Torrefaction of olive mill waste

Abstract

Two-phase olive mill waste (TPOMW) was converted via torrefaction into a carbon rich solid interesting as bioenergy feedstock. TPOMW was characterized and torrefied in an oven at temperatures ranging from 150 to 300 °C for 2 h. Mass and energy losses occurred during torrefaction were measured and the torrefied products were characterized including ultimate analysis, heating value measurements, accelerate solvent extraction (ASE) and FTIR in order to assess the effects of torrefaction on the physicochemical properties of TPOMW. Additionally, ash fouling evaluation was also performed through XRF analysis. The weight fraction of C, defined in percentage as wt.%, improved from 56 to 68 wt.% and the high heating value rose from 26.4 to 30.0 MJ·kg−1 as torrefaction temperature increased, reaching typical values of subbituminous coal and finding the best results at 200 °C in terms of maximizing the heating value and minimizing the energy losses. Accordingly, from FTIR analysis it was observed that the degree of coalification increased during torrefaction of TPOMW. ASE results shown that the residual olive oil in TPOMW was removed during torrefaction, being completely eliminated at 300 °C. The alkali index for TPOMW was found to be 0.66 kg alkali·GJ−1, which implied a high fouling tendency that could be mitigated through co-firing. Finally, t-TPOMW briquettes with good mechanical strength and energy density of 26.7 GJ·m−3 were produced using a hydraulic piston press. Results demonstrated that torrefaction allows transforming TPOMW into a coal-like material, which would imply a profitable way to manage these wastes.