JP3786786B2 - Flue gas desulfurization method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a flue gas desulfurization method. More specifically, the present invention relates to a flue gas desulfurization method including a step of absorbing sulfurous acid gas in flue gas using an absorbing solution containing ammonium ions or ammonium compounds.
[0002]
[Prior art]
Sulfur oxides (especially sulfurous acid gas) generated when burning fuel containing sulfur are air pollutants, and their emissions are regulated. Especially in large boilers that burn coal and heavy oil, the amount of sulfur oxides is high. The flue gas desulfurization equipment is an indispensable facility because of the concentration of sulfurous acid gas. There are various types of flue gas desulfurization methods, but the method that is widely used in large boilers is said to be wet, and it absorbs sulfurous acid gas in the flue gas by bringing the flue gas into contact with an absorbent containing alkali components. To be removed. As the alkali component, calcium ion, magnesium ion, sodium ion or ammonium ion is generally used, and these are combined with the sulfite ion generated in the liquid by the absorbed sulfite gas or the oxidized sulfite ion, and finally. In this case, calcium sulfate (gypsum), magnesium sulfate, sodium sulfate, ammonium sulfate (ammonium sulfate) and the like are formed and separated from the absorbing solution or discharged as they are.
[0003]
In the case of the ammonia-ammonium sulfate method using ammonium ions as an alkali component in the absorbing solution, ammonia is added to the absorbing solution. The ammonia concentration in the absorbing solution is preferably a high concentration of 20 mmol / L or more in order to increase the pH buffer action against sulfurous acid or sulfuric acid. Moreover, when utilizing the pH buffer action of sulfurous acid, it is preferable that pH of an absorption liquid is 4 or more. However, under such high ammonia concentration or high pH conditions, the vapor pressure of ammonia becomes high, and ammonia is contained in the flue gas after the desulfurization treatment, causing a problem of bad odor. For this reason, conventionally, the ammonia concentration and pH of the absorbing solution have been kept low even if the absorption efficiency is somewhat sacrificed.
[0004]
Ammonia is not only added as an alkali component to the absorption liquid of the flue gas desulfurization unit, but is also added to the flue gas for the purpose of suppressing purple smoke due to sulfur trioxide in the flue gas and preventing corrosion of the device material. Sometimes it is done. In recent years, the concentration of sulfur trioxide in the flue gas has increased along with the heaviness of the fuel, so the amount of ammonia added to the flue gas has increased accordingly. Further, ammonia gas leaking from the flue gas denitration device (SCR) may be included in the flue gas, and the amount thereof increases as the denitration rate increases. Therefore, not only in the ammonia-ammonium sulfate method but also in the limestone-gypsum method, the magnesium hydroxide absorption method or the sodium hydroxide absorption method, ammonia accumulates in the absorption liquid and the absorption liquid shows the ammonia vapor pressure. Increasingly, there is a problem that ammonia is released into the atmosphere along with later smoke. In such a case, for example, it is necessary to prevent the release of ammonia into the atmosphere as much as possible by separately providing an ammonia absorption device downstream of the flue gas desulfurization device (absorption tower). Specifically, this is to make the low pH absorption liquid to which sulfuric acid has been added contact the flue gas after the desulfurization treatment, or to provide a liquid circulation line separate from the absorption tower, but only for the removal of ammonia. Providing the device separately has a problem in cost.
[0005]
[Problems to be solved by the invention]
The present invention is intended to improve the removal efficiency of sulfur oxide without causing the problem of atmospheric release of ammonia in the flue gas desulfurization method by the ammonia-ammonium sulfate method. In addition, the present invention also provides that ammonia added to the flue gas accumulates in the absorption liquid of the wet flue gas desulfurization device, and the absorption liquid shows the ammonia vapor pressure, thereby releasing the ammonia into the atmosphere together with the exhaust gas after the desulfurization treatment. It is intended to prevent this without adding a new acid.
[0006]
[Means for Solving the Problems]
The present invention relates to an ammonium-containing liquid desulfurization step in which flue gas containing sulfurous acid gas is brought into contact with an absorbing liquid exhibiting an ammonia vapor pressure, and flue gas containing sulfurous acid gas is brought into contact with a solid catalyst to convert the sulfurous acid gas in the flue gas into the catalyst. A flue gas desulfurization method comprising a catalytic desulfurization step of removing as sulfuric acid by being adsorbed and oxidized at the same, thereby solving the above problems.
[0007]
In a typical embodiment of the present invention, flue gas containing sulfurous acid gas is first treated in the ammonium-containing liquid desulfurization step and then in the catalytic desulfurization step. At this time, the exhaust gas that has undergone the ammonium-containing liquid desulfurization process generally contains ammonia gas, which is neutralized and removed by gas-liquid contact with the sulfuric acid-containing liquid produced in the subsequent catalyst desulfurization process.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The method of the present invention includes two types of desulfurization processes, an ammonium-containing liquid desulfurization process and a catalyst desulfurization process. Among these, in the ammonium-containing liquid desulfurization process, flue gas containing sulfurous acid gas and an absorbing liquid exhibiting ammonia vapor pressure; Comes in gas-liquid contact. At this time, the sulfurous acid gas in the flue gas is dissolved in the absorbing solution to become sulfite ions, and then oxidized in the solution to become sulfate ions. The sulfate ions can be discharged as an aqueous solution from the absorbing solution as ammonium sulfate (ammonium sulfate) under a predetermined condition, or can be separated from the absorbing solution and recovered. For example, Japanese Patent Application Laid-Open No. 10-66826 describes that the produced absorption solution containing ammonium sulfate is extracted and concentrated crystallization or concentration drying with the heat of flue gas to obtain solid ammonium sulfate. Solid ammonium sulfate can also be separated and recovered by cooling and precipitating the extracted absorbent. Alternatively, ammonium sulfate may be precipitated in a supersaturated state in the absorption liquid or in the gas cooling liquid before the absorption step, and a part of the ammonium sulfate may be extracted and separated and recovered while operating the absorption liquid in a slurry state. . These methods are preferable in that the amount of industrial water used is reduced, and solid ammonium sulfate is directly obtained as a by-product of flue gas desulfurization by utilizing the heat of flue gas. In this case, it is preferable that the exhaust gas introduced into the ammonium-containing liquid desulfurization step is sufficiently dust-removed on the upstream side by an electric dust collector, a wet dust removal tower, or the like from the viewpoint of increasing the purity of the produced solid ammonium sulfate. In addition, the sulfate ion in the absorption liquid contains calcium-based compounds such as limestone (calcium carbonate), quicklime (calcium oxide), slaked lime (calcium hydroxide), magnesium-based compounds such as magnesium hydroxide, or sodium hydroxide. By adding a sodium compound such as ammonium sulfate, the ammonium sulfate is metathesized to release ammonia, and the sulfuric acid content can be separated and recovered as gypsum (calcium sulfate), magnesium sulfate, or sodium sulfate. At this time, the generated ammonia gas may be absorbed as it is in the absorption liquid in the ammonium-containing liquid desulfurization step, but if it is absorbed in sulfuric acid generated in the catalyst desulfurization step, the absorption efficiency is good.
[0009]
The absorbing liquid in the ammonium-containing liquid desulfurization process of the present invention exhibits an ammonia vapor pressure. That is, the absorbing solution contains undissociated ammonia or a compound that generates undissociated ammonia from ionic equilibrium. A compound that generates undissociated ammonia from ionic equilibrium does not exist as undissociated ammonia in the absorbing solution, but generates undissociated ammonia by changing the liquid properties under the operating conditions of the ammonium-containing liquid desulfurization process. Refers to the compound. To explain this point in more detail, since the acid dissociation index pKa of ammonia is 4.65, about half of the ammonia molecules in water exist as ammonium ions at pH 4.65, and at higher pH, ammonia molecules in water A part or most of it does not dissociate depending on pH, and it exists in a form showing the vapor pressure of ammonia such as undissociated ammonia, that is, dissolved ammonia, hydrated ammonia molecules, ammonium hydroxide, and the like. Such undissociated ammonia dissociates and generates ammonium ions when the pH of the liquid decreases with the absorption of sulfurous acid gas. Alternatively, in the presence of certain metal ions in water, ammonia molecules may form ammine complexes with the metal ions and may not exist as free undissociated ammonia. In this case, undissociated ammonia may be liberated due to precipitation of the metal ions or masking with other complex-forming species. The present invention includes all cases where the absorbing liquid exhibits ammonia vapor pressure, including the above case. The ammonia in the absorbent may be added to the absorbent as an alkaline component, or the ammonia gas added to the flue gas to neutralize sulfur trioxide in the flue gas Sometimes it is embedded in it.
[0010]
In the ammonium-containing liquid desulfurization process, when the absorbing liquid absorbs sulfurous acid gas in the flue gas, sulfur dioxide and sulfuric acid are generated and the pH of the liquid decreases, so it is necessary to supply the absorbing liquid with an alkali component in an amount to compensate for that. There is. As an alkaline component supplied at this time, ammonium compounds such as ammonia gas, aqueous ammonia, and ammonium carbonate may be used, and calcium compounds such as limestone, quicklime, and slaked lime described above may be used. Furthermore, a magnesium compound such as magnesium hydroxide or a sodium compound such as sodium hydroxide may be used. In addition, when ammonia gas is contained in the flue gas as described above, this is absorbed into the absorbing solution together with the sulfurous acid gas and becomes an alkali component, and accordingly, the alkali component added to the absorbing solution may be small.
[0011]
In addition, in order to oxidize sulfite and generate sulfate ions in the absorbing solution of the ammonium-containing liquid desulfurization step, dissolved oxygen for oxidizing the sulfite ions is required. Since the ammonium-containing liquid desulfurization process is a gas-liquid contact operation between the flue gas and the absorption liquid, some of the oxygen contained in the flue gas dissolves in the absorption liquid, but this is necessary for efficient oxidation of sulfite ions. However, it is not sufficient, and it is necessary to supply a larger amount of oxygen. This oxygen is supplied by blowing a gas containing oxygen, usually air, into the absorbing solution. In addition, when utilizing the pH buffer effect | action by sulfurous acid at an ammonium containing liquid desulfurization process, the gas containing oxygen will be blown into the extracted absorption liquid.
[0012]
As the gas-liquid contact device for performing the ammonium-containing liquid desulfurization step, various types of devices such as a spray tower, a packed tower, a plate tower, and a bubble tower which are usually used as a gas absorption tower can be appropriately employed. As an example, a jet bubbling reactor (JBR) as described in JP-A-10-66826 can be used for the gas-liquid contact operation. JBR extends a number of sparger pipes from the inlet plenum provided above the liquid level of the reaction tank storing the absorption liquid to the level below the liquid, and emits smoke into the liquid from the hole provided near the tip of the pipe. By doing so, the flue gas and the absorbing liquid are vigorously mixed and contacted in the upper region of the absorbing liquid. On the other hand, by introducing air to the bottom of the reaction tank to aerate the lower part of the absorbing liquid, and gently stirring the entire reaction tank to circulate the absorbing liquid, dissolved oxygen is also supplied to the upper part of the absorbing liquid. ing. In this way, JBR can efficiently absorb sulfurous acid gas in the flue gas by gas-liquid contact between the flue gas and the absorbing liquid and supply oxygen to the absorbing liquid with one compact device. It is a thing.
[0013]
In the catalytic desulfurization process, sulfurous acid gas in the flue gas is oxidized by oxygen coexisting in the presence of an oxidation catalyst to become sulfur trioxide (anhydrous sulfuric acid), and reacts with moisture present in the atmosphere to produce sulfuric acid. . This sulfuric acid (or sulfuric acid-containing liquid) is produced on the catalyst charged in the reactor, then flows down the catalyst surface, and eventually flows out of the reactor to be recovered or disposed of. As such a catalyst, activated carbon is usually used. If the activated carbon is hydrophobized, it has a function of strongly pushing out the generated sulfuric acid from the inside of the pores, and the outflow from the reactor is also promoted. So convenient. The activated carbon may be packed in a granular form in the reactor, but for example, a honeycomb or triangle shaped one that has a surface parallel to the direction in which the flue gas circulates can be used as a flow type reactor. Filling is preferable because the flow resistance (or pressure loss) is small and the generated sulfuric acid flows down the catalyst surface and is easily discharged. As the catalyst, activated carbon fiber and activated coke can be used in addition to activated carbon.
[0014]
In the catalyst desulfurization step, not only sulfurous acid gas but also ammonia gas in the flue gas is removed at the same time. This is because, as described above, the sulfuric acid produced by the oxidation of sulfurous acid gas in the flue gas exists on the catalyst surface, so that ammonia gas is removed in a form that is absorbed and neutralized by this. Therefore, if the flue gas that has undergone the ammonium-containing liquid desulfurization step is led to the catalyst desulfurization step, the ammonia gas remaining in the flue gas in the ammonium-containing liquid desulfurization step can be removed. That is, sulfurous acid gas is removed in two stages, an ammonium-containing liquid desulfurization process and a catalyst desulfurization process, and the ammonia gas remaining in the flue gas in the former process is removed in the latter process. Alternatively, a part of the flue gas may be removed in the catalyst desulfurization step, and the sulfuric acid or sulfuric acid-containing liquid generated there may be taken out and brought into gas-liquid contact with the flue gas treated in the remaining ammonium-containing liquid desulfurization step. Even in this case, the ammonia gas remaining in the flue gas in the ammonium-containing liquid desulfurization step can be absorbed and removed.
[0015]
As described above, even if ammonia is contained in the flue gas treated in the ammonium-containing liquid desulfurization process, it will come into contact with the sulfuric acid generated in the catalytic desulfurization process. There is no risk of release. Therefore, since the operating conditions of the ammonium-containing liquid desulfurization step can be set without considering ammonia leakage, this can be performed under the optimum conditions for desulfurization, specifically, at a relatively high pH and high ammonia concentration. .
[0016]
Note that the sulfuric acid or the sulfuric acid-containing liquid produced in the catalyst desulfurization process can be introduced and treated in the absorbing liquid of the ammonium-containing liquid desulfurization process regardless of whether it contains ammonium ions. Conversely, when ammonium sulfate is metathesized in the ammonium-containing liquid desulfurization step, the ammonia gas generated at that time is not only recycled in the ammonium-containing liquid desulfurization step but also absorbed in the sulfuric acid-containing liquid produced in the catalyst desulfurization step. You can also.
[0017]
The absorption tower in the ammonium-containing liquid desulfurization step and the reactor in the catalyst desulfurization step can be appropriately arranged depending on the flow of the exhaust gas through the ammonium-containing liquid desulfurization step and the catalyst desulfurization step. For example, a reactor for the catalyst desulfurization step may be provided in the upper part of the absorption tower for the ammonium-containing liquid desulfurization step, or a part or all of the mist eliminator at the outlet of the absorption tower may be used also as the reactor. Or you may provide the absorption tower of an ammonium containing liquid desulfurization process, and the reactor of a catalyst desulfurization process separately.
[0018]
【Example】
FIG. 1 is a flowchart showing an example of a preferred embodiment for carrying out the present invention. In FIG. 1, for the sake of simplicity, some devices such as a gas gas heater (GGH) that is normally provided are omitted. In this example, ammonium sulfate is deposited in a supersaturated state in the absorption tower of the ammonium-containing liquid desulfurization step. The flue gas is first dust-removed by the electrostatic precipitator 1, and then most of the sulfurous acid gas is removed in the ammonium-containing liquid desulfurization step 2, and then the sulfurous acid gas and ammonia remaining in the flue gas are removed in the catalyst desulfurization step 3, Finally, it is released into the atmosphere via the mist eliminator 4. In the absorption liquid of the ammonium-containing liquid desulfurization step 2, the precipitated ammonium sulfate is dispersed and is in a slurry state, and after extracting a part thereof and separating and recovering ammonium sulfate by the solid-liquid separator 5, it is put in the absorption tower. return. Ammonia in the absorption liquid is removed as ammonium sulfate by combining with sulfate ions derived from sulfurous acid gas absorbed from the flue gas, so that the concentration gradually decreases and the pH of the absorption liquid also decreases. In order to replenish this, ammonia is added to the absorption liquid in the form of ammonia gas or ammonia water. The sulfuric acid produced in the catalytic desulfurization step 3 is added to the absorption liquid in the ammonium-containing liquid desulfurization step and processed together.
[0019]
FIG. 2 shows another example of a preferred form for carrying out the present invention. In this example, a part of the absorbing liquid in the ammonium-containing liquid desulfurization step 2 is extracted, treated with a solid-liquid separator 5 ′ for removing dust, and then removed with dust, and then heated and concentrated with an evaporator 6 by steam heating. As a result, ammonium sulfate is precipitated. The absorption liquid after the ammonia and water vapor evaporated by the evaporator and the precipitated ammonium sulfate are separated and recovered by the solid-liquid separator 5 is returned to the ammonium-containing liquid desulfurization step. In this example, a mist eliminator 4 ′ is also provided between the ammonium-containing liquid desulfurization step 2 and the catalyst desulfurization step 3.
[0020]
FIG. 3 shows still another example of a preferred form for carrying out the present invention. In this example, a part of the absorption liquid in the ammonium-containing liquid desulfurization step 2 is extracted, slaked lime is added to this in a double decomposition tank 7 to double decompose ammonium sulfate, and the generated gypsum is separated and recovered by the solid-liquid separator 5. Both the ammonia generated in the metathesis tank and the absorption liquid after separating and recovering the generated gypsum are returned to the ammonium-containing liquid desulfurization step. Also in this example, a mist eliminator 4 ′ is provided between the ammonium desulfurization step 2 and the catalyst desulfurization step 3.
[0021]
【The invention's effect】
According to the method of the present invention, even if ammonia gas remains in the flue gas treated in the ammonium-containing liquid desulfurization step, it is removed downstream, so that the ammonium ion concentration and pH of the absorbing solution are kept low in this step. This eliminates the need for the sulfur dioxide absorption efficiency.
[0022]
In particular, if the flue gas that has undergone the ammonium-containing liquid desulfurization step is guided to the catalyst desulfurization step, desulfurization is performed in a two-step process, and the removal rate of sulfurous acid gas can be further increased. In this case, in the catalyst desulfurization step, low-concentration sulfurous acid gas is removed using a catalyst. In this case, residual ammonia gas contributes to improvement of desulfurization performance.
[0023]
Furthermore, if activated carbon is used as a desulfurization catalyst in the catalyst desulfurization step, even if gaseous organic substances (benzenes, chlorobenzenes, phenols, etc.) are contained in the flue gas, these can be removed. Further, if ammonia is used as an alkali component in the ammonium-containing liquid desulfurization step, an ammonium sulfate fertilizer can be easily obtained.
[Brief description of the drawings]
FIG. 1 shows an example of a preferred form for carrying out the method of the present invention.
FIG. 2 shows another example of a preferred form for carrying out the method of the present invention.
FIG. 3 shows yet another example of a preferred form for carrying out the method of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Electric dust collector 2 Ammonium containing liquid desulfurization process 3 Catalytic desulfurization process 4, 4 'Mist eliminator 5, 5' Solid-liquid separator 6 Evaporator 7 Metathesis tank

Claims (10)

Ammonium-containing liquid desulfurization process in which flue gas containing sulfurous acid gas is brought into contact with an absorbing liquid exhibiting ammonia vapor pressure, and the flue gas that has passed through the ammonium-containing liquid desulfurization process is brought into contact with a solid catalyst, and sulfurous acid gas remaining in the flue gas And a catalyst desulfurization step of removing ammonia by absorbing and removing ammonia present in the flue gas with a sulfuric acid-containing liquid produced . 2. A part of flue gas containing sulfurous acid gas is led to the catalytic desulfurization step without going through the ammonium-containing liquid desulfurization step, and the sulfuric acid-containing liquid produced therein is brought into contact with the flue gas after passing through the ammonium-containing liquid desulfurization step. The method described. The method according to claim 1 or 2, wherein the sulfuric acid-containing liquid produced in the catalyst desulfurization step is introduced into the absorption liquid of the ammonium-containing liquid desulfurization step. The method according to any one of claims 1 to 3, wherein the catalyst used in the catalyst desulfurization step is activated carbon or activated carbon hydrophobized. The method according to any one of claims 1 to 4, wherein a gas or an aqueous solution containing an ammonium compound is added to the absorbing solution. The method according to any one of claims 1 to 4, wherein a calcium compound, a magnesium compound, or a sodium compound is added to the absorbing solution. The method according to any one of claims 1 to 6, wherein a gas containing oxygen is introduced into the absorbing liquid. The solid ammonium sulfate is obtained by extracting a part of the absorption liquid and heating it to concentrate crystallization or concentration drying, or cooling through an absorption refrigerator. Method. The method according to any one of claims 1 to 7, wherein a part of the absorbing solution is extracted and a calcium compound is added thereto to metathesize ammonium sulfate to produce gypsum. The method according to claim 9, wherein ammonia gas generated during metathesis of ammonium sulfate is absorbed in a sulfuric acid-containing liquid produced in the catalytic desulfurization step.

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