Comparison of steam explosion, dilute acid, and alkali pretreatments on enzymatic saccharification and fermentation of hardwood sawdust

Effects were compared for three low-cost pretreatment methods (dilute acid, alkali, and steam explosion) relative to the effectiveness of environmentally friendly enzymatic hydrolysis and ethanol fermentation of aspen, birch, and oak chips. The highest monomeric sugar yield was achieved with the alkali pretreatment of the aspen chips (22 g/L of glucose and 6 g/L of xylose). Additionally, the concentration of lignocellulose degradation products formed during this pretreatment was relatively low, and so the hydrolysis and fermentation efficiencies were 80% and 85%, respectively. The application of dilute acid pretreatment led to lower yield of enzymatic hydrolysis in comparison with alkali pretreatment, resulting in 41% to 62% of theoretical yield for aspen and birch chips, respectively. Increasing the NaOH concentration led to an increase in the monomeric sugar yield, and consequently increased the hydrolysis and fermentation yields. By contrast, increasing the acid concentration resulted in a higher sugar yield, and the fermentation efficiency decreased. The applied steam explosion conditions resulted in the formation of 6.8 to 15.4 g glucose/L, with hydrolysis yield in the range 34 to 42% of theoretical. The most susceptible for pretreatment and enzymatic hydrolysis was found to be aspen biomass.