Biomass fractionation methods need to be optimized to enable selective separation and recovery of valuable intermediates. The effects of oxygen on the extraction and structure of biomass components are generally not well understood. Molecular oxygen can be used as reagent in fiber or lignin modification processes such as pulp delignification or lignin oxidation. On the other hand, the presence of oxygen during biomass processing may be detrimental, lowering selectivity, inducing formation of lignin-carbohydrate linkages and auto-oxidation of extractives, thus hindering fractionation. In the FROG project, we aim to obtain more fundamental understanding of the molecular oxygen chemistry and control of its content for optimal performance in different types of process methods. Computational Fluid Dynamics will be used to model the mixing process for ensuring the optimal oxygen dissolution, supporting the scale-up of the process units. The results will aid the development of fractionation processes based on controlled concentration of oxygen.
Main results
Birch extractives seem to have an effect on the solubility of wood polymers during alkaline pulping. Removal of the extractives prior to pulping increased the overall yield. At the same time also delignification efficiency decreased; nevertheless, the yield gain was not only due to the increased lignin content of the pulped wood, but increased xylan content was also seen. The postulated effect of oxygen was not seen in the results: the oxidation of the extractive compounds prior to pulping did not seem to have an effect on the delignification chemistry.
Inner birch bark, a residue left after removal of the suberin- and betulin-rich outer bark, contains tannins and other polyphenolic substances that were extracted by an alkaline treatment with yields 10%-25% (on dry inner bark), depending on extraction conditions. The addition of molecular oxygen during alkaline extraction decreased the yield of extracted polyphenols, mainly tannin and some lignin, due to oxidative degradation of phenolic structures. However, the oxidation of the polyphenols considerably increased their solubility, making them almost fully soluble at pH 4.
Publications
Hakala, M., Effect of extractives in birch kraft pulping, Master’s Thesis, Aalto University, 2022. http://urn.fi/URN:NBN:fi:aalto-202210236156
Hirvelä, A., llman happipitoisuuden vaikutus koivun fenolisten uuteaineiden reaktioihin (Effect of atmospheric oxygen on reactions of phenolic extractive compounds in birch), Master’s Thesis, Aalto University, 2021. http://urn.fi/URN:NBN:fi:aalto-202106207555
Eilamo, T., Oxidation reactions of phenolic extractive compounds in wood, Masters Thesis, Aalto University, 2020. http://urn.fi/URN:NBN:fi:aalto-202003222582
Aso, A., Effect of water-soluble extractives on birch wood reactivity in kraft pulping, Masters Thesis, Aalto University, 2021. http://urn.fi/URN:NBN:fi:aalto-202109059023