Anaerobic Digestion Biorefinery to Produce Bioenergy and Biobased Products using High Yielding Tropical Feedstock.

Abstract

A series of batch experiments were conducted to investigate the effects of inoculum type, oxygen (O2) dosage, and incubation time on volatile fatty acids (VFAs) production during anaerobic digestion (AD) of Napier grass (Pennisetum purpureum), a high yielding energy crop. The results showed that anaerobically digested cattle manure (ADCM) was the appropriate inoculum for VFAs production form Napier grass. Additionally, the incubation time of 3 days and O2 dosage of 15 mL/g volatile solids (VS)added showed the highest VFAs production when ADCM was used as an inoculum. The semi-continuous bench-scale experiment was then performed using horizontal acid bioreactor to investigate the effect of micro-aeration on VFAs production from Napier grass. The VFAs produced during micro-aeration condition was significantly higher than that of anaerobic condition. The produced methane was significantly decreased during micro-oxygenation, thus, the methanogens were inhibited by the injected oxygen. The soluble chemical oxygen demand (SCOD) was also significantly enhanced by micro-aeration resulting in more available soluble organic substrates for acidogens. Hemicellulose was the main component of biomass degraded during AD, whereas cellulose and lignin were preserved in the digestate. AD of Napier grass as a mono-substrate was also investigated for biomethane production. Two semi-continuous bench-scale horizontal bioreactors were operated in parallel for over 300 days, and the highest organic loading rate of 6 kgVS/m3-d was achieved during long-term operation with average methane yield of 112.48±9.03 NmL/gVSadded. The methane yield accounted for more than 90% of the methane potential of the raw Napier grass. Similar to the acid bioreactor, hemicellulose was the main component of lignocellulosic biomass contributed to methane production, while cellulose and lignin remained in the digestate. This cellulose-rich fiber and lignin was further examined for bioenergy potential via thermochemical conversion, e.g., torrefaction and hydrothermal carbonization (HTC), and showed the identical energy contents with that of bituminous coal. The techno-economic analysis indicated that torrefaction might be the most appropriate thermochemical process for digestate utilization. Thus, this study provided the first time successful integration of anaerobic digestion and thermochemical treatment for complete utilization of whole plant biomass representing a true biorefinery for lignocellulosic biomass.