Changes on wood powder morphology and flowability due to thermal pretreatment,
Résumé
Torrefaction is a thermal pretreatment of lignocellulosic biomass before gasification. This mild form of pyrolysis, carried out at temperatures between 200 and 300 °C, changes the physical and chemical properties of the material. In particular, it improves the feedstocks homogenisation, enhances the aeration ability and makes the grinding easier. Our project deals with the study of the effects of the combined torrefaction and grinding processes on (i) the major particle behaviour: grindability, surface state, particle size, shape distribution (ii) and the major powder behaviour: bulk density, compressibility, aerability. In this article, the studied parameter is the torrefaction temperature. The characterisation of the particles and powders is performed using the following techniques: optical microscopy, Flodex® methodology and powder rheometry. The rheological behaviours of spruce torrefied at different temperatures then ground in the same apparatus (a knife mill with a sieving grid of 500 µm) are compared. It is shown that spruce torrefied at 240 °C (S240) has similar rheological properties compared to natural spruce (NS) after being ground, while the one torrefied at 300 °C exhibits a different rheological behaviour. The results are correlated to reproduce the coupled effect of the torrefaction and grinding conditions on the powder rheological behaviour.
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