JP3366417B2 - Prevention method of ammonium sulfate precipitation in selective reduction denitration method - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to ammonia (NH ₃ )
The present invention relates to a method for selectively detoxifying NOx in combustion exhaust gas by means of a method, that is, a selective reduction denitration method using ammonia in combustion exhaust gas, and more specifically, unreacted ammonia, that is, slip contained in exhaust gas treated in this denitration method. The present invention relates to a method of oxidatively decomposing ammonia and preventing the production and precipitation of ammonium sulfate in a subsequent step such as subsequent heat recovery.

[0002]

2. Description of the Related Art When incinerating municipal waste, industrial waste, etc., NOx and SOx, hydrogen chloride, odor, etc. are generated in the combustion exhaust gas depending on the origin and type of the waste. For this reason, various measures have been taken against these, further research has been advanced, and the sludge generated at the sewage treatment plant will eventually be incinerated.
Exhaust gas problems similar to those of municipal waste occur.

Further, electricity, mechanical and thermal energy are produced from a single drive source (energy source) and used in various facilities such as buildings, factories, hotels, hospitals, lighting, air conditioning, hot water supply, etc. The so-called cogeneration system, which comprehensively supplies various energy required for the above, has been attracting attention as one that enables energy to be used with high efficiency, and has been successively adopted and has been popularized.

Of these, in a system that uses city gas mainly composed of natural gas as its drive source, NOx, SOx, etc. are generated and emitted at the time of combustion of the gas much less than other fossil fuels. Clean and highly evaluated in terms of energy supply stability,
It has been especially noticed.

FIG. 1 shows an example of the above cogeneration system when city gas is used as its driving source ["Pollution and Countermeasures" Vol. 25 N
o. 4 (1989) p. 25-30]. In this example,
City gas is boosted and combusted by a compressor, a gas turbine is driven to generate electric power, and this is used for power, lighting, etc., while the heat of combustion exhaust gas is exhaust heat boiler and subsequent economizer. It is collected by and is supplied for production, air conditioning, hot water supply, or the like.

However, in this type of cogeneration system, a gas turbine or a gas engine is usually used, and the combustion of the gas fuel is carried out using air. For co-generation facilities that include NOx and are above a certain scale,
It is subject to regulations under the Air Pollution Control Law.

NO from the cogeneration system
In the case of a gas turbine, the power generation scale is about 230 kW, which is the regulation value under the Air Pollution Control Law for x.
(Converted value) or more, 294 ppm, in the case of a gas engine, power generation scale of about 110 kW (converted value) or more, 600
Although it is set as ppm, some local governments provide and provide guidance standards below this value. For example, in the case of Tokyo, the upper limit is 100 pp in gas turbine.
m (power generation scale: 2000 kW or more), 200 ppm for gas engines (power generation scale: converted value, approximately 110 kW or more)
I am trying.

In this respect, in a gas turbine or a gas engine that uses natural gas as its driving source, the amount of NOx contained in the exhaust gas is usually about 50 to 150 ppm, so the regulation under the above Air Pollution Control Act is required. Although it clears much of the value, and most of the above-mentioned guidance and regulation values in local governments that are stricter than this, it contributes to "creating a clean urban environment" in the city gas industry. From the point of view, “development of low NOx technology” is being further promoted.

A method for reducing NOx components in combustion exhaust gas is S
In tandem with flue gas desulfurization technology such as Ox, thermal power generation,
It has been researched and developed as a part of technology for treating exhaust gas from automobiles or various factories, and various proposals have been made. Among them, the reduction denitration method using ammonia is said to be effective in view of its selective performance with respect to NOx in particular, and therefore, this method is also used as a method for treating combustion exhaust gas from a cogeneration system. There is.

This denitration method is basically based on N in exhaust gas.
Ox reacts with ammonia, undergoes oxidation and catalytic reduction, is converted into nitrogen and water vapor (water), is detoxified, and is released into the atmosphere. The reaction used here is
Looking at the case where NOx is NO, the following equation (1)
It is represented by. In this case, a suitable catalyst, that is, a denitration catalyst is used for this reaction. NO + NH ₃ + 1 / 4O ₂ = N ₂ + 3 / 2H ₂ O (1)

As the ammonia source for denitration, liquefied ammonia, ammonia water (ammonium water), urea (or urea water), ammonium hydrogen carbonate aqueous solution and the like are used. All of these are converted to ammonia immediately before the denitration catalyst or in the catalyst layer to act as a reducing agent.

FIG. 2 is a schematic diagram showing an example of applying this denitration method to combustion exhaust gas from a gas turbine or a gas engine. In the figure, 1 is a heat recovery boiler,
2 is a chimney connected to the boiler 1, and 3 and 4 are boiler heat exchangers. The exhaust gas from the gas turbine or the gas engine introduced from the arrow A is recovered by the heat exchangers 3 and 4 of the heat recovery boiler-1 and then discharged from the arrow B through the chimney 2.

Reference numeral 5 is an injection portion of ammonia for denitration (liquefied ammonia, ammonia water, urea water, etc.), and 6 is a denitration catalyst layer. Of these, the denitration ammonia injecting section 5 is constituted by an appropriate means such as a porous gas distributor,
Ammonia is evenly distributed and mixed in the exhaust gas stream, and the denitration catalyst layer 6 is formed in the form of, for example, a honeycomb or the like, by which the reaction represented by the formula (1) or the like is performed,
It converts NOx in exhaust gas into harmless N ₂ , H ₂ O, etc.

A cogeneration system using a natural gas as a driving source and a gas turbine or a gas engine.
When the selective reduction denitration method using ammonia is applied to the combustion exhaust gas from the combustion system, the NOx in the exhaust gas after the denitration is reduced to a value far below the regulation value.

With respect to other harmful components other than NOx, especially SOx, when natural gas is used as the drive source, the SOx amount in the combustion exhaust gas is 0 (zero) even in related documents. It is usually introduced as ppm, and even when natural gas is used as the energy source in a cogeneration system, SOx cannot be a particular problem.

[0016]

However, in the cogeneration system, as described above, natural gas is used as a driving source of the cogeneration system, and the heat recovery of exhaust gas is sufficient for heat generation as well as power generation, thereby improving the overall thermal efficiency. In order to raise the temperature, if the temperature of the exhaust gas is set lower than the ammonium sulfate precipitation limit temperature,
About combustion exhaust gas Ammonia (not limited to ammonia itself, but includes other ammonia-based reducing agents such as urea.)
When the denitration method using is continued for a long period of time, ammonium sulfate is deposited on the surface of the heat exchanger such as the heat recovery boiler and the economizer, and successively accumulated.

The deposition and accumulation impede the thermal efficiency of the heat recovery boiler section, making the heat recovery incomplete and narrowing the exhaust gas passage passing through this section, resulting in pressure loss. Furthermore, it has been found that various inconveniences occur, such as making the operation of the system itself impossible.

As described above, city gas containing natural gas hardly produces SOx such as SO ₂ and cannot be normally detected by a ppm level detector. When I analyzed the ingredients,
Still, in the combustion exhaust gas, 0.0X to 0.1pp
It was found to contain a trace amount of SOx of m. On the other hand, the exhaust gas introduced into the denitration catalyst layer usually contains about 6 to 10% oxygen.

In the present invention, in addition to the facts described above, further observation has revealed that this oxygen is SO 2 during the denitration process.
_It oxidizes and converts ₂ into SO _3, and its exhaust gas contains a small amount of unreacted ammonia, that is, slip ammonia (according to the measurement results, about several ppm). Trace amount of S that is oxidized and contained
It was found that ammonium sulfate was reacted with O ₃ to form and precipitate, which was the cause of the above-mentioned inconveniences.

As a means for solving the above-mentioned problem that occurs in the selective reduction denitration method of ammonia of combustion exhaust gas,
As a denitration catalyst used in the denitration step, a denitration catalyst that does not easily generate SOx is used, the temperature in the heat recovery boiler section following the denitration step, that is, the denitration step is maintained at a temperature equal to or higher than the precipitation temperature of ammonium sulfate, and the precipitated ammonium sulfate is temporarily used. There may be a method of heating and decomposing and removing.

[0021] However, in the above method, there is no catalyst that suppresses the formation of SO ₃ to such an extent that the precipitation of ammonium sulfate can be prevented, and separate research and development are required. I have to sacrifice the recovery,
Along with the loss of energy, problems such as thermo-economic problems occur, and further methods require large-scale heating means, control means, etc., which are also problems in terms of their operation and thermal efficiency. There is.

Therefore, in the present invention, as described above, one of the causes of ammonium sulfate production is that the produced component is a trace amount of ammonia slipping from the denitration step, and that the exhaust gas contains oxygen. In view of the above, the inventors have found that the above problems can be solved all at once by decomposing the slip ammonia itself by oxidatively decomposing the slip ammonia itself with this oxygen, and arrived at the present invention.

According to this, in the operation of the heat recovery step, the temperature in the step can be performed at a lower temperature, so that the heat recovery rate can be increased correspondingly, and in addition, the odor due to ammonia in the exhaust gas can be increased. There can be various advantages such as no or virtually none and weakening.

As a method for preventing the precipitation of ammonium sulfate produced by the reduction denitration apparatus using ammonia, there is a method of removing unreacted ammonia itself. Tokkyo 4-20
Japanese Patent No. 664 is an example, but here, as effective ones for removing the unreacted ammonia, mordenite, clinoptite, or Y-type zeolite Na, K,
A specific catalyst has been proposed in which the Ca element is replaced by H, Fe, Cu or Co.

According to the description, those catalysts are intended for reducing and denitration gas of various exhaust gases such as boiler exhaust gas for thermal power generation. Therefore, in this respect, they are common to the present invention. These catalysts are specific to the catalyst itself, and are not intended for the combustion exhaust gas from the above-described co-generation system or the like containing the extremely small amount of SOx in the present invention.

[0026]

That is, the present invention is
In the selective reduction denitration method of ammonia (NH ₃ ) of combustion exhaust gas, unreacted ammonia from the denitration step, that is, slip ammonia, is removed by using a specific catalyst, so that the boiler, economizer, etc. following this denitration step It is an object of the present invention to prevent generation and precipitation of ammonium sulfate in the heat recovery section, improve heat recovery efficiency at the same time, and prevent slip ammonia from being released to the outside air.

[0027]

The present invention has been made to solve the above-mentioned problems, and in the selective reduction denitration method of ammonia of combustion exhaust gas, after the denitration, before the subsequent heat recovery step. Alternatively, in the process, the denitrated exhaust gas is brought into contact with at least one catalyst selected from platinum, rhodium or palladium and supported on a heat-resistant oxide carrier to oxidatively decompose slip ammonia in the denitrated exhaust gas. The present invention provides a method for preventing ammonium sulfate precipitation in the selective reduction denitration method using ammonia, which is characterized in that it is removed.

The ammonia oxidation reaction used here can be represented by the following reaction formula (2). This reaction can be carried out at a temperature in the range of about 200 to 600 ° C. using an oxidation catalyst, and particularly when this is applied to the exhaust gas from the denitration step using ammonia in a cogeneration system, Since the catalyst is installed after the denitration catalyst, and the higher the temperature is, the faster the oxidation rate is, the temperature is preferably 230 to 450 ° C. 2NH ₃ + 3 / 2O ₂ = N ₂ + 3H ₂ O (2)

Further, in the selective reduction denitration method itself using ammonia of the combustion exhaust gas, which is the object of the present invention,
Liquefied ammonia, ammonia water (ammonium water), urea (or urea water), an aqueous solution of ammonium hydrogencarbonate, etc. are used as the ammonia source for the reductive denitration. These are just before the denitration catalyst or in the catalyst layer. Is converted into ammonia and acts as a reducing agent, and the present invention is also applied to exhaust gas when any of these ammonia sources is used.

Further, as the above-mentioned oxidation catalyst used in the present invention, a SOx resistant catalyst in which iron, copper or cobalt is supported on zeolite such as mordenite is also conceivable, but such ammonia and SOx low concentration conditions are sufficient. Can not exert its activity. Therefore, a catalyst having a high activity is required even under such special conditions, and a platinum group-based catalyst can be used. Of these, a platinum catalyst is particularly desirable.

Regarding this point, for example, platinum and palladium, as described in JP-A-53-102866, these catalysts contain NH _{3 in the} waste gas thereof.
Is effectively applied when it is contained at a high concentration of 15000 ppm, the present invention is effective even when NH _{3 in} the exhaust gas is a very low concentration of several ppm. It was possible to confirm that

Because of such performance of platinum group catalysts, these catalysts are flue gas, ie NO, from cogeneration systems that use natural gas as their driving source.
The exhaust gas obtained by applying the selective reduction denitration method using ammonia to the exhaust gas having a low x concentration can also be effectively used.

The mode of use of the catalyst in the present invention, that is, the shape of use of the catalyst, is such that these are supported on a suitable carrier, for example, activated alumina, silica, magnesia, etc., and are in the form of powder, sphere, pellet, or honeycomb. Although it can be used in a shape such as, it is particularly preferably a honeycomb shape.

The catalyst according to the present invention is prepared, for example, by supporting platinum, rhodium or palladium on a carrier obtained by wash-coating alumina on a cordierite honeycomb.
As the raw material of the platinum group metal, its chloride, nitrate, acetate, ammonium chloride, alkoxide and the like can be used. The supporting method is ion exchange, impregnation, or the like. As the carrier, alumina, silica alumina, silica, zirconia, magnesia, etc. can be used. Of these, alumina is particularly desirable.

FIG. 3 is a schematic view showing an example of its usage. In the figure, 7 is the ammonia oxidation catalyst layer according to the present invention, and the portions indicated by A, B and 1 to 6 in FIG.
Is the same as in. As you can see by comparing with Figure 2,
It is sufficient to configure the ammonia oxidation catalyst layer 7 in front of the heat recovery boiler 4 behind the denitration catalyst layer 6 in such a conventional apparatus, and it is necessary to adopt a special structure or the like. There is no. Further, the heat recovery unit 4 can be multi-staged, and an ammonia oxidation catalyst can be installed between them. It is not always necessary to install immediately after the denitration catalyst.

[0036]

EXAMPLES Examples of the present invention will be described below, but it goes without saying that the present invention is not limited to these examples. In this example, the apparatus shown in FIG. 3 was used.

1.5 g of alumina and platinum were added to a cylindrical cordierite honeycomb having a diameter of 20 mm and a length of 15 to 35 mm.
/ Liter was carried, dried and calcined to prepare a platinum catalyst. The catalyst was filled in a stainless steel reaction tube having an inner diameter of 20 mm, and ammonia removal performance was examined under the following conditions. 1. Test gas composition: Ammonia = 3ppm, 10pp
m and 30 ppm, SO ₃ = 0.1 ppm,
Oxygen = 11%, water (steam) = 10%, nitrogen = balance (balance). 2, space velocity: 20000h ^-1 , 38000 ^-1 , 760
There are three kinds of 00h ^-1 . 3, catalyst layer temperature: 350 ℃

The measurement was carried out under the above conditions, and the ammonia concentration before and after the catalyst layer was measured. This measurement was carried out by the indophenol method, and the results were used as Examples 1 to 5 for the decrease in ammonium sulfate precipitation upper limit temperature calculated by thermodynamic calculation from the removal rate of ammonia and the outlet gas composition. , Shown in Table 1.

Further, in Table 1, Example 6 contains 2.5 g of platinum.
Per liter, Example 7 uses palladium with an amount of 2.5 g / liter supported, and Example 8 shows platinum 1.0 g / liter and rhodium 0.1 g.
/ Liter is supported, and the other points are manufactured in the same manner as in Examples 1 to 5.

On the other hand, as a comparative example, ferric chloride was used and mordenite supporting iron by an ion exchange method was wash-coated on a cordierite honeycomb having the same shape as in Examples 1 to 5, After drying at 115 ° C., it was calcined at 500 ° C. to prepare an iron mordenite catalyst. With this catalyst,
Under the same test conditions as in Example 3, the ammonia removal performance was examined. The results are shown in Table 1.

As shown in Table 1, according to the present invention, it is clear that excellent effects can be obtained in both the ammonia removal rate and the ammonium sulfate precipitation temperature. For example, looking at Examples 1 to 6 using a platinum-supported catalyst, the ammonia removal rate reaches 95.1 to 97.1%, and the ammonium sulfate precipitation temperature is 25.
It can be seen that the temperature is lowered by ~ 28 ° C.

[0042]

[Table 1]

Among them, in Example 3, which was carried out under the same conditions as those of Comparative Example, although the concentration of ammonia at the inlet to the reaction tube was 3.0 ppm, which was extremely low, the ammonia removal rate was 96.3%. As for the precipitation temperature of ammonium sulphate, it decreased by 26 ° C.
Compared with 84.6% and 15 ° C., respectively, the excellent effect of the present invention is clear.

Further, as is apparent from Example 7, when the palladium catalyst is used, excellent activity is exhibited. By contrast with this, Example 6 shows that the platinum catalyst is used under the same conditions. It can be seen that the activity is higher than this. In the case of platinum catalyst, the supported amount is 1.5g
/ Liter (Examples 1 to 5, especially Example 3) shows higher activity than 2.5 g / liter (Example 6). Furthermore, according to Example 8, it can be seen that the activity is further increased by adding a small amount of rhodium to platinum.

[0045]

As described above, according to the present invention, in the selective reduction denitration method of ammonia of combustion exhaust gas, after the denitration step, the heat recovery section of the boiler, the economizer, etc., which follows the denitration step, and the conduit section that follows the heat recovery section Also, the precipitation of ammonium sulfate in the chimney can be prevented and suppressed very effectively. As a result, troubles due to precipitation and deposition of ammonium sulfate at those locations can be substantially eliminated.

Further, in the present invention, the precipitation limit temperature of ammonium sulfate can be significantly lowered. As a result, the heat recovery temperature can be lowered, and the energy recovery rate can be increased accordingly. Further, the odor caused by ammonia can be weakened and eliminated, which can prevent the escape and emission of ammonia into the atmosphere and solve the problem of environmental pollution.

These effects according to the present invention make it possible to continuously operate the reduction denitration method for a long period of time,
Further, since the present invention is also effective for a very small amount of slip ammonia, the present invention particularly relates to a co-drive using natural gas as a driving source.
It can also be effectively applied to combustion exhaust gas from a generation system.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a cogeneration system.

FIG. 2 is a diagram showing an example in which a selective denitration method using ammonia is applied to a heat recovery unit.

FIG. 3 is a view in which the ammonia oxidation catalyst layer according to the present invention is incorporated in the portion of FIG.

[Explanation of symbols]

1 Heat recovery boiler 2 chimney Heat exchanger for 3, 4 boiler 5 Ammonia injection part for denitration 6 DeNOx catalyst layer 7 Ammonia oxidation catalyst layer

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-2-191527 (JP, A) JP-A-53-102866 (JP, A) JP-A-3-221147 (JP, A) JP-A-5- 329334 (JP, A) JP-A-5-269351 (JP, A) JP-A-5-15739 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B01D 53 / 86,53 / 56 B01J 21/00-38/74

Claims (3)

(57) [Claims] 1. A gas engine or a natural gas driven source
Is a cogeneration system using a gas turbine
Selective denitration of combustion exhaust gas from ammonia with ammonia
In the heat recovery part, conduit part and chimney following the denitration process.
Method for preventing the precipitation of ammonium sulfate in
After that, before or after the heat recovery process in the subsequent heat recovery section.
During the process of, the denitrated exhaust gas is supported on an alumina carrier.
By contacting the platinum catalyst with
Oxidative decomposition and removal of slip ammonia in wastewater
Heat recovery part, conduit part and chimney following denitration process characterized by
Prevention method of ammonium sulfate precipitation. 2. Ammonia for the selective reduction denitration is deoxidized.
Immediately before the denitration catalyst in the nitrification process or in the denitration catalyst layer
Ammonia converted to ammonia from monia source
The method for preventing ammonium sulfate precipitation according to claim 1, wherein: 3. The ammonia source is liquefied ammonia,
Ammonia water, urea, urea water or ammonium hydrogen carbonate water
3. Ammonium sulfate precipitation according to claim 2, which is a solution
Prevention method.