Single particle and reactor studies on char combustion



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When coal particles are introduced into a combustor, they experience a rapid temperature rise. As a result, an almost instantaneous evolution of volatiles occxrrs and a solid fuel is generated, called char. This analysis is concerned with the establishment of reliable mathematical models for processes Involving the combustion of char. With this intention, two categories of investigation were conducted, comprising single particle and whole reactor models. Factors such as the distribution of heterogeneous kinetics, competition for active sites, carbon reactivity, carbon-carbon dioxide reaction and particle shape were examined to assess their influence on the pseudosteady-state structure from qualitative and quantitative perspectives. The research on reactor models was initiated with stirred pots, thereby substantiating that the transient path of a single particle burning under fixed conditions is very sensitive to the ambient oxygen concentration in the surroundings. As a natural extension, it was necessary to develop combustor models with diverse degrees of segregation of the gaseous species. This enabled the conventionally employed approach of perfect gas mixing to be evaluated in relation to its suitability for the prediction of actual reactor performance. On the basis of this work, it is recommended that char combustor models take into consideration as much of the detail of internal transport as possible. Indeed, the combustion rates generated by means of distributed models are always larger than those evaluated at the average concentrations in lumped systems, due to considerable changes in the behavior of the ignition-extinction phenomenon.



Combustion--Mathematical models