The purpose of this study was to experimentally test and compare the fast pyrolysis behavior of torrefied wood, peat, and two types of coal. The experiments were conducted in a laboratory-scale drop-tube reactor (DTR) at a temperature range of 700-900 degrees C. Before pyrolysis, the sample particles were sieved with vibration sieves the opening of which was 100-125 mu m. The initial size distribution of the sample particles and their diameter evolution during pyrolysis was studied by using optical techniques. According to the optical measurements particle swelling during pyrolysis occurred with the tested coal types but not with peat or torrefied wood. With torrefied wood the particle shape changed during pyrolysis from elongated to spherical. The density of the samples was measured with a mercury porosimeter. The mass loss of the sample particles during pyrolysis was modeled with two first order models: the single-step one reaction kinetics model and a model in which two reactions competed to form char and volatiles from the virgin matter. The kinetic parameters of the reactions and the diameter evolution equation coefficients were determined with both models and with all fuels. The optical measurement data from the particles was used to discretize the particle size distribution. The discretized size fractions were then used in the model calculations instead of a mono-sized single particle approach. (C) 2013 Elsevier Ltd. All rights reserved.