Ammonia selective catalytic reduction

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. 

Fickel DW, D Addio E, Lauterbach JA et al (2011) The ammonia selective catalytic reduction activity of copper-exchanged small-pore zeolites. Applied Catalysis B Environmental 102 441-448... 

Olsson L, Sjovall H, Blint RJ (2008) A kinetic model for ammonia selective catalytic reduction over Cu-ZSM-5. Applied Catalysis B Environmental 81 203-217... 

An example will be used to illustrate the number of degrees of freedom. In Figme 7.8, the experimental results from one ammonia selective catalytic reduction (SCR) experiment are shown. In this experiment, a copper-zeoUte catalyst was exposed to ammonia NOx (NO + NO2) and oxygen, while varying the NO2 to NOx ratio. The outlet concentrations of NO, NO2, NH3, and N2O were measured and are depicted in Figure 7.8. If these data points were used in a model, what would the number of degrees of freedom be ... 

NHj-SCR activity after the 14-h hydrothermal treatment of Cu-SSZ-13, Cu-SAPO-34, Cu-SSZ-16, and Cu-ZSM-5 [41 ]. Reprinted from Picket DW, D Addio E, Lauterbach JA, Lobo RF. The ammonia selective catalytic reduction activity of copper-exchanged small-pore zeolites. Appl. Catal. B 2011 102 441—448. Copyright... 

Data adapted from Lezcano-Gonzalez I, Deka U, van der Bij HE, Paalanen P, Arstad B, Weckhuysen BM, Beale AM. Chemical deactivation ofCu-SSZ-13 ammonia selective catalytic reduction (NHfSCR) systems. Appl. Catal. B 2014 154-155 339-349. Copyright 2014, with permission of Elsevier. 

Gao F, Walter ED, Kollar M, Wang Y, Szanyi J, Peden CHF. Understanding ammonia selective catalytic reduction kinetics over Cu/SSZ-13 from motion of the Cu ions. J Catal 2014 319 1-14. 

Selective Catalytic Reduction. Selective catalytic reduction (SCR) is widely used in Japan and Europe to control NO emissions (1). SCR converts the NO in an oxygen-containing exhaust stream to molecular N2 and H2O using ammonia as the reducing agent in the presence of a catalyst. 

Selective catalytic reduction (SCR) is cmrently the most developed and widely applied FGT technology. In the SCR process, ammonia is used as a reducing agent to convert NO, to nitrogen in the presence of a catalyst in a converter upstream of the air heater. The catalyst is usually a mixture of titanium dioxide, vanadium pentoxide, and hmgsten trioxide. SCR can remove 60-90% of NO, from flue gases. Unfortunately, the process is very expensive (US 40- 80/kilowatt), and the associated ammonia injection results in an ammonia slip stream in the exhaust. In addition, there are safety and environmental concerns associated with anhydrous ammonia storage. 

Selective Catalytic Reduction (SCR) SCE is a process to reduce NO, to nitrogen and water with ammonia in the presence of a catalyst between 540-840 F (282-449 C). Ammonia is usually injected at a 1 1 molar ratio with the NOx contaminants. Ammonia is used due to its tendency to react only with the contaminants and not with the oxygen in the gas stream. Ammonia is injected by means of compressed gas or steam carriers. Efficiencies near 90% have been reported with SCR. See Exxon Thermal DeNO. ... 

Selective catalytic reduction is based on selective reactions of a continuous gaseous flow of ammonia or similar reducing agents with the exhaust stream in the presence of a catalyst. The reaction that occurs is as follows ... 

Postcombustion processes are designed to capture NO, after it has been produced. In a selective catalytic reduction (SCR) system, ammonia is mixed with flue gas in the presence of a catalyst to transform the NO, into molecular nitrogen and water. In a selective noncatalytic reduction (SNCR) system, a reducing agent, such as ammonia or urea, is injected into the furnace above the combustion zone where it reacts with the NO, to form nitrogen gas and water vapor. Existing postcombustion processes are costly and each has drawbacks. SCR relies on expensive catalysts and experiences problems with ammonia adsorption on the fly ash. SNCR systems have not been proven for boilers larger than 300 MW. 

Emission control from heavy duty diesel engines in vehicles and stationary sources involves the use of ammonium to selectively reduce N O, from the exhaust gas. This NO removal system is called selective catalytic reduction by ammonium (NH3-SGR) and it is additionally used for the catalytic oxidation of GO and HGs.The ammonia primarily reacts in the SGR catalytic converter with NO2 to form nitrogen and water. Excess ammonia is converted to nitrogen and water on reaction with residual oxygen. As ammonia is a toxic substance, the actual reducing agent used in motor vehicle applications is urea. Urea is manufactured commercially and is both ground water compatible and chemically stable under ambient conditions [46]. 

NO, however, can only be removed by adding a reductant, ammonia, and using a catalyst. The process is called selective catalytic reduction, or SCR. The catalyst consists of vanadia and titania and works in the temperature interval 600-700 K according to the overall reaction ... 

Figure 10.17. Principle of selective catalytic reduction using, for example, urea or a solution of ammonia urea as the reducing agent for application of the SCR reaction on mobile diesel units such as ferries or trucks. (Courtesy of HaldorTopsoe A/S.)...

Wet air pollution control (WAPC) devices are used to treat exhaust gases from stainless steel pickling operations, thereby generating wastewater, which are treated using the selective catalytic reduction (SCR) technology in which anhydrous ammonia is injected into the gas stream prior to a catalyst to reduce NO, to nitrogen and water. The most common types of catalysts are a metal oxide, a noble metal, or zeolite. 

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. 

Example 25.5 A gas turbine exhaust is currently operating with a flowrate of 41.6 kg s-1 and a temperature of 180°C after a heat recovery steam generator. The exhaust contains 200 ppmv NOx to be reduced to 60 rng rn 3 (expressed as N02) at 0°C and 1 atm. The NOx is to be treated in the exhaust using low temperature selective catalytic reduction. Ammonia slippage must be restricted to be less than 10 mgm 3, but a design basis of 5 mg-rn 3 will be taken. Aqueous ammonia is to be used at a cost of 300 -1 1 (dry NH3 basis). Estimate the cost of ammonia if the plant operates... 

Romero Sarria, F., Marie, O., Bazin, P. et al. (2006) FT-IR operando study on selective catalytic reduction of NO species by ammonia A comparison between zeolitic and GAPON... 

Busca, G., Lietti, L., Ramis, G. et al. (1998) Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts A review, Appl. Catal. B Environ., 18, 1. 

The actual issues of EuroV standards aim at optimizing engine s design to decrease the engine-out N(), emissions in order to avoid the need for expensive after-treatments in the exhaust line. Only some heavily loaded applications would need such NOx after-treatment. Today, two major technological ways of NOx treatment are identified the NOxTrap and the selective catalytic reduction with ammonia (SCR-NH3). 

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. 

Devadas, M., Krocher, O., Elsener, M. et al. (2006) Influence of N02 on the Selective Catalytic Reduction of NO with Ammonia over FE-ZSM5, Appl. Catal. B, 67, 187. 

Devadas, M. (2006) Selective catalytic reduction (SCR) of nitrogen oxides with ammonia over Fe-ZSM5, PhD. Thesis No. 16524, ETH Zurich. 

Acke, F. and Skoglundh, M. (1999) Comparison between ammonia and propane as the reducing agent in the selective catalytic reduction of NO under lean conditions over Pt black. Appl. Catal. B Environ., 20, 133. 

V-Mo-Zeolite catalysts prepared by solid-state ion exchange were studied in the selective catalytic reduction of NOx by ammonia. The catalysts were characterized by chemical analysis, X-ray powder diffraction, N2 adsorption (BET), DRIFT, UV-Vis and Raman, spectroscopy and H2 TPR. Catalytic results show that upon addition of Mo to V-ZSM-5, catalytic performance was enhanced compared to V-ZSM-5. 

Richter, M., Trunschke, A., Bentrup, U., Brzezinka, K.W., Schreier, E., Schneider, M., Pohl, M.M., and Fricke, R. 2002. Selective catalytic reduction of nitric oxide by ammonia over egg-shell MnOx/NaY composite catalysts. J. Catal. 206 98-113. 

S02 and NOx in flue gas from coal combustion contribute to smog and acid rain. Methods to remove these pollutants include alkaline wet scrubber systems that fix S02 to solid CaS04, and selective catalytic reduction by metal/metal oxide systems of NO/NOz to N2 and steam in the presence of ammonia. Particulate active carbons have also been used in flue gas decontamination, especially as they avoid costly scrubber processes and can operate at lower temperatures. The potential of active carbon fibers in this application has been explored by a... 

SCR [Selective Catalytic Reduction] A general term for processes which destroy nitrogen oxides in gaseous effluents by reacting them with ammonia in the presence of a catalyst ... 

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