Char quality response surfaces from torrefaction of coppiced willow in a horizontal moving bed pilot plant

Abstract

Characterizing the operation of a novel torrefaction plant based on horizontal moving bed reactors was the focus of this research work. Coppiced willow biomass was used as the feedstock for the torrefaction experiments carried out with this plant. The operating temperature and torrefaction residence time were each varied over a wide range in order to understand how these two parameters each influence torrefaction severity. The torrefaction severity was characterized by developing response surface plots for torrefaction metrics, including mass yield, energy yield, milling energy, and elemental composition. The operating conditions were varied from 220–280 °C with an 8–24 min residence time. The coppiced willow biomass was processed into a granular form, resulting in an average length and diameter of 19.3 and 7.5 mm, respectively. The operating conditions were found to produce a wide range of torrefaction severities with a mass yield range from 64.7–91.6 %. Temperature was found to have a greater influence over severity than exposure time. In order to understand the uncertainty of the measurements, experiments were repeated at a central point (250 °C and 16 min) six times. The repeatability was found to be ±0.72 % at the 95 % confidence limit. The response surfaces for mass and energy yield indicate that, for this feedstock, torrefaction severity below 80 % mass yield can be achieved in under 10 min at temperatures of 255 °C and above. This performance compares very favourably with other continuously fed pilot scale torrefaction plants.