Flue gas treatment system and method

FIG. 1 is a schematic view of a flue gas treatment system according to Example 1. As illustrated in FIG. 1 , a flue gas treatment system 10 according to this example includes: a wet desulfurization apparatus 13 which removes sulfur oxides in flue gas 12 A from a boiler 11 ; a mist collection/agglomeration apparatus 14 which is provided on the downstream side of the desulfurization apparatus 13 and forms agglomerated SO 3 mist 51 which is agglomerated and bloated mist by causing particles of SO 3 mist 50 contained in flue gas 12 D from the wet desulfurization apparatus 13 to be bonded together and have bloated particle sizes; a flue gas cooling tower 15 which cools flue gas 12 C from the mist collection/agglomeration apparatus 14 ; a CO 2 recovery apparatus 18 constituted by a CO 2 absorption tower 16 having a CO 2 absorption unit 16 A which removes CO 2 contained in flue gas 12 D from the flue gas cooling tower 15 by being brought into contact with a CO 2 absorbent and an absorbent regeneration tower 17 which recovers CO 2 by releasing CO 2 from the CO 2 absorbent having absorbed CO 2 and regenerates the CO 2 absorbent; a washing unit 16 B which is provided on the gas flow downstream side of the CO 2 absorption unit 16 A and washes flue gas 12 E; and a mist collection unit 16 C which is provided on the gas flow downstream side of the washing unit 16 B and collects CO 2 absorbent bloated mist 53 bloated by the CO 2 absorbent being absorbed by the agglomerated SO 3 mist 51 in the CO 2 absorption unit 16 A and the washing unit 16 B. In addition, the behavior of the mist (the SO 3 mist 50 , the agglomerated SO 3 mist 51 , and the CO 2 absorbent bloated mist 53 ) will be described later with reference to FIGS. 2-1 and 2-2 .

Here, in this example, the SO 3 mist exemplifies the mist. However, the present invention is not limited thereto, and for example, fine mist or the like generated by causing moisture to adhere to solid particles such as fine coal ash may be exemplified.

Here, in the flue gas treatment system according to this example, CO 2 in the combustion flue gas exemplifies an object to be removed. In FIG. 1 , only main facilities are illustrated, and accessory facilities are omitted. In FIG. 1 , reference numeral 12 G denotes a purified flue gas, 16 D denotes a decarbonated combustion flue gas discharge port, 16 E denotes an absorbent supply port, 16 F denotes a liquid distributor, 15 a denotes a cooling water liquid distributor, 15 b denotes a cooling water circulation pump, 15 c denotes a make-up water supply line, 15 d denotes a drainage discharge line, 21 denotes a discharge pump for the absorbent having absorbed CO 2 , 22 denotes a heat exchanger, 17 denotes an absorbent regeneration tower, 17 a is a liquid distributor, 23 denotes an upper filling unit, 24 denotes a reflux water pump, 25 denotes a CO 2 separator, 26 denotes a recovered CO 2 discharge line, 27 denotes a regeneration tower reflux cooler, 28 denotes a nozzle, 29 denotes a regeneration tower reflux water supply line, 30 denotes a combustion flue gas supply blower, 31 denotes a cooler, and 32 denotes a regeneration heater (reboiler).

In FIG. 1 , sulfur oxides in the flue gas 12 A are removed from the flue gas 12 A from the boiler 11 by the wet desulfurization apparatus 13 , and the flue gas 12 D from the desulfurization apparatus 13 is introduced into the mist collection/agglomeration apparatus 14 . Here, particles of SO 3 mist are agglomerated and bloated, thereby forming agglomerated SO 3 mist. Thereafter, the flue gas 12 C containing the agglomerated SO 3 mist is pushed into the flue gas cooling tower 15 by the combustion flue gas supply blower 30 , and is cooled by being brought into contact with the cooling water from the cooling water liquid distributor 15 a in a filling unit 15 e . Next, the cooled flue gas 12 D is guided into the CO 2 absorption tower 16 through a combustion flue gas supply port 16 a of the CO 2 absorption tower 16 . Cooling water contacting the flue gas 12 C accumulates in the lower portion of the flue gas cooling tower 15 and circulates to the cooling water liquid distributor 15 a by the cooling water circulation pump 15 b so as to be used. Here, in a case where the amount of moisture in the flue gas 12 B is small, the cooling water is gradually lost by humidifying and cooling the combustion flue gas. Accordingly, cooling water is replenished by the make-up water supply line 15 c . In a case where the amount of moisture in the flue gas 12 B is large, moisture in the combustion flue gas condenses due to contact with the cooling water and causes an increase in the amount of the cooling water. Accordingly, excess waste water is discharged by the drainage discharge line 15 d.

Next, the flue gas 12 D pushed into the CO 2 absorption tower 16 is brought into countercurrent contact with the CO 2 absorbent at a constant concentration supplied from the liquid distributor 16 F in the filling unit in the CO 2 absorption unit 16 A, CO 2 in the flue gas 12 D is absorbed and removed by the absorbent, and the decarbonated flue gas 12 E is directed to the washing unit 16 B on the gas flow downstream side. The absorbent supplied to the CO 2 absorption tower 16 absorbs CO 2 and typically reaches a temperature higher than the temperature at the combustion flue gas supply port 16 a due to the heat of reaction caused by the absorption, and is sent to the heat exchanger 22 by the discharge pump 21 for the absorbent having the absorbed CO 2 so as to be heated and guided to the absorbent regeneration tower 17 . Temperature control of the regenerated absorbent can be performed by the heat exchanger 22 or, if necessary, by the cooler 31 provided between the heat exchanger 22 and the absorbent supply port 16 E.

In the absorbent regeneration tower 17 , the absorbent is regenerated in a lower filling unit 17 b through heating by the regeneration heater (reboiler) 32 , is cooled by the heat exchanger 22 , and is returned to the CO 2 absorption tower 16 side. In the upper portion of the absorbent regeneration tower 17 , CO 2 separated from the absorbent comes into contact with reflux water supplied from the nozzle 28 in the upper filling unit 23 , is cooled by the regeneration tower reflux cooler 27 , is separated from the reflux water having condensed water vapor with CO 2 entrained therein by the CO 2 separator 25 , and is guided to a CO 2 recovery process by the recovered CO 2 discharge line 26 .

A portion of the reflux water is refluxed to the absorbent regeneration tower 17 by the reflux water pump 24 , and the portion is supplied to the regeneration tower reflux water supply port 29 a of the CO 2 absorption tower 16 via the regeneration tower reflux water supply line 29 .

Since the amount of the absorbent contained in the regenerated reflux water is small, the absorbent comes into contact with the flue gas in the washing unit 16 B of the CO 2 absorption tower 16 and contributes to the recovery of a small amount of absorbent contained in the decarbonated combustion flue gas 12 E.

FIG. 2-1 is a schematic view illustrating a mechanism for bloating mist according to the present invention. FIG. 2-2 is a schematic view illustrating a mechanism for bloating mist according to the related art.

First, as illustrated in FIG. 2-1 , in the flue gas 12 A from the boiler 11 introduced into the wet desulfurization apparatus 13 , the SO 3 mist 50 is generated from a portion of SO 3 gas in the desulfurization apparatus 13 .

The generated SO 3 mist 50 is contained in the flue gas 12 B discharged from the desulfurization apparatus 13 . The SO 3 mist 50 in the flue gas 12 B introduced into the mist collection/agglomeration apparatus 14 provided on the downstream side of the desulfurization apparatus 13 adheres to, for example, a wire mesh, agglomerates, and bloats to form the agglomerated SO 3 mist 51 .

The flue gas 12 C containing the agglomerated SO 3 mist 51 bloated in the mist collection/agglomeration apparatus 14 is then introduced into the flue gas cooling tower 15 . In the flue gas cooling tower 15 , the agglomerated SO 3 mist 51 absorbs water vapor 60 in the flue gas cooling tower 15 and becomes dilute sulfuric acid, thereby forming dilute sulfuric acid mist 52 with the agglomerated SO 3 mist 51 as the nucleus.

The flue gas 12 D containing the dilute sulfuric acid mist 52 with the agglomerated SO 3 mist 51 bloated in the flue gas cooling tower 15 as the nucleus is then introduced into the CO 2 absorption unit 16 A in the CO 2 absorption tower 16 .

The dilute sulfuric acid mist 52 with the agglomerated SO 3 mist 51 in the flue gas introduced into the CO 2 absorption unit 16 A as the nucleus absorbs amine vapor 61 in a case where the water vapor 60 in the CO 2 absorption unit 16 A and an amine compound as the CO 2 absorbent are used, and forms the CO 2 absorbent bloated mist 53 which contains a high concentration of amine and is thus bloated.

The flue gas 12 E containing the CO 2 absorbent bloated mist 53 bloated in the CO 2 absorption unit 16 A is then introduced into the washing unit 16 B.

In the washing unit 16 B, the CO 2 absorbent bloated mist 53 which contains a high concentration of amine and is thus bloated is formed by further absorbing the water vapor 60 and the amine vapor 61 in the washing unit 16 B.

A demister provided with a wire mesh is used in the mist collection unit 16 C on the outlet side in the CO 2 absorption tower 16 , and the bloated CO 2 absorbent bloated mist 53 is collected by the demister.

Contrary to this, in the related art, as illustrated in FIG. 2-2 , the mist collection/agglomeration apparatus 14 as in this example is not provided, and thus the size of the nucleus forming dilute sulfuric acid mist in the flue gas cooling tower 15 is small. As a result, even in a case where there is bloating due to the CO 2 absorbent in the CO 2 absorption tower 16 , CO 2 absorbent bloated mist 53 which is smaller than that in the case of the present invention is formed. In the mist collection unit 16 C on the outlet side in the CO 2 absorption tower 16 , the efficiency of the demister provided with the wire mesh in collecting the bloated CO 2 absorbent bloated mist 53 is low.

Table 1 shown below shows the outlet mist particle size of the desulfurization apparatus 13 , the outlet mist particle size of the mist collection/agglomeration apparatus 14 , and the inlet mist particle size of the mist collection unit 16 C in the related art and Example 1.

In the related art, since the mist collection/agglomeration apparatus 14 is not installed, the SO 3 mist 50 (particle size 0.1 to 1.0 μm) at the outlet of the desulfurization apparatus 13 becomes the nucleus, is introduced into the CO 2 absorption unit 16 A, and is bloated herein such that the inlet mist particle size of the mist collection unit 16 C was 0.5 to 2.0 μm.

Contrary to this, in Example 1, since the mist collection/agglomeration apparatus 14 is installed, in a case where two particles of mist having an outlet mist particle size of 0.1 to 1.0 μm for the desulfurization apparatus 13 agglomerate, the outlet mist particle size of the mist collection/agglomeration apparatus 14 is bloated to become 0.12 to 1.2 μm. The agglomerated SO 3 mist 51 with the two agglomerating particles of mist becomes the nucleus, is introduced into the CO 2 absorption unit 16 A, and is bloated herein such that the inlet mist particle size of the mist collection unit 16 C was 0.6 to 2.3 μm.

In addition, in a case where, for example, five particles of mist having an outlet mist particle size of 0.1 to 1.0 μm for the desulfurization apparatus 13 agglomerate, the outlet mist particle size of the mist collection/agglomeration apparatus 14 is bloated to become 0.17 to 1.7 μm. The agglomerated SO 3 mist 51 with the five agglomerating particles of mist becomes the nucleus, is introduced into the CO 2 absorption unit 16 A, and is bloated herein such that the inlet mist particle size of the mist collection unit 16 C was 0.7 to 2.9 μm.

As a result, according to Example 1, the collection efficiency in the mist collection unit 16 C could be improved.

The reason why the removing means using the wire mesh is selected as the mist collection/agglomeration apparatus 14 will be described below in Table 2.

In a case where mist removal is performed, hitherto, a mist removal apparatus which uses dense filter fabric such as a candle filter using the Brownian diffusion principle is suitable to remove fine mist of several micrometers or smaller. However, it is necessary to perform an operation at a low filtration wind velocity, resulting in an increase in the size of the apparatus, which is not preferable.

In addition, in a case where a corrugated mist removal apparatus is used, it is possible to perform an operation at a high filtration wind velocity, and thus a reduction in the size of the apparatus is possible. However, the inertial forces of fine particles decrease in proportion to mass, and the mist removal efficiency decreases. Therefore, it is not preferable in practice to use the corrugated mist removal apparatus for removing fine particles.

Therefore, the removing means using the wire mesh is preferable as the mist collection/agglomeration apparatus 14 .

FIG. 3 is a schematic view of mist bloating according to the present invention. In addition, in FIG. 3 , the lower side of the figure is the gas flow downstream side and the upper side of the figure is the gas flow upstream side. As illustrated in FIG. 3 , in a case where a wire mesh 70 is used as the mist collection/agglomeration apparatus 14 , particles of the SO 3 mist 50 adhered to the surface of the wire mesh 70 come into contact with each other and agglomerate, thereby forming the agglomerated SO 3 mist 51 . In addition, the agglomerated SO 3 mist 51 flows downward along the wire mesh 70 , and falls dropwise from the vicinity of the lower end portion of the wire mesh 70 as a mist drain to be discharged from the mist collection/agglomeration apparatus 14 .

Here, in a case where the filtration wind velocity of the flue gas 12 B is set to be equal to or higher than a wind velocity value regarded as the limit during typical mist collection, downflow and falling of the agglomerated SO 3 mist 51 are partially disturbed and re-scattering thereof from the upper surface side of the wire mesh 70 occurs, resulting in a reduction in the mist removal efficiency. Therefore, the filtration wind velocity regarded as the limit in the wire mesh type mist removal apparatus is, for example, 2.5 to 5 m/s (varies depending on the mist load in the gas and the wire mesh type).

FIG. 4 is a diagram showing an example of the relationship between the filtration wind velocity and the mist removal ratio. FIG. 5 is a diagram showing an example of the relationship between the filtration wind velocity, the mist adhesion ratio, and the mist re-scattering ratio.

In the example shown in FIG. 4 , the mist removal ratio increases as the filtration wind velocity increases until the filtration wind velocity approaches, for example, 2.8 m/s. However, when the filtration wind velocity exceeds 2.8 m/s, re-scattering of the SO 3 mist collected by the wire mesh 70 and aggregated starts.

Furthermore, as shown in FIG. 5 , the re-scattering ratio of the SO 3 mist adhered to the wire mesh 70 increases from the vicinity of the critical filtration wind velocity (for example, 2.8 m/s).

In a region of higher than the critical filtration wind velocity (for example, 2.8 m/s), the amount of the re-scattering agglomerated SO 3 mist 51 increases as the filtration wind velocity increases, and accordingly, as shown in FIG. 4 , the mist removal ratio of the wire mesh (mist removal apparatus) 70 decreases.

However, agglomeration of the mist adhered to the wire mesh 70 occurs, and the particle size of the re-scattering mist is greater than the particle size of the inflow mist. Therefore, the re-scattering agglomerated SO 3 mist 51 becomes the nucleus and is further bloated in the CO 2 absorption tower 16 , so that the mist recovery ratio of the demister which is the mist collection unit 16 C installed at the outlet of the CO 2 absorption tower 16 is improved. As a result, the amount of mist accompanying the CO 2 absorbent and scattering to the outside of the system can be reduced.

As described above, as the filtration wind velocity (V) of the flue gas in the mist collection/agglomeration apparatus 14 , a wind velocity (for example, V>2.5 m/s) which exceeds the filtration critical wind velocity (for example, 2.5 m/s) for the mist collection or a wind velocity of 1.2 to 1.5 times the filtration critical wind velocity is preferable.

Here, since the filtration wind velocity in the mist collection/agglomeration apparatus 14 varies depending on the mist load in the flue gas and the wire mesh type, in an actual apparatus, it is preferable to set the wind velocity to a wind velocity of 1.2 to 1.5 times the filtration critical wind velocity after the critical filtration wind velocity is determined.

According to this example, by the mist collection/agglomeration apparatus 14 , the agglomerated SO 3 mist 51 which is agglomerated and bloated mist is formed by causing the particles of the SO 3 mist 50 contained in the flue gas 12 B from the desulfurization apparatus 13 to be bonded together and have bloated particle sizes, the agglomerated SO 3 mist 51 is caused to re-scatter and be introduced to the CO 2 absorption unit 16 A side which removes CO 2 by bringing the CO 2 absorbent on the gas flow downstream side into contact therewith. Thereafter, in the CO 2 absorption unit 16 A, when CO 2 contained in the flue gas is removed by being brought into contact with the CO 2 absorbent, the CO 2 absorbent bloated mist 53 bloated by the CO 2 absorbent being absorbed by the agglomerated SO 3 mist 51 as the nucleus is formed, and the CO 2 absorbent bloated mist 53 can be collected by the mist collection unit 16 C. Accordingly, when the treated flue gas from which CO 2 has been removed is discharged to the outside of the system, entrainment of the CO 2 absorbent can be significantly suppressed.

A flue gas treatment system according to Example 2 will be described below. In this example, the mist collection/agglomeration apparatus 14 of Example 1 is caused to bloat mist through charging. FIG. 6 is a schematic view of mist bloating caused by charging. As illustrated in FIG. 6 , in this example, SO 3 mist is charged by using a discharge electrode 102 provided with a high voltage power supply 101 .

In this example, as the mist collection/agglomeration apparatus 14 using charging, there are the discharge electrode 102 for charging the SO 3 mist 50 and a low pressure loss filter 120 which is grounded 111 . The SO 3 mist 50 charged by using electrostatic force electrically neutralizes the adhered SO 3 mist in order to prevent repulsion due to the electrostatic force between particles of the mist adhered to the low pressure loss filter 120 .

In this example, by using the electrostatic force, compared to the case of using only the inertia/interception/diffusion effect (Brownian effect), the collection performance of the SO 3 mist is improved.

Furthermore, by using the electrostatic force, compared to the case of using only the inertia/interception/diffusion effect (Brownian effect), the mist collection/agglomeration apparatus 14 can be downsized.

FIG. 7 is a diagram of the relationship between the SO 3 mist concentration in the inlet flue gas 12 D of the CO 2 absorption unit 16 A and the amine concentration in the gas released from the outlet of the mist collection unit 16 C of the CO 2 absorption tower 16 .

In the case of this example, it was confirmed that by providing the mist collection/agglomeration apparatus 14 using charging, a reduction in the amine concentration in the gas released from the outlet of the mist collection unit 16 C of the CO 2 absorption tower 16 is smaller than that of a comparative example.