Catalyst for the SCR Process

A range of different monolith-catalyst combinations exists to cope with the various sorts of fuel that can be used in a power plant, such as oil, coal, or biomass. 

which is a particular problem, is filtered out of the flue gas by electrostatic precipitators either before (low dust operation) or after the SCR reactor (high dust operation). 

The catalytically active material on the monolith also comes in many forms. Formulations based on iron, chromium, and vanadium as the active components supported on Ti02, AI2O3, Si02, and zeolites have been reported see the review by Bosch and Janssen [H. Bosch and F.J.J.G. Janssen, Catal. Today 2 (1988) 369]. 

More than 3000 units are required for a 300 MW power plant. [Adapted from N.-Y. Topsoe, CaTTech 1 (1997) 125.] 

The commonly used catalyst today is a vanadia on a titania support, which is resistant to the high SO2 content. Usually the titania is in the anatase form since it is easier to produce with large surface areas than the rutile form. Several poisons for the catalyst exist, e.g. arsenic and potassium. The latter is a major problem with biomass fuel. In particular, straw, a byproduct from grain production, seems to be an attractive biomass but contains potassium, which is very mobile at reaction tern- 

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Figure 11. Effect of catalyst pore structure on the NO reduction activity of a monolith catalyst for the SCR process. From Beeckman and Hegedus [32],...

In this paper we have employed the ion-exchange technique to prepare VO " " exchanged titanium phosphates as catalysts for the SCR process. The influence of precursor phase (crystalline or amorphous hydrogen titanium phosphate or half sodium exchanged titanium phosphate) was also investigated. 

Catalyst cost constitutes 15-20% of the capital cost of an SCR unit therefore, it is essential to operate at temperatures as high as possible to maximize space velocity and thus minimize catalyst volume. At the same time, it is necessary to minimize the rate of oxidation of S02 to S03, which is more temperature sensitive than the SCR reaction. The optimum operating temperature for the SCR process using titanium and vanadium oxide catalysts is about 38CM180oC. Most installations use an economizer bypass to provide flue gas to the reactors at the desired temperature during periods when flue gas temperatures are low, such as low-load operation. 

The monolithic supports typically have higher geometric surface areas than do packed beds this is specifically advantageous for the SCR process. Due to its high rates, the DeNO reaction suffers from strong intraporous diffusional limitations and is confined only to a thin outer layer of catalyst, so that the NOx reduction efficiency is controlled by the catalyst geometric area rather than by its volume. 

Various supported platinum group metal systems have been tested for the SCR process.101 Among them, supported platinum systems appear to be the most active when jointly considering the NOx reduction level achieved and the temperature range at which the catalyst is active, while palladium, rhodium and iridium also show catalytic activity for the process and Rh and Ir apparently present higher selectivity to N2.101>i03-i07 Support effects are observed which generally depend on the type of hydrocarbon employed, the presence or absence of SO2 in the reactant mixture or the type of impurities present in the support.101 In this respect, a variety of materials like SiC>2, AI2O3, ZrC>2, sulphated alumina, zeolitic materials and activated carbons have been employed as supports of the metals and tested for the process.101-112... 

Accordingly, the catalysts used for the SCR process should be highly selective, particularly with respect to SO2 oxidation. 

Several excellent reviews have discussed the SCR-NH3 process over metal oxides [6,7,30,34,38,41-43], in terms of both fundamental and technological aspects. Therefore, the discussion here is focused on some additional aspects which deserve attention and are useful for a better understanding of this field. Some background information necessary for the discussion is also included. There are three different classes of commercial catalysts for the SCR-NH3 process (noble metals, metal oxides, and zeolites), but only metal oxide catalysts will be discussed here. 

Understanding these aspects and the reaction mechanism will probably lead to the design development of a new generation of catalysts for the SCR-HC process. 

The SCR of NO with NH3 over Fe- and Cu zeolite-based monolithic catalysts provides another example of washcoat diffusion limitations [129]. Although this work specihcahy considers the main transport phenomena associated with monolithic catalysts for the SCR reaction, it may illustrate the transport processes that may potentially contribute to diffusion limitahons in structured monolithic reactors in other processes. These authors analyzed the characteristic times of hve diffusion processes and parameters that characterize the mass transport phenomena as a function of the temperature according to a model previously proposed by Bhatia et al. [ 132] ... 

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 a 1 1 mixture of NO and NO2 (i.e., NO2/NOx=0,5) is fed to the SCR reactor at low temperature (200 °C) where the thermodynamic equilibrium between NO and NO2 is severely constrained by kinetics, the NO2 conversion is much greater than (or nearly twice) the NO conversion for all three catalysts. This observation is consistent with the following parallel reactions of the SCR process [6] Reaction (2) is the dominant reaction due to its reaction rate much faster than the others, resulting in an equal conversion of NO and NO2. On the other hand, Reaction (3) is more favorable than Reaction (1), which leads to a greater additional NO2 conversion by Reaction (3) compared with the NO conversion by Reaction... 

Understanding the kinetics of the SCR process helps greatly in developing new and better catalysts. For efficient operation, one important issue is to maximize NO con-... 

Fig. IB shows that at all temperatures the rate of CH4 oxidation by O2 alone is lower than the rate of CH4 oxidation during the SCR reaction, e.g., at 400 C with CoZSM-5 catalyst the difference between these rates is about 10 times. With increasing temperature this difference diminishes due to the different activation energies of these reactions (Fig. 2). At high temperatures these rates become comparable (in considering Figs. IB and 2 recall that the rate of CH4 oxidation during the SCR process includes a contribution from the rate of CH4 oxidation by O2 alone). These data suggest that below 500 C O2 does not compete effectively with NO, for CH4, but that at high temperatures such a competition must exist. The data of Table I support this view. At ADO C an increase in 62 concentration results in an increase in conversions of both NO into N2 and CH4 into CO2. At the same time, variation of Oj concentration by a factor of 13 has practically no effect on the...

Montanari el al., for example, studied a Co—H-MFI sample through FT-IR spectroscopy of in situ adsorption and coadsorption of probe molecules [o-toluonitrile (oTN), CO and NO] and CH4-SCR process tests under IR operando conditions. The oTN adsorption and the oTN and NO coadsorption showed that both Co2+ and Co3+ species are present on the catalyst surface. Co3+ species are located inside the zeolitic channels while Co2+ ions are distributed both at the external and at the internal surfaces. The operando study showed the activity of Co3+ sites in the reaction. The existence of three parallel reactions, CH4-SCR, CH4 total oxidation and NO to NOz oxidation, was also confirmed. Isocyanate species and nitrate-like species appear to be intermediates of CH4-SCR and NO oxidation, respectively. A mechanism for CH4-SCR has been proposed. On the contrary, Co2+ substitutional sites, very evident and predominant in the catalyst, which are very hardly reducible, seemed not to play a key role in the SCR process [173],... 

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. 

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. 

SINOx [Siemens NO system] A process for removing nitrogen oxides and dioxins from the exhausts of stationary diesel engines and truck engines, based on the SCR process. The catalyst is based on titania and is in the form of a honeycomb. The reducing agent is ammonia, generated from an aqueous solution of urea. See SCR. 

Orsenigo et al. [47] have proposed an alternative reactor design suitable in principle to exploit NH3 inhibition for minimizing SO3 formation in the SCR process. This is based on the idea of splitting the NOx-containing feed stream in substreams fed separately to the SCR reactor in this way, a portion of the catalyst volume can operate with an excess of ammonia, while the overall NH3/NO feed ratio is still substoichiometric. 

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