Attached-growth bioreactors for syngas fermentation to biofuel
Lignocellulosic biomass is a renewable resource for sustainable production of biofuels and chemicals. Syngas fermentation, a hybrid process integrating the thermochemical (i.e. gasification of feedstock to syngas) and biochemical (i.e. microbial fermentation of syngas) conversions, has been considered as a promising technology for production of lignocellulosic-biomass-derived ethanol. The challenge to commercialize syngas fermentation, though, is to enhance the gas-to-liquid mass transfer rate due to the low solubilities of carbon monoxide (CO) and hydrogen (H2) in an energy-efficient manner. Conventional suspended-growth bioreactors, such as continuous stirred tank reactor (CSTR) and bubble column reactor (BCR), suffer from inefficient mass transfer and unwanted cell washout at high dilution rate, resulting to low productivities. The present study explored the feasibility of applying three innovative attached-growth bioreactor systems, hollow fiber membrane biofilm reactor (HFM-BR), monolithic biofilm reactor (MBR) and rotating packed bed biofilm reactor (RPB-BR), in continuous syngas fermentation, in order to enhance mass transfer of CO and to maximize ethanol production, by optimizing selected operational parameters.
The highest ethanol productivity of HFM-BR, MBR and RPB-BR was achieved at 3.44 g/L/day, 2.35 g/L and 6.70 g/L/day with optimized fermentation operational condition, respectively. HFM-BR showed the highest CO kLa (1096.2 hour-1) among the three bioreactor systems; however, the costly membrane and biofouling issue are the drawbacks to conduct continuous syngas fermentation with high ethanol productivity for extended period of time. MBR showed modest performance of CO mass transfer rate and ethanol productivity, but it has inherent advantages such as high mechanical strength and less biofouling problem. With installation of an in-situ washing device, the microchannel-clogging problem could potentially be resolved, indicating its capability of extended periods of continuous fermentation. RPB-BR gave the highest ethanol productivity with a simple mechanical design, inexpensive packing media and stable operation. The present study demonstrated the great potential of attached-growth bioreactors as efficient systems to obtain syngas fermentation with high productivity of ethanol, making cellulosic ethanol biorefinery move one step forward to technical and economic feasibility. Ultimately, it is believed that this study will contribute to our nation's independence from petroleum fuels.