WET DESULFURIZATION PROCESS FOR TREATING FLUE GAS

Abstract: A process for treating a flue gas containing sulfur dioxide and fly ash is disclosed in which an aqueous absorbent liquid is sprayed into the glue gas stream, and the resulting gas is fed to a vertical, open-ended pipe to form a jet annular stream, so that portions of the SO₂ and the fly ash contained in the flue gas are transferred to the liquid phase. The jet annular stream is then sparged into the aqueous absorbent held in a well-mixed vessel to form a jet bubbling layer in a shallow, upper region of the aqueous absorbent, so that a greater part of the SO₂ and the fly ash are removed from the gas. Air is introduced into a deep lower region of said aqueous absorbent to provide streams of fine oxygen-containing gas bubbles throughout the whole vessel, so that the absorbed SO₂ and other sulfites are oxidized to form coarse gypsum crystals while COD of the absorbent is reduced.

Description: This invention relates to a wet desulfurization process for treating a flue gas and, more particularly, to a method of efficiently removing SO₂ and other air pollutants, especially HF, HCl and dust (fly ash) contained in a flue gas. Wet desulfurization processes in which a flue gas is contacted with a solution or slurry containing an absorbent for removing air pollutants such as SO₂ and fly ash therefrom are widely known. Various such processes have been hitherto proposed and a number of large commercial apparatuses are actually employed for the treatment of flue gas from thermal power boilers, etc. Of these processes, processes in which limestone is used as the absorbent and in which gypsum is produced as a by-product are most commonly used. As the SO₂ absorbing tower which represents the central feature of such processes, many types of absorbing towers have been adopted. Spray towers and packed towers are typical absorbing towers. However, spray towers have an advantage that the gas-liquid contact space is structurally sparse so that they withstand scaling and clogging, whereas packed towers provide a high SO₂ removal efficiency because the gas-liquid contact is effected in a non-linear path. On the other hand, the spray towers consume much electric power and require large installation costs because, for example, it is necessary to recirculate a large amount of absorbent slurry liquid by means of a large, high-water head pump in order to form fine particulates and to effect the gas-liquid contact. Further, since the absorbent slurry liquid recirculating pump must be actuated and stopped according to the start, the stop and the load variation of the boiler, the control of the operation becomes complicated. The packed towers, on the other hand, suffer from drawbacks because they require a large gas-liquid contact volume and because solids are apt to be accumulated, thereby causing clogging due to the complicated passages through which gas and absorbent slurry liquid flow and rendering the cleaning of scaling difficult. Additionally, neither type of tower can allow the apparatus to be made compact and, therefore, they require a large installation area. Especially, flue gas discharged from boilers using coal as fuel contains a relatively large amount of dusts called fly ash and, additionally, air pollutants, such as HCl and HF, depending upon the kind of coal. Thus, it is necessary to establish an economical method of treating flue gas in which the removal by absorption of these pollutants is also taken into consideration in addition to the removal of SO₂. The HCl and HF which are contained in the flue gas in amounts of, for example, as high as about 200 ppm and 150 ppm respectively, can react with the limestone supplied for absorbing SO₂ because of their tendency to react with calcium compounds to form CaCl₂ and CaF₂. During the course of such reactions, the removal by absorption of SO₂, which is the primary object, is obstructed. In wet, flue gas desulfurization systems using the conventional types of absorbing towers, therefore, phenomena of abrupt reduction of SO₂ removal rate and considerable reduction of limestone utilization factor as a result of the lowering of the reactivity of the limestone stone-containing slurry are often observed. The present invention has been made with a view toward overcoming the above-mentioned problems and has as its object the provision of a process by which a flue gas is efficiently purified by treating not only SO₂ but also fly ash, and in preferred instances HCl and HF, and which are contained in a large amount in coal boiler flue gas. In accordance with the present invention air pollutants such as sulfur dioxide and fly ash contained in a flue gas are removed therefrom by a process which comprises the steps of: (a) introducing the flue gas into a humidifying zone into which is densely sprayed a humidifying liquid containing gypsum and an absorbent, said absorbent being a calcium compound capable of reacting with SO₂, such as limestone or slaked lime, so as to bring the flue gas into direct contact with the humidifying liquid; (b) introducing the mixture of flue gas and humidifying liquid which is formed in said humidifying zone into a mixing zone having one or more vertical tubes each extending into a reactor and each having an inside diameter such that said mixture is throttled thereinto and flows therethrough at a high speed to form a jet flow which consists of gas, liquid and solid phases and in which the gas phase forms a continuous phase, so that a portion of the fly ash is caught by the liquid phase with a portion of the SO₂ being absorbed in the liquid phase; (c) injecting said jet flow into a slurry containing said absorbent which is contained in said reactor and which is maintained at a pH of from 3 to 5 to form in an upper portion of the slurry a layer which consists of gas, liquid and solid phases and in which the liquid phase forms a continuous phase, so that the SO₂ contained in the gas phase of said layer is absorbed in the slurry with the fly ash contained in the solid phase of said layer being caught by the slurry; (d) dispersedly introducing air into the deep lower region of said slurry to provide streams of fine oxygen-containing gas bubbles throughout the whole vessel including said layer in an upper portion of the slurry, so that the absorbed SO₂ and other sulfites are oxidized to form coarse gypsum crystals while chemical oxygen demand (COD) of said absorbent is reduced; (e) discharging from said reactor a portion of said slurry beneath said layer in an upper region thereof and recycling same to said humidifying zone as at least part of said humidifying liquid; and (f) withdrawing a portion of said slurry from said reactor to maintain the concentration of gypsum therein in a predetermined range. As the absorbent, there may be used any calcium compound which can react with SO₂, but limestone is generally used. It is preferred that a slurry containing the calcium absorbent such as slaked lime be sprayed densely into the gas which separates from the absorbent-containing slurry in the reactor, in order to substantially completely remove the air pollutants from the gas. The present invention will now be described in detail below with reference to the accompanying drawings, in which: Fig. 1 is a schematic illustration of a flue gas treating apparatus used in the present invention; and Fig. 2 is a schematic illustration of a flue gas treating apparatus used in the Example. Referring to Fig. 1, in which embodiment limestone is used as an absorbent, flue gas, discharged from a boiler, a heating furnace, an incinerator, a sulfuric acid plant or the like plant, is introduced into a flue gas inlet pipe 12 where make up water supplied through a conduit 11 is sprayed from a nozzle 19. Thereafter, the flue gas is humidified and cooled in a gas cooling and humidifying equipment 3 by spraying from a nozzle 20 an absorbing liquid which contains limestone and gypsum and which is supplied from a reactor 1 through conduits 2 and 13. The flue gas, which has generally a high temperature of about 130-200 °C, is first quenched by contact with water. Since, in the present invention, the flue gas is contacted with the absorbent-containing liquid, the pH of the liquid is not significantly lowered. This is advantageous from the standpoint of material quality required for the equipment. That is, when the flue gas is cooled directly with mere water, the pH is decreased to 1 or less so that a high quality material is needed for the treating equipment. On the other hand, in the present invention the pH is reduced only to about 4-5. Further, whilst solids are unavoidably formed as a result of the evaporation of water since liquid in which gypsum is dissolved is used, no clogging need occur because the step is performed in a spray apparatus having an uncluttered interior. Then, the mixture of the flue gas and the absorbent slurry is passed through a vertical pipe 4. Since the mass transfer efficiency is high because of the increase in surface area as a result of the fine liquid particles and because the mixture is passed through the pipe 4 at a high speed, portions of the SO₂, fly ash, HCl and HF are removed from the gas. Further, as a result of the contact with the absorbent slurry liquid, the SO₂ is transferred to the liquid in the form of sulfurous acid or sulfites which, in turn, react with O₂ remaining in the flue gas to form fresh gypsum within the pipe 4. However, since gypsum is originally contained in the liquid within the pipe 4 and since this gypsum serves as seed crystals, scaling of gypsum is prevented. The high speed, gas/liquid/solid three-phase stream in which the gas forms a continuous phase is injected from the lower end of the pipe 4 into an upper, shallow layer of the limestone- and gypsum-containing, absorbent slurry liquid at a pH of 3-5 contained in a reactor 1 to form a liquid/solid/gas three phase mixed layer (so-called "jet bubbling layer") in which the liquid phase forms a continuous phase, so that the SO₂ and the fly ash are removed from the gas almost completely. The jet bubbling layer is a layer in which gas-liquid contact between bubbles of the flue gas and the absorbent liquid is proceeding and in which the liquid phase forms a continuous phase. Both violent gas-liquid contact as a result of the splitting of bubbles caused upon impingement of the gas against the absorbing liquid by its energy and efficient gas-liquid contact as a result of violent deformation of the split bubbles in the jet bubbling layer are effected. Thus, in the jet bubbling layer in which the liquid phase forms a continuous phase and in which the gas-liquid cotact is effected, a high efficiecy of gas-liquid contact is attainable. Further, the height of the gas-liquid contact layer need be only 1 m or less so that compactness may also be attained. As a consequence, even though the pH of the absorbent slurry liquid is kept in the range of 3-5, it is possible to maintain a required SO₂ removal rate of at least 95 %. Thus, the utilization of the limestone becomes high while suppressing the amount of unreacted limestone, generally to no more than 1 % by weight. The presence of HF and HCl scarcely adversely affects the removal rate and utility factor. Moreover, since the flue gas is humidified and saturated, the wall of the pipe is always wet with water, so that no deposition of solids occurs on the inside walls due to the evaporation and concentration of the substances dissolved in the absorbing liquid which are caused in the interface of the gas-liquid phases between the flue gas and the absorbent liquid in the pipe. The removal of the air pollutants such as SO₂ is effected in two stages of absorption mechanism ie in the high speed continuous-gas stream within the pipe and in the jet bubbling layer outside of the pipe. Therefore, the depth of immersion of the gas injecting pipe (sparger pipe) into the absorbing liquid can be relatively shallow, so that the costs of the electric power required to operate the flue gas supplying fan are minimized. Especially, when the absorption is carried out in such a manner that gas bubbles are surrounded by the liquid, probability of the contact becomes so high that the air pollutants are removed effectively and the amount of the fly ash becomes less than 5 mg/Nm³. The limestone is supplied to the apparatus after being formed into a slurry. When, as in Fig. 1, this slurry having a high pH value is sprayed into the gas which separates from the absorbent slurry liquid, the SO₂ remaining in the gas is absorbed by the slurry so that the SO₂ removal rate is improved and a high performace apparatus can be obtained. Thus, to the gas leaving the jet bubbling layer, a mixture of limestone and a mother liquor and/or the absorbing liquid is sprayed from a nozzle 21 via conduits 9, 14 and 18. While the amount of this slurry is small, the SO₂ remaining in the gas is effeciently removed because the pH thereof is high. After removal of entrainments in a mist eliminator 5, the purified gas is discharged to the atmosphere from a discharge pipe 6. Simultaneously with the absorption of the SO₂ in the absorbing liquid, it is oxidized with air injected through a conduit 10 and a nozzle 22 into the absorbing liquid in the bottom of the reactor 1 to form sulfuric acid, so that the absorbing liquid becomes acidic. Because the absorbing liquid is acidic, the limestone supplied from the nozzle 21 is quickly dissolved therein and reacts swiftly with the sulfuric acid to form gypsum. Therefore, the content of unreacted calcium sulfite and unreacted limestone in the absorbing liquid is low. Consequently, there is an absence of the formation of hard scales which would otherwise be caused by the formation of gypsum as a result of the reaction of unreacted materials contained in the absorbing liquid entrainments which deposit in the mist eliminator 5 with SO₂ and O₂ contained in the exahust gas. The gypsum contained in the absorbing liquid in an amount of 5-30 % serves as seed crystals and can reduce the supersaturation of gypsum in the absorbing liquid. Thus, gypsum appears on the surface of the seed crystals rather than on the walls of the equipment, so that scaling by the formation of precipitates of gypsum can be avoided. The gypsum thus formed is discharged through a conduit 15 so as to maintain the concentration thereof in the absorbing liquid in the range 5-30 %, and is introduced into a gypsum separator 7 for recovery. A portion of the mother liquor separated in the gypsum separator 7 is withdrawn through conduits 16 and 17 so as to keep the concentration of calcium chloride, calcium fluoride and fly ash constant. The mother liquor is fed to a fly ash separator 8 for the removal of the fly ash and calcium fluoride and is then discharged from the system as waste water. According to the present invention, HCl, HF and fly ash contained in flue gas in addition to SO₂ can be treated at the same time. Neither HCl nor HF adversely affects the reactivity of CaCO₃. Moreover, since the removal of SO₂ is effected in two stages in the high speed sprayed stream in the vertical pipe and in the high speed bubbling flow outside of the pipe, a high desulfurization rate can be obtained. Since the two-stage mechanism can be effected in a compact space inside and outside of a single pipe, the apparatus becomes simple and economical. In addition, since the two-stage absorbing mechanism permits the gas-liquid contact to be performed effectively, a desired SO₂ removal rate can be attained even when the pH of the absorbing liquid is set in a low level. There is also obtained an advantage that the cost of the apparatus can be reduced since, of the two high speed sprayed flows, one of them also serves to effect gas-liquid direct contact for the purpose of cooling and humidifying the flue gas and the other also serves to provide the system with the absorbent such as limestone. The following example will further illustrate the present invention but does not limit the scope of claims thereto in any way. Example The type of the apparatus used for the treatment of flue gas was as shown in Fig. 2. A reactor 1 having circular cross section had a diameter of 1000 mm. The height of the liquid was 2000 mm. As sparger pipes, 12 vertical tubes with diameters of 1016 mm (4 inches) were used. The injection ports of the tubes were set at a depth of about 100 mm. Flue gas containing 800 ppm of SO₂, 80 mg/Nm³ of dust, 3 % of O₂, 90 mg/Nm³ of HCl and 170 mg/Nm³ of HF was introduced through a flue gas feed pipe 2 at a rate of 4000 Nm³/h into a gas humidifying and cooling device 3 where an absorbing liquid having a gypsum content of 10-20 % and supplied from a reactor 1 through conduits 12 and 13 was sprayed from a spray nozzle 20 at feed rate of 4 m³/h. The flue gas was fed to the sparger pipes 4 together with the sprayed slurry and jet bubbled into the absorbing liquid in the reactor 1 therefrom to remove SO₂, dust, HCl and HF. To the gas leaving the jet bubbling layer, a mixed slurry of about 10 kg/h of limestone, mother liquor and the absorbing liquid fed through conduits 9, 14 and 18 was sprayed from a nozzle 21 at a rate of 2 m³/h. After removing entrainments in a mist eliminator 5, the gas was discharged out of the system as a purified gas. The SO₂ thus absorbed was oxidized by air supplied through a conduit 10 and injected into the absorbing liquid from a nozzle 22 to form sulfuric acid which, in turn, was converted into gypsum by reaction with the above-mentioned limestone. The thus formed gypsum was discharged through a conduit 15 so as to maintain the concentration thereof in the absorbing liquid in the range of 10-20 % and was introduced into a gypsum separator 7 for the recovery thereof. As a result of the foregoing treatment of the flue gas, a high desulfurization rate of 93-96 % was obtained. The concentraton of dust at the exit portion was found to be 1-5 mg/Nm³, indicating that a high dust removal efficiency is obtainable. The concentrations of HCl and HF at the exit portion were trace. The grade of the gypsum recovered was high as shown below: CaSO₄·2H₂O 98.9 % (except dust) CaCO₃ 0.6 % CaSO₃ trace pH 6-7 Average particle size 60-80 µm In order to examine the desulfurization effect by the spraying of the absorbing liquid from the nozzles 20 and 21, the feed of the absorbing liquid to the nozzles 20 and 21 was stopped. Instead, limestone was fed at a position adjacent to the lower end of the sparger pipe through a 2.54 cm pipe. As a result, the desulfurization rate was decreased to 87-90 %. When, the immersion depth of the sparger pipes 4 in the liquid was increased to 200 mm, a desulfurization rate of 93-96 % was able to be obtained. Thus, it was revealed that the spraying of the absorbing liquid from the nozzles 20 and 21 was effective in considerably reducing the consumption of electric power by the flue gas feed fan. Deposits were observed on the inside wall of the sparger pipes 4 when the spraying of the slurry from the nozzle 20 stopped and, therefore, it was impossible to continue the operation for a long period of time.

Claims: A process for treating a flue gas containing sulfur dioxide and fly ash, comprising the steps of: (a) introducing the flue gas into a humidifying zone into which is densely sprayed a humidifying liquid containing gypsum and an absorbent, said absorbent being a calcium compound capable of reacting with SO₂, such as limestone or slaked lime, so as to bring the flue gas into direct contact with the humidifying liquid; (b) introducing the mixture of flue gas and humidifying liquid which is formed in said humidifying zone into a mixing zone having one or more vertical tubes each extending into a reactor and each having an inside diameter such that said mixture is throttled thereinto and flows therethrough at a high speed to form a jet flow which consists of gas, liquid and solid phases and in which the gas phase forms a continuous phase, so that a portion of the fly ash is caught by the liquid phase with a portion of the SO₂ being absorbed in the liquid phase; (c) injecting said jet flow into a slurry containing said absorbent which is contained in said reactor and which is maintained at a pH of from 3 to 5 to form in an upper portion of the slurry a layer which consists of gas, liquid and solid phases and in which the liquid phase forms a continuous phase, so that the SO₂ contained in the gas phase of said layer is absorbed in the slurry with the fly ash contained in the solid phase of said layer being caught by the slurry; (d) dispersedly introducing air into the deep lower region of said slurry to provide streams of fine oxygen-containing gas bubbles throughout the whole vessel including said layer in an upper portion of the slurry, so that the absorbed SO₂ and other sulfites are oxidized to form coarse gypsum crystals while chemical oxygen demand (COD) of said absorbent is reduced; (e) discharging from said reactor a portion of said slurry beneath said layer in an upper region thereof and recycling the same to said humidifying zone as at least part of said humidifying liquid; and (f) withdrawing a portion of said slurry from said reactor to maintain the concentration of gypsum therein in a predetermined range. A process according to Claim 1, further comprising densely spraying the gas leaving the slurry contained in said reactor with a slurry containing said absorbent. A process according to Claim 1 or Claim 2, wherein the gas leaving the slurry contained in said reactor is passed through a mist separator for further purification of the gas.

Inventors: HASHIMOTO, NAOBUMI, KAWAMURA, KAZUSHIGE, KOMATSUBARA, YOSHIAKI, KUMAGAI, AKIRA, OGAWA, YOSHIO, URATA, TOSHIAKI
Applicants: CHIYODA CORPORATION
Application year: 1988
Main classification: B01D53
IPC codes: B01D53