Sequential acid-autoclave and microwave-alkali pretreatment of rice straw for bioethanol production

Two-step acid-autoclave and microwave-alkali pre-treatment process of rice straw have been carried out for bioethanol production. The reaction conditions of both the steps are optimized using Response Surface Methodology. The studies reveal that the optimum conditions for acid-autoclave pretreatment are found to be 1.90 per cent acid concentration, 51.85 min time and 1:17.51 solid:liquid ratio with maximum release of reducing sugars (16.94 g per 100 g) and minimum release of furfurals (0.93 g per 100 g). For second step pretreatment i.e. microwave-alkali, optimized conditions are observed to be 3.75 percent alkali concentration, 9.16 min time and 475W microwave power with minimum lignin content of 2.96 percent in pre-treated straw. Scanning electron micrographs reveal extensive damage of silicified waxy surface and disruption of the cell wall structure of straw after two step pre-treatment process. It has been concluded that sequential pre-treatment of rice straw using acid-autoclave and microwave-alkali process efficiently remove hemicellulose and lignin from straw, thereby increasing accessibility of cellulose to enzymatic hydrolysis for enhanced production of fermentable sugars and bioethanol.