JPS6147128B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for treating exhaust gas, and in detail,
The present invention relates to a method for treating exhaust gas that can efficiently remove nitrogen oxides from exhaust gas containing nitrogen oxides and sulfur oxides at low temperatures and maintain catalytic activity for a long time. Recently, research and development regarding methods for removing nitrogen oxides (NOx) from exhaust gas containing nitrogen oxides (NOx) have made rapid progress, and many proposals have been made. Among these, the method that is becoming mainstream is a method in which nitrogen oxides are reduced and removed using ammonia using a catalyst. This method is usually carried out at a reaction temperature of 200 to 400°C, but if the exhaust gas to be treated contains a high concentration of sulfur oxides (SOx), the activity of the catalyst used may decrease. be.
The cause of this is that sulfur trioxide (SO ₃ ) in sulfur oxides or sulfur mist acts as a catalyst poison, or that ammonium sulfate or acidic ammonium sulfate produced from these substances and ammonia enters the pores of the catalyst. These substances may condense and become clogged, or they may react with catalyst components to produce compounds that reduce the activity of the catalyst. Therefore, in general, when applying the ammonia catalytic reduction method to exhaust gas containing a high concentration of sulfur compounds,
It is necessary to carry out the reaction at a relatively high temperature range of 350°C or higher. Therefore, in order to treat relatively low-temperature exhaust gas of 150 to 250°C discharged from a coke oven or sintering furnace, the temperature must be raised to this reaction temperature, which is extremely disadvantageous economically. . Therefore,
Several methods have been proposed to solve this problem. In other words, one of them is to determine the reaction temperature range.
There is a method in which the denitrification reaction is carried out by alternately changing the temperature from 200 to 350°C and above 400°C. (Refer to Japanese Patent Application Laid-Open No. 52-31970.) Another method has been proposed in which the poisoned catalyst is fired at 350 to 800°C to regenerate it. (Refer to Japanese Patent Application Laid-Open No. 52-26394) However,
According to the former method, it is true that there is considerable savings in terms of thermal economy, but the temperature must be raised to over 400°C even for a short period of time, and if the operation is performed at low temperatures for a long period of time, the denitrification rate will inevitably decrease little by little. In addition, in the latter method, the performance of the catalyst has considerably deteriorated before regeneration, and to prevent this, it is necessary to perform regeneration frequently, which is not very desirable. An object of the present invention is to solve the above-mentioned drawbacks of the prior art, to efficiently remove nitrogen oxides from exhaust gas containing sulfur oxides at low temperatures, and to maintain catalytic activity for a long period of time. The goal is to provide the following. To summarize the present invention, in the exhaust gas treatment method of the present invention, when exhaust gas containing nitrogen oxides and sulfur oxides is catalytically reduced with ammonia to remove nitrogen oxides, the exhaust gas is heated to 200 to 350°C in advance. Contact with an absorbent that selectively absorbs sulfur trioxide, which is made of titanium oxide and at least one member selected from the group consisting of calcium oxide, nickel oxide, and cobalt oxide, at a temperature of It is characterized by doing. As mentioned above, in ammonia catalytic reduction,
The present inventors have learned that the main cause of the decrease in catalyst activity is particularly sulfur trioxide (hereinafter referred to as SO ₃ ) among sulfur oxides (hereinafter referred to as SOx) contained in exhaust gas. In order to confirm this, the inventors investigated SO ₃
The effects of gas containing only sulfur dioxide (hereinafter referred to as SO ₂ ) and gas containing SO ₃ on the life of the catalyst were investigated through experiments. The results are shown below as an example. Example 1 Using a titanium oxide-molybdenum oxide-vanadium oxide (atomic ratio Ti:Mo:V=84:10:6) catalyst, gases A and B having the compositions shown in Table 1 below were heated at a space velocity of 5000 hr ^-1 , and the reaction temperature was 250°C for 200 hours.

[Table] The resulting NOx removal rates are shown in Table 2 below.

[Table] From Table 2, it can be seen that in the case of gas A containing SO ₂ and SO ₃ , the NOx removal rate decreases significantly over time, whereas in the case of gas B, which does not contain SO ₃ , It can be seen that there is almost no decrease in the NOx removal rate, and the activity of the catalyst is maintained for a long time. Furthermore, a vanadium oxide-aluminum oxide (atomic ratio: 6:94) catalyst (particle size 5 mm), which is a general vanadium-based catalyst, was used as another catalyst, and the catalyst was heated under the same conditions as in the previous experiment for gas A in the above experiment. We investigated the lifespan of. The results are shown in Table 3 below.

[Table] As is clear from Table 3, the NOx removal rate of vanadium-based catalysts also decreases significantly over time in the case of gas containing SO ₃ . The above experiments have shown that when SO ₃ is contained in the gas, the activity of the catalyst in the general ammonia catalytic reduction method decreases significantly, making it impossible to maintain a stable NOx removal rate. To prevent
It was confirmed that it was necessary to remove SO ₃ from the gas in advance. The present invention has been made based on the above knowledge. That is, according to the present invention, SO ₃ in the gas is removed in advance by an absorbent that selectively absorbs SO 3 and is made of at least one member selected from the group consisting of calcium oxide, nickel oxide, and cobalt oxide and titanium _oxide . By performing ammonia catalytic reduction after absorption and removal, NOx can be removed at a relatively low temperature, and the catalyst activity and NOx removal rate can be stably maintained. The SO ₃ concentration in exhaust gas from combustion furnaces, etc. is usually about 1 to 10% of the SOx concentration. Therefore, when using the absorbent of the present invention that can selectively remove _SO3 , the required capacity of the absorbent is 1/10 to 1/100 compared to the case where the entire SOx is absorbed. It is sufficient and economically advantageous. In the present invention, the absorbent in which titanium oxide is blended with calcium oxide, nickel oxide, or cobalt oxide has relatively little change in its pore structure even when SO ₃ is absorbed, and there is also little decrease in strength, so it has no effect on absorption performance. It has the advantage of being difficult to change. Further, when titanium oxide and the other components mentioned above are used in combination, the appropriate composition ratio is in the range of 0.1 to 100 moles of the other components per 1 mole of titanium oxide. Also, SO ₃ absorption is 200-350℃, usually 250-300℃
It is carried out at a temperature of about ℃, and the gas space velocity is
It is carried out at 1000 to 100000h ^-1 . The shape of the absorbent is not particularly limited, and may be pellet type,
A ring shape, a spherical shape, a honeycomb shape, etc. can be used as appropriate. Next, regarding several types of absorbents in the present invention,
The results of an SO ₃ removal performance experiment are shown. Example 2 Using a gas with the following composition, 7 as shown in Table 4 below
types of absorbents (TiO ₂ −NiO, TiO ₂ −CoO, TiO ₂
-CaO: particle size 3mm spherical), the absorption reaction temperature
SO ₃ absorption experiments were conducted at 250°C and a space velocity of 5000 h ^-1 . Gas composition SO ₃ 100 ppm SO ₂ 500 〃 O ₂ 3% H ₂ O 12 〃 N ₂ balance The results obtained are shown in Table 4 below. In addition, SO ₃
The removal rate was calculated using the following formula. SO ₃ removal method (%) = SO ₃ concentration absorbed by absorbent / supplied SO ₃ concentration × 100

[Table] From Table 4, it can be seen that all absorbents exhibit high SO ₃ removal rates over a fairly long period of time. In addition, it was confirmed that SO ₂ was hardly absorbed by the absorbent and passed through as it was. The gas from which SO ₃ has been absorbed and removed is denitrified by ammonia catalytic reduction, but the activity of the catalyst used is maintained for a long time, and the denitrification rate does not decrease much. Moreover, according to the present invention, the ammonia catalytic reduction reaction
Same temperature range as in SO ₃ absorption reaction, i.e.
It can be carried out at a low temperature of about 200 to 350°C.
In addition, all catalysts used in conventional ammonia catalytic reduction methods can be used as catalysts for the denitrification reaction, but titanium oxide is particularly effective as a catalyst with excellent activity in the low temperature range and good SOx resistance. and/or catalysts based on tin oxide can be advantageously used. Regarding these catalysts, please refer to Japanese Patent Publication No. 52-6953 and Japanese Patent Publication No. 52-6953.
Publication No. 6954, Japanese Patent Application Laid-Open No. 1989-89291, Japanese Patent Application Publication No. 1973
-89288 Publication, JP-A-51-21569, JP-A-Sho
Those described in JP-A No. 51-52363, JP-A No. 52-42463, etc. can be appropriately selected. In addition,
Moreover, the shape of these denitrification catalysts is not particularly limited;
Any of pellet type, ring type, spherical type and honeycomb type can be adopted. Next, the present invention will be explained with reference to Examples, but the present invention is not limited to these in any way. Example A heavy oil boiler exhaust gas having the following composition, and also,
TiO ₂ −CaO as SO ₃ absorbent (molar ratio 7:3)
100 was used. Gas composition NOx 150-210 ppm SO ₂ 500-700 〃 SO ₃ 10-20 〃 O ₂ 3-6% CO ₂ 10-12 〃 H ₂ O 8-12 〃 Dust 60-90 mg/Nm ³ N ₂ balance above Gas space velocity 10000h ^-1 , absorption reaction temperature 260
It was absorbed into an absorbent at ±10°C. At this time, SO ₃ in the gas after passing through the SO ₃ absorption tower (before entering the denitrification reaction tower)
When we analyzed the concentration of
SO ₃ less than 1ppm, 2-3ppm even after 1000 hours
It was confirmed that more than 80% of SO ₃ was removed by the absorbent. The gas from which SO ₃ was removed was then subjected to denitrification treatment. Ti-Mo-V (atomic ratio 84:
10:6) 200, space velocity 5000h ^-1 , denitrification reaction temperature 260±10℃, ammonia/NOx 1.0 to 1.2
The reaction was carried out for 1000 hours under the following conditions. The results are shown in Table 5 below.

[Table] As is clear from the results in Table 5, by selectively removing SO ₃ in advance, it was possible to maintain a stable NOx removal rate for a long time at a low temperature of about 260°C. As explained above, according to the present invention, nitrogen oxides are efficiently removed at low temperatures by performing ammonia catalytic reduction after previously absorbing and removing sulfur trioxide from exhaust gas containing nitrogen oxides and sulfur oxides. Moreover, the catalytic activity can be maintained for a long time and stable reactions can be carried out.

Claims (1)

[Claims] 1. When exhaust gas containing nitrogen oxides and sulfur oxides is catalytically reduced with ammonia to remove nitrogen oxides, the exhaust gas is preliminarily treated at a temperature of 200 to 350°C from the group consisting of calcium oxide, nickel oxide, and cobalt oxide. A method for treating exhaust gas, which comprises bringing the mixture into contact with an absorbent that selectively absorbs sulfur trioxide, which is composed of at least one selected species and titanium oxide, and then performing catalytic reduction with ammonia in the same temperature range.