Green Liquor Pretreatment for Hardwood chips to Ethanol conversion strategies

Abstract

Modified Green Liquor pretreatment is a novel high sulfide pretreatment method that enhances carbohydrate stability. The influence of hardwood chips pretreatment with green liquor was investigated under alkaline conditions of 12% and 16% TTA, and sulfidities of 0%, 12.5%, 25%, and 37.5% to see its effect on the sugar recovery. Also, ethanol was added in the pretreatment conditions of 12%TTA and 25%S as an Organosolv pretreatment method. Quantitative analyses of glucose, xylose, mannose, and carbohydrates yield is discussed. The influence of pretreatment conditions such as alkalinity and sulfidity charge were the main variables controlled to analyze their influence in the carbohydrate yield. The results suggest that higher sulfidity promotes an increase in delignification, and also that this delignification is more notable in the samples pretreated with ethanol. The pretreated pulps were then hydrolyzed with cellulase, xylanase, and β-glucosidase enzymes. After enzymatic hydrolysis with these commercial cellulases enzymes, higher glucose yields were obtained at higher sulfidity and higher ethanol concentration in the liquor. The results also indicated that higher enzyme activity exhibited higher carbohydrates and glucan yield. The carbohydrates yield was 70.6% for hardwood chips pretreated with 16%TTA, 37.5%S, and hydrolyzed with cellulases at 20 FPU/g pulp. However, the higher carbohydrates yield obtained was 75.7% for the pulps pretreated at 20% ethanol and 20 FPU/g pulp. Different techniques were applied in the samples 16%TTA, 0%S and 37.5%S, and 12%TTA, 25%S, 20%ethanol to determine its pretreatment effect on lignin. ToF-SIMS technique was used to detect the changes in cellulose/lignin composition and then compare the results with ones the obtained from the sugar analysis using Ion Exchange Chromatography. ToF-SIMS suggested that the removal of lignin is mainly localized on the sample surface. The molecular weight distribution of lignin using Size Exclusion Chromatography technique was also studied. The results shown a decrease in the molecular weight when sulfidity was increased. It is hypothesized that this happened mainly due to cleavage of lignin in the β-O-4 linkage that decrease the chain length of the residual lignin. Green liquor has the advantage of altering the lignin structure, and preserving the celluloses and hemicelluloses present in the biomass. An economic analysis based in different assumptions was elaborated to compare all the study cases. In general the results shown that samples pretreated with higher sulfidity always shown better profit per liter compared with lower sulfidity. Also, in the case of the samples pretreated with ethanol, the profit was even better than without ethanol. But, variations in ethanol lost during the process shown that this method is not as attractive as the pretreatment at high sulfidity and high total titratable alkali.