UBC Theses and Dissertations
Adapting a chemi-thermomechanical pulping (CTMP) process as a possible pretreatment/front-end for an enzyme-based biorefinery Wu, Jie
To try to take advantage of existing infrastructure and experience a chemi-thermomechanical pulping (CTMP) process was assessed for its potential as a “front-end” for a biochemical-based bioconversion process. It had been shown that biomass, after mechanical pulping treatment, remained highly recalcitrant to enzymatic hydrolysis. This was largely due to the presence of lignin restricting enzyme accessibility to cellulose. Considering the high costs related to complete delignification, mild chemical treatment such as sulfonation and oxidation under neutral/alkaline conditions were assessed to minimize lignin’s inhibitory influence while maximizing the recovery of hemicellulose in the water-insoluble component. Sulfonation and oxidation were able to incorporate acid groups onto the lignin macromolecule, consequently enhancing substrate swelling. This increased enzyme accessibility to the cellulose while reducing non-productive lignin binding via increased lignin hydrophilicity. CTMP-based pretreatment was shown to be effective on agricultural and hardwood substrates. Mild alkali treatment of agricultural residues induced deacetylation of the hemicellulose and partial delignification. This resulted in enhanced enzyme accessibility to the hemicellulose and cellulose and increased enzymatic hydrolysis. Although hardwood lignin was more resistant to delignification, the incorporation of oxygen treatment into the CTMP treatment of the hardwood substrate substantially reduced the negative effects of lignin on enzymatic hydrolysis. As the lignin present in the CTMP treated substrate was enriched in acid groups, this resulted in increased substrate swelling and a decrease in the non-productive binding of enzymes to the lignin (via hydrophobic interactions). Both softwood chips and pellets where pretreated using the adapted CTMP process to provide both a comparison with hardwood and agriculture feedstocks and to assess any differences between pellets and chips. Alkali addition prior to CTMP pulping enhanced lignin sulfonation. This predominantly occurred within the secondary-cell-wall, consequently increasing cellulose accessibility. However, the pretreated softwood chips and pellets remained relatively recalcitrant to enzymatic hydrolysis. Although the reduced particle size of softwood pellets was anticipated to facilitate chemicals and enzyme access, the high temperatures used during pelletisation resulted in lignin condensation. This was indicated by higher molecular weight and lower β-O-4 linkages of pellet-derived lignin, probably contributing to this substrate’s higher recalcitrance.
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