Abstract In the present study, the phenomenon known as “thermal deactivation” or “thermal annealing”, which describes the influence of heat treatment on the kinetics of gas-solid reactions, is investigated. A literature review revealed a distinct gap in the field of quantitative analysis of thermal deactivation influencing low rank fuels reacting under gasification conditions. Therefore, the reactivity of Rhenish lignite and torrefied poplar wood particles towards carbon dioxide was examined after the process of heat treatment. For both the heat treatment and the gasification step consecutively, a small-scale fluidized bed reactor was utilized. Assessment of thermal deactivation was achieved by varying heat treatment temperature between 1023–1173 K and heat treatment time between 0–1800 s. Flue gases were analyzed using a Fourier-transform infrared spectrometer to deduce the chemical reaction rate from the temporal evolution of the product gas species. The experimental results were reproduced by application of an nth-order power law and a model with distributed activation energies to propose parameters for implementation in gasification models and to compare these model types.