Effects of Biomass Particle Size on Slow Pyrolysis Kinetics and Fast Pyrolysis Product Distribution

Abstract

Thermochemical conversion of lignocellulosic biomass is a promising technique to produce biofuels and intermediates. The effects of important parameters such as biomass particle size, shape, composition, heating rate, and residence time on the kinetics of devolatilization and bio-oil composition need to be understood thoroughly in order to successfully scale up the process. Pyrolysis of mixed wood sawdust of eight different particle sizes (26.5–925 µm) is conducted at nine different heating rates (0.5–100 °C min⁻¹) in a thermogravimetric analyzer, and at fast heating rates (~10,000 °C s⁻¹) in analytical pyrolyzer coupled with gas chromatograph/mass spectrometer. The apparent activation energies (E_α) evaluated by isoconversional Friedman method in the very slow (0.5–3 °C min⁻¹), slow (5–20 °C min⁻¹) and medium heating rate regimes (50–100 °C min⁻¹) were 153–203, 174–251 and 286–380 kJ mol⁻¹, respectively. The yield of phenolics and linear hydrocarbons decreased, while the production of gases like CO and CO₂ increased with particle size during fast pyrolysis. High yield of aromatics was obtained with medium sized particles (362.5, 512.5 μm). This study demonstrates that E_α decreases and increases with particle size in the very slow and slow heating regimes, respectively, which is attributed to the effect of particle shape that induces mass transfer limitations in the transport of volatiles, and intraparticle thermal gradients that induce tar decomposition reactions.