Diffusion effects carbon burning reaction

Some coals contain an ash in addition to carbon, moisture, and volatiles. To obtain a conservative estimate, one should assume that a porous ash shell is retained during the burning of the combustible material. This ash may, of course, have a catalytic effect on the heterogeneous carbon combustion reactions however, it is a cause for additional diffusion resistance. 

Regeneration of coked particles involves both chemical reaction and transport processes, since oxygen must be transported by external mass transfer and by pore diffusion to the internal coked surface. Pore diffusion strongly influences the effective rate of burn-off, at least for particle diameters and temperatures relevant for industrial (fixed bed) processes (>lmm, >400 °C). Consequently, radial gradients of the 02-concentration and, vdth proceeding burn-off also of the carbon content in a particle are established (Figure 6.9.9b). 

In reality, CO is always formed in the initial reaction stages. The CO subsequently diffuses outward and reacts with oxygen. However, if CO bums within the boundary layer, the effective surface product is carbon dioxide, and the first stoichiometry is used. On the other hand, if CO burns outside the boundary layer, the effective surface product is CO and the second stoichiometry is utilized. Carbon dioxide can diffuse back to the char particle and react ... 

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