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 
  • 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
  • F23C10/04—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 the particles being circulated to a section, e.g. a heat-exchange section or a return duct, at least partially shielded from the combustion zone, before being reintroduced into the combustion zone
  • F23C10/08—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 the particles being circulated to a section, e.g. a heat-exchange section or a return duct, at least partially shielded from the combustion zone, before being reintroduced into the combustion zone characterised by the arrangement of separation apparatus, e.g. cyclones, for separating particles from the flue gases
  • F23C10/10—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 the particles being circulated to a section, e.g. a heat-exchange section or a return duct, at least partially shielded from the combustion zone, before being reintroduced into the combustion zone characterised by the arrangement of separation apparatus, e.g. cyclones, for separating particles from the flue gases the separation apparatus being located outside the combustion chamber
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F22—STEAM GENERATION
  • F22B—METHODS OF STEAM GENERATION; STEAM BOILERS
  • F22B31/00—Modifications of boiler construction, or of tube systems, dependent on installation of combustion apparatus; Arrangements of dispositions of combustion apparatus
  • F22B31/0007—Modifications of boiler construction, or of tube systems, dependent on installation of combustion apparatus; Arrangements of dispositions of combustion apparatus with combustion in a fluidized bed
  • F22B31/0084—Modifications of boiler construction, or of tube systems, dependent on installation of combustion apparatus; Arrangements of dispositions of combustion apparatus with combustion in a fluidized bed with recirculation of separated solids or with cooling of the bed particles outside the combustion 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 
  • F23C2206/00—Fluidised bed combustion
  • F23C2206/10—Circulating fluidised bed
  • F23C2206/101—Entrained or fast fluidised bed
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
  • F23J2900/00—Special arrangements for conducting or purifying combustion fumes; Treatment of fumes or ashes
  • F23J2900/15026—Cyclone separators with horizontal axis

Definitions

• the present invention relates to a method of temperature regulation in a fluidized bed reactor where a circulating fluidized bed is maintained so that
• a portion of the particles forming the fluidized bed is discharged with gases from the reaction chamber through the upper section thereof, - said portion of particles is separated from the gases in one or a plurality of adjacently arranged particle separators and,
• the separated particles are returned to the reaction chamber into a lower section thereof, and where the gases and the particles entrained therewith are conducted, at least in the upper section of the reaction chamber, past heat transfer surfaces in contact with said heat transfer surfaces.
• the invention also relates to a circulating fluidized bed reactor, which comprises a vertical reaction chamber, the upper section thereof being provided with heat transfer surfaces.
• a particle separator is also connected to the flue gas passage in the upper section of the reaction chamber, for separating particles from the flue gas flow.
• the particle separator is in communication with a duct for returning the separated particles into the lower section of the reaction chamber and with a gas outlet for leading the gases purified in the particle separator further into a convection section.
• Fluidized bed particles are generally, on one hand, inert bed material such as sand and ash and, on the other hand, process infeed material such as fuel and e.g. absorbents used for desulphurization.
• Continuous circulation of particles in a circulating fluidized bed reactor considerably extends the retention time of the substances introduced into the process, thereby improving e.g. the combustion efficiency and desulphurization.
• a large, circulating mass also has an equalizing effect on the temperature, and the temperature of the circulating fluidized bed is therefore relatively even throughout the reaction chamber and the particle circulation area.
• the temperature and the gas flow in the fluidized bed reactor have to be relatively even in order to achieve optimal results.
• a temperature of appr. 800 to 900°C has proved to be advantageous in combustion processes.
• 850°C is optimal for desulphurization and high enough for combustion. At this relatively low temperature, formation of nitrogen oxides is of minor nature.
• the entire reaction chamber is mainly filled with particles as particles are being entrained with gas into the upper section of the reaction chamber, and further into the particle separator. Therefrom, the separated particles are returned into the lower section of the reaction chamber, wherefrom they again pass, entrained with gas, into the upper section of the reaction chamber and further into the particle separator.
• This cycle is repeated several times.
• Heat transfer is, at a certain load, adjustable in the upper section of the reaction chamber by adjusting the primary/secondary air ratio or the total solids mass. The aim is to adjust the process so that a wide range of alterations would not be necessary in the primary gas flow even though there are fluctuations in the load. Use of excess air is avoided if possible.
• the structure of the bottom plate is so dimensioned as to keep a suitable pressure loss over the bottom plate at normal values of primary gas flows.
• the pressure loss has to be adequate for ensuring even fluidizing across the reaction chamber. Too low a presssure difference may cause uneven fluidization and even clogging of air or gas nozzles. If the pressure loss is too great, fluidizing of the fluidized bed consumes much energy and becomes costly.
• the process temperatures in the combustion reactor have to be maintained even in spite of fluctuations in the load or, e.g. when changing to another fuel. Temperature regulation must not have a harmful effect on the circulating fluidized bed system.
• the temperature of the purified flue gas entering the convection section has to be maintained at a sufficiently high level for the gas to provide energy needed for superheaters in the convection section.
• the temperature difference between the flue gas and the steam should be as great as possible. A small temperature difference in superheating requires very large heat transfer surfaces.
• the superheating efficiency of the convection section is often insufficient. Therefore, additional superheaters have often been installed in the combustion chamber of the fluidized bed reactor itself, more specifically in the upper section of the reactor, for providing sufficient superheating efficiency. Hot superheating surfaces are, however, susceptible to erosion in the conditions prevailing in the fluidized bed. Ash and other circulating material also wear heat transfer tubes. Furthermore, the efficiency of the superheaters disposed in the combustion chamber is dependent on the temperature in the combustion chamber, and it is therefore difficult to ensure an even steam generation independent of the load.
• the thermal power obtained from the fluidized bed reactor is partly self-regulated as a function of load.
• Heat is transferred from the flue gases to the heating surfaces in the combustion chamber either as radiation heat transfer or convection heat transfer.
• Convection heat transfer is to a great extent dependent on the suspension density prevailing in the reactor.
• the heat transfer intensifies as the suspension density grows.
• the amount of circulating mass may be increased in the upper section of the reactor by increasing the amount of primary air, thereby intensifying heat transfer to the heat transfer surfaces arranged in the upper section of the reactor.
• An increase in the total amount of air however, adds the amount of flue gases to be cleaned, which increases costs.
• An increase in the amount of primary air also increases the pressure loss over the bottom plate, thereby also increasing the energy consumption. It is, however, more recommendable to influence the suspension density by regulating the primary/secondary air ratio than by regulating the total amount of air as a certain load usually corresponds to a certain total- amount of air.
• Swedish patent specification 452359 suggests adjustment of the suspension density and consequently the adjustment of heat transfer in the upper section of a fluidized bed reactor by increasing or decreasing the amount of fine or coarse solids in the reactor.
• Coarse material may be directly discharged from the fluidized bed and fine material from the return duct.
• either coarse or fine material may be fed into the reactor.
• suspension density in the upper section of the reactor is decreased at low load levels, for maintaining an optimal temperature.
• a change in mass flow in the fluidized bed may also cause problems as it is, e.g., hard to provide even fluidization with a small mass flow.
• the mass flow is too large, the pressure loss will increase unreasonably.
• US patent specifications 4,312,301 and 4,552,203 suggest that the thermal power obtained from a circulating fluidized bed reactor be regulated in such a manner that the whole heat generated by combustion is recovered in adjustable and parallelly connected heat exchangers of fluidized bed type, located outside the actual reaction chamber.
• the circulating mass flow is divided into two or more adjustable particle flows after the particle separator. One particle flow is directly returned to the reaction chamber.
• the other particle flows are conducted to external fluidized bed type heat exchangers where heat is recovered by evaporators and superheaters.
• the walls of the actual fluidized bed reactor are of refractory or refractory-lined material, whereby the circulating mass flow leaving the reactor is maintained at a high temperature.
• the purpose of this is to ensure that the thermal power obtained from the fluidized bed type heat exchangers is sufficient.
• the fluidized bed type heat exchangers are large, separate constructions. They considerably add to the costs of the whole plant and yet, they do not guarantee sufficient thermal power for the superheaters at minimum loads. At minimum loads, the amount of particles circulating in the circulating fluidized bed reactor system is very restricted. In this case, the efficiency of separate fluidized bed type heat exchangers is also decreased due to lack of circulating mass flow.
• the heat transfer surfaces of the fluidized bed type heat exchangers are highly susceptible to wear, constituting an obvious risk in case of a steam or water tube bursting.
• An ojbect of the present invention is to provide an improved method of temperature regulation in a fluidized bed reactor.
• an object of the present invention is to provide a method of adjusting the temperature in the reactor to an optimum level when considering the chemical reactions taking place in the process, independently of the load and the fuel.
• a still further object of the invention is to provide a more flexible method of stepless regulation of the fuel temperature.
• a still further object of the invention is to provide a method of obtaining adjustable thermal power from heating surfaces arranged in the fluidized bed reactor.
• a still further object of the invention is to provide a fluidized bed reactor which facilitates temperature regulation of the flue gases in the reactor itself, irrespective of the load and fuel, so as to suit superheating.
• the method according to the invention is characterized in that passage of the gases and particles entrained therewith past the heating surfaces in various heat transfer zones in the upper section of the reactor is limited by adjusting said passage of the gases
• partition wall arranged between the upper section of the reaction chamber and at least one particle separator or a passage leading to a particle separator, said partition wall being provided with at least two openings for the gas flow, at least one of said openings being adjustable, or
• gas flow adjusting means disposed in or after at least one gas outlet of a particle separator system having at least two gas outlets.
• the fluidized bed reactor according to the invention is characterized in that - the flue gas passage in the reaction chamber is provided with a partition wall for dividing the upper section of the reaction chamber into an upper extension of the reaction chamber and at least one particle separator or a gas passage leading to a particle separator,
• the partition wall is provided with at least two openings, at least one of said openings being adjustable and which openings are connected to at .
• least two different heat transfer zones arranged in front of or after the opening, or that
• the flue gas passage in the reaction chamber is provided with a partition wall for dividing the upper section of the reaction chamber into two or more heat transfer zones, which lead to different particle separators, and that the gas outlets of the particle separators are provided with adjustable means for regulating the gas flows passing through said gas outlets.
• the heat transfer zones are arranged so that the extension of the reaction chamber side wall constitutes a partition wall between a first heat transfer zone being arranged in the upper extension of the reaction chamber and a second heat transfer zone being arranged in an adjacent gas passage leading downwards to a particle separator.
• the second heat transfer zone may be arranged in the gas passage at substantially the same horizontal level as the first heat transfer zone. The gas flowing upwardly in the reaction chamber changes its direction after the partition wall and starts to flow downwardly.
• the partition wall is provided with openings for leading gas from the first heat transfer zone to the second zone.
• the gas flow passing different heat transfer zones is controlled by adjustment of the gas routes through the partition wall by, e.g. making the openings larger or smaller. If a portion of the gas openings are made smaller in the partition wall, this will also decrease the amount of gas passing a corresponding portion of the heat transfer zones.
• the above-mentioned regulation may be used to decrease or increase heat transfer from gas to the heating surfaces.
• the temperature of the flue gases can, however, be maintained substantially constant.
• the gas flow may be conducted mainly over heat transfer surfaces in one zone by closing the openings corresponding to other heat transfer zones, thereby maintaining the flue gas temperature at the same level as with a heavier load.
• these openings may be opened and gas may be conducted over all heat transfer surfaces, which prevents the temperature from rising too much.
• the temperature of the reactor and the flue gases is regulated by adjusting the area of the total heat transfer surface passed by the gas flow.
• the effective height of the combustion chamber may be adjusted by conducting either a larger or a smaller portion of the gas flow through either upper or lower openings in a vertical partition wall and by conduting the gas flow in this manner through either the whole upper part of the reaction chamber or just a part of the upper part of the reaction chamber. At low load or if poor fuel is used, the effective height of the reactor is low. At a full load and high-grade fuel it is higher.
• the reactor according to the invention facilitates fast and stepless temperature regulation of the flue gases by decreasing the effective heating surface area of the boiler. External heat exchangers for heat recovery are not necessary.
• the walls of a combustion chamber in a fluidized bed reactor in accordance with the invention may be water tube walls, whereby the whole combustion chamber may be efficiently utilized in thermal power generation.
• Chemical or other processes taking place in the fluidized bed reactor require a certain reaction time, which corresponds to a certain combustion chamber volume.
• the entire combustion chamber volume required by the processes may be efficiently utilized in steam generation.
• the combustion .chamber walls need not necessarily be refractory-lined in order to limit heat transfer in the combustion chamber, as is the case in prior art fluidized bed reactors in which the heat regulation is effected in external heat exchangers and which therefore have to be supplied with sufficient heat, brought into the heat exchanger with circulating material from the combustion chamber.
• the fluidized bed reactor of the invention is compact and the temperature of the flue gases is regulated already in the combustion chamber to a level suitable for superheating.
• Fig. 1 is a vertical schematic illustration of a fluidized bed reactor according to the invention.
• Figs. 2, 3 and 4 are vertical schematic illustrations of three other fluidized bed reactors according to the invention.
• Fig. 5 is a cross section taken from point AA of the reactor of Fig. 4
• Fig. 6 is a schematic illustration of an additional fluidized bed reactor according to the invention.
• Fig. 7 is a cross section taken from point BB of the reactor of Fig. 6.
• Fig. 1 shows a circulating fluidized bed reactor 10 comprising a combustion chamber, i.e. a reaction chamber 12, a particle separator 14, a flue gas duct 16 leading from the upper section of the reaction chamber to the particle separator and a return duct 18 connecting the particle separator to the lower section of the reaction chamber.
• a windbox 20 which is in communication with the reaction chamber through openings in a grid plate 22. Primary air or fluidizing gas is introduced from the windbox into the reaction chamber.
• the walls 26 of the lowermost part 24 of the combustion chamber are refractory-lined. Otherwise, the walls 28 of the reaction chamber are water tube walls 30.
• the upper section 32 of the reaction chamber is divided with a vertical partition wall 34 into a first and a second zone 36 and 38.
• the partition wall 34 is an extension of a vertical wall 40 of the reaction chamber.
• the partition wall has openings 42, 44 and 46 one on top of the other, and they are provided with baffles/gates 48, 50 and 52 by which the open area of the openings may be decreased or totally closed.
• baffles/gates 48, 50 and 52 by which the open area of the openings may be decreased or totally closed.
• the zones in the upper section of . the reaction chamber may be provided with separate heat transfer surfaces 54 and 56. Both zones in the reaction chamber may be provided with one or more heating surfaces.
• Separate, vertically extending heat transfer surfaces may additionally be arranged in the upper section of the reaction chamber, not shown in Fig. 1, in connection with the partition wall so as to form separate flow channels that are mainly independent of each other, wherethrough the gases flow from the combustion chamber to the other side of the partition wall.
• the partition wall may be provided with at least one opening in the vicinity of each flow channel. When a portion or at least one of the openings is adjustable, the flow channels may also serve as heat transfer adjusting means.. Closing the openings at one flow duct prevents the flow of gas in this duct and thereby also the heat transfer from the duct to the heat transfer surfaces.
• the second zone 38 in Fig. 1 is connected to the particle separator 14 by a flue gas duct 16.
• the particle separator is a horizontal cyclone provided with a gas outlet 58 at one end thereof.
• the gases are conducted via the gas outlet into a convection section, not disclosed in the Figs.
• the flue gas temperature can be maintained substantially constant, independent of load or fuel variations, when the flue gas enters the convection section, whereby the superheaters disposed in the convection section always provide enough power.
• the particles separated in the particle separator flow from the cyclone into the return duct 18 and further via a return inlet 60 into the lower section of the combustion chamber.
• the combustion chamber is also provided with means 62 and 64 for feeding fuel and other process material and with secondary air nozzles 66.
• the passage of the flue gas may be controlled by adjusting the gates/baffles 48 - 52 so as to cause either a larger or a smaller portion of the flue gas to flow via the openings 42 - 46.
• By leading a larger portion of the flue gases via opening 46 and a smaller portion through openings 44 - 46 decreases heat transfer from flue gases to the water tube walls of the uppermost section of the reactor and to the separate heating surfaces 54 and 56.
• leading a ' larger portion of the flue gases via opening 42 and a smaller through openings 46 and 44 increases the heat transfer from the flue gases to the heating surfaces 54, 56 and to the water tube walls of the uppermost section of the reactor.
• the flue gas temperature in duct 16 may, according to the invention be kept constant by closing the opening 42, and possibly the opening 44, thereby decreasing the heat transfer from the flue gases to the heating surfaces in the upper section of the reactor.
• the heat transfer from the flue gases to the heating surfaces may be correspondingly intensified by bringing the flue gases into contact with all effective heating surfaces 54, 56 and tube walls in the whole combustion chamber. Heat transfer may simply be regulated steplessly by decreasing or increasing the open area of the openings.
• the gas flow may also be regulated on the horizontal level so that the open area of ajdacently disposed openings which are horizontally on the same level is either decreased or increased in different proportions.
• additional vertical partition walls perpendicular to the main partition wall having openings, may be arranged in connection therewith, such additional vertical partition walls controlling passage of gas in vertical ducts formed between the additional partition walls.
• the open area of the openings in the partition wall may be decreased or increased by e.g. a baffle/gate, as shown in the Figs.
• the baffle arrangement is simple and easily arranged as the baffle need not be tight. It is not, however, an intention to limit the invention to the baffle construction. Other arrangements enduring hot gases are also applicable.
• the baffles may be adjustable separately or in groups.
• those baffles which are horizontally on the same level may be arranged on the same axis, whereby they are simultaneously adjustable.
• the baffles on the same vertical axis may be adjusted simultaneously.
• a fluidized bed reactor shown in Fig. 2 deviates from the one described previously in having a different type of particle separator 14.
• the particle separator is a flow- through cyclone, where gas is discharged via a central duct 68 into the convection section, not disclosed.
• the separated particles flow from the bottom of the particle separator into the return duct 18 and via the return inlet 60 into the combustion chamber.
• the flow-through cyclone may be arranged with a vortex forming device 69, which creates an intensive vortex in the cyclone.
• the partition wall 34 provided with openings is arranged as an inclined upper wall in the upper section of the reaction chamber so as to divide the chamber into two zones 36 and 38 one on top of the other.
• additional vertical partition walls 70 and 72 which may be formed of water tube walls. They divide zone 36 into separate subzones 74, 76 and 78.
• the flue gases are conducted via all openings 42, 44, 46.
• the flue gases are introduced into a vertical cyclone separator, which comprises a vortex chamber 80 and gas outlet duct 82.
• the separated particles are returned to the combustion chamber via return duct 18.
• Various cyclone separators as well as e.g. ceramic filters are applicable to the fluidized bed reactor in accordance with the invention.
• the flue gas passage is divided into subzones on the combustion chamber side of the partition wall. If necessary, the flue gas passage may be divided into subzones also on the separator side of the partition wall.
• Figs. 4 and 5 illustrate a reactor with the upper section thereof being in communication with two adjacent cyclones 90 and 92.
• a partition wall 34 provided with openings 42 and 44.
• the openings are equipped with control baffles 48 and 50.
• the uppermost section of the reactor is additionally arranged with an additional partition wall 94, which divides the upper section into two ducts 96 and 98. These ducts are provided with heat transfer surfaces either as water tube walls or as separate surfaces.
• Duct 96 leads flue gases into the first cyclone 90 and duct 98 into the second cyclone 92. Passage of gas in the upper section of the reactor and consequently the heat transfer may be influenced by adjusting the baffles.
• passage of gas may also be regulated by baffles 100 disposed in the gas outlet ducts 82, whereby baffled openings or even partition walls need not be disposed in the gas passage between the upper section of the reaction chamber and the cyclones.
• the control baffles are disposed in a clean gas flow, whereby the baffles wear less.
• the control baffles may also be disposed in cooler parts of the gas duct if the exhaust gas ducts are divided into parallel gas paths.
• the circulating fluidized bed reactor according to the present invention has two circulation systems at different horizontal levels.
• Independently working particle separators 190 and 192 are connected to different levels of the reaction chamber through openings 186 and 196 in a partition wall 134, constituting an extension to reaction chamber side wall 140.
• the particle separators have gas outlets 180 and 182 and particle recirculation passages 118 and 120.
• the gas outlets may be connected to a common gas passage and convection section not depicted.
• the particle recirculation passages recirculate particles into different zones in the reaction chamber.
• Heat transfer surfaces 154 and 156 are arranged in the uppermost part of the reaction chamber. Gases and particles entrained therein being discharged into the upper particle separator pass the heat transfer surfaces. Gases being discharged through the lower particle separator do not substantially come into heat transfer contact with the heat transfer surfaces 154 and 156, but transfer heat only to the cooled side walls of the reaction chamber.
• Each gas outlet 180, 182 and 184 has a baffle with which the open area of the gas outlet may be regulated in order to control the particle circulation in the reactor through an upper or lower circulation path, thereby controlling the heat transfer and temperature in the reactor.
• the gas flow through the lower particle separators is increased and correspondingly decreased in the upper part of the reactor, in order to keep the tempera- ture in the reactor at an optimum level.
• the circualtion is increased in the upper part of the reactor and only a smaller flow is allowed to pass less cooled through the lower particle separator.
• the particle separators depicted in fig. 6 and 7 are con- structed of vertical planar water tube walls forming a non- circular vortex chamber.
• the bottom of the vortex chamber is inclined towards the reactor chamber, thereby sloping towards an opening in the partition wall 134 or towards a return duct 118 leading to an opening 160 in the side wall 140.
• Tangential gas inlets 196 and 198 into the vortex chamber and centrally disposed gas outlets 182 and 184 provide for a particle separating gas vortex being formed in the vortex chamber.
• the non-circular separators can be made of similar tube panels as the reactor chamber and have not to be insulated with heavy layers of refractory as conventional vertical cyclone separators.
• the separators are therefore much less space consuming and provide a very compact circulating fluidized bed reactor construction.

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

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Citations (5)

• 1989
  • 1989-12-28 FI FI896295A patent/FI85417C/fi not_active IP Right Cessation
• 1991
  • 1991-06-26 WO PCT/FI1991/000200 patent/WO1993000553A1/en unknown

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