Fuel oxidation kinetics

Should exhibit catalytic properties to enhance the fuel oxidation kinetics. 

The counterflow configuration has been extensively utilized to provide benchmark experimental data for the study of stretched flame phenomena and the modeling of turbulent flames through the concept of laminar flamelets. Global flame properties of a fuel/oxidizer mixture obtained using this configuration, such as laminar flame speed and extinction stretch rate, have also been widely used as target responses for the development, validation, and optimization of a detailed reaction mechanism. In particular, extinction stretch rate represents a kinetics-affected phenomenon and characterizes the interaction between a characteristic flame time and a characteristic flow time. Furthermore, the study of extinction phenomena is of fundamental and practical importance in the field of combustion, and is closely related to the areas of safety, fire suppression, and control of combustion processes. 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

Table I contains kinetic constants for various fuel-oxidant mixtures from various sources. A different procedure of evaluating kinetic constants and reaction orders has been described (43).

Helling, R.K. Tester, J.W. Oxidation Kinetics of Carbon Monoxide in Supercritical Water, Energy and Fuels, 1 (1987) 417-423. 

A present-day fuel cell depends for its oxidant kinetics on irreversible diffusion for the transport of oxygen, and sacrifices the potential difference generated by the oxygen circulator, as in Figures A.l and A.2. 

Reactions I-IV are global steps for gaseous fuel oxidation. Reactions V-VII, are for char oxidation. The single film model is used here, where the particle is consumed via reactions with oxygen (or carbon dioxide) and no reaction occurs in the boundary layer. CO and CO2 are the two products formed at the particle surface. The first four reactions are treated based on the eddy-dissipation concept [8], which assumes that chemical reactions in the gaseous phase occur rapidly and the mean consumption rate of fuel is limited by the mixing rate of fuel and oxidant. The char reactions are treated using kinetic Arrhenius expression. 

For gaseous flames, the LES/FMDF can be implemented via two combustion models (1) a finite-rate, reduced-chemistry model for nonequilibrium flames and (2) a near-equilibrium model employing detailed kinetics. In (1), a system of nonlinear ordinary differential equations (ODEs) is solved together with the FMDF equation for all the scalars (mass fractions and enthalpy). Finite-rate chemistry effects are explicitly and exactly" included in this procedure since the chemistry is closed in the formulation. In (2). the LES/FMDF is employed in conjunction with the equilibrium fuel-oxidation model. This model is enacted via fiamelet simulations, which consider a laminar counterflow (opposed jet) flame configuration. At low strain rates, the flame is usually close to equilibrium. Thus, the thermochemical variables are determined completely by the mixture fraction variable. A fiamelet library is coupled with the LES/FMDF solver in which transport of the mixture fraction is considered. It is useful to emphasize here that the PDF of the mixture fraction is not assumed a priori (as done in almost all other flamelet-based models), but is calculated explicitly via the FMDF. The LES/FMDF/flamelet solver is computationally less expensive than that described in (1) thus, it can be used for more complex flow configurations. 

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