As previously reported, the extractives contained in eucalyptus woods increase viscosity of concentrated black liquor abnormally and make troubles at burning. However, such extractives are known to be easily dissolved in alkaline cooking liquor and partly decomposed. The authors have applied wet combustion process for the oxidative decomposition of extractives in black liquor in order to improve the properties at evaporation and combustion.
Different from an ordinary Zimmermann process, the partial wet combustion, of which COD reduction is about less than 20%, was found to be effective for lowering the viscosity of concentrated black liquor and the swelling volume, preserving the sufficient calorific value of black liquor, which enables the chemical recovery for the kraft process.
Moreover, the changes of black liquor on the partial wet combustion were examined by gel filtration methods and found the fraction of higher molecular weight was decomposed easily as well as lignin.