METHOD FOR RECOVERING DENITRATING CATALYST FOR AMMONIA CATALYTIC REDUCTION

Abstract: A method for recovering a denitrating catalyst for ammonia catalytic reduction which is characterized by treat­ ing the denitrating catalyst for ammonia catalytic reduction, in which a performance has deteriorated, with at least one selected from the group consisting of SO₄²⁻ compounds, sul­ fur compounds and sulfonic acid compounds in order to re­ plenish the catalyst with a sulfur content.

Description: The present invention relates to a method for recovering a deteriorated denitrating catalyst for ammonia catalytic reduction which is used to treat a combustion exhaust gas such as an exhaust gas from a boiler. The present invention considers that recovering of any deteriorated denitrating catalyst by dipping it into aqueous sulfuric acid is known from JP-A-5227091. As a method for removing nitrogen oxides (hereinafter simply referred to as NOx) from a combustion exhaust gas, there has widely employed an ammonia catalytic reduction system in which NOx is reduced with a catalyst by the use of ammonia as a reducing agent. When a dust is contained in the combustion exhaust gas, a honeycomb, cylindrical or platelike catalyst is disposed in parallel with a stream direction of the exhaust gas for the purpose of preventing the catalyst from being clogged with the dust during operation, and at present, such a constitution has been mainly taken. In particular, the perforated honeycomb catalysts having hexagonal, square, rectangular and triangular shapes are often utilized because of a large contact area with a gas and a great economy. Such a catalyst for the NOx removal will become gradually poor in performance during its use in a combustion exhaust gas, and therefore if it is intended to keep its denitrating performance high, the exchange or recovery of the deteriorated catalyst will be necessary. The exchange of all the deteriorated catalysts for new ones is economically unbeneficial since the catalysts are expensive and their disposal also takes some cost. Accordingly, it is profitable to reuse the deteriorated catalysts by recovering them. The inventors of the present application have intensively researched a performance drop phenomenon of the denitrating catalyst which has been used practically in a plant, and it has resultantly been found that the performance deterioration of the catalyst would mainly be attributable to the accumulation, on the catalyst, of alkaline components such as Na and K contained in the dust in the exhaust gas. The thus deteriorated catalyst can be recovered by water washing to remove Na and K, but its recovery degree is not so high as a performance of the new catalyst and hence such a washing recovery technique is not satisfactory. The present inventors have found as a result of further investigations that the performance of the denitrating catalyst is closely connected to a sulfur content in the catalyst and the catalyst unsatisfactorily recovered by the washing operation contains a less amount of sulfur than in the unused new catalyst. Thus, it has been confirmed that when the denitrating catalyst in which a performance has deteriorated was treated with at least one selected from the group consisting of SO₄²⁻ compounds, sulfur compounds and sulfonic acid compounds in order to thereby replenish the catalyst with a sulfur content, the performance of the catalyst could be recovered effectively. Further, in the case that an exhaust gas is treated at a temperature of 250 to 400°C under the feed of ammonia by the use of the catalyst recovered by water washing, a treatment efficiency will be low at its initial stage and a certain period of time will elapse before its efficiency has reached the level in the vicinity of a design efficiency (a performance of the new catalyst) (which period will be hereinafter referred to, for simplification, as the induction period). Such a phenomenon will lead to the deterioration of the treatment efficiency and the ununiformity of the exhaust gas treatment. Therefore, if the induction period is shortened or desirably eliminated, the treatment efficiency will be able to always remain at a high level without temporarily lowering the effect and efficiency of the exhaust gas treatment, even when the recovered catalyst is used. The present invention permits maintaining the treatment efficiency at a high level. That is to say, the inventors of the present application have found that an effective performance recovery of the catalyst can be accomplished by first impregnating or spraying the deteriorated denitrating catalyst with an aqueous sulfuric acid solution, and calcining it in order to replenish the catalyst with a sulfur content. Further, the present inventors have found that the replenishment of the catalyst with a sulfur content can be accomplished by first impregnating the water-washed catalyst with an aqueous ammonium hydrogensulfate solution and calcining it, whereby the performance of the catalyst can be effectively recovered, and the present invention has been completed. Still further, the present inventors have found that the replenishment of the catalyst with a sulfur content can be accomplished by first impregnating the water-washed catalyst with an aqueous sulfamic acid solution and calcining it, whereby the performance of the catalyst can be effectively recovered, and the present invention has been completed. An object of the present invention is to provide a method for recovering a deteriorated denitrating catalyst for ammonia catalytic reduction up to a performance level comparable to the new unused catalyst. Another object of the present invention is to provide a method for recovering a deteriorated denitrating catalyst for ammonia catalytic reduction which is capable of eliminating the above defined induction period. For the sake of achieving the above-mentioned objects, the present invention is characterized by the features mentioned in claim 1. Further aspects are to be seen from the subclaims 2 to 4. Now, the present invention will be described in detail as examples in reference to accompanying drawings, in which: Figs. 1 and 2 show results of Examples 3 and 4 of the present invention as well as comparative examples corresponding to them, and each abscissa axis therein represents an elapsed time in an exhaust gas treatment and each ordinate axis therein represents a denitration rate in terms of percent. Figs. 3 and 4 show results of Examples 5 and 6 of the present invention as well as comparative examples corresponding to them, and each abscissa axis therein represents an elapsed time in an exhaust gas treatment and each ordinate axis therein represents a denitration rate in terms of percent. In these drawings, solid and dotted lines indicate results of the present invention and comparative examples, respectively. Example 1 A TiO₂-WO₃-V₂O₅ catalyst (91% of TiO₂, 8% of WO₃ and 1% of V₂O₅) of a performance deterioration degree K/Ko = 0.63 (K is a performance of the deteriorated catalyst or the catalyst to be recovered, and Ko is a performance of the new unused catalyst) was dipped into and washed with water for 60 minutes the volume of which was 5 times as much as that of the catalyst. As a result, the catalyst was recovered up to a level of K/Ko = 0.88. The thus treated catalyst was further impregnated with a 1 N H₂SO₄ aqueous solution, and calcination was then carried out at 450°C for 1 hour, so that the catalyst was recovered up to a level of K/Ko = 0.99. Example 2 A deteriorated TiO₂-WO₃-V₂O₅ catalyst (89.7% of TiO₂, 10% of WO₃ and 0.3% of V₂O₅) of K/Ko = 0.50 was dipped into a 1 N H₂SO₄ aqueous solution for 5 minutes and was air dried one night (16 hours), followed by calcination at 450°C for 1 hour. As a result, the performance of the catalyst was recovered up to a level of K/Ko = 0.95. Moreover, the similar catalyst was dipped into a 0.1 N H₂SO₄ aqueous solution for 5 minutes and was air dried one night (16 hours), followed by calcination at 450°C for 1 hour. Thereby, the performance of the catalyst was recovered up to a level of K/Ko = 0.93. As be definite from the foregoing, when the deteriorated catalyst is impregnated or sprayed with the aqueous sulfuric acid solution and is then calcined, an extremely great recovery effect can be obtained. Thus, the catalyst can be repeatedly utilized by the recovery treatment, whereby a cost of the catalyst used in a denitrating apparatus can be decreased to a substantial degree. Example 3 A TiO₂-WO₃-V₂O₅ catalyst (91% of TiO₂, 8% of WO₃ and 1% of V₂O₅), in which a denitrating performance had deteriorated by being exposed in an exhaust gas from an A heavy oil-fired boiler for 16,000 hours, was dipped into and washed with water the volume of which was 5 times as much as that of the catalyst, at ordinary temperature for 60 minutes, and drying was then carried out at 110°C for 3 hours. The thus catalyst was further impregnated with a 1 (mol/l) aqueous ammonium hydrogensulfate solution and was then calcined at 350°C for 1 hour. A denitration performance was measured for the recovered catalyst under test conditions shown in Table 1. A relation between an elapsed time in the exhaust gas treatment and a denitration rate is represented by a solid line in Fig. 1. For comparison, the catalyst which had been recovered with the dip water washing but had not undergone the ammonium hydrogensulfate impregnation was also tested for a denitrating performance under the same conditions as in Table 1, and the test results are denoted by a dotted line in Fig. 1. The results in this drawing clearly indicate that the induction period can be eliminated by impregnating the catalyst with the aqueous ammonium hydrogensulfate solution regarding the present invention. Example 4 A TiO₂-WO₃-V₂O₅ catalyst (89.2% of TiO₂, 9.4% of WO₃ and 1.4% of V₂O₅) in which a denitrating performance had deteriorated was dipped into a 0.2 (mol/l) aqueous ammonium hydrogensulfate solution for 60 minutes the volume of which was 5 times as much as that of the catalyst, and drying was then carried out at 110°C for 3 hours, followed by calcination at 350°C for 1 hour. A denitration performance was measured for the thus recovered catalyst under test conditions shown in Table 2. A relation between an elapsed time in the exhaust gas treatment and a denitration rate is represented by a solid line in Fig. 2. For comparison, the catalyst which had been recovered with the dip water washing was also tested for a denitrating performance under the same conditions as in Table 2, and the test results are denoted by a dotted line in Fig. 2. This drawing makes it apparent that the dip recovery by the use of the aqueous ammonium hydrogensulfate solution regarding the present invention can shorten the induction period more noticeably as compared with a usual water washing recovery technique. Example 5 A TiO₂-WO₃-V₂O₅ catalyst (91% of TiO₂, 8% of WO₃ and 1% of V₂O₅), in which a denitrating performance had deteriorated by being exposed in an exhaust gas from an A heavy oil-fired boiler for 1,600 hours, was dipped into and washed with water the volume of which was 5 times as much as that of the catalyst, at ordinary temperature for 60 minutes, and drying was then carried out at 110°C for 3 hours. The thus catalyst was further impregnated with a 1 N aqueous sulfamic acid solution and was then calcined at 350°C for 1 hour. A denitration performance was measured for the recovered catalyst under test conditions shown in Table 2. A relation between an elapsed time in the exhaust gas treatment and a denitration rate is represented by a solid line in Fig. 3. For comparison, the catalyst which had been recovered with the dip washing but had not undergone the sulfamic acid impregnation was also tested for a denitrating performance under the same conditions as in Table 2, and the test results are denoted by a dotted line in Fig. 3. The results in this drawing clearly indicate that the induction period can be eliminated by impregnating the catalyst with the aqueous sulfamic acid solution regarding the present invention. Example 5 A TiO₂-WO₃-V₂O₅ catalyst (89.2% of TiO₂, 9.4% of WO₃ and 1.4% of V₂O₅) was dipped into a 0.2 (mol/l) aqueous sulfamic acid solution the volume of which was 5 times as much as that of the catalyst for 60 minutes, and drying was then carried out at 110°C for 3 hours, followed by calcination at 350°C for 1 hour. A denitration performance was measured for the recovered catalyst under test conditions shown in Table 2. A relation between an elapsed time in the exhaust gas treatment and a denitration rate is represented by a solid line in Fig. 4. For comparison, the catalyst which had been recovered with the dip water washing was also tested for a denitrating performance under the same conditions as in Table 2, and the test results are denoted by a dotted line in Fig. 4. The results in this drawing clearly indicate that the induction period can be shortened more noticeably by dipping the catalyst into the aqueous sulfamic acid solution regarding the present invention, as compared with the case where the recovery is carried out by the usual water washing technique.

Claims: A method for recovering deteriorated denitrating catalyst for ammonia catalytic reduction which is characterized by treating a TiO₂-containing denitrating catalyst for ammonia catalytic reduction with at least one agent selected from the group consisting of aqueous sulfuric acid, aqueous ammonium hydrogen sulfate, aqueous sulfamic acid and mixture thereof for sufficient time and calcining said catalyst at a temperature ranging from 300 °C to 650 °C to replenish said catalyst with a sulfur content. The method according to claim 1, characterized by treating said catalyst by impregnating or spraying with an aqueous sulfuric acid solution and then calcining said catalyst. The method according to claim 1, characterized by treating said catalyst by impregnating or spraying with an aqueous ammonium hydrogensulfate solution and then calcining said catalyst. The method according to claim 1, characterized by treating said catalyst by impregnating or spraying with an aqueous sulfamic acid solution and then calcining said catalyst.

Inventors: IIDA, KOZO C/O HIROSHIMA T.I., MITSUOKA, SHIGEAKI C/O HIROSHIMA T.I., NAGANO, TSUNEO C/O RYOMEI GIKEN K.K., OBAYASHI, YOSHIAKI C/O HIROSHIMA T.I., YOKOYAMA, NARUO C/O HIROSHIMA T.I.
Applicants: MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1985
Main classification: B01D53
IPC codes: B01D53, B01J23