Abstract The development of a rapidly responding lab-scale fluidized bed reactor (FBR) optimized for the investigation into combustion reactions of solid fuels is presented. The experimental setup represents a novelty in FBR systems, as it quantitatively captures reactions with an apparent 90% carbon conversion time t 90 of Original and improved design are modeled with a 1-D finite volume approach to gain insights into the ideal operating conditions and operating limits. In a second step, the theoretical reaction rate limits are compared with experimental combustion experiments. Reaction rate measurements with chars from two pulverized Colombian coals (Calenturitas, Mina Norte) and one pulverized biomass (beech wood) are undertaken in N2/O2 atmosphere at different temperatures ranging from 823 to 1273 K. It is found that results of the 1-D simulation are a good guidance for actual experiments.