The oxygen content at the surface of a fuel particle can significantly exceed zero during the devolatilization stage of combustion, despite the flux of volatiles from the surface and also gas phase reactions. This implies that char oxidation can take place simultaneously. This overlapping of the devolatilization and char combustion stages is studied by modeling. The rates of gas phase reactions around the particle influence the availability of oxygen at the surface of a burning particle and they are accounted for by using a two-step global model for combustion of volatiles. The effects of particle size, ambient temperature, and oxygen concentration on the degree of overlap are studied. The study provides theoretical and experimental evidence that the combustion time of a particle does not always increase with its size at constant ambient conditions, but there can be a specific particle size giving a maximum combustion rate.
Gurgel-Veras, C., Saastamoinen, J., Carvalho, J., & Aho, M. (1999). Overlapping of the devolatilization and char combustion stages in the burning of coal particles. Combustion and Flame, 116(4), 567-579. https://doi.org/10.1016/S0010-2180(98)00064-9