JPS6147567B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a denitrification process for removing nitrogen oxides contained in exhaust gas containing sulfur oxides and moisture by a so-called ammonia catalytic reduction method. As is well known, ammonia as a reducing agent is a method for removing nitrogen oxides of about 50 to 1000 ppm contained in various industrial exhaust gases emitted from fixed sources such as boilers, heating furnaces, coke ovens, and sintering furnaces. The ammonia catalytic reduction method, in which nitrogen oxides and nitrogen oxides are catalytically reacted and removed under a temperature condition of 350 to 450°C, is particularly representative. A low-temperature ammonia catalytic reduction method is attracting attention in which the above reaction is carried out in a temperature range approximately equivalent to the temperature of exhaust gas discharged from a stationary source, ie, 150 to 350°C. In this low-temperature ammonia catalytic reduction method, in conjunction with the development of a low-temperature denitrification catalyst that has sufficient catalytic activity for the above-mentioned low-temperature reactions, it is possible to maintain the life of the catalyst over a long period of time and achieve industrial performance. The biggest challenge is to find a way to enable continuous operation on a large scale. Currently, the aforementioned low-temperature denitrification catalysts that have already been developed include alumina, silica alumina, silica,
1 selected from copper, titanium, vanadium, chromium, manganese, iron, cobalt, molybdenum, tungsten and nickel on a carrier consisting of one or a mixture of two or more selected from diatomaceous earth and titania.
Examples include solid catalysts supporting an active component consisting of an oxide, sulfate, metal oxyacid and/or heteropolyacid of one or more of the first transition metals. However, it is extremely difficult to use these catalysts continuously over a long period of time without any intermediate treatment as industrial process catalysts for the ammonia catalytic reduction method in the low temperature range mentioned above. It is said to be difficult. This is because when trying to carry out the above reaction at a relatively low temperature of 150 to 350 degrees Celsius, the sulfur oxides contained in the exhaust gas and moisture, as well as the ammonia introduced as a reducing agent, react. As a result, solid substances consisting of various ammonium sulfates, ammonium sulfites, etc. are generated, which deposit on the catalyst and impede the physical properties of the catalyst, such as pore structure, and eventually deteriorate the activity of the catalyst. This is because it brings about In this case, substances deposited on the catalyst include impurities such as ammonium sulfate, ammonium sulfite, and soot, which are composed of ammonium sulfate, acidic ammonium sulfate, triammonium hydrogen disulfite, ammonium sulfite, acidic ammonium sulfite, etc. Furthermore, it is known that ammonium sulfate and acidic ammonium sulfate, which are the main components of the deposited substance, are produced according to the following reaction formula. NH ₃ (gas) + H ₂ O (gas) + SO ₃ (gas) → NH ₄ HSO ₄ (solid) 2NH ₃ (gas) + H ₂ O (gas) + SO ₃ (gas) → (NH ₄ ) ₂ SO ₄ (solid) ) Therefore, in the past, the above solidified substances deposited on the catalyst were heated at 350 to 800°C, more specifically at 400 to 600°C.
A method has been proposed in which the activity of the catalyst is activated and regenerated by decomposing it under temperature conditions of °C. This utilizes the property that ammonium sulfate and the like described above decompose into gas components under temperature conditions of 350°C or higher; known documents disclosing this technology include JP-A-51-3366; JP-A-52-26394 and JP-A-52-30285 can be recognized. However, if the techniques related to the catalyst regeneration operation disclosed in these known documents are attempted to be carried out while the above-mentioned denitrification reaction is being carried out, the pressure loss inside the reactor will rapidly increase as the temperature is increased. Therefore, it will have an adverse effect on exhaust gas generation sources such as coke ovens and other incidental equipment. On the other hand, in the past, the special public
As described in publicly known documents such as Japanese Patent Publication No. 49-13463 and Japanese Patent Publication No. 51-57034, a bypass line that connects directly from the exhaust gas generation source to the exhaust gas release system in a short circuit is installed to prevent sudden occurrences. A method has been proposed for absorbing pressure fluctuations that occur during catalyst regeneration as described above. However, in such a method, during normal operation of the denitrification process, untreated exhaust gas from the exhaust gas generation source flows directly into the exhaust gas discharge system via a bypass line, whereas treated exhaust gas from the exhaust gas discharge system flows directly into the exhaust gas discharge system. This method not only necessitates a powerful absorption fan with a large processing capacity, but also causes untreated exhaust gas to flow directly into the exhaust gas during pressure fluctuations such as those mentioned above. Since it flows into the direction of the exhaust gas discharge system, it has the disadvantage that the apparent denitrification rate measured at the outlet of the exhaust gas discharge system temporarily decreases. It should be noted that a method has also been proposed in which the catalyst regeneration operation described above is carried out by switching the introduction of exhaust gas into the reaction device loaded with the catalyst to be activated and regenerated to another pre-installed reaction device. According to the above, it is not only necessary to prepare at least two reactors large enough to treat the entire amount of exhaust gas generated from the exhaust gas generation source, but also complicated operations associated with switching the exhaust gas introduction mentioned above are required. Become. Therefore, the inventors of the present invention conducted intensive research to solve the problems of the conventional method as described above, and as a result, the above-mentioned exhaust gas was divided and introduced into a catalytic reaction device consisting of a plurality of units arranged in parallel. If the denitrification reaction described above is carried out in the reactors, and the regeneration operation of the denitrification catalyst loaded in each reactor is performed alternately while the above reaction is carried out, the amount of exhaust gas processed inside each reactor can be reduced at all. It was discovered that the pressure fluctuations accompanying the temperature raising operation described above can be easily absorbed without artificial adjustment, and the present invention was accomplished. That is, the present invention is a method for removing nitrogen oxides contained in exhaust gas containing sulfur oxides and moisture by reacting it with ammonia introduced as a reducing agent into the exhaust gas stream at a temperature of 150 to 350°C. In the denitrification process, the mixed gas in which ammonia is introduced into the exhaust gas stream is dividedly introduced into a contact reaction device consisting of a plurality of units arranged in parallel to carry out the denitrification reaction, and each reaction In the equipment, the catalyst bed of the denitrification catalyst loaded in each reactor is alternately exposed to an atmosphere at a temperature of 330 to 650°C higher than the reaction temperature and heated without interrupting the movement of the process in any way. The present invention provides a method for removing nitrogen oxides contained in exhaust gas, which comprises successively activating and regenerating the catalyst whose catalytic activity has deteriorated over time to a level below a desired value due to treatment. The denitrification catalyst that can be used in the method of the present invention is 150
Although it has catalytic activity at temperatures of ~350°C, its catalytic activity tends to decrease due to the deposition of solid substances such as ammonium sulfate, and furthermore, at temperatures above 350°C, It is a low-temperature denitrification catalyst that has the property of not undergoing a change in shape even when subjected to heat treatment under certain conditions, and examples of such a catalyst include the conventionally known low-temperature denitrification catalysts as described above. Incidentally, these catalysts are generally used by being loaded into various reaction apparatuses such as packed bed reactors, parallel passage reactors, and honeycomb reactors, and the method of the present invention can be applied to these reactor structures, as well as It can be applied regardless of the catalyst bed structure, etc. In the method of the present invention, a plurality of these reaction apparatuses are necessarily arranged in parallel and used, but in normal cases, two such reaction apparatuses may be installed in parallel. This is because the larger the number of reactors, the easier it is to absorb the above-mentioned pressure fluctuations, but on the other hand, equipment costs tend to increase. In the regeneration operation of the method of the present invention, the heating treatment of the catalyst bed can be performed by indirectly exchanging heat with a high-temperature fluid as a heat source using an external heating type reactor. It is desirable that this be carried out by direct continuous contact with an exhaust gas stream that has been preheated to a temperature of 330° C. or higher, which is higher than the exhaust gas temperature, using a heater installed upstream of the exhaust gas, such as an in-line heater. In this way, said catalyst bed is heated to 330-650°C, more specifically
Although the catalyst bed is subjected to heat treatment by being exposed to an atmosphere at a temperature of 350 to 450°C, it is desirable that the heating operation of the catalyst bed be carried out at a temperature increase rate of 50°C or less per hour, more specifically, at a temperature increase rate of 20 to 40°C per hour. This is because, as stated in Japanese Patent Application No. 52-95381 filed by the present applicant, if the temperature increase rate is set to 50°C or more per hour, the amount of ammonia generated increases, and the amount of ammonia discharged from the system increases accordingly. This is because the amount of pollution may become large and cause secondary pollution. In addition, the heat treatment temperature of the catalyst bed is 330 to 650.
The reason for adopting the temperature range of ℃ is that the catalyst cannot be completely regenerated below 330℃, and
This is because at temperatures above 650°C, many catalysts undergo morphological changes during the regeneration operation, and furthermore, there is no point in employing temperatures higher than that. Hereinafter, the method of the present invention will be explained in more detail with reference to the drawings. FIG. 1 shows a typical embodiment of the denitrification process according to the method of the present invention, where A is an exhaust gas generation source;
B is the ammonia supply source, C is the suction exhaust fan, D is the exhaust gas release system, H-1 and H-2 are the in-line
A heater such as a heater, and R-1 and R-2 each indicate a reaction device loaded with a denitrification catalyst. Here, the exhaust gas emitted from the exhaust gas generation source A is
The exhaust gas containing nitrogen oxides in a temperature atmosphere of 150 to 350°C is pressurized by suction exhaust fan C, and then
Ammonia as a reducing agent introduced from ammonia supply source B is mixed at a ratio such that NH ₃ /NO (molar ratio) is 2.0 or less, more specifically 0.6 to 1.2. Next, the mixed gas is divided into two at branch point E, and then heated to heaters H-1 and H-2.
(However, it is stopped during normal operation because there is no need to heat the exhaust gas.) The gas superficial velocity (GHSV) in each reactor is 2000~
20000 hr ^-1 into reactors R-1 and R-2. Nitrogen oxides in the exhaust gas introduced into the reactors R-1 and R-2 in this way are decomposed by catalytically reacting with ammonia at a temperature of 150 to 350°C, and then the exhaust gas is released. It is discharged out of the system via system D, but if such operation is continued for a long time, solid substances such as ammonium sulfate as described above will deposit on the catalyst and the catalyst activity will be reduced. It decreases over time, resulting in a decrease in the denitrification rate. If the denitrification rate decreases and you can no longer obtain the denitrification rate below the desired value, use heater H-1,
Alternatively, H-2 is automatically ignited to heat the exhaust gas passing through the heater according to instructions from the program setting device, and the exhaust gas stream is used to heat the catalyst in reactor R-1 or R-2. Heat the floor. The heating temperature and temperature increase rate at this time are as described above, but the heat treatment time is preferably selected from the range of 0.5 to 10 hours. The regeneration operation of the catalysts loaded in the reactors R-1 and R-2 in this way is repeated alternately, and the regeneration operation is performed for each reactor at intervals of 4 to 40 days. It is economical. In this case, the catalyst beds in the reactors R-1 and R-2 must not be heated to the regeneration temperature at the same time for any reason. This is because the pressure loss inside reactors R-1 and R-2 increases rapidly due to such temperature raising operation, which has a negative effect on exhaust gas generation source A, etc., and the original purpose is not achieved. This is because you will become unable to do so. On the other hand, in the method of the present invention, as mentioned above, the catalyst beds in these reactors are alternately heated and raised in temperature, so not only is the pressure loss during the temperature rise not so large; Since the pressure fluctuations occurring inside one reactor, that is, the regeneration reactor, can be easily absorbed inside the other reactor, the above-mentioned problem does not occur. Note that the absorption of pressure loss referred to here means that a portion of the exhaust gas flowing into one reactor, where the pressure loss has increased, is automatically passed into the other reactor without any manual operation. This is done by balancing the pressure inside these reactors. Note that the dotted line from the exhaust gas generation source A to the exhaust gas release system D shown in FIG. 1 indicates a conventionally known bypass line, and as long as the method of the present invention is adopted, such a bypass line is usually There is no need to provide However, if the exhaust gas generation source A is such as a coke oven or the like, where there is a possibility that a small explosion may occur and a large amount of exhaust gas is temporarily released, a bypass line that is only effective in such a sudden event is installed to ensure safety. Of course, it may also be provided for this purpose. Next, examples will be shown. Example 1 Commercially available titanium-vanadium-based low-temperature denitrification catalysts were installed in two rectangular parallel parallel passage reactors R-1 and 2, which were arranged as shown in FIG. 1 (however, no bypass line was provided). R-2
(One side of the inner wall; 19.5 cm) was loaded with 41 kg each, and NOx; 300 ppm, SOx; 30 ppm and
H ₂ O; 12%, O ₂ ; 4.57%, CO ₂ ; 17%, CO; 0.1
% and the balance is nitrogen 400Nm ³ /hr
was introduced in portions, and ammonia as a reducing agent was supplied at a ratio such that NH ₃ /NO (molar ratio) was 1.1 to carry out a low-temperature denitrification reaction at a temperature of about 200°C. In this case, the gas superficial velocity (GHSV) inside the reactor is 3960 hr ^-1 ,
In addition, the initial denitrification rate was 94.0%. However, after conducting this reaction continuously for 100 hours, it was found that the denitrification rate had decreased to 90.0%, so we used the heater shown in Figure 1 (an in-line heater with a built-in burner). H-1
is ignited to raise the temperature of the exhaust gas flow passing through the heater to 380℃, and at the same time
After a period of time, the catalyst loaded in the reactor R-1 was regenerated. Then, the amount of exhaust gas flowing into each reactor (Nm ³ /hr) and the pressure loss inside each reactor (during normal operation) and immediately after (during regeneration operation) the regeneration operation is started mmH ₂ O), and the denitrification rate (%) at the outlet of each reactor and the outlet of the process were determined, and the results were as shown in Table 1 below.

[Table] For comparison, the above-mentioned low-temperature denitrification catalyst
Using one rectangular parallel passage type reactor (one side of inner wall: 27.6 cm) loaded with 82 kg, the above-mentioned exhaust gas of 400 Nm ³ /hr was introduced into it, and ammonia as a reducing agent was added at the above-mentioned rate. The denitration reaction is carried out at a low temperature of approximately 200°C, and when the denitrification rate reaches 90%, the exhaust gas stream is heated to a temperature of 380°C and A period of time was maintained to regenerate the catalyst loaded in the reactor. Similarly to the above case, the pressure loss (mmH ₂ O) inside the reactor and the denitrification rate ( %), the results are shown in Table 2 below. In addition, in this comparative experiment, only one reactor was used, and the above-mentioned bypass line was not installed, so the denitrification rate mentioned here can be considered as the denitrification rate at the outlet of the process. I can do it.

[Table] As is clear from the results in Tables 1 and 2 above, the method of the present invention does not significantly increase the pressure loss inside the reactor (at a pressure that is permissible at the exhaust gas generation source). (within a range of variation), it was found that the catalyst can be easily regenerated while the denitrification reaction is being carried out. Example 2 A commercially available titanium-vanadium-based low-temperature denitrification catalyst was used for two parallel passage type reactions arranged as shown in Figure 1 (however, unlike in Example 1, a bypass line was provided). Vessel R′−
17900 each for 1 and R'-2 (inner wall diameter; 550cm)
Load each kg of NOx; 300ppm, SOx;
50ppm, CO; 0.1%, _CO2 ; 20%, _H2O ; 10%,
Exhaust gas consisting of 2% O ₂ and the balance N ₂ 180000Nm ³ /h
A low-temperature denitrification reaction was carried out at a temperature of approximately 200°C by introducing r in portions and supplying ammonia as a reducing agent at a ratio such that NH ₃ /NO (molar ratio) was 1.1 to 1.2. . In this case, the gas superficial velocity (GHSV) inside the reactor was 4140 hr ^-1 and the initial denitrification rate was 94.0%. However, after carrying out this reaction continuously for 109 hours, it was observed that the denitrification rate had decreased to 91.0%. ) H′-1 is automatically ignited to raise the temperature of the exhaust gas stream passing through the heater to 350°C and maintain the temperature condition for 2 hours in the reactor.
The catalyst loaded in R'-1 was regenerated.
Then, the amount of exhaust gas flowing into each reactor (m ³ /hr) and the pressure loss inside each reactor (during normal operation) and immediately after (during regeneration operation) the regeneration operation is started. mmH ₂ O) and bypass
The leakage rate (%) of untreated exhaust gas that is short-circuited to the exhaust gas release system via the line, and the denitrification rate (%) at each reactor outlet and the relevant process outlet were determined, and the results are shown in Table 3 below. The results were as shown.

【table】

[Table] On the other hand, for comparison, in the above process, when the denitrification rate decreased to 91%,
Both heaters H'-1 and H'-2 shown in Figure 1 are ignited to reduce the exhaust gas flow passing through the heaters.
The temperature was raised to 350°C, and the temperature was maintained for 2 hours to simultaneously regenerate the catalysts loaded in reactors R'-1 and R'-2. Then, in the same way as in the previous case, immediately after performing the regeneration operation (during regeneration operation), the amount of exhaust gas flowing into each reactor (Nm ³ /hr), the pressure loss inside each reactor ( mmH ₂ O) and the leakage rate (%) of untreated exhaust gas short-circuited to the exhaust gas release system via the bypass line, as well as the denitrification rate (%) at each reactor outlet and the process outlet, respectively. The results were as shown in Table 4 below. The results of these measurements immediately before the regeneration operation, ie, during normal operation, were as shown in Table 3 above.

【table】

[Table] As is clear from the results in Tables 3 and 4 above, according to the method of the present invention, even if the bypass line is provided in consideration of safety in the event of an emergency, the pressure loss described above will not occur. It was confirmed that the leakage rate of untreated exhaust gas short-circuited to the exhaust gas release system via the bypass line was extremely low because the leakage rate did not become very large.

[Brief explanation of the drawing]

FIG. 1 shows a typical embodiment of the denitrification process according to the method of the present invention, where A is an exhaust gas generation source;
B is the ammonia supply source, C is the suction exhaust fan, D is the exhaust gas release system, H-1 and H-2 are the in-line
A heater such as a heater, and R-1 and R-2 each indicate a reaction device loaded with a denitrification catalyst.

Claims (1)

[Claims] 1. Nitrogen oxides contained in exhaust gas containing sulfur oxides and moisture are catalytically reacted with ammonia introduced as a reducing agent into the exhaust gas stream at a temperature of 150 to 350°C. In the denitrification process for denitrification and removal, the mixed gas containing ammonia introduced into the exhaust gas stream is dividedly introduced into a contact reaction device consisting of a plurality of units arranged in parallel to carry out the denitrification reaction. At the same time, the catalyst bed of the denitrification catalyst loaded in each reactor was heated in an atmosphere at a temperature of 330 to 650°C higher than the above reaction temperature without interrupting the operation of the process in any way. A method for removing nitrogen oxides contained in exhaust gas, which comprises sequentially reactivating and regenerating the catalyst whose catalytic activity has deteriorated over time to a desired value or less by being alternately exposed to heat treatment and subjected to heat treatment. 2. Claim 1, characterized in that the exhaust gas is exhaust gas discharged from fixed sources such as boilers, heating furnaces, coke ovens, and sintering furnaces, and contains 50 to 1000 ppm of nitrogen oxides. The method described in section. 3 The above exhaust gas has a gas superficial velocity (GHSV) of 2000~2000 in the reactor loaded with the denitrification catalyst.
A method according to claim 1, characterized in that the process system is introduced at a rate of 20000 hr ^-1 . 4 The denitrification catalyst is selected from alumina, silica alumina, silica, diatomaceous earth, and titania.
oxides of one or more first transition metals selected from copper, titanium, vanadium, chromium, manganese, iron, cobalt, molybdenum, tungsten and nickel on a support consisting of a species or a mixture of two or more; 2. The method according to claim 1, characterized in that the catalyst is a solid catalyst supporting an active component consisting of a sulfate, a metal oxyacid and/or a heteropolyacid. 5. Claim 1, characterized in that each line located between the reaction device and the branch point of the mixed gas is provided with a heater for heating the mixed gas. the method of. 6. The method of claim 5, wherein said heater is an in-line heater. 7. The method according to any one of claims 1 and 5, characterized in that the number of reactors is two. 8. The method according to claim 5, characterized in that the ammonia is introduced into the exhaust gas before the heater. 9 The above regeneration operation involves heating the exhaust gas passing through the heater to a temperature of 330 to 650°C, which is higher than the temperature of the exhaust gas, using a heater installed upstream of the reaction device loaded with the catalyst to be activated and regenerated. The method according to claim 5, characterized in that the method is carried out as follows. 10. The method according to claim 9, wherein the heating temperature is in the range of 350 to 450°C. 11 The above regeneration operation is carried out for each reactor,
11. A method according to any one of claims 1, 9 and 10, characterized in that it is carried out for 0.5 to 10 hours. 12 The above regeneration operation is carried out for each reactor,
12. A method according to any one of claims 1 and 9 to 11, characterized in that it is carried out at intervals of ~40 days. 13 The catalyst to be activated and regenerated has ammonium sulfate, ammonium sulfite, etc. produced by the reaction of sulfur oxides contained in the exhaust gas and moisture, as well as ammonia introduced as a reducing agent, which are deposited on the catalyst surface. 2. The method according to claim 1, wherein the catalyst has a catalytic activity that deteriorates over time to a level below a desired value.