Selective catalytic oxidation ammonia

However, besides the SCR activity, the selectivity is an important parameter for the assessment of the catalysts. In case of the SCR reaction, the selectivity with respect to both the product nitrogen and the reactant ammonia has to be considered. The product selectivity is important, as side products such as N20 can be formed and the reactant selectivity is important, as ammonia can be converted to nitrogen not only in the SCR reaction but also by the selective catalytic oxidation with oxygen [54],... 

The Selectoxo (Selective Catalytic Oxidation) Process reduces 1) the hydrogen consumption of the methanation system and 2) the inert gas content of the purified synthesis gas that is fed to the ammonia synthesis loop74. 

Ammonia is sometimes used as a reducing agent for the selective reduction of NOE in emissions from industrial installations, but unreacted ammonia creates a secondary air pollution problem because it is itself hazardous. Consequently, selective catalytic oxidation (SCO) is required to convert traces of ammonia to nitrogen downstream of the reactor 134... 

Another option is to destroy the ammonia in situ, in the gas. Several methods, thermal as well as catalytic, are presently being developed. Tests which used actual fuel gas, carried out at atmospheric pressure on a side-stream experiment at the Vamamo plant, showed that around 80% of the ammonia could be removed using a selective catalytic oxidation (SCO) approach based on porous aluminium oxide catalysts. In addition, laboratory scale experiments have shown the potential of the SCO approach to remove up to 95% of the ammonia in the gas... 

Nassos S, Svensson EE, Boutonnet M, Jaras SG (2007) The influence of Ni load and support material on catalysts for the selective catalytic oxidation of ammonia in gasified biomass. Appl Catal B Environ 74 92... 

The selective catalytic oxidation of ammonia to nitrogen (SCO process) has been proposed for the abatement of ammonia from waste gases. Among different formulations, catalysts based on Cu0-Ti02 offer interesting properties [1]. However, selectivity to nitrogen can be decreased by the parallel formation of N2O and NO. The behaviour of metal oxide catalysts upon these reactions can be influenced by water vapour that is always present in the reacting mixture [2]. 

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). 

Qi G, Gatt J E, Yang R T (2004) Selective catalytic oxidation (SCO) of ammonia to nitrogen over Fe-exchanged zeolites prepared by sublimation of FeCla. J. Catal. 226 120-128. 

The direct comparison between the two combined systems (Double Bed versus Mechanical Mixture) clearly shows that the mechanical mixture of SCR and PGM powders entails the occurrence of interactions between the activities of the two catalysts, as the SCR reaction products can react over the PGM catalyst and vice versa. Such an interaction is not possible in the Double Bed configuration, wherein the two catalysts are segregated. A similar picture was observed by Long et al. [39] who studied a noble metal-promoted Fe-ZSM-5 for Selective Catalytic Oxidation of NH3. Long et al. [39] attributed the reduced N2O and NOx production to the SCR features of the combined catalyst N2O and NO generated by oxidation over the noble metal can be indeed further reduced to N2 by unreacted ammonia, thus improving simultaneously ammonia conversion and N2 selectivity. 

Long RQ, Yang RT (2002) Noble Metal (Pt, Rh, Pd) Promoted Fe-ZSM-5 for Selective Catalytic Oxidation of Ammonia to N2 at Low Temperatures. Catal Lett 78 (l) 353-357... 

Selective catalytic reduction of NOx by ammonia (NH -SCR) and selective catalytic oxidation (SCO) of ammonia... 

Lietti, L., Ramis, G., Busca, G., et al. (2000). Characterization and reactivity of Moo3/Sio2 De-Nox catalysts in the selective catalytic oxidation of ammonia to N2, Catal. Today, 61, pp. 187-195. 

Designing a catalyst for the selective oxidation of CO in the presence of high concentrations of Hz is a major scientific challenge. Nevratheless, the selective catalytic oxidation of CO has been used to increase the production rate of existing ammonia synthesis plants. This reaction network also is a critical element in the developing technology for Hz-powered fuel cells. In this context, the process is referred to as PROX (preferential oxidation). 

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]. 

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. 

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

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 ... 

Rhenium is one of the oxophilic atoms effective for oxidation reactions. ReOx species are likely to have chemical interaction with various oxide supports and exhibit unique catalytic properties that cannot be observed on monomeric rhenium oxides. A new active six-membered octahedral Re cluster in zeolite pores (H-ZSM-5 [HZ]) is produced from inactive [Re04] monomers in situ under selective propene oxidation to acrolein (C3H6+02 - CH2=CHCH0+H20) in the presence of ammonia that is not involved in the reaction equation [16], The cluster is transformed back to the original inactive monomer in the absence ammonia. Note that coexistence of spectator NH3 is indispensable for the selective oxidation. 

The proposed Re6 cluster (8) with terminal and bridged-oxygen atoms acts as a catalytic site for selective propene oxidation under a mixture of propene, Oz and NH3. When the Re6 catalyst is treated with propene and Oz at 673 K, the cluster is transformed back to the inactive [Re04] monomers (7), reversibly. This is the reason why the catalytic activity is lost in the absence of ammonia (Table 8.5). Note that NH3, which is not involved in the reaction equation for the acrolein formation (C3H6+02->CH2=CHCH0+H20) is a prerequisite for the catalytic reaction as it produces the active cluster structure under the catalytic reaction conditions. 

Amiridis, M.D., Duevel, R.V., Wachs, l.E. 1999. The effect of metal oxide additives on the activity of VjOj/TiOj catalysts for the selective catalytic reduction of nitric oxide by ammonia. Appl Catal B Environ 20 111-122. 

Selective Catalytic Reduction (SCR) process is very similar to SNCR with the exception that a catalyst is used to accelerate the reactions at lower temperatures allowing it to be applied to both full and partial-burn regenerators. An SCR system consists of a catalyst bed installed in the flue gas line of a combustion system. Ammonia is injected into the flue gas with air in the presence of a catalyst. The catalyst is typically oxide forms of vanadium and tungsten. The ammonia selectively reacts with NOx to form molecular nitrogen and water via an exothermic reaction that has achieved > 90% reduction in NO when applied to an FCCU. 

Reduction of Nitric Oxide with Ammonia. - Control of the emission of NO from stationary sources is possible by selective catalytic reduction, for which up to now NH3 is the only effective reductant in the presence of excess 02. Beside noble metal catalysts Bauerle etal.101 109 and Wu and Nobe108 studied Al2 03-supported vanadium oxide and found this to be highly effective in NO removal which is considerably enhanced by the presence of 02. Alkali metal compounds which are usually added as promoters for S02 oxidation completely inactivate the catalysts for NO reduction. Adsorption kinetic studies indicated first-order dependence on NH3 adsorption. Similar results were obtained for NO on reduced vanadium oxide, but its adsorption on... 

The operability and reliability of processes using ammonia must also be studied. With the potential for increased ammonia use in these systems (in the selective catalytic reduction of nitrogen oxides and as an absorbent), research documenting ammonia emissions and the effects of ammonia on process equipment should be conducted. Furthermore, additional investigations should be performed to determine whether ammonium salts are formed and to document their effects on both the environment and the flue gas treatment system. 

While the development of flue gas clean-up processes has been progressing for many years, a satisfactory process is not yet available. Lime/limestone wet flue gas desulfurization (FGD) scrubber is the most widely used process in the utility industry at present, owing to the fact that it is the most technically developed and generally the most economically attractive. In spite of this, it is expensive and accounts for about 25-35% of the capital and operating costs of a power plant. Techniques for the post combustion control of nitrogen oxides emissions have not been developed as extensively as those for control of sulfur dioxide emissions. Several approaches have been proposed. Among these, ammonia-based selective catalytic reduction (SCR) has received the most attention. But, SCR may not be suitable for U.S. coal-fired power plants because of reliability concerns and other unresolved technical issues (1). These include uncertain catalyst life, water disposal requirements, and the effects of ammonia by-products on plant components downstream from the reactor. The sensitivity of SCR processes to the cost of NH3 is also the subject of some concern. 

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