Torrefaction of olive tree pruning: Effect of operating conditions on solid product properties

•Fuel properties of the olive tree pruning were improved by torrefaction.•Changes in properties of solid are mainly sensitive to torrefaction temperature.•Higher heating value increased from 17.32 MJ/kg to 20.50 MJ/kg.•The hemicellulose is the component more affected during the torrefaction.•Effect of torrefaction on pyrolytic behavior of material was investigated.

In this work, the effect of experimental conditions of torrefaction on properties of olive tree pruning was investigated. This study has demonstrated that fuel properties of olive tree pruning were improved by torrefaction. For example, torrefaction at 300 °C during 60 min provided to raw biomass an increase on the ratio of fixed carbon to volatiles (from 0.23 to 0.39) and it improved its fuel quality. Elemental analysis revealed that the composition of olive tree pruning moved from lignocellulosic biomass to coal (for example, from O/C and H/C ratios of 1.02 and 0.17 for raw biomass to 0.90 and 0.15 for torrefied sample at 300 °C during 10 min). Also, as a result of torrefaction, a more homogeneous solid with higher heating value was obtained. However, bulk density of solid did not change significantly after torrefaction process. In addition, SEM analysis indicated that the olive tree pruning surface structure was broken and destroyed by torrefaction process. Also, the thermogravimetric analysis of native and torrefied samples in nitrogen atmosphere showed a strong degradation of hemicellulose and, also, in severe torrefaction conditions, a modification on the thermal stability of cellulose. Finally, an investigation on the pyrolysis kinetics of native and torrefied samples was presented. Lower energy activation values of main lignocellulosic pseudo-components were obtained for torrefied samples.

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