Forest residues as a potential feedstock for a biorefinery : material balance and pretreatment strategies

Forest residues represent an abundant and potentially sustainable source of biomass which could be used as a feedstock for a biomass-to-chemicals-and-fuels process (biorefinery). However, due to the heterogeneity of forest residues, one of the expected challenges will be to obtain an accurate material balance of both the starting and pretreated material. As current compositional analysis methods have been developed to quantify more homogenous feedstocks such as whitewood and agricultural crops, it is likely that they will have difficulty in providing a complete material balance for these more diverse substrates. The research work initially assessed the robustness of established methods to quantify a variety of forest residues (bark, hog fuel, forest thinnings, logging residue, disturbance wood) before and after steam pretreatment. It was anticipated that the diverse chemistry and heterogeneity of forest residues would make it difficult to obtain an accurate material balance. Although the NREL recommended methods provided a reasonable estimate of carbohydrate components of the various feedstocks, method revision was necessary to accurately quantify the non-carbohydrate components and thus obtain an acceptable summative mass closure. This was particularly evident for high-extractive containing residues such as bark. After steam pretreatment, the incomplete removal of extractives from the pretreated material proved to be more problematic. The refined material balance methods were subsequently used to evaluate the potential of using pretreated forest residues as a biorefinery feedstock. Acid catalysed steam pretreatment was not as effective on forest residues and poor sugar yields were obtained despite using high enzyme loadings. It was likely that, in the acidic medium resulting from SO₂ catalysed steam pretreatment, the extractives reacted with the lignin and consequently restricted enzyme accessibility to the cellulose. In contrast, an alkaline pretreatment effectively removed most of the extractives and lignin from cellulosic components of the bark. The resulting cellulose-rich, water insoluble component could be almost completely hydrolyzed. It was apparent that established analytical methods will have to be modified to obtain a representative material balance of both the starting and pretreated material and that, even with “tailoring” pretreatment/fractionation strategies, forest residues will prove to be challenging feedstocks for any potential bioconversion process.

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