JPS5850129B2 - Exhaust gas treatment method - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for treating exhaust gas.

Specifically, nitrogen oxides in the exhaust gas discharged from boilers, heating furnaces, etc. are decomposed by adding ammonia at high temperatures in the presence of oxygen in the exhaust gas flow path, and then the nitrogen oxides remain in the exhaust gas. The present invention relates to a method for efficiently decomposing ammonia.

NO and N02 (hereinafter referred to as rNOxJ) are harmful to the human body, and if directly absorbed into the body, they can cause harm such as decreasing internal functions, and recently they have also become a cause of photochemical smock called oxidants. Therefore, there is a strong demand for establishing a method for removing NOx.

In particular, decomposing NOx in exhaust gas from boilers, heating furnaces, etc., which are the main sources of NOx, is an issue that urgently needs to be solved in terms of pollution prevention.

As a method for efficiently and economically decomposing NOx in exhaust gas containing NOx without using a catalyst, the NOx-containing exhaust gas is heated to a temperature higher than 700°C, particularly 800 to 1100°C.
A method has been proposed in which ammonia is added overnight at a moderate temperature to substantially reduce NOx to nitrogen.

However, when using fuel containing sulfur such as C heavy oil in a boiler, if SO3 is present in the exhaust gas and there is ammonia remaining unreacted, a carbon-saving method is used to recover the heat of the exhaust gas at the boiler outlet. When ammonia and SO3 react in the equipment, air preheater, etc., various ammonium salts such as ammonium sulfate and acidic ammonium sulfate are generated, precipitate, and grow, eventually clogging the equipment and preventing the flow of exhaust gas or causing the Due to its strong corrosivity, it may cause problems such as damage to the equipment.

Furthermore, when ammonia is discharged into the atmosphere at high concentrations, it causes a problem of bad odors, so it is very important to take measures to reduce them.

In view of the above-mentioned circumstances, the present inventors have conducted intensive studies on a method for efficiently and economically decomposing ammonia remaining in the exhaust gas containing ammonia after NOx decomposition at high temperatures. The inventors have discovered that ammonia can be decomposed by bringing ammonia-containing exhaust gas into contact with a powdered catalyst made of iron oxide or iron hydroxide in an exhaust gas flow path at a temperature higher than 400°C, and have arrived at the present invention.

That is, the gist of the present invention is to provide a method for removing, cooling and releasing nitrogen oxides from high-temperature exhaust gas containing nitrogen oxides in an exhaust gas flow path of a boiler, heating furnace, etc.
First, the high-temperature exhaust gas is brought into contact with ammonia in the presence of oxygen in a temperature range higher than 700°C in the exhaust gas flow path to decompose nitrogen oxides in the exhaust gas, and then the treated exhaust gas is introduced into the exhaust gas flow path. A method for treating exhaust gas is characterized in that residual ammonia is decomposed by adding a powdered catalyst made of iron oxide or iron hydroxide in a suspended state in a temperature range higher than 400'C.

The present invention will be explained in more detail.

Examples of the NOx-containing exhaust gas that can be applied to the method of the present invention include exhaust gas from various devices such as boilers and heating furnaces.

The temperature of these exhaust gases is usually over 1000℃,
1. As a first step, the exhaust gas flow path 7 in the device is
Temperature range higher than 00°C, preferably 800-1100
Ammonia is injected into the temperature range of ℃ to remove NOx.

The amount of ammonia to be used can be appropriately selected in consideration of both economical efficiency and NOx removal rate, but it is usually appropriate to use a molar ratio of about 0.6 to 10 with respect to the NOx concentration in the total gas.

Ammonia may be used after being diluted with air, water, etc., and an ammonia precursor such as ammonium carbonate or ammonium oxalate may also be used instead of ammonia.

At this time, oxygen may be added in an amount of 0.1 to 20 volumes relative to the total volume of the exhaust gas, and may be added as necessary.

In order to bring the exhaust gas into contact with ammonia as uniformly as possible, it is desirable to adjust the ammonia introduction means and the exhaust gas flow rate.

The reaction residence time is usually selected from about 0.001 to 10 seconds.

The exhaust gas containing unreacted ammonia resulting from the decomposition of NOx by the above-mentioned high-temperature denitrification treatment is then treated with a powdered catalyst made of iron oxide or iron hydroxide suspended in a temperature range higher than 400°C in the exhaust gas flow path inside the device. By adding it in a turbid state, ammonia remaining in the exhaust gas is oxidized and decomposed.

To explain in more detail about the catalyst used in the present invention, iron oxide or iron hydroxide is used as a catalyst in the present invention, and it is thought that iron hydroxide turns into iron oxide under reaction conditions.

Further, the catalyst used in the present invention may contain base metals other than iron such as cobalt, nickel, bismuth, aluminum, beryllium, tungsten, and cerium. Examples of such catalysts include iron oxide-cobalt oxide, Iron-
Examples include bismuth oxide.

These catalysts are used in powder form, but in consideration of catalyst deterioration, pressure loss, etc., the catalyst is suspended in a gas such as air, an inert gas, or water or steam as a medium, and the injection method is used. etc. to uniformly disperse it into the waste gas in the exhaust gas flow path.

The contact temperature between the powdered catalyst and the exhaust gas should be as high as possible, for example, 500° C. or higher, which makes the vehicle heavy.

The amount of catalyst added is 0.1 kg catalyst/k based on the residual NH3.
It is necessary to have at least 3 g of residual NH3, and it is particularly preferable to have 9 residual NH3 per 0.5 to 5 kg of catalyst.

Note that it is desirable to collect and remove the added powdered catalyst by providing a dust collector, such as an electrostatic precipitator or a venturi scrubber, at the boiler outlet.

According to the method of the present invention, residual ammonia can be removed economically and efficiently, which is industrially advantageous.

Next, the method of the present invention will be further explained by examples.

Example 1 Boiler exhaust gas (70,000 N m'/H, NOx 178
When 356 ppm of ammonia was injected into the zone with a temperature of 850 to 900 degrees Celsius (ppm, 0.24%), the resulting NOx was 120
ppm (removal rate: 32.4%), and residual ammonia is 1
It became 34 ppm (ammonia decomposition rate 62.4%).

Next, 19.3 kg/h of ferric hydroxide powder (32 mesh or less) was injected into the 800° C. zone to suspend the mixture with 200 ON m'/h of air, and the ammonia at the outlet was reduced to 1109 pp.

The decomposition rate of ammonia due to the addition of ferric hydroxide is 18.
7% (NOx concentration remained unchanged).

Example 2 When 534 ppm of ammonia was injected under the same conditions as Example 1, NOx was 1109 ppm (NOx removal rate was 38.
5φ), and the residual ammonia was 296 ppm (ammonia decomposition rate 44.6%).

Next, 18.9 kg/H of ferric hydroxide powder was added to the 800°C zone in the same manner as in Example, and the ammonia at the outlet was reduced to 236 ppm.

Ammonia decomposition rate by adding ferric hydroxide is 20.3%
(No change in NOx concentration).

Example 3 Boiler waste gas (70,000 Nm/H, NOx 230pp
m, 024%) When 460 ppm of ammonia was injected into a zone with a temperature of 850 to 900°C, NOx was 158 ppm.
m (removal rate: 31.3%) and residual ammonia is 142p.
pm (ammonia decomposition rate 69.1φ).

Next, 18 kg/h of ferric oxide powder (32 mesh or less) was added to the 8000C zone in the same manner as in Example 1, and the ammonia at the outlet was reduced to 1106 pp.

The decomposition rate of ammonia due to the addition of ferric oxide is 25.4%
(NOx concentration remained unchanged).

Claims (1)

[Claims] 1 In a method of removing nitrogen oxides from high-temperature exhaust gas containing nitrogen oxides in the exhaust gas flow path of a boiler, heating furnace, etc., cooling and releasing the high-temperature exhaust gas, the high-temperature exhaust gas is heated above 700°C in the exhaust gas flow path without being shredded. Nitrogen oxides in the exhaust gas are decomposed by contacting with ammonia in the presence of oxygen in a high temperature range, and then the treated gas is
A method for treating exhaust gas, which comprises adding a powdered catalyst made of iron oxide or iron hydroxide in a suspended state to decompose residual ammonia in a temperature range higher than 00°C.