Selective catalytic reduction ranges

A wide range of catalytic materials have been investigated for the selective catalytic reduction of NOx. For stationary emissions, NH3-SCR using vanadium-tungsten oxides supported on titania is the most used method however, when there is a simultaneous emission of NO and NOz (in tail gas from nitric acid plants), copper-based zeolites or analogous systems have been proven to be preferable [31b], In fact, there are two main reactions for NH3-SCR ... 

The second process is known as selective catalytic reduction (SCR). SCR may also be used in the treatment of nitric acid process tail gas and similar processes, but has achieved prominence through its application to NO removal from electricity-generating power stations, especially those that are coal fired. In SCR, a range of reductants for the NOx can be used the most common is ammonia. The primary reactions involved arc shown in Eqs. (13) and (14). Oxygen is required for this form of NO control, and levels of 2-3% are typically needed for optimum catalyst performance. 

Metastable species derived from (L)Cu(02), where L- = 2,4-di-te/V-butyl-phenolate linked to l,4-di-z, so-propyl-l,4,7-triazacyclononane, show multiple vCuO bands in the wavenumber range 500-550 cm-1.389 The resonance Raman spectrum of [Cu2(p-0)2(flf4-Me2-etpy)2]2+ has a vCu-O-Cu band at 579 cm-1 (551 cm-1 for lsO).390 Selective catalytic reduction of NO by NH3/02 on copper-faujasite catalysts gave rise to IR bands showing changes in copper oxidation states (using the vas[Cu-0-Cu]+ band near 900 cm-1).391... 

In the case of the selective catalytic reduction using ammonia as reductant but in excess 02, Ce-exchanged sodium-type mordenite (CeNa-MOR) has been reported as an active catalyst in the 250-560°C temperature range with respect to non-redox La,Na-and H-mordenite catalysts (Ito et al. 1994). In this case, the reduction of nitric oxide is thought to proceed with crucial involvement of a Ce3+/Ce4+ redox couple, although the intermediate reaction pathway depends on the reaction temperature. 

Selective non catalytic reduction (SNCR) with NH3 is limited to industrial boilers in consequence of the relatively narrow temperature range for the reaction. Selective catalytic reduction (SCR) by ammonia has high efficiency and it can be used for many stationary sources, especially for nitric acid plants [1], and it is based on the catalytic pairing of nitrogen atoms, one fi-om nitric oxide, one fi om ammonia. This method, however, is imsuitable for small sources and vehicles. As far as automotive emission is concerned nonselective catalytic reduction (NSCR) by hydrocarbons, CO and H2 from the exhaust stream has been reported over various catalysts recently [1,3,4]. 

Some methods used to clean off gases from incinerators employ catalytic bag filters with the injection of PAG or selective catalytic reduction (for NO removal) in a combination with catalyzed oxidation of the dioxins [ISO]. The use of activated carbon packed in adsorption units or sprayed into flue gas streams together with lime, has become a standard in gas cleaning liom dioxins on all sizes of plants. Typically, the treatment with activated carbon maintains the dioxin emission range below 0.1 ng m [181]. 

Selective Catalytic Reduction (SCR) using ammonia as the reductant provides NOx reduction levels of greater than 80%. Three types of catalyst systems have been deployed commercially noble metal, base metal and zeolites. Noble metals are typically washcoated on inert ceramic or metal monoliths and used for particulate-free, low sulfur exhausts. They function at the lower end of the SCR temperature range (460-520°F) and are susceptible to inhibition by SOx [14]. Base metal vanadia-titania catalysts may either be washcoated or extruded into honeycombs [11]. Typically washcoated catalysts are only used for treating particulate-free, clean gas exhausts. Extruded monoliths are used in particulate-laden coal and oil-fired applications. The temperature window for these catalysts is 600-750°F. Zeolites may also be washcoated or extruded into honeycombs. They function at relatively high temperatures of 650-940°F [15]. Zeolites may be loaded with metal cations (such as Fe, Cu) to broaden the temperature window [16]. 

NOx (typically 1-3%), present in the stack gas of nuclear waste process plants, is removed in die WINCO Process by two primary reactors to 300-1000 ppm by selective catalytic reduction (SCR) with NH3 over a commercial zeolite catalyst at 300-500°C followed by reduction to low ppm levels in a third cleanup reactor. This study involved laboratory tests on advanced SCR zeolite catalysts, NC-301, ZNX, and Cu-ZSM-5, for the primary SCR reactors over a range of anticipated process conditions using gas mixtures containing 500-5000 ppm NO+NO2, 500-5000 ppm NH3, 1-2% CO, 14% O2, and 20% steam in He. All three catalysts have acceptable levels of performance, i.e. selectively reduce >80% the NOx with NH3 to N2 over the temperature range of 400-500°C at a space velocity of 30,000 h- The Cu-ZSM-5 catalyst is the most active and selective catalyst converting >95% NOx d NH3 (at 500 - 5000 ppm of each) to N2. 

Although the main applications of zeohtic sohds in catalysis will continue to be as solid acids in the synthesis and transformations of petrochemicals and commodity chemicals they continue to be considered as catalysts and catalyst supports for a range of reactions of synthetic and industrial relevance. The most important of these are of titanium- and tin-containing solids in selective oxidations. Other well-studied reactions over zeohtes include light hydrocar-bons-to-aromatics (Ga-zeolites) selective catalytic reduction of NO (transition metal exchanged zeolites) C C bond formation (Pd zeohtes) selective alkane oxyfunctionalisation with air (MAPOs, M Mn, Fe, Co) and chiral catalysis over encapsulated chiral complexes. 

Honeycomb shape used in monolithic catalysts has become the standard catalyst shape in many environmental applications, such as automotive catalytic converter and DeNO catalyst for the selective catalytic reduction of NO. Monolithic catalysts consist of parallel channels with diameter in the millimeter range. They are characterized by low pressure drop, which is roughly two orders of magnitude... 

Chen L, Li J, Ablikim W, Wang J, Chang H, Ma L, Xu J, Ge M, Arandiyan H (2011) Ce02-WO3 Mixed Oxides for the Selective Catalytic Reduction of NO by NH3 Over a Wide Temperature Range. Catal Lett 141 (12) 1859-1864... 

The selective catalytic reduction of NOx by ammonia on Fe-zeolite catalysts displays interesting performance trends over a range of temperatures and NO/NO2 feed ratios. Feeds containing various NOa/NOx inlet ratios (0-1) provide insight into the effect of NO2 which can be appreciable on Fe-exchanged catalysts. 

Trace metal concentrations in herbaceous biomass materials are the final area of concern, particularly with the emphasis on mercury emissions management and the possible concern for selective catalytic reduction catalyst deactivation or poisoning as a result of cofiring straws and other herbaceous biomass fuels [1,17]. The database ftM" herbaceous materials is not extensive. Table 5.10 presents a general range of values. 

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