Anaerobic biohydrogen production by a fluidized granular bed bioreactor under thermophilic condition
Date
2011-07-20
Authors
Masilela, Phumlani
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Abstract
There is now a critical need for development of full-scale practical application of
fermentation technologies for energy generation (e.g. hydrogen production) that would be
dependent on carbon neutral fuels such as biomass or wastewaters containing organic
materials. Thermophilic fermentative biohydrogen production was studied in the
anaerobic fluidized bed reactor (AFBR) operated at 65ÂșC with sucrose as a substrate.
Theoretically, the maximum hydrogen yield (HY) is 4 mol H2.mol-1 glucose when
glucose is completely metabolized to acetate, H2 and CO2. But somehow, under most
bioreactor design and operation conditions the maximum possible hydrogen yield (HY)
has generally been observed not to exceed or reach 70-100% of the maximum theoretical
hydrogen yield. In this study the application of external work in the form of high
temperatures, high dilution rates and high rates of de-gassed effluent recycling were
investigated as a means to overcome the thermodynamic constrains preventing the
simultaneous achievement of high hydrogen yield (HY) and hydrogen productivity (HP)
in an AFBR reactor. Bacterial granulation was successfully induced under a thermophilic
temperature of 65 oC within a period ranging from 7 to 14 days. The bacterial granules
consisted of a multispecies bacterial consortium comprised of thermophilic clostridial and
enterobacter species. At a hydraulic retention time (HRT) of 1.67 h and effluent recycle
rate of 3.5 L min-1, hydrogen production rate (HPR) of 32.7 L H2/h and hydrogen yield
(HY) of 3.91 mol H2/ mol glucose were achieved. The design and operation of our bench
scale AFBR system has also resulted in HYs greater than 4 mol H2/mol glucose. The
maximum substrate conversion efficiency was 95%. However, it was noted that at very
low HRTs (< 1h) the bioreactor substrate conversion efficiency dropped to 55%. This
work demonstrated that the application of external work to a bioreactor in the form of
high temperatures, high dilution rates and high rates of de-gassed effluent recycling could
be used to overcome the thermodynamic constraints preventing the simultaneous
achievement of high HYs and high HPs.