Extrusion followed by ultrasound as a chemical-free pretreatment method to enhance enzymatic hydrolysis of rice hull for fermentable sugars production

•No chemical reagents were used in the pretreatment process, and chemical-free pretreatment method really comes true.•For our knowledge, it is first time to pretreatment rice hull with two kinds of physical methods compared.•Extrusion conditions were optimized by response surface method, and the optimum conditions were obtained.