Biomass torrefaction in a standard retort: A study on oil palm solid residues

Abstract

This study evaluates the torrefaction of oil palm empty fruit bunches (EFB) and mesocarp fiber in a standard retort at laboratory scale (according to standard ISO 647). The influence of torrefaction temperature (220 °C−270 °C), residence time (up to 60 min) and reactor heating rate (5 K/min–15 K/min) are determined on solid, liquid and gas yields, as well as their chemical and fuel properties. Torrefaction yielded between 64 wt-% and 92 wt-% solid that exhibited lower heating value increase between 11 % and 28 % compared to the original biomass. The liquid fraction ranges between 6 wt-% and 26 wt-% of the raw biomass, containing 26 wt-%–42 wt-% of water and LHV up to 11 MJ/kg. The gas fraction represents 1 wt-%–9 wt-% of the original biomass with LHV between 0,6 MJ/kg–2,4 MJ/kg. Optimum trade-offs for solid yields and lower heating values were obtained for torrefaction with a heating rate of 10 K/min and a residence time of 30 min at 272 °C for EFB (62,8 wt-% solid yield and 17,7% increase in the original LHV) and at 282 °C for fiber (65,3 wt-% solid yield and 25,8% increase in LHV). A formal reaction kinetic model for pyrolyis coupled to a thermal model of the retort has been tested and used to simulate the solid mass yield and lower heating value during torrefaction and to determine the optimum trade-offs between both parameters. Simulated temperature gradients inside the retort solid bed are as high as 55 °C during heating up at a torrefaction temperature of Ttorr = 250 °C and around 25 °C after a residence time of 30 min at this temperature. These results are relevant and decisively influence the solid yield during torrefaction in the retort.