Chemistry /chemical kinetic models

E. James Davis, Microchemical Engineering The Physics and Chemistry of the Microparticle Selim M, Senkan, Detailed Chemical Kinetic Modeling Chemical Reaction Engineering of the Future... 

The NIST Chemical Kinetics Model Database web site (http //kinetics.nist. gov/CKMech/) is a good resource for chemical kinetic models, thermochemical property data, and elementary rate coefficients. The book Gas-Phase Combustion Chemistry edited by W. C. Gardiner, Ir. (Springer-Verlag, NY, 1999) also lists many detailed mechanisms for different fuels that are available in technical papers and from the Internet. 

Using a chemical kinetic model is one way to describe the chemistry in reacting flow modeling. The chemical kinetic model offers a comprehensive description of the chemistry, but it requires a larger computational effort than simplified chemical models. 

For a specific application, a chemical kinetic model may either be adopted from literature or developed based on available reaction specific information. Developing a reaction mechanism for a practical process is potentially a tremendous task, since the chemistry may involve hundreds of species and perhaps thousands of reactions. For this reason it is generally preferable to adopt an existing model, at least as a starting point. It is very seldom necessary to develop a reaction mechanism from scratch. 

The detailed chemistry of hydrogen and carbon monoxide oxidation is well established [152,291,442], and chemical kinetic modeling can be used confidently for these reaction systems to predict behavior over a wide range of conditions. 

The SNCR reaction has been studied extensively, and the detailed chemistry is fairly well established. Use the provided chemical kinetic model for SNCR (SNCR.mec [276]) together with a plug-flow code to assess the potential of SNCR in reducing NO in the municipal solid waste (MSW) facility, based on the available technical information. Determine the preferred agent injection location and the optimum NH3/NO ratio, with the restriction that the NH3 slip must not exceed 15 ppm. 

In recent years, it has become possible to extrapolate accurately using detailed chemical kinetic models to predict quantitatively the behavior of some rather complicated chemical systems. The most famous examples of this success are the detailed atmospheric chemistry models whose predictions underlie the Montreal Protocol on ozone-depleting chemicals. However, these atmospheric chemistry models were developed through a huge international effort over several decades, based heavily on a large number of laboratory experiments. Much more rapid and efficient methods of model development are required for detailed predictive chemical kinetics to become a practical everyday design tool for chemical engineering. 

The primary goal of chemical kinetic modeling is to make predictions given our current understanding of chemistry, what do we expect to happen in a particular reacting mixture under specified reaction conditions If desired, these... 

When one begins to construct a chemical kinetic model, there are several different types of required input information, Fig. 4. Obviously, one needs some specification of the initial concentrations of the reactants, and of the reaction conditions (e.g. T, P, timescale) of interest. Normally one wants to numerically solve the kinetic model to predict species and/or temperature profiles, so the inputs must also include some specification of numerical tolerances on these outputs, and options for the differential equation solver. The most complicated input information required to construct a kinetic model is the chemistry what species, reactions, or reaction types will be considered How will all the thermochemical and rate parameters be estimated ... 

The methods described in Section II carry out the first step in Fig. 1, constructing a detailed chemical kinetic model from our current understanding of chemistry. However, this step is only useful if we can numerically solve the chemical kinetic simulation to obtain quantitative predictions. In many cases, solving the model is even more challenging than constructing it. 

There are many ways one can try to reduce the computational burden. Ideally, one would find numerical methods which are guaranteed to retain accuracy while speeding the calculations, and it would be best if the procedure were completely automatic i.e. it did not rely on the user to provide any special information to the numerical routine. Unfortunately, often one is driven to make physical approximations in order to make it feasible to reach a solution. Common approximations of this type are the quasi-steady-state approximation (QSSA), the use of reduced chemical kinetic models, and interpolation between tabulated solutions of the differential equations (Chen, 1988 Peters and Rogg, 1993 Pope, 1997 Tonse et al., 1999). All of these methods were used effectively in the 20th century for particular cases, but all of these approximated-chemistry methods share a serious problem it is hard to know how much error is... 

Nowadays, improved computing facilities and, more importantly, the availability of the Chemkin package (Kee and Rupley, 1990) and similar kinetic compilers and processors have made these complex kinetic schemes more user-friendly and allows the study of process alternatives as well as the design and optimization of pyrolysis coils and furnaces. In spite of their rigorous, theoretical approach, these kinetic models of pyrolysis have always been designed and used for practical applications, such as process simulation, feedstock evaluations, process alternative analysis, reactor design and optimization, process control and so on. Despite criticisms raised recently by Miller et al. (2005), these detailed chemical kinetic models constitute an excellent tool for the analysis and understanding of the chemistry of such systems. 

With laser augmentation at 1 atm, HMX will exhibit a dark zone temperature plateau similar to NC/NG at 1300 to 1500 K. In this case, the single-step gas reaction can be applied to the primary flame the secondary flame will have no appreciable effect on steady burning rate, as in NC/NG. If it is desired to simulate the secondary flame, a more complex kinetic mechanism (at least two-step) must be considered. Complex chemical kinetics models have shown the ability to simulate the two-stage gas flame structure of RDX under laser irradiation. (However, complex chemistry models still have difficulty in predicting the correct temperature sensitivity of HMX, as noted below.)... 

Meeks et al. l for analysis of diamond CVD. Detailed chemical kinetic models of the gas-phase and surface chemistry in diamond CVD have been presented by Tsuda et Frenklachet Harris etal.,t J tioil... 

As discussed above, LES/FMDF can be implemented with (1) nonequilibrium and (2) near-equilibrium combustion models. The former uses a direct ODF solver for the chemistry and can handle finite-rate chemistry effects. In the latter, a flamelet library is coupled with the LFS/FMDF solver in which transport of the mixture fraction is considered. The latter approach has the advantage it is computationally much less intensive and can be conducted with very complex chemical kinetics models. Below, some of the results recently obtained via Fq. (4.2) are presented. The flamelet library is generated with the full methane oxidation mechanism of the Gas Research Institute (GRI) [6] accounting for 53 species and about 300 elementary reactions. 

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