(Invited) Fermentative Hydrogen Production from Biomass in the Cellulose-Degrading Bacterium Clostridium Thermocellum

Hydrogen can be produced from diverse energy sources, using a variety of process technologies. One promising route for biological hydrogen production is via the fermentation of renewable lignocellulosic biomass. However, due to its crystallinity, biomass has to undergo a thermochemical pretreatment step in order for microbes to ferment its cellulose and hemicellulose constituents. Clostridium thermocellum is a thermophilic (55 – 60^oC) and anaerobic bacterium which displays one of the highest growth rates on cellulose.  C. thermocellum is capable of simultaneous biomass hydrolysis and fermentation in one integrated step, which could be a potential cost saver. During cellulose fermentation, the bacterium produces hydrogen, ethanol, formate, lactate, acetate, and CO₂.  To reduce the cost of hydrogen production, we aim to overcome several technical barriers identified by the DOE Fuel Cell Technologies Office. To reduce the feedstock cost barrier, we optimized cellulose fermentation in bioreactors, operating in either batch or fed-batch mode. In the latter mode we tested parameters including hydraulic retention time, and amount and frequency of liquid replacement. To circumvent the low hydrogen molar yield barrier (mol H₂/mol hexose), we conducted genetic engineering by redirecting metabolic pathways toward maximal hydrogen production lieu of reduced carbon byproducts. Lastly, a techno-economic analysis is underway to set the targets and to guide future research direction to deliver low cost hydrogen. This effort would rely on standardization of metrics in the fermentation technology to assess more accurately as to its economic viability.