Integrated production of bioethanol and biogas from agricultural residue : comparison of pretreatment methods using mass flow and energy yields analysis

Abstract: Lignocellulosic biomasses are believed to reduce the conflict for resource between food and energy production that was criticized in the first generation biofuel production. The aims of this study were two; the first being to compare the effects of three lignocellulose pretreatment methods, lime pretreatment and two steam pretreatment methods, either with or without prior acid impregnation, on the production of bioethanol and biogas from oat straw. The second was to analyze the mass flow and compare energy yields of two alternative process routes: direct biogas digestion of the pretreated oat straw, against enzymatic saccharification and bioethanol fermentation of the pretreated oat straw followed by biogas digestion of the ethanol fermentation residues. Thermochemical pretreatments, enzymatic saccharification, ethanol fermentation, and biogas digestion experiments were carried out in laboratory scale batch processes. Soluble sugars were analyzed by HPAE-PAD (Dionex); ethanol, glycerol and acetate by HPLC. Methane yields were estimated using measured biogas pressure and methane content of the biogas as analyzed by gas chromatography.The results show that ethanol production followed by biogas digestion of the fermentation residues provides opportunity for more efficient utilization of the lignocellulose biomass. Methane was in all cases produced faster from fermentation residues than from unfermented material, showing that the ethanol production step increases accessibility of the lignocellulose substrate for biogas digestion. Differences in effect on individual steps were observed between the pretreatments, which may have implications for industrial process design.