Quantification of the torrefaction effects on the grindability and the hygroscopicity of wood chips

- Beech wood chips are torrefied in a continuous pilot-scale rotary kiln.
- A minimum of hygroscopicity is obtained for a mass loss between 1.7 and 7.4%.
- A method is developed to determine accurately the grinding energy requirement.
- The surfacic grinding energy is divided by 6.3 between wet and dry chips.
- Torrefaction can divide surfacic grinding energy by 8.1 for a mass loss of 25%.

In the field of biomass torrefaction, lots of product properties have been widely investigated at the lab scale but some uncertainties remain about the gains in terms of grindability and hygroscopicity of torrefied products. In this study, beech wood chips (with an initial moisture content of 10-12%) have been torrefied in a pilot-scale rotary kiln. The torrefaction severity was controlled by adjusting the temperature, the treatment duration and the solid hold-up in the kiln. Mass losses ranging between 1.7% and 25% have been obtained. Properties of torrefied wood chips were then analyzed in terms of composition, heat content, hygroscopicity and grinding energy requirement. Dynamic vapor sorption measurements show that a minimum of hygroscopicity is reached for a mass loss (ML) between 1.7 and 7.8%. The moisture uptakes for mass losses above this optimum remain stable at values twice lower than that of raw biomass. Finally, a new method is proposed to estimate the grindability of wood chips. This method takes into account the grinding energy consumption and the particle size distribution of ground samples. A reduction by a factor of 6.3 of the apparent specific surface grinding energy is observed between a moisture content of 41% and the dryness. This energy measurement is in turn reduced by a further factor of 8.1 after torrefaction with a 25% mass loss.