Abstract
Lignocellulosic biomass can be utilized to produce ethanol, a promising alternative energy source for the limited crude oil. There are mainly two processes involved in the conversion: hydrolysis of cellulose in the lignocellulosic biomass to produce reducing sugars, and fermentation of the sugars to ethanol. The cost of ethanol production from lignocellulosic material is relatively high based on current technologies, and the main challenges are the low yield and high cost of the hydrolysis process. Considerable research efforts have been made to improve the hydrolysis of lignocellulosic materials. Pretreatment of lignocellulosic materials to remove lignin and hemicellulose can significantly enhance the hydrolysis of cellulose. However, physical and chemical barriers caused by the close association of the main components of lignocellulosic biomass, hinder the hydrolysis of cellulose and hemicellulose and lignin fraction thus, different pretreatment methods and conditions should be chosen according to the process configuration selected for the subsequent hydrolysis and fermentation steps. The present work aims at exploring new pretreatment approaches using sequential combinations of supercritical CO2 (scCO2) and other pretreatment means such as ultrasound, alkaline hydrogen peroxide and ozonolysis at mild conditions (80-180 oC and 45-120 min) to pretreat lignocellulosic biomass in a batch reactor. An innovative method for pretreatment of sugarcane bagasse using sequential combination of supercritical CO2 (scCO2) and alkaline hydrogen peroxide (H2O2) at mild conditions was proposed. Yields of cellulose was found to be two times superior when compared to the individual pretreatment with scCO2, ultrasound, or H2O2 and the sequential combination of scCO2 and ultrasound. Pretreatment with scCO2 obtained high amounts of cellulose and hemicellulose but also acid-insoluble lignin. Pretreatment with ultrasound or H2O2 could partly depolymerize lignin, however, could not separate cellulose from lignin. HPLC analysis of liquid products via enzymatic hydrolysis and characterization of solid residues by SEM revealed strong synergetic effects in the sequential combination of scCO2 and H2O2. A significant increase in glucose recovery from 15-20 to 40-65% was obtained when sugarcane bagasse was sequentially pretreated using scCO2 followed by ozonolysis. The reason for this can be attributed to an increased lignocellulosic material digestibility by scCO2 explosion pretreatment that was conducted in a high pressure autoclave at 80-120 oC, 20.6 MPa of CO2 pressure and 30 min while the ozonolysis of scCO2-pretreated sugarcane bagasse was carried out in a fixed bed reactor at room temperature. The effects of two major parameters, temperature of scCO2 explosion pretreatment and time for ozonolysis, were studied. An increased access of cellulose to the cellulose fiber and an increased enzymatic hydrolysis led to an increase in the cellulose fraction while decreasing the lignin content for the combined pretreatment of sugarcane bagasse. It is also noteworthy that furfural and hydroxymethyl furfural were not detected of the pretreatments. Thus it can be concluded that the sequential pretreatment of sugarcane bagasse using scCO2 explosion followed by ozonolysis is an efficient way to improve glucose recovery from biomass feedstocks. A new pretreatment for wood dust called CO2 compressed aqueous THF solution pretreatment was proposed to enhance cellulose in the subsequent process of bioethanol production. This pretreatment approach employed THF miscible with aqueous acidity solution by CO2-compressed to obtain up 90% glucose recovery from wood dust if coupled with enzymatic hydrolysis at the optimum condition. A central composite design was used to optimize the pretreatment conditions. All factors were found to affect glucose recovery significantly. A quadratic polynomial equation was modelled for glucose recovery by multiple regression analysis using response surface methodology to determine the optimum pretreatment condition. A glucose recovery of 84.1% for the optimum condition at a water:THF ratio of 1:2, a temperature of 190 oC, a CO2 pressure of 11.7 MPa and a pretreated time of 2 h. Experimental verification of the optimum conditions showed glucose recovery well within the estimated value of the model. High efficiency of this pretreatment approach could be attributed to a very high lignin removal which is supported by compositional analysis. Subsequently, nearly pure lignin product can be precipitated by evaporation of volatile THF for recovery and recycling. The other work optimized the effects of pressure, temperature, and pretreated time in the pressurized CO2 pretreatment of water hyacinth to enhance glucose recovery following enzymatic hydrolysis using a central composite design. All of the factors were found to significantly affect glucose recovery. A quadratic polynomial equation was modelled to predict and determine the optimum conditions for glucose recovery by multiple regression analysis using response surface methodology. The optimum conditions for the pretreatment of water hyacinth were obtained at CO2 pressure of 5.8 MPa and 140 oC for 90 min, yielding a glucose recovery of 96.5 %. Experimental verification of the optimum conditions showed that the glucose recovery was well within the predicted value. The obtained results indicate that the pressurized CO2 pretreatment avoids formation of the inhibitors for fermentation such as furfural and 5-hydroxymethylfurfural and constitutes an efficient way to improve enzymatic hydrolysis from water hyacinth. Following a simplified life cycle assessment using all of the required input values from our experimental data, it was demonstrated that a significant fossil fuel reduction, as well as less human health and ecosystem impacts, could be achieved.