Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C7/00—Combustion apparatus characterised by arrangements for air supply
  • F23C7/002—Combustion apparatus characterised by arrangements for air supply the air being submitted to a rotary or spinning motion
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C10/00—Fluidised bed combustion apparatus
  • F23C10/02—Fluidised bed combustion apparatus with means specially adapted for achieving or promoting a circulating movement of particles within the bed or for a recirculation of particles entrained from the bed
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C10/00—Fluidised bed combustion apparatus
  • F23C10/18—Details; Accessories
  • F23C10/20—Inlets for fluidisation air, e.g. grids; Bottoms
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C9/00—Combustion apparatus characterised by arrangements for returning combustion products or flue gases to the combustion chamber
  • F23C9/003—Combustion apparatus characterised by arrangements for returning combustion products or flue gases to the combustion chamber for pulverulent fuel

Definitions

• 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.
• 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.
• 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.
• heat exchangers im ⁇ mersed in the bed have been used, whereby problems arise from their rapid erosion.
• the bed is an extremely harsh site, the conditions in the bed varying from reducing to oxi ⁇ dizing, causing extremely strong erosion and corrosion.
• 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.
• 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.
• 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.
• the method according to the invention is characterized by what is stated in the characterizing part of claim 1.
• 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.
• 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.
• the combustion process parameters can be controlled easier and over a wider range by this method than by prior-art arrangements.
• oxi- dizing conditions are formed that facilitate desulfuriza- tion with limestone in the same manner as in a circulat ⁇ ing fluidized-bed boiler.
• 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.
• Figure 1 is a schematic block diagram of a fluidized-bed boiler arrangement according to the invention.
• FIG. 2 is a more detailed illustration of the fluidized bed of the boiler arrangement according to the invention.
• 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.
• 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.
• the fluidizing gas injection nozzles 4, 5 are divided into two domains.
• 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.
• 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.
• the invention must not be understood to be limited by the proportion of the nozzles in the different areas of the bed.
• 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.
• 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.
• 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.
• 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.
• the air feed rate is adjusted so that the particles can attain at least their terminal velocity.
• 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.
• 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.
• 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.
• 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.
• 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.
• the bed temperature control occurs via position adjust ⁇ ment of the dampers 17 in the air ducts 12, 13.
• the primary air flow through the fluidized bed is kept essentially equal to that in a typical bubbling fluidized-bed boiler.
• the system may be deactivated, whereby fuels of high moisture content may be fired as in a conventional fluidized-bed process.
• the invention makes it possible to fire a number of different fuels in a single boiler.
• 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.
• 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.
• the present invention may have alternative embodiments.
• 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.
• the injection pressure at some nozzles may be fed with a higher pressure, whereby the air flow rate therethrough is increased.
• 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.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Fluidized-Bed Combustion And Resonant Combustion (AREA)

Priority Applications (7)

Applications Claiming Priority (2)

Publications (1)

Family

ID=8544230

Family Applications (1)

Country Status (10)

Cited By (3)

Citations (5)

• 1995
  • 1995-10-20 FI FI955011A patent/FI100064B/fi active IP Right Grant
• 1996
  • 1996-10-18 SK SK487-98A patent/SK48798A3/sk unknown
  • 1996-10-18 AU AU73017/96A patent/AU7301796A/en not_active Abandoned
  • 1996-10-18 EP EP96934842A patent/EP0856129A1/en not_active Withdrawn
  • 1996-10-18 EE EE9800118A patent/EE9800118A/xx unknown
  • 1996-10-18 PL PL96326315A patent/PL326315A1/xx unknown
  • 1996-10-18 WO PCT/FI1996/000553 patent/WO1997015784A1/en not_active Application Discontinuation
  • 1996-10-18 HU HU9903894A patent/HUP9903894A3/hu unknown
  • 1996-10-18 CZ CZ981171A patent/CZ117198A3/cs unknown
• 1998
  • 1998-04-16 LV LVP-98-71A patent/LV12095B/en unknown

Patent Citations (5)

Non-Patent Citations (3)

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