JP2019217429A - Method and device for removal of ammonia in denitrification processed gas - Google Patents

Abstract

To provide a method for removing and recovering ammonia contained in a denitrated gas.SOLUTION: An ammonia removal method includes: (a) a process in which a denitrated gas, which contains ammonia, is brought into contact with a solid absorbent, thereby forming an ammonia occlusion absorbent; (b) a process in which the ammonia occlusion absorbent and the processed gas, from which ammonia has been removed, are separated; (c) a process in which the ammonia occlusion absorbent is brought into contact with an acidic aqueous solution, thereby forming an ammonia-containing aqueous solution; (d) a process in which the ammonia-containing aqueous solution and the processed absorbent from which ammonia has been removed, are separated; and (e) a process in which a strong base aqueous solution is mixed with the ammonia-containing aqueous solution to discharge ammonia, and thus, ammonia and a waste liquid are separated. Further, provided is an ammonia removal device which uses such a method.SELECTED DRAWING: Figure 1

Description

  An embodiment according to the present invention relates to a method and an apparatus for removing ammonia from a gas after denitration of exhaust gas discharged from a thermal power plant or the like using ammonia.

Exhaust gas emitted from a thermal power plant such as a power generation gas turbine or a boiler contains nitrogen oxides (NOx). In order to reduce the burden on the environment, it is necessary to remove such NOx, and ammonia denitration is generally used for that purpose.
For example, conventionally, the concentration of NOx emitted from a gas turbine or the like has been defined only at the rated time. In recent years, with the spread of renewable energy, the partial load and start / stop of a gas turbine and the like have increased, and it has also become important to control the concentration of exhausted NOx other than at the rated time. Generally, the NOx concentration of gas turbine exhaust gas is higher at a partial load than at a rated load, and even at a partial load, the ammonia / NOx ratio is increased in order to achieve the conventionally specified NOx emission concentration level. It is possible. However, when the ammonia / NOx ratio is increased, even if NOx can be reduced, the amount of leaked ammonia tends to increase, which is not preferable from an environmental load.

  Therefore, a measure for reducing leak ammonia is desired. As a method for reducing leaked ammonia, a method using an ammonia decomposition catalyst is generally known. However, in this method, since excess ammonia is decomposed by ammonia decomposition, the amount of ammonia used tends to increase, and the cost tends to increase.

JP 2014-208561 A

As described above, in the case of using an ammonia decomposition catalyst to reduce the leak ammonia contained in the exhaust gas after the denitration treatment in the exhaust gas treatment, excess ammonia is decomposed, so that the ammonia consumption increases. there were.
An embodiment of the present invention has been made to solve such a problem, and an object of the present invention is to provide a method for reducing leaked ammonia without using an ammonia decomposition catalyst.

The method for removing ammonia according to the embodiment of the present invention includes:
A method for removing ammonia contained in a denitrated gas obtained by denitrifying untreated exhaust gas containing nitrogen oxides by ammonia reduction,
(A) contacting the denitrated gas with a solid adsorbent to adsorb ammonia to the solid adsorbent to form an ammonia storage adsorbent;
(B) a step of separating the ammonia storage adsorbent formed in the step (a) and the treated gas from which the ammonia has been removed,
(C) contacting the ammonia storage adsorbent separated in the step (b) with an acidic aqueous solution to elute ammonia to form an ammonia-containing aqueous solution;
(D) a step of separating the aqueous ammonia-containing solution formed in the step (c) from the treated adsorbent from which the ammonia has been removed, and (e) mixing a strong base aqueous solution with the aqueous ammonia-containing solution. And releasing ammonia from the mixed aqueous solution to separate ammonia and waste liquid.

Further, the ammonia removing apparatus according to the embodiment of the present invention is an apparatus for removing ammonia contained in a denitrated gas obtained by denitrifying untreated exhaust gas containing nitrogen oxides by ammonia reduction,
(A) an ammonia adsorbing section for adsorbing ammonia on the solid adsorbent by contacting the denitrated gas with the solid adsorbent to form an ammonia storage adsorbent
(B) a treated gas separator for separating the ammonia storage adsorbent from the treated gas from which ammonia has been removed;
(C) a treated gas separation unit: an acidic aqueous solution mixing unit that elutes ammonia by contacting the separated ammonia storage adsorbent with an acidic aqueous solution to form an ammonia-containing aqueous solution;
(D) an adsorbent separating section formed in the acidic aqueous solution mixing section for separating the ammonia-containing aqueous solution and the treated adsorbent from which ammonia has been removed; and (E) mixing a strong base aqueous solution with the ammonia-containing aqueous solution. In this way, an ammonia recovery section is provided to release ammonia from the mixed aqueous solution and recover ammonia.

  According to the embodiment of the present invention, it is possible to reduce the leak ammonia contained in the exhaust gas after the denitration process.

FIG. 4 is a flowchart for explaining the ammonia removing method according to the first embodiment. FIG. 6 is a flowchart for explaining an ammonia removing method according to a second embodiment. FIG. 9 is a flowchart for explaining an ammonia removing method according to a third embodiment. FIG. 9 is a flowchart for explaining an ammonia removing method according to a fourth embodiment. The conceptual diagram for explaining the ammonia removal device by an embodiment.

  Hereinafter, an ammonia removing method and an ammonia removing apparatus according to an embodiment will be described with reference to the drawings.

First Embodiment A method for removing ammonia according to a first embodiment will be described with reference to FIG. The gas to be processed by the method according to the embodiment is a denitrated gas. In general, gas discharged from a plant such as a boiler in a power generation facility or a heat recovery steam generator (HRSG) for a gas turbine contains nitrogen oxides (NOx). In order to render this NOx harmless, that is, to denitrify it, reduction using ammonia is generally performed. In such a denitration system, mainly the reaction formulas (1) and (2):
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (1)
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (2)
The conversion of NOx (NO and NO ₂ ) into nitrogen and water occurs. In this case, one mole of ammonia is required for the reduction of one mole of NOx, and the stoichiometric ratio of NH ₃ / NOx becomes one.

However, when the ratio of NO to NO ₂ in the exhaust gas fluctuates and the NO ratio increases, the reaction formula (3) further increases:
6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O (3)
Occurs. In the reduction reaction according to the reaction formula (3), the stoichiometric ratio of NH ₃ required to reduce NO ₂ becomes 4/3, and the stoichiometry in the reduction reaction according to the reaction formulas (1) and (2) Since the ratio becomes larger than the ratio of 1, the consumption of NH ₃ increases.

For this reason, in order to always maintain high denitration efficiency, it is necessary to always increase the supply amount of ammonia. As a result, surplus ammonia is included in the denitrated gas.
In the first embodiment, the denitrated gas containing ammonia is brought into contact with the adsorbent in step (a).

  The adsorbent used here needs to be solid for subsequent separation. When the adsorbent is a liquid, it becomes difficult to separate it from the treated gas released to the outside of the system, and it may remain as fine droplets in the gas and cause problems such as corrosion of downstream piping. That's why.

  Various solid adsorbents capable of adsorbing ammonia are known, and any of them can be used. However, in the embodiment, since the adsorbent is treated with an aqueous solution in a later step, it is preferably insoluble or hardly soluble in water. Specific examples include zeolite, activated carbon, silica, titania, alumina, zirconia, ion exchange resins, metal salts capable of forming a complex with ammonia, and metal cyanide complex compounds capable of absorbing ammonia. Among them, the metal cyanide complex compound is an ammonia adsorbent that has recently attracted attention. The metal cyanide complex compound is preferable because it has a characteristic of having a higher ability to adsorb low-concentration ammonia than other adsorbents. Representative examples of such a metal cyanide complex compound include a hexacyanoferrate complex and a hexacyanocobalt complex. The crystals of these complexes are composed of metal ions and cyan ions, and ammonia is adsorbed by being taken into voids in the crystals or replacing the cyan ions. Examples of such a metal cyanide complex compound include Prussian blue and a compound in which iron ions of Prussian blue are substituted with cobalt, copper, or the like, and among them, Prussian blue is particularly preferable.

  The shape of these adsorbents is not particularly limited, and any shape such as fine particles, granules, rods, and plates can be used. In any case, the surface area is preferably large so that ammonia can be adsorbed with high efficiency when mixed with a gas. Further, the particulate adsorbent may be granulated to have a relatively bulky particle shape, or the particulate or granular adsorbent may be packed in a tube or the like and used as a column.

  The shape of the solid adsorbent can be arbitrarily selected depending on the purpose, but the surface area capable of adsorbing ammonia is relatively large, and it is easy to make contact with ammonia in a gas stream. A particulate adsorbent is preferred because the operation of collection and collection is easy. Although the size of the particulate adsorbent can be adjusted according to the purpose, for example, the average particle diameter is preferably from 0.1 to 3 mm, more preferably from 0.3 to 1.5 mm.

  When the particulate adsorbent is used, the adsorbent can be brought into contact with ammonia in the denitrated gas by spraying or scattering fine particles in the gas stream. For example, denitrified gas discharged from a boiler can be efficiently contacted by mixing an adsorbent in a portion where a pipe is relatively thin. The gas flow mixes the adsorbent with the denitrated gas and ammonia is adsorbed by the adsorbent to form an ammonia storage adsorbent. In addition, the supply amount of the adsorbent is adjusted according to the operation status of the release source of the denitrified gas such as a boiler, or the ammonia concentration in the denitrified gas is measured in real time, and the supply amount of the adsorbent is adjusted accordingly You can also.

  The gas containing the ammonia storage adsorbent is then separated in step (b) into an ammonia storage adsorbent and a treated gas from which ammonia has been removed. The separation method is not particularly limited, and when the adsorbent is a particulate adsorbent, a generally known gas separation method can be used. Specific examples include filter separation such as a bag filter and centrifugation using a cyclone separator.

  In addition, when the adsorbent has a relatively large rod-like or plate-like structure, or when the adsorbent is filled in a tube or the like, the gas flows through gaps and surfaces between these adsorbents. As a result, the contact of the adsorbent with the gas and the separation from the gas are performed in parallel.

  The separated treated gas has a reduced ammonia concentration, which is generally less than 10 ppm, preferably less than 5 ppm. The treated gas from which ammonia has been removed in this way is harmless to the environment even if released.

  On the other hand, the separated ammonia storage adsorbent is then brought into contact with an acidic aqueous solution in step (c). The ammonia adsorbed on the adsorbent in this step is eluted into the aqueous solution. As a result, the acidic aqueous solution is converted to an ammonia-containing aqueous solution.

  Although the kind of the acid contained in the acidic solution used here is not particularly limited, hydrochloric acid, sulfuric acid, nitric acid, acetic acid, propionic acid and the like are used. Of these, hydrochloric acid and sulfuric acid are preferred because they easily form harmless salts during wastewater treatment. The concentration of the acidic solution is not particularly limited, but is preferably high in order to elute ammonia efficiently, and is preferably low in terms of safety and handling. From these viewpoints, the concentration of the acidic aqueous solution is preferably from 1 to 10% by mass, and more preferably from 2 to 5% by mass.

  When the adsorbent is a particulate adsorbent, the adsorbent can be introduced into the acidic aqueous solution and stirred in order to efficiently elute ammonia from the adsorbent into the acidic aqueous solution in step (c). When the adsorbent has a relatively large structure in the shape of a rod or plate, or is filled in a tube, etc., these adsorbents can be washed with an acidic aqueous solution or immersed in an acidic aqueous solution. The ammonia can be eluted into the acidic aqueous solution.

  Next, in step (d), the aqueous solution containing ammonia and the treated adsorbent from which ammonia has been removed are separated. The separation method is not particularly limited, and can be arbitrarily selected according to the shape of the tea-feeding agent. When the adsorbent is a particulate adsorbent, a dispersion of the adsorbent is formed in step (c), and this is separated by a method such as filtration, liquid cyclone, or decantation. Of these, filtration is a simple method and is preferred.

  The aqueous ammonia-containing solution separated in step (d) is mixed with a strongly basic aqueous solution in step (e). Here, strong basicity means that basicity is stronger than ammonia. As a result, ammonia is released from the solution, and salts remain in the solution. The strongly basic aqueous solution contains a basic compound, and any base can be used as long as its basicity is stronger than that of ammonia. Specific examples include sodium hydroxide, calcium hydroxide, and potassium hydroxide. When the acid used in step (c) is sulfuric acid or hydrochloric acid, the use of these bases is preferable because the formed salt is harmless. The concentration of the strongly basic aqueous solution is preferably adjusted in accordance with the acidic aqueous solution used in step (b) and the amount of dissolved ammonia, but is generally preferably 1 to 10% by mass, More preferably, it is 5% by mass.

  The released ammonia is collected, dried and purified if necessary.

  In this way, it is possible to remove the ammonia contained in the denitrated gas, purify the exhaust gas, and collect the excess ammonia not used for the denitration reaction.

Second Embodiment A method for removing ammonia according to a second embodiment will be described with reference to FIG.

  The second embodiment is different from the first embodiment in that the treated adsorbent separated in the step (d) is recycled in the step (a). By thus reusing the adsorbent, the running cost can be suppressed.

  The treated adsorbent may be reused after being washed and / or dried, if necessary.

Third Embodiment A method for removing ammonia according to a third embodiment will be described with reference to FIG.
The third embodiment is different from the first embodiment in that the ammonia recovered in the step (e) is circulated and reused for denitration of the NOx-containing gas. By thus reusing the adsorbent, the running cost can be suppressed.

  The recovered ammonia may be reused after drying and / or purification if necessary. Further, the recovered ammonia may be returned to the tank storing ammonia for use in the denitration process, or may be stored in another tank independent from the tank. Further, the recovered ammonia can be directly supplied to the denitration step. At this time, in addition to the recovered ammonia, unused ammonia supplied from the tank is also supplied to the denitration process. Alternatively, it can be adjusted according to the amount of NOx actually contained in the NOx-containing gas.

Fourth Embodiment A method for removing ammonia according to a fourth embodiment will be described with reference to FIG. The fourth embodiment is a combination of the second embodiment and the third embodiment. By reusing the treated adsorbent and the recovered ammonia, it is possible to reduce running costs.

Fifth Embodiment A fifth embodiment is an apparatus for removing and recovering ammonia from a denitrated gas by the above-described ammonia removing method. The ammonia removing device according to the embodiment will be described with reference to FIG.
The ammonia removing device according to the embodiment,
(A) ammonia adsorption section,
(B) a treated gas separator,
(C) an acidic aqueous solution mixing section,
(D) an adsorbent separation section; and (E) an ammonia recovery section. These parts (A) to (E) have functions corresponding to the above-described steps (a) to (e), respectively.

  The NOx gas discharged from the boiler or the like is first introduced into the denitration unit 1 and subjected to denitration by ammonia supplied from the ammonia tank 2. At this time, the supply amount of ammonia can be controlled, for example, by the NOx concentration measured by the NOx concentration meter 2a provided upstream of the denitration unit 1.

  The denitrated gas flowing out of the denitration unit flows into the ammonia adsorption unit A of the ammonia removing device according to the embodiment. In the ammonia adsorbing section A, the adsorbent is mixed with the adsorbent supplied from the adsorbent tank 3. At this time, the supply amount of the adsorbent can be controlled by the ammonia concentration measured by the ammonia concentration meter 3a provided upstream of the ammonia adsorption section 2.

  The gas mixed with the adsorbent in the ammonia adsorption section is introduced into the treated gas separation section B, where it is separated from the treated gas and the ammonia storage adsorbent. The treated gas from which ammonia has been removed is rendered harmless and is generally released to the atmosphere. On the other hand, the ammonia storage adsorbent is introduced into the acidic aqueous solution mixing section C downstream.

  In the acidic aqueous solution mixing section C, the adsorbent is mixed with the acidic aqueous solution introduced from the acidic aqueous solution tank. The mixture of the adsorbent and the aqueous solution containing ammonia formed here is introduced into the adsorbent separation section D.

  In the adsorbent separating section D, the treated adsorbent and the aqueous ammonia-containing solution are separated by filtration or the like.

  The treated grade adsorbent is collected and reused as needed. In the case where the adsorbent is reused, the adsorbent is treated in the drying and / or refining device 6 as necessary, and then is reused in the ammonia adsorption section A. At this time, the amount of the adsorbent recovered and the amount of the adsorbent supplied from the adsorbent tank 3 can be adjusted according to the measurement value of the ammonia concentration meter 3a.

  On the other hand, the ammonia-containing aqueous solution discharged from the adsorbent separation section D is mixed with the strong basic aqueous solution supplied from the strong basic aqueous solution tank 5 in the ammonia recovery section E. As a result, ammonia released from the aqueous solution is recovered. On the other hand, since the aqueous solution generated as a residue generally contains only harmless salts such as sodium chloride, it is released to the environment as it is or after being subjected to a purification treatment as required.

  The recovered ammonia is reused as needed. When reused, the recovered ammonia is treated in the drying and / or purification device 7 as necessary, and then reused in the denitration unit 1. At this time, the amount of the recovered ammonia and the amount of the adsorbent supplied from the ammonia tank 2 can be adjusted according to the measurement value of the NOx concentration meter 2a.

A: Ammonia adsorption part,
B: treated gas separation unit,
C: Acidic aqueous solution mixing section,
D: adsorbent separation section, and E: ammonia recovery section 1: denitration section 2: ammonia tank 2a: NOx concentration meter 3: adsorbent tank 3a: ammonia concentration meter 4: acidic aqueous solution tank 5: strong base aqueous solution tank 6: recovery Adsorbent drying / purification unit 7… Recovery ammonia drying / purification unit

Claims (5)

An ammonia removal method for removing ammonia contained in a denitrated gas obtained by denitrifying untreated exhaust gas containing nitrogen oxides by ammonia reduction,
(A) contacting the denitrated gas with a solid adsorbent to adsorb ammonia to the solid adsorbent to form an ammonia storage adsorbent;
(B) a step of separating the ammonia storage adsorbent formed in the step (a) and the treated gas from which the ammonia has been removed,
(C) contacting the ammonia storage adsorbent separated in the step (b) with an acidic aqueous solution to elute ammonia to form an ammonia-containing aqueous solution;
(D) a step of separating the ammonia-containing aqueous solution formed in the step (c) from the treated adsorbent from which ammonia has been removed; and (e) mixing a strong base aqueous solution with the ammonia-containing aqueous solution. A method of removing ammonia from a mixed aqueous solution to separate ammonia and waste liquid.   The method according to claim 1, wherein the treated adsorbent separated in the step (d) is recycled and reused in the step (a).   3. The method of claim 2, wherein the treated sorbent is reused after being washed and dried.   The method according to any one of claims 1 to 3, wherein the ammonia separated in the step (e) is recovered and reused for the ammonia reduction. An ammonia removing device for removing ammonia contained in a denitrated gas obtained by denitrifying untreated exhaust gas containing nitrogen oxides by ammonia reduction,
(A) an ammonia adsorbing section for adsorbing ammonia on the solid adsorbent by contacting the denitrated gas with the solid adsorbent to form an ammonia storage adsorbent;
(B) a treated gas separator for separating the ammonia storage adsorbent from the treated gas from which ammonia has been removed;
(C) a treated gas separation unit: an acidic aqueous solution mixing unit that elutes ammonia by contacting the separated ammonia storage adsorbent with an acidic aqueous solution to form an ammonia-containing aqueous solution;
(D) an adsorbent separating section formed in the acidic aqueous solution mixing section for separating the ammonia-containing aqueous solution and the treated adsorbent from which ammonia has been removed; and (E) mixing a strong base aqueous solution with the ammonia-containing aqueous solution. Thus, an ammonia removing unit including an ammonia recovering unit that releases ammonia from the mixed aqueous solution and recovers ammonia.

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