Optimization of xylose and glucose yields from Napier grass (Pennisetum purpureum) using hybrid pretreatment and assessment for fermentative hydrogen production using immobilized cells.

Abstract

The dependence on non-renewable fossil fuels has led to the depletion of these energies and global warming. Thus there is a need to search for sustainable energy sources to mitigate these challenges. Lignocellulosic biomass is an excellent potential substrate for renewable biofuel production due to its high carbohydrate content, abundance and sustainability. The polymer interlinks of cellulose, hemicellulose and lignin hinder effective hydrolysis of biomass for the production of fermentable sugars. Efficient and cost effective pretreatment is required for utilization of lignocellulosic materials as substrates for biofuel and biomaterial. In this study, four hybrid techniques of napier grass pretreatment, namely HCl and moist heat (HH), HCl and microwave (HM), NaOH and moist heat (NH) and NaOH and microwave (NM) were modeled and optimized for xylose and glucose production using Response Surface Methodology (RSM). The optimized pretreatment conditions of HH gave 12.83 g/l xylose and 2.28 g/l glucose, and optimized HM pretreatment gave 15.06 g/l and 2.44 g/l xylose and glucose respectively. A xylose to glucose ratio of 5.6:1 was obtained for the optimized HH pretreatment compared to 6.1:1 for the optimized HM pretreatment. For NH and NM hybrid pretreatments, low concentrations of fermentable sugars were observed. The coefficients of determination (R2) of 0.83 and 0.97 were obtained for xylose and glucose production respectively using HH hybrid pretreatment, and 0.90 and 0.80 were obtained for xylose and glucose respectively using HM hybrid pretreatment. The optimum generation of xylose and glucose from napier grass indicates its potential as substrate for the production of renewable biofuels. Furthermore, the optimum physico-chemical set points of Hydraulic Retention Time (HRT) and substrate concentration were investigated for hydrogen production on napier grass using immobilized beads. The optimized set points were 139.97 hours and 19.05% for HRT and substrate concentration respectively, with predicted yield of 5.31ml H2/g napier grass. Model validation gave 6.61ml H2/g napier grass. To assess the dynamics of hydrogen generation at semi-pilot scale, biohydrogen production was carried out in a 13L bioreactor. A peak hydrogen fraction of 28.52% and hydrogen yield of 14.03ml H2/g napier grass was observed at pH 6.3, temperature 37˚C at 62 hours. This optimum generation of biohydrogen using renewable napier grass highlights potential application of this feedstock for large scale biofuel production. Additionally, dark fermentative hydrogen production from napier grass using immobilized microbial consortia combined a cheap hydrogen production method with high unit volumetric production rate thus positively impacting the bioprocess economics.