Combustion properties of spruce black liquor droplets: Sulfur-free pulping and influence of hot-water pretreatment

Hot-water pretreatment of lignocellulosics prior to sulfur-free alkaline pulping is an interesting approach for recovering value-added carbohydrate- and lignin-derived materials. This paper investigated the chemical composition variations and combustion properties of the black liquors (BLs) from three different cooking processes: two sulfur-free soda-anthraquinone (soda-AQ) and oxygen-alkali (O2-NaOH) cooks and kraft cook, before and after hot-water pretreatment of the spruce sawdust. Relatively less aliphatic acids but more lignin were present in the BLs after hot-water pretreatment as cooking time prolonged. In general, the burning time decreased as follows: soda-AQ BL > O2-NaOH BL ≅ kraft BL. With the same cooking time, the increase in the total burning time for soda-AQ BLs compared with kraft BLs was 11-17% (without pretreatment) and 25-39% (with pretreatment). Compared to kraft BLs with the same cooking time, the swelling for soda-AQ and O2-NaOH BLs decreased by 8-25% and 4-23%, respectively.