustor, the feed pipes being pro¬ vided with nozzles at their orifices into the fluidized bed. The principle of such a described fuel flow distributor is that fuel in fluidized state, when the bed surface reaches the discharge pipe, will be fed out of the fuel flow distributor and thus be fed into the combustor bed at these discharge pipes. Also in this case, it is not possible to control the feed quantity for each nozzle in the bed individually. The aim of our invention is to feed and control, by means of a fluidized fuel flow distributor, in an inexpensive manner, the distribution of fuel to a large number of feed points in a fluidized bed, a so-called FBC, for combustion of the fuel. SUMMARY OF THE INVENTION The invention relates to a method and a device for feeding and controlling the distribution of a particulate fuel to a flui¬ dized combustor bed in a combustor, which is supplied with fuel via a fuel flow distributor comprising: - a vessel for receiving a particulate fuel from a fuel storage, from where the fuel is fed into the vessel and flui¬ dized in the vessel in a bed above a bed bottom in the fuel flow distributor, - at least two feed pipes with associated inlets located close to the surface of the bed in the fuel flow distributor where fluidization gas, leaving the vessel via the feed pipe, feeds out fuel to at least one nozzle in the combustor bed, and further that gas is supplied to the feed pipe at the inlet thereof, whereby the flow of fuel can be controlled by controlling the gas flow supplied to the inlet of the feed pipe. The level of the inlets of the feed pipes in the vessel of the fuel flow distributor will be determined by the level of the bed surface in the vessel since new fuel fed into the bed when the surface of the bed has already reached the level of the inlets of the feed pipes automatically results in fuel above this level accompanying fluidizing air escaping from the vessel via the feed pipes. In this way, the total outflow of fuel from the fuel flow distributor is determined by the quantity of fuel fed to the fuel flow distributor. All feed pipes open out into an FBC, which means that the pressure drop across each individual feed pipe is equally great. By adding extra gas close to the inlet of a feed pipe, the pressure drop across this pipe increases and the suction of particulate material to this very pipe decreases. In this way, the flow of fuel through each individual feed pipe and, as a result thereof, the flow of fuel at the associated fuel feed nozzle in the combustor can be individually controlled. In the invention, the additionally supplied air is utilized to trim the distribution of fuel between the different fuel feed nozzles. Without this trimming arrangement, differences in fuel flow within the interval ±10% between different nozzles may occur. This is due to the conditions (pressure, flows, etc.) which prevail locally in the bed at each nozzle. By means of a device according to the invention, the fuel distri¬ bution in the bed may be improved such that acceptable diffe¬ rences in fuel flows within ±2% are obtained. A considerable advantage achieved with a method according to the invention is that the distribution of fuel fed into the fluidized bed is considerably improved. The fuel is more uniformly fed in. This, in turn, leads to other good results. As an example may be mentioned that the maximum working tem- perature of the bed is limited by the highest local tempera¬ ture in the bed. If local temperature deviations can be reduced, the working temperature of the whole bed can thus be increased, which is possible with the proposed method. An additional advantage which is obtained with the described method is that feeding of additional material into a feed pipe which is subjected to a dust plug is prevented, since the additional air moves away particles from the inlet to the pipe which has been plugged. The combustion process may further require that the flows to each nozzle can be controlled, since conditions in the bed require this. Also in this case, a control to different flows according to the invention is useful. A valve which controls the gas flow supplied to the inlet of each individual feed pipe is used for the described control of fuel flow. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically shows a fuel flow distributor according to the invention, wherein the fuel flow distributor receives fuel from a storage and feeds fuel into a fluidized bed (an FBC) in a combustor. Gas is supplied to the feed pipes via valves. Figure 2 suggests different ways of orienting the inlets of the feed pipes at the bed surface in the fuel flow dis- tributor. Figures 3 and 4 illustrate a couple of alternatives to the method of supplying gas to a feed pipe at the bed surface in the fuel flow distributor. Figure 5 illustrates an alternative wherein a freeboard in the fuel flow distributor is divided into several spaces by arranging walls between each feed pipe. DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will be described in greater detail in a number of embodiments with reference to the accompanying drawings. The device and the method according to the invention are intended for use during combustion of a particulate fuel in a fluidized bed in a power plant. The fluidized bed may be under atmospheric pressure, an FBC bed, or under pressure, a PFBC bed. Figure 1 shows, for example, that a combustion of a fuel takes place in a fluidized bed 1 in a combustor 2 in a power plant. The fluidizing air to the combustor is supplied at 3 to an air distribution space 4 below a first bed bottom 5, on which the fluidized bed 1 of the combustor rests. For feeding and distribution of particulate fuel in dry state, nozzles 6 at the outlet of the feed pipe 7 are arranged in a number of two but normally of the order of magnitude of 10 to 150 feed pipes with associated nozzles. A fuel flow distributor 8, hereinafter only referred to a distributor, is also comprised in Figure 1. The distributor comprises a closed vessel 9, to which fuel is fed from a fuel storage 10 in a container 11 for storage of fuel. Fuel from the storage 10 is transported via a flow-controlling conveyor 12, for example a rotary vane feeder, via a conduit 13 to the distributor 8. Fuel fed to the vessel 9 builds up a second bed 14, here referred to as distribution bed, on a second bed bottom 15. This distribution bed is fluidized by means of air or gas introduced at 16 in the figure. The above-mentioned feed pipes 7 have their inlets 18 disposed at a certain level in the vessel 8, which means that fuel fed via the conduit 13 to the distributor 8 will build up a bed 14, the surface of which can only rise to a height on a level with the level of the inlets 18 at the feed pipes 7. When additional fuel is fed in, fluidization gas leaving the closed vessel 9 via the feed pipes 7 will bring along fuel which tends to exceed the level of the inlets 18 of the pipes 7, fuel then being fed by means of the fluidizing air in the fuel flow distributor 8 via the feed pipes 7 and the nozzles 6 to the fluidized first bed 1 in the combustor 2 for combustion. The level of the feed pipes 7 consequently adjusts the level of the bed surface 19 in the distribution bed 14. According to the invention, extra gas is supplied at 20 to the inlet 18 of each feed pipe 7. This extra gas is supplied to each individual inlet 18. The quantity of gas supplied to each inlet 18 is controlled by means of a control valve vi, v2 etc. in the respective conduit 20a, 20b, etc. which conduct extra air to the inlets 18. In the schematic representation according to Figure 1, the proportions between the beds in the fuel flow distributor and the combustor are not clearly shown. Normally, the combustor bed 1 is of considerably larger size than the bed 14 in the fuel flow distributor 8. Figure 2 shows that the orientation of the feed pipes 7 vis-a- vis the vessel 9 in the distributor 8 can be oriented in several alternative ways. The lefthand picture shows an alter¬ native wherein the inlets 18 are facing downwards towards the bed surface 19. The centre picture shows an alternative where¬ in the inlets are oriented such that the openings thereof are facing upwards towards a freeboard 21 above the bed surface 19. The righthand picture indicates that the inlets may also be directed in the lateral direction and lie on a level with the bed surface. Figure 3 shows an example of how the injection of extra gas 22 is performed at an inlet 18. In this example, a feed pipe 7 is used with its associated inlet 18 facing upwards. The gas is injected towards one of the feed points of the distributor 8, that is, towards the opening of the inlet 18. The arrows 23 illustrate fluidization gas and fuel from the bed 14 which, from the bed surface 19, are feedd via the feed pipe 7. The extra gas 22 may be supplied to the inlet 18 of a feed pipe 7 by designing a pipe for the supply of extra gas 22 to constitute a coaxial pipe part or other casing 24 externally of the feed pipe 7, at least at the inlet 18 of the feed pipe. Figure 4 is further used to show that the direction of the inlet may be varied arbitrarily. Figure 5 illustrates an alternative embodiment of the fuel flow distributor 8, where the freeboard 21 has been divided into several spaces 26 by means of partitions 25. Each such divided space 26 comprises the inlet of a feed pipe. The intention of arranging these spaces 26 is to prevent too lively fluidzation of the area at the bed surface 19 at an inlet 18 where a reduction of the fuel flow has been carried out according to the method of the invention. The fluidization around the inlet of a feed pipe 7 is reduced especially if this pipe has been blocked by fuel (plugged) . If fluidization gas is supplied to the bed 14 without these partitions 25 being arranged, the bed surface is fluidized largely equally lively over the entire surface independently of the control of the fuel flow at individual inlets 18. The extra gas which is supplied to a feed pipe 7, at the inlet 18 thereof, preferably consists of air. Instead of air, another gas may, of course, be used. Suitable gases for this purpose are then primarily flue gas or oxygen-reduced air.

Claims: CLAIMS 1. A method for feeding and controlling the distribution of a particulate fuel to a fluidized combustor bed (1) of a com- bustor (2), which is supplied with fuel via a fuel flow dis¬ tributor (8) comprising - a vessel (9) for receiving a particulate fuel from a fuel storage (10) from where the fuel is fed into the vessel (9) and is fluidized in the vessel (9) in a bed (14) above a bed bottom (15) in the fuel flow distributor (8), - at least two feed pipes (7) with associated inlets (18) located close to the surface (19) of the bed (14) in the fuel flow distributor (8) where fluidization gas leaving the vessel (9) via the feed pipes (7) feeds out fuel to nozzles (6) associated with the respective feed pipe in the bed (1) of the combustor (2), characterized in that extra gas is supplied to the feed pipes (7), at the inlets (18) thereof, whereby the flow of fuel can be controlled by controlling the gas flow supplied to the inlets (18) of the respective feed pipe (7). 2. A method according to claim 1, characterized in that the fuel flow of the respective feed pipe (7) is determined by control of a valve (vi) which influences the gas flow which is supplied to the inlets (18) of the respective feed pipe (7). 3. A method according to claim 1, characterized in that the inlets (18) of the respective feed pipe (7) are oriented in an optional direction. 4. A method according to claim 3, characterized in that the supplied gas is injected towards the inlets (18). 5. A method according to claim 3, characterized in that the supplied gas is supplied via pipes which constitute casings (24) for the inlets (18) of the respective feed pipe (7). 6. A method according to any of claims 1-5, characterized in that the fuel in the fuel flow consists of crushed coal or of crushed coal with an addition of other crushed material. 7. A method according to any of claims 1-6, characterized in that the supplied gas which controls the fuel flow through the respective feed pipe (7) consists of any of the alternatives air, flue gas, or oxygen-reduced air. 8. A fuel flow distributor (8) for feeding and controlling the distribution of a particulate fuel to a fluidized combustor bed (1) of a combustor (2), wherein the fuel flow distributor comprises - a vessel (9) for receiving a particulate fuel from a fuel storage (11) from where the fuel is fed into the vessel (9) and is fluidized in the vessel (9) in a bed (14) above a bed bottom (15) in the fuel flow distributor (8), - at least two feed pipes (7) with associated inlets (18) located close to the surface of the bed (14) in the fuel flow distributor (8) where fluidization gas leaving the vessel (9) via the feed pipes (7) feeds out fuel to nozzles (6) associated with the respective feed pipe in the bed (1) of the combustor (2), characterized in that extra gas is supplied to the feed pipes (7) , at the inlets (18) thereof, whereby the flow of fuel can be controlled by controlling the gas flow supplied to the inlets (18) of the respective feed pipe (7). 9. A fuel flow distributor (8) according to claim 8, characterized in that the fluidized combustor bed (1) , to which the fuel flow distributor (8) feeds fuel, consists of an atmospheric FBC or CFB bed, or a PFBC bed, or a pressurized CFB bed. 10. A fuel flow distributor (8) according to claim 8, characterized in that the inlets (18) of the respective feed pipe (7) are oriented in an optional direction. 11. A fuel flow distributor (8) according to claim 8, characterized in that the supplied gas is adapted to be injected towards the inlets (18) . 12. A fuel flow distributor (8) according to claim 8, characterized in that the supplied gas is supplied via pipes which constitute casings (24) for the inlets (18) . 13. A fuel flow distributor (8) according to claim 8, characterized in that a freeboard (26) in the fuel flow distributor (8) is divided by means of partitions (25) into separate spaces (26), wherein each such space (26) houses the inlets (18) of at least one feed pipe (7) .

Inventors: BRAENNSTROEM ROINE, SANDELIN CARL JOHAN, BRAENNSTROEM, ROINE, SANDELIN, CARL, JOHAN
Applicants: ABB CARBON AB, BRAENNSTROEM ROINE, SANDELIN CARL JOHAN, BRAENNSTROEM, ROINE, SANDELIN, CARL, JOHAN
Application year: 1995
Main classification: F23K3
IPC codes: B65G53, F23C10, F23K3, F27B15