Assessment of Coproduction of Ethanol and Methane from <i>Pennisetum purpureum</i>: Effects of Pretreatment, Process Performance, and Mass Balance

To overcome the structural complexity and improve the bioconversion efficiency of <i>Pennisetum purpureum</i> into bioethanol or/and biomethane, the effects of ensiling pretreatment, NaOH pretreatment, and their combination on digestion performance and mass flow were comparatively investigated. The coproduction of bioethanol and biomethane showed that 65.2 g of ethanol and 102.6 g of methane could be obtained from 1 kg of untreated <i>Pennisetum purpureum</i>, and pretreatment had significant impacts on the production; however, there is no significant difference between the results of NaOH pretreatment and ensiling-NaOH pretreatment in terms of production improvement. Among them, 1 kg of ensiling-NaOH treated <i>Pennisetum purpureum</i> could yield 269.4 g of ethanol and 144.5 g of methane, with a respective increase of 313.2% and 40.8% compared to that from the untreated sample; this corresponded to the final energy production of 14.5 MJ, with the energy conversion efficiency of 46.8%. In addition, for the ensiling-NaOH treated <i>Pennisetum purpureum</i>, the energy recovery from coproduction (process III) was 98.9% higher than that from enzymatic hydrolysis and fermentation only (process I) and 53.6% higher than that from anaerobic digestion only (process II). This indicated that coproduction of bioethanol and biomethane from <i>Pennisetum purpureum</i> after ensiling and NaOH pretreatment is an effective method to improve its conversion efficiency and energy output.