Formation of insoluble components during autohydrolysis of Eucalyptus globulus

Autohydrolysis reactions of ligninleading to the formation of insolublecomponents and scaling during waterprehydrolysis and during handling ofthe prehydrolysate were studied. Scalingrepresents a major problem for theextraction of valuable hemicellulosedegradation products from theprehydrolysate.Various lignin fractions isolated fromautohydrolysis experiments wereextensively characterised. In addition,lignin mass balances were established.During autohydrolysis, lignin isdegraded to a significant extent throughcleavage of aryl-ether bonds resulting ina considerable reduction of themolecular weight. Some of the dissolvedlignin degradation products remainsoluble in the prehydrolysate, whiledegradation products characterised by ahigher molecular weight precipitateupon cooling. Exposure of waterprehydrolysates to 170°C leads to theformation of highly condensedprecipitates characterised by anincreased molecular weight. Theseprecipitates, denoted as PT-fraction,form scaling on the reactor wall.Condensation reactions of soluble andinsoluble lignin degradation productsdissolved in the prehydrolysate wereidentified as the origin of the PT-fraction.Keywords: