Process for Reducing an Eliminating Nitrogen Oxides in an Exhaust Gas

This invention relates to a process for reducing and eliminating nitrogen oxides in a waste gas which contains the nitrogen oxides and which is emitted from various industrial plants and processes such as a combustion furnace, a nitric acid-producing plant, a fertilizer manufacturing plant and a metal pickling process, thereby to render the waste gas innocuous.

An exhaust gas generated by the combustion of various fossil fuels contains very small quantities of nitrogen oxides NO.sub.x, for example, NO (nitrogen monoxide), NO.sub.2 (nitrogen dioxide), N.sub.2 O.sub.4, N.sub.2 O.sub.5 and N.sub.2 O, which provide for air pollution. Most of nitrogen oxides in the exhaust gases of petroleum fractions (such as naphtha, kerosene, light oil and heavy oil), coal etc. are nitrogen monoxide NO, and a variety of denitrating methods are being developed at present. As one of the methods of eliminating NO, there is a process in which NO is decomposed into nitrogen and water in the presence of a catalyst by employing ammonia NH.sub.3 as a reductant.

The reductive reaction of NO with NH.sub.3 has been considered to proceed according to the following reaction formula:

It has recently been verified by the inventors, however, that in the presence of oxygen the reduction proceeds according to the reaction of the following formula:

(Refer to Official Gazette of Japanese Unexamined Published Patent Application No. 51-56284). As catalysts for promoting the reaction (2), many have been proposed. There can be mentioned, for example, catalysts of mixed oxides containing vanadium (Japanese Unexamined Published Patent Application No. 49-122473); catalysts with molybdenum, tungsten and vanadium added to tin oxides (Japanese Unexamined Published Patent Application Nos. 50-71564 and 51-45689); and catalysts, some of the inventors having participated in the inventions thereof, which contain titanium oxides as their principal constituents and in which are added oxides of iron (Japanese Examined Published Patent Application No. 52-6954), copper, nickel and cobalt (Japanese Examined Published Patent Application No. 52-6953), molybdenum (Japanese Unexamined Published Patent Application No. 50-89291), tungsten (Japanese Unexamined Published Patent Application No. 50-89264), vanadium (Japanese Unexamined Published Patent Application No. 50-128680), uranium (Japanese Unexamined Published Patent Application No. 51-21568), chromium (Japanese Unexamined Published Patent Application No. 51-68473), cerium (Japanese Unpublished Patent Application No. 50-117753), etc. Excepting the catalysts which contain titanium oxides as their principal constituents, alumina or silica alumina is generally used as a catalyst support. In case of the treatment of an exhaust gas containing sulfur oxides, the catalysts employing the alumina type support have the catalytic activity quickly degraded because the alumina turns into aluminum sulfate. The catalysts containing titanium oxides as their principal constituents to which the oxides of Fe, Cu, Ni, Co, Mo, W, V, U, Cr and Ce are added in a range of, for example, at most 50 weight % are not vitiated by the sulfur oxides. Besides, their activities for the reaction of NO and NH.sub.3 are very excellent.

With the catalysts as stated above and under ideal conditions, approximately 90% or more of NO.sub.x can be eliminated from within the exhaust gas. However, when the temperature of the exhaust gas is 250.degree. C. or below, or when the space velocity is increased, the rate of eliminating NO.sub.x lowers, and problems are involved in the application to a large-scale actual plant. If the range of effective reaction temperature can be expanded to a lower temperature region, it will become unnecessary to preheat the exhaust gas to the optimum temperature, which is economically advantageous. If the reaction rate can be raised, the reaction will proceed with a smaller quantity of catalyst, and the required power for transferring the exhaust gas will decrease, which also enhances economy.

An object of this invention is to provide a process for eliminating nitrogen oxides in an exhaust gas efficiently at a high denitrating rate.

Another object is to provide an economical treatment process capable of eliminating nitrogen oxides at temperatures lower than used in the prior art processes.

The process for eliminating nitrogen oxides in an exhaust gas according to this invention is effected so that nitrogen monoxide and nitrogen dioxide in the exhaust gas are put into substantially equal mol concentrations, and the resulting gas is held in contact with a metallic oxide catalyst together with a reducing agent.

More concretely, the mol ratio of NO/(NO+NO.sub.2) in the nitrogen oxides contained in the exhaust gas is adjusted so as to become approximately 0.4-0.6 by changing part of the nitrogen oxides in the exhaust gas or by externally adding a necessary type of nitrogen oxide, whereupon the exhaust gas is held in contact with the metallic oxide catalyst, especially a titanium oxide type catalyst, together with the reducing agent of ammonia at a temperature in the range of 100.degree. to 550.degree. C. preferably 150.degree. C. to 350.degree. C.

Besides the reaction between NO and NH.sub.3 in the heretofore described case of using the various metallic oxide catalysts, the inventors studied in detail the reaction between NO.sub.2 and NH.sub.3 and the reaction between (NO+NO.sub.2) and NH.sub.3. The reaction between NO and NH.sub.3 proceeds according to the reaction formula (2). The reaction between NO.sub.2 and NH.sub.3 proceeds under coexistence or non-coexistence with oxygen according to the following reaction formula as has hitherto been proposed:

Further, the reaction between NH.sub.3 and a gas in which NO and NO.sub.2 and contained at equal molar proportions proceeds according to the following formula:

Although the reaction rates of the reactions (2), (3) and (4) greatly vary depending on the reaction temperature and the sort of the catalyst used, that of the reaction (4) is, in general, 2 to 10 times as high as those of the reactions (2) and (3). Especially when the titanium oxide type catalyst is used, the reaction (4) proceeds 4 to 10 times faster than the reactions (2) and (3).

As regards the reaction temperature, it has been experimentally verified that the reaction (4) exhibits the small denitrating rate at temperatures which are 100.degree. to 150.degree. C. lower than in the reactions (2) and (3).

According to this invention, the mol concentrations of nitrogen monoxide and nitrogen dioxide in an exhaust gas to be treated are adjusted so as to become substantially equal, whereupon the nitrogen oxides are reacted by the use of a reductant of ammonia and a catalyst into nitrogen gas and water. Usually, the relative percentage of NO and NO.sub.2 in the nitrogen oxides being treated is more than 90%.

In the ordinary combustion exhaust gases, most of nitrogen oxides are in the form of nitrogen monoxide, and the mole ratio of NO.sub.2 /NO is 0.1 or below. In treating such an exhaust gas, part of NO is oxidized into NO.sub.2 so that the concentrations of NO and NO.sub.2 in the exhaust gas after the oxidation may usually be made NO/(NO+NO.sub.2)=0.4 to 0.6 (mole ratio) though this value varies depending on the required percentage of eliminating nitrogen oxides (percentage of denitration). By adjusting the rates of NO and NO.sub.2 in this manner, the denitrating percentage by NH.sub.3 is easily made 80% or higher.

As a method of oxidizing NO into NO.sub.2, there is a process wherein the oxidation is carried out with oxygen in the presence of a catalyst. Usable as the catalyst is any of the manganese oxides (refer to the Official Gazette of Japanese Unexamined Published Patent Application No. 50-62859) which have already been known or TiO.sub.2 --MnO.sub.2, TiO.sub.2 --CoO, etc. which the inventors have found out. The temperature of the oxidizing reaction may be to the same extent as a temperature at which the nitrogen oxides are reduced with ammonia, that is, 100.degree. to 350.degree. C.

Another method of oxidizing NO into NO.sub.2 is a process wherein ozone (O.sub.3) is employed. The reaction of the oxidation with ozone is represented by the following formula, and the reaction rate thereof is very high:

At temperatures at which the self-decomposition of ozone does not take place, the reaction (5) proceeds very selectively. Regarding the quantity of addition of ozone, therefore, 0.1 to 1, preferably 0.4 to 0.6 mol, is suitable per mol of NO in the exhaust gas. The place in which the ozone is added may be the side of a reaction tower in which the nitrogen oxides are reduced by the use of NH.sub.3. It is not always necessary that NO and O.sub.3 have perfectly reacted before entering the reaction tower. That is, the reaction between NO and O.sub.3 is considered to proceed also within the denitrating reaction tower. In this case, the two stages of reactions in which NO turns into NO.sub.2 and further the three of NO, NO.sub.2 and NH.sub.3 reaction will take place substantially successively, and the reaction formula will be represented as follows:

Although this reaction formula resembles the reaction formula (2), the reaction rate of the reaction (6) is much higher than that of the reaction (2).

Another expedient for adjusting the concentrations of NO and NO.sub.2 in the exhaust gas to be treated consists in adding nitric acid to the gas and converting nitrogen monoxide into nitrogen dioxide according to the following reaction:

In accordance with this reaction, 3 mols of NO.sub.2 are produced from 2 mols of nitric acid, and besides, 1 mol of NO is decreased. Accordingly, when the nitric acid added amounts to a half of the concentration of NO in the gas to be treated, a gas in which NO and NO.sub.2 are in equal molar amounts is formed. The theoretically required quantity of ammonia becomes 1.5 times as large as the quantity of original NO in terms of molar proportions. The concentration of the nitric acid to be used is not especially restricted, and either dilute nitric acid or undiluted nitric acid may be used. The nitric acid is added to the exhaust gas in such a way that 1 normal aqueous solution of HNO.sub.3 is prepared and then injected into a reactor. Alternatively, the undiluted nitric acid is added into a reactor by spraying it with air. It is suitable to make the quantity of the additive nitric acid 0.2 to 1 times (mol ratio), preferably 0.4 to 0.6 times, as large as that of NO in the exhaust gas.

As a further method of adjusting the concentrations of NO and NO.sub.2, there is considered a process in which a predetermined amount of NO or NO.sub.2 is added to the gas to be treated. This process is easy, but when compared with the adjusting methods thus far described, it has the disadvantage that the amount of NO.sub.x to be reduced by NH.sub.3 increases.

Since the reaction between NH.sub.3 and the exhaust gas containing NO and NO.sub.2 at substantially equal molar concentrations proceeds as indicated by the reaction formula (4), the proper quantity of the additive NH.sub.3 is 0.8 to 2.0 mol times, preferably 0.8 to 1.2 mol times, as large as the total quantity of NO and NO.sub.2. By making the quantity of the additive NH.sub.3 as small as 1.+-.0.2 mol times with respect to the quantity of NO.sub.x (=NO+NO.sub.2), unreacted NH.sub.3 contained in the processed exhaust gas can be suppressed to a small quantity.

The exhaust gas in which the concentrations of NO and NO.sub.2 have become substantially equal and to which NH.sub.3 has been added is held in contact with a catalyst. Then, the nitrogen oxides in the exhaust gas are decomposed into nitrogen and water in accordance with the reaction (4).

As the catalysts which are employed for reducing NO.sub.x in the presence of ammonia, there have been known oxides of platinum group metals, vanadium, iron, copper, molybdenum, tungsten, chromium etc. or mixed oxides thereof. These metallic oxide catalysts are usually used in the form in which they are carried by a support such as alumina and silica alumina. As the reducing catalysts for use in this invention, however, ones are the most appropriate which contain titanium oxides as their principal constituents and in which is added an oxide of at least one metal selected from the group consisting of iron, copper, nickel, cobalt, molybdenum, tungsten, vanadium, chromium and cerium. The reason therefor is that these catalysts, the principal constituents of which are titanium oxides, are especially suitable in case where the exhaust gas to be treated contains sulfur oxides (SO.sub.2, SO.sub.3, etc.). The catalysts whose principal constituents are titanium oxides are not poisoned or vitiated by the sulfur oxides, and their activities for the reaction (4) are ordinarily higher than the alumina support catalysts. In case where the exhaust gas contains quite no, or substantially no sulfur oxide, it is also possible to use catalysts of iron oxide--tungsten oxide (refer to the Official Gazette of Japanese Unexamined Published Patent Application No. 50-57946), iron oxide-tin oxide (Japanese Unexamined Published Patent Application No. 50-89287), etc. These catalysts, however, are inferior in the denitrating percentage to the titanium type catalysts.

The reduction of the nitrogen oxides by NH.sub.3 is executed at temperatures of 100.degree. to 550.degree. C., preferably 150.degree. to 350.degree. C. Although the reaction temperature may be higher than in the preferable range, also the reaction (2) between NO and NH.sub.3 proceeds speedily at high temperatures above, e.g., 400.degree. C., which makes it meaningless to provide the additional step of oxidizing part of NO into NO.sub.2 and rendering their concentrations in the exhaust gas substantially equal. In case where the reaction temperature is lower than 100.degree. C., the reaction rate is slow. Therefore, the required amount of the catalyst becomes large uneconomically. Further, troubles such as precipitation of ammonium nitrate take place unfavorably.

The space velocity of the exhaust gas in the case of reducing the nitrogen oxides with ammonia needs to be made 2,000 to 100,000 h.sup.-1 (calculated for an empty tower, N.T.P.) and to be selected depending on the reaction temperature and the shape of the catalyst. As regards the configuration of the reaction tower, any of a stationary bed, a movable bed, a fluid bed, etc. can be used. As regards the shape of the catalyst, there can be used any of columnar, cylindrical and honeycomb ones or flat and meshy ones, etc.

Now, an example of manufacture of catalysts will be given as a reference example. (These catalysts were used in examples to be described later).

Slurry of metatitanic acid [TiO(OH).sub.2 ] was employed as the raw material of titanium oxide. Respective predetermined amounts of ammonium meta-vanadate, chromium nitrate, ammonium para-molybdate, ammonium para-tungstate, iron nitrate, copper nitrate and cerium nitrate were mixed into the slurry, and the mixtures were sufficiently kneaded. After drying and pulverizing the mixtures, they were molded into a diameter of 5 mm and a length of approximately 5 mm by a tablet compressing machine. The molded compacts were baked at 400.degree.-550.degree. C. to obtain the catalysts. The metallic atom compositions of the respective catalysts acquired were as follows:

Now, this invention will be described in connection with examples which are illustrative but not limitative of said invention.

The reaction rates of the reaction between NO and NH.sub.3, the reaction between (NO+NO.sub.2) and NH.sub.3 and the reaction between NO.sub.2 and NH.sub.3 were compared. Each of the catalysts A, C, E, F, G and H manufactured in the foregoing reference example and amounting to 40 ml was packed in the central part of a reaction tube having an inside diameter of 50 mm and a length of 50 cm, and three sorts of exhaust gases having compositions listed in Table 1 were caused to flow through the reaction tube at a space velocity of 12,000 h.sup.-1. The concentrations of NO and NO.sub.2 at the inlet and outlet of the reaction tube were analyzed by the chemiluminescence process, to evaluate the rate of eliminating nitrogen oxides. The results are indicated in Table 2.

As apparent from Table 2, the exhaust gas B (a gas containing NO and NO.sub.2 at a mol ratio of 1:1) exhibits a particular fast reaction with NH.sub.3 when compared with the exhaust gas A (a gas containing only NO) and the exhaust gas C (a gas containing only NO.sub.2).

O.sub.3 generated by an ozonator was added into an exhaust gas at the inlet of a reaction tube, whereupon the reaction was caused to proceed. The exhaust gas had a composition indicated in Table 3. The catalysts A, B, D, E and I produced in the reference example were used, and the reaction temperatures were 150.degree. C., 200.degree. C., and 300.degree. C. In the same way as in Example 1, the rate of eliminating the nitrogen oxides was assessed. The results are given in Table 4.

It is understood from Table 4 that, in case where the oxidation of NO by O.sub.3 and the reduction of NO.sub.x by NH.sub.3 are carried out in the identical reaction tube, an excellent effect of denitration is achieved.

Both the reactions of the oxidation of NO by oxygen and the reduction of (NO+NO.sub.2) by NH.sub.3 were executed in two stages, and the denitrating rate was measured by the same method as in Example 1. An exhaust gas used had a composition given in Table 5 below.

For the oxidation of NO, the following 4 sorts of catalysts were employed:

The exhaust gas E was passed through an NO-oxidizing catalyst layer at a desired space-velocity value of 2,000-5,000 h.sup.-1, and after adding a predetermined amount of NH.sub.3, it was caused to flow through a NO.sub.x -reducing catalyst layer at a space velocity of 20,000 h.sup.-1. The reaction temperature was made 300.degree. C. for the NO-oxidizing step, and 200.degree. C. for the NO.sub.x -reducing step. By controlling the passing velocity through the oxidizing catalyst layer, NO/(NO+NO.sub.2) was adjusted to 0.4-0.6.

The results obtained are indicated in Table 6.

As apparent from Table 6, even when the oxidizing reaction and the reducing reaction are executed in the two stages, the nitrogen oxides can be eliminated at a denitrating rate higher than 70%.

A stream of the exhaust gas containing 300 ppm of NO and 10% of O.sub.2 as indicated in Table 5 was divided into two streams. One stream was passed through a catalyst layer packed with the oxidizing catalyst b composed of MnO.sub.2 --Fe.sub.2 O.sub.3 (atomic ratio, Mn:Fe=95:5), under conditions of a space velocity of 3,000 h.sup.-1 and a reaction temperature of 250.degree. C. The other stream was caused to bypass the oxidizing catalyst layer, and downstream of the oxidizing catalyst layer, it was caused to join the exhaust gas having passed through the oxidizing catalyst layer. Then, 330 ppm of NH.sub.3 was added to the exhaust gas.

Subsequently, the exhaust gas with NH.sub.3 added thereto was passed through a layer of the nitrogen oxides-reducing catalyst composed of Ti.Mo (Catalyst C) under conditions of a space velocity of 15,000 h.sup.-1 and a reaction temperature of 250.degree. C. The rate of reducing the nitrogen oxides was higher than 98%.

A test of denitration was made by the use of an exhaust gas containing sulfur oxides as seen from its composition given in Table 7. The reaction was carried out continuously for 500 hours under conditions of a space velocity of 30,000 h.sup.-1 and a reaction temperature of 350.degree. C. The results are indicated in Table 8.

As apparent from Table 8, according to this example, even in case where the invention is applied to the exhaust gas containing sulfur oxides, good denitrating rates are attained without being vitiated by the sulfur oxides.

In this example, comparisons were made of the reaction rates of the reaction between nitrogen monoxide and ammonia and the reaction between a gas with nitric acid added to nitrogen monoxide and ammonia. Each of the catalysts A-J amounting to 40 ml was packed in the central part of a reaction tube made of quartz glass and having an inside diameter of 50 mm as well as a length of 50 cm, and each of exhaust gases G and H of compositions indicated in Table 9 was caused to flow through the reaction tube at a space velocity of 15,000 h.sup.-1. The NO.sub.x (NO+NO.sub.2) concentrations at the inlet and outlet of the reaction tube were analyzed by the chemiluminescence process, to evaluate the percentage of removal or elimination of the nitrogen oxides.

The addition of HNO.sub.3 in the exhaust gas H was done by injecting 1 normal aqueous solution of HNO.sub.3 into the reactor from a pipe having an inside diameter of 1 mm.

The results of the experiments employing the catalysts A-J are given in Table 10.

As indicated in Table 10, the gas with HNO.sub.3 added thereto is particularly fast in the reaction with NH.sub.3 when compared with the gas containing only NO.

In this example, an exhaust gas containing sulfur oxides was used, the exhaust gas having a composition as indicated in Table 11. A continuous test for 100 hours was carried out under conditions of a space velocity of 30,000 h.sup.-1 and a reaction temperature of 350.degree. C.

The catalysts A-J were used, and the same equipment as in Example 6 was employed. The results are given in Table 12.

As understood from the results of Table 12, even when the exhaust gas contains the sulfur oxides, good denitrating rates are achieved without the degradation of the catalytic activity.