SCR Catalysts

Performance criteria for SCR are analogous to those for other catalytic oxidation systems NO conversion, pressure drop, catalyst/system life, cost, and minimum SO2 oxidations to SO. An optimum SCR catalyst is one that meets both the pressure drop and NO conversion targets with the minimum catalyst volume. Because of the interrelationship between cell density, pressure drop, and catalyst volume, a wide range of optional catalyst cell densities are needed for optimizing SCR system performance. 

Reactions. The SCR process is termed selective because the ammonia reacts selectively with NO at temperatures >232° C in the presence of excess oxygen (44). The optimum temperature range for the SCR catalyst is determined by balancing the needs of the redox reactions. 

When sulfur dioxide is also present there are important side reactions in which SO2 is oxidized to SO. The main side reaction in the SCR catalyst is the conversion of SO2 to SO, thus faciUtating the reaction above. The SO in turn reacts with ammonia to form ammonium sulfates. 

The ACR Process. The first step in the SCR reaction is the adsorption of the ammonia on the catalyst. SCR catalysts can adsorb considerable amounts of ammonia (45). However, the adsorption must be selective and high enough to yield reasonable cycle times for typical industrial catalyst loadings, ie, uptakes in excess of 0.1% by weight. The rate of adsorption must be comparable to the rate of reaction to ensure that suitable fronts are formed. The rate of desorption must be slow. Ideally the adsorption isotherm is rectangular. For optimum performance, the reaction must be irreversible and free of side reactions. 

Types ofSCT Catalysts. The catalysts used in the SCR were initially formed into spherical shapes that were placed either in fixed-bed reactors for clean gas apphcations or moving-bed reactors where dust was present. The moving-bed reactors added complexity to the design and in some appHcations resulted in unacceptable catalyst abrasion. As of 1993 most SCR catalysts are either supported on a ceramic or metallic honeycomb or are direcdy extmded as a honeycomb (1). A typical honeycomb block has face dimensions of 150 by 150 mm and can be as long as one meter. The number of cells per block varies from 20 by 20 up to 45 by 45 (39). 

No SCR catalyst can operate economically over the whole temperature range possible for combustion systems. As a result, three general classes of catalysts have evolved for commercial SCR systems (44) precious-metal catalysts for operation at low temperatures, base metals for operation at medium temperatures, and 2eohtes for operation at higher temperatures. 

The most popular SCR catalyst formulations are those that were developed in Japan in the late 1970s comprised of base metal oxides such as vanadium pentoxide [1314-62-1J, V20, supported on titanium dioxide [13463-67-7] Ti02 (1). As for low temperature catalysts, NO conversion rises with increasing temperatures to a plateau and then falls as ammonia oxidation begins to dominate the SCR reaction. However, peak conversion occurs in the temperature range between 300 and 450°C, and the fah-off in NO conversion is more gradual than for low temperature catalysis (44). 

R. Craig, G. Robinson, and P. Hatfield, "Performance of High Temperature SCR Catalyst System at Unocal Science and Technology Division," paper 92-109-08, in Ref. 15. 

Morita, I., Ogasahara, T., and Franklin, H.N. (2002) Recent Experience with Hitachi Plate Type SCR Catalyst, The Institute of Clean Air Companies Fomm 02, Febmary 12-13. 

In general, NO and NO2 are mutually beneficial for NOx reduction over the SCR catalysts tested. That is, the presence of NO enhances the NO2 conversion, and vice versa. This results in the synergistic effects of NO and NO2 in the catalytic reduction of NOx with NH3 over CuZSMS, FeZSMS and V20s/Ti02 catalysts. 

As with the automotive exhaust converter, the SCR catalyst is designed to handle large flows of gas (e.g. 300 N s for a 300 MW power plant) without causing a significant pressure drop. Figure 10.12 shows a reactor arrangement with about 250 m of catalyst in monolithic form, sufficient for a 300 MW power plant. 

The SCR catalyst is considerably more complex than, for example, the metal catalysts we discussed earlier. Also, it is very difficult to perform surface science studies on these oxide surfaces. The nature of the active sites in the SCR catalyst has been probed by temperature-programmed desorption of NO and NH3 and by in situ infrared studies. This has led to a set of kinetic parameters (Tab. 10.7) that can describe NO conversion and NH3 slip (Fig. 10.16). The model gives a good fit to the experimental data over a wide range, is based on the physical reality of the SCR catalyst and its interactions with the reacting gases and is, therefore, preferable to a simple power rate law in which catalysis happens in a black box . Nevertheless, several questions remain unanswered, such as what are the elementary steps and what do the active site looks like on the atomic scale ... 

For the abatement of NO with NH3 in the absence of O2, ZV catalysts give low, but stable activity. The activity is strongly enhanced in the presence of O2- This is a common feature to all SCR catalysts, including the ZSM-5 based system [30]. In the presence of O2. our results show the formation of bidentate nitrates and, on NH3 addition, their transformation into chelating nitrates. Our results also show that the... 

Therefore, the claim of new more active catalysts is correct only when all these aspects are considered, but it should also be taken into account that there are not main motivations and incentives from the commercial point of view to develop new NHj-SCR catalysts. However, some specific cases may be interesting to develop new catalysts, especially when considering different characteristics of the feed and the reaction temperature of operations. 

Analysis of the dynamics of SCR catalysts is also very important. It has been shown that surface heterogeneity must be considered to describe transient kinetics of NH3 adsorption-desorption and that the rate of NO conversion does not depend on the ammonia surface coverage above a critical value [79], There is probably a reservoir of adsorbed species which may migrate during the catalytic reaction to the active vanadium sites. It was also noted in these studies that ammonia desorption is a much slower process than ammonia adsorption, the rate of the latter being comparable to that of the surface reaction. In the S02 oxidation on the same catalysts, it was also noted in transient experiments [80] that the build up/depletion of sulphates at the catalyst surface is rate controlling in S02 oxidation. 

Low-temperature activity promotion is an issue in mobile (diesel) applications, but may not be a critical issue in several stationary applications, apart from those where the temperature of the emissions to be treated is below 200°C (for example, when a retrofitting SCR process must be located downstream from secondary exchangers, or in the tail gas of expanders in a nitric acid plant). In the latter cases, a plasmacatalytic process [91] could be interesting. In the other cases, the use of NTP together with the SCR catalyst is not economically viable. However, the synergetic combination of plasma and catalysts has been shown to significantly promote the conversion of hazardous chemicals such as dioxins [92], Although this field has not yet been explored, it may be considered as a new plasmacatalytic SCR process for the combined elimination of NO, CO and dioxins in the emissions from incinerators. 

Larrubia, M.A., Ramis, G. and Busca, G. (2000) An FT-IR study of the adsorption of urea and ammonia over V205-Mo03-Ti02 SCR catalysts, Appl. Catal. B, 27, L145. 

To have a mixture N0 + N02 at the SCR catalyst intake, the presence of a DOC upstream of the system is required (oxidation of NO into N02). The DOC must be sized according to the optimal ratio NO/N02 = 1. 

Another characteristic of this solution is its proneness to crystallization and polymerization. When parts of the exhaust system are constantly welted by Adblue on the same spot, undesired urea crystals or polymers may form if the exhaust line temperature is lower than 300°C. This phenomenon will result in uncontrolled ammonia production when the crystals or polymers melt or sublimate after being heated at significantly higher temperatures (T > 350°C). This may result in ammonia release. Furthermore, the crystals or polymers can also have an impact on the SCR catalyst cells by reducing the catalyst surface and thus reducing the catalyst performances. 

The ammonia necessary for the reaction is the main hindrance to the SCR-NH3 process because pure ammonia is an irritating and toxic gas which cannot be released in the exhaust line. Particular care must be taken to ensure that the maximal NH3 content released in the exhaust does not exceed the threshold of 10 ppm. NH3 release in the exhaust line can be prevented by keeping the overall urea/NOx ratio significantly below stoichiometry or by installing an NH3 clean-up catalyst behind the SCR catalyst. 

When speaking about SCR system, one should keep in mind that the SCR catalyst is not the single component and that several elements are necessary for its correct operation. The complete system includes (see Figure 7.15) ... 

Mixing device integration -> Homogeneous composition at the SCR catalyst inlet... 

SCR systems at stationary diesel engines profit from the high exhaust gas temperatures of about 350-400 C, caused by the usually constant high load operation conditions of the diesel engine. In this temperature window nearly all known SCR catalysts are very active. Moreover, weight and size of the exhaust gas catalyst are usually not strictly limited, which results in a good NO, reduction efficiency (DeNOJ. However, DeNO, is not the only criterion for an SCR catalyst. Further requirements are excellent selectivities regarding NO and urea/ammonia as well as low ammonia slip, which is an undesired secondary emission of the SCR process. Therefore, all SCR catalysts exhibit surface acidity, which is necessary to store ammonia on the catalyst surface and, thus, to prevent ammonia slip. 

For reasons of safety and toxicity, urea is the preferred selective reducing agent for mobile SCR applications. Under the hydrothermal conditions in the exhaust system, urea decomposes to ammonia which reduces the nitrogen oxides on the surface of the SCR catalyst [18,19], If urea is used instead of ammonia, the DeNO chemistry involves isocyanic acid as an important intermediate which will lead to a complication of the SCR chemistry [20],... 

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