Influence of temperature and particle size on structural characteristics of chars from Beechwood pyrolysis

This work investigates the effect of temperature and particle size on the product yields and structure of chars obtained from the pyrolysis of Beechwood Chips (BWC), a lignocellulosic biomass. BWC of three different size fractions (0.21-0.50 mm, 0.85-1.70 mm and 2.06-3.15 mm) were pyrolyzed at atmospheric pressure and temperatures ranging from 300 to 900 °C in a fixed bed reactor. Tar and gas yields increased with increasing temperature, while char yield decreased, particularly between 300 and 450 °C. The effect of particle size was mostly observed at temperatures lower than 400 °C as a larger char yield for larger particles due to intraparticle reactions. At higher temperatures the larger surface area in the char fixed bed favoured reactions increasing char and gas yields from the smaller particles. Pyrolysis chars were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Raman spectroscopy. Loss in oxygenated functional groups and aliphatic side chains with increasing temperature was revealed, along with an increase in the concentration of large aromatic systems, leading to a more ordered char structure but no significant graphitization. The changes in char nature at high temperature led to a loss in their combustion reactivity. Raman spectra indicated that the temperature needed to completely decompose the cellulose structure increased with biomass particle size and the enhanced intraparticle reactions in pyrolysis of large particles was likely to give rise to amorphous carbon structures with small fused ring systems.