Properties and relevance of the volatile flame of an isolated coal particle in conventional and oxy-fuel combustion conditions

A model that accounts for the volatile matter release and its subsequent combustion in the surroundings of an isolated coal particle is presented and used to analyze the characteristics and relevance of the flame so generated in a broad range of conditions. The devolatilization kinetics were experimentally determined in a previous work; the volatile composition is given by the chemical percolation model; and the GRI 3.0 mechanism accounts for the homogeneous reactions. Both conventional and oxy-fuel combustion conditions are considered. In all the cases, three consecutive (although not fully independent) flames are identifiable around the particle, corresponding to the consumption of CH4 and C2H2, first, and the oxidation of CO, further on. A more intense chemistry is generally observed in N2 than in CO2-rich environments; in any case, those reactions result in significant heat transfer towards the particle, and thus in an acceleration of the devolatilization, more marked for higher bulk oxygen concentrations. The predictions of the model regarding devolatilization duration and peak flame temperature and position are in good qualitative agreement with the (rather scarce) experimental data available in the literature. Finally, simulations with different particle diameters from 30 to 120 µm show that only for the finest particles an overlapping of the devolatilization and the char oxidation steps during combustion is foreseeable.