METHOD AND APPARATUS FOR CONTROLLING THE TEMPERATURE OF THE BED OF A BUBBLING BED BOILER

Abstract: The present invention relates to a method and assembly for controlling the bed temperature in a fluidized-bed boiler, particularly in boilers fired with coal or other fuels of high heat value but difficult to gasify. Above the fluidized bed (6), but below the freeboard (8), is arranged a cooling zone (19) and the fluidizing gas is injected into a portion of the bed at an excess rate per unit area, whereby over the area of increased fluidizing gas injection, the bed is expanded upward into the cooling zone, wherefrom unburned combustible material and the particulate matter of the bed can fall back in the bed. The gas injection (9) with the higher flow rate is arranged to the center of the fluidized bed, whereby the return circulation can occur past the waterwall portions enclosing the cooling zone, thus facilitating efficient heat transfer from the refluxed bed material in the form of radiation absorbed by the heat transfer surfaces of the cooling zone waterwalls.

Description: METH O D A ND A PPARATUS FOR CONTROLLING THE TEMPERATURE OF THE BED OF A BUBBLINGBEDBOILER The invention relates to a method according to the pre- amble of claim 1 for controlling the bed temperature in a fluidized-bed boiler, particularly in boilers fired with coal or other fuels of high heat value but difficult to gasify. The invention also concerns an assembly suited for imple¬ menting the method. A fluidized-bed boiler is a boiler structure in which the fuel is combusted and partially gasified substantially above the boiler bottom in a fluidized bed layer formed by the mixture of incombustible particulate bed material with the fuel. The bed is kept fluid by high-velocity injection of a fluidizing gas, usually air, thereto from nozzles placed in the boiler bottom. Fluidized-bed boilers are intended for firing solid fuels, and they are particularly efficient in firing easily gasifiable fuels such as wood and peat, whereby the bed temperature can be controlled by adjusting the degree of gasification through controlled air distribution. The present invention is applicable to a bubbling fluidized-bed boiler, usually operated with two zones, the lower one being formed by an approx. 1 m high bed in which the fuel is combusted and partially gasified. The upper zone is the freeboard for postcombustion. Fuel gasified in the fluidized bed rises up to the freeboard zone, where the fuel is combusted completely, whereby the postcombustion process is controlled by means of blowing secondary air into the postcombustion zone to ensure complete burning of the fuel. The walls of the freeboard zone are conventionally water-cooled and heat transfer to the waterwalls occurs mainly by absorption of radiation emanating from the gases of the combustion process. The temperature within the fluidized bed is approx. 750 - 900 °C. On one hand, to avoid loss of combustion efficiency, the bed temperature must not fall too low. On the other hand, if the bed temperature is allowed to rise above a certain upper limit, sintering will occur rapidly in the bed as the melting ash binds the bed material into clumps. When burning easily gasifiable fuels, the bed temperature can be controlled by altering the flow rate of air injected into the bed, whereby a smaller air flow favours fuel gasification to its combustion in the bed, resulting in a lower bed temperature. Then, a larger portion of the fuel is combusted in the freeboard zone. Because fuels of problematic gasification properties such as coal often cause uncontrolled temperature rise in the bed, these fuel types cannot be used as the main fuel. Problems in bed temperature control will also occur when fuel quality and boiler output vary. Therefore, coal is conventionally combusted in a circulating fluidized-bed boiler, where a portion of the bed material and fuel are returned via the boiler top part back to the bed. As this arrangement requires a cyclone or other efficient par¬ ticle collector to capture the circulating bed material from the flue gases, its construction becomes more expen¬ sive than that of conventional fluidized-bed boilers. Circulating fluidized-bed boilers are discussed in, e.g., US Pat. Nos. 5,054,436 and 4,766,851. Conventionally, bed temperature control has been imple¬ mented by, e.g., altering the air coefficient through ad- justing the air feed rate to the fluidized bed or circu¬ lating flue gases back to the boiler bottom in order to cool down the bed. In some cases, heat exchangers im¬ mersed in the bed have been used, whereby problems arise from their rapid erosion. From the standpoint of wear of boiler components, the bed is an extremely harsh site, the conditions in the bed varying from reducing to oxi¬ dizing, causing extremely strong erosion and corrosion. Particularly, the combination of heat and the above¬ mentioned factors with the erosive effect of fuel and bed material circulation results in fast wear of structures located in the bed. Due to the multifaceted nature of the different wear mechanisms acting in the bed, it is almost impossible to find a material for heat exchangers that would resist the combination effect of all the above¬ mentioned wear mechanisms and simultaneously could offer a sufficiently high heat transfer efficiency. Use of additional air injection is hampered by the limited control range of bed temperature available thereby. While a satisfactory result of combustion parameter control can be achieved by flue gas circulation, thiε arrangement requires the erection of a separate circulation system. It is an object of the present invention to provide a method capable of controlling the temperature of a fluid¬ ized bed in a superior manner with respect to the conven¬ tional methods described above. The goal of the invention is achieved by arranging a cooling zone above the bed, but below the freeboard, and then feeding the fluidizing gas into a portion of the bed with an excess rate per unit area, whereby over the area of increased fluidizing gas injection rate, the bed is expanded upward from teh bubbling bed into said cooling zone, wherefrom unburned combustible material and the particulate matter of the bed can fall back in the bed. In particular, the gas injection with the higher flow rate is arranged to the center of the fluidized bed, whereby the return circulation can occur past the water¬ wall portions enclosing the cooling zone, thus facilitat¬ ing efficient heat transfer from the bed material in the form of radiation absorbed by the heat transfer surfaces of the cooling zone waterwalls. More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the assembly according to the invention is characterized by what is stated in the characterizing part of claim 8. The invention offers significant benefits. The invention makes it possible to combust fuels of high heat value in a fluidized-bed system without the need for expensive particulate matter collectors such as those used in circulating fluidized-bed techniques. Temperature in the postcombustion zone, that is, the freeboard zone above the fluidized bed can be elevated sufficiently high thus permitting elimination of obnoxious nitrogen com¬ pounds such as nitrous oxide from flue gases of a normal fluidized-bed combustion process. Hence, the present method is excellently suited for, e.g., converting pul- verized-coal fired boilers into fluidized-bed boilers. Without compromising a sufficient cooling rate, installa¬ tion of expensive and rapidly wearing heat exchangers into the fluidized bed zone becomes unnecessary. Due to the variable air injection rate in the different zones of the fluidized bed, the combustion process parameters can be controlled easier and over a wider range by this method than by prior-art arrangements. In the high- velocity fluidization domain of the fluidized bed, oxi- dizing conditions are formed that facilitate desulfuriza- tion with limestone in the same manner as in a circulat¬ ing fluidized-bed boiler. With the help of secondary-air injection or circulating gas, or both, the cooling zone can be provided with a gas vortex that stabilizes the gas flow pattern of the boiler. Resultingly, among other things, nitrogen oxides are reduced. The size of the gasification region of the fluidized bed can be made smaller and the maximum temperature in the boiler lowered owing to the improved heat transfer rate. This factor, too, contributes to easier removal of nitrogen and sulfur oxides. In the following the invention is described in greater detail by making reference to the appended drawings in which Figure 1 is a schematic block diagram of a fluidized-bed boiler arrangement according to the invention; and Figure 2 is a more detailed illustration of the fluidized bed of the boiler arrangement according to the invention. Referring to the diagrams, the boiler arrangement accord¬ ing to the invention is basically similar to that of con¬ ventional fluidized-bed boilers. The boiler 1 has a furnace region 1 above which is located a heat transfer region 2 in which the heat released by the combustion of the fuel is transferred by means of heat exchangers 3 into the heat conveying medium. The heat conveying medium conventionally is water which is evaporated and super¬ heated in the heat exchangers for use in steam turbines and other equipment. From the heat transfer region 2, the cooled flue gases are taken to the stack either directly or via cleaning equipment, depending on the need for additional cleaning. According to the present method, nitrogen and sulfur oxides can be substantially removed already in the boiler 1 during the combustion process. The furnace region of the boiler 1 is divided into a number of zones. Lowermost in the boiler 1 are located fluidizing gas injection nozzles 4, 5, above which is maintained the fluidized bed 6. Above the fluidized bed 6 is provided a cooling zone 7 according to the invention, and above that, a freeboard 8 for postcombustion. The fluidizing gas, conventionally air or flue gas or a mixture thereof, is blown to the nozzles 4, 5 by means of a fan 10 along a duct 11. The duct 11 is on its way branched into a channel 12 leading to the lateral nozzles 5 and into a channel 13 leading to the center nozzles 4. The fuel is fed into the bed via a duct 14 using a con¬ ventional feed arrangement. Into the region between the cooling zone 7 and the freeboard 8 are adapted secondary air nozzles 15, whose function may be complemented by additionally placing in the cooling zone 7 a return nozzle 16 for circulating flue gas or particulate matter from the region of the boiler 1 following the heat transfer region 2. According to the invention, the fluidizing gas injection nozzles 4, 5 are divided into two domains. In a certain domain of the boiler 1, advantageously in the center as illustrated in this embodiment, the nozzles are grouped denser than in the periphery, whereby more air will be injected centrally into the bed 6 in proportion to the greater number of the nozzles 4, 5 in the center. The amount of air to the nozzles 4, 5 may be regulated by means of control dampers 17 located in the air ducts 12, 13. When both dampers 17 are fully open, the amount of air injected into the bed 6 will be determined by the numerical ratio of the center domain nozzles to the peripheral ones. If the flow rate of air taken to the center domain nozzles 4 is regulated smaller than that taken to the peripheral domain nozzles 5 in inverted proportion to the number of nozzles in the two domains, the air flow injected into the entire area of the fluid¬ ized bed 6 will behave as in a conventional fluidized-bed boiler. The nozzles can be located so that, e.g., the number of nozzles is the same in the center and peri- pheral domains, but the area of the high-velocity fluid¬ ization domain is designed to be approx. 1/9 of the peripheral domain area. However, the invention must not be understood to be limited by the proportion of the nozzles in the different areas of the bed. When air is injected into the fluidized bed 6, the opera- tion of the boiler is as follows. The overall flow rate of air injected into the fluidized bed can be regulated equal to that used in conventional fluidized-bed combus¬ tion. As more air is now injected into the domain of high-velocity fluidization, the bed material will therein rise up as a center pillar 9 reaching into the cooling zone 7. The cooling zone 7 is the region formed above the fluidized bed, but below the freeboard 8, whereby the height of this zone is determined by the maximum height from the top surface of the fluidized bed 6 reached by the particles thrown upward therefrom. In the boiler 1, this zone extends partially to the upper part of the protective lining 18 of the fluidized-bed zone 6, par¬ tially above that. Above the protective lining 18, the walls of the boiler according to the invention are formed into heat-transferring walls 19 cooled with water or steam. The secondary-air nozzles 15 are located in the region of the heat-transferring walls 18. In the fluidized bed 6 and the cooling zone, the internal circulation occurs in the form of a center pillar rising from the central high-velocity fluidization domain of the bed 6, whereby the pillar is formed by combustibles and gases entrained with the bed particulate matter. In that domain, where the particulate matter rises above the bubbling bed, the air feed rate is adjusted so that the particles can attain at least their terminal velocity. In the present context, the term terminal velocity refers to that velocity at which the particulate matter of the bed in the furnace starts rising up entrained with the upward moving gas flow. The particle size of the bed material is selected such that the particles can fall back in the bed without totally becoming carried up in the furnace. Thus, the bed material is forced to rise as a center pillar 9 upward and then is returned back in the bed as a reflow along the inner walls of the boiler 1, thus efficiently transferring heat by radiation to the heat-transferring wall surfaces 19 of the cooling zone. As the emittivity of the bed material is approximately three-fold in com¬ parison to that of furnace gases, the internal circula¬ tion of the bed material provides extremely efficient cooling of the bed. The temperature in the cooling zone 7 is approx. 850 °C, while the temperature in the freeboard 8 is approx. 1100 °C. These values are only given as an example eluci¬ dating the operating conditions used in the invention. Obviously, each boiler design will have different optimal temperatures for its components and operating conditions. Secondary air required to assure complete combustion of the fuel is injected into the boiler 1 advantageously tangentially in the region of the cooling zone 7, whereby its flow will cause a corresponding vortex in the center pillar and the reflux passing down along the boiler walls. The thus induced vortical flow forces the par¬ ticles of the refluxed bed material close to the heat- transferring surfaces 19, whereby improved heat transfer is attained. Additionally, the vortex equalizes the par¬ tial gas flows in the different parts of the furnace. The injection of secondary air may be complemented with injection of flue gas that also can be injected into the cooling zone when additional cooling is desired. The method according to the invention for cooling the fluidized bed is thus based on concentrated injection of the air flow taken into the bed so that a certain domain of the bed is formed into a high-velocity fluidization area in which the particulate matter is ejected upward substantially higher than the particles of the surround- ing bed surface. By arranging above the bed a tangential flow with the help of gas jets, which may be fed with secondary air or circulating gas, the overall gas flow in the furnace is forced into a vortical motion that conveys the entrained particles in the gas flow to the inner walls of the boiler. Then, the lower portion of the furnace is provided in the vicinity of its water-cooled walls with a domain of dense suspension in which the fluidized bed material is cooled efficiently during its downward flow. In this fashion, into the fluidized bed is formed a vigorous internal circulation in which the particles located in the center area of the bed are forced to rise higher and return back in the bed after being cooled in a reflux occurring close to the furnace walls, thereby lowering the bed temperature particularly when fuels of high heat value are combusted. Resultingly, the bed temperature can be regulated by altering the internal reflux circulation, whereby with an increased gas injection at the center of the bed, a higher cooling effect is attained. In the above-described embodiment, the bed temperature control occurs via position adjust¬ ment of the dampers 17 in the air ducts 12, 13. In the method, the primary air flow through the fluidized bed is kept essentially equal to that in a typical bubbling fluidized-bed boiler. Under favourable condi¬ tions, the system may be deactivated, whereby fuels of high moisture content may be fired as in a conventional fluidized-bed process. Thus, the invention makes it possible to fire a number of different fuels in a single boiler. As the need for the bed temperature control arises, a major portion of the injection gas is directed into the center area of the fluidized bed, simultaneously reducing the flow through the peripheral areas of the bed. However, such a minimum flow rate of fluidizing gas must at all times be maintained in all parts of the bed that is sufficient to keep the bed in a fluidized state. Besides those described above, the present invention may have alternative embodiments. In principle, the injection nozzles of the fluidizing gas could be divided into a larger number of control domains, but such an arrangement is hardly practical due to the complicated construction and minimal extra benefit involved. The gas being inject¬ ed into the separate nozzle domains may be taken using separate main ducts and fans, whereby air and circulating flue gas can be mixed. The principle of the invention based on regulated injection of different amounts of air to the different domains of the bed can be implemented in varied manners. When using equal-size nozzles distributed homogeneously, the injection pressure at some nozzles may be fed with a higher pressure, whereby the air flow rate therethrough is increased. Alternatively, larger-diameter nozzles may be used in certain areas of the nozzle field or a higher density of nozzles can be arranged through complementing a conventional nozzle system in a certain domain with circulating gas nozzles, whereby the fluid- ized bed is operated in a conventional manner, while the particulate matter reflux is driven with circulating gas. The cooling zone can be formed in existing boilers by adjusting the height of the fluidized bed center pillar at the lower part of the freeboard, whereby the cooling effect is accomplished via the cooled walls of the freeboard.

Claims: Claims : 1. A method of controlling the bed temperature in a bubbling fluidized-bed boiler, in which method, - upward from the bottom of a boiler (1) containing bed material is injected fluidizing gas via fluidizing gas nozzles (4, 5) in order to suspend the bed material as a fluidized layer (6) above the boiler bottom, - into the fluidized bed (6) is fed fuel which is allowed to partially gasify and combust in this bed, - the gasified fuel and emerging combustion gases are allowed to rise upward in the boiler (1) into a freeboard (8) for postcombustion, and - above the fluidized bed into the boiler (1) is injected air in order to effect complete combustion of the gasified fuel in the freeboard (8) , c h a r a c t e r i z e d in that - the fluidizing gas is injected into the boiler (1) so that the amount of injected gaε per unit area is over at least one domain (4) of the area equipped with the fluidizing gas injection nozzles (4, 5) larger than on the other domains equipped with the fluidizing gas injection nozzles (5) , and bed par¬ ticles located above said domain (4) of higher gas injection rate are given at least such a terminal speed that makes the particles to rise up (9) from the fluidized bed (6) proper, and - the amount and injection velocity of the fed fluidizing gas are kept such that the higher rising bed material particles can fall back in the fluidized bed (6) after being carried to an area (5) fed with a smaller amount of the fluidizing gas. 2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that at least a portion of the inner wall of the boiler (1) is used as a heat exchanger over a furnace region extending from the top surface of the fluidized bed (6) up to the level of the maximum rising height of bed material particles. 3. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that a relatively larger amount of the fluidizing gas is injected through a domain located in the center of the boiler (1) . 4. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that air is injected into the furnace of the boiler (1) tangentially from the inner walls of the boiler. 5. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that circulating gas is fed to the furnace region remaining between the top surface of the fluidized bed (6) of the boiler (1) and the level of the maximum rising height of the bed material particles. 6. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that the fluidizing gas is air. 7. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that the fluidizing gas is air and circulating gas. 8. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that into the area injected with a relatively larger amount of the fluidizing gas is fed limestone for sulfur removal. 9. An assembly for controlling the bed temperature in a fluidized-bed boiler, said assembly comprising - a boiler (1) containing bed material - fluidizing gas nozzles (4, 5) adapted to the bottom of the boiler (1) for injecting fluidizing gas into the bed material in order to suspend the bed material as a fluidized layer (6) above the boiler bottom, - means (14) for feeding fuel into the fluidized bed (6), - a freeboard (8) for postcombustion located above the fluidized bed (6) of the boiler (1) , and - at least one nozzle (15) for feeding secondary air into the freeboard (8) , c h a r a c t e r i z e d by - means for controlling the amount of injected fluid¬ izing gas by domains so that the amount of injected gas per unit area over at least one domain (4) of the area equipped with the fluidizing gas injection nozzles (4, 5) is larger than on the other domains equipped with the fluidizing gas injection nozzles (5) , whereby bed particles located above said domain (4) of higher gas injection rate are given at least such a terminal speed that makes the particles to rise up (9) from the fluidized bed (6) proper, and - the particle size of the bed material is selected such that the higher rising particles can fall back in the bed (6) after being carried to an area (5) fed with a smaller amount of the fluidizing gas. 10. A method as defined in claim 9, c h a r a c t e r ¬ i z e d by at least one heat transfer surface (18) formed to that furnace wall of the boiler (1) which extends from the top surface of the fluidized bed (6) up to the level of the maximum rising height of bed material particles. 11. A method as defined in any of foregoing claims 9 - 10, c h a r a c t e r i z e d in that said means for controlling the amount of injected fluidizing gas comprise a first nozzle domain (4) in the center of the boiler bottom and a second nozzle domain (5) in the peri¬ pheral areas of the boiler bottom and that the density of the nozzles per unit area in the central domain of high- velocity fluidization is relatively larger than in the peripheral domain of low-velocity fluidization. 12. A method as defined in any of foregoing claims 9 - 10, c h a r a c t e r i z e d in that said means for controlling the amount of injected fluidizing gas comprise a first nozzle domain (4) in the center of the boiler bottom and a second nozzle domain (5) in the peripheral areas of the boiler bottom and that the gas injection flow rate through the nozzles per unit area in the central domain of high-velocity fluidization is relatively larger than in the peripheral domain of low- velocity fluidization. 13. A method as defined in any of foregoing claims 9 - 10, c h a r a c t e r i z e d in that said means for controlling the amount of injected fluidizing gas comprise a first nozzle domain (4) in the central high- velocity fluidization area of the boiler bottom and a second nozzle domain (5) in the peripheral areas of the boiler bottom, additionally complemented with means for regulating the pressure of the gas fed to the nozzles. 14. A method as defined in any of foregoing claims 9 - 13, c h a r a c t e r i z e d in that the nozzles for feeding secondary air are adapted to the boiler (1) tangentially approximately to the level of the maximum rising height of the bed material particles. 15. A method as defined in any of foregoing claims 9 - 14, c h a r a c t e r i z e d in that at least one nozzle (16) for feeding circulating gas is adapted to the boiler (1) in the region between the top surface of the fluidized bed (6) and the level of the maximum rising height of the bed material particles.

Inventors: HIRVENOJA JUKKA, HULKKONEN SEPPO, RAIKO MARKKU, HIRVENOJA, JUKKA, HULKKONEN, SEPPO, RAIKO, MARKKU
Applicants: HIRVENOJA JUKKA, HULKKONEN SEPPO, IMATRAN VOIMA OY, RAIKO MARKKU, HIRVENOJA, JUKKA, HULKKONEN, SEPPO, RAIKO, MARKKU
Application year: 1996
Main classification: F23C11
IPC codes: F23C7, F23C9, F23C10