Katariina Kemppainen's thesis examined two abundant and underutilized biomass streams spruce bark and recovered fibres as biorefinery feedstocks for the production of sugars, ethanol and tannin.Valorisation of the 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.
The production of lignocellulosic biofuels and chemicals is driven by the increasing global consumption of energy, food and feed, the depletion of fossil oil reservoirs and efforts to slow down climate change. On the other hand the forest industry faces challenges derived from the digitalization of media, increasing global competition and rising costs of energy and raw materials.
Spruce bark was found to contain 11-12% tannin and 48-51% carbohydrates, mainly cellulose, pectin and non-cellulosic glucose. The effects of temperature, solids content and the use of selected chemicals on the extraction yield of bark components were investigated. Up to 21% of spruce bark dry matter could be solubilized by hot water extraction at 60-90°C, and an increase in temperature was found to have a significant positive effect on extraction yield. However, the results indicated that a selective extraction of only tannins or carbohydrates is not possible in these conditions. The resulting spruce bark extracts were characterized in detail and found to contain up to 58% tannin and 22-32% carbohydrates, which were mainly present as oligo- and polysaccharides and as glycosylated compounds. Enzymes having hemicellulase, pectinase and β-glucosidase activities could be used to hydrolyse a minimum of 55% of the carbohydrates in the extract to monosaccharides, which could possibly enable their size-based separation by ultrafiltration from larger tannin molecules.
The effects of hot water extraction, steam explosion, and sequential combination of the two on the composition of the insoluble solids, and enzymatic digestibility and fermentation of spruce bark carbohydrates were studied. Steam explosion solubilized pectin and hemicellulose, and increased the enzymatic digestibility of spruce bark carbohydrates from 36% to 75%. A treatment at 190°C without an acid catalyst was found to improve the hydrolysis yield more than a treatment at 205°C or a treatment using an acid catalyst. 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. Furthermore, the results indicated that enzymatic hydrolysates from hot water extracted and steam exploded bark can be fermented to ethanol even at 15% solids loading.
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, but it was estimated that a higher yield could be obtained from high quality SRF with industrial scale equipment. The recovered fibres contained at least 46% hexose polysaccharides and 12-17% ash partially derived from non-biomass sources such as inks and coating materials. The enzymatic digestibility of recovered fibres was found to be high without pretreatment (>82% hydrolysis yield in 24 h).
he hydrolysis yield of recovered fibres in high consistency conditions (15-25% dry matter content) 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 effect was higher at low solids loading. The effects of surfactants on the three feedstocks were compared over a wide range of solids loadings (1-25%), 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 and recovered fibres have technical potential for industrial application.
Public examination on 20 March
The public examination of the doctoral dissertation of Katariina Kemppainen is held in Aalto University School of Chemical Technology on 20 March at 12. The address is Kemian tekniikan talo, hall Ke2, Kemistintie 1 (entrance on the side of Biologinkuja). The title of the dissertation is ”Production of sugars, ethanol and tannin from spruce bark and recovered fibres”. Opponent is Prof. Gunnar Lidén from the University of Lund (Sweden).
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