Characterisation of grindability of biomass and residues torrefied at pilot scale

Torrefaction and pelletisation are key enabling processes for the deployment of Biomass-to-Liquid technologies for sustainable aviation fuel production, relying on the use of entrained gas flow gasifiers. This work investigates the impact of torrefaction followed by pelletisation on the grindability (specific grinding energy and comminution parameters) of various biomass feedstocks (sawdust, bark, manure digestate, paper waste) at industrially-relevant conditions. All torrefied and pelletised materials showed lower specific grinding energies (8–93 kJ/kg) than the corresponding untorrefied (either pelletised or unpelletised) materials (26–499 kJ/kg). When considering the whole dataset, a negative correlation was observed between the torrefaction severity factor and the specific grinding energy of the pellets, the milled product particle size and the Holmes work index. However, data from torrefied sawdust samples alone did not show any correlation between their grindability and the torrefaction severity, suggesting that pelletisation conditions may influence significantly the mechanical properties of pellets from torrefied sawdust. This work demonstrates the improved grindability (i.e. specific grinding energy decrease) that the torrefaction plus pelletisation line-up brings to the production of pulverised torrefied biomass. Further research is still needed to elucidate any combined complex effects that torrefaction conditions, changes in the biochemical matrix of treated biomass and pellet formulation/processing can have on the final mechanical properties of torrefied biomass pellets.