Hemicellulose degradation

Table 3 shows that mainly the hemicellulose was converted and/or degraded in the steam pretreatment. About 60% of the original hemicellulose could be recovered, except when 2% H2S04 was combined with high reaction temperature (210°C). In this case, the hemicellulose recovery was only 55.9%. This relatively low recovery could be explained by the hemicellulose degradation to other byproducts during steam pretreatment. 

A mild severity parameter was chosen to reduce the hemicellulose degradation while ensuring an adequate cellulose fiberization. The steam pretreatment was therefore carried out at 215°C for 3 min corresponding to Log R0 3.86. 

The hemicellulose-degrading enzymes (Ultraflo L) and the cellulases (Celluclast 1.5L) were kindly donated by Novozymes A/S. Celluclast 1.5L had a filter paper activity of 80 filter paper units/mL, determined according to the procedure of Mandels (8-9). For the present study the most important side activities of Ultraflo L were endo-l,4-P-xylanase, P-xylo-sidase, and a-arabinofuranosidase (10). 

The pyrolysis oil collected between 200 to 275 C in the second step was 18.14 % by wt. on an anhydrous wood basis and corresponded to the hemicelluloses degradation . In general, hemicelluloses decompose to yield furans and its derivatives as well as a series of aliphatic carboxylic acids. Hardwoods are composed of pentosan-based hemicelluloses, while hexosan-based hemicelluloses are found in softwoods. Hardwood produces more low molecular weight carboxylic acids and furans than softwood upon decomposition . In contrast, hardwood contains a lower quantity of fatty acids than softwood. Cl - C6 carboxylic acids, fatty acids (e.g. Cia), fiiran and derivatives were identified in this fraction. In the second pyrolysis step, a little rearrangement of the lignin polymer occurred which was based on the formation of small amount of phenolic compounds 0,395 % by wt., anhydrous wood basis). The main components of this fraction are listed in Table 2. Betulin and lupeol were abundant in this fraction. Furthermore, levoglucosan which has been reported earlier as a major compound in the cellulose-derived pyrolysis oils, was identified in the second... 

Steam explosion Causes lignin transformation and hemicellulose solubilization Cost-effective Higher yield of glucose and hemicellulose in the two-step method Generation of toxic compounds Partial hemicellulose degradation... 

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