Nitrogen oxide removal catalyst

Abstract: This invention effectively utilizes a solid type honeycomb-shaped NO x removal catalyst which has been used in a catalytic ammonia reduction process and hence reduced in NO x removal power, and thereby provides an inexpensive catalyst having NO x removal power equal to that of a fresh catalyst. The nitrogen oxide removal catalyst of this invention, which is useful in a process wherein ammonia is added to exhaust gas and nitrogen oxides present in the exhaust gas are catalytically reduced, has a two-layer structure composed of a lower layer comprising a spent solid type nitrogen oxide removal catalyst and an upper layer comprising a fresh TiO 2 -WO 3 -V 2 O 5 ternary or TiO 2 -WO 3 binary catalyst powder which has been applied to the lower layer so as to give a coating thickness of 100 to 250 µm. Moreover, the catalyst power constituting the upper layer is a catalyst powder obtained by pulverizing a spent solid type nitrogen oxide removal catalyst.

Description: This invention relates to a nitrogen oxide removal catalyst which is useful for the removal of nitrogen oxides present in combustion exhaust gases such as exhaust gas from boilers and a method of producing such catalyst. Conventionally, in order to remove nitrogen oxides (hereinafter referred to as NO x ) produced in boilers and various combustion furnaces for the purpose of preventing air pollution, a catalytic ammonia reduction process wherein ammonia is used as a reducing agent and nitrogen oxides are catalytically decomposed to nitrogen and water with the aid of a catalyst is being widely employed. Most of the NO x removal catalysts currently used for practical purposes are honeycomb-shaped catalysts which have through-holes of square cross section in order to prevent clogging with dust present in exhaust gas and increase the gas contact area. With respect to catalyst components, titanium oxide is highly suitable for use as a principal component, and vanadium, tungsten and the like are used as additional active components. Thus, TiO 2 -WO 3 binary catalysts and TiO 2 -V 2 O 5 -WO 3 ternary catalysts are popularly used. However, when a NO x removal catalyst is used for the treatment of combustion exhaust gas, its NO x removal power tends to be gradually reduced. Of various NO x removal catalysts, those used for the treatment of exhaust gas from oil-fired boilers pose no problem because their reduction in NO x removal power is insignificant. In contrast, NO x removal catalysts used for the treatment of exhaust gas from coal-fired boilers show a considerable reduction in NO x removal power with the lapse of time, and hence require some counter-measure. In order to maintain the required NO x removal power, it is necessary to replace the spent catalyst with a fresh one. However, this is disadvantageous from an economic point of view because a fresh catalyst is expensive and the spent catalyst requires a cost of disposal. DE 40 06 918 A discloses a process for the reconditioning of catalysts; deactivated catalysts, especially monolithic catalysts, which have been used for the reduction of nitrogen oxides in flue gases by reaction with ammonia, are reprocessed by grinding the catalysts down to grain sizes of 5 to 20 µm and, in the preparation of catalysts, adding the resulting powder in quantities of up to 80 % by weight, relative to the total material used, to fresh starting material before forming. EP 0161 206 A disclose a method for regenerating a denitration catalyst which is characterized by washing, with an aqueous oxalic acid solution, the used tungsten-titania or the tungsten-titania-vanadium denitrating catalyst which dust components have adhered to or have accumulated an and in which an S02-oxidizing power has thus risen; impregnating the catalyst with a tungsten compound; and drying and calcining the thus treated catalyst. JP 57 078 949 A discloses to regenerate catalytic activity by simple treating operation by a method wherein a poisoned catalyst with a honeycomb structure of which catalytic activity is lowered is subjected to heat treatment at a specific temperature for a definite time or more and, thereafter, a catalyst substance is supported on the resultant carrier. A poisoned catalyst of which catalytic activity is lowered by using a catalyst for removing nitrogen oxide comprising a honeycomb structure for a long time is subjected to heat treatment in a temperature range of 1 150-1 450 °C for 10 minutes or more and, thereafter, a catalyst substance such as titanium, tungsten or the like is supported an the resultant carrier. As described above, by simple treating operation, the poisoned catalyst is recovered as a catalyst carrier and the catalyst substance is supported on said recovered carrier to regenerate said poisoned catalyst to a catalyst having high denitrating characteristics. Accordingly, an object of the present invention is to effectively utilize a spent solid type (i.e., the type in which the catalysts themselves form a shaped body) honeycomb-shaped NO x removal catalyst for use in a catalytic ammonia reduction process (in particular, a spent NO x removal catalyst which has been used for the removal of NO x from the exhaust gas of a coal-fired boiler) and thereby provide an inexpensive catalyst having NO x removal power equal to that of a fresh catalyst. In reusing solid type honeycomb-shaped NO x removal catalysts which had been practically used for the removal of NO x from the exhaust gas of coal-fired boiler plants for about 500,000 hours, the present inventors investigated the strength and NO x removal power of the honeycomb-shaped catalysts. As a result, it was confirmed that their strength remained unchanged as compared with fresh catalysts, but their NO x removal power was reduced to less than 1/2 of that of fresh catalysts. As a result of further investigation on the cause for the reduction in the NO x removal power of these catalysts, it was found that, among various fly ash components attached to the catalyst surface, principally calcium oxide (CaO) was gradually converted into gypsum (CaSO 4 ) which covered the catalyst surface and thereby prevented the reaction gases (i.e., NO x and NH 3 ) from diffusing into the catalyst. It was also confirmed that this CaSO 4 layer lay on the catalyst surface to a depth of several micrometers to several tens of micrometers and the catalyst remained fresh at greater depths and that the NO x removal action of the solid type honeycomb-shaped NO x removal catalysts was effectively used only in a surface layer having a thickness of the order of 100 µm. Then, the present inventors hit on the idea that a solid type honeycomb-shaped NO x removal catalyst having sufficiently high denitration power might be obtained by coating the surface of a solid type honeycomb-shaped NO x removal catalyst having reduced NO x removal power with a catalytic component having NO x removal power. On the basis of this conception, the present inventors have made an intensive investigation and have now completed the present invention. Thus, the present invention provides (1) a nitrogen oxide removal catalyst as defined in claim 1 and dependent claims and a method of producing such catalyst as defined in claim 8. The present invention relates to a NO x removal catalyst which is useful in a process wherein ammonia is added to exhaust gas and NO x present in the exhaust gas are removed by catalytic reduction. This NO x removal catalyst has a two-layer structure composed of a lower layer comprising a spent solid type NO x removal catalyst and an upper layer comprising a catalyst powder applied to the lower layer. In the lower layer, a spent solid type honeycomb-shaped catalyst which has been used for the removal of NO x from combustion exhaust gas, and has become useless because of a reduction in NO x removal power is used directly in the form of a shaped product. The catalyst powder constituting the upper layer may be a fresh titania-based (e.g., TiO 2 -V 2 O 5 -WO 3 ) catalyst powder or a catalyst powder obtained by pulverizing a spent solid type honeycomb-shaped catalyst which has been used for the removal of NO x , for example, from the exhaust gas of a coal-fired boiler and has become useless. Although the composition of the catalyst used in the upper layer may vary according to the exhaust gas conditions (i.e., the type of fuel used, the temperature of the exhaust gas, and the like) under which the catalyst of the present invention is used, it is selected from TiO 2 -V 2 O 5 -WO 3 ternary and TiO 2 -WO 3 binary NO x removal catalysts which are currently used for practical purposes. Typically, this catalyst consists essentially of 0 to 10% by weight of V 2 O 5 , 5 to 20% by weight of WO 3 and the balance being TiO 2 . The thickness of the upper catalyst layer is preferably in the range of 100 to 250 µm. If the thickness thereof is less than 100 µm, the resulting catalyst will have low NO x removal power, and if it is greater than 250 µm, the upper catalyst layer will have poor adhesion to the lower catalyst layer and may be separated therefrom. As described previously, the reduced NO x removal power of a spent solid type honeycomb-shaped NO x removal catalyst is attributable to a gypsum layer covering the catalyst surface to a depth ranging from several micrometers to several tens of micrometers. At greater depths, the catalyst is scarcely reduced in NO x removal power. Consequently, if a solid type honeycomb-shaped NO x removal catalyst having reduced NO x removal power (i.e., a spent catalyst) is pulverized, the gypsum concentration is decreased to substantially the same level as in a fresh binary or ternary NO x removal catalyst. As a result, there is obtained a NO x removal catalyst powder having NO x removal equal to that of a fresh catalyst. In the present invention, therefore, a catalyst powder obtained by pulverizing a spent solid type honeycomb-shaped NO x removal catalyst may be used as a fresh binary or ternary NO x removal catalyst. This makes it possible to produce a NO x removal catalyst having the same degree of strength and NO x removal powder as a fresh solid type honeycomb-shaped NO x removal catalyst. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a solid type honeycomb-shaped NO x removal catalyst in accordance with the present invention; and FIG. 2 is an enlarged cross-sectional view illustrating the honeycomb configuration of the solid type honeycomb-shaped NO x removal catalyst in accordance with the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The structure of a NO x removal catalyst in accordance with the present invention is explained with reference with FIGs. 1 and 2. FIG. 1 is a perspective view thereof and FIG. 2 is an enlarged cross-sectional view illustrating its honeycomb configuration. In FIG. 1, numeral 1 designates a solid type honeycomb-shaped NO x removal catalyst in accordance with the present invention, and p and L stand for the pitch (i.e., the center distance between adjacent walls) and length of the honeycomb, respectively. In FIG. 2, numeral 2 designates a lower catalyst layer (comprising a spent solid type NO x removal catalyst) and numeral 3 designates an upper catalyst layer (comprising a powder of a fresh NO x removal catalyst or a spent solid type NO x removal catalyst). In order to demonstrate the effects of nitrogen oxide removal catalysts in accordance with the present invention, the following examples are given. Example 1 A solid type honeycomb-shaped catalyst which had been used in exhaust gas from coal-fired boiler plant A for about 45,000 hours and had become useless because of a reduction in NO x removal power was provided. This catalyst had a honeycomb configuration as shown in FIG. 1 and its pitch (P) was 7.4 mm. This catalyst was composed of 90.9% by weight of TiO 2 , 8.5% by weight of WO 3 and 0.6% by weight of V 2 O 5 . On the other hand, powdered titania (MC-50; manufactured by Ishihara Sangyo Kaisha, Ltd., Japan) as a raw material for TiO 2 was impregnated with a methylamine solution of ammonium metavanadate as a raw material for V 2 O 5 and a methylamine solution of ammonium paratungstate as a raw material for WO 3 , followed by drying and firing. Thus, there was obtained a powder composed of 90.9% by weight of TiO 2 , 8.5% by weight of WO 3 and 0.6% by weight of V 2 O 5 . This powder had a particle diameter range of 0.2 to 23 µm and a median diameter of 1.1 µm. Then, a slurry was prepared by adding water, a silica sol and an alumina sol to the above powder, and applied to the above honeycomb-shaped catalyst so as to give a coating thickness of 100 µm. The catalyst thus obtained was called Catalyst 1. Example 2 A solid type honeycomb-shaped catalyst (with a pitch of 7.4 mm) which had been used in exhaust gas from coal-fired boiler plant A for about 45,000 hours and had become useless because of a reduction in NO x removal power was provided. This catalyst had the same composition as that used in Example 1. On the other hand, a similar catalyst (composed of 90.9% by weight of TiO 2 , 0.6% by weight of V 2 O 5 and 8.5% by weight of WO 3 ) having reduced NO x removal power was pulverized to obtain a catalyst powder (having a particle diameter range of 0.1 to 20 µm and a median diameter of 2.3 µm). Then, a slurry was prepared by adding water, a silica sol and an alumina sol to this catalyst powder, and applied to the above honeycomb-shaped catalyst so as to give a coating thickness of about 100 µm. The catalyst thus obtained was called Catalyst 2. Example 3 A solid type honeycomb-shaped catalyst (with a pitch of 7.4 mm) which had been used in exhaust gas from coal-fired boiler plant A for about 45,000 hours and had become useless because of a reduction in NO x removal power was provided for use as the underlying catalyst. This catalyst had the same composition as that used in Example 1. On the other hand, a catalyst similar to the underlying catalyst was pulverized to obtain a catalyst powder having a particle diameter range of 0.1 to 20 µm and a median diameter of 2.3 µm. This catalyst powder was impregnated with a methylamine solution of ammonium metavanadate as a raw material for V 2 O 5 , and a methylamine solution of ammonium paratungstate as a raw material for WO 3 so as to increase its V 2 O 5 content by 2.35% by weight, followed by drying and firing. Thus, a powder composed of 88.5% by weight of TiO 2 , 8.5% by weight of WO 3 and 3.0% by weight of V 2 O 5 was obtained for use as the upper catalyst layer. A slurry was prepared by adding water, a silica sol and an alumina sol to this powder, and applied to the surface of the above honeycomb-shaped catalyst so as to give a coating thickness of about 100 µm. The catalyst thus obtained was called Catalyst 3. Example 4 A solid type honeycomb-shaped catalyst (with a pitch of 7.4 mm) which had been used in exhaust gas from coal-fired boiler plant B for about 60,000 hours and had become useless because of a reduction in NO x removal power was provided for use as the underlying catalyst. This catalyst had the same composition as that used in Example 1. On the other hand, a catalyst similar to the underlying catalyst was pulverized to obtain a catalyst powder having a particle diameter range of 0.1 to 26 µm and a median diameter of 2.5 µm. Then, a slurry was prepared by adding water, a silica sol and an alumina sol to this catalyst powder, and applied to surface of the above honeycomb-shaped catalyst so as to give a coating thickness of about 100 µm. The catalyst thus obtained was called Catalyst 4. Experiments In these experiments, fresh catalysts for use in coal-fired boiler plants A and B, and the spent catalysts used in the examples were used as control catalysts. By comparison with these control catalyst, Catalyst 1-4 obtained in the foregoing examples were tested for NO x removal power under the conditions shown in Table 1. And the test results were shown in Table 2. Testing conditions Shape of catalyst 6 holes x 7 holes x 500 mm long Flow rate of gas 20.1 Nm 3 /m 2 •hr SV value 16,600 hr -1 NH 3 /NO x ratio 1.0 Temperature of gas 380°C Composition of gas NO x = 150 ppm NH 3 = 150 ppm SO x = 800 ppm O 2 = 4% CO 2 = 11% H 2 O = 11% N 2 = Balance Test results Catalyst Degree of NO x removal (%) Example 1 67.0 2 65.4 3 75.6 4 64.2 Comparative Example Spent catalyst used in plant A 49.8 Spent catalyst used in plant B 26.2 Fresh catalyst for use in plant A 65.3 Fresh catalyst for use in plant B 64.5 Degree of NO x removal (%) = {[(Inlet NO x content) - (Outlet NO x content)]/(Inlet NO x content)} x 100 These results reveal that, when catalysts which had been used in coal-fired boiler plants A and B and hence had reduced NO x removal power were reconstructed according to the present invention, there were obtained catalysts having NO x removal power equal to that of fresh catalysts. Moreover, they also reveal that the NO x removal power of fresh catalysts was surpassed by supporting an active component (such as V 2 O 5 ) on a spent catalyst. Thus, the present invention makes it possible to effectively utilize spent solid type honeycomb-shaped NO x removal catalysts (especially for use with coal-fired boilers) which have conventionally been useless and disposed of, and thereby provide inexpensive NO x removal catalysts while reducing the amount of industrial waste. Consequently, the present invention can produce marked industrial effects.

Claims: Nitrogen oxide removal catalyst for use in a process for catalytically reducing nitrogen oxides present in exhaust gas by adding ammonia to the exhaust gas; said catalyst comprising a two-layer structure having a lower layer and an upper layer; characterized in that the lower layer comprises a first spent solid type nitrogen oxide removal catalyst having fly ash components attached to the catalyst surface, and that the upper layer comprises a fresh TiO 2 -WO 3 -V 2 O 5 ternary or TiO 3 -WO 3 binary catalyst powder which has been applied to the lower layer so as to give a coating thickness of 100 to 250 µm. Catalyst according to claim 1, wherein the fresh catalyst powder consists essentially of 0 to 10% by weight of V 2 O 5 , 5 to 20% by weight of WO 3 and the balance being TiO 2 . Catalyst according to any of claims 1 to 3, wherein said structure is a honeycomb-shaped structure. Catalyst according to any of claims 1 to 4, wherein the catalyst powder constituting the upper layer is a catalyst powder obtained by pulverizing a second spent solid type nitrogen oxide removal catalyst having fly ash components attached to the catalyst surface. Catalyst according to any of claims 1 to 5, wherein the catalyst surface of the lower layer is covered with a layer of gypsum. Catalyst according to any of claims 1 to 6, wherein the upper layer has a coating thickness of 100 µm. Method for producing a nitrogen oxide removal catalyst according to claims 1-7 for use in a process for catalytically reducing nitrogen oxides present in exhaust gas by adding ammonia to the exhaust gas; the method comprising the steps of: taking a first spent solid type nitrogen oxide removal catalyst having fly ash components attached to the catalyst surface, the spent solid type nitrogen oxide removal catalyst being a lower layer of the catalyst; and applying to the lower layer an upper layer comprising a fresh TiO 2 -WO 3 -V 2 O 5 ternary or TiO 2 -WO 3 binary catalyst powder and having a coating thickness of 100 to 250 µm. Method according to claim 8 or 9, wherein the fresh catalyst powder consists essentially of 0 to 10% by weight of V 2 O 5 , 5 to 20% by weight of WO 3 and the balance being TiO 2 . Method according to any of claims 8 to 10, further comprising the step of obtaining the catalyst powder constituting the upper layer by pulverizing a second spent solid type nitrogen oxide removal catalyst having fly ash components attached to the catalyst surface. Method according to any of claims 8 to 13, wherein the upper layer has a coating thickness of 100 µm.

Inventors: IIDA KOZO, MORII ATSUSHI, NAITO OSAMU, NOJIMA SHIGERU, YOSHIAKI OBAYASHI, IIDA, KOZO, MORII, ATSUSHI, NAITO, OSAMU, NOJIMA, SHIGERU, Iida, Kozo, Mitsubishi Heavy Ind., LTD., Morii, Atsushi c/o Nagasaki Ship. &amp; Mach. Works, Naito, Osamu c/o Nagasaki Ship. &amp; Mach. Works, Nojima, Shigeru, Mitsubishi Heavy Ind., LTD., Yoshiaki Obayashi, Mitsubishi Heavy Ind., LTD.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI HEAVY INDUSTRIES, LTD.
Application year: 1997
Main classification: B01D53
IPC codes: B01D53, B01J38, B01J23, B01J37