Kinetics of the thermal decomposition of biomass and the influence of alkali metals on these kinetics

The thermal degradation of biomass has received extensive consideration due to its central role in biomass combustion. Biomass decomposition is also a major step in fast pyrolysis and other thermal processing methods involved in the production of chemicals. Detailed understanding of the kinetics of biomass decomposition is vital for reactor kinetics and combustion processes modeling. Analysis methods were studied to determine which method is best suited for reliable kinetic parameter extraction based TGA derived data, kinetics most applicable to industrial applications were explored. This paper then goes on to develop a preliminary expression (involving only SRC Willow and only potassium) directly linking biomass degradation kinetics to the inherent alkali metal content. The Senum-Yang, the Murray and White, and the reaction rate constant methods all yield apparent first-order kinetics that give excellent predictions of pyrolysis under slow heating rate conditions. For higher heating rates, as encountered under flash pyrolysis conditions, kinetics expressions with high E and A values typically give more sensible predictions of conversion. A Langmuir-Hinshelwood relation can be applied to describe the catalytic effect of potassium on biomass pyrolysis. The maximum reaction rate constant of 3.26 x 10-3 (s-1) and the potassium saturation constant of 0.56 (wt%) can be accurately used to derive the pyrolysis reaction rate at 300 °C of any willow sample with a known potassium concentration. A leveling off of the catalytic effect is seen with regard to potassium concentration and the apparent first-order reaction rate at ca. 4.5 wt%