The present study focuses on the exploitation of sweet sorghum biomass as a source for
hydrogen in continuous and batch systems. Sweet sorghum is an annual C4 plant of tropical
origin, well-adapted to sub-tropical and temperate regions and highly productive in biomass.
Sweet sorghum biomass is rich in readily fermentable sugars and thus it can be considered
as an excellent raw material for fermentative hydrogen production. Extraction of free sugars
from the sorghum stalks was achieved using water at 30°C. After the extraction process, a
liquid fraction (sorghum extract), rich in sucrose, and a solid fraction (sorghum cellulosichemicellulosic
residues), containing the cellulose and hemicelluloses, were obtained.
Hydrogen production from sorghum extract was investigated using mixed acidogenic
microbial cultures, coming from the indigenous sorghum microflora and Ruminococcus albus,
an important, fibrolytic bacterium of the rumen. Hydrogen productivity of sorghum residues
was assessed as well, using R. albus.
The highest hydrogen yield obtained from sorghum extract fermented with mixed microbial
cultures in continuous system was 0.86 mol hydrogen per mol of glucose consumed, at a
hydraulic retention time of 12 hours. This corresponded to a hydrogen productivity of 10.4 l
hydrogen per kg of sorghum biomass and was comparable with those obtained from batch
experiments. On the other hand, the hydrogen yield obtained from sorghum extract treated
with R. albus was as high as 2.1-2.6 mol hydrogen per mol of glucose consumed. Hydrogen
productivity of sorghum residues fermented with R. albus reached 2.6 mol hydrogen per mol
of glucose consumed. In total, the productivity of sorghum biomass (that of sorghum extract
plus that of sorghum residues) could be 60 l hydrogen per kg of sorghum biomass if R. albus
is used.