Enhanced coproduction of hydrogen and butanol from rice straw by a novel two-stage fermentation process

Lignocellulosic biomass has been increasingly used for biofuel production, such as biohydrogen and biobutanol. However, the enzymes used in hydrolysis result in a large increase in production cost. To reduce the enzyme involvement in lignocellulosic butanol, a novel two-stage fermentation process was constructed by combining a Carboxylate Platform and acetone-butanol-ethanol (ABE) fermentation. First, without enzyme involvement, lignocellulose was converted to volatile fatty acids (VFAs) using the Carboxylate Platform. Subsequently, the VFAs were used as feedstock to generate butanol by ABE fermentation of Clostridium beijerinckii NCIMB 8052. Fed with 20 g/L pretreated rice straw, the acidogenic fermentation produced 660 mL/L hydrogen and 6.87 g/L butyric acid, with a total VFAs of 9.52 g/L in the supernatant of fermentation culture (SFC). With 40% rice straw hydrolysate and 60% SFC as co-substrate, hydrogen and butanol productions of 5897 mL/L and 13.8 g/L, respectively, were obtained in the ABE fermentation. Mass balance and energy generation analysis showed that the specific butanol and energy yields from rice straw reached 230 g/kg and 9633.7 kJ/kg, respectively. Additionally, the carbon-flow distribution analysis indicated that the SFC supplement decreased the metabolic flux of acetone producing pathway and increased the metabolic flux of butanol producing pathway. The novel two-stage fermentation process was a more effective approach for energy generation from lignocellulose that avoided external addition of commercial butyric acid and reduced enzyme involvement for fermentation product.