Combustion behavior of kraft black liquor droplets from hot water pretreated hardwood and softwood chips

This paper describes the combustion properties of birch and spruce kraft black liquors obtained from an integrated forest biorefinery concept in which a hot-water extraction of chips was performed prior to pulping. This pretreatment, aiming mainly at the recovery of various hemicelluloses-derived materials, increased the concentrations of lignin and hydroxy acids in black liquors, compared to those in the reference black liquors without any process modification. On the other hand, the pretreatment decreased the concentrations of volatile acids and other organics (extractives and hemicellulose residues). Due to these characteristic changes, the total burning times (pyrolysis time plus char burning time) of the reference black liquors were somewhat longer than those of black liquors from the modified cooking process. It was also shown that the novel biorefinery-based black liquors swell more than the conventional ones. This phenomenon was concluded to be primarily associated with the combined effect of high-molarmass lignin fragments and hydroxy acids. All of the detected changes in combustion behavior were more intense for birch black liquors than for spruce black liquors. Application: This study clarifies various effects caused by the typical hot-water pretreatment of hardwood and softwood chips prior to kraft pulping on the combustion properties of black liquor.