Combustion and gasification characteristics of chars from raw and torrefied biomass

Torrefaction is a mild thermal pretreatment (T < 300 °C) that improves biomass milling and storage properties. The impact of torrefaction on the gasification and oxidation reactivity of chars from torrefied and raw biomass was investigated. Thermogravimetric analysis was used to study the differences in O2 and steam reactivity, between chars prepared from torrefied and raw willow, under both high- and low-heating-rate conditions. High-heating-rate chars were prepared at 900 °C with a residence time of 2 s. Low-heating-rate chars were prepared with a heating rate of 33 °C/min, a maximum temperature of 850 or 1000 °C, and a residence time of 30 min or 1 h, respectively, at the maximum temperature. Pretreatment by torrefaction consistently reduced char reactivity. Torrefaction’s impact was greatest for high-heating-rate chars, reducing reactivity by a factor of two to three. The effect of torrefaction on a residence time requirements for char burnout and gasification was estimated.