Combustion Properties of Birch (Betula pendula) Black Liquors From Sulfur-Free Pulping

Sulfur-free pulping has an environmental advantage over the traditional kraft process. This article describes the combustion properties of the black liquors produced from silver birch (Betula pendula) sawdust using three different cooking processes: two sulfur-free cooks (soda-anthraquinone and oxygen-alkali), and one reference kraft cook. It also considers the corresponding black liquors from an integrated forest biorefinery, in which a hot-water pretreatment of feedstock was performed prior to pulping. With the same cooking time, the total burning times for the sulfur-free black liquors were higher (15–55%) than those for the conventional kraft black liquors. However, no significant differences were noted between the total burning times for black liquors from pretreated feedstock and those from untreated feedstock. Especially in the case of untreated feedstock, the results showed that the kraft black liquors typically swelled more (25–45%) than the sulfur-free black liquors. It was further observed that the kraft and soda-anthraquinone black liquors from the untreated feedstock swelled more than those from the pretreated feedstock, while the oxygen-alkali black liquors swelled less.