Biomass pyrolysis kinetics: A comparative critical review with relevant agricultural residue case studies

Publisher Summary This chapter discusses the thermodynamic activity of an atomic species on the surface of a catalyst and its determination under steady-state conditions of a catalytic reaction. If the rate of an individual step of a catalytic reaction depends on the concentration of adsorbed atoms occurring as intermediates, it is imperative to determine this dependence. The determination of the surface concentration of molecules adsorbed on a catalyst is feasible under favorable conditions. In the case of atomic species, it has been found to be convenient to use the thermodynamic activity rather than the surface concentration as the relevant variable. The primary problem of chemical kinetics is the formulation of an empirical rate law that represents the rate of a reaction as a function of the concentrations or partial pressures of reactants, products, and catalysts present in a gaseous or liquid phase. The knowledge of the mechanism of a reaction may be used to formulate a rational rate law that may represent the empirical rate data in a more logical form than an empirical rate law.

Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis

Publisher Summary This chapter discusses the correspondence between kinetics and mechanism, addresses Gibbs' fundamental rule of stoichiometry, intermediate products and sequences, calculation of stationary velocities and concentrations, and integration of the velocity expressions and comparison with experiments. The chemical kinetics, like any other science, can be developed only through the cooperation of the members of a team comprising all contemporary workers in that field and that any advance rests on the work of former and contemporary scientists. Such considerations form the background for the presentation of problems concerning the elucidation of the mechanisms of chemical reactions. The determination of the mechanism of a chemical reaction is an experimental problem. The methods used for the interpretation of the experiments may vary, but the experiments themselves form the fixed basis.

The Elucidation of Reaction Mechanisms by the Method of Intermediates in Quasi-Stationary Concentrations

The description of chemically reacting systems leads very often to reaction mechanisms with far above hundred chemical species (and, therefore, to more than a hundred partial differential equations), which possibly react within more than a thousand of elementary reactions. These kinetic processes cover time scales from nanoseconds to seconds. Due to these scaling problems the detailed simulation of three-dimensional turbulent flows in practical systems using detailed kinetics is beyond the capacity of even today’s super-computers. Using simplified sub-models for the chemical kinetics is a way out of this problem. The question arising in mathematical modeling of reactive flows is then: how detailed, or down to which scale has the chemical reaction to be resolved in order to allow a reliable description of the entire process? The aim is the development of models, which should be as simple as possible in the sense of an efficient description, and also as detailed as necessary in the sense of reliability. In particular, an oversimplification of the coupling processes between chemical reaction and turbulent flow should be avoided by all means to allow a predictive character. This chapter describes different methodologies for the reduction of chemical mechanisms for subsequent use in reacting flow calculations.

S. C. Lind

Papers

Geschwindigkeit der Bildung des Bromwasserstoffs aus seinen Elementen