SCR experiments

Accordingly, we conclude that the dual-site MR approach is compatible with the ammonia inhibition effects observed during unsteady SCR experiments, as well as with the oxygen dependence of the SCR kinetics at low temperatures, and can be successfully applied to simulate the complex dynamic behavior of... 

K. Goldschmidt, VKR full-scale SCR experience on hard coal fired boilers. Proceedings Joint Symposium on Stationary NO Control, New Orleans. Louisiana, March 23-26, 1987. 

P. A. Lowe, W. Ellison, and M. Perlsweig, "Understanding the German and Japanese Coal-Fired SCR Experience", presented at EPA/EPRI Joint Symposium on Stationary Combustion NOx Control, Washington, DC, March, 1991. 

Fig. 10.6 Transient SCR experiments at different temperatures with high frequency NH3 feed pulses at 200 °C ...

Fig. 10.7 Transient SCR experiments at different temperatures with high frequency NH3 feed pulses data as in Fig. 10.6b. Symbols experimental solid lines model fit using the M(9 rate law Eq. (10.19). Adapted from Ref. [5]...

Fig. 11.14 Comparison of N2O evolved obtained during the temperature programmed desorption (TPD) after catalyst was exposed to four different reaction conditions at 180 °C. A temperature ramp of 10 °C/min was applied evolve the N2O from the catalyst. The Fast SCR experiments involved a feed mixture containing 500 ppm NO, 500 ppm NO2, 1,000 ppm NH3, 5 % O2 fed to the reactor for durations of 30 min, 1, and 2 h. The NO2 CR experiments involved a feed mixture of 1,000 ppm NO2, 1,000 ppm NH3, and 5 % O2 for a duration of 2 h...

Fig. 12.24 SCR experiments when varying NO2 to NO ratio over a Cu-zeolite at different temperatures. The inlet feed gas consists of 500 ppm NOx, 500 ppm NH3, 3 % H2O and 2 % O2. Solid lines (kinetic model) and symbols (experiment) [8]. Reprinted with permission from Grossale et al. [8]. Copyright (2009) Springer...

Figure 7.8. NO, NO2, NH3, and N2O concentration after an ammoma selective catalytic reduction (SCR) experiment over a copper-zeolite catalyst. The inlet gas composition is 400 ppm NOx (NO orNOz), 400 ppmNHs, and 8% O2. From Sjdvall, H., Olsson, L., Fridell, E., andBUnt,...

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 ... 

This is applicable to thyristor (SCR) circuits to protect all the semiconductor devices used in the switching circuit, such as diodes (also power diodes) or IGBTs, in addition to SCRs. The same protection can be applied to all the semiconductor circuits likely to experience high dv/di. 

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. 

Morita, I., Ogasahara, T., and Franklin, H.N. (2002) Recent Experience with Hitachi Plate Type SCR Catalyst, The Institute of Clean Air Companies Fomm 02, Febmary 12-13. 

Analysis of the dynamics of SCR catalysts is also very important. It has been shown that surface heterogeneity must be considered to describe transient kinetics of NH3 adsorption-desorption and that the rate of NO conversion does not depend on the ammonia surface coverage above a critical value [79], There is probably a reservoir of adsorbed species which may migrate during the catalytic reaction to the active vanadium sites. It was also noted in these studies that ammonia desorption is a much slower process than ammonia adsorption, the rate of the latter being comparable to that of the surface reaction. In the S02 oxidation on the same catalysts, it was also noted in transient experiments [80] that the build up/depletion of sulphates at the catalyst surface is rate controlling in S02 oxidation. 

SCR for heavy-duty vehicles reduces NOx emissions by 80%, HC emissions by 90% and PM emissions by 40% in the EU test cycles, using current diesel fuel (<350 ppm sulphur) [27], Fleet tests with SCR technology show excellent NOx reduction performance for more than 500000 km of truck operation. This experience is based on over 6 000 000 km of accumulated commercial fleet operation [82], The combination of SCR with a pre-oxidation catalyst, a hydrolysis catalyst and an oxidation catalyst enables higher NOx reduction under low-load and low-temperature conditions [83],... 

It is believed that SCR by hydrocarbons is an important way for elimination of nitrogen oxide emissions from diesel and lean-burn engines. Gerlach etal. [115] studied by infrared in batch condition the mechanism of the reaction between nitrogen dioxide and propene over acidic mordenites. The aim of their work was to elucidate the relevance of adsorbed N-containing species for the F>cNOx reaction to propose a mechanism. Infrared experiments showed that nitrosonium ions (NO+) are formed upon reaction between NO, NOz and the Brpnsted acid sites of H—MOR and that this species is highly reactive towards propene, forming propenal oxime at 120°C. At temperatures above 170°C, the propenal oxime is dehydrated to acrylonitrile. A mechanism is proposed to explain the acrylonitrile formation. The nitrile can further be hydrolysed to yield... 

Figure 9.15. Comparison of the total ammonia adsorption of coated and extruded V2O5/WO3—Ti02 catalysts. Catalyst volume = 7 cm3. Model gas for loading 10% 02, 5% H20, NH3 = 1000ppm, and balance N2. GHSV = 52000h 1. Model gas for temperature-programmed desorption (TPD) experiment 10% 02, 5% H20, NO = 1000 ppm, NH3 = 1000 ppm, and balance N2. NH3 desorbed is calculated as the sum of thermally desorbed NH3, directly measured at the catalyst outlet, and chemically desorbed NH3, measured by the reduction of the NO concentration due to the SCR reaction.

Since most transfer hydrogenation catalysts employ precious metals, a high number of turnovers are required in order to make their use economic. As the ligands are simply made they are generally of low cost. In our experience, for the average pharmaceutical intermediate, a substrate catalyst ratio (SCR) of about 1000 1 is sufficient for the catalysts contribution to the product cost to be minor. These SCRs are regularly achieved, and so from an economic standpoint there has been little incentive to recover and recycle the catalyst, unless a low-cost product is required. The recovery of precious metals from waste streams provides another way in which costs can be minimized. 

Reaetion (31) suggests acrolein as a key intermediate in SCR of NO by propylene. The formation of nitro species by this reaetion was already diseussed in the literature and evideneed by a significant reduction in the surface concentration of propylene adspecies in the presenee of a NO2 + O2 mixture[133, 134]. Note that the role of organie nitro species as active intermediate in the SCR of NO over Cu-ZSM-5 was already diseussed by Hayes et al.[135]. In addition, in TPR experiments, we observe Cu forming predominantly on the surfaee of Cu-Al-MCM-41 after exposure to CsHg, and a redox of Cu and Cu during the reaetion. It is therefore possible to postulate that the divalent eopper ion is reduced to monovalent in the conditions of the reaetion between adsorbed propylene and NO2 species. 

The study of the mechanism of the fast SCR over V-W-Ti-0 catalysts was addressed first by Koebel and co-workers [65-68]. They suggested that (i) the reoxidation ofthe catalyst is rate determining at low temperature in the redox cycle of standard SCR catalyst, (ii) NO2 reoxidizes the catalyst faster than O2 the NO2-enhanced reoxidation of the catalyst was demonstrated by in situ Raman experiments, (hi) the reaction occurs via the nitrosamide intermediate in both standard and fast SCR and (iv) ammonium nitrate is considered an undesired side-product. 

Ammonium nitrite is unstable above 100 °C and the sum of reactions (13.25) and (13.26) results in the fast SCR reaction (13.24). This reaction scheme can explain the optimal 1 1 NO NO2 feed ratio of the fast SCR on the basis of well-known chemistry however, it cannot explain all of the several products (N2, NH4NO3, N2O) observed in experiments covering the full range of NO NO feed ratios [71]. 

Tronconi and co-workers, through an extensive study of the reactivity of NH3-NO/ NO2 mixtures with different NO NO2 ratios over V-W-Ti-O SCR catalysts, proposed a novel mechanism for the fast SCR reaction that has been validated step by step by dedicated experiments [71-74]. 

The model was validated against heavy duty and passenger car diesel engine test bench experiments. A good correlation was obtained between ESC and ETC experiments and simulation with 0 and 0.5% NO2 NO ratios and a virtual oxidation catalyst. The virtual oxidation catalyst model was realized by placing an oxidation catalyst model in front of the SCR catalyst. 

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