NEW KINETIC-MODEL FOR THERMAL-DECOMPOSITION OF HETEROGENEOUS MATERIALS

In the kinetic studies of thermal decomposition of lignocellulosic materials using dynamic TG, relationships between the biomass fraction ''w'' and the time ''t'' of the form dw/dt = -k(w - w infinity)(n) are usually admitted, and the residue fraction at infinite time (w infinity) is considered constant. However, in heterogeneous solids such as lignocellulosic materials, the different polymers decompose at different temperatures, and so the value of w infinity is not constant, Therefore, the previous equation must be considered approximate. To illustrate this feature, experiments with kraft lignin, which decomposes in an interval of temperatures between 150 and 750 degrees C, have been carried out. A kinetic model is proposed, bearing in mind that there is a maximum pyrolyzable fraction at each temperature. This model considers that the thermal decomposition of a heterogeneous material occurs through a great number of reactions and that at a given temperature only some fractions can decompose. The kinetic parameters (activation energy and preexponential factor) can change during the decomposition process as functions of the reactions taking place. Under some assumptions, it is deduced that this model is equivalent to assume the kinetic law dw/dt = -k(w - w infinity) for first-order reaction, where the residue yield w infinity, is a function of the temperature.