JPH0256129B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a flue gas desulfurization method, and in particular to a flue gas desulfurization method that recovers the deterioration in desulfurization performance caused by aluminum compounds contained in flue gas and further improves the desulfurization performance. Regarding.

In the absorption process of wet lime plaster method flue gas desulfurization equipment,
Exhaust gas containing SO ₂ and CaCO ₃ , CaSO ₃・1/2H ₂ O,
SO ₂ is absorbed from the exhaust gas by contacting it with a slurry containing a calcium compound with low solubility such as CaSO ₄ .2H ₂ O. The overall reaction formula for the SO ₂ absorption reaction is SO ₂ + CaCO ₃ ＋1/2H ₂ O →CaSO ₃・1/2H ₂ O＋CO ₂ (1) The following oxidation reaction also occurs due to the oxygen in the exhaust gas.

CaSO ₃・1/2H ₂ O＋1/2O ₂ +3/2H ₂ O →CaSO ₄・2H ₂ O (2) Although the overall reaction formula is simple, the actual reaction mechanism is not as simple as this.
It is said that various ions are involved in an extremely complex manner, and there are a wide variety of chemical factors that affect the desulfurization performance in the absorption process. This is still evident from the fact that various analyzes are being conducted based on various studies and various desulfurization processes are being proposed.
In addition to the complexity of SO ₂ absorption reactions in phase systems, research into the effects of a wide variety of trace elements is endless.

In addition to SO _2, combustion exhaust gas contains many compounds such as ammonia compounds, halogen compounds, and dust as trace components, and the composition of dust varies depending on the source of the exhaust gas, making the compounds even more complex and diverse. It's crossing.

CaCO ₃ used as an absorbent for SO ₂ is prepared from naturally occurring limestone, but it also contains large amounts of impurities such as aluminum, silicon, and iron. Furthermore, it goes without saying that the make-up water, which is indispensable in the wet vision method, also contains impurities.

As shown above, aluminum compounds are included as trace components other than SO ₂ in the slurry in the absorption process of the wet lime gypsum method and flue gas desulfurization equipment. Aluminum compounds originate from dust in exhaust gas, absorbents, make-up water, etc., and the exhaust gas, make-up water, and absorbent that flow into the absorption process each contain aluminum compounds, albeit in varying concentrations.

The present inventors have discovered that aluminum compounds, especially _complex salts ^of aluminum (hereinafter referred to as aluminum ion Faced with the problem of deterioration in desulfurization performance due to this problem, various countermeasures have been investigated. The mechanism of the reaction that aluminum compounds have on desulfurization performance is not clear, but it is thought that the aluminum complex envelops absorbents such as CaCO ₃ and hinders the dissolution rate of these absorbents.

In addition, some of the aluminum compounds that flow into the absorption process are dissolved and ionized, but the rest exists as solid phase aluminum compounds, and the amount dissolved depends on the history of the aluminum compounds and the atmospheric conditions at the site. are different and cannot be determined uniquely. For example, we have experienced that the degree of dissolution during the absorption process of aluminum compounds in the dust generated by burning coal varies depending on the source of the coal. Currently, it is necessary to theoretically solve the proportions of aluminum compounds contained in substances that flow into the absorption process, such as absorbents, make-up water, and exhaust gas, in the liquid phase and solid phase of the slurry in the absorption process. is not possible, but
At least empirically, it is thought that most of it does not exist in the liquid phase, but rather in the solid phase.

As a countermeasure against the above deterioration in desulfurization performance, a method of promoting the discharge of aluminum ions to the outside of the system can be considered. This is done by neutralizing aluminum ions and separating them by precipitation, or by increasing the amount of water discharged, thereby keeping the aluminum ion concentration of the slurry in the absorption process low and reducing the drop in desulfurization performance. However, the precipitation separation treatment operation by neutralizing aluminum ions is troublesome and impairs economic efficiency, and the increase in wastewater volume is
From the perspective of preventing secondary pollution caused by wastewater, there is a problem that it does not meet the needs of the times. especially,
The main component of dust contained in coal combustion exhaust gas is aluminum, and when such exhaust gas is treated with a wet lime-gypsum flue gas desulfurization equipment, aluminum dissolves and becomes aluminum ions, which increases the amount of aluminum ions sent to the absorption process. However, in order to prevent deterioration of desulfurization performance, the amount of neutralization treatment increases and the amount of waste water increases.

Therefore, the present inventors conducted intensive research on a method to prevent the deterioration of desulfurization performance due to aluminum ions without using the above-mentioned neutralization treatment, and found that a manganese compound was added to the slurry despite the presence of aluminum ions. As a result, the desulfurization performance was found to be significantly recovered as shown in FIG. 2, and this was proposed earlier (see Japanese Patent Application Laid-Open No. 57-209624).

The present invention was made to further improve the previously proposed method, and was based on the discovery that adding sodium hydroxide in addition to the manganese compound further improves the desulfurization performance. This led to the completion of the present invention.

In other words, when the slurry is desulfurized by contacting the exhaust gas containing SO ₂ with the slurry containing calcium compounds, the aluminum concentration in the slurry is detected and an amount of manganese compound and sodium hydroxide is removed according to the detected concentration. The present invention relates to a flue gas desulfurization method characterized by adding the present invention to the slurry.

In the present invention, the supply amount M of the manganese compound
(mol/hr as Mn) is preferably supplied so that the ratio M/A to the inflow amount A (mol/hr as Al) of aluminum ions flowing into the absorption process is 0.1<M/A≦1. .

In addition, the supply amount of sodium hydroxide N (mol/hr
As Na) is preferably supplied so that the ratio N/A to the inflow amount A (nol/hr as Al) of aluminum ions flowing into the absorption step satisfies 5<N/A≦50.

The supply amount M of manganese compound and the supply amount N of sodium hydroxide are equal to the inflow amount A of aluminum ions.
When the ratio M/A and N/A of the manganese compound is 0.1 to 1 and 5 to 50, the adverse effect of aluminum ions on the desulfurization performance is completely prevented. Sodium oxide N/
It is wasteful to supply more than 50 A.

Manganese compounds have long been well known as catalysts that promote oxidation reactions, and in the present invention, Mn compounds are used to promote the oxidation reaction of the above-mentioned formula (2) and the following oxidation reaction of the formula (3) in the overall reaction. is promoted in the gas-liquid contact part of the absorption tower, the pH of the absorption liquid is lowered, and the dissolution of the absorbent CaCO ₃ is promoted in the gas-liquid contact part,
This results in improved desulfurization performance.

H ₂ SO ₃ +1/2O ₂ →H ₂ SO ₄ (3) In addition, sodium hydroxide in the present invention increases the pH of the absorption liquid and suppresses the dissolution of aluminum in the flyash (aluminum compounds become aluminum ions). (suppressing) In order to effectively carry out this action, it is preferable to supply sodium hydroxide to the tank of the absorption tower.

Generally, manganese compounds can be easily supplied quantitatively to the absorption process if used as an aqueous solution of manganese sulfate, but manganese ions are not limited to manganese sulfate, such as manganese chloride, manganese dioxide, potassium permanganate, manganese acetate, etc. It is effective if it contains the manganese compound, and it is suitable to supply the manganese compound to the circulation line of the absorption tower.

The amount of manganese compound and sodium hydroxide to be supplied is determined based on the value of the aluminum compound present in the liquid phase determined from analytical data during operation of the desulfurization equipment. That is, the concentration of aluminum dissolved in the liquid phase of the slurry in the absorption process is detected by analysis, and the concentration of manganese and
It is possible to manage the operation so that the sodium concentration is maintained.

_CaCO3 as an absorbent is different from Ca(OH) ₂ ,
The pH of the absorption liquid is low, especially calcium utilization (absorbed SO ₂ (mol/H)/supplied calcium (mol/H)).
When H)) is increased to 0.9 or more, the pH of the absorption liquid will generally be 5.5 or less. In addition, in the oxidation tower downstream of the absorption tower, the pH is generally lowered to 4.0 to increase oxidation efficiency. Therefore, sodium hydroxide is added to the absorption tower tank to increase the pH of the absorption tower tank and oxidation tower to prevent aluminum in the flyash from dissolving.

At the gas-liquid contact section of the absorption tower, the manganese compound increases the reaction density (amount) of converting calcium sulfite into gypsum and sulfite into sulfuric acid in the absorption solution, thereby lowering the pH of the absorption solution. From this, dissolution of CaCO ₃ at the gas-liquid contact portion in the absorption tower proceeds as shown in FIG. 5 below, and SO ₂ is efficiently absorbed in the absorption tower. This effect is
It is highest when manganese compounds are supplied to the absorption tower circulation line.

Example An embodiment of the present invention will be explained with reference to FIG.

The exhaust gas 2 leaving the coal combustion equipment 1, which is composed of a coal combustion furnace, a gas cooler, and a dry dust collector, is
Approximately including SO ₂ 1200ppm, dust 10-2000mg/m ² N
It is a gas at 70°C and is led to the absorption tower 3 at approximately 2000 m ² N/H. In the absorption tower 3, the slurry injected from the spray nozzle 6 is brought into contact with the exhaust gas, and the exhaust gas 4 from which SO ₂ and dust have been removed is discharged from the absorption tower 3. In addition, in the absorption tower 3, CaSO ₃ 1/2H ₂ O produced by absorbing SO ₂ and CaSO ₄ produced by oxidation are
A slurry containing 2H ₂ O, an absorbent calcium compound such as CaCO ₃ , and dust is circulated through a spray nozzle 6 by an absorption tower circulation pump 5 . The circulation flow rate was 30 m ² /H.

The dust contained in exhaust gas 2 was analyzed to contain a large amount of aluminum compounds, and the dust contained
It contained about 29wt% as Al ₂ O ₃ . This dust dissolves in the circulating slurry and becomes Al ³⁺ ions.

The slurry containing calcium compounds and aluminum compounds is circulated by the absorption tower circulation pump 5, while a portion is sent from the line 7 to the solid-liquid separator 8 at a rate of about 100/H, where it is mainly composed of CaSO ₄ .2H ₂ O. It was divided into a solid substance 14 and a supernatant liquid 9. The supernatant liquid 9 is sent to an absorbent tank 10, and a calcium carbonate slurry 11 is sent to the absorbent tank 10.
It is mixed with about 8wt% calcium carbonate slurry from line 12 to absorption tower 3 at about 125/H.
It was supplied by

Figure 1 shows the correlation data between the Al ³⁺ ion concentration in the absorption tower circulating slurry and the SO ₂ concentration at the absorption tower outlet in exhaust gas 4 when the dust concentration, that is, the aluminum compound concentration contained in exhaust gas 2, was varied. .
It can be seen that as the Al ³⁺ ion concentration increases, the SO ₂ concentration at the outlet of the absorption tower increases, and the desulfurization performance decreases.

Next, a Mn compound was supplied to the absorption tower 3 through the line 13, and FIG. 2 shows how the desulfurization performance improved as the manganese concentration in the absorption tower slurry changed.

A remarkable recovery effect on desulfurization performance was obtained.

The Mn compounds supplied to the absorption tower 3 from the line 13 were manganese sulfate, manganese chloride, manganese dioxide, potassium permanganate, and manganese acetate, all of which exhibited similar effects. This shows that the Mn compound supplied to the absorption tower is effective no matter what the anion is as long as the cation is manganese.

As is clear from Fig. 2, the decrease in desulfurization performance caused by aluminum ions can be recovered in the region where the molar ratio M/A (mol/mol) between the supply amount M of manganese compounds and the inflow amount A of aluminum ions is 1 or less. can.

Desulfurization performance when the Na concentration in the slurry was changed by supplying Mn compound from line 13 so that the concentration in the slurry was 2 mmol/and then supplying sodium hydroxide to the absorption tower tank from line 15. Figure 4 shows the improvement.

By adding sodium hydroxide, a more significant effect of improving desulfurization performance was obtained than using the Mn compound alone.

As described in detail above, according to the present invention, when processing exhaust gas containing SO ₂ , operations to eliminate aluminum ions, that is, precipitation separation due to neutralization treatment and an increase in the amount of waste water, are not required, and the desulfurization performance can be effectively improved. The decline can be prevented.

[Brief explanation of the drawing]

Figure 1 is a correlation diagram of experimental results showing that the desulfurization performance decreased with an increase in the Al ³⁺ ion concentration in the absorption tower circulating slurry, and Figure 2 shows that the absorption tower circulating slurry containing Al ³⁺ ions decreased in desulfurization performance. A correlation diagram of experimental results showing that the desulfurization performance was recovered when a Mn compound was added. Figure 3 is a diagram showing an embodiment of the present invention. Figure 4 is an absorption tower circulating slurry containing Al ³⁺ ions. It is a correlation diagram of experimental results showing that the desulfurization performance was recovered when 2 mmol/Mn compound was added to the sample and sodium hydroxide was further added. Fifth
The figure shows the relationship between the pH of the absorption tower tank slurry and the dissolution rate of CaCO ₃ .

Claims (1)

[Claims] 1. When desulfurizing exhaust gas containing SO ₂ by contacting it with a slurry containing a calcium compound, the aluminum concentration in the slurry is detected and an amount of manganese compound and sodium hydroxide is added to the slurry according to the detected concentration. A flue gas desulfurization method characterized by adding.