Carbon dioxide torrefaction of oil palm empty fruit bunches pellets: characterisation and optimisation by response surface methodology

The carbon dioxide (CO 2 ) torrefaction of oil palm empty fruit bunch (OPEFB) pellets was investigated at various temperatures from 250 to 300 °C and residence times from 15 to 60 minutes. The objective was to investigate the effects of CO 2 torrefaction on the yield and characteristics of the torrefied products. The optimal conditions for maximum mass yield (M Y ) of CO 2 torrefaction were also examined by response surface methodology (RSM) using full factorial design. Results revealed that temperature and time significantly influenced the mass (M Y ), liquid (L Y ) and gas (G Y ) yields. The M Y and energy yield (E Y ) decreased with increasing severity of torrefaction, whereas the L Y , G Y , energy density (D E ) and higher heating value (HHV) increased during the process. Characterisation revealed substantial improvements in the microstructure, pH, hydrophobicity and grindability of the torrefied pellets compared with the raw pellets. The thermal ignition and degradation characteristics of the OPEFB pellets were also significantly transformed after torrefaction. The liquid torrefaction products contained an acidic, turbid and pungent mixture of water and organic compounds. RSM optimisation revealed the optimal conditions: temperature of 275 °C and residence time of 35 minutes with the predicted M Y of 50.54%, mass loss of M L = 49.46% and HHV = 24.47 MJ/kg. The findings revealed that CO 2 torrefaction is a practical approach to clean energy recovery.