JPS6351733B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for treating exhaust gas containing nitrogen oxides. Conventionally, the method of treating nitrogen oxides (hereinafter referred to as _NOx ) in exhaust gas containing sulfur oxides (hereinafter referred to as _SOx ) has been to use a catalyst to reduce and remove _NOx with ammonia. There is. However, if the exhaust gas to be treated contains a high concentration of SO _x , there is a problem that the activity of the catalyst decreases. Possible reasons for this are as follows. The first cause is that SO _x contained in exhaust gas, especially sulfur trioxide (hereinafter referred to as SO ₃ ), reacts with catalyst components to produce sulfates, which leads to a decrease in the specific surface area and pore volume of the catalyst. This results in a decrease in activity. This phenomenon is remarkable when an iron oxide catalyst or a copper oxide catalyst is used, and is also likely to occur when alumina is used as a carrier. The second cause is that SO ₃ adsorbs to the active sites of the catalyst and exerts a poisoning effect, resulting in a decrease in activity. This is particularly noticeable in vanadium oxide catalysts. Another cause is that SO ₃ reacts with ammonia added as a reducing agent, producing ammonium sulfate ((NH ₄ ) ₂ SO ₄ ), acidic ammonium sulfate (NH ₄ HSO ₄ ), etc. (hereinafter, these compounds are collectively referred to as ammonium sulfate). ) and gradually accumulate on the catalyst, leading to a decrease in activity.
Note that the production of ammonium sulfate is thought to occur according to the following equation. NH ₃ +SO ₃ +H ₂ O→NH ₄ HSO ₄ (1) 2NH ₃ +SO ₃ +H ₂ O→(NH ₄ ) ₂ SO ₄ (2) The SO ₃ concentration in the combustion furnace exhaust gas is usually 1 of the SO _x concentration.
It is said to be ~10%. Therefore, when the SO _x concentration is 1000 ppm, the SO ₃ concentration is 10 to 100 ppm. When ammonium sulfate is generated and accumulated on the catalyst, the activity decreases regardless of the type of catalyst. Conventionally, 350% was used to prevent ammonium sulfate from depositing on the catalyst.
Methods have been used in which the reaction is carried out at a temperature of 0.degree. C. or higher, or the temperature is raised to 400 to 600.degree. C. each time the activity decreases to perform regeneration. However, in the conventional method, the fuel cost for raising the exhaust gas temperature to the reaction temperature and the fuel cost for regenerating the catalyst by raising the temperature account for a considerable amount of cost, which is undesirable because the cost for treating NO _x becomes large. In order to keep processing costs low, it is desirable to keep the reaction temperature as low as possible. In view of the shortcomings of the prior art, the present inventors studied methods to solve these shortcomings,
As a result of research, we have arrived at the present invention. That is, an object of the present invention is to eliminate the drawbacks of the above-mentioned conventional techniques, and to provide an economical method for efficiently removing nitrogen oxides in exhaust gas containing sulfur oxides and nitrogen oxides at low temperatures, while continuing to maintain catalyst activity. Our goal is to provide the following. The method of the present invention is characterized in that ammonia gas is added to exhaust gas containing nitrogen oxides and sulfur oxides, and the gas is brought into contact with a _SOx- resistant catalyst containing titanium oxide and/or tin oxide as a main component. In the method of reducing oxides to nitrogen and water, in addition to adding ammonia in an amount of 0.8 to 2 times the concentration of nitrogen oxides in the exhaust gas, ammonia in an amount of 1 to 5 times the concentration of sulfur trioxide is added. A residence time of 1 second or more is provided before the gas is brought into contact with the catalyst, and after the SO ₃ contained in the exhaust gas is reacted with ammonia to produce ammonium sulfate, the exhaust gas to be treated is placed in a parallel structure such as a honeycomb or plate shape. The method is to bring it into contact with a flow type catalyst at a temperature of 150°C to 330°C. In the method of the present invention, sufficient time is allowed for SO ₃ in the exhaust gas to react with ammonia gas, and the SO ₃ in the catalyst layer is converted into ammonium sulfate mist.
to avoid reaching it as By doing this, the catalyst component reacts with SO ₃ and becomes sulfated,
It can prevent a decrease in activity due to SO ₃ adsorption and poisoning. Moreover, since the reaction for producing ammonium sulfate is completed before the catalyst layer, accumulation of ammonium sulfate due to the reaction between SO ₃ and ammonia on the catalyst is unlikely to occur. Note that addition of ammonia water in place of ammonia gas should be avoided, as this leads to a decrease in exhaust gas temperature and a decrease in denitrification efficiency. Although the reactivity between SO ₂ and ammonia gas is poor, the reactivity between SO ₃ and ammonia gas is extremely good, so unreacted SO ₃ can be prevented from reaching the catalyst. Further, in the method of the present invention, it is preferable to use a parallel flow catalyst having a honeycomb shape or a plate shape so that the ammonium sulfate mist generated in advance can easily pass through the catalyst layer. In the method of the present invention, it is necessary to react SO ₃ and ammonia gas to form a mist of ammonium sulfate before entering the catalyst layer. It is preferable to provide time. For example, when the flow rate of exhaust gas is 10 m/sec, it is necessary to provide a distance of 10 m or more between the NH ₃ inlet and the catalyst layer of the reactor. If this interval is too short, the reaction between SO ₃ and ammonia gas will not proceed sufficiently, and some
Activity decreases because SO ₃ reaches the catalyst layer as it is. The reaction between ammonia and NO, which is the main component of NO _x , is thought to be expressed by the following equation. NO + NH ₃ + 1/4O ₂ →N ₂ + 2/3H ₂ O Theoretically, NO can be reduced if ammonia is at the same concentration as NO, but as mentioned above, SO ₃ in the exhaust gas is reacted with ammonia in advance. Since it is converted into ammonium sulfate, there is a corresponding shortage of ammonia. Therefore, in the exhaust gas
In addition to ammonia at an amount of 0.8 to 2.0 times the NO _x concentration
It is necessary to add ammonia in an amount of 1.0 to 5.0 times the SO ₃ concentration. The denitrification reaction is carried out at 150-330℃, preferably
It is carried out at 200-300℃. If the reaction temperature is too low, the activity is insufficient and it is not practical, and if the reaction temperature is too high, it is disadvantageous in terms of thermoeconomics. Space velocity is 1000 to 50000h ^-1 (sky tower equivalent, NTP)
range is used. As a component of the catalyst, a catalyst containing titanium oxide and/or tin oxide as a main component is advantageously used, as it has good activity particularly in a low temperature range and has good SO _x resistance. Regarding these catalysts, for example, Japanese Patent Publication No. 52-6953, Japanese Patent Publication No. 52-6954, Japanese Patent Application Laid-open No. 89291-1989, Japanese Patent Application Publication No. 89288-1988, Japanese Patent Publication No. 51-1989
-21569, JP-A-51-52363, JP-A-52-
Please refer to each publication No. 42463. The shape of the catalyst is such that the generated ammonium sulfate mist can easily pass through the catalyst layer, and so-called parallel flow catalysts such as honeycomb and plate shapes are preferred. The present invention will be explained below by giving specific examples. Example 1 Using a titanium oxide-molybdenum oxide-vanadium oxide (Ti:Mo:V=84:10:6; atomic ratio) catalyst, the following gas composition was prepared under the conditions of a space velocity of 5000 h ^-1 and a reaction temperature of 250°C. A continuous test of denitrification reaction was conducted for 200 hours. The shape of the catalyst is honeycomb type, and the dimensions of one honeycomb are 105 mm in height, 105 mm in width, and 100 mm in length.
mm, with a total of 196 6mm square holes lined up in 14 vertical and horizontal rows. The wall thickness is 1.5mm. A total of eight honeycombs were stacked in two rows, two rows each, and packed into a reactor. The packing volume of the catalyst is 8.8. The linear velocity of the gas is 2 m/s. NH ₃ gas was injected 2 m before the catalyst layer. Therefore, there is a residence time of 1 second before reaching the catalyst layer, and SO ₃ in the gas fully reacts with NH ₃ and enters the catalyst layer as ammonium sulfate mist. Reaction gas {NO 200ppm, NH ₃ 300ppm SO ₂ 500ppm, SO ₃ 10ppm O ₂ 3%, H ₂ O 12% CO ₂ 12%, N ₂ balance} The test results are shown in Table 1. From Table 1, it can be seen that according to this example, the NO _x removal rate shows a high value of 80% or more over 200 hours.

[Table] Comparative Example 1 A test was conducted using the same catalyst as in Example-1 and under the same reaction conditions as in Example-1, except that the injection of NH ₃ gas was changed to 1 m before the catalyst layer. In this case, the residence time from when NH ₃ is added to the reaction gas containing SO ₃ to the catalyst layer is only 0.5 seconds. The test results are shown in Table 2. Table 2 shows that by shortening the residence time to 0.5 seconds, there was not enough time for SO ₃ to react with NH ₃ and generate ammonium sulfate mist, and a large proportion of SO ₃ reached the catalyst layer, resulting in NO It can be seen that the reduction in _x removal rate is large.

[Table] Comparative Example 2 The same test as in Example 1 was carried out by filling an 8.8 reactor with the same catalyst composition as in Example 1 but changing the shape to a 6 mm diameter sphere. The test results are shown in Table 3. From Table 3, it can be seen that by changing the shape of the reactor to a fixed bed system using a granular catalyst, it becomes difficult for ammonium sulfate mist to pass through the catalyst layer, and the NO _x removal rate is greatly reduced.

[Table] As explained above, the present invention is effective in efficiently removing nitrogen oxides in exhaust gas containing sulfur oxides and nitrogen oxides at low temperatures.

Claims (1)

[Claims] 1. A method of reducing nitrogen oxides by adding ammonia gas to exhaust gas containing nitrogen oxides and sulfur oxides, and bringing the exhaust gas into contact with an _SOx- resistant catalyst mainly composed of titanium oxide and/or tin oxide. In the exhaust gas, the concentration of nitrogen oxides in the gas is
After adding 0.8 to 2 times the amount of ammonia gas and 1 to 5 times the amount of sulfur trioxide concentration,
Before bringing the exhaust gas into contact with the catalyst, a residence time of 1 second or more is provided, and sulfur trioxide in the sulfur oxide contained in the exhaust gas is reacted with ammonia to produce acidic ammonium sulfate or ammonium sulfate, and then the exhaust gas to be treated is Parallel flow catalyst and
A method for treating exhaust gas containing nitrogen oxides, which comprises contacting at 150°C to 330°C.