A study on NOx removal performance of SCR catalystwashcoated on the metal foam substrate

Transcription

1 A study on NOx removal performance of SCR catalystwashcoated on the metal foam substrate Woo-Jin Na, Jong-Seong Bang, Min-Kee Jeon, Hea-Kyung Park* Research Institute of Technology, Hanseo University, Seosan, Chungnam, South Korea. Abstract This study has evaluated the performance of removing NOx and analyzed the properties of the catalyst prepared by washcoating on the metal foam substrate. BET, porosimeter and EDX have been applied to analyze the properties of the prepared catalyst. And also, catalytic performance test of de- NOx has been carried out in an atmospheric pressure reactor with changing the space velocities and temperatures by simulating of exhaust gas from coal fired power plant. The metal foam SCR catalyst has the merit of light-weight and extraordinary responsibility of reaction temperature. This metal foam catalyst made it possible to confirm that the performance of removing NOx is almost the same even when the required volume of it is decreased by one-third of that of the existing SCR catalyst. In addition, it was considered that the cost of purchasing and maintaining the SCR catalysts will be remarkably reduced because of the possibility of its remanufacturing of several times. In order to estimate the thermal deactivation and washcoat loss, excel deactivation experiment has been carried out in which BET, porosimeter and EDX showed that there were not considerable changes at surface component composition, TiO 2 crystal structure and specific surface area. Keywords: metal foam substrate, NOx removal, SCR catalyst Introduction Recently the issue of air pollution has been seriously acknowledged over the past 30 years around the world and in particular a lot of researches and technology developments have been carried out so as to solve the problem of air pollution by NOx which is one of serious air pollutants 1-6. Today, SCR (Selective Catalytic Reduction) technologies which have been commercialized for the removal of NOx are applied into NOx emission sources such as power plants and incineration plants all around the world The SCR catalyst is mostly composed of V 2 O 5 -WO 3 -TiO 2 and the types of commercial catalyst are extrusion type, plate type and corrugated type. The extrusion type has the shortcomings of being heavy and fragile, the plate type has that of large volume of the catalyst because of low space velocity and lastly the corrugated catalyst has the hazard of fire Furthermore, all of these catalysts have the biggest problem of difficulty in remanufacturing more than two times. In this study, a new method was attempted by selecting the metal foam as the catalyst substrate for manufacturing SCR catalyst by wash coating the SCR catalytic material on the metal foam substrate. Thin coating on the surface of metal foam substrate decreased amount of catalytic material. And this catalyst has such merits as easy handling and mounting on reactors and excellent temperature responsibility due to a light weight metal foam substrate. In particular, the possibility of being remanufactured many times is thought to lead to the large reduction of maintenance cost of the catalyst when it is applied to SCR systems in power plants or incineration plants 17,18. This 6835study is to prove these merits by preparing metal foam supported SCR catalyst, for which BET, porosimeter and EDX employed for the property analysis of the prepared catalyst and the atmospheric pressure reactor was used for the SCR performance test with the changes of space velocity and temperature at the simulated exhaust gas from coal fired power plant. Through the result of this study comparing of it with commercial extrusion SCR catalyst for coal fired power plant by employing this metal foam catalyst, it was confirmed that even the reduction of the volume for the conventional SCR catalyst by one third had the same performance of removing NOx. What is more, since the coating amount of the used SCR catalytic material is not much, much less amount of SCR catalytic material than any existing catalysts could be used and also because multi-times of remanufacturing is possible, it is thought that the cost of purchasing and maintaining SCR catalyst used at power plants and incineration plants will be reduced considerably. For the estimation of thermal deactivation and washcoat loss related to the life time of this catalyst, the experiment of excel deactivation has been executed, and such surface analysis devices as BET, porosimeter and EDX showed no changes of surface condition, surface components, TiO 2 crystal structure and specific surface area before and after excel deactivation along with nearly no change at the performance of NOx. Experimental procedure Preparation of metal foam supported SCR catalyst Silicasol (AS-40 manufactured by S-chemtech Co. Ltd. in Korea) was added to be 3% of the entire weight which was applied as binder with the composition ratio V 2 O 5 (5wt%) - WO 3 (5wt%) -TiO 2 (90wt%) of the SCR catalytic materials of TiO2 (DT-51 manufactured by Millenium Inorganic Chemicals Inc. in USA), AMV (ammonium meta vanadate, manufactured by Daejung Chemical Co. Ltd. in Korea), AMT (ammonium meta tungstate, manufactured by Hanbo Chemical Co. Ltd. in Korea) to the metal foam of Fe-Cr-Al alloy with the pore size of 3,000μm for manufacturing SCR catalyst used for this study. Solid % was adjusted to 40 by adding DI water to make wash coating slurry. This wash coating slurry was applied for wash coating of metal foam in 6835

2 the conventional dip-sipping method. And then it was dried at 120 for 24h and followed by calcination at 450 for 3h for preparing catalyst. Also, in order to test its durability such as washcoat loss and thermal shock, it went through excel deactivation in air at 700 for 4h, 6h and 10h, after processing it which results are shown in Table 1. Analyzer (Model 200EH manufactured by Teledyne technologies Co. in USA) were used. For precise activity test, stabilization was executed for a certain time in the reacting condition. Table 1: Prepared SCR catalyst supported on metal foam substrate notation excel deactivation dry coating weight to catalyst volume Fresh-catalyst - 4hrs at 700 6hrs at h-agedcatalyst 10hrs at 700 Characterization of prepared catalysts In order to estimate the property change of the prepared catalyst, various kinds of surface analysis devices were used. For checking the shape of catalyst surface, analysis of BET(ASAP 2400 manufactured by Micromeritics Co. in USA) was carried out by the adsorption of liquid nitrogen at 77K for the analysis of specific surface area of the catalyst. In addition, for the check of components of catalyst surface, the analysis of EDX (Energy dispersive X-ray spectrometer: Stereo scan 440/Link ISIS manufactured by Leica Cambridge Ltd. in UK) was marked by 4 points per catalytic sample and shown average value of four points. Catalytic activity measurement In this study, the experimental device for NO conversion reaction has been composed as seen in Fig.1. Catalytic activity for selective catalytic reduction (SCR) of NO by ammonia has been measured in a tubular, downflow, fixed-bed reactor operated under isothermal and at slightly above atmospheric pressure and inserted into an electric furnace. Also, the flow of all the gases used for this experiment was controlled with MFC (Mass Flow Controller: F-100C, manufactured by Bronkhorst Co. in UK) by the simulated exhaust gas from the commercial SCR catalyst reactor in power plants, when 300ppm (v/v) of NO gas (Union Gas Co %) was made to flow quantitatively along with NH 3 gas (Sungkang specialty gas Co. 99.9%) flowed in by MFC to be consistent with the concentration of NO. The concentration of O 2 gas (Sungkang specialty gas Co. 99.9%) was kept to be 3.6% (v/v) and that of SO 2 gas (Sungkang specialty gas Co. 99.9%) was set to be 150ppm and N 2 (Sung Kang specialty gas, Co. 99.9%) was balanced for the total flow to be kept. The temperature range of reactor was , which was controlled by PID controller. The space velocity was set to be 4,167 41,670h -1. For the analysis of reacting gas, Gas Analyzer (Greenline D max. II 9000 manufactured by Eurotron Inc. in UK) and Chemiluminescent NO/NOx Figure 1: Schematic diagram of catalyst performance test unit. Results and Discussion Results of space velocities of prepared catalyst In order to confirm if the catalyst supported on metal foam would decrease to be less than the volume of the commercial extruded SCR catalyst while maintaining the same SCR catalytic performance, the space velocities were made to be subject to the changes at the temperature range of , which results are shown in Fig. 2. The commercial SCR catalyst being used a lot in the power plant for the comparison of catalyst volumes was verified for its performance under this study condition. When the volume of metal foam catalyst was made to be half that of commercial catalyst (S.V. 8,334h -1 ), the activity of the catalyst was found to be very high at the entire range of temperature, and in particular, it was a lot higher at which is the temperature operated most at power plants. While it has a similar value at the temperature range of when the volume of metal foam catalyst was reduced to one third, it showed higher activity at other ranges. In case of reducing the catalyst volume to one-fifth (S.V. 20,835h -1 ), the activity was found to be 10% lower at , while at other ranges it was higher.even when reduced to one tenth, the performance was generally low but considerably high at lower and higher temperatures. It was, consequently, found that the volume of metal foal catalyst should be reduced to 1/3 so that its performance may be the same as that of any existing extruded SCR catalyst. 6836

3 Figure 2: NO conversion of prepared metal foam SCR catalysts with the variation of S.V. compared to commercial extrusion SCR catalyst Results of excel deactivation of prepared catalyst and its characterization In order to find out the thermal durability and washcoat loss of SCR catalyst supported on the metal foam 19, 20, after excel deactivation at 700 for 4h, 6h and 10h, activities of the catalyst at the three temperatures were tested, which is shown in Fig.3 and activity test of NO conversion performed at the space velocity of 20,835h -1 revealed that there was not a big difference in activities among the fresh catalyst before excel deactivation and those which underwent it. However, the NO conversion of the catalyst by excel deactivation time shows that the activity a little drops. Table 2 showing washcoat loadings, BET, total pore volume and average pore width indicate that the surface areas of the excel deactivated catalysts are a little lower than that of a new catalyst but that there are not any big differences in washcoat loadings, total pore volumes and average pore widths. Table 3 are showing the results of analyzing the surface components of catalysts by EDX, where there are not seen any big differences on the catalyst surfaces and also the surface components does not have any substantial change of the active component material of the catalyst such as Ti, V and W. The results of analyzing the characteristics of catalyst leads to the inference that there is not a big change to the activity of the catalyst in that there is neither serious change to physical specific surface area, pore volume and average pore size after excel deactivation nor substantial chemical change on catalyst surface and to the composition of active components of the catalyst in bulk. Figure 3: NO conversion of excel deactivated metal foam SCR catalysts Table 2: Results of BET & pore analysis of prepared catalysts Washcoat loadings (g/ 9cc of catalyst) BET surface area (m 2 /g) Total pore volume (single point adsorption, cm 3 /g) Adsorption average pore width (nm) Table 3: EDX analysis results of prepared catalysts Freshcatalyst 10hagedcatalyst Element Freshcatalyst 10h-agedcatalyst C O Al Si Ti V Cu Zn W Total Temperature responsibility of metal foam SCR catalyst When the SCR system in most power plants and incineration plants is started up, the reaction temperature of the catalyst in SCR systems become room temperature. Since it takes a lot of time for the temperature of the catalyst layer to be able to have the proper performance of the catalyst and the catalyst 6837

4 layer does not have an appropriate NO conversion reaction for this time, most of power plants and incineration plants can t help discharging the NO in the air, which can not be efficiently removed. The government, also recognizing it as the limit of technology, are not punishing any of them even though they exceed the allowable temperature for this time. In order to find out the responsibility of this metal foam catalyst, the temperature at the rear end of the catalyst was measured while the temperature of gas at the inlet of the reactor was kept to be 350. Fig. 4 shows the measurements and their comparisons with the extruded SCR catalyst. It was found that while the metal foam catalyst had the temperature of 250 from the beginning of gas input through the rear end of the reactor, which is compared with about 120 of the extruded SCR catalyst. Furthermore, it also was found that the time for the metal foam SCR catalyst to get the proper temperature of 350 at SCR reaction in power plants was about 12 minutes while it was almost 50 minutes for the commercial extruded SCR catalyst, which indicates that the responsibility to the temperature of this metal foam catalyst was almost four times faster than that of the extruded catalyst. This finding is thought to provide an opportunity for an appropriate disposal of plenty of NOx which is discharged in the air at the initial start up of SCR systems. which makes it have the same performance as that of any new catalyst. From this viewpoint, the metal foam SCR catalyst was remanufactured twice for performance test, which is shown in Fig. 5. The process of remanufacturing is the same as that of making a new catalyst. As seen in Fig. 5, all the performances of the initial fresh catalyst, the catalyst remanufactured once and that remanufactured twice are almost the same. It is natural for them to have the same performance because the catalytic material which is the same as that for a fresh material was coated with the same amount and in the same way. Figure 5: NO conversion of remanufactured metal foam SCR catalysts Figure 4: Comparison of temperature responsibility of metal foam catalyst with commercial extrusion catalyst Potentials of remanufacturing of metal foam SCR catalyst It is not avoidable to take account of the maintenance cost of SCR catalysts asconsumables in every power plant and incineration plant 7-9. Accordingly, most of the current power plants use remanufactured SCR catalysts (extrusion type, plate type and corrugated type) rather than purchase new ones 17,18,21,22.It, in reality, is very difficult to remanufacture these SCR catalysts more than two times. Fortunately, the metal foam SCR catalyst can be remanufactured more than ten times. What is better, when its life time is over, only the catalytic material, whenever needed, can be cleaned out for the metal foam to be coated again with catalytic material, Conclusion This study carried out the experiment by changing the space velocities to confirm if the SCR catalyst supported on the metal foam would come to have less volume than that of commercial extruded SCR catalyst while maintaining the same SCR catalyst performance, which resulted in the reduction of the volume to one third. And, when the responsibility of the metal foam SCR catalyst to temperature was tested, it was found that the temperature responsibility of this metal foam SCR catalyst was four times faster than that of the extruded catalyst, which will provide the opportunity for proper disposal of the large amount of NOx discharged at the initial operation of SCR system. Moreover, since the metal foam SCR catalyst has the same performance as that of any fresh ones when it is remanufactured even multiple times, it will make a great contribution to the economy of SCR catalytic performance. For the estimation of thermal durability and washcoat loss of SCR catalyst supported on the metal foam, after excel deactivation was attempted at 700 for 4h, 6h and 10h. And then, chemical analysis to find out the compositions on the catalyst surface and in catalyst bulk using EDX and physical analysis to characterize specific surface area and pore characteristics of each catalyst by BET revealed that there was not any big difference before and after the excel deactivation and therefore the activity of the catalyst did not have any 6838

5 remarkable change. Since this is just an initial phase of researching metal foam SCR catalyst, a lot more researches will have to be carried out relating to the activity depending on the coating thickness of catalyst material and the characterization of the catalyst itself. Acknowledgements This work was supported by the Global Excellent Technology Innovation of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No ) References [1] Vargas M. A.L., Casanova M., Trovarelli A., and Busca G., 2007, An IR study of thermally stable V 2 O 5 -WO 3 -TiO 2 SCR catalysts modified with silica and rare-earths(ce, Tb, Er), Appl.Catal.B: Environ, 75, pp [2] Lietti L., Nova I., Ramis G., Busca G., Giamello E., Forzatti P., and Bregani F., 1999, Characterization and reactivity of V 2 O 5 -MoO 3 /TiO 2 De-NOx SCR catalysts, J. Catal.,187(2), pp [3] Kim M. Y., Lee K. W., Park J. H., Shin C. H., Lee J. and Seo G., 2010, Catalytic decomposition of nitrous oxide over Fe-BEA zeolites: Essential components of iron active sites, Korean J. Chem. Eng., 27(1), pp [4] Seo P. W., Kim S. S., and Hong S. C., 2010, A study of the increase in selective catalytic reduction (SCR) activity of the V/TiO 2 catalyst due to the addition of monoethanolamine (MEA), Korean J. Chem. Eng., 27(4), pp [5] Zhang X., Shen B., Wang K., and Chen J., 2013, A contrastive study of the introduction of cobalt as a modifier for active component and supports of catalysts for NH 3 -SCR, J. Ind. Eng. Chem., 19(4), pp [6] Koh H. L., and Park H. K., 2013, Characterization of MoO 3 -V 2 O 5 /Al 2 O 3 catalysts for selective catalytic reduction of NO by NH 3, J. Ind. Eng. Chem., 19(1), pp [7] Nah W. J., and Park H. K., 2012, A study on remanufacturing of deactivated commercial DPF using diesel engine dynamo system in ND-13 mode, J. Ind. Eng. Chem., 18(4), pp [8] Hong S. C., 2006, The effect of SO 2 in flue gas on the SCR activity of V/TiO 2, J. Korean Ind. Eng. Chem., 17(5), pp [9] Park H. K., Study on the regeneration effects of commercial V 2 O 5 -WO 3 /TiO 2 SCR catalyst for the reduction of NOx, J. KSEE, 27(8), pp [10] Lee S. J., and Hong S. C., 2008, Characterization of V/TiO 2 catalysts for selective reduction, J. Korean Ind. Eng. Chem., 19(5), pp [11] Nakajima F., and Hamada I., 1996, The state-of-theart technology of NOx control, Catal. Today, 29, pp [12] Sun S., Zhang J., Hu X., Qiu P., Qian J. and Qin Y., 2009, Kinetic analysis of NO-Char reaction, Korean J. Chem. Eng., 26(2), pp [13] Kim M. H., 2013, Formation of N 2 O in NH 3 -SCR De-NOx reaction with V 2 O 5 /TiO 2 -based catalysts for fossil fuels-fired power stations, Korean Chem. Eng. Res., 51(2), pp [14] Ryu S. H., 2011, Recycling of the Waste TiO 2 - WO 3 -V 2 O 5 system Honeycomb type SCR for De-NOx, Ph.D. Thesis, Gyeongsang National University, Korea. [15] Kobayashi M., and Miyoshi K., 2007, WO 3 -TiO 2 monolithic catalysts for high temperature SCR of NO by NH 3 : Influence of preparation method on structural and physic-chemical properties, activity and durability, Appl. Catal. B: Environ.,72, pp [16] Blanco J., Avila P., Suares S., Yates M., Martin J. A., Marzo L., and Knapp C., 2004, CuO/NiO monolithic catalysts for NOx removal from nitric acid plant flue gas, J. Chem. Eng., 97(1), pp [17] Yoon G. K., Yoo M. S., Lim J. S., Kim T. W., and Park H. K., 2005, Remanufacturing of Commercial V 2 O 5 -WO 3 /TiO 2 used in the SCR Process of Incinerator, J. KSEE, 27(9), pp [18] Park H. K., Jun M. K., and Kim M C., 2012, A Study on the In situ Regeneration Effects of Commercial Deactivated SCR,J. KSEE, 34(10), pp [19] Madia G., Elsener M., Koebel M., Raimondi F., and Wokaun A., 2002, Thermal stability of vanadiatungsta-titania catalysts in the SCR process, Appl. Catal. B: Environ.,39(2), pp [20] IngemarOdenbrand C. U., 2008, Thermal stability of vanadia SCR catalysts for the use in diesel applications, Chem. Eng. Research & Design, 86(7), pp [21] Choi K. Y., and Park H. K., 2010, Study on the Effectiveness of Remanufacturing Technology for the Catalyzed Diesel Particulate Filter-trap(DPF) Deactivated by Diesel Exhaust Gas, J. KSEE, 32(10), pp [22] Raziyeh K., and Ingemar C. U., 2001, Regeneration of commercial SCR catalysts by washing and sulphation: effect of sulphate groups on the activity, Appl. Catal. B: Environ.,33(4), pp