Further Insight into Carbohydrate Degradation and Dissolution Behavior during Kraft Cooking under Elevated Alkalinity without and in the Presence of Anthraquinone

The polysaccharide degradation and dissolution behavior during high liquor-to-wood ratio (200:1) kraft cooking of Scots pine wood meal was studied at high (1.55 M) and moderate (0.50 M) hydroxide ion concentrations at a constant sulfidity of 33%. Both alkalinity levels were studied in and without the presence of anthraquinone (AQ) (0.05, 0.15, and 0.25 g AQ/L). High alkalinity experiments without AQ at 130–160 °C clearly confirmed significant galactoglucomannan stabilization (in respect to lignin content) throughout initial and bulk delignification phases. Additionally, at high alkali compared to moderate alkali concentration, lower amounts of low molecular weight carboxylic acids originating from the degradation of carbohydrates were detected in spent black liquor. The presence of AQ provided significant hemicellulose stabilization against endwise degradation reactions, being more pronounced at moderate 0.50 M concentration than at 1.55 M hydroxyl ion concentration. In all cases, higher alkalinity promoted carbohydrate removal via dissolution, and the addition of AQ reduced the degradation of the dissolved carbohydrate fraction, thus further increasing the amount of dissolved polysaccharides found in black liquor

Modification of the Lignin Structure during Alkaline Delignification of Eucalyptus Wood by Kraft, Soda-AQ, and Soda‑O₂ Cooking

The modification of the lignin structure of an eucalyptus feedstock during alkaline delignification by kraft, soda-AQ, and soda-O₂ cooking processes has been investigated by different analytical techniques (size exclusion chromatography (SEC), pyrolysis gas chromatography–mass spectroscopy (Py-GC/MS), ¹H–¹³C two-dimensional nuclear magnetic resonance (2D-NMR), and ³¹P NMR). The characteristics of the lignins were compared at different pulp kappa levels, and with the native lignin isolated from the wood. The structural differences between the kraft, soda-AQ, and soda-O₂ residual lignins were more significant at earlier pulping stages. At the final stages, all the lignin characteristics were similar, with the exception of their phenolic content. Strong differences between lignins from pulps and cooking liquors were observed, including enrichment in guaiacyl units in pulp residual lignin and enrichment in syringyl units in black liquor lignin. A comparison of the alkaline cookings indicate that soda-O₂ process produced higher lignin degradation and provided promising results as pretreatment for the deconstruction of eucalyptus feedstocks for subsequent use in lignocellulose biorefineries