Experiments on torrefaction of Dichrostachys cinerea wood: two-level factorial design and thermogravimetric analysis

The aim of this study was to determine the effect of the temperature and reaction time variables on the solid yield and the increment of heating values and composition of torrefied Dichrostachys cinerea wood by using a factorial experiment design and thermogravimetric analysis. The significant factors were identified by means of a two-level factorial design type (2²), for which the statistical software Design Expert version 10 was used. Torrefaction and thermogravimetric experimental runs were carried out in a fixed-bed reactor and thermobalance TGA–DTA, respectively. Torrefaction temperature, residence time and their interaction have a significant effect on solid yield, whilst the effect of the temperature was the only statistically significant factor on increment at a high heating value (HHV). The R-Squared values for both response variables were greater than 95% in each case. An increase in torrefied biomass was achieved at HHVs of 14.92 and 30.31% under the conditions of 120 min at 250 and 290 °C, respectively. Thermogravimetric characterisation and DTG–TG curves of the torrefied material suggest that the pre-treated material has been modified chemically and structurally.