JP3748861B2 - Exhaust gas desulfurization method by metathesis method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for desulfurization of various combustion exhaust gases containing sulfur oxide (SOx) discharged from equipment (boiler, furnace, etc.) combusting liquid fuel, solid fuel, and gaseous fuel.
[0002]
[Prior art]
Numerous methods have already been known and put into practical use as exhaust gas desulfurization techniques. Among such desulfurization methods, a method using a magnesium-based compound such as magnesium hydroxide having a high sulfur oxide absorption capability is known. As a specific example, as shown in FIG. 3, for example, Patent Documents 1, 2, and 3 disclose a desulfurization technique called a magnesium hydroxide gypsum method. This technique has a feature that it is possible to collect an excellent quality gypsum.
[0003]
[Patent Document 1]
JP-A-8-155263 [Patent Document 2]
Japanese Patent Laid-Open No. 11-70316 [Patent Document 3]
Japanese Patent Laid-Open No. 2000-84351
[Problems to be solved by the invention]
However, in the conventional magnesium hydroxide gypsum method, (1) process flow, use of absorption tower with special structure, COD control of absorption liquid, reduction management of by-product sulfite gypsum, high to gas absorption liquid ratio (L / G ) Improvements, improvements, or groundbreaking new technologies related to operation, complex operation management, long-term stable operation requirements, reduction of construction and operation costs, environmental friendliness, and technologies without secondary disasters due to emissions Process development is required.
Furthermore, (2) environmentally friendly processes that can be easily constructed not only near the coast but also inland and can be operated and maintained are also required, and it is not always possible to recover particularly good quality plaster. And the development of a process that focuses on (2) is desired.
[0005]
Therefore, the present invention is based on the exhaust gas treatment technology using the Moretana tower and the Moretana tower type lime gypsum method technology disclosed in JP-A-11-290643, JP-B-51-31036, and JP-B-60-18208. The existing magnesium hydroxide gypsum method has been restructured and features high desulfurization efficiency, long-term stable operation, easy operation, inexpensive equipment, easy operation management system, easy maintenance, environmentally friendly equipment, etc. An object of the present invention is to provide a method for desulfurizing exhaust gas.
[0006]
[Means for Solving the Problems]
According to the present invention, the pretreatment process for removing impurities in the exhaust gas containing sulfur oxide and conditioning the exhaust gas, and the exhaust gas containing sulfur oxide and the absorbing slurry liquid are brought into contact with each other in the Moretana absorption tower to exhaust the exhaust gas. A desulfurization process for absorbing and removing sulfur oxides contained therein, an oxidation process for treating the treatment liquid after the desulfurization process with a gas containing oxygen, and a treatment liquid after the oxidation process are separated into dihydrate gypsum and a supernatant liquid A separation process, and a metathesis process in which the supernatant after separating the dihydrate gypsum is reacted with a slurry liquid mainly composed of calcium hydroxide to generate dihydrate gypsum and regenerate magnesium hydroxide. An exhaust gas desulfurization method in which the slurry liquid generated in the process is directly returned to the desulfurization process as an absorption slurry liquid. (1) The pH of the absorption slurry liquid to the Moretana absorption tower is set to the overflow of the Moretana absorption tower outlet slurry liquid. Receiving A slurry liquid from the metathesis step that is supplied to the tank and returned to the tank is held at a predetermined value. (2) When the pH of the slurry liquid at the outlet of the moletana absorption tower is not at the predetermined value, a part of the absorbed slurry liquid Is supplied into the Moretana absorption tower and maintained at a predetermined pH. (3) The pH of the treatment liquid from the oxidation step is adjusted by the supply amount of oxygen-containing gas and / or the residence time to vary the oxidation efficiency. (4) The pH of the slurry liquid from the metathesis step is supplied to the metathesis step using the slurry liquid mainly composed of calcium hydroxide as an auxiliary agent, and is kept at the predetermined value. An exhaust gas desulfurization method is provided.
[0007]
According to the present invention, there is also provided the exhaust gas desulfurization method that maintains the pH value of each step in the following range.
(1) Absorption slurry liquid to Moretana absorption tower 6-8
(2) Moretana absorption tower outlet slurry liquid 4-6
(3) Treatment liquid from oxidation process 4-5
(4) Slurry liquid from double decomposition process 9-12
[0008]
According to the present invention, there is provided the exhaust gas desulfurization method according to claim 1 or 2, wherein aluminum hydroxide is further present in the absorbing slurry liquid.
[0009]
As a result of intensive studies to achieve the above object, the present inventors have
(B) Preferably, scaling to the lower surface of the Moretana plate can be reduced by using a pretreatment step of conditioning the supplied exhaust gas containing sulfur oxide.
(B) Stable operation can be achieved by controlling the pH of the absorption slurry liquid to the Moretana absorption tower, the Moretana absorption tower outlet slurry liquid, the oxidation tank outlet slurry liquid, the slurry liquid in the metathesis tank within a certain value,
(C) Preferably, the presence of aluminum hydroxide in the order of several ppm in the solid state in the desulfurization system where the clogging is concerned can contribute to prevention of scaling and clogging.
(D) Preferably, the magnesium hydroxide and aluminum hydroxide added to the absorbent slurry are in principle a closed system, but it is only necessary to replenish the portion discharged with the byproduct gypsum slightly out of the system.
(E) Each step of the desulfurization is independent as a unit operation, and it is not necessary to perform operation management to return a part to the previous step against the flow of the process flow.
(F) Solids mainly composed of byproduct dihydrate gypsum can be easily separated by ordinary sedimentation separation,
(G) Preferably, L / G (liter / m ³ ) in the absorption tower is capable of high absorption efficiency (95 to 99%) and stable operation in the order of 3-5.
(H) If the pH of the slurry liquid at the outlet of the Moretana absorption tower does not fall within the specified value, a part of the absorption slurry is supplied into the Moretana tower and maintained at the specified pH, and a stable buffer is maintained and stabilized. Being able to drive,
(I) When there are few impurities in the raw exhaust gas, the inventors have found that stable operation can be performed without periodically blowing down a part of the wastewater from the pretreatment process, leading to the present invention.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the accompanying drawings.
[0011]
As shown in FIGS. 1 and 2, the main part of the process for carrying out the present invention is a pretreatment tank 1 corresponding to the pretreatment step used in the preferred embodiment of the present invention, a moletana absorption tower 2 corresponding to the desulfurization step, and Liquid receiving tank 3, oxidation tank 4 corresponding to the oxidation process, sedimentation tank 5 and supernatant liquid storage tank 6 corresponding to the separation process, metathesis tank 7 and auxiliary agent supply storage tank 8 corresponding to the metathesis process, and general-purpose accessories attached to these. Consists of blower, pump, stirrer, instrumentation, electricity, piping, etc.
[0012]
In the liquid receiving tank 3, the oxidation tank 4, and the metathesis tank 7, a slurry liquid pH measuring device is installed. Management of the pH of the liquid receiving tank 3 and the metathesis tank 7 is an important part of the present invention, and can be controlled by an automatic control method by a general method.
[0013]
When the pH of the slurry liquid at the outlet of the Moretana absorption tower becomes unstable due to changes in the properties of the raw exhaust gas at the start of the desulfurization equipment, fluctuations during operation of the raw material gas generation equipment, etc., part of the absorbed slurry liquid is put into the Moretana tower A predetermined pH is maintained by supplying temporarily.
[0014]
A mixed slurry liquid of magnesium hydroxide regenerated in the metathesis tank 7, byproduct dihydrate gypsum, aluminum hydroxide, and calcium hydroxide is introduced into the liquid receiving tank 3, and the pH of the absorbed slurry liquid to the Moretana absorption tower is introduced. Is supplied to the absorption tower (this adjusted liquid is the “absorbing slurry liquid”).
[0015]
Supply the absorption slurry to the top of the Moretana absorption tower 2 and contact the exhaust gas and the absorption slurry with the multi-stage Moretana in the Moretana absorption tower 2 dynamically (please tell me what it means) Thus, the sulfur oxide is absorbed and fixed from the exhaust gas to the absorbing slurry.
[0016]
Although the present invention is not limited to the following mechanism, the chemical principle of the process of the present invention will be described below in the case where the sulfur oxide component is sulfurous acid gas (SO ₂ ).
That is, sulfurous acid gas (SO ₂ ) is absorbed in the absorbing slurry liquid in the Moretana absorption tower 2 (hereinafter referred to as “absorbing tower treatment liquid”) and then fixed as magnesium bisulfite mainly as shown in the formula (I). .
[0017]
2SO ₂ + 2H ₂ O → 2H ₂ SO ₃
2H ₂ SO ₃ → 2H ⁺ + 2HSO ₃ ⁻
Mg (OH) ₂ + 2HSO ₃ ⁻ → Mg (HSO ₃ ) ₂ + 2OH ⁻ (I)
[0018]
The absorption tower treatment liquid is sent to the liquid receiving tank 3, and a part thereof is sent to the oxidation tank 4 before overflowing from the partition plate installed in the liquid receiving tank 3. In the oxidation tank, for example, after being oxidized by air (hereinafter referred to as “oxidation tank treatment liquid”), it is converted into readily soluble magnesium sulfate and sulfuric acid mainly as in the formula (II).
Mg (HSO ₃ ) ₂ + O ₂ → MgSO ₄ + H ₂ SO ₄ (II)
[0019]
The oxidation tank treatment liquid is sent to the sedimentation tank 5, where it is separated into a solid and a supernatant mainly composed of dihydrate gypsum, for example, by a natural sedimentation method. This supernatant is a liquid mainly containing magnesium sulfate.
[0020]
Magnesium sulfate in the supernatant separated in the settling tank 5 is sent to the metathesis tank 7, where it reacts with calcium hydroxide in the slurry liquid supplied from the auxiliary agent supply storage tank 8, and mainly of the formula (III) Then, it is converted into magnesium sulfate and dihydrate gypsum and returned to the Moretana absorption tower 2 again.
MgSO ₄ + Ca (OH) ₂ + 2H ₂ O →
CaSO ₄ .2H ₂ O + Mg (OH) ₂ (III)
[0021]
Thus, according to the present invention, a multi-stage Moretana tower is applied, magnesium hydroxide is used as a main absorbent, aluminum hydroxide is effectively used as a scaling inhibitor, etc., and a high desulfurization rate is achieved as a desulfurization facility. simplification, downsizing of the equipment, environmentally friendly technologies of long-term stable operation of, ease of operation and management, in which have succeeded in providing a means for the reduction of such construction and operating costs.
[0022]
The Moretana tower used in the present invention does not have a weir, an overflow part, etc., and a perforated plate or a countersunk plate having an opening ratio Fc of 0.25 to 0.5, preferably 0.3 to 0.4. A tower comprising at least one stage, preferably a plurality of stages, is already widely used in exhaust gas treatment technology. Exhaust gas treatment technology using such a Moretana (leakage shelf) tower is described in Japanese Patent Publication Nos. 51-31036 and 60-18208. Specific operating conditions are the flow rate of gas supplied to the tower. The ratio L / G of G (m ³ / hr) to the absorption liquid flow rate L (liter / hr) is 2.5 or more, preferably 3 to 5, and the gas superficial velocity Ug (m / sec) passing through the Moretana tower Is 2 or more, preferably 2.5-4.
[0023]
According to the present invention, most of the sulfur content and a part of impurities (Hg, Pb & F, etc.) in the raw exhaust gas are finally separated out of the system as solids mainly composed of dihydrate gypsum, and the raw exhaust gas. Most of the impurities (Hg, Pb & F, etc.) therein are preferably separated outside in the pretreatment step. In principle, since the wastewater is not discharged out of the system as industrial wastewater, the present invention can effectively prevent damage such as acid rain and air pollution caused by sulfur oxides as an environmentally friendly technology. Furthermore, since this desulfurization facility is based on environmentally friendly technology, it has sufficient feasibility economically without being restricted by location regulations such as near the coast or inland areas.
[0024]
【Example】
Hereinafter, the exhaust gas desulfurization method of the present invention will be described in detail by way of examples. However, the present invention is not limited to the following examples.
[0025]
Example 1
Experiments were conducted with the equipment shown in FIG. Since the performance of the absorbing slurry liquid was insufficient at the start of operation, the operation was started using magnesium hydroxide, and the operation was performed under steady operating conditions after the buffer as the absorbent was stabilized.
As a safety measure to prevent environmental pollution problems to the surroundings during the experimental test, the exhaust gas treated from the Moretana absorption tower 2 and the air supplied to the oxidation tank 4 are all introduced into the sodium hydroxide solution tank. After sufficient desulfurization, it was released into the atmosphere.
[0026]
Since the raw exhaust gas is close to the actual exhaust gas conditions, air pressurized by a blower is heated in a heating furnace, sulfurous acid gas is added from a cylinder to the raw exhaust gas, 136 ° C, sulfurous acid gas 950 ppm, 520 mmH ₂ O raw exhaust gas Made and used.
This raw exhaust gas 320 m ³ / hr (wet) was introduced into the pretreatment tank 1 to adjust the humidity, and the exhaust gas cooled to 42 ° C. was introduced into the moletana absorption tower 2.
[0027]
The slurry liquid in the metathesis tank (pH set value 10.80) is automatically supplied to the liquid receiving tank 3 to adjust the pH of the absorbent slurry liquid (pH set value 7.0) to the Moretana absorption tower within a certain value. Then, 1.4 m ³ / hr was sent to the upper part of the absorption tower to make dynamic contact in the multi-stage Moretana tower, and by repeating this, the absorption slurry was continuously circulated in the Moretana absorption tower.
[0028]
A part of the slurry at the outlet of the Moretana absorption tower (0.1 m ^{3 /} hr, pH 5.7 (not necessarily constant as a guide)) was periodically extracted and supplied to the oxidation tank 4.
[0029]
The slurry liquid supplied to the oxidation tank is oxidized by supplying air, for example. The degree of oxidation depends on the slurry properties of the Moretana absorption tower outlet slurry liquid, the residence time, the amount of air, the air blowing mechanism, etc. , but is not necessarily constant. Instead, the amount of air was controlled as ³ Nm ³ / hr, the residence time was 1.5 hr, and the pH was 4.4.
[0030]
The slurry at the outlet of the oxidation tank is supplied to the settling tank 5 and separated into a solid and a supernatant mainly composed of dihydrate gypsum by a natural sedimentation method, and the supernatant is introduced into the supernatant storage tank 6. This supernatant storage tank 6 also serves as a buffer tank. The amount of slurry at the oxidation tank outlet was not necessarily constant, and 0.1 m ³ / hr was supplied to the settling tank 5 as a guide.
The supernatant liquid separation time in the sedimentation tank 5 is about 17 minutes, and the solid sedimentation time from the concentrate is about 16 minutes, so that natural sedimentation separation can be performed relatively easily.
The water contained in the concentrate is recovered and supplied to the supernatant storage tank 6.
[0031]
The supernatant liquid in the supernatant liquid storage tank 6 is supplied to the metathesis tank 7, and the basic slurry liquid in the auxiliary agent supply storage tank 8 is supplied by an automatic control system so that the slurry liquid in the metathesis tank is kept at a constant pH value (pH setting). Control within the value 10.80). The pH was measured continuously with a pH meter.
[0032]
The temperature of the absorption liquid in the absorption tower was 34 ° C., the concentration of sulfurous acid gas in the treated exhaust gas was 20 ppm, and the desulfurization rate was about 98%.
[0033]
【The invention's effect】
As described above, according to the present invention, a multi-stage Moretana tower is used, magnesium hydroxide is a main absorbent, aluminum hydroxide is added to prevent scaling, and calcium hydroxide is used to immobilize sulfur oxides in exhaust gas. And control the operation by controlling the pH, thereby increasing the desulfurization rate, simplifying the process, making the equipment compact, environmentally friendly technology, long-term stable operation, facilitating operation management, construction and operation It can be a reduction or the like of the cost, can be implemented at low cost by using a simple and compact device environmental conservation.
[Brief description of the drawings]
FIG. 1 is a block flow diagram showing an embodiment of the present invention.
FIG. 2 is a block flow diagram showing an embodiment of the present invention.
FIG. 3 is a block flow diagram showing an example of the prior art.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Pretreatment tank 2 ... Moretana absorption tower 3 ... Receiving tank 4 ... Oxidation tank 5 ... Sedimentation tank 6 ... Supernatant storage tank 7 ... Metathesis tank 8 ... Auxiliary agent supply storage tank

Claims (6)

Oxidation of sulfur contained in exhaust gas by contacting the exhaust gas containing sulfur oxide with the absorption slurry liquid in a pretreatment process for removing impurities in the exhaust gas containing sulfur oxide and conditioning the exhaust gas, and in the Moretana absorption tower A desulfurization step for absorbing and removing substances, an oxidation step for treating the treatment liquid after the desulfurization step with a gas containing oxygen, a separation step for separating the treatment liquid after the oxidation step into dihydrate gypsum and a supernatant, Slurry liquid produced in the metathesis process, including a metathesis process in which the supernatant after separating the hydrogypsum is reacted with a slurry liquid mainly composed of calcium hydroxide to produce dihydrate gypsum and regeneration of magnesium hydroxide Is an exhaust gas desulfurization method in which it is returned to the desulfurization process as an absorption slurry liquid. This (2) When the pH of the slurry liquid at the outlet of the moletana absorption tower is not at the predetermined value, a part of the absorption slurry liquid is put into the moletana absorption tower. (3) The pH of the treatment liquid from the oxidation step is adjusted with the supply amount of oxygen-containing gas and / or the residence time to vary the oxidation efficiency to a predetermined value. (4) Exhaust gas desulfurization characterized in that the pH of the slurry solution from the metathesis step is supplied to the metathesis step using a slurry solution mainly composed of calcium hydroxide as an auxiliary agent and maintained at a predetermined value. Method. The exhaust gas desulfurization method according to claim 1, wherein the pH value of each step is maintained within the following range.
(1) Absorption slurry liquid to Moretana absorption tower 6-8
(2) Moretana absorption tower outlet slurry liquid 4-6
(3) Treatment liquid from oxidation process 4-5
(4) Slurry liquid from metathesis process 9-12 The exhaust gas desulfurization method according to claim 1 or 2, wherein aluminum hydroxide is present in the absorbent slurry. A small amount of an aluminum salt solution and a small amount of magnesium oxide powder are added to an auxiliary slurry liquid containing calcium hydroxide as a main component, and aluminum hydroxide and magnesium hydroxide are circulated substantially in a closed system. The exhaust gas desulfurization method according to any one of the above. The exhaust gas desulfurization method according to any one of claims 1 to 4, wherein the desulfurization, oxidation, separation, and metathesis steps are operated in this order by controlling the pH for each step in a forward flow manner. 6. The exhaust gas desulfurization method according to any one of claims 1 to 5, which is operated without discharging waste water from each step in principle.

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