Global kinetic models

In this section we shall discuss the development of a global kinetic model for DOC. The basic model reactions considered in the DOC model are summarized in Table II. Here the real HC mixture is modeled by two characteristic hydrocarbons—propene and decane. Propene represents more reactive, light hydrocarbons, which practically do not adsorb during cold start, while decane is a representative of heavier hydrocarbons with significant adsorption on... 

For a comprehensive kinetic description of the NH3 + N0/N02 reacting system in a wide range of temperatures and N02/N0X feed ratios a global kinetic model was developed, based on the whole set of reactions in Table V. 

Global Kinetic Models for Supercritical Water Oxidation of Organic Substances... 

Global kinetic models for high-temperature gas-phase reactions were reviewed by Abdalla et al. [193] and Babushok and Dakdancha [194]. Empirical global kinetic schemes for the low-temperature oxidation of hydrocarbons have been recently reviewed by Griffiths [11]. Here, only some characteristic models are mentioned and for a full account the reader is referred to the review by Griffiths. 

Concerning other catalysts, Olsson and co-workers (132) have recently published a global kinetic model for a Cu-zeolite, which includes account of NH3 adsorption-desorption and oxidation, standard, fast, and NO2-SCR reactions, NO oxidation and N2O formation. Their model could adequately reproduce the effects of NO2/NO feed ratio in short transient experiments. 

Roduit et al. [1] proposed a global kinetic model for the standard SCR reaction based on V-based catalysts. The kinetic model accounts for three different reactions and intraparticle diffusion. The three reactions are Langmuir-Hinshelwood LH-type SCR, Eley-Rideal ER-type SCR, and direct NH3 oxidation. The main SCR pathway proceeds via the ER-type mechanism, but in the low temperature region T < 200 °C), LH-type reaction occurs. Furthermore, at high temperatures (T > 300 °C), NH3 oxidation and intraparticle mass transfer also takes place [1]. 

Concerning the rate expressions, extending the dual-site Redox kinetics already presented for the NH3-NO/O2 reacting subsystem, a new global kinetic model for the full NH3-NO/NO2 SCR reacting system was derived. 

These steps increase the coverage of surface nitrites which rapidly convert to N2. The differential rate data for temperatures below 250 °C presented earlier show clear evidence for multiple reaction pathways The differential rate of NO2 consumption exceeds that of NO at lower temperatures. This points to the formation of NH4NO3 and its inhibition of N2 formation, but also the mitigation of the inhibition by and AN reduction by NO. It can be shown that an overall rate based on the reduction of HNO3 and/or NH4NO3 as the RDS has the functional features to predict the main trends in the experimental data. Further analysis of microkinetic models that include these steps Sl lO and S19-S26 is needed. Later we describe global kinetic models that predict these data as a first step toward this goal. 

Progress has been made towards the development of monolith reactor models that predict SCR performance under both steady state and transient operation. Guth-enke et al. [33]. provided a thorough review of SCR reactors. Most of the earlier work in this area was done for the more established Vanadia-based catalysts and involved the use of global kinetic models [81-83]. More recent works by Nova et al. provided detailed transient model for the SCR reaction system on Vanadia-based catalyst [8, 45]. Olsson and coworkers developed both global and detailed kinetic models for NH3-SCR reactions on Cu-ZSM-5 catalysts [14, 15, 49]. More recent works have communicated models for NH3-SCR reactions on Fe-zeoUte catalysts [25, 57, 76, 84]. 

Global Kinetic Model for Ammonia Storage and Desorption... 

At high temperature, ammonia is oxidized by oxygen, which decreases the selectivity of the standard SCR reaction. In a kinetic model that should describe a broad temperature interval, it is therefore crucial to have a good description of the ammonia oxidation. Accordingly, we investigated ammonia oxidation separately in order to decrease the correlation between the parameters. In the global kinetic model, the following reaction was added in which ammonia stored on the surface is oxidized to produce Na and H2O ... 

Metkar et al. [9] developed a global kinetic model from a derivation of detailed reaction steps. They used reactions described in (12.11-12.14 [11]) and combined two reactions (see Eqs. 12.15-12.16 [11]). They received the best results when assuming the Eley-Rideal step, during the NO oxidation (Eq. 12.14) as rate determining. The resulting rate expression was ... 

We developed a global kinetic model for ammonia SCR over Cu-ZSM-5 [10], consisting of seven reaction steps ... 

Nova et al. [37] presented a global kinetic model for ammonia SCR over a Cu-zeolite catalyst. They used the same seven reactions as presented above (Eqs. 12.21-12.27). In addition, they added two more reactions. In the first reaction (Eq. 12.28), N2O is formed from a reaction between ammonia on the surface (denoted NH3) and NO and in the second reaction, ammonium nitrate is formed... 

Different SCR reaction occurs depending on the NO2 to NOx ratio, where standard SCR occurs with NO only, rapid SCR with equimolar amount of NO and NO2, and slow NO2 SCR with NO2 only. In several global kinetic models, these three reactions are added. In more detailed models, more surface species are considered, for example, nitrites, nitrates, HNO3, oxygen, and hydroxyls. N2O is an unwanted by-product during the SCR process over copper zeolites that are increasing with the NO2 content. The mechanism for the N2O production is suggested to be from decomposition of ammonium nitrate. In addition, there are models available that incorporate the urea decomposition and hydrolysis, in addition to the SCR reactions. 

Koci P, Plat F, Stepanek J, BSrtovS , Marek M, Kubicek M, SchmeiBer V, Chatteijee D, Weibel M (2009) Global kinetic model for the regeneration of NOx storage catalyst with CO, H2 and C3H6 in the presence of CO2 and H2O. Catal. Today 147 257-264... 

Olsson L, Blint RJ, Fridell E (2005) Global kinetic model for lean NOx traps. Industrial Engineering Chemistry Research, 44(9), 3021-3032... 

The combustion reaction is more complex than summarized in the above reaction. Several reactions occur in sequence and in parallel. A global kinetic model establishes that the reaction rate can be evaluated by ... 

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