Efficient Conversion of Rice Straw to Bioethanol Using Sodium Carbonate Pretreatment

Abstract

In this study, rice straw was treated with sodium carbonate prior to enzymatic hydrolysis and fermentation. All pretreatments were performed in a high-pressure reactor at 90, 120, 150, and 180 °C by 0.25, 0.5, and 1 M sodium carbonate (Na2CO3) solution. The reactor was designed to inject the straw to the reactor at a desired temperature without any preheating effects. The reactor content was continuously mixed, and samples were taken at different time periods. Afterward, enzymatic hydrolysis of the untreated and all treated straws were conducted at 45 °C for 72 h with enzyme loading of 20 FPU cellulase and 30 IU β-glucosidase per gram of substrate. The best pretreatment conditions were obtained to be 0.5 M Na2CO3 at 180 °C for 120 min. This pretreatment improved the released glucose from 9.5 g/L for the untreated straw to 43.5 g/L for the treated one. On the other hand, the treatment showed significant improvement on ethanol production from rice straw applying simultaneous saccharification and fermentation. Ethanol production was enhanced from 90.2 to 351.4 g/L by the treatment. The analysis showed that the treatment with sodium carbonate at elevated temperature can significantly reduce the lignin and xylan contents and the cellulose crystallinity and also convert cellulose type I to type II, which is more amenable for enzymatic hydrolysis.