Method and apparatus for treating absorbent solution slurry containing fly ash

Abstract: Disclosed are, in the wet lime desulfurization of exhaust gases containing SO₂, a method for treating absorbent solution slurry containing fly ash which is characterized by the steps of diverting at least a part of an absorbent solution slurry containing fly ash particles; after the solid and liquid components of the diverted absorbent solution slurry are separated by centrifugal separation, adjusting the pH value of a suspension containing fine fly ash particles remaining in a filtrate to 6 to 9 by adding an alkali to it; and then supplying the suspension to a thickener, and an apparatus adopted to carry out the above method. Ill effects caused by fly ash and HF in exhaust gases can be avoided, and the activity of CaCO₃ may be maintained, and highly pure gypsum is recovered.

Description: The present invention relates to a method and an apparatus for treating absorption slurry containing fly ash. More specifically, it relates to a method and an apparatus for separating fly ash which is present in an absorbent slurry out of a desulfurization apparatus for exhaust gases by a wet lime method. When an exhaust gas from coal burning is desulfurized according to a wet lime method, an absorbent solution absorbs SO x present in the exhaust gas. Also, an exhaust gas often includes HF as a harmful component. When such exhaust gas is treated with CaCO₃ as an SO₂ absorbent in a wet type exhaust gas absorption tower, the following reactions occur: CaCO₃ + SO₂ -> CaSO₃ + CO₂, (1) CaCO₃ + 2HF -> CaF₂ + CO₂ + H₂O. (2) However, the exhaust gas contains fly ash as well as SO x and HF, and this fly ash also mixes into the absorbent solution. If an absorbent solution, as a result, contains a large amount of fly ash, the aluminum (Al) component in the fly ash dissolves and produces fluorine compounds of aluminum (called AlF x hereinafter) by reacting with HF present in the gas. The AlF x thus produced is known to inhibit the dissolution effect of limestone (CaCO₃) (Japanese Patent Provisional Publication No. 16702/1970 (or Showa55-16702)). To cope with this, it is effective to prevent fly ash, a source of aluminum, from mixing into. It is thus common to dispose a dust separator, such as an electrostatic precipitator, in an upstream position from the exhaust gas desulfurization apparatus. However, it is difficult to remove fly ash completely, and the mixing of fly ash into the absorbent solution is unavoidable in reality. Also, the fly ash which mixes into the absorbent solution is fine particles whose average diameter is 5µm or less, and since the speed of precipitation is slow, removal by precipitation separation requires a thickener with an extremely large sectional area. The separation is difficult with a thickener only. Furthermore, if the absorbent solution contains a large amount of fly ash, it has been a disadvantageous problem that collected gypsum will also contain fly ash and the recovery of highly pure gypsum is impossible. It is an object of the present invention to solve the above-mentioned problems and provide a method and an apparatus for treating an absorbent solution slurry containing fly ash which method makes it possible to avoid ill effects caused by fly ash and HF in exhaust gases, and maintain the activity of CaCO₃, and collect gypsum with high purity. In the wet lime desulfurization of exhaust gases, the present invention concerns itself with a method for treating absorbent solution slurry containing fly ash by diverting a part of an absorbent solution slurry containing fly ash particles; after removing essentially gypsum from the liquid components by centrifugal separation, adjusting the pH value of a suspension containing fine fly ash particles remaining in the filtrate to 6 to 9 by adding an alkali to it; then supplying the suspension to a thickener; collecting the overflow water from the thickener as a clarified solution, using it as a make-up feed for the preparation of an absorbent slurry and supplying said slurry to the absorbent tower as a make-up feed; and at the same time, discharging an underflow solution from the thickener out of the system as concentrated sludge. Also, the present invention concerns itself with an apparatus for carrying out the above method. According to the present invention, the absorbent solution slurry coming out of an absorption tower and containing fly ash particles is first supplied to a centrifugal separator and separated between solid and liquid. In this case, the absorbent solution slurry is mostly, 20 wt% for example, gypsum and contains small amounts of fly ash, less than 1 wt% for example. A centrifugal separator can easily separate particles with larger specific density and with greater grain size because of its operating principle. Therefore, while it can separate gypsum with larger grain size and specific density completely, a centrifugal separator cannot completely remove fly ash with smaller grain size and specific density, leaving some residue of fly ash in the solution. For example, while gypsum can be separated 90% or more, only 20% or less of fly ash can be separated. Therefore, further procedures are required to remove fine particles of fly ash remaining in the filtrate. In removing fine particles by sedimentation from a suspension containing such particles, while thickeners are widely used, the speed of sedimentation becomes a problem. The speed of sedimentation of fine particles whose average diameter is 5µm or less, such as fly ash, is extremely slow as it is, and a thickener with a very large sectional area is required. Thus the present inventors have carried out an extensive study and found that if an alkali, such as Ca(OH)₂ and NaOH, is added to the suspension containing fly ash and coming out of the centrifugal separator to adjust its pH to a value between 6 and 9, the speed of sedimentation increases significantly, and therefore a small thickener can perform the separation. This is considered to be caused by some change in the zeta-potential due to an electric double layer formed on the surface of fly ash particles depending on pH values. Particles which do not cohere on the acidic side because of electric repulsive forces between them do cohere when the solution is neutral or alkaline because the repulsive forces disappear, and their sedimentation is accelerated because of this cohesion. Because the pH of an absorbent solution slurry in normal operation is between 4 and 6, i.e. in a weakly acidic region, only a small amount of an alkali is necessary for adjusting the pH as described above, and this is very advantageous in putting the method of the present invention into practice. The overflow water from the thickener, which has been treated as described above, is collected as a clarified solution which contains almost no fly ash particles used as a make-up feed for the preparation of an absorbent slurry which is supplied to the absorption tower as a make-up feed. The underflow solution from the thickener is discharged from the system as concentrated sludge, so that the concentration of fly ash in the absorbent solution slurry is maintained to be less than a certain value. Also, because the separation ratio of fly ash particles is smaller than that of gypsum with the centrifugal separator mentioned above, the collected gypsum is highly pure and contains only a small amount of fly ash mixed in. FIG. 1 shows a flow chart of an embodiment of the present invention. In this figure, numeral 1 indicates a supply line for exhaust gas to be treated, 2 an absorption tower, 3 a discharge line, 4 a slurry tank, 5 an absorption solution pump, 6 a supply line for oxidizing air, 7 a line for supplying the absorption solution to a centrifugal separator, 8 a centrifugal separator, 9 separated gypsum, 10 a supply line for the suspension, 11 a pH-adjusting tank, 12 pH control meter, 13 a control valve, 14 a supply line for an alkali, 15 a thickener, 16 an overflow water line, 17 a tank for preparing limestone slurry, 18 an underflow line, 19 line for supplying limestone powder, 20 a line for supplying make-up feed, and 21 a pump for supplying limestone slurry. With reference to FIG. 1, we shall describe an embodiment of the present invention. A part of an exhaust gas diverted from a small pulverized coal firing equipment (not shown) was treated according to the embodiment of the present invention shown in FIG. 1 at a rate of 200m³N/h. The entrance temperature of an exhaust gas to be treated was adjusted to 110°C by a heat exchanger (not shown), and the concentration of fly ash was adjusted to about 200mg/m³N by a bag filter (not shown). The entrance concentration of SO₂ was 3000ppm, and the concentration of HF 40ppm. According to this embodiment, the gas to be treated is introduced into an absorption tower 2 through line 1. After SO₂, HF and fly ash are removed from it, the gas is discharged from line 3. A slurry tank 4 is disposed under the absorption tower 2 to hold an absorbent solution slurry, and air for oxidation is supplied to the bottom of the tank through line 6. The absorbent solution slurry is circulated to the top portion of the absorption tower 2 from the slurry tank 4 by way of an absorbent solution pump 5. A part of the absorbent solution slurry is diverted through line 7 and supplied to a centrifugal separator 8. The centrifugal separator 8 separates highly pure gypsum 9, and a suspension containing fine particle fly ash is supplied to a tank 11 for adjusting pH through line 10. The pH of the suspension in the pH-adjusting tank 11 is adjusted to a prescribed range between 6 and 9 by changing the amount of alkalis supplied via line 14 and by controlling this amount by varying the opening of a control valve 13 via a pH control meter 12. After the pH is adjusted, the suspension is supplied to a thickener 15, and the overflow water is sent to a tank 17 for preparing limestone slurry via line 16. The underflow solution from the thickener 15 is discharged out of the system through line 18 as concentrated sludge. A powder of limestone is supplied to the limestone slurry preparation tank 17 through line 19. After a limestone slurry of a given concentration is prepared, it is supplied to a slurry tank 4 at a constant rate by way of a limestone slurry supply pump 21. Also, a make-up feed, as well as the overflow water from the thickener, is supplied to the limestone slurry preparation tank 17 from the outside of the system via line 20. Under the conditions described above, a stable operation was carried out. The concentration of SO₂ in the outlet gas was 120ppm with the stable operation, and the pH of the absorbent solution slurry in the slurry tank 4 was 5.6. The pH of the suspension in the pH-adjusting tank 11 was controlled to 7.0. Also, the concentration of CaCO₃ in the absorbent solution turned out to be 0.05mol/l, and the reactivity of CaCO₃ was good. The concentrations of aluminum (Al) and fluorine (F) in the filtered absorbent solution were measured to be 1.0mg/l and 17mg/l, respectively, and small. Also, the purity of the gypsum thus obtained was 96% or more and its color was white. (Comparative Example) Using the same apparatus and gas conditions as the above embodiment, another stable operation was carried out with the supply of an alkali from line 14 stopped and without controlling the pH in the pH-adjusting tank 11. In this comparative example, the pH of the absorbent solution slurry in the slurry tank 4 gradually decreased after the starting of the operation and finally reached 4.5. While the CaCO₃ concentration remained unchanged at 0.05mol/l simply because it was controlled to be the same as in the above embodiment, the reactivity of CaCO₃ was greatly reduced compared to the above embodiment due to the decrease of the pH value. Also, the SO₂ concentration in the outlet gas was 250ppm, and the desulfurization efficiency became degraded. The concentrations of aluminum (Al) and fluorine (F) in the filtered absorbent solution were 72mg/l and 125mg/l, respectively. Also, the color of the gypsum thus obtained was grey due to the mixing of fly ash. We have described in detail the method and the apparatus of the present invention for separating fly ash in the absorbent solution in the a wet lime desulfurization method for exhaust gases by way of the embodiments. It should be noted that the present invention can also be applied to the case in which fly ash in a suspension has to be precipitated and removed in an ordinary water treatment apparatus. As we have described above, according to the present invention, ill effects caused by fly ash and HF in exhaust gases can be avoided, and the activity of CaCO₃ may be maintained, and highly pure gypsum can be collected.

Claims: A method for treating absorbent solution slurry containing fly ash from wet lime desulfurization of exhaust gases, which is characterized by the steps of: diverting at least a part of an absorbent solution slurry containing fly ash particles; removing substantially gypsum from the liquid components of the diverted absorbent solution slurry by centrifugal separation, adjusting the pH value of a suspension containing fine fly ash particles remaining in a filtrate to 6 to 9 by adding an alkali to it, then supplying the suspension to a thickener, collecting the overflow water from the thickener as a clarified solution, using it as a make-up feed for the preparation of an absorbent slurry and supplying said slurry to the absorption tower as a make-up feed; and discharging an underflow solution from the thickener out of a system as a concentrated sludge. The method as described in claim 1, wherein the alkali is one or both of Ca(OH)₂ and NaOH. An apparatus for wet lime desulfurization of exhaust gases which comprises an absorption tower (2), a centrifugal separator (8) for separating gypsum from an absorbent solution slurry diverted from a circulation line of the absorption tower, a pH-adjusting tank (11) for adjusting the pH of a suspension from the centrifugal separator (8) to a value between 6 and 9, a thickener (15) for the pH-adjusted suspension from the pH-adjusting tank and a tank (17) which is for preparation of a limestone solution slurry to be supplied to the absorption tower (2) and which uses overflow water from the thickener (15) as make-up feed.

Inventors: OKINO SUSUMU HIROSHIMA TECHNIC, ONIZUKA MASAKAZU HIROSHIMA TEC, TAKASHINA TORU HIROSHIMA TECHN, UKAWA NAOHIKO HIROSHIMA TECHNI, OKINO, SUSUMU, HIROSHIMA TECHNICAL INSTITUTE, ONIZUKA, MASAKAZU, HIROSHIMA TECHNICAL INSTITUTE, TAKASHINA, TORU, HIROSHIMA TECHNICAL INSTITUTE, UKAWA, NAOHIKO, HIROSHIMA TECHNICAL INSTITUTE
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: B01D53
IPC codes: B01D53, C01F11