METHOD FOR OPERATING A FLUIDIZED BED COMBUSTION

Abstract: A fluidized bed system comprises at least three cells (6a, ll, l6) constituting a fluidized bed. One (l6) of the cells interposed between other cells (6a, ll) may function as a non-fluidized bed cell in response to the operating condition of the system.

Description: BACKGROUND OF THE INVENTION This invention relates to a fluidized bed system in which bed material, such as coal, coal ash, limestone, cement clinker or sand, is burned, gasified or dryed by being fluidized. In the conventional fluidized bed system, a plurality of fluidized bed cells of which operational conditions are different from each other are formed by providing partition plates. Usually, in the system, temperature is raised to 500 - l000°C, so that the partition plate has inevitably been complex in construction and high in cost because of the need to withstand the high temperature. Also, provision of a partition plate has made the fluidized bed system unable to meet various different operating conditions. In other words, the system does not have a wide use. In a fluidized bed combustion system for burning, for example, coal or coal ash, it has hitherto been usual practice to keep the temperature of the fluidized bed in the range between 800 and l000°C to burn a fuel, such as coal, for reasons stated hereinafter. (l) Burning the fuel at a high temperature of over l000°C produces oxides of nitrogen (NOx) as the bonding of nitrogen to oxygen contained in the air used for combustion takes place. When the concentration of NOx emissions in the flue gas rises, the atmosphere is polluted and this would give rise to a problem with regard to environmental disruption. (2) When a fuel of high sulfur content is burned, it is now usual practice to mix the bed material with limestone particles to remove sulfur oxides (SOx). To enable this desulfurization reaction effectively requires maintaining the temperature of the fluidized bed in the range between 850 and 1000°C. When particles of coal or EP ash (ash collected by electric dust collectors) are burned in a fluidized bed, fuel particles of small diameter are scattered by gas, such as air, for achieving fluidization of the bed material, and fly to a hollow body above the fluidized bed, with the result that such fuel particles are conveyed to the flue by exhaust gases of combustion before they are fully combusted. Fuel particles of relatively large diameter are broken down to small particles by combustion in the fluidized bed. However, with the temperature of the fluidized bed being at a relatively low level of 800 to 1000°C, the fuel particles are conveyed to the flue by the exhaust gases of combustion through the hollow body before being burned to the core of each particle. Thus exhaust gas from fluidized bed combustion systems contains ash of high non-combusted fuel component. Accordingly, the present practice is to separate the ash content of flue gas by a cyclone dust collector and to feed the separated ash into a re-combustion furnace of the fluidized bed system maintained at a high temperature of about ll00°C so as to achieve a complete combustion of the non-combusted fuel components contained in the ash to turn the same into ash of the type that can be utilized as fly ash. Fig. 2 shows one example of the system for re-combusting the non-combusted fuel components. The system comprises a furnace l of a main combustion chamber, a cyclone dust collector 2 for collecting solid masses of ash released from the main combustion chamber, an air heater 3 for supplying combustion gas of high temperature to a start-up wind box 4, when the main combustion chamber is started up, to heat bed material above the wind box 4, a wind box 5 for the main combustion chamber, heat transfer tubes 6 located in a fluidized bed 6 in the main combustion chamber, a furnace 7 of a re-combustion chamber for re-combusting solid masses of ash containing non-combusted fuel components released from the main combustion chamber and collected by the cyclone dust collector 2, a cyclone dust collector 8 for collecting ash released from the re-combustion chamber, an air heater 9 having the same function as the air heater 3 used when the main combustion chamber is started up and used when the re-combustion chamber is started up, a wind box l0 for the re-combustion chamber for supplying air for combustion to set in motion bed material forming a fluidized bed 11a and to achieve a complete combustion of the solid masses of ash containing non-combusted fuel components which are supplied to the fluidized bed 11a, an ash transporting tube 12 for transporting the solid masses of ash containing non-combusted fuel components collected by the cyclone dust collector 2 and feeding the same into the fluidized bed 11a through ash feeding nozzles 13 and an exhaust duct 14 for releasing exhaust gases to the atmosphere. When coal is used as a fuel, for example, in the fluidized bed combustion system of the aforesaid construction, costs are high because it is necessary to use an additional fluidized bed combustion system for collecting and re-combusting solid masses of ash containing non-combusted fuel components. JP-A-52 149 639 discloses the deposition of the bed material extended above the chambers 3B. The deposition is for preventing the heat transfer to the heat transfer tubes 6 in response to the load variations and is not for separating the furnace into a plurality of fluidized bed zones. "Chemical Engineering Progress", Vol. 80, no. 1, Jan. 1984, pages 35-40, discloses a fluidized bed combustion system, comprising a main combustion cell, a start-up cell and at least a partition cell interposed between said main combustion cell and start-up cell and a dust collector for collecting solid masses of ash containing noncombusted fuel components from exhaust gases produced by combustion and released from a combustion section of the fluidised bed and an ash transporting tube connecting said dust collector to said start-up cell for recombusting the solid masses of ash. SUMMARY OF THE INVENTION The object of the invention is to provide a method for operating a fluidised bed combustion for burning low combustibility coal, where non-combusted coal components have to be recirculated to the bed, which method uses an improved means for separating the reinjection cell from the rest of the bed. Said object is achieved according to the invention by a method for operating a fluidized bed combustion system, the system comprising a furnace, a distributor plate supported in said furnace, bed material supported on said distributor plate, a plurality of wind boxes arranged side by side below said distributor plate for feeding fluidized gas through portions of said distributor plate into corresponding portions of said bed material, each of said wind boxes having a gas damper, dividing thus the furnace in a plurality of cells constituting a fluidised bed, one of the cells at one end of the distributor plate being operable as a start-up cell, and a dust collector for collecting solid masses of ash containing non-combusted fuel components from exhaust gases produced by combustion and released from a combustion section of the furnace and an ash transporting tube connecting said dust collector to said start-up cell for re-combusting said solid masses of ash, the method comprising opening the damper associated with the wind box of the start-up cell at one end of the distributor plate, and supplying air to said said start-up cell as to fluidise the bed material therein; supplying heated air to said start-up cell; supplying fuel to be burned to said start-up cell; burning said fuel supplied to said start-up cell; opening the dampers associated with the wind boxes adjacent the wind box for supplying air to said start-up cell, and supplying air to the entire distributor plate and to fluidize the entire bed material; and operating the start-up cell and the main combustion cell at different operating conditions; the method being characterized by closing the dampers associated with intermediary wind boxes, so as to form a partition cell in which the bed material remains deposited on the distributor plate, the partition cell separating the start-up cell from the main combustion cells. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view of the fluidized bed combustion system showing an embodiment of a fluidized bed system used according to the invention; and Fig. 2 is a schematic view of a fluidized bed combustion system of the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will now be described by referring to a fluidized bed combustion system by way of example. A furnace designated by the reference numeral l is formed at its bottom portion with three cells constituting a fluidized bed, which are a start-up cell ll for starting up the combustion system, a main combustion cell 6a for burning low combustibility coal, and a partition cell l6 interposed between the main combustion cell 6a and start-up cell ll. Wind boxes 4, 5 and l7 are located under the cells ll, 6a and l6 respectively for feeding a gas, i.e., air in this embodiment, into the respective cells for setting bed material, such as a fuel, in the cells in motion until the bed material reaches a state of high turbulence. The wind boxes 4, 5 and l7 are independent of each other and are maintained in communication with respective cells through a distributor plate 24 formed of porous plate, and are connected through an air duct l9 to a blower, not shown. Air dampers 20, 2l and 22 are each mounted in one of branch air ducts connecting the wind boxes 4, 5 and 17 to the air duct 19 for regulating the amount of air fed into the respective wind boxes. An air heater 3 for feeding a hot blast of air of high temperature into the wind box 4 for the start-up cell ll when the combustion system is started up is connected to the wind box 4. Located in the fluidized bed are heat transfer tubes 6 and evaporators or super heaters for removing heat from the fluidized bed which are reduced in number in the start-up cell ll to economize on fuel for the air heater 3 when the combustion system is started up. A heater assembly l8 including a superheater and an economizer is located in a flue in a rear portion of the furnace l for heating exhaust gases. Located also in the flue is a cyclone dust collector 2 for collecting solid masses of ash containing non-combusted fuel components from the exhaust gases in the flue which is connected through the ash transporting tube l2 to ash feeding nozzles l3 opening in the start-up cell ll. A feed nozzle l5 is opened in an upper portion of each of the cells ll, 6a and l6. An operation will be described. When starting, the air damper 2l is opened to supply air to the start-up cell ll via the wind box 4. So as to set the bed material, for example, limestone on the distributor plate 24 in motion. At the same time, heated air is fed from the air heater 3 to the start-up cell ll. When the bed material (limestone) reaches a predetermined temperature, the coal is supplied into the start-up cell from the feed nozzles l5 and, firstly, the start-up cell is self-burned. Then, the main combustion cell 6a and the partition cell l6 are operated in turn. Subsequently, the air damper 20 is closed so that no air is fed to the wind box l7 for the partition cell l6 which is interposed between the wind box 4 for the start-up cell ll and the wind box 5 for the main combustion cell 6a. As a result, the coal in the partition cell l6 remains deposited on the distributor plate 24 without moving. In other words, the fluidized bed in the lower portion of the furnace l is split by the stack of bed material in the partition cell l6 into a main combustion fluidized bed section and a start-up fluidized bed section. Gases containing non-combusted fuel components are exhausted from the main combustion cell 6a and flow through the flue to heat the heater assembly l8 before being led to the dust collector 2 which collects solid masses of ash containing non-combusted fuel components from the exhaust gases and feeds the same into the start-up cell ll through the ash transporting tube l2 and ash feeding nozzles l3. Since the start-up cell ll is separated from the main combustion cell 6a by the partition cell 16, it is possible to operate the start-up cell ll in a condition which is distinct from the condition of operation of the main combustion cell 6a, i.e., at a temperature of ll00°C which is higher than the temperature in the main combustion cell 6a. As described hereinabove, the heat transfer tubes 6 are smaller in number in the start-up cell ll than in other cells 6a and l6, so that temperature inevitably becomes higher in the start-up cell ll than in other cells 6a and l6 during operation. This advantageously enables the solid masses of ash containing non-combusted fuel components to be burned in the start-up cell ll, thereby eliminating the need to provide an additional fluidized bed combustion system for re-combusting the solid masses of ash. When the coal used as a fuel is of the type which produces solid masses of ash containing non-combusted fuel components whose amount is not large enough to warrant re-combustion of the ash in the start-up cell ll at all times during operation, the start-up cell ll and partition cell l6 are first operated as coal burning cells while the ash collected by the dust collector 2 is temporarily stored in a tank, not shown. When the amount of the collected ash has reached a level which enables the operation of the start-up cell ll as a re-combustion cell to be continuously performed for a predetermined period of time, the ash stored in the tank is fed into the start-up cell ll to burn the non-combusted fuel components therein. When the collected ash becomes empty, the start-up cell ll is used again as a coal burning cell. When the ash stored in the tank has increased in amount again, the start-up cell ll is changed into a re-combustion cell again. By performing the operation in which the start-up cell ll temporarily functions as a re-combustion cell as described hereinabove, it is possible for the fluidized bed combustion system to handle any one of different types of coal as a fuel and to achieve a perfect combustion of the non-combusted fuel components in the ash by burning the ash at a high temperature. This is conducive to increased fuel efficiency of the system. In another embodiment of the invention, the fluidized bed combustion system may be provided with a plurality of partition cells so that one of the partition cells adjacent the start-up cell may be used as an additional start-up cell when the coal used as a fuel is of the type which produces a large amount of solid masses of ash containing non-combusted fuel components. The start-up cell may be used for re-combusting the non-combusted fuel components in the ash when the coal used is of the type which produces a small amount of ash in the form of solid masses while letting the partition cell adjacent the start-up cell perform its original function of partitioning and using another cell adjacent the main combustion cell as an ordinary coal burning cell. By this arrangement, it is possible to eliminate the need to use a tank for temporarily storing the ash in the form of solid masses or to reduce the size of such tank. In short, by using a plurality of partition cells temporarily as a start-up cell and a combustion cell in response to the operating condition, it is possible to provide the fluidized bed system with wide use which greatly increases the usefulness of the system. When the partition cell l6 is small in width as shown in Fig. l or when the speed of the hollow body is so high during operation that it is impossible to satisfactorily effect partitioning of the bed by the bed material, a partition plate may be mounted to extend from a portion of the distributor plate 24 located at the top of the wind box l7 for the partition cell l6 into the fluidized bed. This ensures that the bed material is positively deposited on the distributor plate 24. In the embodiment shown and described hereinabove, the air fed into the wind boxes 4, 5 and l7 has been described as being supplied via the common air duct l9. However, the invention is not limited to this specific form of the embodiment and each wind box may be connected to a separate source of gas supply to feed a different type of gas to each wind box. Also, it is possible in the invention to vary the air ratio, i.e., the ratio of actual air volume to theoretical air volume for each cell. In the embodiment shown and described hereinabove, the fluidized bed system has been described as having application in a fluidized bed combustion system. However, the invention is not limited to this specific form of the embodiment and the fluidized bed system according to the invention also has application in a system designed to dry bed material or gasify same.

Claims: A method for operating a fluidized bed combustion system, the system comprising: a furnace (1), a distributor plate (24) supported in said furnace, bed material supported on said distributor plate, a plurality of wind boxes (4, 5, 17) arranged side by side below said distributor plate for feeding fluidized gas through portions of said distributor plate into corresponding portions of said bed material, each of said wind boxes having a gas damper (21, 22, 20), dividing thus the furnace in a plurality of cells (6a, 11, 16) constituting a fluidized bed, one of the cells (11) at one end of the distributor plate being operable as a start-up cell, and a dust collector for collecting solid masses of ash containing non-combusted fuel components from exhaust gases produced by combustion and released from a combustion section of the furnace and an ash transporting tube (12) connecting said dust collector to said start-up cell for re-combusting said solid masses of ash; the method comprising: opening the damper (21) associated with the wind box (4) of the start-up cell at one end of the distributor plate (24), and supplying air to said start-up cell as to fluidize the bed material therein; supplying heated air to said start-up cell (11); supplying fuel to be burned to said start-up cell (11); burning said fuel supplied to said start-up cell (11); opening the dampers (20, 22) associated with the wind boxes (17, 5) adjacent the wind box (4) for supplying air to said start-up cell (11), and supplying air to the entire distributor plate (24) and to fluidize the entire bed material; and operating the start-up cell (11) and the main combustion cell (6a) at different operating conditions; the method being characterized by closing the dampers (20) associated with intermediary wind boxes (17), so as to form a partition cell (16) in which the bed material remains deposited on the distributor plate (24), the partition cell (16) separating the start-up cell (11) from the main combustion cells (6a). A method according to claim 1, wherein the start-up cell (11) operates at a higher temperature than the main combustion cell (6a). A method according to claim 1 wherein there is only one intermediary wind box (17). A method according to claim 1, wherein the main combustion cell (6a) may be extended by opening the damper (20) associated with the intermediary wind box (17) adjacent the wind box (5) for supplying air to the main combustion cell (6a), and supplying air to the distributor plate (24) so as to fluidize the bed material above the open intermediary wind box (17), while leaving further dampers associated with further intermediary wind boxes closed. A method according to claim 1, wherein the start-up cell (11) may be extended by opening the damper (20) associated with the intermediary wind box (17) adjacent the wind box (4) for supplying air to the start-up cell (11), and supplying air to the distributor plate (24) so as to fluidize the bed material above the open intermediary wind box (17), while leaving further dampers associated with further intermediary wind boxes closed.

Inventors: NOYA, TAKEO
Applicants: BABCOCK-HITACHI KABUSHIKI KAISHA
Application year: 1986
Main classification: F23C11
IPC codes: F23C10