Catalyst and process for removing nitrogen oxides

Abstract: Disclosed herein are an inexpensive catalyst for removing nitrogen oxides effectively from an exhaust gas by either catalytic reduction or catalytic decomposition, and a process for removing nitrogen oxides from an exhaust gas by the aid of said catalyst. The invention will eliminate the disadvantages -- high material cost and complex process for catalyst preparation -- involved in the conventional technology. The catalyst contains as the major constituent a multiple oxide of the CaO-Fe x O type containing 5-50 wt% CaO, said catalyst reducing or decomposing nitrogen oxides. The process is designed for denitration of nitrogen oxides by the aid of said catalyst.

Description: The present invention relates to a catalyst for removing nitrogen oxides from a combustion exhaust gas (such as from a sintering machine). More particularly, the present invention relates to a catalyst for removing nitrogen oxides from a combustion exhaust gas by reduction or decomposition. (The catalyst promotes reduction by a reducing agent which is originally present in an exhaust gas or is added to an exhaust gas afterward. The catalyst also performs catalytic decomposition by itself.) The present invention also relates to a process for removing nitrogen oxides from an exhaust gas by the aid of said catalyst. One of the most important problems associated with air pollution is the removal of nitrogen oxides. There are several means to remove nitrogen oxides from exhaust gases. They are broadly classified as (1) denitration by catalytic reduction, (2). denitration by catalytic decomposition, (3) denitration without catalysts, and (4) denitration by electron beam irradiation. The first two are generally preferred because of low operation cost and high performance. Exhaust gas from a sintering machine needs the reduction of nitrogen oxides. This object is usually achieved by denitration with a special equipment attached to the sintering machine. Other means to this end include "exhaust gas recycling" to control fuel combustion (as disclosed in Japanese Patent Laid-open No. 70008/1978) and "multi-stage charging" (as disclosed in Japanese Patent Laid-open No. 80202/1979). These technologies have the disadvantage of requiring additional equipment for denitration, exhaust gas recycling, or multi-stage charging. On the other hand, an attempt has been made to carry out denitration by mixing an exhaust gas with a substance, such as an ammonium compound, as disclosed in Japanese Patent Laid-open No. 82304/1979. This attempt, however, is not successful because of the possibility of ammonia leaking from the system to cause another environmental pollution and the difficulties of process control. Now, the "Scale charging process" as disclosed in Japanese Patent Laid-open No. 53704/1977 is attracting attention from the standpoint of environmental safety and high denitrating performance. To improve this technology, the present inventors paid their attention to denitration with a catalyst. US 5 124 305 describes a ternary mixture of iron oxide, calcium oxide and vanadium oxide as a catalyst for destroying NO x . A minimum of 30% vanadium oxide is present and is said to be necessary for activity. The conventional catalyst for denitration are base mostly on noble metals,m and hence they are expensive per se and need to be supported on a carrier by a complex process. Therefore, they are not generally used in the field of air pollution control. In view of the foregoing, the present invention was completed to eliminate the disadvantages -- high material cost and complex process for catalyst preparation -- involved in the conventional technology. It is an object of the present invention to provide an inexpensive catalyst for removing nitrogen oxides effectively from an exhaust gas by either catalytic reduction or catalytic decomposition. It is another object of the present invention to provide a process for removing nitrogen oxides from an exhaust gas by the aid of said catalyst. Fig. 1 is a schematic representation of the experimental apparatus used in the present invention. Fig. 2 is a graph showing the relationship between the ratio of denitration and the concentration of CaO (in catalytic reduction). Fig. 3 is a graph showing the relationship between the reaction temperature and the ratio of denitration (in catalytic reduction). Fig. 4 is a graph showing the relationship between the ratio of denitration and the concentration or CaO (in catalytic decomposition). Fig. 5 is a graph showing the relationship between the reaction temperature and the ratio of denitration (in catalytic decomposition). Fig. 6 is a graph showing the relationship between the value of x in Fe x O and the ratio of denitration (in catalytic reduction). Fig. 7 is a graph showing the relationship between the value of x in Fe x O and the ratio of denitration (in catalytic decomposition). Fig. 8 is a graph showing the relationship between the reaction temperature and the ratio of denitration (in catalytic reduction). Fig. 9 is a graph showing the relationship between the reaction temperature and the ratio of denitration (in catalytic decomposition). Fig. 10 is a schematic representation of the experimental apparatus simulating a sintering machine. Fig. 11 is a graph showing the effect of denitration in the sintering machine model. According to the present invention there is provided a catalyst for removing nitrogen oxides which consists essentially of a multiple oxide of the Ca0-Fe x 0 type containing 5-50 wt% CaO, said catalyst promoting the reduction of nitrogen oxides. According to the present invention there is provided a process for removing nitrogen oxides which comprises treating a nitrogen oxides-containing gas with a reducing agent in the presence of the catalyst of the invention. According to the present invention there is provided a catalyst for removing nitrogen oxides which consists essentially of a multiple oxide of the CaO-Fe x 0 type containing 5-50 wt% CaO, said catalyst decomposing nitrogen oxides. According to the present invention there is provided a process for removing nitrogen oxides which comprises treating a nitrogen oxides-containing gas with the catalyst of the invention. The functions and affects of the present invention will be described with reference to the following experimental results. Fig. 1 is a schematic representation of the apparatus used for experiments. There is shown a nitrogen oxides-containing gas generator (1), in which nitrogen oxides, carrier gas, optical reducing gas, etc. are mixed in a prescribed ratio. The thus prepared nitrogen oxides-containing gas passes through a conduit tube (2) to enter a sealed tune (crucible) (3). The gas jets out from the nozzle (4) and comes into contact with a catalyst (6) heated to a prescribed temperature by a heater (5). The temperature of the catalyst is monitored by a thermocouple (7). Where it is necessary to test the catalyst for ability to promote reduction, the nitrogen oxides-containing gas should be incorporated with a reducing agent such as ammonia, carbon monoxide, and RX gas. After reduction or decomposition by the catalyst (6), the resulting gas is discharged through a conduit tube (9). It is partly caught by a gas sampling bag (8) and the remainder is tested for the concentration of nitrogen oxides by an analyzer (10). Tables 1 to 3 show the experimental conditions, the gas composition, and the catalyst composition, respectively. Crucible diameter 24 mm Gas flow rate 2 N L/min Nozzle diameter 4 mm Crucible material 99.9% Al 2 O 3 Calcium ferrite 10 g Type of reaction Gas composition Catalytic reduction  360 ppm·NO-2.5% CO-7.5% CO 2 -Ar ▴ 360 ppm·NO-5.0% CO-5.0% CO 2 -Ar ▪ 360 ppm·NO-7.5% CO-2.5% CO 2 -Ar Catalytic decomposition 400 ppm·NO-Ar Sample CaO content Iron oxide Value of x in Fe x O C-W 25% FeO 1.00 C-M 25% Fe 3 O 4 0.75 C-H 25% Fe 2 O 3 0.67 The catalyst used in the experiment was prepared by fusing CaO and an iron oxide specified in Table 3. It was used in the pore-free state. The experimental results are shown in Figs. 2 to 5. The ratio of denitration (EL%) was calculated as follows from the amount of NO (C NO in ) charged and the amount of NO (C NO out ) discharged. EL% = [(C NO in - C NO out ) ÷ C NO in ] × 100 Figs. 2 and 3 show the ratio of denitration by catalytic reduction with CaO-FeO as an example of the catalyst. It is noted from Fig. 2 that the high ratio of denitration is achieved when the CaO content is in the range of 5-50%, especially in the neighborhood of 25%. It is noted from Fig. 3 that the high ratio of denitration is achieved when the reaction temperature is higher than 800°C if the CO/CO 2 ratio is lower than 0.6. (A sufficiently high ratio of denitration is achieved even at 500°C if the CO/CO 2 ratio is higher than 0.6.) Incidentally, it is amazing that the catalyst remains effective even in its molten state at a high temperature. Figs. 4 and 5 show the ratio of denitration by catalytic decomposition with CaO-FeO as an example of the catalyst. It also varies depending on the CaO content and reaction temperature as mentioned above. In other words, it is noted from Fig. 4 that if the CaO content is 5-50 wt%, the ratio of denitration is high, but otherwise it is low. It is also noted from Fig. 5 that the high ratio of denitration is achieved when the reaction temperature is higher than 900°C. Experiments were carried out under the same conditions as shown in Fig. 1 and Tables 1 and 2 (at 1300°C) to see how the catalyst varies in denitrating capability depending on the degree of oxidation of the calcium ferrite. Figs. 6 and 7 show the results of catalytic reduction and catalytic decomposition, respectively. It is noted that the maximum ratio of denitration is achieved when x = 1 (FeO) in either cases. Although good results are obtained in catalytic reduction even though the value of x is low (Fe 2 O 3 ) so long as the CO/CO 2 ratio is high, it is desirable that the value of x be in the range of 0.67-1.00 for catalytic reduction and in the higher range of 0.9-1.0 for catalytic decomposition. Incidentally, the ratio of denitration shown in Fig. 7 will be greatly improved if the reaction area is increased. For this reason, it is desirable from the standpoint of industrial production that the catalyst be formed in lumps of proper size from fine powder. The catalyst becomes more effective as the reaction temperature increases, and it is effective even when it is in molten state at about 1200°C and above although it has a reduced specific surface area. Experiments on denitration were carried out using iron oxide pellets as the catalyst containing a multiple oxide of the CaO-Fe x O type They were prepared by mixing finely divided iron ore with a CaO-containing mineral, making the mixture into pellets, and heating the pellets at about 1300°C. The experimental conditions are shown in Tables 4 and 5. Chemical composition Fe 62 % CaO 4.4 % SiO 2 3.0 % Al 2 O 3 1.4 % Porosity 27.0 % Diameter 9.2-9.8 mm Gas composition For catalytic reduction 360 ppm·NO-5% CO-5% CO 2 -Ar For catalytic decomposition 400 ppm·NO-Ar Gas flow rate 2 N L/min Weight of catalyst 140 g Material of crucible 99.9% Al 2 O 3 The results of catalytic reduction are shown in Fig. 8. It is noted that the catalyst is effective when it is above 500°C. The results of catalytic decomposition are shown in Fig. 9. It is noted that the catalyst is effective when it is above 850°C. EXAMPLE Since CaO-Fe x O is one of the major constituents of sintered products, it is expected that if it is fed to the bed along with the raw material for sintering, denitration will take place as soon as NO x occurs, without any adverse effect on the sintered products. To simulate the combustion in a sintering machine, coke breeze (as the major fuel for a sintering machine) was burned together with CaO-Fe x O and the amount of NO x was measured, using an apparatus schematically shown in Fig. 10. The experimental conditions are shown in Tables 6 and 7. The experimental results are shown in Table 8 and Fig. 11. It is noted that CaO-Fe x O is twice as effective in denitration as scale. Items Description Amount of coke 0.5 g Diameter of coke 1-3 mm Diameter of catalyst 1-3 mm Diameter of reactor tube 50 mm Preheating temperature 800°C Gas flow rate 2 N L/min Diameter of crucible 12 mm Material of crucible Mullite Sample Composition CaO-Fe x O 25% CaO-75% FeO Scale By-product from steel making Sample charged Ratio of catalyst to coke (by weight) Amount of NO x discharged (ml) Undecomposed NO x in discharged gas (wt%) Coke (control) - 2.77 - CaO-Fe x O 10 2.44 11.9 20 2.25 18.8 Scale 10 2.60 6.1 20 2.43 12.3 The present invention provides a catalyst which, on account of its constitution mentioned above, is economical materialwise and processwise and is effective in the removal of nitrogen oxides by either reduction or decomposition.

Claims: A catalyst for removing nitrogen oxides by promoting reduction of nitrogen oxides or by decomposing nitrogen oxides which consists essentially of a multiple oxide of the CaO-Fe x O type containing 5-50 wt% CaO. A catalyst as claimed in claim 1 for removing nitrogen oxides by promoting reduction of nitrogen oxides wherein the multiple oxide of the CaO-Fe x O type has a value of x in the range of 0.67 to 1.00. A catalyst as claimed in claim 1 for removing nitrogen oxides by decomposition of nitrogen oxides wherein the multiple oxide of the CaO-Fe x O type has a value of x in the range of 0.9 to 1.0. A catalyst as claimed in any one of the preceding claims which is used in molten state or in lumps formed by sintering from fine powder. The use of a catalyst as claimed in any one of the preceding claims for removing nitrogen oxides by promoting reduction by nitrogen oxides or by decomposing nitrogen oxides. A process for removing nitrogen oxides by promoting reduction of nitrogen oxides which comprises treating a nitrogen oxides - containing gas with a reducing agent in the presence of the catalyst as claimed in claim 1, claim 2 or claim 4 when dependent on claim 1 or claim 2 or/a process for removing nitrogen oxides which comprises treating a nitrogen oxides-containing gas with the catalyst as claimed in claim 1, claim 3 or claim 4 when dependent on claim 1 or claim 3. A process for removing nitrogen oxides by promoting reduction as claimed in claim 6 in which the catalyst is used at a reaction temperature higher than 500°C if the CO/CO 2 ratio in the nitrogen oxides containing gas is greater than 0.6 or at a reaction temperature higher than 800°C if the CO/CO 2 ratio in the nitrogen oxides containing gas is smaller than 0.6. A process as claimed in claim 6 wherein the catalyst is used at a reaction temperature higher than 850°C.

Inventors: MORIOKA KOUICHI, SHIROUCHI SHOJI, SUGIYAMA TAKESHI, MORIOKA, KOUICHI, SHIROUCHI, SHOJI, SUGIYAMA, TAKESHI, Morioka, Kouichi, c/o Kakogawa Works, Shirouchi, Shoji, c/o Kakogawa Works, Sugiyama, Takeshi, c/o Kakogawa Works
Applicants: KOBE STEEL LTD, KABUSHIKI KAISHA KOBE SEIKO SHO ALSO KNOWN AS KOBE STEEL LTD.
Application year: 1993
Main classification: B01J23
IPC codes: B01J23, B01D53