A unique approach and proposed methodology was implemented to determine char burning rates in conjunction to their prevailing structure from detailed results on the combustion histories of small biomass fuel particles, exposed to elevated temperatures (>1000 K) at very high heating rates (10(4)-10(5) K/s). Analogous conditions typically prevail in pulverized fuel utility boilers for power generation. Individual particles of pre-measured size, shape, aspect ratio and mass from five different types of raw and torrefied biomass were selected for this study. The particles were injected into a transparent drop-tube furnace, electrically heated to 1400 K, where they were rapidly heated, ignited and burned in air. Temperature time histories of the individual particles were recorded pyrometrically and were used to assess their individual combustion rates. These particles burned in distinct volatile and char phases. A published phenomenological combustion model for carbonaceous fuel particles was enhanced and applied to the experimental data to calculate the char burning rates. Important information on the nature of these chars, which formed inside volatile matter envelope flames, was obtained by juxtaposition of the knowledge of their original properties with the pyrometric observations and the numerical simulations. It was concluded that, under the conditions of these experiments, chars of most types of biomass consisted of thin wall cenospherical particles. The modeling results show that this predominant particle structure needs to be considered for reliable burnout predictions. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.