JPS605328B2 - Method of detoxifying high concentration nitrogen oxides using coke - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention utilizes thermal decomposition treatment of mixtures containing nitrates such as calcium nitrate or magnesium nitrate that require disposal, and nitrates such as calcium nitrate or magnesium nitrate that are by-produced during wet denitrification waste liquid treatment. Nitrogen oxides generated at a high concentration of 1% by volume or more (hereinafter referred to as N○×)
The present invention also relates to a method for reducing and rendering harmless NO2 at a high concentration of 1% by volume or more, which is generated by treating an aqueous solution of a functional nitrate such as calcium nitrite or magnesium nitrite with a strong acid.

The exhaust gas discharged from industrial furnaces such as boilers, combustion furnaces, and pseudo-drying furnaces contains several hundred units of N○×.
x is extremely harmful and must be removed from the exhaust gas.

Conventionally, methods for removing NOx from exhaust gas include an equimolar method in which NO is oxidized to N02 so that the molar ratio of NO and NO2 is approximately 1 and absorbed in an alkaline absorption liquid, or a There are so-called wet methods such as the ozone method, which oxidizes to N205, etc. and absorbs it in an alkaline absorption liquid or a box nitrate absorption liquid.However, in these methods, nitrates and nitrites are produced as by-products. It cannot be disposed of as is; it must be rendered harmless. The inventors have already made the Samurai request in 1973-106.
In No. 797 (Special Seki No. 52-30298), he proposed a method for thermally decomposing nitrates. The gist is that calcium nitrate and quicklime, which is a thermal decomposition product of calcium nitrate, are mixed in an appropriate ratio, and
By performing thermal decomposition while maintaining the thermal decomposition temperature at 0.00 to 1200 qo, quicklime with good activity is continuously produced and highly concentrated N○x gas is generated.
Thus, by this pyrolysis method, it is possible to generate N○x gas with a high concentration of 1 volume % or more. This highly concentrated NO gas must then be delivered to a reduction zone to be reduced and rendered harmless. A method for reducing N○x gas is the so-called dry denitrification method, in which a reducing gas such as ammonia, hydrogen sulfide, hydrogen, carbon monoxide, methane, etc. is added and brought into contact with a catalyst to reduce the N○x to N2. A typical example is a selective reduction method using ammonia.

However, as mentioned above, the concentration of NOx gas in general combustion exhaust gas is several hundreds, and even in the exhaust gas from a nitric acid production device, the concentration of NOx gas is several thousand. As a companion, a selective reduction/detoxification method has been proposed in which ammonia is used as a reducing gas and brought into contact with a catalyst within these concentration ranges. However, there are some difficulties in reducing the high concentration N○× generated by the above method using a catalytic method. That is 1% by volume
When the concentration of N○× gas is as high as above, the same reduction method as in the case of low concentration N○× gas cannot be adopted. The following four points can be considered as reasons for this. First of all,
In the high concentration region, reduction under the condition of several hundred sails cannot be achieved,
This means that large amounts of dinitrogen monoxide (N20, laughing gas) are likely to be generated. . For example, Fe-Cr catalyst (y-N20
3 carrier), the exhaust gas containing 500 N○× was
When catalytic reduction is carried out at 030, no generation of N20 is detected, but when the N○× concentration reaches 5% by volume, the generation of N20 of about 1% by volume is observed. The analysis of N20 was performed using a Shimadzu gas chromatography (Polarpack Q, 40o0). The second reason is that the presence of dust in the exhaust gas containing high concentration N○× gas generated by thermal decomposition in a direct heating furnace has a significant adverse effect on the operability of reduction using a catalyst.

Dust collectors such as cyclones and electrostatic precipitators cannot completely remove dust in exhaust gas over a long period of time, and the dust in exhaust gas coats the surface of the catalyst, reducing catalyst performance. In particular, when nitrates are thermally decomposed using a device such as a direct-heating rotary furnace, the amount of dust removed from the furnace reaches more than 5% of the supplied amount, and when this dust is introduced into the reduction equipment while being kept at a high temperature. This is extremely difficult, and wet dust collection in which water is dropped into the exhaust gas is not suitable because the N02 in the exhaust gas is absorbed by moisture. Third
In this case, the concentration of nitrogen gas in the gas generated from the pyrolysis furnace fluctuates frequently, and it is necessary to track and adjust ammonia to match this fluctuation.
There is a problem with follow-up operability that it is difficult to input. Fourthly, if the temperature of the catalyst layer is maintained at around the normal 300°C for reduction, even in a normal reduction reaction, NO12'Karihi 315 / Fence 20 Day 20171. Ball caIN02 14'
Karihi 3-7' Carp 22LO+108. When an exothermic reaction such as bulb caIN 02 12' state day 317/state 0 13' QLO + 7.7 kcal occurs, and when processing a high concentration of NO, the heat generation in the temperature zone of the catalyst layer is significant. The catalyst layer must be cooled, which affects the catalyst life and shortens the catalyst life.

The four drawbacks of the catalyst method explained above make stable continuous reduction operation impossible.

Therefore, the present inventors have conducted repeated experiments and research in order to eliminate these drawbacks and render the high-concentration, high-temperature NOx gas of 1% by volume or more harmless, and as a result, they have arrived at the present invention. The gist of the present invention is to reduce the temperature in the coke oven by using coke with a high melting point of ash so as not to cause scale trouble.
Keep it below 400oo, preferably 700~140pi○
, more desirably maintained at 900 to 1100°C to avoid the generation of clinker scale due to combustion in the coke oven, and further to separate gas components such as N○× gas and oxygen (02) in the exhaust gas from carbon (C). If the above reaction is an endothermic reaction as a whole, an appropriate amount of air is introduced to promote combustion. On the other hand, if the reaction is an exothermic reaction as a whole, an appropriate amount of steam is introduced to promote the aqueous formation of the steam and coke. Cooling using the endothermic reaction of the gasification reaction and the sensible heat of water vapor (hereinafter referred to as
air steam injection method),
The object of the present invention is to provide a method for freely controlling the maximum temperature in a coke oven to prevent clinker and scale troubles in coke ash, and for highly efficiently rendering highly concentrated N○x gas harmless. In this way, when detoxifying N○x gas using coke (hereinafter referred to as the coke method), no generation of N20 (laughing gas) is observed, and there is no concern that dust will impede detoxification. ×
It is insensitive to changes in gas concentration, there is no problem of shortening the catalyst life due to heat generation, and it can be rendered harmless very efficiently. That is, the present invention
When nitrates or mixtures containing nitrates are thermally decomposed in a pyrolysis device, or when an aqueous solution of a mixture containing nitrates or nitrites is treated with a strong acid, gas containing a high concentration of N○k of 1% by volume or more is generated. , by supplying nitrogen oxides and coke into a coke oven, maintaining the temperature in the coke oven at 700 to 140,000, and introducing water vapor and air into the coke oven in an appropriate ratio. , is characterized by preventing clinker scale caused by coke ash and freely controlling the reaction temperature to be in the range of 700 to 1400 qC to continuously or intermittently render high concentration N○× harmless. It is something that does. Specifically, the temperature of the coke layer in the coke oven is an appropriate temperature (700 to 1400
℃), no special operation is performed, and when the temperature of the coke layer is lower than the appropriate temperature (700 to 1400 degrees Celsius), air is introduced into the coke layer to burn the coke and raise the temperature of the coke layer. In, when the temperature of the coke layer is higher than the appropriate temperature (700 to 1400 qo),
Steam is introduced from the upstream side of the coke layer, and the coke layer is cooled using the endothermic reaction of the water gasification reaction between the steam and coke and the sensible heat of the steam, and the reaction temperature is controlled to be within a certain range. clinker and coke ash.
Reduces scale formation. Regarding the reduction of N○× gas by carbon, for example, G. Technique d Pietc;1. E
．． C50 1165 (1958). According to this, using activated carbon, space velocity (hereinafter referred to as SV)
1500/Hr, NO gas 0.53% (helium dilution)
At 70,000, NO gas is almost removed.
According to the present invention, the coke layer temperature is set at 700 to 140 pi.
SV by keeping it preferably at 900-11000C
(See Figure 1). To explain this in more detail, coke ovens can be made more compact by reducing the amount of coke charged, and air injection can be used to compensate for the exhaust gas composition in which the entire system undergoes an endothermic reaction. A thermal method is used, and if the entire system is an exothermic reaction, a cooling method using steam injection is used, or an air cooling method is used.
By adopting a balanced steam injection method and freely controlling the maximum temperature inside the coke oven, clinker trouble caused by high temperature overheating is prevented, and long-term stable and high-performance operation is possible. be. Thus, FIG. 1 shows the relationship between the SV and the conversion rate (removal rate) of N○× gas in the vicinity of 100 p0. It is clear from FIG. 1 that the highly concentrated N○× gas is rendered harmless at an extremely high efficiency in the range of 900 to 1100 qo. In the method of the present invention, the reaction formula of the exhaust gas containing NO at a high concentration of 1% by volume or more, which is carried out in the coke oven, is as follows.

NC+C (Coke) 1 CO+1' 248.4kc
al/mol (1) Q+ main (coke) primary O+
58. Autumn cal/mol (2) QO+C(
coke) → CO + Koichi 28.4 kcal'mol
(3) C020C (coke) primary 0-38.2k
caVmol (4) 020C (coke) →
C02197.0kcal/mol (5
1NO11'2C→1'th 0211/is 2167. ball c
al'mol (6) If the above elementary reaction equations overlap and the system as a whole is an exothermic reaction, water vapor is introduced to maintain heat balance in order to suppress clinker and scale troubles in the ash and control the reaction. In addition, if the entire system is an endothermic reaction, increase the amount of air introduced to control the reaction to maintain heat balance, and appropriately adjust and control the introduction of water vapor and air, i.e. oxygen, to ensure smooth operation. .

The method of the present invention targets nitrogen oxides with a high concentration of 1% by volume or more, and in particular, regardless of the presence of oxygen,
) and {1' equations, the so-called solid-gas reaction in which NOx and coke are directly reacted is carried out by utilizing the oxidation properties of NO.

In addition, oxygen (air) can be introduced into the coke layer as needed using only the heat of reaction between NO and coke.
Oxygen (air) is introduced when the temperature of the reduction layer (reduced layer) cannot be maintained at the optimum temperature (700 to 1400 oo), and depending on the N○× concentration, it may not be necessary to introduce oxygen (air) at all. The purpose of introducing oxygen (air) in this way is to control the temperature of the coke layer. Also, N○ in the processing gas
× When the concentration is high and the temperature of the coke layer rises due to the reaction between NOx and coke, in addition to introducing steam and simply utilizing the heat generated by introducing the steam, it is possible to , the temperature of the coke layer is controlled by utilizing the endothermic reaction of the so-called water gasification reaction. Furthermore, by introducing water vapor to reduce the temperature, CO and nitrogen as a reducing agent are generated, and NO
A secondary effect of promoting the N○× reduction reaction can be expected due to the reaction of O→1/2N20C02NO+day21'state20ratio0.

Note that if economic efficiency is ignored, oxygen may be introduced instead of air. For the prototype of such a furnace, reference is made to the description of coke generating furnace gas (for example, written by Sakae Yagi, Industrial Kiln Furnace, p. 267, Oh Kyoritsu Shuppan).

Since the coke oven exhaust gas made harmless by the method of the present invention contains carbon monoxide and hydrogen, if necessary, it is subjected to secondary combustion in an afterburning furnace, etc., and after cooling,
Release into the atmosphere as a completely harmless gas. Furthermore, the exhaust gas discharged from the coke oven can be recycled and used as a heat source. The type of coke oven may be a fixed bed type, a fluidized bed type, or a moving bed type. Next, a flow sheet of an example of an apparatus for carrying out the method of the present invention is shown in FIG.

This is a flow sheet of a device that combines a NO gas generation device by thermal decomposition of calcium nitrate and a detoxification treatment device using a coke oven. Explanations of the numbers in the figures will be given in the examples described later. Note that the gas to be treated in the present invention is not limited to the high concentration N○× gas generated by thermal decomposition of nitrates and the high concentration N○× gas generated by strong acid treatment of nitrite aqueous solution; This gas can be applied as long as it contains a high concentration of N○× of % or more. Examples of the present invention will be shown below.

Example - Using the apparatus shown in Figure 2, Ca○/Ca(N03)2=4/1(
(weight ratio), a sample of two grains is fed from feeder 1 into rotary kiln 2 for thermal decomposition, and C0212.3%
An exhaust gas having a gas composition of , Q5.8%, and N○×2.5% was obtained.

This exhaust gas was introduced into a coke oven 4 after removing dust through a dust collector 3. The exhaust gas temperature was 310°C.
Melting point of coke ash as coke: 1350o0, particle size: 1
Steelmaking coke with 0 ribs and a density of 0.67 coke was used.
The coke in the coke oven has an SV of 2,500 nohr, and this coke is ignited with a burner to raise the maximum temperature zone to 110
If reduction is carried out by introducing exhaust gas while keeping it at 0qo, N○×
The concentration was 5 winds. If you continue to drive for a long time, 1
With this gas composition, the temperature of the coke layer decreased, so air injection was performed from the air supply pipe 5. In this case, the amount of air is 50 som/min for the introduced exhaust gas of 110 som/min, and the temperature inside the furnace reaches 900 m within a few minutes after introducing the air.
Air injection was stopped as it rose from 30 to 1050 qo. Further, when the temperature rose due to excessive heat generation, water vapor was introduced from the water vapor supply pipe 6. The N○k gas in the process gas obtained in this way is less than 5 degrees, and NO
The conversion rate of x was 98% or more, and no N20 (laughing gas) was detected. In addition, the ash content, which is a by-product of about 1% by weight of coke, does not cause clinker phenomenon due to fusion because the furnace temperature is below the melting point of the ash content, allowing the coke oven to continue operating smoothly. High concentration NOx gas could be effectively rendered harmless. The melon concentration was measured by chemiluminescence using an ECL-77 device manufactured by Yanagimoto. 7 is a cooling tower.

As explained above, according to the method of the present invention, 1% by volume
It is possible to efficiently convert gas containing a high concentration of NO into a detoxified gas that does not contain any N20, and since it does not use a catalyst or a reducing gas such as ammonia as in conventional methods, it is detoxified by dust in the exhaust gas. This solves problems such as the following problems with reducing gas such as ammonia and injection due to fluctuations in N & gas concentration, and the shortening of catalyst life due to heat generation.Furthermore, when the temperature of the coke layer in the coke oven increases, the coke layer Steam is supplied from the upstream side of the coke layer, and the coke layer is cooled using the endothermic reaction of the water gasification reaction between the steam and coke and the rat heat of the steam, so it is possible to prevent the formation of clinker scale due to coke ash. It has this effect.

[Brief explanation of the drawing]

Figure 1 shows N0x concentration 890-980 skin gas, coke amount 106, coke layer temperature 900oo, 10000
Graph showing the relationship between space velocity (SV) and NO heritization rate (removal rate) when processed under the conditions of 0 and 110030, Figure 2 is a flowchart showing an example of an apparatus for carrying out the method of the present invention It is a sheet. 1... Feeder, 2... Rotary kiln, 3... Dust collector, 4... Coke oven, 5... Air supply pipe, 6... Steam supply pipe, 7... Cooling tower. Hair 'Z diagram

Claims (1)

[Claims] 1. Gas containing nitrogen oxides with a high concentration of 1% by volume or more, which is generated when nitrates or mixtures containing nitrates are thermally decomposed in a pyrolysis device, or when an aqueous solution of nitrites or mixtures containing nitrites is treated with strong acid. is fed into the coke oven to cause nitrogen oxides and coke to react directly, and when the temperature of the coke layer in the coke oven is at an appropriate temperature, no special operation is performed, and when the temperature of the coke layer is lower than the appropriate temperature, the coke layer is reacted directly. In this method, when the temperature of the coke layer is higher than the appropriate temperature, water vapor is introduced from the upstream side of the coke layer to initiate a water gasification reaction between the water vapor and coke. A method of detoxifying highly concentrated nitrogen oxides using coke, which is characterized by cooling a coke layer using the endothermic reaction of .