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Model of Ash Size Distributions from Coal Char Oxidation

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Model of Ash Size Distributions from Coal Char Oxidation

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A flexible model of final ash size distributions resulting from coal char oxidation under varying conditions is presented. To include the important effects of fragmentation during char oxidation, the authors have modeled three-dimensional char particles as porous spatially random percolation clusters on a cubic bond lattice. Random walkers represent oxygen diffusion to the surface of the char particles. A reaction probability r governs whether a random walker reaching the surface will react with a unit mass of char. Thus, values of r from near zero to unity take the model from a chemical-reaction-limited to a diffusion-limited regime. Fragmentation events occur when reactions break the connectivity of the clusters, leading to a distribution of ash particle sizes after burnout. In the chemistry-limited regime, penetration of the random walkers into the pores of the cluster results in major fragmentation events and a correspondingly large number of ash particles. In the diffusion-limited regime, the lack of such penetration results in fewer ash particles with a larger maximum size. Model parameters are related to experimental conditions such as temperature, particle diameter, char porosity, and ash content. Results are compared with experiments.


Proc. Symp. on Math. Sim. of Coal Conversion Processes at the American Chemical Society meeting, June 5-10 in Toronto, Canada

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