Ethanol production from forest residues: the effect of bark content

While forestry biomass is already a feasible choice for large scale heat- and power production, utilization of forest residues for the production of fuel ethanol is hindered by economic and technical challenges. Softwood is generally considered as one of the most recalcitrant lignocellulosic feedstock for biochemical conversion to ethanol, and the presence of bark in forest residues constitutes further challenges for the wood-to-ethanol process.

In this work, the effect of bark content on ethanol production was investigated in different process configurations after SO₂-catalyzed steam pretreatment of bark mixed with spruce at different ratios. The experimental results were used to study the ethanol production process utilizing forest residues by performing techno-economic analysis in Aspen Plus. The bark content of the forest residues was found to have a substantial impact on the ethanol production cost, mostly due to the impaired enzymatic hydrolysability. At current market prices, lower feedstock costs of forest residues could not offset the yield loss, as utilizing forest residues with higher bark content resulted in increasing minimum ethanol selling price.

Hot-water extraction prior to acid-catalyzed steam pretreatment was also investigated as an inexpensive option to improve enzymatic hydrolysis of bark-containing forest residues. By removing the extractives of the bark prior to steam pretreatment, the condensation of these phenolic compounds during acid-catalyzed steam pretreatment could be avoided, to some extent, which would increase the accessibility of the cellulose to the hydrolytic enzymes.