6-03: Pellets, a globally traded biomass feedstock that can be used for a biomass-to-sugars process. Why are they not as recalcitrant as anticipated?

Steam pretreatment is recognised as a promising method for opening up the cellulosic fraction to enzymatic hydrolysis while allowing good recovery of the hemicellulose and lignin components. Typically pulp chips or undensified biomass substrates such as straws are used as the biomass feedstock. However, this limits the cost of transportation. In earlier work, we evaluated the potential of SO₂-catalyzed steam treatment to help in the densification of wood pellets. We also assessed the subsequent recovery of the original wood sugars and the ease of hydrolysis of the cellulosic component. We have shown that steam pretreatment substantially reduced the particle size of the woodchips reducing the requirement for grinding prior to pelletisation. The resulting pellets had a substantially higher density and mechanical strength compared to regular pellets and they remained intact under high humid conditions. Surprisingly, pelletisation did not significantly influence the recovery of the water soluble hemicellulose fraction while the cellulosic component was almost as readily hydrolysed as the non-pelletised substrate. The mechanism by which the SO₂ catalysed steam pretreatment facilitated both pelletisation and subsequent enzymatic hydrolysis will be discussed. It appears that the redistributed lignin on the fibre surface, the presence of water soluble hemicellulose and the smaller particle size all contributed to enhancing the binding characteristics during densification, resulting in durable pellets. It is likely that the soluble hemicellulose present in the steam pretreated material minimised the extent of fibre collapse during drying and subsequent palletisation, resulting in sufficient accessibility to the cellulose required for effective enzymatic hydrolysis.