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Abstract
BackgroundForest residues represent an abundant and sustainable source of biomass which could be used as a biorefinery feedstock. Due to the heterogeneity of forest residues, such as hog fuel and bark, one of the expected challenges is to obtain an accurate material balance of these feedstocks. Current compositional analytical methods have been standardised for more homogenous feedstocks such as white wood and agricultural residues. The described work assessed the accuracy of existing and modified methods on a variety of forest residues both before and after a typical pretreatment process.ResultsWhen “traditional” pulp and paper methods were used, the total amount of material that could be quantified in each of the six softwood-derived residues ranged from 88% to 96%. It was apparent that the extractives present in the substrate were most influential in limiting the accuracy of a more representative material balance. This was particularly evident when trying to determine the lignin content, due to the incomplete removal of the extractives, even after a two stage water-ethanol extraction. Residual extractives likely precipitated with the acid insoluble lignin during analysis, contributing to an overestimation of the lignin content. Despite the minor dissolution of hemicellulosic sugars, extraction with mild alkali removed most of the extractives from the bark and improved the raw material mass closure to 95% in comparison to the 88% value obtained after water-ethanol extraction. After pretreatment, the extent of extractive removal and their reaction/precipitation with lignin was heavily dependent on the pretreatment conditions used. The selective removal of extractives and their quantification after a pretreatment proved to be even more challenging. Regardless of the amount of extractives that were originally present, the analytical methods could be refined to provide reproducible quantification of the carbohydrates present in both the starting material and after pretreatment.ConclusionDespite the challenges resulting from the heterogeneity of the initial biomass substrates a reasonable summative mass closure could be obtained before and after steam pretreatment. However, method revision and optimisation was required, particularly the effective removal of extractives, to ensure that representative and reproducible values for the major lignin and carbohydrate components.
Highlights
Various national and global incentives have been used to try and reduce our dependency on fossil derived transportation fuels while encouraging the production and use of renewable biofuels such as ethanol [1,2]
We investigated how a “typical” pretreatment such as steam pretreatment might influence the robustness of the refined methods to provide a reasonable material balance including the reproducibility and accuracy of the mass closure and the recovery of the various biomass components
Physical characterisation and preparation of the forest derived residues The residues were predominantly derived from Pacific Northwest softwoods species such as Douglas-fir, Western Hemlock and Lodgepole Pine
Summary
Various national and global incentives have been used to try and reduce our dependency on fossil derived transportation fuels while encouraging the production and use of renewable biofuels such as ethanol [1,2]. While virtually all of the ethanol currently used in automobiles is derived from sugar or starch crops there has been a considerable investment in biomass-to-ethanol processes. A typical biomass-to-ethanol process involves the three major steps of pretreatment and fractionation, enzymatic hydrolysis of the cellulosic fraction and fermentation of the derived sugars to ethanol. One way to try and reduce these costs is by making use of underutilised biomass materials such as the residues obtained at forestry and saw/pulp mill sites. Due to the heterogeneity of forest residues, such as hog fuel and bark, one of the expected challenges is to obtain an accurate material balance of these feedstocks. The described work assessed the accuracy of existing and modified methods on a variety of forest residues both before and after a typical pretreatment process
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