The thermal decomposition of wood chips (pine and beech samples) and an energy crop (thistle) is studied using thermogravimetric analysis. Assuming the addition of three independent parallel reactions, corresponding to three pseudocomponents linked to the hemicellulose, cellulose, and lignin, the weight loss associated with the pyrolysis process is simulated. The kinetic constants are based on the assumption of an irreversible, single-step, first-order process for each pseudocomponent, except for the third pseudocomponent, for which a kinetic model of third order is applied. The weight loss evolution simulated by means of the summative model has been compared with the experimental data. This summative nth-order model has been able to reproduce the experimental results satisfactorily and has evidenced the influence of the biomass composition on the pyrolysis process.