A larch biorefinery producing pulp and lactic acid

Hanna Hörhammer

    Research output: ThesisDoctoral ThesisCollection of Articles


    In a forest based biorefinery pulp and paper are the main products, while side-streams are utilized for value-added products. Hemicelluloses can be removed from wood by extraction prior to pulping and converted into biofuels or chemicals. In this thesis a proof of concept for a process utilizing larch for production of papermaking pulp and biochemically obtained lactic acid was determined. Larch is a softwood species, which has a high content of water-soluble arabinogalactan. Therefore, water extraction suits especially larch wood. By removing the water-soluble carbohydrates with pre-extraction (PE) prior pulping, an additional sugar stream is formed. In this study wood chips of Siberian larch (Larix sibirica Lebed.) were pre-extracted with water prior pulping. The optimal PE conditions were determined. Both the sugar-rich extract and the pre-extracted wood chips were analyzed in detail. PE at 150 °C for 90 min dissolved about 11% sugars on wood. A mild wash was needed to wash out the dissolved carbohydrates from the wood chips. The pre-extracted and washed wood chips were used for pulping, while the extract was fermented into lactic acid. Polysulfide (PS) and anthraquinone (AQ) were used as pulping additives to compensate for the yield loss caused by the PE. Without PS and AQ addition the pulp yield loss after PE was 5% relative to conventional kraft pulp. At a PS charge of 2% and 0.1% AQ on wood the pulp yield loss was reduced to 2%. PE allows less alkali and lower H-factor. The larch pulp was then bleached, and paper sheets were made. The unbleached PE-PSAQ larch pulp had lower tear strength and optical properties compared to kraft larch pulp. The PE-PSAQ pulp was easier to bleach, and therefore reached higher brightness. The bleached PE-PSAQ pulp had lower sheet density, poorer tensile strength, better tear strength, higher opacity, higher light scattering, and higher air permeability. PE-PSAQ pulps could be used in similar applications as other softwoods. The hydrolyzed extract was fermented with different bacteria strains. Fermentation with Bacillus coagulans MXL-9 gave the most attractive results. Larch extract containing 36 g/L sugars was fermented into 28 g/L lactic acid. The lactic acid yield for 97 h cultivation time was 78.5% on initial sugar and 5.8% on wood. A process flowchart for a larch biorefinery was constructed, mass and energy balances were obtained through simulation, and the process was economically evaluated. An existing kraft pulp mill could relatively easy be converted into a larch biorefinery. The cash flow analysis showed that an additional lactic acid production for a kraft pulp mill would be profitable. The investment costs could be paid back within 16 months, when the lactic acid production was added to an existing kraft pulp mill.
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    • van Heiningen, Adriaan, Supervising Professor
    • van Heiningen, Adriaan, Thesis Advisor
    Print ISBNs978-952-60-5868-9
    Electronic ISBNs978-952-60-5869-6
    Publication statusPublished - 2014
    MoE publication typeG5 Doctoral dissertation (article)


    • biorefinery
    • larch
    • Larix sibirica
    • pre-extraction
    • fermentation
    • lactic acid


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