METHOD AND APPARATUS FOR CLEANING FLUE GAS

Abstract: In a method and an apparatus for cleaning a process gas, such as flue gas, of gaseous pollutants, such as sulphur dioxide, the gas is contacted in a space (5) with a particulate absorbent, such as lime, which is reactive with said gaseous pollutants, for conversion of said pollutants into separable, particulate pollutants. First the particles of the absorbent are dispersed by means of a means (20) in a carrier gas, such as air, said means communicating with at least one Venturi device (6) positioned in said space (5) and having a convergent inlet portion, at least one intake opening arranged downstream of the inlet portion and an outlet portion. Then the carrier gas and the particles dispersed therein are accelerated in said inlet portion and are at high velocity by means of said intake opening contacted with a pressurised flow of liquid, such as water, which is finely divided on the surface of said particles, before the particles are supplied to the process gas by means of said outlet portion.

Description: METHOD AND APPARATUS FOR CLEANING FLUE GAS Field of the Invention The present invention relates to a method and an apparatus for cleaning a process gas, such as flue gas, of gaseous pollutants, such as sulphur dioxide, sulphur trioxide, hydrogen chloride and hydrogen fluoride, by contacting the gas with an absorbent, such as lime, which is reactive with the gaseous pollutants, for conversion of these pollutants into separable, particulate pollutants. Description of the Prior Art The emission of acidifying gases, such as sulphur dioxide, from e.g. coal-fired power plants has become a major environmental problem. Several different methods have been tried in order to reduce emissions of this type. To this end, it has been suggested both to purify the fuel and to take measures during the combustion and/or to clean the flue gas produced. Certain pollutants, e.g. nitrogen oxides, can be dissolved in harmless substances. The formation of pollutants can also be counteracted or obviated by optimising the combustion process. Other substances, such as sulphur, can however be taken care of only as a residual product and must not be emitted in any form in an uncontrolled manner. In order to remove e.g. sulphur, an absorbent is generally added, containing sub¬ stances which together with sulphur form stable compounds, either in the combustion chamber or in a specially design¬ ed flue gas cleaning system. Limestone or dolomite in the form of a finely milled or slightly coarser powder is generally added in the com¬ bustion chamber. In the flue gas cleaning system, finely milled limestone or slaked lime in water suspension is most often added. It has also been suggested to supply a dry absorbent to the flue gases. However, the reactivity of most absorbents that are available at reasonable costs is low at low temperatures and low relative humidity, which makes these techniques using a dry absorbent insuf¬ ficiently effective. Examples of prior art methods are given in SE-8005571-8 describing the injection of absorbent into the combustion chamber, DE-36,07,357 describing wet flue gas cleaning, SE-7904382-4 describing wet-dry flue gas cleaning, EP-0,177,896 describing completely dry flue gas cleaning, and SE-8505269-4 describing a combined solution. When supplying absorbent to the combustion chamber, the major drawback is that the degree of utilisation of the absorbent is very low. The wet flue gas scrubbers are far more efficient in this respect, but involve high investment costs and maintenance problems. Wet-dry flue gas cleaning involves lower investment costs but necessitates a more refined and thus more expen¬ sive absorbent as compared with the wet flue gas scrub¬ bers. To avoid precipitation of moisture in the flue gas cleaning system, the absorbent is supplied, in the wet-dry methods, loosely or suspended in an amount of water which is smaller than the amount which is necessary to cool the flue gas to a temperature below the saturation tempera¬ ture. Since the suspended absorbent must be supplied to the flue gas in the form of fine water droplets so that the formed surface of evaporation will be large enough to ensure complete evaporation of the supplied amount of water, the suspension cannot be made thicker than as to allow fine division. The supplied amount of liquid and, consequently, the amount of absorbent added thus are limited by the heat content of the flue gas. The completely dry methods entail the lowest invest¬ ment cost for the flue gas cleaning systems, but most often require a highly refined and, thus, expensive absor¬ bent and do not permit high-grade cleaning. The main reason for this is that solid phase reactions are slow at the temperatures which are prevalent after a coal-fired boiler. For high-grade cleaning by means of a simple flue gas cleaning system which does not comprise means for produc¬ tion, transport or fine division of an absorbent suspended in water, different techniques have been suggested which are something between the dry and the wet-dry methods. For example, SE-7908674-0 suggests separate moisten¬ ing of the flue gases before being introduced in a bag filter whose bags have been coated with a dust cake of calcium hydroxide. SE-8504675-3 suggests instead that the absorbent be moistened with water, before being supplied to the flue gas. The amount of water must then not be greater than as to allow maintaining the powder form of the absorbent. Otherwise the absorbent cannot be dis¬ charged to the flue gas by means of the feed screw shown in the drawing. However, none of these techniques makes the flue gas cleaning sufficiently effective according to current environmental standards. Summary of the Invention Technical Problem It thus constitutes a problem to provide satisfactory separation of the gaseous pollutants, such as sulphur dioxide, of the flue gas, without using an absorbent sus¬ pended in water. One object of the present invention therefore is to provide a simple method for effective separation of gaseous pollutants from e.g. flue gas, without supplying an absorbent suspended in water to the polluted gas. A further object of the present invention is to pro¬ vide a simple apparatus for carrying out this method. Solution of the Problem According to the present invention, the above-men¬ tioned problem of providing satisfactory separation o f gaseous pollutants, such as sulphur dioxide, from a pro¬ cess gas, such as flue gas, is solved in that a particu- late absorbent, such as lime, which is reactive with the gaseous pollutants, is dispersed in a carrier gas, such as air, and that said carrier gas and the absorbent particles dispersed therein are accelerated and subsequently are at high velocity contacted with a pressurised flow of liquid, such as water, which is finely divided on the surface of said particles, before the particles are contacted with the process gas for conversion of the pollutants thereof into separable, particulate pollutants. Since the carrier gas and the particles dispersed therein are accelerated to a high velocity, before being contacted with a pressurised flow of liquid, a most finely divided flow of liquid is obtained on the surface of the particles. The absorbent which is contacted with the flue gases thus consists of fine, moistened high reactivity particles which quickly react with the gaseous pollutants of the gas and, by an absorption process, convert these into separable particulate pollutants. The carrier gas and the moistened particles dispersed therein are preferably retarded, before being supplied to the process gas. The flow of liquid is preferably supplied in such an amount to the carrier gas that the mass ratio of the sup¬ plied liquid to the absorbent particles dispersed in the carrier gas is in the range of 0.1-10, especially about 0.5. For separation of the gaseous pollutants of the gas according to the method described above, a means for dis¬ persing the particles of the absorbent in a carrier gas, such as air, is arranged in connection with a space for contacting a process gas, such as flue gas, containing gaseous pollutants with a particulate absorbent, such as lime, which is reactive with the gaseous pollutants. This means communicates with at least one Venturi device posi¬ tioned in said space and having a convergent inlet portion for accelerating the carrier gas and the particles dis¬ persed therein, at least one intake opening arranged down- stream of the inlet portion and adapted to supply a pres¬ surised flow of liquid, such as water, to the carrier gas and the accelerated particles dispersed therein, and an outlet portion for supplying the carrier gas and the mois¬ tened particles dispersed therein to said space. The Venturi device is preferably formed with a throat having a constant cross-sectional area, i.e. a constant hydraulic inner diameter. The outlet portion of the Venturi device can be divergent in order to retard the carrier gas and the moistened particles dispersed therein before they are supplied to the process gas, and its intake opening can be positioned in the outlet portion, preferably immediately downstream of the throat. The outlet portion of the Venturi device can have a constant cross-sectional area and its intake opening can be formed in the throat, preferably immediately upstream of its outlet portion. The Venturi device preferably is of circular cross- section and is formed with intake openings which can be symmetrically arranged in the inner surface of the diver¬ gent outlet portion or in the throat inner surface in a plane perpendicular to the symmetry axis of the Venturi device. Each intake opening preferable forms a mouth of an intake duct whose centre line makes an angle in the range of 10°-80°, especially 45°, with the symmetry axis of the Venturi device. Description of Proposed Embodiments The invention will now be described in more detail with reference to the accompanying drawings. Fig. 1 is a schematic view of a plant for cleaning flue gases from a coal-fired central boiler plant, said flue gas cleaning plant comprising an apparatus according to the present invention. Fig. 2 is a detailed view of a portion of the appa¬ ratus in Fig. 1. Fig. 3 is a detailed view of a different embodiment of the portion in Fig. 2. The flue gas formed during the combustion of coal in the central boiler plant 1 shown in Fig. 1 is conducted to an air preheater 2 adapted to transfer heat from the hot flue gas to combustion air which through a duct 2a is sup- plied to the' central boiler plant by means of a fan 3. The flue gas is then, without preceding separation of its fly ash, led through a duct 4 to an elongate tubular reactor 5 in which the flue gas is mixed with an absorbent which is reactive with the gaseous pollutants of the flue gas. The absorbent is supplied to the reactor via a Ven¬ turi device 6 positioned in the lower part of the reactor, see Fig. 2 or 3, in the form of fine, moistened particles. The production of these particles will be described fur¬ ther down in the specification. In the tubular reactor 5, the absorbent particles are effectively mixed with the flue gas, the amount of mois¬ ture supplied with the particles being evaporated during its contact with the flue gas, at the same time as the gaseous pollutants, such as a sulphur dioxide, of the flue gas react with the particles and are converted into parti¬ culate pollutants. The amount of moisture supplied is selected relative to the temperature of the flue gas so that the flue gas is not cooled to a temperature below the saturation temperature, thereby avoiding precipitation of moisture. As the tubular reactor 5 has approximately the same cross-sectional area as the duct 4, the velocity of the flue gas does not significantly decrease when conducted into the tubular reactor. The flue gas thus maintains a sufficiently high velocity to entrain also the heavier particles of the particulate pollutants formed during the reaction, the unreacted absorbent particles and the fly ash of the flue gas to an electrostatic precipitator 7 positioned downstream of the tubular reactor. In the precipitator, the particles of the flue gas are separated, whereupon the flue gas cleaned of particulate and gaseous pollutants is passed through a duct 8 to a flue gas fan 9. The fan 9 discharges via a duct 10 the cleaned flue gas to a chimney 11 for emission into the atmosphere. The particles which are separated in the precipitator 7 are collected in three dust hoppers 12, 13 and 14 formed in the bottom of the precipitator. The particles collected in the first dust hopper 12, about 90% of the total amount of dust, are supplied via a conduit 15 to a recycling silo 16. The particles collected in the hoppers 13 and 14 are instead conducted via a conduit 17 to a storage silo (not shown). The separated particles which are stored in the recycling silo 16 are, in a conduit 19, mixed with fresh, dry absorbent particles, preferably calcium hydroxide, which are stored in a silo 18. The amount of fresh absor- bent supplied from the silo 18 to the conduit 19 is deter¬ mined in conventional manner by the residual emission which is measured in the duct 8 after the precipitator. The amount of recycled absorption material supplied to the conduit 19 is determined according to the amount of fresh absorption material supplied, so that, at a certain load, a constant amount of absorption material is supplied to the tubular reactor 5. A fan or compressor 20 supplies fresh air to the con¬ duit 19 at such a pressure, about 0.5 bar, that the air can mix the fresh absorbent particles with the recycled particles and supply these in dispersed form to the Ven¬ turi device 6 which is mounted in the upper end of the conduit 19. In the embodiment shown in Fig. 2, the Venturi device 6 consists of two circular parts 21 and 22 which are interconnected by means of a nut 23. The first part 21 which is fixedly mounted in the upper end of the conduit 19, comprises a convergent inlet portion 21a having a length which is between 40 and 90 mm, usually about 60 mm, and an inner diameter which is between 40 and 120 mm,, usually about 80 mm, at its upstream end, and a throat portion 21b having a constant inner diameter which is between 20 and 60 mm, usually about 40 mm. The part 22 also comprises a throat portion 22a having the same inner diameter as the throat portion 21b and forming, together with the throat portion 21b, a throat having a length which is between 20 and 40 mm, usually about 30 mm, and a divergent outlet portion 22b having a length which is between 20 and 80 mm, usually about 30 mm, and an inner diameter which is between 30 and 80 mm, usually about 5.0 mm, at its downstream end. The throat portion 22a is formed with a circumferential cavity 24 into which a water conduit 25 opens. The cavity 24 communicates with the interior of the Venturi device by means of four intake ducts 26 whose intake openings 27 are symmetrically arranged in the inner surface of the divergent outlet por- tion 22b immediately downstream of the throat portion 22a. When the air and the absorption particles dispersed therein flow into the convergent portion 21a of the Ven¬ turi device, the air and, thus, the particles are acce¬ lerated to a velocity which is in the range of 70-150 m/s, especially 100 m/s, before it flows into the throat 21b and 22a. When the air passes the intake openings 27, it is contacted with a flow of water which is, at a high pres¬ sure, about 15 bar, supplied to the interior of the Ven¬ turi device via the conduit 25, the cavity 24, the intake ducts 26 and the intake openings 27. This results in an extremely high degree of fine division of the flow of water adhering to the surface of the absorbent particles as very small droplets. The powder form of the absorbent thus is maintained, although the absorbent now contains such an amount of moisture as to have high reactivity. The moistened absorbent particles are then supplied via the divergent outlet portion 22b to the tubular reactor 5, see Fig. 1. Since the intake ducts are formed in the divergent outlet portion in such manner that their centre lines make an angle of about 45° with the symmetry axis of the Ven¬ turi device, see Fig. 2, and since their intake openings are positioned immediately downstream of the throat por¬ tion 22a, water is prevented from being sprayed on the inside of the Venturi device and, consequently, no dust is built up thereon. The building-up of dust is also counter- acted by the fly ash which is present in the recycled absorption material and contains aluminium and silicon dioxide which act as abrasives. In the embodiment shown in Fig. 3, the Venturi device 6 consists of two circular parts 31 and 32. The first part 31 which is fixedly mounted in the upper end of the con- duit 19 comprises a convergent inlet portion 31a having a length which is between 40 and 90 mm, usually about 60 mm, and an inner diameter which is between 40 and 120 mm, usually about 80 mm, at its upstream end, and a throat portion 31b having a constant inner diameter which is between 20 and 60 mm, usually about 40 mm. The part 32 also comprises a throat portion 32a which has the same inner diameter as the throat portion 31b and forms, together with the throat portion 31b, a throat having a length which is.between 20 and 60 mm, usually about 40 mm, and an outlet portion 32b having a length which is between 1 and 5 mm, usually about 2 mm, and a constant inner dia¬ meter which is between 20 and 60 mm, usually about 40 mm. The throat portion 32a is formed with a circumferential -cavity 34 into which a water conduit 35 opens. The cavity 34 communicates with the interior of the Venturi device by means of four intake ducts 36 whose intake openings 37 are symmetrically arranged in the inner surface of the throat portion 32a immediately upstream of the outlet portion 32b. Further the intake ducts are formed in the throat portion 32a in such manner that their centre lines make an angle of about 45° with the symmetry axis of the Venturi device. The parts 31 and 32 are interconnected by means of the left end portion 33 of the part 32, said end portion being formed as an internally threaded sleeve which is fastened on threads made on the outer surfaces of the throat portion 31b and the conduit 19. The embodiment of the Venturi device 6 which is shown in Fig. 3 functions in essentially the same manner as the Venturi device in Fig. 2, except that the moistened absor¬ bent particles are not retarded in a divergent outlet por¬ tion, before being supplied to the tubular reactor 5. Example 3 A flow of flue gas of 4900 N /h having a temperature of 166°C, a velocity of 11 m/s and a sulphur dioxide con- 3 centration of 1020 mg/Nm was supplied to a tubular reac¬ tor having a diameter of 0.5 and a length of 16 m. The sojourn time of the flue gas in the tubular reactor thus was about 1.6 s. The moistened absorbent supplied to the tubular reactor consisted of 7 kg of calcium hydroxide, 483 kg of recycled dust and 240 1 of water an hour, and the mass ratio of the supplied water to the absorbent thus amounted to 0.49. The saturation temperature in the tubu¬ lar reactor amounted to 52°C. Downstream of the electrostatic precipitator, the 3 sulphur dioxide concentration amounted to 95 mg/Nm , while the temperature was 75°C. The moisture content of the dust separated in the precipitator amounted to 0.8% by weight. The degree of separation for sulphur dioxide thus was 91%. Conclusion The invention is, of course, not limited to the embo¬ diment described above, but can be modified in various ways within the scope of the " appended claims. For example, the tubular reactor 5 can be provided with several, e.g. four, Venturi devices instead of a single Venturi device. However, they should be arranged on the same level in the reactor, thereby eliminating the risk that they spray dust on each other. For example, the Venturi device 6 can be of rectan- gular cross-section, instead of circular. For example, the reactor can be a large vessle having a ratio of the reactor height/length to the hydraulic dia¬ meter in the order of 2-5, instead of an elongate tube having a corresponding ratio in the order of 10-40. The Venturi devices will in this variant be positioned in the upper part of the reactor, instead of in the lower part thereof. For example, the precipitator 7 can be a fabric fil¬ ter, such as a bag filter, instead of an electrostatic precipitator. For example, the fresh absorbent can be calcium oxide instead of calcium hydroxide. For example, the flue gas cleaning plant can also be provided with a precipitator positioned upstream of the tubular reactor.

Claims: CLAIMS 1. Method for cleaning a process gas, such as flue gas, of gaseous pollutants, such as sulphur dioxide, said gas being contacted with a particulate absorbent, such as lime, which is reactive with said gaseous pollutants, for conversion of said pollutants into separable, particulate pollutants, the particles of the absorbent being dispersed in a carrier gas, such as air, c h a r a c t e r ¬ i s e d in that said carrier gas and the particles dis¬ persed therein are accelerated and subsequently are at high velocity contacted with a pressurised flow of liquid, such as water, which is finely divided on the surface of said particles, before the particles are supplied to said process gas. 2. The method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that said carrier gas and the moistened particles dispersed therein are retarded before being sup- plied to said process gas. 3. The method as claimed in claim 1 or 2, c h a ¬ r a c t e r i s e d in that the flow of liquid is sup¬ plied in such an amount to said carrier gas that the mass ratio of the supplied liquid to the absorbent particles dispersed in the carrier gas is in the range of 0.1-10, especially about 0.5. 4. Apparatus for carrying out the method according to claim 1, comprising a space (5) for contacting a process gas, such as flue gas, containing gaseous pollutants with a particulate absorbent, such as lime, which is reactive with said gaseous pollutants, for conversion of said pol¬ lutants into separable, particulate pollutants, and a means (20) for dispersing the particles of the absorbent in a carrier gas, such as air, c h a r a c t e r - i s e d in that said means communicates with at least one Venturi device (6) positioned in said space (5) and having a convergent inlet portion (21a; 31a) for accelerating said carrier gas and the particles dispersed therein, at least one intake opening (27; 37) arranged downstream of the inlet portion and adapted to supply a pressurised flow of liquid, such as water, to said carrier gas and the accelerated particles dispersed therein, and an outlet portion (22b; 32b) for supplying said carrier gas and the moistened particles dispersed therein to said space (5) . 5. The apparatus as claimed in claim 4, c h a ¬ r a c t e r i s e d in that said Venturi device (6) is formed with a throat (21b, 22a; 31b, 32a) having a con¬ stant cross-sectional area. 6. The apparatus as claimed in claim 5, c h a ¬ r a c t e r i s e d in that the outlet portion (22b) of said Venturi device (6) is divergent, and that its intake opening (27) is arranged in said outlet portion, prefer¬ ably immediately downstream of said throat (21b, 22a). 7. The apparatus as claimed in claim 5, c h a ¬ r a c t e r i s e d in that the outlet portion (32b) of said Venturi device (6) has a constant cross-sectional area, and that its intake opening (37) is arranged in said throat (31b, 32a), preferably immediately upstream of its outlet portion (32b). 8. The apparatus as claimed in claim 6, c h a ¬ r a c t e r i s e d in that said Venturi device (6) is of circular cross-section and is formed with intake openings (27) which are symmetrically arranged in the inner surface of said divergent outlet portion (22b) in a plane perpen¬ dicular to the symmetry axis of said Venturi device. 9. The apparatus as claimed in claim 7, c h a - r a c t e r i s e d in that said Venturi device (6) is of circular cross-section and is formed with intake openings (37) which are symmetrically arranged in the inner surface of said throat (31b, 32a) in a plane perpendicular to the symmetry axis of said Venturi device. 10. The apparatus as claimed in any one of claims 4-9, c h a r a c t e r i s e d in that each intake open¬ ing (27; 37) forms a mouth of an intake duct (26; 36) whose centre line makes an angle in the range of 10°-80° especially 45°, with the symmetry axis of said Venturi device (6).

Inventors: AAHMAN STEFAN, BRINGFORS NILS, JOHANSSON LARS-ERIK, AAHMAN, STEFAN, BRINGFORS, NILS, JOHANSSON, LARS-ERIK
Applicants: FLAEKT AB, ABB FLAEKT AKTIEBOLAG
Application year: 1990
Main classification: B01D53
IPC codes: B01D53, F23J15