| Significant amounts of hydrogen (H2) may be produced from cellulosic feedstock such as straw, wood chips, grass residue, paper waste, saw dust, etc. using anaerobic thermophilic fermentation process and can contribute to reducing greenhouse gas emissions alleviating global warming. This study investigated the effects of substrate loading on cell-growth, metabolic flux distribution among various end products, and hydrogen production potential from cellulosic substrates (such as cellobiose, pure cellulose, paper, delignified wood) relative to a soluble substrate, cellobiose. Rates of cell growth were unaffected by different substrate concentrations. H2, carbon dioxide (CO2), acetic acid, formic acid, lactic acid, pyruvic acid and ethanol were the main products of fermentation. Formic acid detection as fermentation end product was a surprising observation but not unexpected as C. thermocellum has the gene pyruvate-formate-lyase present in its sequence. It was concluded that C. thermocellum is capable of (i) maintaining carbon flow to acetic acid and H2-production under limited, sufficient and excess-substrate conditions, (ii) balancing oxidation/reduction (O/R) ratios over the entire period of fermentation being independent of substrate concentrations. Under excess-substrate conditions Carbon flow shifted away from acetate pathway as H2 partial pressures exceeded 50 kPa and pH dropped below 7.0. Clostridium thermocellum possess a very active cellulase system to hydrolize insoluble cellulose and is capable of obtaining a competitive yield of 1.6 mol H2/mol glucose equivalent producing H2 at a high specific rate of 14.6 mmol/g dry cell/h. Sustainable H2 production from cellulosic biomass by C. thermocellum may be possible if bioreactor conditions were able to maintain a near neutral pH (approx. 7.0) and gas products (H2 and CO2) are removed rapidly to maintain a low enough H2 partial pressure. |
PDF Full Text Request