Utilization of forest harvest residues in a biofuel production process was studied. The chemical composition of forest biomass and its effects on processability were evaluated. The biomass was fractionated using the SO2-ethanol-water (SEW) pulping technology and optimal processing conditions were determined. Conversion of the released cellulosic fibers into sugar monomers through enzymatic hydrolysis was also studied. The raw materials included hardwood (HW) and softwood (SW) biomass which consisted mainly of branches and tree tops. The chemical composition differed clearly from that of stem wood due to the presence of bark and special tissues, such as reaction wood, in biomass. SEW fractionation efficiently dissolved lignin and hemicelluloses from biomass while cellulose remained mostly intact. The degradation of dissolved hemicelluloses was minimal and thus, compounds inhibiting the fermentation were not produced in notable quantities at normal operating conditions. However, sugar degradation was observed at severe treatment conditions. Hemicellulose dissolution and HW delignification were comparable to those of stem wood but SW delignification was clearly inferior. Especially the presence of polyphenolic acids, typical for coniferous bark, was found to reduce the degree of delignification. The negative effect of SW bark was also demonstrated by purposely adding bark to the feedstock, resulting in increased amounts of undigested wood rejects. Similar effect was not observed with HW biomass since its rejects consisted mainly of undigested bark. Both HW and SW SEW fibers were effectively hydrolyzed by commercial enzymes although SW biomass fibers required significantly higher enzyme dosages. Especially a high residual lignin content in the SEW treated fibers reduced the enzymatic hydrolysis and this explains in part the high recalcitrance of SW biomass. Besides lignin, polyphenolics present in SW bark also bind to the enzymes thereby reducing their hydrolytic activity. SW bark was found to notably impair hydrolysis while HW bark only had a negative effect on enzymatic hydrolysis at very high bark content. Based on this study it was estimated that the best method for overcoming the negative effects of bark was to apply surfactants during enzymatic treatment. It was also speculated that lignosulfonates produced in the SEW fractionation could be utilized as enzyme enhancers instead of commercial surfactants.
|Translated title of the contribution||Metsäbiomassan SO2-etanoli-vesi-fraktiointi ja entsymaattinen hydrolyysi|
|Publication status||Published - 2014|
|MoE publication type||G5 Doctoral dissertation (article)|
- enzymatic hydrolysis
- forest biomass
- sulfur dioxide -ethanol-water (SEW) fractionation