Implications of feedstock diversity on forest-based ethanol production
Sammanfattning: The utilization of lignocellulosic biomass to produce biofuels, such as bioethanol, has the potential to provide a sustainable alternative to fossil fuels, and thus mitigate greenhouse gas emissions from the transportation sector.Forest biomass is expected to be a significant source of such biomass, as it can serve as an abundant and sustainable feedstock for bioethanol production. It is unlikely that white wood chips will be used as a sole commercial feedstock for the production of bioethanol, due to increasing feedstock competition and requirements to meet large scale. The high demand for biomass means that other forestry assortments, not traditionally utilized by the forest industry, such as harvesting residues, will have to be exploited. However, the presence of bark in these forest residues is expected to pose a challenge in the traditional wood-to-ethanol process and adversely affect the conversion efficiency. Ethanol production from softwoods was investigated with the main objective of assessing the potential of expanding the feedstock base of an ethanol plant to include not only white wood, but also other forestry residues from a process perspective. Bark was found to be significantly more difficult to hydrolyze to monomeric sugars than white wood. This could mainly be attributed to the condensation reactions of bark extractives during acid-catalyzed steam pretreatment, which rendered the otherwise water-soluble extractives insoluble, and altered the structure of the solid fraction, resulting in impaired enzymatic hydrolysis. Techno-economic evaluations showed decreasing profitability of ethanol production with increasing bark content in the feedstock. Thus, the utilization of bark-containing forestry residues will not lead to significant cost reductions compared to higher-value pulpwood at current market prices, unless the conversion of cellulose and hemicellulose to monomeric sugars is improved.Another alternative to increase the future biomass supply for large-scale bioethanol production is the use of fast-growing trees such as willow and poplar. Although the production of ethanol from these hardwood species is welldocumented, the inclusion of biomass from fast-growing tree species in a softwood feedstock base for bioethanol production has not previously been investigated. The structural differences between hardwood and softwood couldbe expected to reduce the pretreatment efficacy when treating a mixture of the two. However, it was found that the use of a mixture of poplar and spruce would presumably be constrained more by the performance of the fermenting microorganism, than the efficacy of steam pretreatment, and that the ethanol production process could be sufficiently robust to allow small amounts of hardwood in a softwood-to-ethanol process.
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