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Biohydrogen production by fermentative processes
Biohydrogen production by fermentative processes
Biohydrogen production by fermentative processes Freda R Hawkes Wastewater Treatment Research Unit School of Technology and School of Applied Sciences University of Glamorgan http://www.glam.ac.uk/wastewater Biologically, hydrogen may be produced: • Photosynthetically • Fermentatively Photosynthesis in green plants and green and blue-green algae light 6H2O + 6CO2 C6H12O6 + 6O2 + cellular energy Bacterial photosynthesis light Reduced cell material + organic cellular energy compounds Photosynthetic H2 production in green and blue-green algae - biophotolysis light H2O 0.5O2 + H2 H2 production in green algae hydrogenase 2H+ + 2 electrons H2 Biophotolysis in green algae H2O 2H+ 2e 0.5O2 H2 photosynthesis hydrogenase Hydrogenase in green algae • is induced in low amounts by dark anaerobic pre-incubation • its role - managing dark/light transition • is O2 sensitive, so H2 production decreases in light • hence two-stage processes suggested Biophotolysis “There is a need to increase the yield of H2 production by a factor of ten or more and to develop a protocol that works with minimum media before the process can be viable in commercial applications.” Ghirardi et al. Trends in Biotechnology Dec 2000 Biophotolysis “biophotolysis processes must achieve close to a 10% solar energy conversion efficiently to be competitive with alternative sources of renewable hydrogen…Another challenge is to scale up with economically viable bioreactors” Benneman J Appl Phycol Dec 2000 Challenges for biophotolysis Photobioreactor design • construction • economics Operating protocol • inducing hydrogenase • maintaining low O2 concentration • avoiding culture contamination Photosynthetic H2 production in blue-green algae and bacteria nitrogenase N2 + 8H+ + 8e- + energy 2NH3 + H2 Nitrogenase in blue-green algae and bacteria • produces more H2 if N2 is absent • is inhibited by NH3, O2 • is energy requiring • turnover rate 1000x slower than hydrogenase H2 production by photosynthetic bacteria • uses nitrogenase • requires organic compounds • does not produce O2 H2 production by blue green algae “…nitrogen-fixing Cyanobacteria can be dismissed as potential candidates for future development as they could not, even with additional research and development, achieve the necessary efficiency goals mandated by the economic analysis”. Benneman J. Appl. Phycol. Dec 2000 Biologically, hydrogen may be produced: • Photosynthetically • Fermentatively Fermentative H2 production • property of many species of bacteria, particularly Clostridia • dark, anaerobic process • carbohydrates are favoured substrate • involves hydrogenase • produces CO2 • maximum H2 yield with acetate as fermentation product Fermentative H2 production - H2 yield C6H12O6 + 2 H2O 2CH3COOH + 2CO2 + 4 H2 yield: 0.5 m3 H2 / kg carbohydrate thermodynamically unfavourable as H2 conc rises Fermentative H2 production technology Process similarities with • classical acetone/butanol process conditions favouring solvents depress acetate/H2 production • anaerobic digestion methanogens consume H2 Optimisation, design and operating parameters not established Fermentative H2 Production R & D Research has used: Commercialisation requires: pure microbial stable mixed microflora cultures, sterile feedstock, non-sterile feed-stocks batch operation continuous processes Effect of lowering dissolved H2 conc Non-Sparging Sparging H2 yield ( mol/mol gluc) 0.85 1.43 Acetic mgl-1 773 785 n-butyric mgl-1 1742 1929 Acetone mgl-1 0 1 Butanol mgl-1 4 6 Mizuno et al. Bioresource Technol. (2000) Sustainable biohydrogen production: process optimisation EPSRC grant 2001-2004, Hawkes DL, Dinsdale R, Hawkes FR Objectives: • to determine optimal operating parameters giving stable long-term generation of H2 on high strength complex biomass substrates • to develop strategies to increase H2 yield Sustainable biohydrogen production: process optimisation EPSRC grant 2001-2004, Hawkes DL, Dinsdale R, Hawkes FR H2 yield optimisation by: • sparging with anaerobic digester biogas • sparging with fuel cell exhaust • operation at thermophilic temperature • on-line H2 measurement in off-gas with PEM fuel cell. Economics of fermentative H2 production • Unit cost of fermentative H2 from sugar cane 0.26 DM/kWh (Tanisho, 1996). • Unit cost for wind-powered electrolysis plants 0.5-0.2 DM/kWh (Dutton et al., 2000) Challenges for fermentative H2 production • what organic material can be used as substrate? • how may yield and rate of H2 production be optimised? • what use may be made of fermentation end products? • what are the overall process economics? References Beneman J R. (2000) Hydrogen production by microalgae. J. Applied Phycology 12, 291-300. Ghirardi ML, Zhang L, Lee JW, Flynn T, Seibert M, Greenbaum E and Melis A. (2000) Microalgae: a green source of renewable energy. Trends in Biotechnology 18, 506-511. Mizuno O, Dinsdale R, Hawkes FR, Hawkes DL, Noike T. (2000) Enhancement of hydrogen production from glucose by nitrogen gas sparging. Bioresource Technology 73 (1) 59-65.
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