Studies on Dry and Wet Torrefaction of Sugarcane Bagasse to Maximize the Recovery of Sugars and Bioethanol Yield

Abstract

Biomass size disruption is a key operation in the process of ethanol production from lignocellulosic materials since enzymatic saccharification of carbohydrate polymers occurs at the molecular level. Various pretreatments of biomass have been extensively studied for the production of sugars and ethanol. However, torrefaction was less explored in the production of sugars. Dry and wet torrefaction techniques have been employed for the production of solid biofuels for many years. Torrefaction technique has been used widely in increasing energy density, hydrophobicity, grindability, enhancing pore structure, and reducing the cost of transportation. In this work, the effect of dry torrefaction of sugarcane bagasse on the production of sugars and ethanol was studied. Moreover, the optimizations of acid (HCl, H2SO4, H3PO4, and HNO3) and alkali (NaOH, KOH, and NH3OH) impregnated sugarcane bagasse (SB) torrefaction were conducted using response surface methodology. Statistically significant models that can predict yields of total reducing sugars were generated. The dry and wet torrefaction experiments of SB were performed at temperatures of 160 °C, 180 °C, 200 °C and 220 °C and residence times of 20 min, 40 min, and 60 min. The sugarcane bagasse pretreated via dry torrefaction at 200 °C for 20 min was observed to produce the highest glucose yields of 199.62 mg g-1 of biomass (dry basis) after saccharification. Furthermore, the pretreated SB under anaerobic fermentation with supplementation of cysteine hydrochloride was noticed to produce ethanol yield up to 81.85 mg g-1 of biomass (dry basis). Moreover, the ethanol yield represents a 19.34% increase for SB when compared with ethanol yields of untreated biomass fermented under anaerobic conditions with cysteine hydrochloride supplement. Moreover, cysteine hydrochloride supplementation enhances ethanol yields through anaerobic fermentation. The crystallinity and chemical nature of biomass materials were analyzed by SEM, XRD, and FTIR. The sulfuric acid impregnated biomass torrefaction using model generated optimized conditions viz temperature (220 °C), sulphuric acid concentration (0.63%w/w) and residence time (10 min) has produced the highest amount of sugars up to 705 mg g-1 of biomass (dry basis) and ethanol up to 320 mg g-1 of biomass (dry basis).