Production of sugars, ethanol and tannin from spruce bark and recovered fibres: Dissertation

Valorisation of forest industry-related side- and waste streams in a biorefinery context could help to reduce dependence on fossil resources and introduce new value chains and sources of income for the forest industry. This thesis examined two abundant and underutilized biomass streams spruce bark and recovered fibres as biorefinery feedstocks for the production of sugars, ethanol and tannin. Spruce bark was found to contain 11-12% tannin and 48-51% carbohydrates, mainly cellulose, pectin and non-cellulosic glucose. Up to 21% of spruce bark dry matter could be solubilised by hot water extraction at 60-90°C and the results indicated that a selective extraction of only tannins or carbohydrates is not possible in these conditions. The resulting spruce bark extracts were found to contain up to 58% tannin and 22-32% carbohydrates, out of which a minimum of 55% could be enzymatically hydrolysed to monosaccharides. Steam explosion solubilised pectin and hemicellulose, and increased the enzymatic digestibility of spruce bark carbohydrates from 36% to 75%. Hot water extracted bark could be hydrolysed efficiently (80% hydrolysis yield) without steam explosion when an enzyme mixture containing pectinase activity was used, indicating that an additional pretreatment step is not needed. Recovered fibres were fractionated in pilot scale from solid recovered fuel (SRF), a standardised combustion fuel composed mainly of packaging waste, and the composition and enzymatic digestibility of the material were determined. A fibre yield of 25-45% was obtained in the pilot-scale fractionation of three different SRF samples, and produced material contained at least 46% hexose polysaccharides and 12-17% ash. The enzymatic digestibility of recovered fibres was found to be high without pretreatment and the hydrolysis yield of recovered fibres in high consistency conditions was found to be higher than the hydrolysis yield of steam pretreated wheat straw and spruce. Non-ionic surfactants improved the hydrolysis yield of recovered fibres and the results indicated that their effect is dependent on the botanical source, pretreatment and lignin content of the feedstock as well as the mixing regime. Selected steps for processing spruce bark and recovered fibres were scaled up from laboratory- to small pilot scale. Up to 22 kg of crude tannin powder was produced from spruce bark representing a 9% yield from dry bark. Ethanol production was demonstrated from recovered fibres in seven up to 12-day long continuous pilot experiments according to the FibreEtOH concept. The results of the work carried out in this thesis indicate that the biorefinery concepts presented for spruce bark nd recovered fibres have technical potential for industrial application.