Near theoretical saccharification of sweet sorghum bagasse using simulated green liquor pretreatment and enzymatic hydrolysis

Sweet sorghum is a tropical grass that can tolerate harsh conditions and has great potential as a feedstock for biofuel production. The green liquor, which is an intermediate liquid stream containing Na2CO3, Na2S and other impurities generated in a kraft pulp mill, has been shown to remove lignin and preserve carbohydrates during pretreatment. The recoveries of key components and reconstitution of green liquor have also been demonstrated in kraft pulp mills. In this study, the potential use of a simulated green liquor (SGL), which contained only Na2CO3 and Na2S, for pretreatment of sweet sorghum bagasse to improve its enzymatic hydrolysis to produce sugars for use in further bioconversion to biofuel was investigated. To study the effects of key process parameters (time, temperature, liquid/solid (L/S) ratios, total titratable alkali (TTA) charges, and sulfidity), the SGL pretreatment was optimized by using response surface methodology. The optimum pretreatment conditions for the highest total sugar yield were determined to be 110 min, 160 °C, L/S ratio of 7, TTA of 18%, and sulfidity of 40%. The optimal total sugar yield predicted by the model was 83.2%. The predicted value was confirmed in an experiment where the overall sugar yield of 82.6% was obtained.