JPS5817644B2 - Treatment method for exhaust gas containing nitrogen oxides - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing nitrogen oxides and sulfur oxides from exhaust gas.

Currently, research and development on methods for removing nitrogen oxides (hereinafter referred to as NOx) from exhaust gas containing sulfur oxides (hereinafter referred to as SOx) are making rapid progress, and many methods have been proposed.

Among these, the method that is becoming mainstream is a method that uses a catalyst to reduce and remove nitrogen oxides with ammonia.

In this method, the reaction temperature is usually 200-450°C.
It is.

However, there are several problems when the exhaust gas to be treated contains a high concentration of sulfur oxides.

The most important problem is that sulfur oxides in the exhaust gas, especially sulfur trioxide, react with the added ammonia to form ammonia sulfates, such as ammonium sulfate ((NH4)2804
), ammonium hydrogen sulfate (NH4H8O4), etc. (
These compounds (hereinafter collectively referred to as ammonium sulfate) are produced and gradually accumulate on the catalyst, resulting in a decrease in the activity of the catalyst.

From the viewpoint of thermoeconomics, it is desirable that the reaction temperature be lower.

However, the lower the temperature, the lower the vapor pressure of ammonium sulfate, which makes accumulation on the catalyst more likely to occur, particularly at temperatures below 330°C.

The production of ammonium sulfate occurs according to the following equation.

. NH3+SO3+H2O-4NH4H8042NH3
+SO3+H20→(NH4)2S04 SO in the combustion furnace exhaust gas (degrees is said to be usually 1 to 10% of the SOx concentration).

Therefore, when the SOx concentration is oooppm, the SO3 concentration is 10-stooppm.

At low temperatures below 330° C., there is a strong possibility that ammonium sulfate will precipitate on the catalyst due to the reaction between ammonia added as a NOx reducing agent and S03, and the activity will gradually decrease.

In particular, ammonium sulfate easily precipitates in the pores of the catalyst due to a phenomenon called "capillary condensation."

For the above reasons, in the treatment of exhaust gas containing a high concentration of SOx, the denitrification reaction is often carried out at a temperature of 350° C. or higher to prevent ammonium sulfate from accumulating in order to prevent a decrease in the activity of the catalyst.

But first. As mentioned above, in the exhaust gas denitrification process, it is desirable to perform the denitrification reaction at a temperature of 150 to 330°C from the viewpoint of thermoeconomics.

Conventionally, the denitrification reaction was carried out at low temperatures, and each time the performance deteriorated, the
A method of decomposing and removing ammonia sulfate by increasing the temperature to 00 to 6000C has been proposed (Japanese Patent Application Laid-Open No. 1983-1993-1).
3366).

Using this method, ammonium salts accumulated on the catalyst can be removed.

However, the conventional technology has several problems.

First, since the gas after regenerating the catalyst contains a high concentration of ammonium sulfate, if it is released into the atmosphere as it is, it will cause new secondary pollution.

Therefore, new equipment is needed to remove ammonium sulfate.

Second, since the ammonia added as a NOx reducing agent partially reacts with S03, the amount of ammonia is insufficient and an excess amount of ammonia is required.

The reaction between ammonia and NO, which is the main component of NOx, is expressed by the following equation.

Theoretically, NO can be reduced if NH3 is at the same concentration as NO, but for the reasons mentioned above, more than the theoretical amount of ammonia is required, and this amount varies depending on the S03 concentration and reaction conditions, but at least 10 % or more.

This is unfavorable from the economical point of view of ammonia consumption.

Another problem is that the catalyst usually has an alumina carrier.

If such a substance that easily becomes sulfated is used as a catalyst, the catalyst itself becomes sulfated and its activity decreases.
It is difficult to regenerate completely at temperatures between 0 and 600°C.

For example, as a method for regenerating sulfated alumina carriers, 6
A method of heating to 50 to 900°C has been proposed (Japanese Patent Application Laid-Open No. 51-2694).

It is possible to decompose aluminum sulfate by raising the temperature to this level.

However, it is very disadvantageous from the point of view of thermal economy.

Furthermore, if the temperature is raised too much, there is a risk that the catalyst will sink.

From the above, it is desirable that the regeneration be performed at a temperature of about 350 to 600°C at most.

An object of the present invention is to provide a method that can efficiently remove nitrogen oxides from exhaust gas containing sulfur oxides at low temperatures.

The feature of the method of the present invention is that the metal oxide catalyst whose activity has decreased due to the precipitation of ammonia and SOx compounds is
The catalyst is regenerated by scattering the compound of ammonia and SOx by heating it to a temperature of 0°C, and the gas containing ammonia and SOx scattered from the top of the catalyst is passed through an SOx removal column filled with an SOx absorbent to form SO3. is selectively absorbed and removed, the remaining gas containing ammonia is added to the exhaust gas to be treated and returned to the reaction tower, and the ammonia is reused as a NOx reducing agent.

Next, the present invention will be explained in detail with reference to the figures.

After adding a predetermined amount of ammonia 2 to the exhaust gas 1, in the denitrification reaction tower 3, it comes into contact with a denitrification catalyst 4 mainly composed of TiO2 and/or SnO2 at a temperature of 150 to 330°C, and NOx is reduced to N2 and H20. .

At this time, SOx, especially SO3, and ammonia contained in the exhaust gas 1 react to generate ammonium sulfate, which is deposited on the catalyst, and the amount of deposit increases with time, causing a decrease in catalyst activity.

Therefore, a part of the denitrification catalyst 4 is intermittently or continuously extracted from the denitrification reaction tower 3 and sent to the regeneration tower 5.

A high temperature gas 6 is passed through the regeneration tower 5 to evaporate the accumulated ammonium sulfate and regenerate the catalyst.

The high temperature gas containing ammonia and SOx is sent to the sulfur oxide removal tower 7, and only SO3 is selectively absorbed by the SO3 absorbent 8.

Since the gas discharged from the SO3 removal tower contains ammonium, this gas is added to the untreated exhaust gas 1 and used again in the denitrification reaction tower 3.

Further, the catalyst regenerated in the regeneration tower 5 is returned to the denitrification reaction tower 3 via a catalyst circulation line 9.

According to the above method, there are the following advantages.

1(1) A predetermined denitrification rate can be maintained by regenerating the catalyst and circulating it to the denitrification reaction tower.

(2) The SO3 contained in the gas after the catalyst regeneration process is S
Since it is absorbed and removed by an O3 absorbent, there is no need to worry about it being released into the atmosphere.

2(3) The ammonia released when the ammonium sulfate accumulated on the catalyst is evaporated is returned to the denitrification reaction tower and reused as a NOx reducing agent, so the amount of ammonia consumed can be saved.

Another feature of the method of the present invention is the use of a catalyst whose main components are titanium oxide 2 and/or tin oxide.

Titanium oxide and tin oxide are highly sulfated.
Strong SOx resistance.

Therefore, the activity can be easily restored as long as the ammonium sulfate accumulated on the catalyst is removed.

The catalyst in method 3 of the present invention has titanium oxide and/or tin oxide as the main component, and the second component is copper, vanadium, chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, lanthanum, cerium, Contains one type of uranium oxide or a combination of five or more types of uranium oxides.

The ratio of the first component, titanium oxide and/or tin oxide, to the second component is such that the first component is 50 to 99%, preferably 80 to 98%, based on the atomic percentage of the metal component.

If the first component is less than 50%, the SOX resistance of the catalyst will be poor, and if it is more than 99%, the activity of the catalyst will be low.

For the preparation of the catalyst, any of the precipitation methods, kneading methods, impregnation methods, etc. used in normal production can be used, and there is no particular limitation.

The reaction between NOx and ammonia in the denitrification reaction tower is 150
The reaction temperature is preferably 200 to 3000C, preferably 200 to 3000C.

NHs 7'N Ox ratio in the exhaust gas entering the denitrification reaction tower (
molar ratio) is adjusted to 0.7-1.2.

During the denitrification reaction, ammonium sulfate accumulates on the catalyst, causing a gradual decrease in activity.

When the denitrification rate falls below the required level, -.part of the catalyst is extracted and regenerated in a regeneration tower.

Alternatively, it is also possible to continue to maintain a constant level of denitrification rate by extracting a portion of the catalyst at regular intervals, regenerating it, and returning it to the denitrification reaction tower.

The catalyst with decreased activity in the regeneration tower is heated to 350-600℃
, preferably by passing hot gas at 375-600°C.

The regeneration time varies depending on the regeneration temperature and the space velocity (SV) of the regeneration gas.

The higher the regeneration temperature, the shorter the regeneration time, but 0.5
~5.0 hours are required.

The SV of the gas is preferably 100 to 10,000 v/v/h, preferably 100 to 1,000 v/v/h.

The regeneration gas is preferably a gas containing oxygen, especially 10% oxygen.
A gas containing the above is good.

The high-temperature gas leaving the regeneration tower contains ammonia and SO3, but is led to the S03 removal tower filled with S03 absorbent.
SO3 is selectively removed.

In the method of the present invention, alkali metal oxides, alkaline earth metal oxides, and 350~
Transition metal oxides are available that form sulfates that are stable at 600°C.

Particularly preferred are calcium oxide, iron oxide, and copper oxide.

Of course, even with these hydroxides and carbonates, 35
Any material that becomes an oxide at 0 to 600°C can be used.

Further, as a recyclable absorbent, titanium oxide-iron oxide, titanium oxide-copper oxide, etc. can be used.

The reaction of SO3 with these absorbents is usually quite fast in the reaction temperature range mentioned above.

Examples of the present invention are shown below.

Example 1 Catalyst composition was Ti-Mo-V (84:10:6
, molar ratio) of catalyst 1001, heavy oil boiler exhaust gas (
100ONm'/h).

1,000 for a denitrification pyrant lamp with the same arrangement as in the figure.
Time processed.

The reaction temperature was 290±10°C, and the space velocity was 5.000.
v/v/h, the reaction gas composition is as follows:

NOx 150~210p NH3220~280p1m So2 10001000-130 0pp 30~70p Water vapor 8-11% 02 3; 6% CO211-14% Amount of soot and dust 60~90mi; t/NrrlN2 remaining The denitrification reaction tower is of a catalyst moving bed type.

The denitrification catalyst is l[3? 2 from the hopper at the bottom of the denitrification tower
The air at 450°C is extracted in the regeneration tower by 2
time, at a space velocity of 1.000 v/v/h, and returned to the denitrification reaction tower.

Further, the gas leaving the regeneration tower contains S03 and NH3, and Sq in this gas is absorbed and removed in the SO3 removal tower filled with 1Ol of CaO.

The gas containing NH3 was returned to the denitrification reaction tower in addition to the exhaust gas to be treated.

The results of the 1,000 hour continuous test are shown in Table 1.

The soot and dust deposited on the catalyst layer was removed by mechanical vibration during removal of the catalyst, and the pressure loss of the process gas passing through the catalyst layer was also restored.

Comparative Example A long-term test was conducted under the same conditions as in Example I except that the catalyst was changed to Al-V (94:6, molar ratio).

The results are shown in Table 2.

Example 2 The same test as shown in Example 1 was carried out with the catalyst at 5 n-V (90
:10, molar ratio).

The results obtained are shown in Table 3.

[Brief explanation of drawings]

The figure is a schematic diagram of the apparatus used in the method of the invention. 1... exhaust gas, 2... ammonia, total... denitrification reaction tower, 4... denitrification catalyst, 5...
... Regeneration tower, 6 ... High temperature gas, 1 ...
... Sulfur oxide removal tower, 8 ... Sulfur oxide absorbent, 9 ... Catalyst circulation line.

Claims (1)

[Claims] 1. Ammonia is added to exhaust gas containing nitrogen oxides and sulfur oxides, and the mixture is brought into contact with a metal oxide catalyst packed in a reaction tower at a temperature of 150 to 330°C to convert nitrogen oxides into nitrogen gas. In a method for treating exhaust gas in which sulfur oxides are precipitated on a catalyst as sulfates of ammonia and water, the catalyst on which the sulfates have been precipitated is extracted from the reaction tower and heated to a temperature of 350~
The ammonia sulfate deposited on the catalyst is evaporated by contacting with an oxygen-containing gas at a temperature of 600°C, and the gas containing the vapor is passed through a sulfur oxide absorption tower to remove sulfur oxides. A method for treating exhaust gas containing nitrogen oxides, characterized by reusing the gas as a NOx reducing agent. 2. In the method for treating exhaust gas as set forth in claim 1, the metal oxide catalyst packed in the reaction tower contains at least one of titanium oxide and tin oxide as a first component in an amount of 50 to 9
9 atoms, 1 of the oxides of copper, vanadium, chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, lanthanum, cerium, and uranium as the second component
A method for treating exhaust gas containing nitrogen oxides, characterized by containing nitrogen oxides or more. 3. The method for treating exhaust gas according to claim 1, wherein the absorbent filled in the sulfur oxide absorption tower is at least one of calcium oxide, copper oxide, and iron oxide. Treatment methods for exhaust gases, including