GB2169888A - Wet desulfurization column for exhaust gases - Google Patents

Abstract

A wet desulfurization column comprises an absorbing section 2 in which exhaust gas containing sulfur dioxide and oxygen gas is brought into contact with absorbant resulting in desulfurization, a first tank 5 for receiving the absorbing solution falling through the absorbing section, a second tank 7 for receiving the solution from the first tank 5, circulating means 9 for feeding the solution in the second tank 7 to the absorbing section, an exhaust gas inlet 1 above, and outlet 4 below the absorbing section, oxidizing gas inlets 6 above the surface of the solution in the first tank 5 and below the exhaust gas outlet, and an inlet 8 for introducing an SO2 absorbent into the solution in the second tank. The first tank 5 is directly under the absorbing section 2 so that the solution can fall directly onto the solution in the tank. <IMAGE>

Description

SPECIFICATION Wet desulfurization column for exhaust gases The present invention relates to a wet desulfurization column for exhaust gases, and more specifically to a wet desulfurization column for absorbing a sulfurous acid gas (hereinafter referred to as SO2) and an oxygen gas (hereinafter referred to as 02) simultaneously in order to convert a sulfite into a sulfate and to thereby carry out desulfurization.

A wet desulfurization method for exhaust gases, which has most prevalently been put into practice of late, comprises first absorbing SO, with calcium carbonate (CaCO3) which is an absorbing agent, then recovering it in the form of calcium sulfite, and further oxidizing the latter with air to form calcium sulfate (gypsum).For the purpose of oxidizing the sulfite to the sulfate in a desulfurization column in which SO, is absorbed, there has been suggested a process where an 02 gas blowing device for generating fine gas bubbles in an absorbing solution tank is provided and oxidization is accomplished by blowing an 02 containing gas such as air or an exhaust gas therethrough (Japanese Patent Publication No. 17318/1975 and Japanese Patent Provisional Publication No. 18269/1976). In other absorbing columns which have been suggested, a desulfurization column, which is a desulfurization apparatus, is provided with an inlet for directly blowing an oxygen containing gas therethrough into an absorbing solution in order to oxidize a sulfite produced by a desulfurization treatment (Japanese Patent Publication Nos.

37295/1980 and 25617/1984 and Japanese Patent Provisional Publication No. 95216/1983).

In these conventional methods, the inlet for blowing the oxygen containing gas into the absorbing solution therethrough to oxidize the absorbing solution is disposed under the surface of the absorbing solution in the absorbing tank. Therefore, a scale of gypsum will adhere to the inlent, and consequently the dispersion of the oxidizing gas will be poor, with the result that the oxidization will be incomplete and the slight sulfite will remain therein disadvantageously. An amount of this unwelcome scale will increase with time, and thus the removal of the scale adhered to the inlet will be necessary. During removing the scale, however, the drive of the desulfurization apparatus must be interrupted, which fact will lead to economically great losses.

In particular, a factor important to heighten an oxidization efficiency is that the oxidizing gas is uniformly dispersed into the absorbing solution. For this, pipes having many orifices and atomizers having intricate mechanisms have been used as devices for dispersing gas bubbles therein. In this case, however, the adhesion of the scale will easily take place and will be liable to bring about the ununiformed dispersion of the oxidizing gas in the absorbing solution disadvantageously.

With the intention of preventing the formation of the scale on the gas bubble dispersing device, another method has been suggested in which air is blown onto the surface of the absorbing solution in the absorbing tank by means of an air blower and the absorbing solution is then finely sprayed into an air layer on the absorbing solution therein in order to bring the absorbing solution into contact with air and to thereby oxidize a sulfite produced by a desulfurization treatment (Japanese Patent Publication No. 37252/1983).

According to this method, the oxidizing gas dispersing device can be omitted, and thus, needless to say, the clogging trouble by the scale can be avoided. However, a spray device is additionally required and will be still clogged with a gypsum scale, and thus in the abovementioned method, any fundamental solution cannot be expected, either. In addition thereto, since a solubility of oxygen in water is extremely poor (the absorption of oxygen into the absorbing solution is the movement of substances which dominate resistance in the solution), the oxidation method of finely spraying the absorbing solution is inconveniently lower in oxidation efficiency than the oxidation method of blowing the fine gas bubbles into the absorbing solution.

It is further important and preferable to promptly oxidize the absorbing solution in which the sulfite produced by the desulfurization treatment is present. This reason is that dissolved sulfite ions in the absorbing solution will combine easily with calcium ions to thereby become solid calcium sulfite and the formed calcium sulfite will scarcely be oxidized unless sulfuric acid will be added thereto to forcedly dissolve the solid calcium sulfite.

In consequence, the method in which a long time is taken to bring the absorbing solution into contact with oxygen by means of the spray is disadvantageous for the oxidation, because it allows the sulfite to become the solid.

For this reason, a suitable manner of carrying out the prompt oxidation of the absorbing solution is to simultaneously accomplish the absorption of the sulfurous acid gas and the absorption of oxygen. However, there is a big difference between solubilities of the sulfurous acid gas and the oxygen gas in water, and the absorption of the oxygen gas will thus delay. As a result, for example, when an exhaust gas containing the sulfurous acid gas is subjected to the desulfurization treatment by the use of the oxygen gas, the sulfite will remain inevitably in the absorbing solution.However, if such specific conditions as a low concentration of the sulfurous acid gas and a contrarily high concentration of the oxygen gas are provided, and if a gas/liquid contact efficiency is heightened by using, for example, a sufficiently tall packed column, the oxidation of the sulfite can be accelerated (Japanese Patent Provisional Publication No.

130697/1975).

Such a method of simultaneously carrying out the absorption and the oxidation of the sulfurous acid gas requires no facilities for introducing the oxidizing gas, but has the following serious drawback. That is to say, since the oxidation is performed while the sulfurous acid gas is absorbed, a small amount of the sulfite will remain inevitably in the absorbing solution after the desulfurization treatment. This reason is as follows: If it is attempted to carry out the desulfurization till the sulfurous acid gas it not in the least present any more in an exhaust gas, too much costs of equipments will be taken, which fact means that such an attempt is impossible from an econical viewpoint.Accordingly, a bit of the remaining sulfurous acid gas will be discharged into the atmosphere as a purified exhaust gas, and thus the absorbing solution just separated from the exhaust gas will naturally contain the sulfite, but sparingly soluble oxygen will not be fed any more. In consequence, when the desulfurization and the oxidation are carried out simultaneously by the contact with the exhaust gas, the small amount of the sulfite will inevitably remain in the absorbing solution after the desulfurization treatment. This tendency wil be more remarkable in the case that the absorbing operation is carried out by a counter flow system than by a parallel flow system.

The inventors of the present application have confirmed on the basis of experiments that the small remainin sulfite in the absorbing solution will noticeably decline the solubility of a CaCO3 crystal (which is, in general, called calcium carbonate or limestone) which is an absorbing agent for the sulfurous acid gas. Inherently, the CaCO3 crystal is sparingly soluble in water and its solubility is as low as 2 mmol/l or so. In the absorbing solution in which the sulfite is dissolved in a small amount of about 0.1 to about 10 mmol/l, the solubility of the CaCO3 crystal will be still poorer. The higher the concentration of the dissolvingly remaining sulfite will be, the lower the solubility of the CaCO3 crystal will be inversely.As a result, it has been experimentally confirmed by the inventors of the present case that if the sulfite is present in a small amount of 1 to 10 mmol/l in the absorbing solution, the solubility of the CaCO3 crystal will decline and the performance of the desulfurization will thus deteriorate disadvantageously.

Fig. 2 attached hereto shows measured results of dissolving velocities of the CaCO3 crystal in the case that a coal-fired exhaust gas containing SO, and 03 brought into contact with a suspension containing the CaCO3 crystal as the absorbing agent in a grid packed column.

SO, will become H2503 when absorbed, and the latter will become sulfuric acid when oxidized.

These conversions can be represented by the following formulae (1) and (2): SO2+H20 ,H2SO3 (1) H,SO, + 21 O2#H3SO4 (2) The reactions of the formulae (1) and (2) make successive progress in the grid packed column, and when the column is tall, most of the SO, will be converted with H2SO4. However, since the infinitely tall column cannot be constructed as mentioned above, H2503 will remain a little therein.

When the sparingly soluble CaCO3 crystal is dissolved and a neutralization reaction takes place, gypsum CaCO4 will be produced in accordance with the following formula (3).

H2SO4+CaCO3~CaSO4 (3) Since a steady state of this formula (3) decides the performance of the desulfurization, accelerating the dissolution of the CaCO3 crystal is important to improve the desulfurization performance.

By changing an oxygen concentration and a column height which are gas/liquid contact conditions, a concentration of the unoxidised sulfite [which is a generic expression and includes HSO;, SO32- and other ions in addition to H2503 shown in the formula (1)] which is present in the absorbing solution can be varied on the level of a trace concentration. Fig. 2 exhibits dissolving velocities of the CaCO3 crystal shown in the formula (3), which results have been obtained by changing the concentrations of the remaining sulfite in accordance with the procedure of the undermentioned embodiment.

As be definite from the results shown in Fig. 2, the concentration of the remaining sulfite is as low as about 1 to about 10 mmol/l in contrast to the fact that the concentration of the reaction product CaSO4 contained in the absorbing solution is about 1,000 mmol/l and that of the CaCO3 crystal is about 100 mmol/l, but the sulfite reduces the dissolving velocity of the CaCO3 crystal remarkably.

Now, reference will be made to the concentration of the small remaining sulfite to which much attention should be paid in the present invention. CaSO4 which is the main component of the absorbing solution is the product obtained by absorbing SO, and then carrying out the oxidation, and has the concentration of about 1,000 mmol/l. If the absorption of SO, is carried out in the absence of 03, the main component of the absorbing solution will be calcium sulfite naturally and the concentration of the latter ought to be about 1,000 mmol/l.However, when the concentrations of CaSO4 and the unoxidized sulfite are about 1,000 mmol/l and 1 to 10 mmol/l, respectively, it is meant from these concentrations that the absorbed SO, compound has been oxidized as much as 99 to 99.9%. Accordingly, it is to be noted that the concentration of the remaining sulfite which is taken up as the problem in the present invention is on a negligible small level.

It seems that the expression "all of SO2 has conventionally been oxidized" all corresponds to the case that the small amount of the remaining sulfite is still detected in the present invention.

In other words, if the quantitative analysis of the sulfite is carried out particularly without taking its trace concentration range into consideration, such a low concentration of the sulfite will not be detected.

Repeadedly speaking, the expression that the absorbed SO2 has been all oxidized means the case where the small amount of the sulfite still remains under the conditions of the present invention.

The oxidation proportion of 99% signifies the fact that the sulfite remains as much as about 10 mmop/p, and this amount of the latter is the problem to be solved in the present invention, as be apparent from Fig. 2.

It is not economical to dispose an additional oxidizing device for the purpose of oxidizing the small amount of the sulfite completely. Therefore, economically preferable is a method which comprises causing the absorbing solution, in which SO, has already been absorbed, to pass through an air layer in order to thereby accomplish its oxidization (Japanese Patent Provisional Publication No. 178326/1983). However, this conventional method also has drawbacks of employing a gas/liquid counter flow system and feeding the absorbing agent CaCO3 to an absorbing solution tank into which the absorbing solution is falling.

First, when the gas/liquid counter flow system is employed, the concentration of SO2 in the absorbing solution will reach a maximum level in the vicinity of a boundary through which the absorbing -solution flows out from the contact range where the latter is brought into contact with the exhaust gas, and thus the absorbing solution containing a great deal of the unoxidized sulfite (rather, containing the scarcely oxidized sulfite) will fall into the absorbing tank, with the result that an oxidation efficiency will be poor and thus the perfect oxidation will be extremely difficult.

Although the oxidation efficiency can be heightened by fairly enlarging the air layer, such a manner will lead to an economical disadvantage.

Next, the drawback of feeding the absorbing agent to the absorbing solution tank, into which the absorbing solution is falling, is that a dissolving velocity of CaCO3 will be lowered because of feeding CaCO3 prior to the perfect oxidation of the sulfite. In other words, since CaCO3 is fed thereto before the oxidation of the sulfite has been completed, the dissolving velocity of CaCO3 will lag, so that the oxidation efficiency will be worsened. After all, if the increase in the oxidation efficiency is attempted, an absorbing solution tank having a large volume will be required inconveniently.

Summary of the Invention An object of the present invention is to provide a wet desulfurization apparatus for exhaust gases by which the above-mentioned conventional technical drawbacks can be overcome.

That is to say, the present invention provides a wet desulfurization column for an exhaust gas which is characterized by comprising (1) an absorbing section in which the exhaust gas containing a sulfurous acid gas and an oxygen gas is brought into contact with an absorbing solution in order to carry out desulfurization, (2) a first absorbing solution tank for receiving the absorbing solution falling through the absorbing section, (3) a second absorbing solution tank for receiving the absorbing solution coming from the first absorbing solution tank, (4) a circulating means for feeding the absorbing solution in the second absorbing solution tank to the absorbing section, (5) an exhaust gas inlet, for introducing the exhaust gas therethrough, disposed above the absorbing section and an exhaust gas outlet, for discharging the exhaust gas therethrough, disposed under the absorbing section, (6) one or more oxidizing gas inlets, for introducing a gas containing oxygen therethrough, disposed above the surface of the absorbing solution in the first solution tank and under the exhaust gas outlet, and (7) a sulfurous acid gas absorbing agent inlet for introducing a sulfurous acid gas absorbing agent therethrough into the absorbing solution in the second absorbing solution tank, (8) the first aborbing tank being disposed just under the absorbing section so that the absorbing solution may fall through the absorbing section directly onto the surface of the absorbing solution in the first absorbing solution tank.

The reason why the gas/liquid parallel flow system of the present invention can provide a satisfactory effect is that the absorbing solution falls while absorbing SO, and simultaneously being brought into contact with oxygen in the exhaust gas in order to increase the oxidation rate, and that a kind of filler and a height of the column are regulated suitably in order to sufficiently carry out the oxidation with 03 present in the exhaust gas.

Further, according to the present invention, in order to achieve the perfect oxidation of the slightly residual sulfite, the high concentration layer of the oxygen gas is formed on the surface of the absorbing solution in the first absorbing solution tank, and when the absorbing solution falls onto the solution surface, the oxygen gas will be embraced into the absorbing solution. In consequence, the amount of the oxidizing gas to be used may be smaller than in the conventional case.

In such a way, by employing the parallel flow system, by forming the high concentration layer of the oxygen gas on the surface of the absorbing solution, and by utilizing the phenomenon of embracing the fine gas bubbles at the time when the absorbing solution is falling, the sulfite can be oxidized even until a level of less than 1 mmol/l.

Afterward, the absorbing solution in which the sulfite has been oxidized to an enough degree is guided to the second absorbing tank, and the CaCO3 crystal which is the SO2 absorbing agent is then fed to the second absorbing tank. In this case, the dissolving velocity of CaCO3, since not affected by the sulfite, can be maintained effectively at a high level.

The most important constitution of the present invention is that the first absorbing tank is separated from the second absorbing tank; the oxygen gas is fed onto the surface of the absorbing solution in the first absorbing solution tank and is then dispersed into the absorbing solution by the utilization of the falling droplets of the absorbing solution; and CaCO3 is fed to the second absorbing solution tank. Needless to say, since a nozzle for dispersing the gas bubbles in the absorbing solution is not inserted into the latter as in the case of the conventional technique, any clogging of the nozzle by a scale cannot take place.

As be definite from the foregoing, in the present invention, such an oxidizing device as the spray nozzle for oxidization can be cancelled, and additionally the oxidization can be achieved completely. In consequence, the present invention can advantageously prevent the deterioration in the dissolving velocity of the CaCO3 absorbing agent which cannot be solved by any conventional methods.

Brief Description of the Drawings Figure 1 is a schematic view illustrating a wet desulfurization tower for exhaust gases regarding the present invention; and Figure 2 shows experimental data regarding an influence of a dissolved sulfite on the reaction of CaCO3 crystal with H2SO4.

Description of the Preferred Embodiment In Fig. 1, reference numeral 1 is an exhaust gas inlet, and numeral 2 is an absorbing section.

A gas containing 1,000 ppm of SO, and 4% of 03 was introduced into the absorbing section 2 through the exhaust gas inlet 1, and the gas was brought into contact with the absorbing solution containing the CaCO3 crystal in a gas/liquid parallel system.

A nozzle 3 for spraying the absorbing solution therethrough was provided above the absorbing section 2, and the sprayed absorbing solution fell through the absorbing section 2 which was packed with grids. During its passage through the absorbing section 2, the absorption of SO, and an oxidation reaction with 03 took place simultagenously.

Under the absorbing section 2, an exhaust gas outlet 4 for allowing the exhaust gas to be discharged therethrough was disposed, and the exhaust gas was separated from the absorbing solution and was then discharged from the column. The concentrations of SO2 and 03 present in the exhaust gas at the exhaust gas outlet 4 were 50 ppm and 4%, respectively.

The absorbing solution separated from the exhaust gas fell downward straightly onto the surface of the absorbing solution in the first absorbing solution tank 5 in order to embrace a gas, so that the upper portion of the absorbing solution in the first absorbing solution tank 5 contained fine gas bubbles.

In order to oxidize a sulfite with the embraced gas bubbles, air was introduced into the column through an oxidizing gas inlet 6 so as to form a high concentration layer of the oxygen gas on the surface of the absorbing solution.

Since the absorbing section 2 was packed with the grids, each droplet of the absorbing solution which fell from the lower end portion of the absorbing section had a diameter of 5 mm, and the absorbing solution in the first absorbing tank 5 was in a state vigorously mixing with the gas in the vicinity of the surface thereof.

Next, the absorbing solution was delivered to the second absorbing solution tank 7, and limestone (CaCO3 crystal) which has been ground to 325 meshes or less was then fed as an SO2 absorbing agent to the second tank 7 through a sulfurous acid gas absorbing agent inlet 8 together with supply water, and in this case, the amount of the limestone was regulated so that a pH of the absorbing solution in the second absorbing solution tank might be within the range of 5.0 to 6.2. The concentration of the sulfite in the absorbing solution in the second absorbing solution tank was 1 mmol/l or less, and a reactivity of the CaCO3 crystal was food. Further, the reaction proportion of CaCO3 was 95% or more.

The absorbing solution in the second absorbing solution tank 7 was then forwarded to the absorbing solution spray nozzle 3 by the use of a circulating pump 9.

CaSO4 produced by the absorption and the oxidation of SO, had the morphology of a dihydrate gypsum crystal and was accumulated in the absorbing solution, and the product was then discharged from the system by means of a drawing pump 10 in accordance with a material balance. The discharged absorbing solution was treated in a centrifugal separator (not shown) in order to recover the di-hydrate gypsum crystal as a by-product.

For comparison, experiments were carried out by diminishing the amount of air coming through the oxydizing gas inlet 6 or by stopping its feed. The concentration of the sulfite dissolved in the absorbing solution was changed by regulating the amount of air, and at this time, the dissolving velocities of the CaCO3 crystal were obtained on the basis of excessive feeds of CaCO2. The results are shown in Fig. 2.

According to the present invention, it was confirmed that the concentration of the sulfite could be lowered to 1 mmol/l or less, and the reduction in the dissolving velocity of the CaCO3 absorbing agent could be prevented.

In the above embodiment of the present invention, CaCO3 was employed as the absorbing agent, but for the purposes of accomplishing the perfect oxidation of the sulfite and minimizing the loss of the absorbing agent, absorbing agents other than CaCO3, for example, Ca(OH)2, dolomite, Mg(OH)2 and the like can, needless to say, be utilized.

The following facts were confirmed by experiments: Even when a gas/liquid counter flow system was employed, the oxidation of the sulfite could be accelerated by separating the absorbing solution tank into two tanks; one of the tanks receives an absorbing agent and another tank promotes the oxidation. Thus, the loss of the absorbing agent could be decreased conveniently. However, the parallel flow system was superior to the oxidation effect to the counter flow system.

Claims (4)

1. A wet desulfurization column for an exhaust gas which is characterised by comprising an absorbing section in which said exhaust gas containing a sulfurous acid gas and an oxygen gas is brought into contact with an absorbing solution in order to carry out desulfurization, a first absorbing solution tank for receiving said absorbing solution falling through said absorbing section, a second absorbing solution tank for receiving said absorbing solution coming from said first absorbing solution tank, a circulating means for feeding said absorbing solution in said second absorbing solution tank to said absorbing section, one or more oxidizing gas inlets, for introducing a gas containing oxygen, disposed above the surface of said absorbing solution in said first tank and an absorbing agent inlet for introducing a sulfurous acid gas absorbing agent into said absorbing solution in said second absorbing solution tank, said first absorbing tank being disposed just under said absorbing section so that said absorbing solution will fall though said absorbing section directly onto the surface of said absorbing solution in said first absorbing solution tank.

2. A wet desulphurisation column as claimed in claim 1 having an exhaust gas inlet above the absorbing section, and an exhaust gas outlet below the absorbing section, whereby the exhaust gas inward flows downward through the absorbing section, the oxidising gas inlet or inlets being below the exhaust gas outlet.

3. A wet desulphurisation column for an exhaust gas containing sulphur dioxide, substantially as herein described with reference to Fig. 1 of the accompanying drawings.

4. A method of desulphurising an exhaust gas which contains sulphur dioxide and oxygen, which method comprises passing the exhaust gas in contact with the downwardly flowing absorbing solution in an absorbing region, allowing the absorbing solution to fall from the absorbing region into a first absorbing solution tank, passing an oxygen-containing gas into the falling absorbing solution above the solution surface in the first tank, transferring the absorbing solution from the first tank to a second tank, recirculating the absorbing solution from second tank to the absorbing region, and adding to the absorbing solution in the second tank an absorbing agent for sulphur dioxide.