GB2163417A - Controlling slurry concentration in wet flue gas desulphurization apparatus - Google Patents

Abstract

In an absorption tower 102 a flue gas 101 comprising SO2 is brought into contact with a slurry suspending a Ca compound for absorbing the SO2. The method comprises withdrawing the slurry from the absorption tower 102 in such a way that the slurry is divided into at least two components with different concentrations, one component (line 110) being a slurry having a higher concentration of the Ca compound, the other component (line 117) being a slurry having a lower concentration of the Ca compound. The rates of withdrawal of the component slurries withdrawn from the absorption tower are regulated so that the concentration of the Ca compound in the slurry being contacted with the flue gas is controlled. <IMAGE>

Description

SPECIFICATION Controlling slurry concentration in wet flue gas desulphurization apparatus This invention relates to a method of controlling the concentration of a slurry in wet flue gas desulphurization apparatus in which SO, in the flue gas is absorbed in a slurry suspending solid matter and the solid sulphur compounds are recovered as by-products.

At present, the mainstream of flue gas desulphurization systems is a flue gas desuphurization apparatus using a so-called wet lime process in which the flue gas is desulphurized using CaCO3 or Ca(OH)2 as an absorbant in order to recover sulphur in the form of calcium sulphite or calcium sulphate (gypsum). For instance, this process is described in detail in Japanese Laid-open Patent Application 57-63117 and numerous other publications.

Reference is now made to Figure 1 of the accompanying drawings, illustrating a currently, industrially, widely adopted flue gas desulphurization apparatus using wet lime processes.

Flue gas 1 comprising SO, is passed into the body of an absorption tower 2. At the lower portion of the absorption tower 2 is provided a tank 3 containing a slurry in which a Ca compound is suspended. The slurry is agitated by means of an agitator 4 to prevent the solid matter from settling.

The slurry suspending the Ca compound is fed to the top of the tower 2 by means of a circulation pump 5, by which it is sprayed throughout the tower and flows down while coming into contact with the flue gas, and returned to the tank 3. The flue gas from which SO, has been removed by contact with the slurry is discharged through a mist eliminator 6 as a purified gas 7. On the other hand, to the tank 3 is fed a slurry of CaCO3 or Ca(OH)2 through line 8 in an amount which depends on the amount of SO, being absorbed. The slurry which contains calcium sulphite produced by absorption of SO, with the absorbent is fed from line 9 to an oxidizing column 10.Air 12 is blown from a bubble generator 11 provided at the bottom of the oxidizing column and sulphuric acid is fed from line 13, by which calcium sulphite as well as unreacted CaCO3 or Ca(OH)2 is oxidized into gypsum. The gypsum slurry from the oxidizing column 10 is passed through line 14 into a thickener 15 and the resulting concentrated gypsum slurry is fed through line 16, a tank 17, and a pump 18 into a centrifugal separator 19 to obtain gypsum 20.

The filtrate is fed to a tank 21 and then through a pump 22 and line 23 into the thickener 15. The supernatant liquid in the thickener 15 is passed from line 24 to a tank 25 and may be used in adjustment of, for example, the absorbent in the flue gas desulphurization apparatus or may be discharged from a pump 26.

In view of these circumstances, the present inventors made intensive studies to simplify the existing flue gas desulphurization apparatus for economy. As a result, it was found that by analysis of experimental data of the velocity of reaction be tween crystals of CaCO3 or Ca(OH)2 and SO3, the oxidation reaction velocity of calcium sulphite pro duced by absorption of SO2, and the settling rate of gypsum, the underlying concept of the known one operation in one step process in which the re spective reactions were carried out in individual steps could be overcome.More particularly, operations including absorption and oxidation of SO3, settlement and concentration of gypsum, and recovery of supernatant liquid could be collectively carried out in an absorption tower having a tank in a simple and collective manner by proper control of concentrations of slurries in the desulphurization system.

Accordingly, the present invention provides a method of controlling a concentration of slurry in an absorption tower in which a flue gas comprising SO, is brought to contact with a slurry suspending a Ca compound therein to absorb the SO, with the compound, the method comprising withdrawing the slurry from the absorption tower in such a way that the slurry is divided into at least two groups with different concentrations, one group being a slurry having a higher concentration of the Ca compound, the other group being a slurry having a lower concentration of the Ca compound, and regulating rates of the slurries being withdrawn from the absorption tower whereby the concentration of the Ca compound in the slurry being contacted with the flue gas is controlled within a predetermined range.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 schematically shows a flue gas desulphurization apparatus using a wet lime process which has been hitherto adopted industrially; and Figure 2 is a schematic view of a wet flue gas desulphurization apparatus suitable for carrying out the method of the invention.

Reference is now made to Figure 2 illustrating wet flue gas desulphurization apparatus suitable for the method of the invention. Flue gas 101 comprising SO, is introduced into an absorption tower 102. In Figure 2, the flue gas and an absorption slurry are shown to be contacted with each other by parallel current flow, but the countercurrent flow gas-liquid contact system as shown in Figure 1 may be also used. In the practice of the invention, the parallel current flow is preferably used because of the advantage that the efficiency of the oxidization reaction into gypsum is improved as described hereinafter.

At the lower portion of the absorption tower 102 is provided a tank receiving a slurry in which a Ca compound is suspended. The slurry is agitated by means of an agitator 104 so that solid matter is prevented from settling. The slurry suspending the Ca compound is passed to the top of the tower by means of a circulation pump 105 and is sprayed throughout the tower, followed by flowing down while contacting with the flue gas and returning to the tank 103. The glue gas from which SO, has been removed by contact with the slurry is dis charged through a mist eliminator 106 as a purified gas 107.

On the other hand, to the tank 103 is fed through a powder transport line 108 powder of CaCO3 or Ca(OH)3 in an amount corresponding to the absorption of 503. CaCO2 or Ca(OH)3 serving as an absorbent may be fed to the tank in the form of a slurry in water. Calcium sulphite which is produced by absorption of SO, in the absorbent is oxidized with oxygen contained in the flue gas into gypsum crystals because the slurry in the gas-liquid zone of the parallel flow system is kept acidic.

However, in case where the content of oxygen in the flue gas is low, a gas containing oxygen gas is fed from an air nozzle 109, by which the absorbed SO, can be fixed as gypsum.

In the manner as described above, because the slurry of the Ca compound in which gypsum crystals are suspended is kept in the tank 103, the slurry containing gypsum crystals is passed through a slurry discharge port 110 and a pump 119 to a separator 111 in which a gypsum cake 112 is obtained. The resulting filtrate is returned from line 113 to the tank 103.

In the tank 103 is provided a partition wall 114 extending from the liquid level to a lower portion of the slurry to establish a liquid chamber 115 which is separated from the agitated slurry. The partition wall 114 has an open lower end, which permits the slurry agitated by the agitator 104 to be passed to the liquid chamber 115 partitioned by the partition wall 114 through the open lower portion. As will be seen from Figure 2, a baffle plate 116 is provided so that the supernatant liquid in the liquid chamber 115 is not disturbed by the movement of the agitated slurry. The supernatant liquid in the liquid chamber 115 is discharged through a supernatant discharge port 117 and a pump 118. The filtrate from line 113 is passed from the upper to lower portions in the lower part of the liquid chamber 115 in order to prevent gypsum crystals from rising.The slurry discharge port 110 is so located that gypsum crystals are settled and concentrated by means of an inclined plate 120 which is provided at the end of the tank 103 positioned at the lower portion of the liquid chamber 115.

In this manner, the concentration of gypsum crystals in the slurry which is withdrawn by means of a pump 119 becomes high, thus leading to saving of energy required for transportation of liquid.

In wet flue gas desulphurization apparatus, it is usual to use a large quantity of water, for example, by passing wash water from a washing nozzle 121 in order to prevent the crystals of the Ca compound in the mist collected in the mist eliminator 106 from being deposited and accumulated to render the gas flow path narrow, or by flowing sealing water for the pumps in the apparatus. Such water will disturb the concentration of the slurry contained in the tank 103. Variation in concentration of the slurry will unstabilize the operation control of the wet flue gas desulphurization apparatus and invite scaling troubles accompanied by the variation in concentration of seed crystals. These problems have not been hitherto solved. In the wet flue gas desulphurization apparatus using a slurry of, especially, Ca compounds, the prevention of scaling is one of important problems.According to our studies, the main cause of the scaling troubles was found to be due to variation in concentration of the slurry, which is attributed to the inflow of water.

In the practice of the present method, the variation in concentration of the slurry is suitably prevented. More particularly, the concentration of a Ca compound in the slurry in the wet flue gas desulphurization apparatus can be stably controlled by carrying out, simultaneously and arbitrarily without a delay of response, two operations including an operation of discharging the slurry suspending crystals of the Ca compound from the tank of the desulphurization apparatus and an operation of discharging the supernatant liquid having a low concentration of the crystals of the Ca compound.

The above-described good effects of the invention can be obtained while simplifying the arrangement of the invention over those of known wet desulphurization apparatus and can produce the above good effects.

The present invention is described in more detail by way of example.

Example The apparatus shown in Figure 2 was used.

The tank 103, which contained a slurry comprising gypsum crystals had a section of 1000 mm x 2000 mm with a depth for liquid of 2000 mm. By the use of the circulation pump 105,50 m3/h of the slurry was sprayed from the top of the absorption tower 102. In the tower were filled grids, and 3,000 NmCh of a flue gas was treated by a gas-liquid parallel current procedure and desulphurized until 1200 ppm of SO, at the inlet was reduced to 60 ppm of SO, at the outlet.

To the tank 103 was fed CaCO3 powder as absorbent from the line 108 in an amount corresponding to the amount of the absorbed 502. In the inside of the tank 103 was provided a circular partition plate 114 having an inner diameter of 400 mm and a height of 2500 mm and opened at the lower end thereof. The supernatant liquid was withdrawn from the liquid chamber 115 established by the partition wall 114. The supernatant liquid from the line 117 contained a slight amount of solid matters. However, by control of the flow rate of the supernatant liquid and the flow rate of the slurry at the discharge port 110 with a microcomputer, the concentration of the Ca compound in the slurry contained in the tank 103 could be controlled within a desired concentration ranging from 1 to 35 wt.%.

In this test, the solid matter discharged from the separator 111 had a composition of 97 wt.% of CaSO4.2H30, 0.5 wt.% of CaCO3, and 2.5 wt.% of other substances and was thus made substantially of gypsum, no calcium sulphite being detected.

During the operation, no air was fed from the air nozzle 109 provided in the tank 103. When the gasliquid contact of the parallel current system was effected in the grids-filled tower, SO, which was ab sorbed in the top of the absorption tower in the case of flue gas desulphurization by the wet lime process was all oxidized with oxygen in the exhaust upon pasing downward through the gridsfilled portion. Accordingly, no oxidation with air in the tank 103 was necessary.

During the operation of the test, although water was charged from the washing nozzle 121 and water for sealing the pumps was used, the concentration in the slurry could be controlled within a desired range according to the method of the invention.

According to the present invention, the oxidizing column, thickener, tank for filtrate, tank for supernatant liquid, and pumps, valves, and measuring instruments required for these devices needed in known wet flue gas desulphurization apparatus are not necessary, giving evidence that the operation procedure can be remarkably simplified.

Claims (5)

1. A method of controlling the concentration of slurry in an absorption tower in which a flue gas comprising SO, is brought into contact with a slurry suspending a Ca compound for absorbing the SO2, the method comprising withdrawing the slurry from the absorption tower in such a way that the slurry is divided into at least two components with different concentrations, one component being a slurry having a higher concentration of the Ca compound, the other component being a slurry having a lower concentration of the Ca compound, and regulating the rates of withdrawal of the components being withdrawn from the absorption tower in such a manner that the concentration of the Ca compound in the slurry being contacted with the flue gas is controlled within a predetermined range.

2. A method as claimed in claim 1, in which the absorption tower has a tank which has a partition with an open lower end to establish a liquid chamber between the partition and the absorption tower, a filtrate from which gypsum is separated from the slurry is passed downwards in the liquid chamber, and a supernatant liquid in the liquid chamber is withdrawn as the component having a lower concentration of the Ca compound.

3. A method as claimed in claim 2, in which an inclined plate is provided in the bottom of the tank beneath the liquid chamber and the component having a higher concentration of the Ca compound is withdrawn from a position adjacent the inclined plate.

4. A method of controlling the concentration of slurry substantially as described with reference to Figure 2 of the accompanying drawings.

5. A method as claimed in claim 4, substantially as described in the Example given.