Effect of oxidative torrefaction on high temperature combustion process of wood sphere

Abstract

The present work studies the effects of oxidative torrefaction (OT) (200–290 °C, 0–8 vol% O2) on the combustion process of wood sphere in a single particle combustor (SPC, 1226 °C, 3.0 vol% O2, wet basis), together with such effects on the yield, reactivity and morphology of the high temperature produced char. Increased torrefaction O2 concentration and temperature both result in higher char yield and prolonged combustion time, and the SPC char yield can be described by a linear regression equation (R2 = 0.85) as a function of the decreased mass yield caused by the OT process. The correlation (R2 = 0.83) between the low heating rate char yield (i.e., fixed carbon content) and the torrefaction mass yield is also validated against literature data, indicating that mass yield is a good indicator for both the OT and traditional inert torrefaction. The TGA analysis suggests that the char reactivity is significantly reduced by raised OT temperature up to 290 °C, while is only limited influenced by the increase in O2 concentration (up to 8 vol% at OT temperature of 260 °C). The NMR analysis of the OT wood and the SEM observation of the OT char produce from SPC show some evidences that OT result in a more cross-linked aromatic structure, which may hinder the volatile release in the subsequent pyrolysis and thereby lead to less porous char surface and a denser and heavier char particle.