Non-Selective Catalytic Reduction

Platinum, palladium and rhodium (either in pellet form or as a honeycomb) are typically used as catalysts in these systems. The minimum temperature for inlet tail gas depends on the fuel and its ignition temperature. Hydrogen, which has the lowest ignition temperature, requires about 150 to 200PC104. 

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Rather than selective non-catalytic reduction, the reduction can be carried out over a catalyst (e.g. zeolite) at 150 to 450 C. This is known as selective catalytic reduction. Figure 25.31 shows a typical selective catalytic reduction arrangement10. Either anhydrous or aqueous ammonia can be used. This is mixed with air and injected into the flue gas stream upstream of the catalyst. Removal efficiency of up to 95% is possible. Again, slippage of excess ammonia needs to be controlled. 

The oxidation of NO to N02 can be considered an advantage when combining plasma with selective catalytic reduction, as it is well-known that at low temperature the efficiency of NO reduction by SCR strongly depends on the N02 concentration in the gas. An oxidation pre-treatment of the gas by non-thermal plasma, prior to the SCR reactor greatly enhances the performance of SCR at temperatures below 500 K. Therefore, the PE-SCR technique is very promising for efficient NO reduction. 

Broer, S. and Hammer, T. (2000) Selective catalytic reduction of nitrogen oxides by combining a non-thermal plasma and a V205-W03/Ti02 catalyst, Appl. Catal. B Env. 28, 101-11. 

At present the most effective available after-treatment techniques for NO, removal under lean conditions are ammonia selective catalytic reduction (SCR) [1-3] and NO, storage reduction (NSR) [4—6]. Indeed, three-way catalysts (TWCs) are not able to reduce NO, in the presence of excess oxygen, because they must be operated at air/ fuel ratios close to the stoichiometric value. Also, non-thermal plasma (NTP) and hydrocarbon-selective catalytic reduction (HC-SCR) are considered, although they are still far from practical applications. 

The primary pollution problem in nitric acid plants is the abatement of NOx in tail gases. Three options exist to reduce these emissions to acceptable levels 1) Capture the NOx and convert it to additional nitric acid, 2) Capture the NOx and convert it to nitrate-nitrite sales, or 3) Render the NOx harmless by converting it to non-polluting compounds. The processes that have been developed to reduce emissions at existing and new plants can be classified into four general categories Absorption, Adsorption, Selective Catalytic Reduction (SCR) and Non-Selective Catalytic Reduction91. 

Van Swaaij and coworkers [5,6] proposed the use of nonpermselective macroporous membranes for gas phase reactions, which by their nature require strict stoichiometric conditions. Such reactions include selective catalytic reduction of NO by NH3, and the Clauss reaction. The principle for the CNMR operation is shown in Fig. 11.7. By creating a sharp reaction front within the membrane one avoids the slip by either reactant (NH3 or NO, SO2 or H2S) on either side of the membrane. Small perturbations in the feed could be accommodated in principle by a shift in the reaction front within the membrane. The success of the CNMR concept depends, as one would expect, on the sharpness of the reaction front created within the membrane. For a non-instantaneous reaction the front created is rather diffuse (see Fig. 11.7) and there is, as a result, a reactant slip. 

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]. 

A synergistic effect leading to the increased catalyst activity and selectivity in selective catalytic reduction (SCR) of NO with methane or propane-butane mixtures was found when cobalt, calcium and lanthanum cations were introduced into the protic MFl-type zeolite. This non-additive increase of the zeolite activity is attributed to increased concentration of the Bronsted acid sites and their defined location as result of interaction between those and cations (Co, Ca, La). Activation of the hydrocarbon reductant occurs at these centers. Doping the H-forms of zeolites (pentasils and mordenites) with alkaline earth metal and Mg cations considerably increased the activity of these catalysts and their stability to sulfur oxides. 

The most important secondary measures include the effective selective catalytic reduction (SCR) and the less effective selective non-catalytic reduction (SNCR). The major drawbacks with SCR are the high capital and operating costs, the large bulky catalytic reactor, NOx/NH3 analytical equipment as well as an ammonia or urea storage and distribution system. SNCR on the other hand has a lower capital cost, but is less effective in NOx reduction. 

Titania-supported vanadia catalysts have been widely used in the selective catalytic reduction (SCR) of nitric oxide by ammonia (1, 2). In an attempt to improve the catalytic performance, many researchers in recent years have used different preparation methods to examine the structure-activity relationship in this system. For example, Ozkan et al (3) used different temperature-programmed methods to obtain vanadia particles exposing different crystal planes to study the effect of crystal morphology. Nickl et al (4) deposited vanadia on titania by the vapor deposition of vanadyl alkoxide instead of the conventional impregnation technique. Other workers have focused on the synthesis of titania by alternative methods in attempts to increase the surface area or improve its porosity. Ciambelli et al (5) used laser-activated pyrolysis to produce non-porous titania powders in the anatase phase with high specific surface area and uniform particle size. Solar et al have stabilized titania by depositing it onto silica (6). In fact, the new SCR catalyst developed by W. R. Grace Co.-Conn., SYNOX , is based on a titania/silica support (7). 

In 2005, the EPA passed the Clean Air Interstate Rule, which requires a 61% cut in nitrogen oxide emissions from power plants by 2015. This level of emissions reduction requires a different technology. Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) both convert NOx into water (H2O) and nitrogen (N2). SCR is capable of reducing NOx emissions by approximately 90%. SNCR is a simpler and less expensive technology than SCR, but it also provides a lower level of NOx reduction. 

Sloot, H., Versteeg, G., Smolders, C., et al. (1991). A Non-Permselective Membrane Reactor for the Selective Catalytic Reduction of NOx with Ammonia, in Proc. 2nd Int. Conf. on Inorganic Membranes, Montpellier (France), Trans. Tech. Publ., Zuerich (Switzerland). 

Selective Non-Catalytic Reduction (SNCR) Processes, 888 Selective Catalytic Reduction (SCR) Processes, 904 Combined N0x/S02 Post-Combustion Processes, 928... 

Note BACT stands for Best Available Control Technology. LAER for Lowest Achievable Emission Rate. NSPSfor New Source Performance Standard, PSD for Prevention of Significant Deterioration. SCR far Selective Catalytic Reduction. SIP far State Implementation Plan. SNCR for Selective Non-Catalytic Reduction, and WEPCOfor Wisconsin Electric Power Company. 

SCAQMD is the only non-attainment area in the U.S. Virtually all boilers in the SCAQMD are affected and must reduce NO, by an average of 90%. Selective Catalytic Reduction (SCR) has already been retrofitted on over 4,000 MW of boiler capacity in the SCAQMD. In the Northeast. NESCAUM (Northeast States for Coordinated Air Use Management), which is made up of representatives from eight northeastern states Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, and Vermont, was formed to solve the non-attainment problems of this ozone transport area and has proposed Reasonably Available Control Technology (RACT) values and other rules stricter than those of the EPA, including SCR for NO control. 

The method results in less NO reduction than selective catalytic reduction, although higher consumption of chemicals is required. There are now eight commercial installations (1700 MWe) in operation in the FRG and Austria. The first circulating fluidised bed boiler to be equipped with selective non-catalytic reduction began operating in the USA in 1988. This has been followed by other similar installations in the USA. Selective non catalytic reduction is expected to reduce NO by 30-50%. Higher reduction, up to 70-80%, is supposed to be attainable under favourable conditions. 

Many other examples are known of non-selective reactions of halo groups in pyridopyridazines with amines, alkoxides, sulfur nucleophiles such as hydrosulfide and thiolate ions, or thiourea, hydrazine(s), cyanide ion and dimethyl sulfoxide, or on catalytic reduction. 

Figure 25.30 Removal of NO using selective non-catalytic reduction.

Thus, the technique can become counterproductive. A typical arrangement for selective non-catalytic reduction is shown in Figure 25.30. Aqueous ammonia is vaporized and mixed with a carrier gas (low-pressure steam or compressed air) and injected into nozzles located in the combustion device for optimum temperature and residence time10. NO, reduction of up to 75% can be achieved. However, slippage of excess ammonia must be controlled carefully. 

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