Influence of alkali metals on the kinetics of the thermal decomposition of biomass

Potassium and sodium are well-known catalysts in the thermal reactions of biomass. This paper develops a modified Langmuir-Hinshelwood expression linking the rate of thermal degradation of a biomass to its potassium or sodium content. Willow samples impregnated with different potassium or sodium concentrations were studied for their pyrolysis behaviour by thermogravimetric analysis, and apparent first order kinetics derived. The developed relation yields a maximum reaction rate constant and a metal saturation constant that can predict the rate of willow pyrolysis based on temperature and concentration of potassium or sodium. The apparent first-order reaction rate reaches a limit at ca. 3.5 wt.% (Na) and 4 wt.% (K); this is attributed to maximum active site availability. Ab initio modelling of possible chelated structures indicates that the metal ions form multiple interactions with hydroxyl and ether groups in the cellulose structure. Metal ions chelated at the C6 position in the ring, have interactions with four oxygen atoms, while metal ions at the C2 position have interactions with only two oxygens. Structures are more stable when the metals can coordinate to more oxygen groups. It is speculated that changes in geometry upon chelation favours certain ring conformations, which might enhance reactivity to cracking reactions.