An experimental study of single particle combustion is performed on a high temperature particle reactor. The particle degradation is monitored by simultaneous high magnification direct and shadowgraph imaging techniques giving access to the full behaviour of the particle even when enveloped by a flame. This allows tracing the time-resolved evolution of the particle shadow during its degradation as a function of its burnout. The new method provides the process timeline, especially the onset of the heterogeneous oxidation even during the flame phase. It is observed to occur earlier for larger particles containing lower volatile matter. The latter occupies around 40% of the initial particle shadow, which decreases with devolatilisation progress following a power trend. The char burns at the surface and then switches to internal combustion leaving an ash matrix. Effects of particle size and torrefaction levels on the different steps of particle combustion are pointed out.