The pyrolysis of biomass occurs via several parallel/serial decomposition reactions. The kinetic parameters, namely the activation energy (E) and the pre-exponential factor (k(o)), do not remain constant during the pyrolysis process. A modified empirical method is introduced for calculating the activation energy (E) and the pre-exponential factor (k(o)) based on the Friedman analysis [Friedman HL. Kinetics of thermal degradation of char-forming plastics from thermogravimetry - application to a phenolic plastic. J Polym Sci C 1963;6: 183-95]. The kinetic parameters are expressed as a function of the conversion (x) during the biomass pyrolysis process. The reactions are assumed to be of first order. At least three data sets obtained at different dynamic heating rates are required. From the Friedman analysis, the conversion (x) related functions E = E(x) and k(o) = k(o)(x) can be obtained by a B-spline regression method. The pyrolysis can hence be described as: dx/dt = k(l - x) = k(o)(x).exp(-E(x)/RT)(1 -x). In this paper, the adapted method is applied to pyrolysis of cellulose and two biomass fuels (meat & bone meal, chicken litter). Experiments were carried out at 2, 10 and 50 K min(-1) thermogravimetric analysis. A good fit of the calculated conversion with experimental data was found. (C) 2009 Elsevier Ltd. All rights reserved.