JPH0478418A - Method for desulfurizing waste gas - Google Patents

Abstract

PURPOSE:To completely prevent the deposition of scales and residue in a circulating line as a factor of clogging in a desulfurization tower and to reduce the cost by using inexpensive soft-burned magnesium oxide as the material for the desulfurizing agent. CONSTITUTION:A slurry of the fine particles of high-purity and highly reactive magnesium hydroxide is supplied to the latter part A2 of a desulfurization tower A, the processing soln. is kept at pH 6-8, hence the sulfur oxides remaining in the waste gas G2 are sufficiently absorbed, and an almost perfect desulfurization reaction is attained. The processing soln. having absorbed the sulfur oxides in the tower A, ordinarily the processing soln. in the latter part A2, is introduced into the former part A1 through a pump P6 and a pipeline L6, moved countercurrently to the waste gas and discharged outside the system from the former part A1 through a discharge pipe 10. The discharged soln. is ordinarily introduced into an oxidation lower 12, the MgSO4 and Mg(HSO3)2 in the soln. are oxidized in the tower, and an aq. soln. of MgSO4 is obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a method for desulfurizing flue gas containing sulfur oxides such as combustion flue gas from heavy oil and coal, particularly a desulfurizing method using lightly burnt magnesium oxide as a desulfurizing agent. Regarding.

[Conventional technology]

Generally, desulfurization of flue gas is carried out in a vertical desulfurization tower by flowing down a treatment liquid consisting of an aqueous solution or slurry of a desulfurizing agent in a shower form from the top, and continuously connecting this treatment liquid with the flue gas introduced from below. Through contact, sulfur oxides in the exhaust gas are fixed as sulfates and sulfites. Normally, the treatment liquid after flowing down, that is, the desulfurization liquid, is pumped up and circulated together with a continuously supplied new treatment liquid. On the other hand, the increased amount due to the above-mentioned supply is discharged.

Conventionally, the above desulfurization agents include sodium hydroxide,
Hydroxides such as ammonium hydroxide, magnesium hydroxide, calcium hydroxide, and base oxides such as calcium oxide are known, and among these, magnesium hydroxide has been widely used in recent years. In the case of magnesium hydroxide, the desulfurization product is easily soluble in water and does not suffer from the scaling problem of calcium-based desulfurization agents, and magnesium hydroxide can be produced by adjusting the pH of the treatment solution to about 6. This is because it can be used in the form of a dissolved aqueous solution.

However, since magnesium hydroxide is obtained by reacting calcium hydroxide with magnesium ions in seawater, it requires a large amount of seawater, requires a large amount of water to wash after construction, and is expensive to transport as a slurry. etc., are relatively expensive.

Therefore, light-burned magnesium oxide, which produces magnesium hydroxide through a hydration reaction, or magnesium carbonate (MgCO,) ore such as magnesai, is used at a relatively low temperature (8
Magnesium hydroxide obtained by calcination and pulverization at a temperature of about 0.0 to 1,000° C. is attracting attention as it can be obtained at a lower cost than magnesium hydroxide.

[Problem to be solved by the invention]

However, this lightly calcined magnesium oxide is converted into magnesium hydroxide through a hydration reaction as a pre-desulfurization step, but the drawback is that this hydration reaction is slower than that of calcium oxide, which is a similar basic oxide. There is.

In addition, light calcination-resistant magnesium is made by calcining coarsely crushed ore lumps of various sizes and then pulverizing them. While hard clinker-like areas with low reactivity occur due to excessive calcination, uncalcined magnesium carbonate remains in the center of large ore blocks, and CaC is present in the ore.
There are impurities such as , 5i02, Alx Oz,
As a result, the powder is a non-uniform powder containing a mixture of magnesium oxide and other components with large variations in reactivity.

For this reason, if such lightly burnt magnesium oxide is supplied as a water slurry into a desulfurization tower, the desulfurization efficiency will be low, and in addition to the slow reaction rate of magnesium oxide, the highly reactive components will be prioritized. Unreacted substances, mainly magnesium oxide with low reactivity, and other components are deposited as residue in the circulation system, and this increase in the amount of deposits can cause scale build-up on circulation pumps and piping. This poses a problem in that clogging is likely to occur, making it impossible to continue the smooth operation of the desulfurization equipment.

In view of the above-mentioned circumstances, this invention uses lightly burnt magnesium oxide as a desulfurizing agent raw material and increases its utilization rate.
Another object of the present invention is to provide a method that prevents the accumulation of residue in the circulation system of the desulfurization tower and thereby performs stable and efficient exhaust gas desulfurization at low processing costs.

Means for Solving Problem C] In the course of intensive studies to achieve the above object, the inventor first added water to lightly burnt magnesium oxide to cause a good hydration reaction, and then subjected it to wet classification. When placed in a container, the magnesium hydroxide produced from the highly reactive magnesium oxide component has an average particle size of 10μ or less, usually 1 to 2μ.
It separates into the upper layer as a fine particle slurry, and in the lower layer, unreacted magnesium oxide, magnesium carbonate, calcium carbonate, and other impurities with poor reactivity remain in the original shape of the pulverized product and separate as a coarse particle slurry. It turned out to be.

Therefore, if the desulfurization tower is divided into a front stage where the pH of the treated liquid is low and a rear stage where the pH is high, and the coarse particle slurry from the lower layer is supplied to the front stage side, the reactivity will be poor due to the low pH of the liquid. Sufficient reactivity can be obtained even with unreacted magnesium oxide, and on the other hand, by supplying the fine particle slurry in the upper layer to the later stage side where the pH of the processing solution is higher, high reactivity can also be obtained due to its activity. The inventors have discovered that the desulfurization reaction can be carried out efficiently without producing residual metal in the circulation system of the desulfurization tower, and that it is possible to perform stable and efficient exhaust gas desulfurization at low cost. I came to do this.

That is, the present invention provides a desulfurization tower for absorbing sulfur oxides into the treatment liquid by continuously bringing the exhaust gas containing sulfur oxides into gas-liquid contact with the treatment liquid containing the desulfurization agent. Consisting of a front part on the inlet side and a rear part on the exhaust gas outlet side,
The pH of the treatment liquid on the front stage side is set to 3 to 5, and the pH of the treatment liquid on the rear stage side is set to 6 to 8, and outside the desulfurization tower composed of both stages, Furthermore, an external treatment system is provided for supplying the above-mentioned treatment liquid into the desulfurization tower, and here, a) water is added to light burnt magnesium oxide to generate a slurry containing magnesila hydroxide 1, and this is wet-processed. A classifier separates the fine particle slurry into a coarse particle slurry, b) the coarse particle slurry is supplied as the treatment liquid to the front stage of the desulfurization tower, and the fine particle slurry is supplied as the treatment liquid to the rear stage of the desulfurization tower, respectively. The present invention relates to an exhaust gas desulfurization method characterized by:

[For production]

In this invention, a slurry produced by adding water to lightly burnt magnesium oxide is separated into fine particle slurry of magnesium hydroxide and other coarse particle slurry using a wet classifier. The latter coarse particle slurry is supplied to the latter stage where the pH is high.
Since the desulfurizing agent component is supplied to the first stage where H is low, the desulfurizing agent component reacts efficiently in both stages, and as a result, the amount of residue deposited in the circulation system of the desulfurization tower is reduced.

〔Example〕

An embodiment of the exhaust gas desulfurization method according to the present invention will be described below with reference to the drawings.

In FIG. 1, A is a two-column type desulfurization tower, which is composed of a front stage section A on the exhaust gas introduction side and a rear stage section A2 on the exhaust gas output side. Reference numeral B designates an external treatment system provided outside the desulfurization tower A to supply a treatment liquid containing a desulfurization agent into the desulfurization tower A.

In the pre-stage AI of the desulfurization tower A, the treatment liquid containing the desulfurization agent supplied from the external treatment system B is made to flow down from above in the form of a shower, and this treatment liquid and the exhaust gas G1 containing sulfur oxides introduced from below are combined. By bringing the sulfur oxide into gas-liquid contact, sulfur oxides are absorbed and fixed in the treatment liquid as magnesium sulfite and the like through a desulfurization reaction, and the exhaust gas G2 in which the sulfur oxides have been absorbed is discharged from above.

In addition, in the rear stage A2, the exhaust gas G2 discharged from the front stage AI is guided from below and brought into gas-liquid contact with the treatment liquid containing the desulfurization agent supplied from the external treatment system B in the same manner as described above. The same absorption and fixation reaction as described above is carried out, and the exhaust gas G, in which the remaining sulfur oxides have been almost completely absorbed and removed, is discharged from above to the outside of the tower.

The treatment liquid flowing down into the lower tank a of the front stage A and the lower tank a of the rear stage A2, that is, the desulfurization liquid that has absorbed sulfur oxides, is usually Mg SO3, Mg SOa
, M g (HS 03)2, etc., are sent to the upper part through pump P1 and piping Ll and pump P2 and piping L2, respectively, together with the newly supplied desulfurizing agent, and by repeating this process, the previous stage It is configured to circulate continuously within section A and within rear section A2.

External treatment system B is a hydration reaction tank for lightly burnt magnesium oxide.
, a wet classifier 2 consisting of a liquid cyclone, a storage tank 3 for coarse particle slurry, and a storage tank 4 for fine particle slurry, and a stirrer 5.6.7 is attached to each of the reaction tank 1 and storage tank 3.4. There is.

In the hydration reaction tank 1, lightly burnt magnesium oxide supplied from the raw material supply pipe 8.9 and water or warm water are stirred and mixed well to cause a uniform hydration reaction, and a slurry containing magnesium hydroxide produced by this reaction is produced. is sent to a wet classifier 2 by a pump P3 and a pipe L3, where a fine particle slurry of magnesium hydroxide and a coarse particle slurry other than this are separated into upper and lower layers and stored in storage tanks 3 and 4, respectively.

The coarse particle slurry in storage tank 3 is pumped through pump P4 and piping L4.
is supplied as the treatment liquid into the front part A of the desulfurization tower A, and the amount is such that the pH of the treatment liquid flowing down to the lower tank a1 of the front part AI is maintained in the range of 3 to 5. Automatically controlled by valve Vl and pH controller PHC.

Further, the fine particle slurry in the storage tank 4 is supplied as the treatment liquid into the rear stage A2 of the desulfurization tower via the pump P5 and the pipe L5, and the amount of the fine particle slurry is determined by the amount of the treatment liquid flowing down into the lower tank a of the rear stage A2. so that the pH of is maintained in the range of 6 to 8,
Automatically controlled by valve V2 and pH controller PHC.

In such a desulfurization system, the front stage A1 to the desulfurization tower
A large amount of unreacted components such as unreacted magnesium oxide, magnesium carbonate, and calcium carbonate contained in the coarse particle slurry will be supplied to the tank, but these components will be supplied when the pH of the treatment liquid in the lower tank a increases. Since it has a low value of 3 to 5, it easily dissolves in this liquid and reacts, thereby sufficiently contributing to the desulfurization reaction.

On the other hand, in the latter part A2 of the desulfurization tower A, a fine particle slurry of highly pure and highly reactive magnesium hydroxide is supplied as a desulfurization agent, and the pH of the treated liquid is set to a high value of 6 to 8. , the sulfur oxides remaining in the exhaust gas G2 are well absorbed, and a nearly complete desulfurization reaction is achieved.

Therefore, in the desulfurization tower A, both the front stage section A and the rear stage section A2 are pumps PI in each circulation system.

This means that stable operating conditions can be maintained without scaling or blockage caused by residues in P2 and the pipes Ll and L2.

The treated liquid in the desulfurization tower A that has absorbed sulfur oxides in the exhaust gas in this way is normally pumped into the downstream part A2 by the pump P.
6 and piping L6 to the front stage A1, that is, the gas is moved countercurrently to the exhaust gas, and is discharged from the front stage A1 side through the discharge pipe 10 to the outside of the system. The amount of water in the lower tank a depends on the amount of desulfurization agent supplied from the external treatment system B and the amount of water supplied from the water supply pipe 11 to the downstream section A2.

Adjustment is made so that the water level of a2 remains constant. Note that the countercurrent movement from the rear section A2 to the front section A may be stopped, and the liquid may be discharged from both sections An and At at the same time.

Such discharged liquid is normally led to an oxidation tower 12, where Mg5O and Mg(HS0x)z components in the liquid are oxidized to an aqueous solution mainly containing Mg5O, and a desulfurizing agent is passed through a filter 13. After removing impurities such as A 1103 and S 10 z that did not participate in the reaction in the raw material, the raw material is discharged or reused as a sulfate radical.

Here, since the effluent after oxidation produced in the oxidation tower 12 has a low p)l due to the by-product sulfuric acid, a part of the fine particle slurry of magnesium hydroxide is supplied from the storage tank 4 to the effluent as appropriate. It is preferable to perform a neutralization treatment to pH.

In addition, in the above embodiment, the desulfurization tower A is divided into the front and rear stages A+, A
x is a two-column type, but as shown in Fig. 2, one column may be partitioned into upper and lower parts, and the upper and lower parts may be used as the front stage part A on the exhaust gas introduction side and the rear stage part A on the exhaust gas outlet side. In addition, as shown in Fig. 3, two reaction towers with different absorption mechanisms are connected on the left and right, and both towers are connected to the front section A on the exhaust gas introduction side.
It is also possible to use the latter part A2 on the exhaust gas outlet side. Each reference numeral in FIGS. 2-3 is the same as in FIG. 1, and the explanation thereof will be omitted.

As the lightly calcined magnesium oxide used in this invention, those obtained by low-temperature calcination of the above-mentioned magnesium carbonate ore are preferably used, but those obtained from other raw materials, such as those generated from a rotary kiln during the manufacture of magnesium oxide clinker, are preferably used. Collected dust can also be used.

〔Effect of the invention〕

As described above, according to the exhaust gas desulfurization method of the present invention,
In addition to being able to use inexpensive lightly burnt magnesium oxide as a raw material for the desulfurization agent, its utilization rate can be greatly increased, making it possible to significantly reduce processing costs compared to conventional desulfurization methods using general-purpose magnesium hydroxide. It is possible to completely prevent the accumulation of residues that cause scale adhesion and blockage in the circulation system, thereby enabling stable and efficient exhaust gas desulfurization at low cost.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a configuration example of an apparatus used in the exhaust gas desulfurization method of the present invention, and FIGS. 2 and 3 are schematic diagrams showing other configuration examples of the desulfurization tower used in the present invention. A...Desulfurization tower, A...front stage section, A! ...Late stage section, B...External processing system, G+, Gz, Gs...
・Exhaust gas,...hydration reaction tank for lightly burnt magnesium oxide,
2 Wet classifier, 3... Coarse particle slurry storage tank,...
Particulate slurry storage tank

Claims (1)

[Claims] (1) A desulfurization tower is installed in the front part of the exhaust gas introduction side to bring the exhaust gas containing sulfur oxides into continuous gas-liquid contact with the treatment liquid containing the desulfurization agent so that the sulfur oxides are absorbed into the treatment liquid. The pH of the processing liquid on the front stage side is set to 3 to 5, and the pH of the processing liquid on the rear stage side is set to 6.
- 8, respectively, and an external treatment system is provided outside the desulfurization tower composed of both stages to supply the above-mentioned treatment liquid into the desulfurization tower, and here, a) light oxidation Water is added to magnesium to generate a slurry containing magnesium hydroxide, and this is separated into a fine particle slurry and a coarse particle slurry using a wet classifier, and b) the coarse particle slurry is sent to the front stage of the desulfurization tower as the treated liquid. . A method for desulfurizing exhaust gas, characterized in that the fine particle slurry is supplied as the treatment liquid to a later stage of a desulfurization tower.