Pyrolysis and gasification processes of three types of lignocellulosic biomass (Eucalyptus wood, fir wood and pine bark) and biomass main components (cellulose, xylan and lignin) were studied by thermogravimetric-mass spectrometric analysis. Biomass samples were pyrolyzed between 30 degrees C and 1000 degrees C obtaining a solid fuel (char) that was later gasified using steam as the reacting agent (5% vol.). The gasification temperature was set at 900 degrees C. Biomass samples reactivity profiles showed a catalytic effect at high conversion values, which was correlated with their ash composition. Three models were used to reproduce the gasification process. Cellulose and pine bark samples were the only ones that properly fitted to these models. This fact was attributed to their low ash content. This way, a semi-empirical model for predicting the gasification rates including the catalytic effect of ashes was proposed, which highly improved the obtained fitting. H-2, CO and CO2 were the main products obtained. Furthermore, the detection of CH4 indicated the existence of methanation reactions. NOx were also observed, indicating that nitrogen was retained in the char after the pyrolysis process. (C) 2014 Elsevier Ltd. All rights reserved.