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

Abstract

This paper describes the combustion behavior of birch and spruce kraft black liquors obtained from an integrated forest biorefinery concept in which a hot water extraction of chips was performed before pulping. This pretreatment, aiming mainly at the recovery of various hemicellulose-derived materials, increased the concentrations of lignin and hydroxy acids in black liquors, compared with 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). Because of 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. The novel biorefinery based black liquors also swell more than the conventional ones. This phenomenon was primarily associated with the combined effect of high-molar-mass 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.