CHAPTER 3: HyDROGEN GENERATION BY MICROBIAL CULTURES Dr Ku Syahidah Ku Ismail Adapted from Drapcho et al. (2008) Biofuels Engineering Process Technology

Introduction Hydrogen – an environmentally friendly alternative fuel because it combusts to form only water and energy. It has high energy content of 122 kJ/g, which is 2.75 times higher than hydrocarbon fuels. H2 + ½ O2  H2O Currently, 95% of global hydrogen production is from fossil fuel sources, via thermocatalytic reformation of natural gas Biological hydrogen production can be carried out by 2 main mechanisms: photobiological production and fermentation

Important enzymes Biological hydrogen production are dependent on hydrogenase or nitrogenase enzymes that catalyze the reduction of protons: 2H+ + 2e-  H2 Several classes of enzymes have been identified: nitrogenases, Fe- hydrogenases, NiFe-hydrogenases and nonmetal hydrogenases Fe-hydrogenases are found primarily in Bacteria and Eucarya, while the NiFe-hydrogenases are found in Archae and Bacteria. Cyanobacteria contain two types of hydrogenase enzyme, an uptake hydrogenase induced under nitrogen fixing conditions, and reversible hydrogenase able to both produce and oxidize hydrogen production. Eg of cyanobacteria; Aphanocapsa variabilis, Synechocystis sp. and Anacystis nidulans

Photobiological H2 PRODUCTION

Direct biophotolysis A biological process that can produce hydrogen directly from water using microalgae photosynthesis system to convert solar energy into chemical energy in the form of hydrogen. 2H2O + Solar energy  2H2 + O2 Light energy that strikes Photosystem II (PSII) is used to split water molecules, generating oxygen, protons and electrons. The electrons then flow to Photosystem I (PSI) to reduce ferrodoxin Problem with direct biophotolysis: Fe-hydrogenase that catalyze the reaction is sensitive to oxygen

Direct biophotolysis (cont.)

INDIRECT BIOPHOTOLYSIS A biological process that can produce hydrogen from water using microalgae and cyanobacteria photosynthesis to convert solar energy into chemical energy in the form of hydrogen through several steps: Biomass production by photosynthesis Biomass concentration Dark aerobic fermentation produces 4 mol hydrogen + 2 mol acetate Conversion of 2 mol acetate into hydrogen

INDIRECT BIOPHOTOLYSIS (cont.) 6H2O + 6CO2 + light  C6H12O6 + 6O2 C6H12O6 + 2H2O  4H2 + 2CH3COOH + 2CO2 2CH3COOH + 4H2O + light  8H2 + 4CO2 The overall reaction is as follows: 12H2O + light  12H2 + 6O2

HYDROGEN PRODUCTION BY FERMENTATION

ADVANTAGES of hydrogen production by fermentation Can produce hydrogen continuously without the need of light Hydrogen production by fermentation has higher stability and efficiency compared to hydrogen production by biophotolysis. Fermentation process is more appropriate in industrial scale because it uses a simple control system, thus operational cost is minimized. Hydrogen production through fermentation can play a dual role – waste reduction and energy production because it can utilize a variety of organic waste as substrate.

FOR MORE NOTES ON HYDROGEN PRODUCTION BY FERMENTATION, PLEASE REFER TO READING MATERIAL GIVEN.

Test yourself Highlight the differences between direct and indirect biophotolysis.