Combined ethanol and methane production using steam pretreated sugarcane bagasse

Efficient energy production relies on complementary use of crop residues, to enhance the amount of energy obtained per unit biomass. In this frame, sugarcane bagasse (SB) was pretreated and the resulting slurry and liquid fraction served, respectively, for simultaneous saccharification and fermentation (SSF) at high solid concentration (15%), and anaerobic digestion (AD). More specifically, SB was subjected to twelve pretreatments to enhance fiber deconstruction and subsequent energy output: steam explosion alone (195 °C for 5, 10 and 15 min), after impregnation with 0.4% and 0.7% Ca(OH)2, and at 205 °C for the same three times after 0.7% Ca(OH)2 addition. After pretreatment, enzymatic hydrolysis was carried out on washed solid fraction; glucose and xylose were determined on this fraction as well as residual liquid fraction. On this latter, inhibitors (acetic and formic acid, furfural and 5-hydroxymethylfurfural) were also determined. Based on high glucose yield in enzymatic hydrolysis, three pretreatments were selected for SSF of the slurry. The same pretreatments underwent AD of the liquid fraction. Inhibitors augmented at increasing time and temperature, although never achieved critical levels. Lignin removal (range, 17-38%) was enhanced by lime addition, whereas increasing temperature and time did not contribute to delignification. Glucose yield in washed solid fraction varied accordingly. SSF exhibited the highest ethanol yield with mild lime addition (60% of theoretical) vs. steam alone (53%). However, modest yields were generally evidenced (average, 55%) as a result of high viscosity, especially in the case of high lime dose in SSF at high solid concentration. Combined energy yield (ethanol, methane and solid residue) proved lime effectiveness as catalyst in steam explosion of SB, beside two intrinsic advantages: low water consumption in SSF at high solid concentration, and the possibility of lime removal from downstream effluents through carbonation.