UBC Theses and Dissertations
Investigation of a two-stage steam/organosolv pretreatment approach for the fractionation of softwood biomass Lin, Mandy
In order to improve the economic viability of a bioconversion process it would be extremely beneficial to maximize the recovery of all of the lignocellulosic components while enhancing enzyme accessibility to the cellulosic component. Wood residue derived pellets are already a prominent Canadian commodity and the existing supply-chain produces a high density, low moisture feedstock suitable for mass collection and transport. However, pellets are almost exclusively used for combustion and not as a possible biorefinery feedstock. As a result, there is limited information on the influence of the pelletization process (e.g. grinding, drying, compressing) on the susceptibility of pellets, as opposed to chips, to the various pretreatment, fractionation and cellulose hydrolysis steps that are components of a typical bioconversion process. The work described in the thesis assessed the potential of a two-stage steam/organosolv pretreatment process to fractionate and isolate the hemicellulose and lignin components from softwood pellets, yielding a more accessible, cellulose-rich substrate. Various steam pretreatment conditions were compared for their ability to enhance hemicellulose solubilisation while minimizing lignin condensation (first-stage), to improve subsequent organosolv delignification (second-stage). Carbocation scavengers were compared for their ability to minimize lignin condensation during either stage. When softwood chips and pellets were compared, the effectiveness of the pretreatment was determined by hemicellulose solubilisation, delignification capability and the ease of enzymatic hydrolysis of the cellulosic component. It was apparent that pellets were more responsive than chips to pretreatment due to their smaller particle size, which facilitated both hemicellulose solubilisation and delignification. At conditions that solubilized and recovered hemicellulose, acid-catalyzed steam pretreatment induced lignin condensation. This impeded subsequent organosolv delignification and enzymatic hydrolysis. The addition of lignosulfonates as a potential carbocation scavenger during acid-catalyzed steam pretreatment resulted in increased hemicellulose solubilisation and carbohydrate recovery while improving delignification during subsequent organosolv treatment. Adding lignosulfonates during acid-catalyzed steam pretreatment also enhanced enzymatic hydrolysis. It was likely that the added lignosulfonates increased lignin hydrophilicity which facilitated lignin dissolution and decreased non-productive enzyme inhibition. It was apparent that the addition of lignosulfonates prior to pretreatment reduced the detrimental effects of lignin condensation which benefited subsequent fractionation of the pretreated biomass.
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