SCR Reaction Kinetics

Fig. 6. Behavior of De-NOx flow reversal reactor. (Computer simulation based on model SCR reaction kinetics). (I, , , ), (2,2 ,2 ,2 ) and (3,3 ,3 ,3 ) temperature, gas phase ammonia concentration, ammonia coverage, and NOx conversion profiles at the beginning, middle and end of flow reversal period.

Unsteady-State Kinetics of the SCR Reaction. Kinetic studies of the SCR reaction have also been performed under transient conditions. These investigations have been motivated by the fact that in the past few years there is a growing interest concerning unsteady operation of monolith SCR catalysts, due to the following reasons ... 

The parameters of the reaction rates, consisting of pre-exponential factors and activation energies have to be calibrated in order to attain an optimal fit between simulated and experimental data. A step-by-step procedure is followed based on experiments which are specifically designed to minimize the number of parameters that have to be calibrated simultaneously. The sequence can be organized as follows First, parameters related to ammonia adsorption and desorption processes are identified, in the absence of SCR reactions. Second, NO oxidation parameters are calibrated without ammonia in the feed stream. Finally, the calibration of the remaining parameters is performed all together [39]. Different experimental methods and procedures involved in the derivation of global SCR reaction kinetics will be presented in Sect. 13.5. 

The kinetic parameters estimated by the experimental data obtained frmn the honeycomb reactor along with the packed bed flow reactor as listed in Table 1 reveal that all the kinetic parameters estimated from both reactors are similar to each other. This indicates that the honeycomb reactor model developed in the present study can directly employ intrinsic kinetic parameters estimated from the kinetic study over the packed-bed flow reactor. It will significantly reduce the efibrt for predicting the performance of monolith and estimating the parameters for the design of the commercial SCR reactor along with the reaction kinetics. 

Sazama, P., Capek, L., Drobna, H. et al. (2005) Enhancement of decane-SCR-N()x over Ag/alumina by hydrogen. Reaction kinetics and in situ FTIR and UV-vis study, J. Catal. 232, 302. 

The dynamic study of the ammonia SCR reactions was typically addressed by investigating the adsorption-desorption kinetics of ammonia and then the surface reactions. 

Also, the MR kinetics provided a much better description than the modified ER kinetics of fast SCR transients originated by high-frequency NHj feed pulses in a stream of 1000 ppm NO, 2% v/v O2 and 1% v/v H2O and similar to those associated with the operation of SCR after treatment devices for vehicles [56]. Indeed, the MR model is definitively more chemically consistent than the modified ER model in view of the redox character of the standard SCR reaction. 

To describe the NH3 + NO/NO2 reaction system over a wide range of temperatures and NO2 NOxfeed ratios in addition to ammonia adsorption-desorption, ammonia oxidation and standard SCR reaction with the associated kinetics already discussed in Section 2.3.2, the following reactions and kinetics have been considered by Chatterjee and co-workers [79] ... 

Another engine cycle and gas exchange simulation software tool which has been extended for exhaust aftertreatment simulation is GT-POWER (2006). This software includes models for engine components as well as templates for DOC, SCR catalyst, NSRC and TWC. Reaction kinetics can be provided by the user, based on templates. Kinetic parameters adaptation is supported with a built-in optimizer tool. 

The following reactions were included in the kinetic model NH3 adsorption (R3 in Table V), NH3 desorption (R4 in Table IV), NH3 oxidation (R5 in Table IV) and standard SCR (R6 in Table V). Mass balances for adsorbed ammonia and nitrogen now include the standard SCR reaction. Moreover, the mass balance of gaseous NO was introduced, too... 

The MR rate law relies on the assumption that the SCR reaction is governed by a redox mechanism and therefore predicts a kinetic dependence on oxygen. It has been derived assuming that (i) two types of sites for NH3 adsorption (acidic non-reducible sites) and for NO + NH3 activation/reaction (redox sites, associated with vanadium), respectively, prevail on the catalyst surface (ii) NH3 blocks the redox sites (iii) reoxidation of the redox sites is rate controlling. 

In order to develop a suitable kinetic model of the full NH3-N0-N02/02 SCR reacting system, first the active reactions depending on N0/N02 feed ratio and temperature were identified then a dedicated study was performed aimed at clarifying the catalytic mechanism of the fast SCR reaction on the basis of such a reaction chemistry a detailed kinetic model was eventually derived, whose intrinsic rate parameters were estimated from global non-linear regression of a large set of experimental transient runs. 

It is worth emphasizing that the reaction scheme above is able to explain not only the stoichiometry of the fast SCR reaction, and specifically the optimal equimolar NO to N02 feed ratio, but also the selectivity to all of the observed products, namely N2, NH4NO3 and N20, which derives from thermal decomposition of ammonium nitrate (Ciardelli et al., 2004b, 2007a Nova et al., 2006b) furthermore it is in agreement with the observed kinetics of the fast SCR reactions, which at low temperature is limited by the rate of the reaction between nitrate and NO. 

Notably, the kinetic scheme herein presented is in complete agreement with the findings on the role of nitrates in the chemistry of the fast SCR reaction presented before. 

Fig. 4A, B C show the activity change of mordenite catalysts as a function of copper content on catalyst for the reduction of NO with the sulfur content deposited on catalyst surface. Note that catalytic activity was defined as the ratio of the reaction rate for a deactivated catalyst to that for a fresh catalyst based on the first-order reaction kinetics a = k/k. The effect of sulfur compounds deposited on the catalysts due to the presence of S02 in the feed gas stream on SCR activity significantly depends on both the reaction temperatures and the copper content of the catalyst. For HM catalyst, the catalytic activity varies with its sulfur content depending on reaction temperatures, i.e., an exponential relationship at 250 °C and a linear relationship at 400 DC as shown in Fig.4A. It has already been investigated that the surface area of deactivated HM catalyst exponentially decreases with sulfur content at lower temperature of 250 °C, while it linearly decreases at higher temperature of 400 aC as shown in Fig. 1 A. Judging from these results between catalytic activity and surface area with their catalyst sulfur content at two different reaction temperatures, the decline of the catalytic activity for deactivated HM catalyst occurs simply due to the decrease of surface area.

The rate of the DeNO, reaction is first order in respect to NO concentration and essentially independent of NH3 concentration when ammonia is in excess. However, in SCR industrial applications a substoichiometric feed ratio (a = NH3/NO < 1) is employed in order to minimize the slip of unconverted ammonia and the formation of ammonium sulfates. A kinetic dependence on ammonia is apparent when NH3 becomes the limiting reactant. Several authors have proposed kinetic expressions for the SCR reaction that account for the observed dependences [31-37]. In particular, the simplest expressions are based on Eley-Rideal kinetics in line with the mechanistic studies, they assume that the reaction occurs between strongly adsorbed ammonia and gas-phase NO. Beckman and Hegedus [36] have proposed and fitted to experimental data obtained over commercial SCR catalysts the following kinetic expression ... 

Turco et al. [81] studied the influence of water on the kinetics of the SCR reaction over a vanadia on titania catalyst in more detail and found that water inhibits the reaction. The influence of water on the SCR reaction is largest at low temperatures (523-573 K) and low at 623 K. They considered a power rate law... 

Kinetic parameters for the SCR reaction on V2O5 on TiCb on alumina plate [80] ... 

Beeckman and Hegedus [50] determined the intrinsic kinetics over two commercial vanadia on titania catalysts. A mathematical model was proposed to compute NO and SO2 conversions and the model was validated by experimental values. Slab-shaped cutouts of the monolith and powdered monolith material were used in a differential reactor. The cutouts contained nine channels with a length of 15 cm and with a channel opening and wall thickness of 0.60 and 0.13 cm, respectively. The SCR reaction over a 0.8 wt% V2O5 on titania catalyst was first-order in NO and zero-order in NH3. 

In Table 3.1 C is the initial acetylene alcohol concentration, Q is the catalyst concentration, S is the selectivity (%), A is the acetylene alcohol conversion (%), turnover frequency (TOP) is the mole of substrate converted over a mole (Pd) of the catalyst per second, Xt is the relative concentration Xi= Q /Q (where Q is the current concentration of the substrate at i= 1 and product at i=2). Strictly speaking TOP should be calculated per Pd atoms participating in the catalytic reaction (available surface atoms), but for the sake of comparison with hterature data, in this chapter we will use the TOP definition given above. To find the kinetic relationships, we have studied the reaction kinetics at different substrate-to-catalyst ratio SCR=Co /Q. Kinetic curves for DHL hydrogenation with Pd and bimetallic catalysts are presented in Pig. 3.4. 

Steady-State Kinetics of the SCR Reaction. The kinetics of the standard SCR reaction over vanadia-based catalysts has been investigated by several authors (64,66,67,86,87,92,93), and different kinetic rate expressions have been proposed. Most of them refer to steady-state conditions, but kinetic studies performed under transient conditions have been reported as well (94-102). 

As previously reported, an Eley-Rideal mechanism is generally accepted for the standard SCR reaction, which implies the reaction between adsorbed NH3 and gas-phase NO accordingly, a kinetic expression, which is in agreement with the observed dependencies of ammonia, NO, oxygen and water on the rate of reaction, is given by... 

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