Exhaust gas treatment method and exhaust gas treatment device

Abstract

A simple method for uniformly spraying a humidifying liquid into exhaust gas in a liquid-phase continuous wet exhaust gas treatment method for removing sulfur oxides from exhaust gas and recovering them as gypsum is provided. The method comprises injecting the humidifying liquid downward in a region where the exhaust gas flows vertically downward.

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas treatment method and an exhaust gas treatment device. [Background technology]

[0002] One flue gas treatment (flue gas desulfurization) method for removing sulfur oxides, especially sulfur dioxide (sulfur dioxide), from exhaust gases such as combustion exhaust gas (flue gas) is the wet method, in which the exhaust gas is brought into contact with an absorbing solution containing an alkaline agent to absorb and remove sulfur oxides.The wet method is suitable for treating large quantities of exhaust gas with high sulfur oxide concentrations at low cost, and is therefore widely used to treat exhaust gases from large-scale waste incinerators and coal-fired boilers in thermal power plants.

[0003] Wet exhaust gas treatment methods include a method (gas phase continuous type) in which the exhaust gas comes into contact with the absorbing liquid by spraying the absorbing liquid into the exhaust gas flow or by flowing the exhaust gas through a packing bed while letting the absorbing liquid flow down the surface of the packing, and a method (liquid phase continuous type) in which the exhaust gas comes into contact with the absorbing liquid by blowing the exhaust gas into the absorbing liquid contained (filled) in a container. Generally, the liquid phase continuous type has the advantage that it is more resistant to load fluctuations than the gas phase continuous type, has high gas-liquid contact efficiency, allows for the equipment to be made more compact, and does not require a large-capacity liquid circulation pump, thereby reducing power costs.

[0004] Known liquid-phase continuous wet exhaust gas treatment devices include jet bubbling reactors (JBRs) described in Patent Documents 1 and 2. FIG. 1 is a schematic diagram of the JBR described in Patent Document 1. As shown in FIG. 1 , a JBR generally has the following configuration: the internal space of an upright cylindrical vessel (reaction tank) 101 (not limited to a cylindrical vessel with a circular cross section but including a rectangular tubular vessel with a rectangular cross section) is vertically divided into three sections; a lower chamber (reaction chamber) 102 contains an absorbing liquid 104, leaving a space 103 above the liquid surface; exhaust gas introduced into a middle chamber (exhaust gas inlet space) 105 is blown below the surface of the absorbing liquid 104 through a plurality of exhaust gas dispersion pipes (exhaust gas introduction pipes) 106 extending from the floor of the middle chamber 105 into the absorbing liquid 104 in the lower chamber 102; the blown exhaust gas rises through the absorbing liquid 104 as fine bubbles and separates into space 103 above the liquid surface; the gas after gas-liquid contact treatment (hereinafter referred to as "treated gas") is introduced into an upper chamber (exhaust gas outlet space) 107 and discharged.

[0005] The absorption liquid is prepared by adding an alkaline agent (usually a slurry of finely ground limestone) to water (usually industrial water). Dissolved oxygen is supplied to the absorption liquid by injecting an oxygen-containing gas, such as air, through an oxygen supply (oxygen-containing gas injection nozzle) 108 located near the bottom of the lower chamber. Sulfur oxides (sulfur dioxide gas) contained in the exhaust gas that comes into contact with the absorption liquid are absorbed into the absorption liquid and converted to sulfurous acid. This sulfur dioxide is then oxidized by dissolved oxygen in the absorption liquid to sulfuric acid, which then reacts with the alkaline agent to produce calcium sulfate (gypsum). The resulting gypsum has low solubility and precipitates in the absorption liquid. The gypsum can be separated and recovered from the absorption liquid by withdrawing a portion of the absorption liquid containing the precipitated gypsum and performing an appropriate solid-liquid separation procedure. In the JBR, all processes—absorption, oxidation, neutralization, and crystallization—occur simultaneously within the lower chamber of the JBR main vessel (reaction tank). This allows the entire flue gas desulfurization system to be designed simply and compactly.

[0006] Furthermore, the JBR is characterized by its particularly high gas-liquid contact efficiency, even among liquid-phase continuous flue gas desulfurization systems. This is because, when flue gas is injected into the absorbing liquid at high speed, strong shear forces break down the flue gas bubbles into fine particles that mix with the absorbing liquid. The fine bubbles are densely dispersed in a region several to several tens of centimeters thick between the flue gas injection level (a plane at a predetermined depth from the liquid surface) and the liquid surface, forming a jet bubbling layer 109 with a large gas-liquid contact area. A stirring means 110 is installed below the jet bubbling layer formation region. This agitates and circulates the absorbing liquid up and down to evenly disperse the alkaline agent throughout the absorbing liquid and to supply dissolved oxygen supplied to the lower layer of the absorbing liquid (near the bottom of the lower chamber) to the upper jet bubbling layer.

[0007] In wet exhaust gas treatment systems like JBR, the exhaust gas must be cooled to its saturation temperature to efficiently absorb sulfur oxides (sulfur dioxide) in the exhaust gas into the liquid. Therefore, a dust removal tower (spray tower) is often installed upstream of the exhaust gas treatment system to also remove dust particles such as soot. Alternatively, as described in Patent Documents 1 and 2, a humidifying liquid is sprayed onto the exhaust gas entering the exhaust gas treatment system to humidify and cool the exhaust gas to its saturation temperature before contacting the absorbing liquid. In these cases, the humidifying liquid is typically sprayed into the middle chamber (exhaust gas introduction chamber) or into the exhaust gas introduction duct connected to the middle chamber. Some of the sprayed humidifying liquid is entrained in the exhaust gas and absorbed into the absorbing liquid in the lower chamber (reaction chamber), while some is separated from the exhaust gas in the middle chamber or the exhaust gas introduction duct and becomes a humidifying liquid drain.

[0008] The JBR of Patent Document 1 shown in Fig. 1 sprays water (first humidifying liquid) as a coolant from a coolant injection nozzle (coolant atomization nozzle) 112 provided in an exhaust gas introduction duct 111, and sprays water (second humidifying liquid) containing an alkaline agent as an absorption liquid from an absorption liquid injection nozzle (absorption liquid atomization nozzle) 113 provided in (the ceiling of) a middle chamber 105. Note that a portion of the absorption liquid in the lower chamber is extracted from an absorption liquid extraction line (drain pipe) 114, and after gypsum is separated and recovered by a solid-liquid separation operation (not shown), most of the remaining mother liquor is replenished with an alkaline agent and returned to the lower chamber from an absorption liquid supply line (absorbent supply pipe) 115, and a portion is extracted and sent to a wastewater treatment device to prevent the accumulation of dissolved salts, etc. in the absorption liquid. On the other hand, the treated gas introduced into the upper chamber (treated gas discharge chamber) 107 is discharged from a treated gas discharge duct (exhaust gas lead-out duct) 116 via a gas-liquid separator (mist eliminator) 117.

[0009] In Figure 1, the humidifying liquid (cooling liquid and absorbing liquid) sprayed into the exhaust gas, whether it is entrained in the exhaust gas or separated as drain from the exhaust gas (humidifying liquid drain), passes through the exhaust gas dispersion pipe 106 and is taken into the absorbing liquid 104 in the lower chamber 102. However, the humidifying liquid drain may be handled separately from the exhaust gas (and the humidifying liquid accompanying it) so as not to be mixed directly into the absorbing liquid in the lower chamber. Figure 2 is a schematic diagram of the JBR described in Patent Document 2. In Figure 2, reference numbers obtained by adding 100 to the reference numbers in Figure 1 indicate components in Figure 2 that correspond to the components indicated by the reference numbers in Figure 1. In the JBR of Figure 2, the cooling liquid (industrial water) serving as the first humidifying liquid is sprayed into the exhaust gas from a cooling liquid injection nozzle (industrial water supply pipe) 212 provided in the exhaust gas introduction duct (treated gas inlet) 211, as in the JBR of Figure 1, but the absorption liquid serving as the second humidifying liquid is a portion of the absorption liquid (alkali agent-containing liquid) 204 in the lower chamber (alkali agent-containing liquid chamber) 202 extracted from an absorption liquid extraction line 214, and this is sprayed not only from an absorption liquid injection nozzle (second humidifying liquid supply pipe) 213b provided in the middle chamber (humidifying liquid contact chamber) 205, but also from an absorption liquid injection nozzle (first humidifying liquid supply pipe) 213a provided in the exhaust gas introduction duct 211. At this time, most of the drain of the absorbing liquid sprayed from the absorbing liquid injection nozzle 213b provided in the middle chamber (humidifying liquid drain generated in the middle chamber) passes through the exhaust gas dispersion pipe (gas downcomer) 206 together with the exhaust gas and is mixed into the absorbing liquid 204, but the drain of the cooling liquid and absorbing liquid sprayed from the cooling liquid injection nozzle 212 and the absorbing liquid injection nozzle 213a provided in the exhaust gas introduction duct 211 (humidifying liquid drain generated in the exhaust gas introduction duct) flows down through the liquid downcomer 218, which is a passage separate from the exhaust gas dispersion pipe (gas downcomer) 206.

[0010] The tip of the liquid downcomer 218 penetrates the jet bubbling layer 209 and extends to a position deeper than the tip of the exhaust gas dispersion pipe (gas downcomer) 206, and due to the principle of a communicating pipe, the liquid (absorption liquid and humidifying liquid drain) is filled from the tip outlet upward to a certain depth, so that the humidifying liquid drain that flows down through it temporarily remains inside the liquid downcomer 218 and does not immediately mix with the main body of the absorption liquid. The liquid (mainly humidifying liquid drain) remaining inside the liquid downcomer 218 is then withdrawn by pump 220 through gas venting means (air separator) 219, and gypsum is separated and recovered from the withdrawn liquid by solid-liquid separation means 221. A portion of the remaining mother liquor is returned to the lower chamber via absorption liquid supply line 215 via circulation line 222, and the remainder is sent to the wastewater treatment device. In order to compensate for the decrease in the alkaline agent concentration in the absorption solution due to the recovery of gypsum, gypsum slurry is added from alkaline agent introducing means 223. The absorption solution in the lower chamber contains a large amount of dissolved oxygen, and may contain peroxides (such as hexavalent selenium) generated thereby, but when the absorption solution is sprayed into the flue gas in the flue gas introduction duct, it absorbs the sulfur dioxide gas in the flue gas and reduces these peroxides, so recovering gypsum only from the separated humidifying liquid drain as shown in Figure 2 has the advantage of preventing adverse effects on the wastewater treatment device. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 18429 / 1983 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-71141 Summary of the Invention [Problem to be solved by the invention]

[0012] In a liquid-phase continuous wet flue gas desulfurization plant like the JBR, flue gas is generally introduced into the plant almost horizontally from the side wall of the main unit (cylindrical vessel), as shown in Figures 1 and 2. For this reason, when humidifying liquid (coolant or absorption liquid) is sprayed onto the flue gas in the flue gas inlet duct connected to the main unit, the spray nozzles that spray the humidifying liquid are installed inside the flue gas inlet duct, which extends horizontally. In other words, the humidifying liquid is sprayed into the horizontal flow of flue gas. In order to spray the liquid uniformly into the flue gas flow, conventionally, multiple nozzles were uniformly arranged within the vertical cross section of the horizontal flue gas flow to spray the humidifying liquid horizontally.

[0013] However, when the cross-sectional area of ​​the horizontal flow of exhaust gas is large, a difference in hydraulic head occurs between the nozzles located at the top and the bottom, resulting in a higher injection pressure from the lower nozzle, resulting in uneven injection volume and velocity in the vertical direction, and also in uneven diameter of the injected droplets in the vertical direction (droplets injected from the lower nozzle are finer). One possible solution to this problem would be to adjust the nozzle diameter and arrangement density to make the injection volume and velocity uniform in the vertical direction, but such correction requires complex adjustments, and the amount of correction must also be adjusted depending on the properties of the humidifying liquid (viscosity and solids concentration), making such adjustments difficult. Furthermore, it is difficult to adjust the droplet diameter to be uniform using such methods.

[0014] Therefore, there is a demand for a method and apparatus that can spray humidifying liquid almost uniformly onto the flow of exhaust gas at all times with a simple configuration, without the need for the above-mentioned complicated adjustments. [Means for solving the problem]

[0015] The present invention provides an exhaust gas treatment method for removing sulfur oxides from exhaust gas and recovering the removed sulfur oxides as a solid reaction product, the method comprising: a gas-liquid contacting step in which exhaust gas containing sulfur oxides is introduced into an aqueous absorption liquid containing an alkaline agent and dissolved oxygen and a gas-liquid contacting operation is carried out to precipitate a reaction product as a solid in the aqueous absorption liquid resulting from a reaction between the alkaline agent, dissolved oxygen, and sulfur oxides; and a solid-liquid separation step in which a solid-liquid separation operation is carried out on the aqueous absorption liquid containing the precipitated solid reaction product to recover the solid reaction product. The method includes a humidifying and cooling step, prior to the gas-liquid contact step, in which a humidifying liquid is sprayed into the flow of the exhaust gas to humidify and cool the exhaust gas, and in the humidifying and cooling step, the humidifying liquid is sprayed vertically downward within a region in which the exhaust gas flows vertically downward, thereby solving the above-mentioned problem.

[0016] Further, the present invention provides a method for treating a tubular vessel having an internal space that is upright and is divided vertically into three compartments, namely, an upper compartment, a middle compartment, and a lower compartment, the lower compartment containing an aqueous absorption liquid containing an alkaline agent and dissolved oxygen, the side wall of the middle compartment being provided with an exhaust gas inlet for introducing exhaust gas, the floor of the middle compartment being provided with a plurality of exhaust gas dispersion pipes that open into the space within the middle compartment and penetrate the floor to extend vertically so that their tips enter the aqueous absorption liquid contained in the lower compartment, each exhaust gas dispersion pipe being provided with an ejection hole near its tip for ejecting exhaust gas below the surface of the aqueous absorption liquid, oxygen supply means being provided near the bottom of the lower compartment for blowing oxygen-containing gas into the aqueous absorption liquid, the ceiling of the lower compartment being provided with a treated gas ascending path that opens into the space within the lower compartment and penetrates the middle compartment to open into the space within the upper compartment, and the upper compartment being provided with a treated gas outlet for discharging the treated gas, In an exhaust gas treatment device configured such that exhaust gas introduced into the middle chamber from the exhaust gas inlet passes through the plurality of exhaust gas dispersion pipes and is ejected as bubbles below the liquid surface of the aqueous absorption liquid contained in the lower chamber, and then separated as a treated gas in a space formed above the liquid surface of the aqueous absorption liquid, and then flows into the upper chamber through the treated gas ascending path and is discharged from the upper chamber through the treated gas discharge port, The above-mentioned problem is solved by providing an apparatus characterized in that an exhaust gas inlet duct having a region extending vertically so that exhaust gas flows downward is connected to the exhaust gas inlet, and the region extending vertically is provided with humidifying liquid spray means for spraying humidifying liquid downward into the exhaust gas flowing downward within the exhaust gas inlet duct. [Effects of the Invention]

[0017] According to the present invention, in wet treatment of flue gas including a liquid-phase continuous gas-liquid contacting operation in which flue gas is introduced into an absorption liquid, a humidifying liquid is uniformly sprayed into the flue gas to be subjected to the gas-liquid contacting operation, so that the flue gas is uniformly humidified and cooled, and it is possible to prevent a part of the flue gas from being sprayed into the aqueous absorption liquid in a state where it is insufficiently humidified and cooled.

[0018] Furthermore, according to the present invention, contact between the exhaust gas and the humidifying liquid occurs in the region where the exhaust gas flows vertically, so even if the contact region is made long and sufficient contact time is provided to increase the humidifying and cooling effect of the exhaust gas, it is possible to prevent the site area (including the contact region) of the exhaust gas treatment device from becoming excessively large. [Brief explanation of the drawings]

[0019] [Figure 1] An example of a conventional liquid-phase continuous wet type exhaust gas treatment device is shown below. [Figure 2] Another example of a conventional liquid-phase continuous wet type exhaust gas treatment device is shown. [Figure 3] A preferred example of the liquid phase continuous wet type exhaust gas treatment device of the present invention will be described below. [Figure 4] An example of a gas dispersion pipe used in the apparatus shown in FIG. 3 is shown. [Figure 5] A form in which a plurality of moisturizing liquid spray nozzles are evenly arranged will be exemplified. [Figure 6] Another preferred example of the liquid-phase continuous wet-type exhaust gas treatment device of the present invention will be described. [Figure 7] The figure shows a comparison of the site area when the exhaust gas introduction duct is vertical and when it is horizontal. [Figure 8] 1 shows the portion of the exhaust gas inlet duct used in the examples that connects to the reaction tank. [Figure 9] The temperature distribution of the exhaust gas at cross sections S1, S2, and S3 in Fig. 8 is shown. [Figure 10] 1 shows the portion of the exhaust gas introduction duct used in the comparative example that connects to the reaction tank. [Figure 11] The temperature distribution of the exhaust gas at the X0 cross section in Figure 10 is shown. DETAILED DESCRIPTION OF THE INVENTION

[0020] The method of the present invention is carried out by filling a vessel with an aqueous absorbing solution containing an alkaline agent and dissolved oxygen, introducing a sulfur oxide-containing exhaust gas into the aqueous absorbing solution, and carrying out a gas-liquid contact operation. Such an aqueous absorbing solution can be prepared by adding an alkaline agent to water and then blowing an oxygen-containing gas into the solution.

[0021] Industrial water is typically used as the water, but natural or reclaimed water can also be used if the water quality is acceptable. However, if the goal is to recover high-quality gypsum, it is preferable to minimize the presence of polyvalent cations other than calcium, heavy metals, and organic matter. The alkaline agent added to the water neutralizes the sulfurous acid and sulfuric acid produced by the dissolution of sulfur oxides (mainly sulfur dioxide) in the aqueous absorption solution, ultimately forming gypsum. Therefore, a basic calcium salt such as calcium carbonate or calcium hydroxide is preferred. From a cost perspective, finely ground limestone (calcium carbonate) is typically used. Because calcium carbonate is practically insoluble in water, finely ground limestone is added to the water (aqueous absorption solution) as a slurry. However, if added to the aqueous absorption solution whose pH has become slightly acidic (around 5–7) after absorbing sulfur dioxide, the finely ground limestone added as a slurry quickly dissolves. The oxygen-containing gas is used to supply dissolved oxygen, which oxidizes sulfurous acid to sulfuric acid, to the aqueous absorption solution. Air is generally used, but other gases (e.g., pure oxygen or oxygen-enriched air) may also be used as long as they contain oxygen and do not contain any components that interfere with the reaction that forms gypsum.

[0022] In the gas-liquid contact step of the present invention, flue gas containing sulfur oxides is injected as fine bubbles into an aqueous absorption solution containing an alkaline agent and dissolved oxygen. The sulfur oxides in the flue gas dissolve into the aqueous absorption solution through the large gas-liquid contact interface. Most of the sulfur oxides are sulfur dioxide (sulfur dioxide), which dissolves in the aqueous absorption solution and combines with water molecules to form sulfite. This sulfur dioxide is partially ionized in the aqueous absorption solution to produce sulfite ions and bisulfite ions (and hydrogen ions), which are then oxidized by dissolved oxygen to sulfate ions (some of which are hydrogen sulfate ions). These sulfate ions combine with calcium ions provided by the dissolution of the alkaline agent (finely powdered limestone) to form calcium sulfate. Due to its relatively low solubility, gypsum (calcium sulfate hydrate) precipitates in the absorption solution as a solid reaction product. However, the actual reaction sequence does not necessarily proceed in the order described above and is thought to be more complex. For example, the oxidation of sulfite to sulfate (in either the liquid or solid phase), the formation of calcium sulfate or calcium sulfite, and the precipitation of solid reaction products (ultimately gypsum) may occur simultaneously.

[0023] In any case, once the gas-liquid contact step of the method of the present invention has progressed to a certain extent, the aqueous absorption liquid becomes an aqueous suspension containing gypsum as a solid reaction product. Therefore, if an appropriate solid-liquid separation operation is performed on the aqueous absorption liquid in the solid-liquid separation step of the method of the present invention, the gypsum precipitated in the absorption liquid can be separated and recovered. The mother liquor from which gypsum has been separated and removed can be reused as the aqueous absorption liquid by replenishing an alkaline agent (limestone slurry). In practice, the method of the present invention is often performed as a continuous operation rather than a batch operation, and the solid-liquid separation step of the method of the present invention involves withdrawing a portion of the aqueous absorption liquid from the gas-liquid contact step, subjecting it to solid-liquid separation operation to separate and recover gypsum, and then returning the mother liquor from which gypsum has been separated and removed (after replenishing an alkaline agent) to the gas-liquid contact step. That is, in the case of continuous operation, the gas-liquid contact step and the solid-liquid separation step of the method of the present invention are performed simultaneously in parallel.

[0024] The method of the present invention includes a humidifying / cooling step in which a humidifying liquid is sprayed into the flow of exhaust gas to be subjected to the gas-liquid contact step to humidify and cool the exhaust gas, and the aqueous absorption liquid (including the precipitated solid reaction product) extracted from the gas-liquid contact step is used as at least a part of the humidifying liquid to be sprayed.The humidifying liquid is characterized in that it is sprayed vertically downward into the flow of exhaust gas in a region where the exhaust gas flows vertically downward.In this case, it is preferable that the aqueous absorption liquid (including the precipitated solid reaction product) extracted from the gas-liquid contact step is used as at least a part of the humidifying liquid to be sprayed, the humidifying liquid sprayed into the exhaust gas is separated from the exhaust gas and recovered as a humidifying liquid drain, and only the recovered humidifying liquid drain is subjected to a solid-liquid separation operation to separate and recover the solid reaction product (mainly gypsum).

[0025] In the method of the present invention, humidifying liquid is sprayed vertically downward into flue gas flowing vertically downward, so that droplets of humidifying liquid sprayed from, for example, one nozzle placed at the center of the flow are dispersed symmetrically about the central axis of the flow. Furthermore, even when multiple nozzles are provided, if the nozzles are located on the same cross section, there is no difference in head between the nozzles. Therefore, by arranging the nozzles at an equal density relative to the cross section of the flow, it is possible to easily spray humidifying liquid evenly across the cross section of the flow. This ensures uniform humidification and cooling of the flue gas, preventing part of the flue gas from being sprayed into the aqueous absorption liquid in an insufficiently humidified and cooled state.

[0026] Furthermore, to improve the humidifying and cooling effect of the exhaust gas, it is necessary to ensure sufficient contact time between the exhaust gas and the humidifying liquid, but in the method of the present invention, the exhaust gas comes into contact with the humidifying liquid in an area where the exhaust gas flows vertically, so even if the contact area is long and sufficient contact time is provided, the site area of ​​the exhaust gas treatment device (including the contact area) can be prevented from becoming excessive. In particular, at JBR, the compactness of Device A is a feature, so the effect of the present invention, which prevents the site area from becoming excessive even if the contact area between the exhaust gas and the humidifying liquid before entering the device is long, can be said to be an extremely advantageous effect.

[0027] As described above, it is preferable that at least a part of the humidifying liquid sprayed into the flue gas is the aqueous absorbing liquid in the gas-liquid contact step. Moreover, when the gas-liquid contact steps are performed in a plurality of apparatuses, the aqueous absorbing liquid used as (a part of) the humidifying liquid does not have to be extracted from the gas-liquid contact step to which the flue gas to which the humidifying liquid is sprayed is supplied. For example, the aqueous absorbing liquids extracted from a plurality of gas-liquid contact steps may be pooled together and supplied from there as (a part of) the humidifying liquid to be sprayed into the flue gas to be supplied to each gas-liquid contact step. However, unless an amount of aqueous absorbing liquid corresponding to the amount of aqueous absorbing liquid extracted is replenished for each gas-liquid contact step, the amount of aqueous absorbing liquid will fluctuate, which is not preferable.

[0028] In a particularly preferred embodiment of the present invention, at least a part of the humidifying liquid sprayed into the flue gas is an aqueous absorption liquid in the gas-liquid contact step, and the part of the humidifying liquid sprayed into the flue gas before the gas-liquid contact step is an aqueous absorption liquid in the gas-liquid contact step. contact The humidifying liquid is separated and recovered from the exhaust gas flow before being supplied to the process, and only the separated and recovered humidifying liquid is supplied to the solid-liquid separation process for recovering the solid reaction product (gypsum). In other words, it is preferable that the absorption liquid in the gas-liquid contacting process is not directly supplied to the solid-liquid separation process (i.e., without being sprayed into the exhaust gas before being supplied to the gas-liquid contacting process).

[0029] In this embodiment, all of the aqueous absorption liquid subjected to the solid-liquid separation step comes into contact with the exhaust gas before being subjected to the gas-liquid contact step, and the solid reaction product (gypsum) in the humidification liquid drain is reduced by absorbing sulfur dioxide gas in the exhaust gas, making it possible to recover a gypsum slurry that does not elute peroxides or mercury ions generated by contact with dissolved oxygen in the gas-liquid contact step. This reduces the load on the wastewater treatment device that treats the mother liquor separated from the gypsum slurry. In other words, the same effects as those of the invention described in Patent Document 2 can be obtained.

[0030] However, in the method of the present invention, the humidifying liquid is sprayed vertically downward into the exhaust gas flow within the region where the exhaust gas flows vertically downward. Therefore, if a drain tank for collecting humidifying liquid drain is placed directly below that region and the exhaust gas flow is bent sideways just before the drain tank, the humidifying liquid present as a mist in the exhaust gas flow is separated from the exhaust gas flow toward the drain tank by inertial force (centrifugal separation effect). Therefore, the method of the present invention can improve the separation efficiency of humidifying liquid drain. In other words, the amount of humidifying liquid drain carried into the gas-liquid contact process can be reduced compared to the invention of Patent Document 2. Furthermore, by reducing the amount of gypsum carried into the gas-liquid contact process along with the exhaust gas, the effort required to clean gypsum adhering to the exhaust gas inlet portion of the equipment performing the gas-liquid contact process is reduced.

[0031] FIG. 3 shows a preferred example of a liquid-phase continuous wet exhaust gas treatment device (JBR) for carrying out the method of the present invention, in which the region just before the exhaust gas introduction duct connects to the exhaust gas inlet chamber (middle chamber) is configured vertically.

[0032] 3, the internal space of an upright cylindrical vessel 1 is vertically partitioned into three sections: an upper chamber (treated gas discharge chamber) 2, a middle chamber (exhaust gas introduction chamber) 3, and a lower chamber (reaction chamber) 4. An aqueous absorption liquid 11 containing an alkaline agent and dissolved oxygen is contained in the lower chamber 4. An exhaust gas inlet 12 for introducing exhaust gas is provided in the side wall of the middle chamber 3. A plurality of exhaust gas dispersion pipes 13 are provided in the floor of the middle chamber 3, each of which opens into the space within the middle chamber and penetrates the floor, extending vertically so that its tip enters the aqueous absorption liquid 11 contained in the lower chamber 4. An ejection hole 14 (see FIG. 4) is provided near the tip of each exhaust gas dispersion pipe for ejecting exhaust gas below the surface of the aqueous absorption liquid 11. An oxygen supply means 15 is provided near the bottom of the lower chamber 4 (at a position lower than the ejection holes 14) for blowing in an oxygen-containing gas (e.g., air) to supply dissolved oxygen to the aqueous absorption liquid, and a stirring means 16 is provided immediately above the oxygen supply means for stirring the aqueous absorption liquid in the vertical direction.

[0033] Exhaust gas containing sulfur oxides is introduced into the middle chamber 3 through an exhaust gas inlet 12, and then passes from the middle chamber through multiple exhaust gas dispersion pipes 13 to be injected below the liquid surface (at a depth of several to several tens of centimeters in a stationary state without blowing bubbles) of an aqueous absorbing liquid (hereinafter simply referred to as "absorbent liquid") 11 contained in the lower chamber 4. The exhaust gas injected below the liquid surface of the absorbing liquid 11 turns into numerous fine bubbles that slowly rise through the absorbing liquid, forming a jet bubbling layer 17 containing numerous fine bubbles as the uppermost layer of the absorbing liquid 11 (the layer region between the height of the liquid surface and the height of the nozzles). In this jet bubbling layer 17, sulfur oxides contained in the exhaust gas that form bubbles are dissolved and absorbed into the absorbing liquid through the interface between the bubbles and the absorbing liquid. In other words, gas absorption is carried out by gas-liquid contact. The bubbles in the jet bubbling layer are minute bubbles formed by repeated breakup of the bubbles due to shearing forces generated when the exhaust gas is blown in at high speed, and have an extremely large gas-liquid contact area (i.e., high gas-liquid contact efficiency). Moreover, because these bubbles rise slowly in the jet bubbling layer 17, the efficiency of absorption of sulfur oxide gases by the absorbing liquid through the gas-liquid contact interface is high.

[0034] The sulfur oxides captured in the absorbent 11 by the jet bubbling layer 17 are mostly sulfur dioxide gas (sulfur dioxide), which combines with water molecules to form sulfite. Sulfite generates sulfite ions or hydrogen sulfite ions in the absorbent, which are then oxidized by dissolved oxygen produced when oxygen injected from the oxygen supply means 15 dissolves in water to form sulfate ions (some of which are hydrogen sulfate ions). These sulfate ions combine with calcium ions supplied by the dissolution of the alkaline agent (limestone) to form calcium sulfate. Because calcium sulfate has a relatively low solubility, gypsum (calcium sulfate hydrate) precipitates in the absorbent as a solid reaction product. However, the actual reaction sequence does not necessarily proceed in the order described above and is likely to be more complex. In particular, the oxidation of sulfite ions to sulfate ions (in the liquid or solid phase), the generation of calcium sulfate (or calcium sulfite as a precursor), and the precipitation of the solid reaction product (ultimately gypsum) are likely to occur simultaneously to some extent.

[0035] The absorption liquid is agitated by the agitation means 16 and circulated vertically. Therefore, sulfur dioxide formed by the absorption of sulfur oxides in the uppermost jet bubbling layer 17, dissolved oxygen supplied by the oxygen supply means 15 installed at the bottom of the lower chamber 4, and gypsum fine particles formed and precipitated by the reaction of these with calcium ions of the alkaline agent circulate throughout the entire liquid phase consisting of the absorption liquid 11. However, these concentrations are not necessarily uniform throughout the liquid phase. Generally, the concentrations of dissolved oxygen and gypsum fine particles are higher in the lower layer, while the sulfur dioxide concentration is higher in the upper layer. Therefore, the oxidation-reduction potential (ORP) of the absorption liquid is generally higher in the lower layer (closer to the jet bubbling layer where sulfur dioxide gas dissolves) than in the upper layer. Therefore, the main body of the absorption liquid 11, excluding the jet bubbling layer 17, is generally oxidizing. Therefore, peroxides such as persulfuric acid and hexavalent selenium tend to be generated in the lower layer of the absorption liquid. On the other hand, the pH of the absorption liquid is generally higher in the lower layer than in the upper layer, but the pH is usually kept slightly acidic even in the lower layer to quickly dissolve the limestone slurry and prevent the precipitation of new calcium carbonate.

[0036] In the system shown in Figure 3, flue gas flows downward through flue gas inlet duct 18, which extends vertically beside the flue gas desulfurization system, turns horizontally (in an L-shape) beside flue gas inlet 12, and flows from flue gas inlet 12 into middle chamber 3. Within this vertically extending area, multiple humidifying liquid spray nozzles 19 are arranged horizontally at equal intervals in one to three rows (two rows in the figure), and humidifying liquid is sprayed downward from each nozzle into the downward flow of flue gas. Because the same spray pressure (pump head - spray nozzle head) is applied to multiple horizontally arranged humidifying liquid spray nozzles, if these spray nozzles have the same structure, humidifying liquid will, in principle, be sprayed in the same amount and speed and with the same droplet diameter. In this embodiment, a plurality of humidifying liquid spray nozzles are arranged in two stages, and therefore there is a difference in water head between the upper and lower stage nozzles, which may result in a difference in spray pressure, but if the nozzles are arranged evenly in each stage, the humidifying liquid will be sprayed uniformly overall in a cross section perpendicular to the flow of exhaust gas.In the device shown in Figure 3, the humidifying liquid sprayed from the humidifying liquid spray nozzle 19 is the lower layer absorption liquid extracted from an absorption liquid extraction outlet 22 provided at the bottom of the lower chamber 4, and is transferred to the humidifying liquid spray nozzle by a pump 23.

[0037] It is preferable to use a single or multiple identical humidifying liquid spray nozzles, each having a structure that sprays the liquid in a conical shape at a uniform density. However, this is not necessarily limited to this, and it is sufficient that the nozzles are configured to spray the liquid uniformly across the horizontal cross section of the vertically extending exhaust gas inlet duct as a whole. For example, nozzles having different structures may be combined. Typical methods for uniformly arranging multiple humidifying liquid spray nozzles in each row include an equilateral triangular arrangement as shown in FIG. 5(a) and a concentric circular arrangement as shown in FIG. 5(b), but this is not necessarily limited to these. In general, a concentric circular arrangement is preferable when the cross section of the exhaust gas inlet duct is circular, and an equilateral triangular arrangement is preferable when the cross section is other than that. Regardless of which arrangement is used, care should be taken to arrange the nozzles so that the nozzle density is uniform across the horizontal cross section.

[0038] The humidifying liquid spray nozzle 19 is provided in the region where the exhaust gas introduction duct 18 extends vertically, and it is preferable that this vertically extending region have a linear length of approximately 2.5 to 5.0 m. Furthermore, by having a linear length of approximately 5 m downstream of the humidifying liquid spray nozzle, the flow of exhaust gas flowing through the exhaust gas introduction duct is regulated, the flow velocity distribution in a cross section perpendicular to the flow is symmetrical about the center of the flow (the flow velocity distribution from the center to the periphery is the same in all directions), and the flow velocity is approximately uniform except for the portion close to the inner wall of the duct (the portion known as the "boundary film"). Therefore, when multiple nozzles with the same structure are evenly arranged in a cross section perpendicular to the flow, the humidifying liquid is sprayed approximately uniformly into the flow.

[0039] Furthermore, for the water content (part of it) of the humidifying liquid sprayed into the exhaust gas flow to evaporate and effectively humidify and cool the exhaust gas, the humidifying liquid droplets must remain suspended in the exhaust gas flow for at least approximately 0.5 seconds (when the 50% mean particle size of the droplets is 2000 μm or less). To achieve this, assuming an exhaust gas flow velocity of approximately 15 m / s, a distance of approximately 8 m is required from the point where the humidifying liquid is sprayed into the exhaust gas flow to the point where it is separated from the exhaust gas flow as a humidifying liquid drain. Therefore, when a vertical exhaust gas flow is bent into an L-shape and the humidifying liquid drain is separated from the exhaust gas flow by centrifugal force, a linear distance of approximately 8 m is required from the position where the humidifying liquid spray nozzle is installed to the L-shaped bend. Based on the above, it is preferable that the distance from the point where the humidifying liquid is sprayed into the exhaust gas flow to the point where it is separated from the exhaust gas flow as a humidifying liquid drain be at least approximately 8 m.

[0040] The sprayed humidifying liquid becomes fine droplets that float in the flue gas. The heat of the flue gas evaporates the water, humidifying and cooling the flue gas. Some of the remaining droplets accompany the flue gas and flow into the middle chamber 3. Others are separated from the flue gas flow (particularly at the L-shaped bend due to the principle of centrifugal separation) and fall, colliding with the bottom plate 20 of the vertically extending region to become humidifying liquid drain. In the device shown in Figure 3, the humidifying liquid drain flows along the bottom surface of the duct, enters the middle chamber 3 of the cylindrical vessel 1, and flows down through the liquid downcomer 13a installed near the flue gas inlet on the floor of the middle chamber. Like the flue gas dispersion pipe 13, the liquid downcomer 13a opens into the space within the middle chamber 3 and extends vertically, penetrating the floor of the middle chamber 3 so that its tip enters the aqueous absorption liquid 11 contained in the lower chamber 4. However, because it extends deeper than the tip of the exhaust gas dispersion pipe, exhaust gas does not spray from its tip. Instead, humidifying liquid drain that has flowed down from the floor of the middle chamber 3 accumulates at the tip and gradually flows out into the absorbing liquid 11 in the lower chamber 4. Therefore, by using pump 24 to extract the humidifying liquid drain that has accumulated near the tip of liquid downcomer pipe 13a, it is possible to recover a liquid consisting almost entirely of humidifying liquid drain. It is preferable that the extraction rate is not greater than the rate at which the humidifying liquid drain flows down. If the extraction rate were greater, absorbing liquid 11 in the lower chamber 4 would enter from the tip of liquid downcomer pipe 13a and mix with the humidifying liquid drain. Note that a gas venting means 28 is provided midway to prevent air bubbles from entering pump 24.

[0041] The humidifying liquid drain extracted from near the tip of the liquid downcomer 13a is transferred by pump 24 to solid-liquid separation means 25, where the solids (gypsum) are recovered. The mother liquor after the solids have been recovered and removed is mostly returned to the lower chamber 4 through a circulation line 26 via an absorption liquid supply port 27 and reused as the absorption liquid. An alkaline agent (limestone slurry) is added to the circulating mother liquor to replenish the gypsum recovered. However, a portion of the mother liquor is withdrawn to prevent the accumulation of salts and dust in the absorption liquid and sent to a wastewater treatment device. If the mother liquor contains peroxides such as persulfuric acid or hexavalent selenium, this can adversely affect the wastewater treatment device (especially the ion exchange device for removing COD components from the wastewater). However, in the preferred embodiment of the method and apparatus of the present invention shown in Figure 3, only the humidifying liquid drain is subjected to solid-liquid separation. Therefore, the mother liquor after the gypsum has been recovered contains almost no peroxides, and therefore, it has almost no adverse effect on the wastewater treatment device.

[0042] Figure 6 shows another suitable example of a liquid-phase continuous wet exhaust gas treatment system (JBR) for carrying out the method of the present invention. In the system shown in Figure 6, droplets of humidifying liquid separated from the exhaust gas flow collide with the bottom plate 20 of the vertically extending region and are collected as humidifying liquid drain in a drain tank 21. As shown in Figure 6, the drain tank 21 may be connected to the lower chamber 4 of the cylindrical vessel 1. In that case, care must be taken to ensure that the amount of humidifying liquid drain flowing into the drain tank 21 exceeds the amount of humidifying liquid drained from the drain tank by a pump 24, otherwise the absorption liquid 11 in the lower chamber 4 will be directly withdrawn and mixed with the humidifying liquid drain.

[0043] When the humidifying liquid drain separated from the exhaust gas is separated and collected at an L-shaped bend provided directly below the vertical area where the humidifying liquid spray nozzles are provided, the humidifying liquid drain is collected by colliding with the bottom plate 20 of the L-shaped bend. In this case, therefore, it is preferable to form the bottom plate in a funnel shape or a V-shaped valley in cross section, as shown in Figure 6, so that the humidifying liquid drain that has collided with the bottom plate can be smoothly collected. However, the portion where the humidifying liquid drain is collected from the exhaust gas flow does not necessarily have to be formed as an L-shaped bend; it may also be a more gradual curve that bends the vertical exhaust gas flow sideways.

[0044] The drain tank 21 is a container for storing the collected humidifying liquid drain, and therefore does not need to have a particularly special structure; however, when the drain tank 21 is connected to the lower chamber 4 of the cylindrical container 1, the liquid level in the drain tank will be approximately the same as the liquid level in the lower chamber, so the piping between the drain tank 21 and the exhaust gas introduction duct 18 must be sealed to prevent liquid leakage. The height at which the drain tank 21 communicates with the lower chamber 4 is preferably at least lower than the bottom surface of the jet bubbling layer so as not to affect the high gas-liquid contact efficiency in the jet bubbling layer. Furthermore, as the solid-liquid separation means 25 for separating and recovering the gypsum, a belt-traveling vacuum filter, a centrifuge, or the like is usually used, but the present invention is not limited to these, and any appropriate solid-liquid separation means can be used.

[0045] Figure 7 is a schematic diagram showing that the use of the method of the present invention can reduce the footprint of an exhaust gas treatment system, including the exhaust gas inlet duct. Figure 7(a) shows the case of using the conventional method of injecting humidifying liquid horizontally into the horizontal flow of exhaust gas, while Figure 7(b) shows the case of using the method of the present invention of injecting humidifying liquid downward into the vertical downward flow of exhaust gas. As mentioned above, to effectively humidify and cool the exhaust gas, the exhaust gas and humidifying liquid must be in contact for a sufficient period of time. This requires a certain distance (several meters) between the point where the humidifying liquid is injected and the point where the exhaust gas enters the system. In this case, the conventional method shown in Figure 7(a) requires that the straight section of the piping be extended horizontally. However, with the method of the present invention shown in Figure 7(b), the piping can be extended vertically, eliminating the need to unnecessarily increase the footprint. This is particularly advantageous for JBR, which prides itself on its compact design.

[0046] The above has described a suitable apparatus for carrying out the method of the present invention, but the practice of the method of the present invention is not limited to using the apparatus described above, and the method can be carried out using any apparatus that is a liquid phase continuous wet exhaust gas treatment apparatus that is configured to spray humidifying liquid downward into a vertical region of an exhaust gas inlet duct that has a region in which exhaust gas introduced into an upright cylindrical container flows vertically downward. [Example]

[0047] (Example) A simulation experiment was conducted using a full-scale JBR experimental device to spray humidifying liquid into exhaust gas. The exhaust gas inlet duct of a standard JBR device was modified to create a vertically downward flow area, with humidifying liquid injection nozzles arranged in two rows midway through the area. As shown in Figure 8, the cross section of the exhaust gas inlet duct in question was a rectangle measuring 8500 mm x 2500 mm. The first row (upper row) of nozzles consisted of 12 nozzle tubes with injection holes (nozzles) on the underside, arranged horizontally and parallel with approximately 700 mm intervals. The second row (lower row) of nozzles consisted of 13 nozzle tubes with the same shape as the first row, arranged horizontally and parallel with the same intervals. The first and second row nozzle groups were spaced vertically approximately 1000 mm apart, with the nozzles in the first row and the second row arranged alternately (staggered) horizontally.

[0048] This vertical section of the exhaust gas introduction duct extends from approximately 1,000 mm upstream (above) of the first-stage nozzle group to approximately 1,000 mm downstream (below) of the second-stage nozzle group. As shown in Figure 8, the cross-sectional shape of this vertical section of the exhaust gas introduction duct is not uniform throughout the entire section; the downstream side, including the nozzle arrangement area, is rectangular with a greater degree of flatness than the upstream side. Furthermore, the downstream side bends in an L-shape at the end of this vertical section and connects to an exhaust gas introduction port installed in the side wall of the middle chamber of the JBR experimental device.

[0049] Approximately 1 million m of air at a temperature of approximately 160°C is introduced into the exhaust gas inlet duct mentioned above. 3 / h, and water at a temperature of approximately 56°C is discharged from the first and second nozzles at a flow rate of approximately 1,300 m 3 The temperature distribution at cross sections S1, S2, and S3 shown in Figure 8 was measured with thermometers at multiple measurement points. Figure 9 shows a schematic diagram of the temperature distribution at cross sections S1, S2, and S3 using isothermal lines.

[0050] As shown in Figure 9(a), air at a temperature of approximately 160°C was cooled to 100-120°C by the injection of water from the first-stage nozzle group, and further cooled to below 70°C by the injection of water from the second-stage nozzle group. At this time, the air was cooled almost uniformly without any bias in the cross-sectional (horizontal) direction. Furthermore, as shown in Figure 9(b) and (c), the temperature of the airflow was cooled to below 60°C in the entire area, including the vicinity of the inner wall of the duct, in the cross-section just before the exhaust gas inlet, and there were no locally high-temperature gas areas.

[0051] (Comparative Example) As in the example, a full-scale JBR experimental device was used to conduct a test of spraying humidifying liquid into exhaust gas. In the comparative example, the exhaust gas inlet duct portion of a standard-spec JBR device was used as is. That is, a vertically downward flow region of exhaust gas was not provided, and humidifying liquid spray nozzles were arranged in two rows midway through the horizontal flow region. As shown in Figure 10, the cross section of the exhaust gas inlet duct at the nozzle arrangement portion was a rectangle measuring 8500 mm x 2500 mm, the same as in the example. The first row (left side) of the nozzle group consisted of 12 nozzle tubes with injection holes (nozzles) on the side, arranged vertically and parallel with approximately 700 mm intervals. The second row (right side) of the nozzle group consisted of 13 nozzle tubes with the same shape as the first row, arranged vertically and parallel with the same intervals. The first row of nozzles and the second row of nozzles were arranged approximately 1000 mm apart horizontally, with the nozzles in the first row and the second row alternately arranged (staggered) horizontally.

[0052] As shown in Figure 10, the cross-sectional shape of the vertical region of the exhaust gas inlet duct is not uniform throughout the entire region, and the downstream side, including the nozzle arrangement region, is rectangular with a greater degree of flatness than the upstream side. The downstream side is connected to the exhaust gas inlet port on the side wall of the middle chamber of the JBR experimental device, approximately 7000 mm beyond the second-stage nozzle group.

[0053] Approximately 1 million m of air at a temperature of approximately 160°C is introduced into the exhaust gas inlet duct mentioned above. 3 / h, and water at a temperature of approximately 54°C is discharged from the first and second nozzles at a flow rate of approximately 1,300 m 3The temperature distribution at the X0 cross section shown in Figure 10 was measured in the same manner as in Example 1. Figure 11 shows a schematic diagram of the temperature distribution at the X0 cross section using isothermal lines.

[0054] The temperature distribution of the airflow in the cross section just before the exhaust gas inlet shown in Figure 11 indicated that there were localized gas regions of approximately 57°C or higher that were not sufficiently cooled near the inner wall of the duct, particularly in the peripheral cross-sectional area along the ceiling and side walls of the duct. [Explanation of symbols]

[0055] 1 cylindrical container 2. Upper chamber (processed gas exhaust chamber) 3 Middle chamber (exhaust gas introduction chamber) 4 Lower chamber (reaction chamber) 11 Aqueous absorption liquid 12 Exhaust gas inlet 13 Exhaust gas dispersion pipe 14 Spout hole 15 Oxygen supply means 16 Stirring means 17 Jet bubbling layer 18 Exhaust gas introduction duct 19 Humidifying liquid spray nozzle 20 Vertical Area Base Plate 21 Drain tank 22 Absorbent liquid outlet 23 Pump 24 Pump 25 Solid-liquid separation means 26 Circulation Line 27 Absorbent liquid supply port 28 Gas venting means 101, 201 container 102, 202 Lower chamber (reaction chamber) 104, 204 Absorbent 105, 205 Middle chamber (exhaust gas inlet chamber) 106, 206 Exhaust gas dispersion pipe (gas downcomer) 107, 207 Upper chamber (treated exhaust gas outflow chamber) 108, 208 Oxygen supply means 109, 209 Jet bubbling layer 110, 210 Stirring means 111, 211 Exhaust gas introduction duct 112, 212 Coolant injection nozzle 113, 213 Absorbent liquid injection nozzle 214 Absorbent liquid extraction pipe 218 Liquid Downcomer 221 Solid separation means

Claims

1. A method for treating an exhaust gas for removing sulfur oxides from the exhaust gas and recovering the removed sulfur oxides as a solid reaction product, the method comprising: a gas-liquid contacting step in which exhaust gas containing sulfur oxides is blown into a liquid phase containing an aqueous absorption liquid containing an alkaline agent and dissolved oxygen to perform a gas-liquid contacting operation, thereby precipitating a reaction product as a solid resulting from a reaction between the alkaline agent, dissolved oxygen, and sulfur oxides in the aqueous absorption liquid; and a solid-liquid separation step in which the aqueous absorption liquid containing the precipitated solid reaction product is subjected to a solid-liquid separation operation to recover the solid reaction product, the method comprises, prior to the gas-liquid contacting step, a humidifying and cooling step of spraying a humidifying liquid into a flow of the flue gas in an exhaust gas inlet duct for introducing the flue gas into the aqueous absorbent liquid to humidify and cool the flue gas, wherein in the humidifying and cooling step, the humidifying liquid is sprayed downward in a vertical region where the flue gas flows vertically downward within the exhaust gas inlet duct, and the aqueous absorbent containing a precipitated solid reaction product is extracted and used as at least a part of the humidifying liquid, the humidifying liquid sprayed into the flue gas is separated from the flue gas and recovered as a humidifying liquid drain, and the humidifying liquid drain is recovered by colliding with a bottom plate formed in the shape of a funnel or a valley with a V-shaped cross section immediately below the vertical region in which the humidifying liquid spray nozzle is provided, and the solid-liquid separation step performs a solid-liquid separation operation only on the recovered humidifying liquid drain, thereby separating and recovering the solid reaction product precipitated in the aqueous absorbent by the reaction of the alkaline agent, dissolved oxygen and sulfur oxides.

2. 2. The method according to claim 1, wherein the vertically downward flow of the exhaust gas after the humidifying liquid is sprayed is bent horizontally, and the humidifying liquid sprayed into the exhaust gas is separated and recovered immediately below the point where the flow bends, utilizing the centrifugal separation effect caused by the bending of the exhaust gas flow.

3. 3. The method of claim 1 or 2, wherein the alkaline agent in the aqueous absorption solution is provided as a limestone slurry.

4. the interior space of an upright cylindrical vessel is vertically partitioned into three compartments, namely, an upper compartment, a middle compartment, and a lower compartment, the lower compartment contains an aqueous absorption liquid containing an alkaline agent and dissolved oxygen, the side wall of the middle compartment is provided with an exhaust gas inlet for introducing exhaust gas containing sulfur oxides, the floor of the middle compartment is provided with a plurality of exhaust gas dispersion pipes which open into the space within the middle compartment and penetrate the floor to extend vertically so that their tips enter the aqueous absorption liquid contained in the lower compartment, and ejection holes are provided near the tips of each exhaust gas dispersion pipe for ejecting exhaust gas below the surface of the aqueous absorption liquid, oxygen supply means is provided near the bottom of the lower compartment for blowing oxygen-containing gas into the aqueous absorption liquid, the ceiling of the lower compartment is provided with a treated gas ascending path which opens into the space within the lower compartment and penetrates the middle compartment to open into the space within the upper compartment, and the upper compartment is provided with a treated gas outlet for discharging the treated gas, In an exhaust gas treatment device configured such that exhaust gas introduced into the middle chamber from the exhaust gas inlet passes through the plurality of exhaust gas dispersion pipes and is ejected as bubbles below the liquid surface of the aqueous absorption liquid contained in the lower chamber, and then separated as a treated gas in a space formed above the liquid surface of the aqueous absorption liquid, and then flows into the upper chamber through the treated gas ascending path and is discharged from the upper chamber through the treated gas discharge port, the exhaust gas inlet is connected to an exhaust gas introduction duct having a vertical region extending vertically so that the exhaust gas flows downward, and the vertical region is provided with humidifying liquid spray means for spraying humidifying liquid downward into the exhaust gas flowing downward inside the exhaust gas introduction duct, the lower chamber has an absorption liquid outlet for withdrawing aqueous absorption liquid from the lower chamber, and a liquid path is formed from the absorption liquid outlet to the humidifying liquid spray means, and the aqueous absorption liquid withdrawn from the absorption liquid outlet passes through the liquid path and is sprayed into the exhaust gas from the humidifying liquid spray means as humidifying liquid, and the humidifying liquid recovery means has a bottom plate formed in the shape of a funnel or a valley with a V-shaped cross section just below the vertical region, and only humidifying liquid drain that collides with the bottom plate and is recovered is sent to solid-liquid separation means, and a solid reaction product precipitated in the aqueous absorption liquid by the reaction of the alkaline agent, dissolved oxygen, and sulfur oxides is separated and recovered.

5. 5. The device according to claim 4, wherein said moisturizing liquid spray means includes a plurality of moisturizing liquid spray nozzles arranged on a horizontal cross section of said vertical region.

6. 6. The device according to claim 4, wherein the exhaust gas introduction duct is connected to the exhaust gas introduction port at the end where it bends laterally below the vertical region, and a drain tank for collecting humidifying liquid drain is provided immediately below the position where the exhaust gas introduction duct bends laterally.

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