JPH0698266B2 - Method and apparatus for treating combustion exhaust gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to municipal waste incinerators, industrial waste incinerators, other waste incinerators, coal combustion boilers, and other combustion exhaust gas as pollutants. When sulfur oxides (SOx), hydrogen chloride gas (HCl) and nitrogen oxides (NOx) are contained, these pollutants can be easily removed by the dry method, thereby contributing to the improvement of air pollution. The present invention relates to a method for treating combustion exhaust gas and an apparatus therefor.

[Prior Art and Problems] Examples of flue gas containing SOx, HCl, and NOx pollutants include flue gas from incinerators such as municipal waste and industrial waste. In other cases, the exhaust gas from a boiler that burns coal, especially overseas coal, may contain SOx, HCl, and NOx. In the case of normal gas combustion and petroleum-based liquid fuel combustion, HCl is not contained. Therefore, the exhaust gas treatment method according to the present invention is intended for the incinerator combustion exhaust gas and the coal combustion exhaust gas of various types of waste as described above.

There are many methods of removing pollutants for each of the simple components of SOx, HCl and NOx as a method for treating combustion exhaust gas, and a practical machine is currently in operation. That is, as a SOx removal method, there are a wet lime gypsum method, an alkali absorption method, a lime milk spray dryer method, and the like, and in particular, the lime gypsum method has many achievements as a highly completed technology. The removal of HCl has been developed and put into practical use as an exhaust gas from an MSW incinerator, and there are methods such as an alkali absorption method and a slaked lime blowing method. Many NOx treatments have been developed and put to practical use for boiler exhaust gas treatments, and dry ammonia selective catalytic reduction method and ammonia non-catalytic denitration method are typical examples.
In the case of NOx treatment, the wet method has hardly been put to practical use. As described above, although there are excellent methods for treating SOx, HCl, and NOx pollutants, the technology for simultaneous removal of these pollutants is not found at all. When the flue gas contains two of these three pollutants, the removal of the pollutants is done by a combination of techniques required to remove each pollutant. For example, in the case of the most common boiler exhaust gas treatment, SOx and NOx contained in the same gas
Is removed by a combination of a wet lime gypsum method and a dry ammonia selective catalytic reduction method. However, as is clear from the above, combining these two exhaust gas treatment devices with completely different configurations, specifically installing them in a series and operating them is an economic measure for the maintenance and treatment cost of the device. It is not desirable from any aspect of sex. Therefore, it is strongly desired to develop and put into practical use a method and an apparatus for treating exhaust gas containing these pollutants in the same apparatus.

Also, regarding the treatment of HCl and NOx, there is a dry method as an already completed technique, but regarding the SOx treatment, the dry method has not been completed, and in most cases, the wet method is applied. The semi-dry method is used in practice in the United States and West Germany, but it is rarely used in Japan. This is mainly because the denitration rate required in Japan cannot be achieved by the semi-dry method. Now, the major problem of this exhaust gas wet treatment method is that this treatment lowers the exhaust gas temperature to around 60 ° C and accompanies the generation of white smoke.It also requires water and wastewater treatment. That is the point. Therefore, there are strong demands for development of the dry method even in Japan where the conditions for adopting the wet method are relatively well-adapted, that is, even in Japan where water resources are abundant and discharge of wastewater to the sea is possible.

There is currently no complete dry process for simultaneous SOx, HCl and NOx simultaneous removal. As a method for removing SOx and HCl, a limestone blowing method and a lime slurry blowing method (semi-dry method) are known, but it is hard to say that a sufficient removal rate is obtained. SOx and N
The electron beam irradiation method and the activated carbon method are known as simultaneous Ox removal methods, and both have reached the test stage in the pilot plant, but they are still problematic for practical application both technically and economically. Is left.

The present invention has been made in view of the actual situation as described above, and an object thereof is to provide an exhaust gas treatment method and an apparatus therefor capable of achieving simultaneous removal of SOx, HCl and NOx, which has heretofore been unattainable. is there. Both SOx and HCl can be treated in the same way from the viewpoint of reaction with salts of alkali metals or alkaline earth metals, that is, they can be treated in the same reaction example, but NOx requires a completely different reaction. To do. Therefore, the present invention is effective not only in the SOx-HCl-NOx system but also in the SOx-NOx, HCl-NOx system, and is effective in improving the removal rate even in the SOx-HCl system. An object of the present invention is to provide a method for treating exhaust gas and an apparatus therefor.

[Problem Solving Means] The present invention is an extension of the conventional semi-dry desulfurization and desalination processes, aims to improve the desulfurization rate and desalination rate in the semi-dry exhaust gas treatment method, and is impossible with the conventional process. It is possible to achieve the reduction and removal of existing NOx at the same time.

That is, the method for treating flue gas according to the present invention comprises adding bromine gas, ammonia gas, and slurry-like or powdery calcium hydroxide and / or calcium carbonate to the flue gas to obtain sulfur oxides and hydrogen chloride. It is characterized in that gas and nitrogen oxides are removed at the same time.

Here, the addition order of each additive is preferably bromine gas, then ammonia gas, and then slurry or powdery calcium hydroxide and / or calcium carbonate.

When using a slurry of calcium hydroxide and / or calcium carbonate, slurry calcium hydroxide and / or calcium carbonate is sprayed from the top in the reaction zone and brought into contact with the exhaust gas passing through the reaction zone. When powdered calcium hydroxide and / or calcium carbonate is used, the powder is injected into the exhaust gas passage downstream of the ammonia injection point and brought into contact with the exhaust gas.

The optimum temperature range in the method of the present invention is 150 to 300 ° C. when using slurry calcium hydroxide and / or calcium carbonate, and when using powdery calcium hydroxide and / or calcium carbonate. Is 130
~ 180 ° C.

The flue gas treatment apparatus according to the present invention is provided with a bromine gas supply device on the downstream side of the cooling device and an ammonia gas supply device on the downstream side of the cooling device in the combustion exhaust gas passage, and the downstream side thereof. Is provided with a slurry or powdery calcium hydroxide and / or calcium carbonate supply device.

When using slurry-like calcium hydroxide and / or calcium carbonate, a slurry spray tower is used as a reaction zone. When powdered calcium hydroxide and / or calcium carbonate is used, the powder is directly injected into the exhaust gas passage on the downstream side of the ammonia injection point.

Hereinafter, the present invention will be specifically described with reference to the case of treating exhaust gas from an municipal waste incinerator. The constituents of the exhaust gas from the municipal waste incinerator vary greatly depending on the type of waste and the combustion status, but mainly O ₂ = 8-15%, SO ₂ = 10-100ppm, HCl =
100～1000Pm, a NOx = 50~200ppm, CO ₂ = about 6% to 8%, with the balance being moisture and N ₂ gas. Here, not to mention harmful gas components according to the present invention, SO ₂ , HCl and NOx
Is.

The waste is burned at about 950 ° C in the combustion furnace, but the exhaust gas generated is heat-exchanged and then discharged from the incinerator at 200 to 300 ° C. In the method of the present invention, this exhaust gas is used as a target, and the gas is cooled to an appropriate temperature range in the reaction process. This cooling can be done by air cooling, but a water spray quencher is practical. By this cooling, the temperature of the exhaust gas becomes 150 to 300 ° C. when using slurry-like calcium hydroxide and / or calcium carbonate, and when using powdery calcium hydroxide and / or calcium carbonate. Made at 130-180 ℃. Wastewater no longer occurs in this temperature range, and this temperature range is appropriate in terms of SOx and NOx removal reactions described later. First, bromine gas (Br ₂ ) is added to the exhaust gas from the quencher. Next, ammonia gas (NH ₃ ) is injected into the exhaust gas.

After that, when slurry-like calcium hydroxide and / or calcium carbonate is used, a slurry spray tower is used as the reaction zone, and the slurry is sprayed from the top of the tower and brought into contact with the exhaust gas passing through the reaction zone. Here, the exhaust gas temperature is cooled to 130 to 250 ° C., but the slurry is converted into solid particles by evaporation of the water content.
Then, the exhaust gas discharged from the slurry spray tower is introduced into a bag filter to remove dust originally contained in the exhaust gas and lime compound particles generated in the lime slurry tower. At this time, the exhaust gas temperature is 130 to 180 ° C, the condensation of water does not occur, and the entire system is operated in a dry system.

When powdered calcium hydroxide and / or calcium carbonate is used, powdered calcium hydroxide and / or calcium carbonate is injected into the exhaust gas passage on the downstream side of the NH ₃ injection point to form an exhaust gas. Contact. The exhaust gas temperature at this time is still 130 to 180 ° C.

This process flow is as described above. The mechanism of SOx, HCl and NOx removal in this process flow will be described in detail below.

First, when Br ₂ is injected and added into the exhaust gas, this Br ₂ reacts with NOx (mainly NO) to form nitrosyl bromide as shown in the following reaction formula.

NO + 1 / 2Br ₂ = NOBr ...... (1) SO ₂ seems to react with Br ₂ to form an intermediate, but this has not been confirmed so far. No reduction in the direct reaction of HCl with Br ₂ has been observed. Generated here
NOBr is very reactive, and is catalytically reduced and decomposed by NH ₃ as shown in the following reaction formula.

NOBr + NH ₃ → N ₂ + H ₂ O + HBr (2) This reaction occurs even at a very low temperature, and NOBr is reductively decomposed even in an aqueous ammonia system at about 60 ° C, for example.
This reaction is considered as follows.

NOBr + NH ₄ OH = NH ₄ NO ₂ + HBr (3) NH ₄ NO ₂ = N ₂ + 2H ₂ O (4) The reaction rates of the above reaction formulas (1) and (2) are extremely fast, and the NOx concentration is 50 to 200 ppm. In the low concentration range, not only in the low temperature range of 60 ° C or higher, but also in the optimum temperature range of 150 to 300 ° C when using slurry calcium hydroxide and / or calcium carbonate, and powdery calcium hydroxide and / or Alternatively, it is possible to obtain a denitration rate of 90% or more in the optimum temperature range of 130 to 180 ° C. when using calcium carbonate. At this time, the addition amount of Br ₂ and NH ₃ may be slightly in excess of the equivalence ratio shown by the reaction formulas (1) and (2) (equivalence ratio of 1.5 or less), and the reaction time is 1 to 10 seconds. It is a degree. This reaction is a very specific reaction and does not occur with Cl ₂ (chlorine gas), which has properties very similar to those of Br ₂ , and therefore NOx removal is not achieved. The HBr produced by the reaction formula (2) produces NH ₄ Br when excess NH ₃ is present.

Action of Br ₂ with respect to _{HBr + NH 3 = NH 4 Br} ...... (5) SO 2 are, as in the following reaction scheme, the oxidation of SO _2.

SO ₂ + Br ₂ + 2H ₂ O = H ₂ SO ₄ + HBr (6) The presence of intermediates has not been confirmed in this reaction. This SO ₂ oxidation reaction also proceeds at a sufficient rate even at a low temperature, for example, about 60 ° C. However, the SO ₂ oxidation equilibrium is on the SO ₂ side in the high temperature region, and the SO ₂ oxidation rate begins to decrease in the region over 400 ° C. As shown in reaction formula (6), the amount of Br ₂ to be added needs to be 1 or more in terms of an equivalent ratio to SO ₂ .

Here, if NH ₃ is present in excess, ammonium sulfate and NH ₄ Br are produced as in the following reaction formula. However, regarding ammonium sulfate, the optimum temperature range is 150 to 300 ° C when using slurry-like calcium hydroxide and / or calcium carbonate, and the optimum temperature range when using powdery calcium hydroxide and / or calcium carbonate. 13
Considering 0 to 180 ° C, it should exist as acid ammonium sulfate in phase equilibrium theory.

H ₂ SO ₄ + 2NH ₃ = effect of the _{(NH 4) 2 SO 4 ......} (7) (NH 4) 2 SO 4 = (NH 4 · H · SO 4 + NH 3 ...... (8) Br against HCl ₂ is Unknown.

As described above, NOx is reduced and removed in the exhaust gas into which Br ₂ and NH ₃ are injected and added, SO ₂ is oxidized to SO _3, and HBr is also added.
Or, in the case of an excess condition of NH ₃ , NH ₄ Br is produced in the exhaust gas.

In this state, this exhaust gas is passed through a slurry spray tower and brought into contact with the slurry sprayed from the top of the slurry when calcium hydroxide and / or calcium carbonate in the form of slurry is used. NOx-related reactions are no longer present in this slurry spray tower. The oxidation product SO ₃ in the exhaust gas reacts with the lime slurry as in the following reaction formula (9) to generate calcium sulfate, and at the same time, the water is evaporated and solid particles of calcium sulfate are generated. Of course, partially unoxidized SO ₂ also reacts with the lime slurry to form solid particles of calcium sulfite as shown in reaction formula (10).

When powdered calcium hydroxide and / or calcium carbonate is used, the powder is injected into the exhaust gas passage downstream of the NH ₃ injection point and brought into contact with the exhaust gas. As a result, the powdery calcium hydroxide and / or calcium carbonate is reacted with the oxidation product SO ₃ in the exhaust gas in the exhaust gas passage and / or the dust collector such as a bag filter on the downstream side by the reaction formula (9). As described above to produce calcium sulfate. It reacts with the powder of partially unoxidized SO ₂ as shown in the reaction formula (10) to form solid particles of calcium sulfite.

H ₂ SO ₄ + Ca (OH) ₂ = CaSO ₄ + 2H ₂ O (9) SO ₂ + Ca (OH) ₂ = CaSO ₃ + H ₂ O (10) Here, the solid particles of CaSO ₄ are chemically Is an inert substance that can be disposed of. When the amount of CaSO ₃ is large, it is necessary to oxidize it, but in the case of this process, the SO ₂ concentration itself is low in the waste incinerator exhaust gas, and SO ₂ may be oxidized to sulfuric acid. The amount of CaSO ₃ is small and does not pose a problem.

HCl reacts with calcium hydroxide and / or calcium carbonate according to the following reaction formula to form calcium chloride, which is almost completely removed.

2HCl + Ca (OH) ₂ ＝ CaCl ₂ ＋ 2H ₂ O …… (11) 2HCl ＋ CaCO ₃ ＝ CaCl ₂ ＋ H ₂ O ＋ CO ₂ …… (12) The bromine added to the exhaust gas produces HBr and then NH Conversion to ₄ Br and CaBr ₂ occurs, and these are collected together with dust, CaSO ₄ , CaSO ₃ , and CaCl ₂ by a dust collector such as a bag filter on the downstream side. The added bromine can be recovered and reused by circulation if necessary. That is, since NH ₄ Br and CaBr ₂ are both easily soluble in water, water extraction is performed. The recovery of the Br ₂ is, as in the following reaction scheme, to maintain the position of the pH 3.5 or less Cl ₂
Made by blowing.

2Br ⁻ + Cl ₂ = Br ₂ + 2Cl ⁻ (13) If inexpensive electric power can be used, Br ₂ can also be produced by electrolysis as in the following reaction formula.

2Br ⁻ = Br ₂ + 2e ⁻ (anode) (14) [Effect of the invention] According to the dry simultaneous removal method of SOx, HCl and NOx of the present invention, simultaneous removal of these substances, which could not be achieved in the past, is enhanced. It can be done literally dry with efficiency. Moreover, the product is a solid and SOx is recovered as inactive CaSO ₄ .

In order to remove SOx, HCl and NOx with high efficiency by the conventional technology, a combination of a wet alkaline cleaning method and an ammonia selective catalytic reduction method has been performed.However, according to the method of the present invention, wastewater in the wet method is used. Efficient treatment is achieved without problems such as treatment problems, generation of white smoke in the chimney, necessity of a catalyst in the ammonia selective catalytic reduction method, maintenance of the catalyst, and complexity of operation.

[Examples] Next, the present invention will be described with reference to Examples and Comparative Examples.

Comparative Example 1 This comparative example is a pilot plant for circulating exhaust gas from an municipal waste incinerator (exhaust gas treatment amount: 1500 to 1900 Nm ³ / h, constructed for the purpose of semi-dry desalination / desulfurization by spraying lime slurry). ) Was used to carry out exhaust gas treatment.

The exhaust gas composition of this municipal solid waste incinerator is shown in Table 1 below. As can be seen from this table, the exhaust gas composition of the municipal solid waste incinerator shown here is not a special one, but is quite normal.

The attached FIG. 1 shows the flow of the pilot plant and exhaust gas used in this comparative example. In the figure, the exhaust gas generated in the incinerator (1) is cooled in the heat exchanger (2),
It is introduced into the electrostatic precipitator (3) at about 250 ° C. Then, the exhaust gas is dedusted here and then diffused from the chimney (5) to the atmosphere through the blower (4).

As shown in FIG. 1, the apparatus used in this comparative example is designed to extract the exhaust gas in the middle part of the duct from the incinerator (1) to the electrostatic precipitator (3).
It is 1400 to 1700 Nm ³ / h (dry gas base). This exhaust gas is first introduced into an exhaust gas cooling tower (6), where it is cooled to 150 to 270 ° C. by spraying water coming from a water tank (7).

After that, the exhaust gas is introduced into the reaction tower (8). Here, about 5-8% of the lime slurry coming from the slurry tank (9) is atomized. The spray amount is 35 ~ 55 kg / h, the amount of effective lime in this is 1.0 ~ in equivalent number shown by the following formula.
The same slurry was injected so as to be 1.3. In the reaction tower (8), the exhaust gas temperature is lowered to 130 to 250 ° C. due to the evaporation of the water content of the slurry.

The exhaust gas is finally introduced into the bag filter (10), and the generated calcium chloride and calcium sulfate as well as the dust inherent in the refuse incinerator are separated and removed to the outside of the system. Here, the reaction of HCl and SOx with Ca (OH) ₂ may occur more significantly in the reaction tower (8), but the exhaust gas moves from the tower (8) to the bag filter (10) in the low temperature range. When this occurs, the reaction rate increases, and HCl and SO ₂ are highly removed. After that, the exhaust gas is blower (4) (11)
To the chimney (5).

As a result, the removal rate of HCl was 93 to 95%, and the removal rate of SO ₂ was also 92 to 95%. However, this method was very effective for removing HCl and SO ₂ , and NOx was not removed at all.

Example 1 Using the same pilot plant as in Comparative Example 1, an experiment under the same conditions was performed. However, in this example, as shown in FIG. 1, between the exhaust gas cooling tower (6) and the reaction tower (8), Br ₂ gas was injected into the exhaust gas on the upstream side and N ₂ on the downstream side.
H ₃ gas was injected. The injection amount of Br ₂ gas and NH ₃ gas is 1.1 to 1.2 for both Br ₂ and NH ₃ at the equivalent number represented by the following formula.
I tried to become.

As a result, HCl removal rate is 95-98%, SO ₂ removal rate is almost 100%.
%, The NOx removal rate was 70-85%.

Comparative Example 2 In this comparative example, a pilot plant having the flow shown in FIG. 2 was used. This plant has an exhaust gas treatment capacity of 1400 to 1700 Nm.
³ / h, without the reaction tower (8) and the slurry tank (9) in the plant of FIG. 1, and having the injection point of powdered slaked lime in the exhaust gas passage on the downstream side of the NH ₃ injection point. . This plant has the same structure as the plant of FIG. 1 in other points.

In the flow shown in Fig. 2, the exhaust gas generated in the incinerator (1) is cooled in the heat exchanger (2) and introduced into the electrostatic precipitator (3) at about 250 ° C, and the exhaust gas is removed here. Then, it is diffused into the atmosphere from the chimney (5) through the blower (4).

Exhaust gas is extracted in the middle of the duct from the incinerator (1) to the electrostatic precipitator (3), and the exhaust gas extraction amount is 1400 to 1700.
It is Nm ³ / h (dry gas base). This flue gas is first introduced into the flue gas cooling tower (6), where it is cooled to 130-180 ° C by the spray of water coming from the water tank (7). After that, powdered slaked lime is injected into the exhaust gas. The injection amount was 2 to 5 kg / h, and the slaked lime was injected so that the amount of effective lime in this was 1.0 to 1.3 in terms of the number of equivalents represented by the formula described in Comparative Example 1.

The exhaust gas is finally introduced into the bag filter (10), and the generated calcium chloride and calcium sulfate as well as the dust inherent in the refuse incinerator are separated and removed to the outside of the system. Then, the exhaust gas is sent to the chimney (5) via the blowers (4) (11).

As a result, the removal rate of HCl was 85 to 92%, and the removal rate of SO ₂ was 80 to 90%. However, this method is not
It was an effective method for removing SO ₂ and SO ₂ , and NOx was not removed at all.

Example 2 Using the same pilot plant as in Comparative Example 2, an experiment under the same conditions was performed. However, in this embodiment, as shown in FIG. 2, between the exhaust gas cooling tower (6) and the injection point of slaked lime, Br ₂ gas is injected into the exhaust gas on the upstream side and NH ₃ gas is injected on the downstream side. did. The injection amounts of Br ₂ gas and NH ₃ gas are Br ₂ and N in the equivalent numbers shown in the formula described in Example 1.
Both H ₃ were set to 1.1 to 1.2.

As a result, HCl removal rate is 90-95%, SO ₂ removal rate is almost 100%.
%, The NOx removal rate was 70-85%.

[Brief description of drawings]

1 and 2 are flow sheets showing an embodiment of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiharu Kobayashi 1-6-14 Edobori, Nishi-ku, Osaka City, Osaka Prefecture Hitachi Shipbuilding Co., Ltd. (56) Reference JP-A-2-214524 (JP, A)

Claims (2)

[Claims] 1. Sulfur oxides, hydrogen chloride gas and nitrogen oxides are simultaneously removed by adding bromine gas, ammonia gas and slurry or powdery calcium hydroxide and / or calcium carbonate to the combustion exhaust gas. A method for treating combustion exhaust gas, comprising: 2. In the combustion exhaust gas passage, a bromine gas supply device is provided on the downstream side of the cooling device, an ammonia gas supply device is provided on the downstream side thereof, and a slurry or powder is provided on the downstream side thereof. A device for treating flue gas used in the method according to claim 1, characterized in that a supply device for calcium hydroxide and / or calcium carbonate is provided.