Selective catalytic reduction deNOx

Figure 7-14 Large duct static mixer (SMV) for application in mixing ammonia with hot flue gas selective catalytic reduction DeNox process. (Courtesy of Koch-Glitsch, LP.)...

The second DeNOx technology, the selective catalytic reduction with ammonia (SCR-NH3) commercially available in heavy-duty vehicles since 2006, seems to present an interesting potential in terms of efficiency, reliability, HC penalties, etc. 

Koebel, M. and Elsener, M. (1998) Selective Catalytic Reduction of NO over Commercial DeNOx-Catalysts Comparison of the Measured and Calculated Performance, Ind. Eng. Chem. Res., 37, 327. 

The selective catalytic reduction (SCR) of NO by NH3 using DENOX catalysts is the best developed and most widely used method for treating flue gases of thermic power plants in order to reduce the emission of toxic compoimds. The efficiency, selectivity, and economics of the method are favorable. It was developed in Japan. 

The aim of this paper is to understand the influence of zinc on platinum catalytic behaviour. The added metal can either deactivate or provoke an increase in the catalytic activity of platinum either for reforming reactions or depollution reactions respectively, even when the gas atmosphere is always reductive. We shall study the influence -i) of the mode of preparation, -ii) of the zinc loading and -iii) of the kinetic parameters, on the activity of S-[Pt-Zn] catalysts in DeNOx reactions.The catalysts have been characterised by TPR, chemisorption and EXAFS and tested in the reaction of selective catalytic reduction (SCR) using diesel conditions. 

In a related set of catalysts a loading of 1% vanadium(v) oxide on titanium dioxide is used. These eatalysts are applied in selective catalytic reduction (SCR) reactions, i.e. the removal of NOx through selective reaction with ammonia from streams eontaining large volumes of o g gen. These eatalysts require the redox aetivity as diseussed above, but also an acidic functionality (to activate the seleetive reduetant (ammonia)). It transpires that the produet of reduetion of the surface V=0 groups (V-OH) is sufficiently acidie for this purpose. This means these V=0 species oriented into the reaction medium beeause of their interaction with Ti02 can act as both redox and acidic surface sites. The mechanism of the deNOx reaction has been extensively studied over these materials. ... 

Secondly, the product of unselective NH oxidation reaction may itself be NOx (indeed, the oxidation of NH to NO is the first step in the industrial preparation of HNOj). Therefore, an unselective reaction renders the entire selective catalytic reduction (SCR)-NHj deNOx process an expensive method of increasing the NOx concentration of the emitted gas, NH.+ O. N.ORNO. 

The Cu-exchanged zeolite ZSM-5 revealed a particularly high and steady activity for the direct catalytic conversion of NO into N2 and O2 ( DeNOx activity) [8612]. The same system is also active for the selective catalytic reduction of NO by hydrocarbons in the presence of excess oxygen [95 S2, 96T1 ]. 

Reaction (9.12) involves the selective catalytic reduction of NO by its reaction with ammonia and with nitrate species, supplied, e.g., in the form of aqueous solution of ammonium nitrate sprayed into the gaseous feed stream to the SCR catalyst. Reaction (9.12) results in very high DeNO efficiencies at low temperatures, similar to those of Fast SCR, even though no NO2 is fed to the SCR catalyst. Accordingly, it can in principle replace reaction (9.6) in boosting the DeNOx... 

Currently, the major deNOx after-treatment technologies under consideration include the so-called Lean-NOx Traps (LNT), which are used with direct injection gasoline and Diesel engines, and the Selective Catalytic Reduction (urea-SCR) process. 

DeNOxing of waste gases from stationary sources can be achieved efficiently by using the so-called SCR process, i.e. the selective catalytic reduction using ammonia as the reductant ... 

Figure 11.2. DeNOX catalyst for use in SCR (Selective Catalytic Reduction) plants. Reproduced with permission from Haldor Topsoe A/S.

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