Janssen, M.G.J., Hoekema, S., Tramper, J. and Wijffels, R.H.

1. 75 1500 2.5 1200 60 2.0 900 45 ml OD (660 nm) mM Gas Production (ml) 1.5 Ac Concentration (mM) 600 30 Optical Density (-) 1.0 300 15 0.5 0 0 0 2 4 6 8 10 0.0 day Photobiological hydrogen production from acetic acid with purple bacteria Janssen, M.G.J., Hoekema, S., Tramper, J. and Wijffels, R.H. Introduction Hydrogen is a very attractive and clean energy carrier. Therefore sustainable ways of hydrogen production have to be developed to meet future demands. Within a EU-project, named “Biohydrogen”, the complete conversion of biomass (energy crops or organic wastes) into hydrogen gas (H2) and carbon dioxide (CO2) is studied, see process scheme on the right. The carbon dioxide liberated was originally fixed into the organic biomass and this makes the process CO2 neutral. Also a Dutch EET-project has started on the same subject. At the food and Bioprocess Engineering group the photo-heterotrophic fermentation of acetic acid (acetate at pH 7) is studied and optimized. Acetate (Ac) is converted according to the following reaction: 2CH3COO- + 8H2O + ”sunlight”  4HCO-3 + 2H+ + 8H2 (1) A photosynthetic purple non-sulfur bacteria is used for this conversion. Because this reaction is energitically unfavourable (G0 = positive), sunlight (Esolar) has to be used to generate extra chemical energy by anoxygenic photosynthesis. In other words light energy is stored inside hydrogen. The gas mixture produced consists of a H2/CO2 mixture with 90 % v/v H2. Optimisation parameters Two parameters need to be optimized to make this process feasible: the yield of hydrogen on acetate; the yield of hydrogen energy on light energy. rH2 = volumetric H2 production [mol l-1 h-1] rAc = volumetric Ac consumption [mol l-1 h-1] H0c(H2) = H2 enthalpy of combustion [kJ mol-1] rE,solar = volumetric light absorption [kJ l-1 h-1] Results Batch experiments have been done to clarify the kinetics of the biomass growth and hydrogen production. In the Figure on the right it can be seen that biomass growth ( increase of Optical Density, OD) is accompanied by acetate consumption and gas production. From the data the yield parameters could be calculated: YH2,Ac = 0.63 and YH2,E = 0.013 at a light intensity of 210 W m-2. It is suspected that not all hydrogen gas was collected in these lab-scale batch experiments. In that case the actual hydrogen yields on acetate and light energy will be better. Future According to equation 1 the theoretical maximal yield of hydrogen on acetate (YH2,Ac) is 4. In the following experiments, therefore, we are aiming at a yield between 3 and 4 under optimal conditions. The theoretical maximum yield on light energy (YH2,E) is somewhere between 0.15 and 0.20. As an end result of this study we hope to be able to reach an energetic yield of 0.10 in a small-scale ( 10 liter) pilot system. 1 Food and Bioprocess Engineering Group P.O. Box 8129, 6700 EV, Wageningen, The Netherlands Marcel.Janssen@algemeen.pk.wau.nl  +31 317 483396