Effects of heat/mass transfer limitations and process exothermicity on the kinetic parameters of the devolatilization and oxidation reactions of wood chars

Abstract

Thermogravimetric curves of wood char oxidation consist of devolatilization and oxidation, described with a linear and a power-law rate reaction, respectively. The lack of measurement and control of the sample temperature in standard commercial thermogravimetric systems (STGA) can introduce serious deviations from the heating program, originated from heat and mass transfer limitations and reaction exothermicity. This study aims at quantifying the consequent effects on the estimated kinetic constants for beech, fir and spruce wood chars. A first set of measurements is based on a special purpose designed, homemade thermo-analyzer (HMTGA), permitting for an accurate sample temperature control and a rigorous kinetic regime. The second set makes use of a standard commercial thermobalance (STGA). It is found that for heating rates up to 10 K/min spikes in the rate curves disappear for 1 mg samples and the deviations on the activation energies become null or very small (maximum around 1–1.5%), compared with the HMTGA data. However, the order of the oxidation reaction is always lower (by about 15%). Improvements are not observed following further sample mass reduction.