Environmental Engineering Reference
In-Depth Information
Fig. 11.1
Metabolic pathways in fermentative H
2
production. (Hallenbeck and Ghosh
2012
)
production of reduced products and rest is required for ATP synthesis. Even though
many organic compounds enable the production of hydrogen during dark fermenta-
tion, estimations of potential yields are mostly based on hexose conversions. The
theoretical yield per mole of glucose is described as follows:
−
+
−
1
CHOHO HCOO
+ → +
4
2
2
HCOHHG kjmol
+ ++=−
4
4
∆
206
6 26 2
3
3
2
Theoretically, a maximum of 4 moles of H
2
per mole of glucose can be produced
concurrently with the production of 206 kJ energy per mole of glucose in acetic
acid fermentations. The remainder of the hydrogen in the hexose is conserved in
the byproduct acetate, and under non ideal circumstances, more reduced products
such as ethanol, lactate, or alanine (at high H
2
partial pressures). The production
of more reduced organic acids and/or alcohols results in lower H
2
yield. For ex-
ample, the conversion of 1 mol of glucose into butyrate is accompanied by the
production of only 2 moles of H
2
. Generally, a mixture of products is produced
especially by
Clostridia
and the available H
2
from glucose is determined by the
butyrate/acetate ratio. The complete oxidation of glucose to hydrogen and carbon
dioxide yields 12 mol H
2
mol
−1
of glucose without taking the metabolic energy
needed. In practice, hydrogen yield is 2 mol H
2
/mol glucose (2 mol H
2
mol
−1
glu-
cose only at PH
2
< 0.1 kPa) for both PFOR and PFL pathways. The overall yields in
these metabolisms are relatively low. This is a natural consequence of the fact that
