Selective Catalyst Reduction Process

Also for oxidation reactions, the choice of the alumina support mainly depends on two criteria the stability of the phase at the reaction temperature and the reactivity (or better the lack thereof) toward feed components and products. For example, ethylene oxychlorination to ethylenedichloride is performed at approximately 220-250° C and 5-6 atm in the presence of a y-Al203-supported catalyst, which has a surface area of from 100 to 200 m and contains 10wt% CUCI2 and 3 wt% KCl, (423,424). Another example is a process called ammonia selective oxidation (ASO, or also selective catalytic oxidation, SCO), which converts small amounts of NH3 from waste gases to N2 at reaction temperatures of 150—300 °C. The process is used to abate the ammonia sHp after a selective catalytic reduction process with ammonia or urea in diesel-engine-exhaust after-treatment (425). The patented catalyst consists of Y-AI2O3 (60—300 m g ) loaded with 0.5-4 wt% platinum and 0.5—4 wt% vanadia and is coated onto the surface of a ceramic or metallic monoftthic structure (426). 

Chen J P, Buzanowski M A, Yang R T, Cichanowicz J E (1990) Deactivation of the Vanadia Catalyst in the Selective Catalytic Reduction Process. J Air Waste Manage Assoc 40 1403-1409... 

Park J-H, Park HJ, Baik JH et al (2006) Hydrothermal stability of CuZSM5 catalyst in reducing NO by NH3 for the urea selective catalytic reduction process. Journal of Catalysis 240 47-57... 

The aim of the project is to evaluate the perspective of a modified selective catalytic reduction process to reduce NO emissions from flue gas. In a previous study the activity of the equimolar NO/NO2 reduction was found to be higher then the NO reduction. The modified process is based on the equimolar NO/NO2 reduction. NO being the predominant NO component in flue gas from stationary sources, must then be partly oxidized to NO2. A desk study is carried out to review the literature on methods for flue gas composition modification with respect to the NO/NO2 ratio. The evaluation of the modified SCR process also requires more knowledge about the increase in catalytic activity of the equimolar NO/NO2 reduction as compared to the NO reduction and about the effect of catalyst composition on the reaction rate. In order to investigate these aspects, an experimental programme is carried out, using four different catalyst materials two commercial preparations and two self-prepared vanadium-titaniumdioxide catalysts. The results of activity measurements for NO, NO2 and the equimolar NO/NO2 reduction are reported, under conditions relevant for large scale applications. From these results some prehminary conclusions can be drawn with respect to the perspective of the modified process. 

In the selective catalytic reduction process, NO, in flue gas is selectively reduced to nitrogen and water vapor by reaction with HH3 in the presence of a catalyst. The SCR process is described by the following overall chemical reactions (Blair et al., 1981) ... 

Investigations to develop base metal catalysts have led to the use of selective catalytic reduction processes with supported vanadium pentoxide catalysts... 

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. ... 

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. 

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

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 ... 

Data reported in Figure 6.18 indicated the almost stoichiometric occurrence of reaction (4) (the N-balance closed within 5%), while very small amounts of NO (not shown in the figure) were formed during the catalyst regeneration and the reduction process appeared to be very selective (Figure 6.19). 

Presently the catalytic selective NOx reduction by ammonia is efficient and widespread through the world for stationary sources. The remarkable beneficial effect of 02 for the complete reduction of NO into nitrogen is usually observed between 200 and 400°C. However, such a technology is not applicable for mobile sources due to the toxicity of ammonia and vanadium, which composes the active phase in vanadia-titania-based catalysts. Main drawbacks related to storing and handling of ammonia as well as changes in the load composition with subsequent ammonia slip considerably affect the reliability of such a process. On the other hand, the use of urea for heavy-duty vehicles is of interest with the in situ formation of ammonia. 

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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