Enhancement and inhibition of the oxidation rates of pyrolytic chars from wood loaded with potassium compounds

Abstract

The oxidation is studied of chars produced from the highly exothermic packed-bed pyrolysis, carried out at moderate temperatures, of potassium-loaded beech wood (additives KOH, K2CO3, KC2H3O2 and KCl, for potassium contents in char up to about 5 wt%). In all cases, for temperatures up to 750 K, chars undergo the kinetically controlled stages of oxidative devolatilization and oxidation. For the latter, as the potassium content increases, the process dynamics shift from a single to two conversion zones, corresponding to thermal and mixed catalytic-thermal activation, respectively. The ignition and burnout temperatures initially decrease, testifying an enhancement of carbon conversion, the more the basicity of the additive is high. However, for potassium contents above certain threshold limits, effects appear first of saturation and then of inhibition. Moreover, small amounts of char are always oxidized during ash cooking, at high temperatures. Potassium enhances the char oxidation rates up to maximum contents around 1–1.5 wt% (KOH, K2CO3, KC2H3O2) and 2 wt% (KCl), resulting from alkaline catalysis and modifications in the char structure, caused by interactions between additives and wood during both aqueous impregnation and pyrolysis.