PROCESS FOR REMOVING NITROGEN OXIDES AND CARBON MONOXIDE SIMULTANEOUSLY

Abstract: A process for removing NO x and CO simultaneously is characterized in that an exhaust gas comprising NO x , CO and O₂ is contacted with a catalyst in the presence of steam and ammonia to reduce NO x into harmless N₂ and also to react CO with H₂O to convert them into CO₂ and H₂ over the same cata­ lyst. The catalyst comprises titanium oxide, oxide of at least one metal selected from the group consisting of molybd­ enum, tungsten, vanadium, cerium, nickel, cobalt and man­ ganese, and at least one metal selected from the group con­ sisting of platinum, palladium, rhodium and ruthenium.

Description: Title of the Invention PROCESS FOR REMOVING NITROGEN OXIDES AND CARBON MONOXIDE SIMULTANEOUSLY Background of the Invention The present invention relates to a process for removing nitrogen oxides (hereinafter referred to as NOx) and carbon monoxide from exhaust gases e.g. of fixed sources such as combustion furnaces such as boilers of electric power plants and gas turbines, nitric acid plants and iron-manufacturing plants, and converting them into harmless substances in an efficient and economical manner. Various processes have been proposed for removing NOx and CO from exhaust gases. Among them, processes for removing NOx can be classified roughly into adsorption, absorption and reduction processes. In another aspect, these processes are classified into dry and wet processes. Among them, a process wherein NO, is reduced into harmless nitrogen with a reducing gas such as ammonia, hydrogen, carbon monoxide or a hydrocarbon is employed most ordinarily. An example of a process of reducing NO, into harmless nitrogen with ammonia is disclosed in US patent No. 4,085,193. On the other hand, examples of the processes for removing CO include absorption, adsorption and oxidation processes. For removing NO, and CO simultaneously, a process has been proposed which comprises reacting NOx with CO directly on a catalyst to convert them into harmless nitrogen and CO2, respectively, which method is disclosed in "SURFACE", Vol. 23, No.9 (1985), pages 510-511. This process is effective to remove NO, and CO from an exhaust gas in which 02 is not almost contained. However, the process has a defect that when the exhaust gas contains Oz, the reaction of CO with 02 occurs in preference to the reaction of CO with NOx and, therefore, the amount of CO usable for reducing NOx becomes insufficient and the NOx-removing capacity of the process is reduced. Thus, the most effective conventional process for removing NOx and CO from an exhaust gas containing 2 is thought to be one comprising oxidizing CO into CO2 over an oxidation catalyst and then adding ammonia thereto to remove NOx by reduction carried out in the presence of a denitration catalyst. However, this process is uneconomical, since both CO-oxidation catalyst and NOx-removal catalyst are necessitated. Further, it has been difficult to effect the reduction of NOx with NH3 and oxidation of CO simultaneously in the presence of only one catalyst in the prior art, since the catalyst effective for the oxidation of CO is also effective for the oxidation of NH3. Summary of the Invention An object of the present invention is to provide a process for removing CO and NOx simultaneously froi an exhaust gas containing 02 from a fixed source in an economical and efficient manner by overcoming the above-mentioned defects of the conventional techniques under these circumstances. The process of the present invention developed for the purpose of attaining the above-mentioned object is characterized in that a gas comprising NOx, CO and 02 is contacted with a catalyst in the presence of steam and ammonia to reduce NOx into harmless N2 and also to react CO with H20 to convert them into CO2 and H2 over the same catalyst. To carry out the reactions capable of attaining the above-mentioned purpose of the invention, a catalyst effective for the reactions of NO, with NH 3 as shown in the following formulae (1) and (2) and the reaction of CO with H20 as shown in the following formula (3) is necessitated: 1 3 NO + NH3 + 4O2 = N2 + 3H2O ......................... (1) 3NO2 + 4NH3 = #N2 + 6H2O ................ (2) CO + H2O = CO2 + H2 ..................... (3) A catalyst comprising the following components satisfies the above-mentioned conditions: titanium oxide as a main component, oxide of at least one metal selected from the group consisting of molybdenum, tungsten, vanadium, cerium, nickel, cobalt and manganese as the second component and at least one metal selected from the group consisting of noble metals, i.e. platinum, palladium, rhodium and ruthenium as the third component. Preferable components of the catalyst are titanium oxide as a main component, oxide of one metal selected from the group consisting of molybdenum, tungsten, vanadium and manganese as the second component, and platinum as the third component. Further, a catalyst including oxide of at least one metal selected from the group consisting of iron, chromium and copper in addition to the main component, the second component and third component is preferable. Now, the description will be made on the reasons why this catalyst has the desired properties. Though a catalyst comprising the above-mentioned main component, i.e. titanium oxide, and also the above-mentioned second component, i.e. oxides of molybdenum, tungsten, vanadium, cerium, nickel, cobalt andyor manganese, is remarkably effective on the denitration of the above formulae (1) and (2), its effect on the CO conversion of the above formula (3) is poor. However, when the above-mentioned third component, i.e. at least one of the above-mentioned noble metals is added to this catalyst, the effects of the catalyst on the CO conversion are improved . According to the inventors' experiments, atomic percentages of the respective metal components to the total of metals of the main, second and third components are preferred as follows: (a) 50 to 97% of titanium of the main component, (b) 1 to 50% of the second component, and (c) 0.02 to 5% of the noble metal used as the third component. It is preferable to add, to the above-mentioned noble metal or metals, 1 to 20% of iron, chromium or copper as the additional third component in the form of their oxide, whereby the COconversion is further improved. More than 50% of components of the catalyst Preferablv consists of the main component in the form of oxide, the second component, and the third component (optionally including the additional third component in the form of their oxide), as aL###-###nti'oned Preferably, the catalyst is to be an intimate mixture of the main component in the form of oxide, the second component in the form of oxide and the third components. The most preferable catalyst comprises titanium as the main component in the form of oxide ; vanadium, manganese, tungsten or molybdenum as the second component in the form of oxide ; and platinum as the third component. Atomic percentages of metal components included in the catalyst as the main, second and third components are as follows; Ti : 68-97% ........................... main component V, Mo, W, or Mn : 2-30% ............... second component Pt : 0.02-2% .......................... third component more than 50% by weight of this catalyst also consists of Ti one of V, Mo, W, and Mn ; and Pt. Preferably, they are in a state of an intimate mixture. In the process of the present invention, NO, is converted into harmless N2 and H20 as shown in the above formulae (1) and (2) and CO is converted into CO2 and H2 as shown in the above formula (3). Usually, H2 is further reacted with 02 over the catalyst to form H20. Therefore, when the gas to be treated contains 02, usually only a very small amount of H2 is contained in the gas after it is passed through the catalyst bed. In the process of the present invention, the molar ratio of steam to carbon monoxide contained in the gas is preferably at least 5/1 prior to se contact thereof with the catalyst, since when the molar ratio is below 5/1, the reaction of the formula (3) does not proceed sufficiently and a high CO-removal rate cannot be obtained easily. The molar ratio of ammonia to NOx is preferably in the ranqe of 0.5 to 2/1, since when this ratio is below 0.5/1, a high NOx-removal rate cannot be obtained and when it exceeds 2/1, a large mount of unreacted NHs is discharged and, therefore, ammonia is required in a large amount economically disadvantageously. The reaction temperature in the step of removing nitrogen oxides by the reduction and converting carbon monoxide in the Drocess of the present invention is preferably selected in the range of 150 to 600 C, more preferably 250 to 4000C. When the reaction temperature is below 1500C, both of the denitration velocity and carbon monoxide conversion velocity may be insufficient and a large amount of the catalyst is necessitated economically disadvantageously, while when the reaction temperature exceeds 6000C, ammonia used as the reducing agent for NOx is oxidized into NOx to seriously reduce the NOx-removal rate. In the removal of NOx and CO from an exhaust gas by the process of the present invention, the space velocity (in terms of superficial velocity under conditions of NTP) is for example selected in the range of 1,000 to 100,000 h-l, preferably 2,000 to 60,000 h l, since when the space velocity is below 1,000 h-l, a large amount of the catalyst is necessitated economically disadvantageously, while when it exceeds 100,000 h""1, high NOx and CO-removal rates cannot be attained. The reaction column may be of any of known fixed, moving or fluidized bed systems. The catalyst having quite a high capacity of removing NOx and CO can be obtained by a process employed usually in the production of catalysts such as precipitation, kneading or impregnation process. The catalyst may be molded into a final form by any process selected suitably depending on the use thereof such as ordinary extrusion, tableting or rolling process. The shape of the catalyst is not particularly limited and it may be globes, cylinders, rings, honey-combs or plates. The titanium sources used in the preparation of the catalyst of the present invention include titanium oxides as well as compounds which form titanium oxides by heating, such as titanium hydroxides and titanic acids. A preferred process for preparing titanium oxides comprises reacting a titanium halide, titanium sulfate or organotitanium compound witd aqueous ammonia or an alkali hydroxide or alkali carbonate to form a titanium hydroxide precipitate and then thermally decomposing it to form a corresponding titanium oxide. The sources of molybdenum, vanadium, tungsten, cerium, nickel, cobalt and manganese used as the second component include oxides thereof and various compounds capable of forming these oxides by heating. In case hydroxides of these components can be prepared by adding an alkaline precipitating agent to an aqueous solution of its nitrate, sulfate or chloride, the obtained hydroxide can be thermally decomposed to obtain the second component. As the starting materials for the noble metal used as the third component, chlorides, nitrates and complex salts of them can be used. The iron, chromium and copper sources include oxides of them as well as compounds which form these oxides by heating such as hydroxides, nitrates, chlorides and sulfates. A preferred process comprises adding an alkaline precipitating agent to an aqueous solution of such a slat to form a hydroxide and then thermally decomposing the same. Brief Description of the Drawing Fig. 1 is graphs showing NOx-removal and CO-removal rates obtained by a process of removing NOx and CO simultaneously according to the present invention. Example 1 A catalyst used in the process of the present invention was prepared as follows: 500 g of titanium tetrachloride (Tici4) was dissolved in about 1 Q of distilled water. It was then neutralized with about 2 Q of 5 N aqueous ammonia to form a precipitate, which was washed thoroughly by decantation and filtered. A solution prepared by dissolving 54 g of ammonium paramolybdate in 500 mll of hot water was added to the precipitate and the mixture was kneaded well with a mixing machine. The obtained slurry was dried and further mixed thoroughly with graphited of 1 wt.% of the resultant substance and the mixture was molded into tablets having a diameter of 6 mm and a height of 6 mm. The tablets were calcined at 5000C for 2 h. The oxide had an atomic ratio of titanium to molybdenum (ti:Mo) of 9:1. The tablets were pulverized and 100 g of a 10- to 20-mesh fraction was taken and impregnated with 40 mQ of an aqueous hexachloroplatinic acid solution. After drying at 1200C for 5 h followed by calcination at 5000C for 2 h, a catalyst 1 containing 0.5 wt.% of platinum was obtained. The capacities of the catalyst for removing NOx and CO were determined by means of a flow reaction device of fixed bed type under atmospheric pressure. The reaction tube used was a quartz glass tube having an inner diameter of 20 mm and also having a thermocouple-protecting quartz tube having an outer diameter of 5 mm. The reaction tube was heated in an electric furnace and the temperature was measured with the thermocouple. About 4 m# of the above-mentioned, sized catalyst was placed in the center of the reaction tube and a reaction gas of the following composition was introduced therein at a space velocity of 60,000 h 1: NOx: 200 ppm NH3: 240 ppm CO: 1,000 ppm 2 1% H20: 10% N2: the balance NOx was analyzed with a chemiluminescent NOx analyzer and CO was analyzed with a non-dispersion infrared CO analyzer. The removal rates of NOx and CO were calculated according to the following formulae: NOx-removal rate = EMI8.1 CO-removal'rate = EMI8.2 The capacities of the catalyst prepared in Example 1 are shown in Fig. 1. It is apparent from this figure that the catalyst 1 of the present invention is effective in removing both NOx and CO at a temperature of above 2000 C. Example 2 A catalyst 2 was prepared in the same manner as in Example 1 except that ammonium paramolybdate was replaced with ammonium paratungstate. The catalyst contained titanium and tungsten in the form of their oxides and in an atomic ratio of Ti to W of 9:1 and it contained also 0.5 wt.%, based on the oxides, of platinum. The capacities of the catalyst 2 are shown in Fig. 1. It is apparent from this figure that, though its capacity of removing NOx is slightly lower than that of the catalyst 1 at low temperatures, it exhlbits high NOx- and CO-removal rates. Example 3 A catalyst 3 was prepared as follows: 7 g of ammonium metavanadate (NH4VO3) was added to 500 g of a metatitanic acid slurry (150 g of Tin), 500 m# of distilled water was added to the mixture and the obtained mixture was kneaded thoroughly with a kneader. The obtained pasty mixture was pre-calcined at 3000C for 2 h. 1 wt.% of graphite was added thereto and the mixture was molded into tablets having a diameter of 6 mm and a height of 6 mm under a molding pressure of about 500 kg/cm2. The tablets were calcined at 5000C for 2 h. The oxide had an atomic ratio of titanium to vanadium (Ti:V) of 97:3. The tablets were pulverized and 100 g of 10- to 20-mesh fraction was taken and impregnated with 40 mi of an aqueous hexachloroplatinic acid solution. After drying at 1200C for 5 h followed by calcination at 5000C for 2 h, the catalyst 3 containing 0.5 wt.% of platinum was obtained. The capacities of the catalyst 3 are shown in Table 1. Table 1 EMI9.1 <tb> <SEP> Reaction <SEP> temperature <SEP> ( C) <SEP> <tb> <SEP> 200 <SEP> 250 <SEP> 300 <SEP> 400 <SEP> 500 <SEP> <SEP> 600 <tb> NOx-removal <SEP> rate <SEP> (%) <SEP> 72 <SEP> 82 <SEP> 91 <SEP> 82 <SEP> 54 <SEP> 31 <tb> CO-removal <SEP> rate <SEP> (%) <SEP> <SEP> 50 <SEP> 83 <SEP> 90 <SEP> 98 <SEP> 99 <SEP> 97 <SEP> <tb> Example 4 A catalyst 4 was prepared in the same manner as in Example 1 except that hexachloroplatinic acid was replaced with palladium nitrate. The catalyst contained titanium and molybdenum in the form of their oxides and in an atomic ratio of Ti to Mo of 9:1 and it further contained 0.5 wt.% of palladium. The capacities of the catalyst 4 determined in the same manner as in Example 1 are shown in Table 2. Table 2 EMI9.2 <tb> <SEP> Reaction <SEP> temperature <SEP> (owe) <SEP> <tb> <SEP> 200 <SEP> 250 <SEP> 300 <SEP> <SEP> '400 <SEP> 500 <SEP> 600 <tb> NOx-removal <SEP> rate <SEP> (%) <SEP> <SEP> 58 <SEP> 80 <SEP> 86 <SEP> 90 <SEP> 1 <SEP> <SEP> 73 <SEP> 50 <tb> CO-removal <SEP> rate <SEP> (*) <SEP> <SEP> 53 <SEP> 81 <SEP> 92 <SEP> 99 <SEP> 99 <SEP> 98 <tb> Example 5 A catalyst 5 was prepared in the same manner as in Example 1 except that hexachloroplatinic acid was replaced with rhodium chloride. The catalyst contained titanium and molybdenum in the form of their oxides and in an atomic ratio of Ti to Mo of 9:1 and it further contained 0.5 wt*E of rhodium. The capacities of the catalyst 5 determined in the same manner as in Example 1 are shown in Table 3. Table 3 EMI10.1 <tb> <SEP> Reaction <SEP> temperature <SEP> (OC) <SEP> <tb> <SEP> 200 <SEP> 250 <SEP> 300 <SEP> 400 <SEP> 500 <SEP> 600 <tb> NOx-removal <SEP> rate <SEP> (%) <SEP> 52 <SEP> 81 <SEP> 86 <SEP> 81 <SEP> 66 <SEP> 51 <tb> CO-removal <SEP> rate <SEP> (%) <SEP> 41 <SEP> 82 <SEP> 87 <SEP> 91 <SEP> 98 <SEP> 97 <tb> Example 6 A catalyst 6 was prepared in the same manner as in Example 1 except that hexachloroplatinic acid was replaced with ruthenium chloride. The catalyst contained titanium and molybdenum in the form of their oxides and in an atomic ratio of Ti to Mo of 9:1 and it further contained 0.5 wt.% of ruthenium. The capacities of the catalyst 6 determined in the same manner as in Example 1 are shown in Table 4. Table 4 EMI10.2 <tb> <SEP> Reaction <SEP> temperature <SEP> (OC) <SEP> <tb> <SEP> 200 <SEP> 250 <SEP> 300 <SEP> 400 <SEP> 500 <SEP> 600 <tb> NOx-removal <SEP> rate <SEP> (%) <SEP> <SEP> 60 <SEP> 80 <SEP> 88 <SEP> 90 <SEP> 71 <SEP> 52 <tb> CO-removal <SEP> rate <SEP> (%) <SEP> 51 <SEP> 81 <SEP> 90 <SEP> t <SEP> <SEP> 96 <SEP> 97 <SEP> 95 <tb> Comparative Example 1 A platinum-free titanium oxideymolybdenum oxide catalyst (atomic ratio of Ti to Mo = 9:1) (comparative catalyst 1) was prepared in the same manner as in Example 1. The capacities of the catalyst were determined in the same manner as in Example 1., The results are shown in Table 5. Table 5 EMI11.1 <tb> <SEP> Reaction <SEP> temperature( C) <SEP> <tb> <SEP> 209 <SEP> 390 <SEP> 406 <SEP> 500 <SEP> 600 <tb> <SEP> 200 <SEP> 300 <SEP> 400 <SEP> 500 <SEP> 600 <tb> NOx-removal <SEP> rate <SEP> (%) <SEP> 21 <SEP> 62 <SEP> 94 <SEP> 91 <SEP> 62 <SEP> <tb> CO-removal <SEP> rate <SEP> (%) <SEP> 2 <SEP> 7 <SEP> 15 <SEP> 28 <SEP> 48 <SEP> <tb> It is apparent from this table that the comparative catalyst 1 had only a poor CO-removal capacity, though its NOx-removal capacity was high. (Comparative Example 2) 5 m# of an aqueous hexachloroplatinic acid solution (10 g Pt/100 g solution) was# diluted with distilled water to obtain 70 ml of a solution. 100 g ofactivealumina carrier pulverized into a 10- to 20-mesh size was impreg- nated with the solution, dried at 120 C for 2 h and calcined at 5000C for 2 h. The catalyst contained 0.5 wt.% of plati- num carried thereon. The capacities of the catalyst were determined in the shame manner as in Example 1 to obtain the results shown in Table 6. Table '6 EMI11.2 <tb> <SEP> Reaction <SEP> temperature <SEP> (bC) <SEP> <tb> <SEP> 200 <SEP> 300 <SEP> 400 <SEP> <SEP> 500 <SEP> 600 <tb> NOx-removal <SEP> rate <SEP> (%) <SEP> 81 <SEP> 48 <SEP> 12 <SEP> - <SEP> - <SEP> <tb> CO-removal <SEP> rate (%) <SEP> <SEP> 68 <SEP> - <SEP> 92 <SEP> 99 <SEP> 99 <SEP> 99 <tb> It is apparent from this table that the comparative catalyst 2 had only a poo,r#NOx-removal capacity, though its CO-removal capacity was high.

Claims: CLAIMS 1. A process for removing nitrogen oxides and carbon monoxides simultaneously from a gas including nitrogen oxides, carbon monoxide and oxygen characterized in that the gas is contacted with a catalyst in the presence of steam and ammonia at a temperature of 150-600tC to remove the nitrogen oxides by the reaction with ammonia and to remove carbon monoxide by the reaction with steam, said catalyst comprising titanium as a main component in the form of oxide, at least one metal selected from the group consisting of molybdenum, tungsten, vanadium, cerium,nickel, cobalt and manganese as the second component in the form of oxide and at least one metal selected from the group consisting of platinum, palladium, rhodium and ruthenium as the third component. 2. A process for removing nitrogen oxides and carbon monoxides simultaneously according to Claim 1, wherein said catalyst further includes oxide of at least one metal selected from the group consisting of iron, chromium and copper as the third component. 3. A process for removing nitrogen oxides and carbon monoxides simultaneously according to Claim 1, wherein said main, second and third components of said catalyst are, by atomic ratio, 50 to 97%, 1 to 50% and 0.02 to 5%, respectively, and more than 50% by weight of said catalyst consists of oxides of said main and second components, and said third component. 4. A process for removing nitrogen oxides and carbon monoxides simultaneously according to Claim 2, wherein said main, second, third and additional third components of said catalyst are by atomic ratio, 50 to 97%, 1 to 50%, 0.02 to 50% and 1 to 20%, respectively, and more than 50% by weight of said catalyst consists of oxides of said main, second, and additional third components, and said third component. 5. A process for removing nitrogen oxides and carbon monoxides simultaneously according to Claim 1,2,3 or 4, wherein said main, second and third components are by atomic percentages, 68-97% of titanium, 2-30% of one metal selected from the group consisting of vanadium, molybdenum, tungsten and manganese, and 0.02-2% of platinum, respectively, and more than 50% by weight of said catalyst consists of oxides of said main and second components, and said third component. 6. A process for removing nitrogen oxides and carbon monoxides simultaneously according to Claim 5, wherein said gas is contacted with said catalyst at a temperature of 2504000C, at a space velocity of 1,000-100,000/hour. 7. The process according to Claim 6, wherein the molar ratio of the steam to carbon monoxide contained in the mixture of nitrogen oxides, carbon monoxide and oxygen is at least 5 and the molar ratio of ammonia to the nitrogen oxides is 0.5 to 2 prior to the reactions. 8. A catalyst for use in the process of any one of the preceding claims, said catalyst being as defined herein.

Inventors: KATO, AKIRA, KAWAGOSHI, HIROSHI, MATSUDA, SHINPEI, TACHI, TAKAHIRO, WATANABE, NORIKO, YAMASHITA, HISAO
Applicants: BABCOCK-HITACHI KABUSHIKI KAISHA, HITACHI, LTD.
Application year: 1986
Main classification: B01D53
IPC codes: B01J23, B01D53