Influence of torrefaction as pretreatment on the fast pyrolysis of sugarcane trash

Abstract

This study aims to perform a comprehensive analysis of the two-step pyrolysis process, including the three-phase products obtained from torrefaction followed by fast pyrolysis (FP), with particular attention to the redistribution of mass, elements and energy. Torrefaction pretreatment of sugarcane trash (ST) was carried out in a tube furnace at 220 °C, 260 °C and 300 °C for 60 min. Torrefaction results showed that the quality improvement of ST was in the increase in carbon content and the removal of low-calorific volatiles. Fast pyrolysis of raw and torrefied ST was carried out in a mechanically stirred fluidized bed reactor. The results showed that torrefaction pretreatment led to an increased biochar yield and a decreased bio-oil yield in the subsequent FP step. Nevertheless, the quality of obtained bio-oil was enhanced by the higher phenolic content and the lower water and acid contents. On the other hand, the total yield of liquid (including torrefaction liquid and FP bio-oil) of the two-step pyrolysis process decreased while the solid (biochar) yield increased and the total gas yield changed less, compared to the direct FP of raw ST. Furthermore, the elemental redistribution results revealed that most of the carbon (65.21–93.02 wt%) remained in ST after torrefaction, while considerable amounts of oxygen (26.14–69.58 wt%) and hydrogen (23.53–61.25 wt%) were transferred into the torrefaction liquid and non-condensable gases, including H2O, CO, and CO2, amongst others. Consequently, most of the feedstock energy (67.14–89.02 %) was transferred into FP products for the two-step pyrolysis process. In view of the redistribution properties and product quality (especially bio-oil), torrefaction prior to fast pyrolysis is recommended and 260 °C is regarded as a suitable temperature to balance product yield and quality. Overall, this study provides valuable data for a better understanding of the role of torrefaction pretreatment on biomass fast pyrolysis.