WO2015113627A1 - A method of and a scrubber for removing pollutant compounds from a gas stream - Google Patents

A method of and a scrubber for removing pollutant compounds from a gas stream Download PDF

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Publication number
WO2015113627A1
WO2015113627A1 PCT/EP2014/051941 EP2014051941W WO2015113627A1 WO 2015113627 A1 WO2015113627 A1 WO 2015113627A1 EP 2014051941 W EP2014051941 W EP 2014051941W WO 2015113627 A1 WO2015113627 A1 WO 2015113627A1
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Prior art keywords
gas stream
vertical
particles
section
scrubber
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PCT/EP2014/051941
Other languages
French (fr)
Inventor
Winfried GUTPERL
Rolf Graf
Harald KESSLER
Original Assignee
Amec Foster Wheeler Energia Oy
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Priority to PCT/EP2014/051941 priority Critical patent/WO2015113627A1/en
Publication of WO2015113627A1 publication Critical patent/WO2015113627A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/81—Solid phase processes
    • B01D53/83—Solid phase processes with moving reactants
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/46—Removing components of defined structure
    • B01D53/48—Sulfur compounds
    • B01D53/50—Sulfur oxides
    • B01D53/508—Sulfur oxides by treating the gases with solids
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/46—Removing components of defined structure
    • B01D53/68—Halogens or halogen compounds
    • B01D53/685—Halogens or halogen compounds by treating the gases with solids
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/40—Alkaline earth metal or magnesium compounds
    • B01D2251/404—Alkaline earth metal or magnesium compounds of calcium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2253/00—Adsorbents used in seperation treatment of gases and vapours
    • B01D2253/10—Inorganic adsorbents
    • B01D2253/102—Carbon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/20—Halogens or halogen compounds
    • B01D2257/204—Inorganic halogen compounds
    • B01D2257/2045—Hydrochloric acid
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/20—Halogens or halogen compounds
    • B01D2257/204—Inorganic halogen compounds
    • B01D2257/2047—Hydrofluoric acid
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/60—Heavy metals or heavy metal compounds
    • B01D2257/602—Mercury or mercury compounds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/70—Organic compounds not provided for in groups B01D2257/00 - B01D2257/602
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2258/00—Sources of waste gases
    • B01D2258/02—Other waste gases
    • B01D2258/0233—Other waste gases from cement factories
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2258/00—Sources of waste gases
    • B01D2258/02—Other waste gases
    • B01D2258/0241—Other waste gases from glass manufacture plants
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2258/00—Sources of waste gases
    • B01D2258/02—Other waste gases
    • B01D2258/0283—Flue gases
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B17/00—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement
    • F26B17/10—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement with movement performed by fluid currents, e.g. issuing from a nozzle, e.g. pneumatic, flash, vortex or entrainment dryers
    • F26B17/101—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement with movement performed by fluid currents, e.g. issuing from a nozzle, e.g. pneumatic, flash, vortex or entrainment dryers the drying enclosure having the shape of one or a plurality of shafts or ducts, e.g. with substantially straight and vertical axis
    • F26B17/102—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement with movement performed by fluid currents, e.g. issuing from a nozzle, e.g. pneumatic, flash, vortex or entrainment dryers the drying enclosure having the shape of one or a plurality of shafts or ducts, e.g. with substantially straight and vertical axis with material recirculation, classifying or disintegrating means

Definitions

  • the present invention relates to a method of and a scrubber for re- moving pollutant compounds from a gas stream. More particularly, the present invention relates to dry CFB scrubbers, used for removing pollutants, such as SO2, HCI and HF, from a stream of exhaust gas.
  • pollutants such as SO2, HCI and HF
  • Dry CFB scrubbers are well-known systems for removing pollutants, especially acid gases, from a pollutant laden gas stream, such as exhaust gas emanating from a combustion boiler.
  • Dry CFB scrubbers generally comprise a gas channel for introducing the gas stream to a lower portion of the scrubber, an inlet chamber at the lower portion of the scrubber for directing the gas stream upwards, a constriction section arranged above the inlet chamber for accelerating the gas, a reaction chamber arranged above the constriction section, means for introducing at least one reagent to the reaction chamber for converting pollutant compounds in the gas stream to reaction products, a discharge channel for discharging gas and particles including the reaction products from the reaction chamber, a particle separator for separating particles including the reaction products from the gas, and a return channel for recycling a portion of the separated particles from the particle separator to the reaction chamber.
  • the reagent used in dry CFB scrubbers is usually alkaline material, such as Ca(OH) 2 , CaO, CaCO3, and NaHCO3.
  • alkaline material such as Ca(OH) 2 , CaO, CaCO3, and NaHCO3.
  • other reagents such as powdered activated carbon, lignite coke or bentonite, can be introduced into the scrubber system.
  • the reagent can be injected into different locations, for example, in the combustion process, in the exhaust gas duct upstream of the scrubber, directly into the reaction chamber of the scrubber or into the return channel.
  • the pollutant compounds in the gas stream react with the reagent material to form reaction products, generally solid salts, which are removed from the gas in the particle separator, usually a fabric filter.
  • the reaction chamber of a dry CFB scrubber comprises a bed of particulate material, consisting mainly of fly ash and reagent particles, fluidized by a vertical stream of pollutant laden gas. The reaction between the reagent and the pollutant compounds takes place mostly on the surface of the bed particles, mainly in the reaction chamber. In order to maintain the bed in the reaction chamber, a portion of the material collected by the particle separator is usually recycled back to the reaction chamber.
  • WO 2006/032288 discloses another construction in which the height of the scrubber system is decreased by connecting the recycling channel by a feeding nozzle to a side wall of the inlet chamber, upstream of the constriction section.
  • This solution has the disadvantage that because the velocity of the gas stream in the inlet chamber may be relatively low, especially near the location of the feeding nozzle, a too large portion of the recycled particles tends to sink to the bottom of the inlet chamber, especially at low loads. This effect can be minimized by recycling gas to the bottom of the inlet chamber, which, however, makes the system complicated and increases the costs.
  • Patent documents CN 201760230 and CN 101402019 show a dry fluidized bed desulfurization reactor comprising a sloped channel for conveying absorbent particles to an absorbent distributor at a central zone of an inlet cham- ber, at the vertical axis of a constriction section comprising multiple circumferen- tially arranged venturi nozzles. Even with these solutions, the recycled particles are introduced to the inlet chamber at a zone in which the gas velocity is relatively slow and the entrainment of particles and their distribution to the reaction chamber may be less than optimal.
  • An object of the present invention is to provide a method of and a scrubber for removing pollutant compounds from a gas stream in which at least a part of the above mentioned problems of prior art are minimized.
  • the present invention provides a scrubber for removing pollutant compounds from a gas stream, comprising at least one gas channel for introducing the gas stream to a lower portion of the scrubber; an inlet chamber at the lower portion of the scrubber for directing the gas stream upwards to form a vertical gas stream; a constriction section having a vertical axis of symmetry and comprising one or more venturi nozzles, the constriction section being above and in flow connection with an upper end of the inlet chamber for accelerat- ing the vertical gas stream; a reaction chamber arranged above and in flow connection with the constriction section; means for introducing at least one reagent to the reaction chamber for converting pollutant compounds in the gas stream to reaction products; a discharge channel in flow connection with the reaction chamber for discharging gas and particles including the reaction products from the reac- tion chamber; a particle separator in flow connection with the discharge channel for separating particles including the reaction products from the gas; and a return channel for continuously recycling a portion of
  • the present invention provides a method of removing pollutant compounds from a gas stream in a scrubber, comprising the continuous steps of introducing the gas stream through at least one gas chan- nel to an inlet chamber at a lower portion of the scrubber; directing the gas stream upwards in the inlet chamber to form a vertical gas stream; accelerating the vertical gas stream in a constriction section arranged above and in flow connection with the inlet chamber, the constriction section having a vertical axis of symmetry and comprising one or more venturi nozzles; conveying the gas stream from the constriction section to a reaction chamber arranged above the constriction section; introducing at least one reagent to the reaction chamber for converting the pollutant compounds to particulate reaction products; discharging gas and particles including the reaction products from the reaction chamber through a discharge channel to a particle separator; separating particles including the reaction products from the gas in the particle separator; recycling a portion of the separated particles from the particle separator through a
  • the present invention relates to dry CFB scrubbers having a constriction section with one or more venturi nozzles.
  • a venturi nozzle is a well-known flow velocity changing component, which has a variable cross sectional area.
  • the horizontal cross section of a vertical venturi is convergent or upwards decreasing in a lower portion of the venturi and divergent or upwards increasing in an upper portion of the venturi. Thereby, the velocity of the vertical gas stream accelerates at the lower portion of the venturi.
  • the shape of the venturi is advantageously streamlined so as to avoid any sharp edges which could cause turbulence or even dead spaces in the flow path.
  • the constriction section comprises only a single venturi nozzle and the vertical end section of the return channel is arranged within the single venturi nozzle.
  • Such a construction is especially useful in relatively small CFB scrubbers, in which there is no need for multiple venturi nozzles.
  • the constriction section comprises multiple venturi nozzles.
  • a large CFB scrubber is herein meant the size of a CFB scrubber which handles the exhaust gases of a utility boiler of at least about 100 MWe.
  • venturi nozzles are advantageously arranged circumfer- entially around the vertical symmetry axis, so that the arrangement does not have a venturi nozzle at the symmetry axis.
  • the vertical end section of the return channel is advantageously arranged between the multiple venturi nozzles.
  • An important feature of the present invention is that the recycled par- tides are not released at a side wall of the inlet chamber but at a portion which is central with respect to the axis of symmetry of the constriction section. Thereby the particles are released evenly or symmetrically with respect of the constriction section. Also it is important that the recycled particles are not released at a vertically central position of the inlet chamber, but at a top portion of the inlet chamber, immediately below the constriction section. In practice the lower end of the vertical end section is preferably within the highest 10 % of the inlet chamber, even more preferably within the highest 5 % of the inlet chamber.
  • the lower end of the vertical end section is at a vertical level which is less than 0.2 m, even more preferably less than 0.1 m, lower than the lower end of the constriction section.
  • the lower end of the vertical end section of the return channel is vertically aligned with the lower end of the lower portion of the one or more venturi nozzles.
  • the lower end of the vertical end section is arranged so close to the lower end of the constriction section that the gas stream has a well- defined flow velocity upwards, directed to the constriction section.
  • the gas stream efficiently entrains particles to the reaction chamber. If the particles were released on the side or at a lower part of the inlet chamber, the particles could be affected by a gas stream, which has an unstable velocity and direction, and the entrainment of particles would be less effective.
  • the vertical end section may direct the recycled particles down towards a central zone of the inlet chamber. How- ever, in many cases the mixing of the recycled particles with the upwards flowing gas stream is more efficient if the direction of the particles is not directly downwards, but the velocity also has a horizontal component outwards, away from the axis of symmetry of the constriction section. This is especially useful in cases in which the constriction section comprises multiple venturi nozzles. Therefore, ac- cording to a preferred embodiment of the present invention, at the lower end of the vertical end section is connected a dispersion piece for causing horizontal momentum to the recycled particles.
  • the dispersion piece comprises an upwards directed right circular cone having an aperture of 60 - 120 degrees, preferably of 80-100 degrees.
  • the return channel may be a single channel, but according to an advantageous embodiment of the present invention, the return channel comprises, or the downstream end of the return channel is divided to, multiple in-parallel connected parallel pipe sections, which pipe sections are connected to a common vertical end section.
  • the return channel may comprise only two pipe sections, which are connected at different angles to the vertical end section, preferably to opposite sides of the end section.
  • the return channel comprises more than two pipe sections which are arranged symmetrically in flow connection with the vertical end section.
  • the constriction section comprises multiple circumferentially arranged venturi nozzles and the pipe sections are arranged between the venturi nozzles.
  • a main feature of the present invention is that a coarse fraction of the recycled particles can be advantageously separated. Separation of coarse particles from the recycled particles and not allowing them to enter into the reaction chamber is especially important because the pollutant reducing efficiency of coarse particles is lower than that of fine particles due to their generally lower reagent content and their lower area-to-volume ratio. The adverse effect of coarse particles is especially enhanced due to their tendency to accumulate into the particle bed of the reaction chamber.
  • the bottom of the inlet chamber is advantageously connected to a discharge duct for removing the separated coarse fraction of the recycled particles from the system.
  • the shares of particles being separated and entrained with the vertical gas stream, respectively can be controlled by adjusting the vertical position of the dispersion piece.
  • An especially useful aspect of the present arrangement is that by releasing the recycled particles at a horizontally center portion of the inlet chamber, the arrangement enables to distribute the recycled particles evenly to the re- action chamber.
  • the constriction section comprises multiple venturi nozzles
  • the present invention enables to distribute the recycled particles evenly to all the venturi nozzles. Even distribution of particles is very important for the efficiency of the reactions taking place in the reaction chamber.
  • Even distribution of particles to the reaction chamber is especially important because the stream of recycled particles may comprise unreacted reagent.
  • the reaction of the reagent with the pollutant compounds may be far from complete during a single pass through the scrubber. In such a case, it is important to recycle a relatively large portion of the material collected by the particle separator in order to obtain a high utilization rate of the reagent.
  • the reagent is introduced directly to the reaction chamber. Then, however, multiple feeding points may be needed to guarantee even distribu- tion of the reagent. It is also possible to introduce the reagent, or a portion of the reagent, through the return channel, i.e., the return channel may comprise means for introducing particulate reagent to the stream of recycled particles. When the reagent is introduced to the scrubber through the return channel, it is naturally es- pecially important that the recycled particles are evenly distributed to the vertical gas stream.
  • Non-symmetrical releasing of the particles also increases the risk that the fluidized bed forming in the reaction chamber becomes non-symmetrical. This decreases the efficiency of the removal of pollutants, and may even cause that the bed does not stay in the reaction chamber but falls down to the inlet chamber.
  • Desired even or symmetrical distribution of separated particles to the constriction section is made possible by arranging the vertical end section of the return channel at the vertical symmetry axis of the constriction section. By releasing the recycled particles symmetrically with respect of the constriction section, it is possible to distribute the recycled particles evenly to the reaction chamber.
  • the constriction section comprises multiple venturies
  • the present invention enables to distribute the recycled particles evenly to all the venturies. Even distribution of particles is very important for the efficiency of the reactions taking place in the reaction chamber.
  • the particles are released to a downstream portion of the constriction section.
  • the particles are almost immediately mixed with the particle bed in the reaction chamber, having typically a temperature of 65-85 °C, which is about 10 °C to 40 °C above the water and acid condensation temperatures, causing a risk of the above mentioned harmful effects.
  • the injection location according to the present invention has also proved to be advantageous for mercury reduction by using suitable adsorbents, for example, carbon particles.
  • the recycled particles are released in a location adjacent the lower end of the one or more venturi nozzles.
  • the releasing location is at a top portion of the inlet chamber, and not at a center or lower portion of the inlet chamber.
  • the gas has a velocity of about 10 m/s at the center portions of the inlet chamber, but it may be as high as 60 m/s in the constriction section.
  • the lower end of the vertical end section is at a location which is so close to the lower end of the constriction section that the gas stream already has a streamlined, well- defined flow velocity upwards, towards the constriction section.
  • the recycled particles are preferably released in a location in which the vertical velocity of the gas is at least 20 m/s, even more preferably at least 30 m/s.
  • the recycled particles advantageously impinge with a high velocity vertical gas stream in a nearly counter-current flow which renders a very efficient heat transfer from the vertical gas stream to the recycled particles possible.
  • the vertical gas stream also efficiently entrains a fine fraction of the recycled particles to the constriction section and to the reaction chamber.
  • a coarse fraction of the recycled particles i.e., the fraction consisting of particles which are too heavy to be entrained with the gas stream, is separated and sinks to the bottom of the inlet chamber.
  • a further advantage of releasing recycled particles according to the present invention is achieved when feeding carbon particles, such as lignite coke particles, through the return channel as a reagent to remove e.g. mercury pollutants.
  • the carbon particles are caused to head-on collisions with SO2 and SO3, and possible sulfur acid molecules, before removing the sulfur oxides in the reaction chamber. Thereby, the sulfur molecules further activate the carbon particles and the pollutant removal is improved.
  • the inlet chamber may generally be of different shapes and sizes. It can be a separate chamber at the end of a gas channel or multiple gas channels, or it can be just an end section of a gas channel in which the direction of gas stream is changed from mainly horizontal to vertical.
  • Fig. 1 shows the schematic diagram of boiler with a dry CFB scrubber according to prior art.
  • Fig. 2 shows the schematic cross sectional diagram of a detail of a scrubber according to a first embodiment of the present invention.
  • FIG. 3 shows the schematic cross sectional diagram of a detail of a scrubber according to a second embodiment of the present invention.
  • the dry CFB scrubber (18) shown in FIG. 1 comprises an inlet chamber (20) arranged at a lower portion of the scrubber for directing the exhaust gas stream upwards to form a vertical gas stream.
  • the inlet chamber may be actual separate chamber or it may be, for example, a curved end section of the exhaust gas channel (14). It is also possible that there are two or even more than two exhaust gas channels leading to a single inlet chamber. Such multiple exhaust gas channels may convey exhaust gas either from a single plant or from multiple separate plants.
  • the reaction chamber (28) comprises conventional feeding means (30), such as a lance or a feeding screw, for introducing reagent, such as calcium hydroxide, Ca(OH) 2 , to the reaction chamber for converting pollutant compounds in the exhaust gas to harmless compounds, i.e., reaction products of the scrubber, such as CaSO 4 .
  • a discharge channel (34) is connected in the upper portion of the reaction chamber (28) for discharging gas and particles, including the reaction products and possible unreacted reagent, from the reaction chamber to a particle separator (36).
  • the particle separator is usually a fabric filter, but in some cases it may alternatively be other type of separator, for example, an electrostatic precipitator. Particles, including the reaction products and unreacted reagent, are separated from the exhaust gas in the particle separator (36), and the remaining cleaned exhaust gas is directed through a stack (38) to the environment.
  • the return channel (46) is connected to the side of an upper portion of the constriction section (24). Alternatively it can be connected directly to a lower portion of the reaction chamber (28). It has, however, been found that such arrangements for connecting the return channel may in some applications lead to harmful effects, such as fouling, agglomeration and coarsening of the bed in the reaction chamber (28).
  • FIGS. 2 and 3 show a vertical cross section of two alternatives for a detail to be used in a conventional dry CFB scrubber according to FIG. 1 . More particularly, FIGS. 2 and 3 show the arrangement for returning particles from the particle separator (36) back to the reaction chamber (28), according to two embodiments of the present invention.
  • Particles returned from the particle separator flow down in the vertical end section (54) towards the inlet chamber (58).
  • the recycled particles arriving in the inlet chamber encounter a vertical high velocity gas stream (60) flowing from the inlet chamber (58) towards the venturi nozzle (50).
  • a fine fraction (62) of the recycled particles is then entrained with the vertical gas stream to a reaction chamber (64) above the constriction section.
  • a coarse fraction (66) of the recycled particles sinks to a bottom hopper (68) at the lower portion of the inlet chamber.
  • the coarse fraction is advantageously discharged through a discharge duct (70) from a bottom hopper of the inlet chamber to a silo.
  • Even distribution of particles to the venturi nozzle (50) is advantageously improved by arranging a dispersion piece (72) below the lower end of the vertical end section (54) so as to cause an outwards directed horizontal momentum to the recycled particles.
  • the dispersion piece (72) comprises advantageously an upwards directed right circular cone having an aperture of 60 - 120 degrees, preferably of 80-100 degrees.
  • the dispersion piece is connected to a lifting mechanism (74), whereby the vertical position of the dispersion piece can be adjusted. When the dispersion piece is raised higher, the recycled particles hit the upper surface of the dispersion piece at a higher level and encounter a higher velocity gas stream, and more particles are entrained by the vertical gas stream.
  • the vertical position of the dispersion piece it is possible to control the portion of particles to be separated to the bottom of the inlet chamber to be removed from the scrubber.
  • Present invention differs from the prior art shown in FIG. 1 , for ex- ample, in that the return channel (52) is connected to the constriction section (48) at a relatively lower level.
  • the vertical end section (54) should be short.
  • the vertical end section (54) has to have a sufficient height (76).
  • the height is at least as large as the minimum width (78) of the venturi nozzle.
  • This embodiment differs from that shown in FIG 2 especially in that the vertical end section (82) of the return channel (52) is not within a venturi nozzle but between the multiple circumferentially ar- ranged venturi nozzles (80).

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Abstract

A method of and a scrubber (18) for removing pollutant compounds from a gas stream, the scrubber comprising at least one gas channel (90) for introducing the gas stream to a lower portion of the scrubber; an inlet chamber (58) for directing the gas stream upwards to form a vertical gas stream; a constriction section (48) having a vertical axis of symmetry and comprising one or more venturi nozzles (50, 80), the constriction section being above and in flow connection with an upper end of the inlet chamber (58); a reaction chamber (64) arranged above and in flow connection with the constriction section; means for introducing at least one reagent (30, 30', 30'') to the reaction chamber for converting pollutant compounds in the gas stream to reaction products; a discharge channel (34) in flow connection with the reaction chamber for discharging gas and particles including the reaction products from the reaction chamber; a particle separator (36) in flow connection with the discharge channel; and a return channel (52, 52') for continuously recycling a portion of separated particles from the particle separator to the reaction chamber; wherein the return channel comprises a vertical end section (54, 82) arranged at the axis of symmetry of the constriction section so as to direct recycled particles down towards the inlet chamber (58), wherein the vertical end section comprises a lower end arranged immediately below the constriction section (48) so as to impinge the recycled particles with the vertical gas stream therein to entrain a fine fraction (62) of the recycled particles with the vertical gas stream and to separate a coarse fraction (66) of the recycled particles to a lower portion of the inlet chamber (58).

Description

A METHOD OF AND A SCRUBBER FOR REMOVING POLLUTANT
COMPOUNDS FROM A GAS STREAM BACKGROUND OF THE INVENTION [0001 ] Field of the Invention
[0002] The present invention relates to a method of and a scrubber for re- moving pollutant compounds from a gas stream. More particularly, the present invention relates to dry CFB scrubbers, used for removing pollutants, such as SO2, HCI and HF, from a stream of exhaust gas.
[0003] Description of related art
[0004] Dry CFB scrubbers are well-known systems for removing pollutants, especially acid gases, from a pollutant laden gas stream, such as exhaust gas emanating from a combustion boiler. Dry CFB scrubbers generally comprise a gas channel for introducing the gas stream to a lower portion of the scrubber, an inlet chamber at the lower portion of the scrubber for directing the gas stream upwards, a constriction section arranged above the inlet chamber for accelerating the gas, a reaction chamber arranged above the constriction section, means for introducing at least one reagent to the reaction chamber for converting pollutant compounds in the gas stream to reaction products, a discharge channel for discharging gas and particles including the reaction products from the reaction chamber, a particle separator for separating particles including the reaction products from the gas, and a return channel for recycling a portion of the separated particles from the particle separator to the reaction chamber. [0005] The reagent used in dry CFB scrubbers, especially when removing SO2, SO3, HCI and HF from exhaust gases, is usually alkaline material, such as Ca(OH)2, CaO, CaCO3, and NaHCO3. Especially when removing pollutants like mercury, dioxins or furans, existing in low concentrations in some flue gases, also other reagents, such as powdered activated carbon, lignite coke or bentonite, can be introduced into the scrubber system.
[0006] The reagent can be injected into different locations, for example, in the combustion process, in the exhaust gas duct upstream of the scrubber, directly into the reaction chamber of the scrubber or into the return channel. The pollutant compounds in the gas stream react with the reagent material to form reaction products, generally solid salts, which are removed from the gas in the particle separator, usually a fabric filter.
[0007] Water is usually also injected to the reaction chamber to control temperature and humidity therein, for example, to enhance the reaction of SO2 with Ca(OH)2. However, in dry scrubbing systems, unlike in wet scrubbers, the gas stream that is being treated is not saturated with moisture. To the contrary, only the amount of moisture that can be evaporated in the exhaust gas without condensing is added. [0008] The reaction chamber of a dry CFB scrubber comprises a bed of particulate material, consisting mainly of fly ash and reagent particles, fluidized by a vertical stream of pollutant laden gas. The reaction between the reagent and the pollutant compounds takes place mostly on the surface of the bed particles, mainly in the reaction chamber. In order to maintain the bed in the reaction chamber, a portion of the material collected by the particle separator is usually recycled back to the reaction chamber.
[0009] In a commonly used construction, shown, for example, in the patent document WO 2005/030368, recycled material is introduced from a bottom hopper of the particle separator through a sloping recycling channel and a feeding nozzle to a side of an expanding, downstream portion of the vertical constriction section arranged above the inlet chamber. This solution has the disadvantage that because the feeding nozzle is at a relatively high vertical level in the scrubber, the total height of the scrubber system tends to become relatively large.
[0010] WO 2006/032288 discloses another construction in which the height of the scrubber system is decreased by connecting the recycling channel by a feeding nozzle to a side wall of the inlet chamber, upstream of the constriction section. This solution has the disadvantage that because the velocity of the gas stream in the inlet chamber may be relatively low, especially near the location of the feeding nozzle, a too large portion of the recycled particles tends to sink to the bottom of the inlet chamber, especially at low loads. This effect can be minimized by recycling gas to the bottom of the inlet chamber, which, however, makes the system complicated and increases the costs.
[001 1 ] Patent documents CN 201760230 and CN 101402019 show a dry fluidized bed desulfurization reactor comprising a sloped channel for conveying absorbent particles to an absorbent distributor at a central zone of an inlet cham- ber, at the vertical axis of a constriction section comprising multiple circumferen- tially arranged venturi nozzles. Even with these solutions, the recycled particles are introduced to the inlet chamber at a zone in which the gas velocity is relatively slow and the entrainment of particles and their distribution to the reaction chamber may be less than optimal.
[0012] An object of the present invention is to provide a method of and a scrubber for removing pollutant compounds from a gas stream in which at least a part of the above mentioned problems of prior art are minimized. SUMMARY OF THE INVENTION
[0013] According to one aspect, the present invention provides a scrubber for removing pollutant compounds from a gas stream, comprising at least one gas channel for introducing the gas stream to a lower portion of the scrubber; an inlet chamber at the lower portion of the scrubber for directing the gas stream upwards to form a vertical gas stream; a constriction section having a vertical axis of symmetry and comprising one or more venturi nozzles, the constriction section being above and in flow connection with an upper end of the inlet chamber for accelerat- ing the vertical gas stream; a reaction chamber arranged above and in flow connection with the constriction section; means for introducing at least one reagent to the reaction chamber for converting pollutant compounds in the gas stream to reaction products; a discharge channel in flow connection with the reaction chamber for discharging gas and particles including the reaction products from the reac- tion chamber; a particle separator in flow connection with the discharge channel for separating particles including the reaction products from the gas; and a return channel for continuously recycling a portion of the separated particles from the particle separator to the reaction chamber, wherein the return channel comprises a vertical end section arranged at the axis of symmetry of the constriction section so as to direct recycled particles down towards the inlet chamber, wherein the vertical end section comprises a lower end arranged immediately below the constriction section so as to impinge the recycled particles with the vertical gas stream therein to entrain a fine fraction of the recycled particles with the vertical gas stream and to separate a coarse fraction of the recycled particles to a lower por- tion of the inlet chamber.
[0014] According to another aspect, the present invention provides a method of removing pollutant compounds from a gas stream in a scrubber, comprising the continuous steps of introducing the gas stream through at least one gas chan- nel to an inlet chamber at a lower portion of the scrubber; directing the gas stream upwards in the inlet chamber to form a vertical gas stream; accelerating the vertical gas stream in a constriction section arranged above and in flow connection with the inlet chamber, the constriction section having a vertical axis of symmetry and comprising one or more venturi nozzles; conveying the gas stream from the constriction section to a reaction chamber arranged above the constriction section; introducing at least one reagent to the reaction chamber for converting the pollutant compounds to particulate reaction products; discharging gas and particles including the reaction products from the reaction chamber through a discharge channel to a particle separator; separating particles including the reaction products from the gas in the particle separator; recycling a portion of the separated particles from the particle separator through a return channel to the reaction camber; directing recycled particles towards the inlet chamber through a vertical end section of the return channel arranged at the axis of symmetry of the constriction section, wherein the vertical end section comprises a lower end arranged immediately below the constriction section, and impinging the recycled particles discharged through the lower end of the vertical end section with the vertical gas stream to entrain a fine fraction of the recycled particles with the vertical gas stream and to separate a course fraction of the recycled particles to a lower portion of the inlet chamber.
[0015] The present invention relates to dry CFB scrubbers having a constriction section with one or more venturi nozzles. A venturi nozzle is a well-known flow velocity changing component, which has a variable cross sectional area. The horizontal cross section of a vertical venturi is convergent or upwards decreasing in a lower portion of the venturi and divergent or upwards increasing in an upper portion of the venturi. Thereby, the velocity of the vertical gas stream accelerates at the lower portion of the venturi. The shape of the venturi is advantageously streamlined so as to avoid any sharp edges which could cause turbulence or even dead spaces in the flow path.
[0016] According to a preferred embodiment of the present invention, the constriction section comprises only a single venturi nozzle and the vertical end section of the return channel is arranged within the single venturi nozzle. Such a construction is especially useful in relatively small CFB scrubbers, in which there is no need for multiple venturi nozzles. [0017] According to another preferred embodiment of the present invention, which is especially useful in large CFB scrubbers, the constriction section comprises multiple venturi nozzles. By a large CFB scrubber is herein meant the size of a CFB scrubber which handles the exhaust gases of a utility boiler of at least about 100 MWe. Multiple venturi nozzles are advantageously arranged circumfer- entially around the vertical symmetry axis, so that the arrangement does not have a venturi nozzle at the symmetry axis. Thereby the vertical end section of the return channel is advantageously arranged between the multiple venturi nozzles.
[0018] An important feature of the present invention is that the recycled par- tides are not released at a side wall of the inlet chamber but at a portion which is central with respect to the axis of symmetry of the constriction section. Thereby the particles are released evenly or symmetrically with respect of the constriction section. Also it is important that the recycled particles are not released at a vertically central position of the inlet chamber, but at a top portion of the inlet chamber, immediately below the constriction section. In practice the lower end of the vertical end section is preferably within the highest 10 % of the inlet chamber, even more preferably within the highest 5 % of the inlet chamber. Preferably the lower end of the vertical end section is at a vertical level which is less than 0.2 m, even more preferably less than 0.1 m, lower than the lower end of the constriction section. [0019] According to a preferred embodiment of the present invention, the lower end of the vertical end section of the return channel is vertically aligned with the lower end of the lower portion of the one or more venturi nozzles. According to the present invention, the lower end of the vertical end section is arranged so close to the lower end of the constriction section that the gas stream has a well- defined flow velocity upwards, directed to the constriction section. Thus, the gas stream efficiently entrains particles to the reaction chamber. If the particles were released on the side or at a lower part of the inlet chamber, the particles could be affected by a gas stream, which has an unstable velocity and direction, and the entrainment of particles would be less effective.
[0020] According to the present invention, the vertical end section may direct the recycled particles down towards a central zone of the inlet chamber. How- ever, in many cases the mixing of the recycled particles with the upwards flowing gas stream is more efficient if the direction of the particles is not directly downwards, but the velocity also has a horizontal component outwards, away from the axis of symmetry of the constriction section. This is especially useful in cases in which the constriction section comprises multiple venturi nozzles. Therefore, ac- cording to a preferred embodiment of the present invention, at the lower end of the vertical end section is connected a dispersion piece for causing horizontal momentum to the recycled particles.
[0021 ] Preferably the dispersion piece comprises an upwards directed right circular cone having an aperture of 60 - 120 degrees, preferably of 80-100 degrees. When the recycled particles hit the upper surface of the cone, they spread circularly around the dispersion piece to as to efficiently interact with the upwards flowing gas stream. [0022] The return channel may be a single channel, but according to an advantageous embodiment of the present invention, the return channel comprises, or the downstream end of the return channel is divided to, multiple in-parallel connected parallel pipe sections, which pipe sections are connected to a common vertical end section. The return channel may comprise only two pipe sections, which are connected at different angles to the vertical end section, preferably to opposite sides of the end section. However, according to an especially advantageous embodiment of the present invention, the return channel comprises more than two pipe sections which are arranged symmetrically in flow connection with the vertical end section. Such a construction is especially advantageous when the constriction section comprises multiple circumferentially arranged venturi nozzles and the pipe sections are arranged between the venturi nozzles.
[0023] A main feature of the present invention is that a coarse fraction of the recycled particles can be advantageously separated. Separation of coarse particles from the recycled particles and not allowing them to enter into the reaction chamber is especially important because the pollutant reducing efficiency of coarse particles is lower than that of fine particles due to their generally lower reagent content and their lower area-to-volume ratio. The adverse effect of coarse particles is especially enhanced due to their tendency to accumulate into the particle bed of the reaction chamber. The bottom of the inlet chamber is advantageously connected to a discharge duct for removing the separated coarse fraction of the recycled particles from the system. [0024] According to an embodiment of the present invention, the shares of particles being separated and entrained with the vertical gas stream, respectively, can be controlled by adjusting the vertical position of the dispersion piece. Therefore, the dispersion piece is advantageously connected to a lifting mechanism for controlling the vertical position of the dispersion piece. [0025] In case the dispersion piece comprises an upwards directed circular cone, the dispersion piece may alternatively, or additionally, comprise means for controlling the aperture of the cone. Thereby, the horizontal momentum caused to the particles can be increased by increasing the aperture of the cone.
[0026] An especially useful aspect of the present arrangement is that by releasing the recycled particles at a horizontally center portion of the inlet chamber, the arrangement enables to distribute the recycled particles evenly to the re- action chamber. Especially when the constriction section comprises multiple venturi nozzles, the present invention enables to distribute the recycled particles evenly to all the venturi nozzles. Even distribution of particles is very important for the efficiency of the reactions taking place in the reaction chamber. [0027] Even distribution of particles to the reaction chamber is especially important because the stream of recycled particles may comprise unreacted reagent. In some applications of the present invention, the reaction of the reagent with the pollutant compounds may be far from complete during a single pass through the scrubber. In such a case, it is important to recycle a relatively large portion of the material collected by the particle separator in order to obtain a high utilization rate of the reagent.
[0028] Usually the reagent is introduced directly to the reaction chamber. Then, however, multiple feeding points may be needed to guarantee even distribu- tion of the reagent. It is also possible to introduce the reagent, or a portion of the reagent, through the return channel, i.e., the return channel may comprise means for introducing particulate reagent to the stream of recycled particles. When the reagent is introduced to the scrubber through the return channel, it is naturally es- pecially important that the recycled particles are evenly distributed to the vertical gas stream.
[0029] Even distribution of recycled particles to the reaction chamber de- pends on the entrainment of particles with the gas stream. Therefore, in order to achieve desired distribution, it is also important that the velocity of the gas stream in the region where the recycled particles contact with the gas stream is even. In order to have such a desired gas velocity, it is in some cases advantageous that the system does not have only a single gas channel for introducing the gas stream to the inlet chamber, but there are multiple gas channels connected to the inlet chamber. There may preferably be two gas channels connected to opposite sides of the inlet chamber, but it is also possible that there are more than two gas channels connected in rotational symmetry with the inlet chamber. [0030] The releasing location of the particles and their distribution to the gas stream are important for optimal functioning of the scrubber. If the particles were released on the side or at a lower portion of the inlet chamber, the particles could be affected by a gas stream, which has a relatively low velocity and possibly unstable direction, and the heat transfer to the particles and their entrainment with the gas would not be effective. Non-symmetrical releasing of the particles also increases the risk that the fluidized bed forming in the reaction chamber becomes non-symmetrical. This decreases the efficiency of the removal of pollutants, and may even cause that the bed does not stay in the reaction chamber but falls down to the inlet chamber.
[0031 ] Desired even or symmetrical distribution of separated particles to the constriction section is made possible by arranging the vertical end section of the return channel at the vertical symmetry axis of the constriction section. By releasing the recycled particles symmetrically with respect of the constriction section, it is possible to distribute the recycled particles evenly to the reaction chamber. When the constriction section comprises multiple venturies, the present invention enables to distribute the recycled particles evenly to all the venturies. Even distribution of particles is very important for the efficiency of the reactions taking place in the reaction chamber.
[0032] A further important feature of the present invention is that the particles are released to a gas stream which has a relatively high velocity and high temperature. Typically a gas stream entering the inlet chamber has a temperature of 100-250 °C, preferably 130-160 °C. Correspondingly, according to the present invention, the recycled particles are distributed in the inlet chamber (58) in a location in which the temperature of the gas stream is at least 100 °C. Thereby, the recycled particles, which have been humidified and cooled down in the reaction chamber, are efficiently and rapidly dried and heated with the hot gas stream while re-entering the constriction section. It has been noticed that such initial drying and heating minimizes harmful effects, such as agglomeration due to CaCI, which may take place in a conventional scrubber. The present construction should be compared with a conventional construction disclosed, for example, in WO
2005/030368, in which the particles are released to a downstream portion of the constriction section. In such a conventional construction, the particles are almost immediately mixed with the particle bed in the reaction chamber, having typically a temperature of 65-85 °C, which is about 10 °C to 40 °C above the water and acid condensation temperatures, causing a risk of the above mentioned harmful effects. The injection location according to the present invention has also proved to be advantageous for mercury reduction by using suitable adsorbents, for example, carbon particles.
[0033] According to the present invention, the recycled particles are released in a location adjacent the lower end of the one or more venturi nozzles. Thereby, the releasing location is at a top portion of the inlet chamber, and not at a center or lower portion of the inlet chamber. Typically the gas has a velocity of about 10 m/s at the center portions of the inlet chamber, but it may be as high as 60 m/s in the constriction section. According to the present invention, the lower end of the vertical end section is at a location which is so close to the lower end of the constriction section that the gas stream already has a streamlined, well- defined flow velocity upwards, towards the constriction section. According to the invention, the recycled particles are preferably released in a location in which the vertical velocity of the gas is at least 20 m/s, even more preferably at least 30 m/s.
[0034] The recycled particles advantageously impinge with a high velocity vertical gas stream in a nearly counter-current flow which renders a very efficient heat transfer from the vertical gas stream to the recycled particles possible. The vertical gas stream also efficiently entrains a fine fraction of the recycled particles to the constriction section and to the reaction chamber. On the other hand, a coarse fraction of the recycled particles, i.e., the fraction consisting of particles which are too heavy to be entrained with the gas stream, is separated and sinks to the bottom of the inlet chamber. [0035] A further advantage of releasing recycled particles according to the present invention is achieved when feeding carbon particles, such as lignite coke particles, through the return channel as a reagent to remove e.g. mercury pollutants. By the present arrangement the carbon particles are caused to head-on collisions with SO2 and SO3, and possible sulfur acid molecules, before removing the sulfur oxides in the reaction chamber. Thereby, the sulfur molecules further activate the carbon particles and the pollutant removal is improved.
[0036] The inlet chamber may generally be of different shapes and sizes. It can be a separate chamber at the end of a gas channel or multiple gas channels, or it can be just an end section of a gas channel in which the direction of gas stream is changed from mainly horizontal to vertical.
[0037] The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the presently preferred, but nonetheless illustrative, embodiments in accordance with the present invention, when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS
[0038] Fig. 1 shows the schematic diagram of boiler with a dry CFB scrubber according to prior art. [0039] Fig. 2 shows the schematic cross sectional diagram of a detail of a scrubber according to a first embodiment of the present invention.
[0040] Fig. 3 shows the schematic cross sectional diagram of a detail of a scrubber according to a second embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0041 ] The schematic diagram of FIG 1 shows a boiler (10) comprising a furnace (12) and an exhaust gas channel (14) with a heat recovery area (HRA) (16). The exhaust gas channel is connected downstream of the HRA to a conventional dry CFB scrubber (18) for removing pollutant compounds, such as sulfur oxides, from the exhaust gas. The temperature of the exhaust gas entering into the scrubber, i.e. the temperature downstream the HRA, is typically about 150 °C [0042] Conventional dry CFB scrubbers, as well as dry CFB scrubbers according to present invention, can be used in connection with any types of boilers, such as CFB boilers or PC boilers, or with many other types of plants, such as cement kilns, glass furnaces and waste incineration plants. It is a common feature to the plants that they all produce a stream of exhaust gas comprising similar pollutant compounds, such as SO2, SO3, HCI, HF, dioxins, furans and heavy metals including Hg.
[0043] The dry CFB scrubber (18) shown in FIG. 1 comprises an inlet chamber (20) arranged at a lower portion of the scrubber for directing the exhaust gas stream upwards to form a vertical gas stream. The inlet chamber may be actual separate chamber or it may be, for example, a curved end section of the exhaust gas channel (14). It is also possible that there are two or even more than two exhaust gas channels leading to a single inlet chamber. Such multiple exhaust gas channels may convey exhaust gas either from a single plant or from multiple separate plants.
[0044] Above and in flow connection with an upper end (22) of the inlet chamber is arranged a vertical constriction section (24), actually a venturi nozzle, for accelerating the vertical gas stream. The constriction section (24), which is symmetrical about a vertical axis (26), renders possible to maintain a fluidized bed in a vertical reaction chamber (28) arranged above and in flow connection with the constriction section. [0045] The reaction chamber (28) comprises conventional feeding means (30), such as a lance or a feeding screw, for introducing reagent, such as calcium hydroxide, Ca(OH)2, to the reaction chamber for converting pollutant compounds in the exhaust gas to harmless compounds, i.e., reaction products of the scrubber, such as CaSO4. The reagent can alternatively be introduced into the scrubber, for example, by using feeding means (30') connected to the exhaust gas channel (14) upstream of the scrubber (18). In order to facilitate the conversion of the pollutant compounds to the reaction products, the reaction chamber usually also comprises means (32), such as one or more nozzles, for feeding water to the reaction cham- ber for humidifying and cooling down the fluidized bed forming into the reaction chamber.
[0046] A discharge channel (34) is connected in the upper portion of the reaction chamber (28) for discharging gas and particles, including the reaction products and possible unreacted reagent, from the reaction chamber to a particle separator (36). The particle separator is usually a fabric filter, but in some cases it may alternatively be other type of separator, for example, an electrostatic precipitator. Particles, including the reaction products and unreacted reagent, are separated from the exhaust gas in the particle separator (36), and the remaining cleaned exhaust gas is directed through a stack (38) to the environment.
[0047] The particles separated in the particle separator (36) are collected to a collecting device (40), such as a series of bottom hoppers, at the bottom of the particle separator. A portion of the separated particles, the share of which can be controlled by a particle flow controlling device (42), is directed to a silo (44) to be removed from the system. In order to maintain a particle bed in the reaction chamber (28), and also because the separated particles may still contain a considerable portion of unreacted reagent, another portion of the separated particles is continuously recycled through a return channel (46) back to the reaction cham- ber (28). The return channel may also comprise reagent feeding means (30"), to be used alternatively or in addition to the reagent feeding means described above.
[0048] According to the conventional construction shown in FIG. 1 , the return channel (46) is connected to the side of an upper portion of the constriction section (24). Alternatively it can be connected directly to a lower portion of the reaction chamber (28). It has, however, been found that such arrangements for connecting the return channel may in some applications lead to harmful effects, such as fouling, agglomeration and coarsening of the bed in the reaction chamber (28).
[0049] According to another conventional construction, the return channel is connected to the side of the inlet chamber (20) (not shown in FIG. 1 ). This, however, has the disadvantage that it may be necessary to add to the stream of exhaust gas a stream of recirculated gas (not shown in FIG. 1 ) in the inlet chamber to maintain a gas velocity which is needed to entrain a sufficient amount of recycled particles to the reaction chamber, especially at low loads.
[0050] The schematic diagrams of FIGS. 2 and 3 show a vertical cross section of two alternatives for a detail to be used in a conventional dry CFB scrubber according to FIG. 1 . More particularly, FIGS. 2 and 3 show the arrangement for returning particles from the particle separator (36) back to the reaction chamber (28), according to two embodiments of the present invention.
[0051 ] FIG. 2 shows a constriction section (48) of a dry CFB scrubber comprising a single venturi nozzle (50) and a return channel (52) for recycling particles from a particle separator. The return channel comprises a vertical end section (54) arranged within the venturi nozzle (50). The lower end of the vertical end section (54), comprising an opening (56) to the upper end of an inlet chamber (58), is vertically aligned with the lower end of the venturi nozzle (50).
[0052] Particles returned from the particle separator flow down in the vertical end section (54) towards the inlet chamber (58). The recycled particles arriving in the inlet chamber encounter a vertical high velocity gas stream (60) flowing from the inlet chamber (58) towards the venturi nozzle (50). A fine fraction (62) of the recycled particles is then entrained with the vertical gas stream to a reaction chamber (64) above the constriction section. At the same time, a coarse fraction (66) of the recycled particles, which is not entrained with the vertical gas stream, sinks to a bottom hopper (68) at the lower portion of the inlet chamber. The coarse fraction is advantageously discharged through a discharge duct (70) from a bottom hopper of the inlet chamber to a silo.
[0053] Even distribution of particles to the venturi nozzle (50) is advantageously improved by arranging a dispersion piece (72) below the lower end of the vertical end section (54) so as to cause an outwards directed horizontal momentum to the recycled particles. The dispersion piece (72) comprises advantageously an upwards directed right circular cone having an aperture of 60 - 120 degrees, preferably of 80-100 degrees. [0054] According to an advantageous embodiment of the present invention the dispersion piece is connected to a lifting mechanism (74), whereby the vertical position of the dispersion piece can be adjusted. When the dispersion piece is raised higher, the recycled particles hit the upper surface of the dispersion piece at a higher level and encounter a higher velocity gas stream, and more particles are entrained by the vertical gas stream. Thus, by adjusting the vertical position of the dispersion piece it is possible to control the portion of particles to be separated to the bottom of the inlet chamber to be removed from the scrubber.
[0055] Present invention differs from the prior art shown in FIG. 1 , for ex- ample, in that the return channel (52) is connected to the constriction section (48) at a relatively lower level. In order to maximize the thus obtained benefit of decreasing the total height of the scrubber system, the vertical end section (54) should be short. However, in order to distribute particles evenly to the venturi noz- zle (50), the vertical end section (54) has to have a sufficient height (76). Preferably the height is at least as large as the minimum width (78) of the venturi nozzle.
[0056] The distribution of particles to the venturi nozzle (50) can also be improved by having multiple return channels (52, 52'), or at least end sections of the return channels, so-called pipe sections, connected to the vertical end section (54). The number of return channels or pipe sections is preferably at least two, even more preferably at least three or four, most preferably at least six. [0057] FIG. 3 shows a second embodiment of the present invention, which is applicable in dry CFB scrubbers having a constriction section with multiple cir- cumferentially arranged venturi nozzles (80). This embodiment differs from that shown in FIG 2 especially in that the vertical end section (82) of the return channel (52) is not within a venturi nozzle but between the multiple circumferentially ar- ranged venturi nozzles (80). The lower end of the vertical end section (82), comprising an opening (84) to the upper end of the inlet chamber (58), is vertically aligned with the lower end of the venturi nozzles (80).
[0058] Particles returned from a particle separator flow down in the vertical end section (82) towards the inlet chamber (58). As in FIG. 2, the recycled particles arriving in the inlet chamber encounter a vertical high velocity gas stream (60) flowing from the inlet chamber (58) towards the venturi nozzles (80). A fine fraction (62) of the recycled particles is then entrained with the vertical gas stream to a reaction chamber (64) above the constriction section. At the same time, a coarse fraction (66) of the recycled particles, which is not entrained with the vertical gas stream, sinks to the bottom of the inlet chamber to be removed from the system.
[0059] Even distribution of particles to the venturi nozzles (80) is advantageously improved by arranging a dispersion piece (86) below the lower end of the vertical end section (82) so as to cause an outwards directed horizontal momentum to the recycled particles. At least most of the particles falling down from the vertical end section (82) hit the upper surface dispersion piece (86) and bounce outwards. The dispersion piece (86) comprises advantageously an upwards di- rected right circular cone having an aperture of 60 - 120 degrees, preferably of 80-100 degrees.
[0060] According to an advantageous embodiment of the present invention the dispersion piece (86) comprises a construction (88) having an upper surface with a front portion partially overlapping the end portion and means for adjusting the amount of overlapping, or other suitable construction which enables controlling of the aperture of the cone. By increasing the aperture, the horizontal momentum caused to the particles is increased. [0061 ] The return channel (52) may in the embodiment shown in FIG. 3, as well as in the embodiment shown in FIG. 2, advantageously comprise multiple in- parallel connected pipe sections (52, 52') in flow connection with a common vertical end section (54). In the embodiment of FIG. 3, the multiple in-parallel connected pipe sections are advantageously arranged symmetrically between the circum- ferentially arranged venturi nozzles (80). Typically there are six venturi nozzles (80) and six pipe sections (52, 52') arranged between the venturi nozzles.
[0062] The embodiment of FIG. 3 shows two gas channels (90) for introducing the pollutant laden gas to the inlet chamber (58). Generally the number of gas channels can be one or two, or even higher than two, such as three or four. Multiple gas channels provide the advantage that the distribution of gas in the inlet chamber is relatively even. Thereby the vertical gas streams directing to the different circumferentially arranged venturi nozzles (80) are nearly identical, and even- ness of the distribution of recycled particles to the circumferential venturi nozzles is improved.
[0063] While the invention has been described herein by way of examples in connection with what are at present considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but it is intended to cover various combinations or modifications of its features and several other applications included within the scope of the invention as defined in the appended claims. For example, as is clear to a person skilled in the art, the lifting mechanism of the dispersion piece shown in FIG. 2 can be used also in the embodiment of FIG. 3, and a dispersion piece with adjustable aperture, as shown in FIG. 3, can also be used in the embodiment of FIG. 2. It is also possible that the both controlling mechanisms are used in any of the embodiments. Also it is possible to use multiple gas channels, as shown in the embodi- ment of FIG. 3, in the embodiment of FIG. 2.

Claims

Claims
1 . A scrubber (18) for removing pollutant compounds from a gas stream, comprising
- at least one gas channel (90) for introducing the gas stream to a lower portion of the scrubber;
- an inlet chamber (58) at the lower portion of the scrubber for directing the gas stream upwards to form a vertical gas stream;
- a constriction section (48) having a vertical axis of symmetry and comprising one or more venturi nozzles (50, 80), the constriction section being above and in flow connection with an upper end of the inlet chamber (58) for accelerating the vertical gas stream;
- a reaction chamber (64) arranged above and in flow connection with the constriction section;
- means for introducing at least one reagent (30, 30', 30") to the reaction chamber for converting pollutant compounds in the gas stream to reaction products;
- a discharge channel (34) in flow connection with the reaction chamber for discharging gas and particles including the reaction products from the reaction chamber;
- a particle separator (36) in flow connection with the discharge channel for separating particles including the reaction products from the gas; and
- a return channel (52, 52') for continuously recycling a portion of the separated particles from the particle separator to the reaction chamber;
characterized in that the return channel comprises a vertical end section (54, 82) arranged at the axis of symmetry of the constriction section so as to direct recycled particles down towards the inlet chamber (58), wherein the vertical end section comprises a lower end arranged immediately below the constriction section (48) so as to impinge the recycled particles with the vertical gas stream therein to entrain a fine fraction (62) of the recycled particles with the vertical gas stream and to separate a coarse fraction (66) of the recycled particles to a lower portion of the inlet chamber (58)
2. A scrubber according to claim 1 , characterized in that the one or more ver- tical venturi nozzles (50, 80) comprise a lower portion with an upwards decreasing cross sectional area, the lower portion having a lower end, wherein the lower end of the vertical end section (54, 82) is vertically aligned with the lower end of the lower portion of the one or more venturi nozzles.
3. A scrubber according to claim 1 , characterized in that a dispersion piece (72, 86) is arranged adjacent the lower end of the vertical end section (54, 82) for causing horizontal momentum away from the axis of symmetry to the recycled particles.
4. A scrubber according to claim 3, characterized in that the dispersion piece (72, 86) comprises an upwards directed circular cone having an aperture of 60 - 120 degrees, preferably of 80-100 degrees.
5. A scrubber according to claim 4, characterized in that the dispersion piece (86) comprises a construction (88) for controlling the aperture of the circular cone.
6. A scrubber according to claim 3, characterized in that the vertical position of the dispersion piece (72) is adjustable by a lifting mechanism (74).
7. A scrubber according to claim 1 , characterized in that the return channel comprises multiple in-parallel connected pipe sections (52, 52') in flow connection with the vertical end section (54, 82).
8. A scrubber according to claim 1 , characterized in that the return channel comprises means (30") for introducing at least one particulate reagent for converting the pollutant compounds to reaction products.
9. A scrubber according to claim 1 , characterized in that the constriction section (48) comprises a single venturi nozzle (50) and the vertical end section (54) is arranged within the single venturi nozzle.
10. A scrubber according to claim 1 , characterized in that the constriction sec- tion (48) comprises multiple circumferentially arranged venturi nozzles (80), and the vertical end section (82) is arranged between the multiple circumferentially arranged venturi nozzles.
1 1 . A method of removing pollutant compounds from a gas stream in a scrub- ber (18), comprising the continuous steps of
- introducing the gas stream through at least one gas channel (90) to an inlet chamber (58) at a lower portion of the scrubber;
- directing the gas stream upwards in the inlet chamber (58) to form a vertical gas stream;
- accelerating the vertical gas stream in a constriction section (48) arranged above and in flow connection with the inlet chamber (58), the constriction section having a vertical axis of symmetry and comprising one or more venturi nozzles (50, 80);
- conveying the gas stream from the constriction section (48) to a reaction
chamber (64) arranged above the constriction section;
- introducing at least one reagent (30, 30', 30") to the reaction chamber for converting the pollutant compounds to particulate reaction products;
- discharging gas and particles including the reaction products from the reaction chamber through a discharge channel (34) to a particle separator (36); - separating particles including the reaction products from the gas in the particle separator (36), and
- recycling a portion of the separated particles from the particle separator
through a return channel (52, 52') to the reaction camber
characterized by
- directing recycled particles towards the inlet chamber (58) through a vertical end section (54, 82) of the return channel arranged at the axis of symmetry of the constriction section (48), wherein the vertical end section comprises a lower end arranged immediately below the constriction section, and
- impinging the recycled particles discharged through the lower end of the vertical end section (54, 82) with the vertical gas stream (60) to entrain a fine fraction (62) of the recycled particles with the vertical gas stream and to separate a course fraction (66) of the recycled particles to a lower portion of the inlet chamber (58).
12. A method of removing pollutant compounds from a gas stream according to claim 1 1 , characterized by causing a horizontal momentum to the separated particles by a dispersion piece (72, 86) arranged adjacent the lower end of the vertical end section (54, 82).
13. A method of removing pollutant compounds from a gas stream according to claim 12, characterized by the constriction section (48) comprising a single ven- turi nozzle (50) and the vertical end section (54) is arranged within the single ven- turi nozzle.
14. A method of removing pollutant compounds from a gas stream according to claim 12, characterized by the constriction section (48) comprising multiple cir- cumferentially arranged venturi nozzles (80) and the vertical end section (82) is arranged between the multiple circumferentially arranged venturi nozzles.
15. A method of removing pollutant compounds from a gas stream according to claim 1 1 , characterized by distributing the recycled particles the inlet chamber (58) in a location in which the temperature of the gas stream (60) is at least 100 °C and the vertical design velocity of the gas stream is at least 20 m/s, preferably at least 30 m/s.
PCT/EP2014/051941 2014-01-31 2014-01-31 A method of and a scrubber for removing pollutant compounds from a gas stream WO2015113627A1 (en)

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CN110585881A (en) * 2019-10-14 2019-12-20 苏州仕净环保科技股份有限公司 Flue gas desulfurization device

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EP0200695A1 (en) * 1985-04-23 1986-11-05 Abb Fläkt Ab Contact reactor
WO2005030368A1 (en) 2003-08-15 2005-04-07 Wuhan Kaidi Electric Power Co. Ltd. One tower-multibeds circulating fluidization large-scale dry desulfurizating process for flue gas
WO2006032288A1 (en) 2004-09-22 2006-03-30 Lurgi Lentjes Ag Flue-gas purification system
CN101402019A (en) 2008-11-06 2009-04-08 北京博朗环境工程技术股份有限公司 Flue gas desulfurization technique of return current circulating fluidized bed in flow equalizing field
CN201760230U (en) 2010-07-19 2011-03-16 北京博朗环保工程技术股份有限公司 Flue gas desulfurization reactor for circulating fluidized bed of uniform flow field

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EP0027398A1 (en) * 1979-10-08 1981-04-22 Air Industrie Process and installation for the treatment of a gas by contacting a stream of this gas with particles of solid matter
EP0200695A1 (en) * 1985-04-23 1986-11-05 Abb Fläkt Ab Contact reactor
WO2005030368A1 (en) 2003-08-15 2005-04-07 Wuhan Kaidi Electric Power Co. Ltd. One tower-multibeds circulating fluidization large-scale dry desulfurizating process for flue gas
WO2006032288A1 (en) 2004-09-22 2006-03-30 Lurgi Lentjes Ag Flue-gas purification system
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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN110585881A (en) * 2019-10-14 2019-12-20 苏州仕净环保科技股份有限公司 Flue gas desulfurization device
CN110585881B (en) * 2019-10-14 2023-09-26 苏州仕净科技股份有限公司 Flue gas desulfurization device

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