Biohydrogen Generation and Other Methods - Hydrogen Generation, Storage, and Utilization - page 66

indirect photolysis systems produce pure H 2 , dark fermentation processes
produce a mixed biogas containing primarily H 2 and CO 2 , but which may
also contain lesser amounts of methane (CH 4 ), CO, and/or hydrogen sulfide
(H 2 S) [75]. Glucose yields different amount of hydrogen depending on the
fermentation pathways and liquid end products. A maximum of 4 mol hydro-
gen is theoretically produced from 1 mol of glucose with acetic acid as the
end product, while a maximum of 2 mol hydrogen is theoretically produced
from 1 mol of glucose with butyrate as the end product [78]:

C H O + 2 H O → 2 CH COOH + 2 CO + 4 H

(4.16)

6 12 6 2 3 2 2

C H O → CH CH COOH + 2 CO + 2 H .

(4.17)

6 12 6 3 2 2 2

Thus, the highest theoretical yields of H 2 are associated with acetate as
the fermentation end product. In practice, however, high H 2 yields are usually
associated with a mixture of acetate and butyrate fermentation products, and
low H 2 yields are with associated propionate and reduced end-products
(alcohols, lactic acid). Clostridium pasteurianum , Clostridium butyricum ,
and Clostridium beijerinkii are high H 2 producers, while Clostridium propi-
onicum is a poor H 2 producer [75, 79, 80].
The amount of hydrogen production by dark fermentation highly depends
on the pH value, hydraulic retention time (HRT) and gas partial pressure.
For the optimal hydrogen production, pH should be maintained between 5
and 6 [23]. Hydrogen production from the bacterial fermentation of sugars
has been examined in a variety of reactor systems. Hexose concentration has
a greater effect on H 2 yields than the HRT. Flocculation also was an important
factor in the performance of the reactor [40]. The hydrogen yield from
sucrose by dark fermentation could be difficult to increase above 2.5-mol/
mol hexose, owing to the formation of fatty acids during the fermentation
process [81].

4.4.4 Two-Stage Process: Integration of Dark and Photofermentation

A combination of dark and photofermentation in a two-stage hybrid system
could be expected to reach as close to the theoretical maximum production
of 12 mol of H 2 (mol glucose) -1 equivalent as possible, according to the fol-
lowing reactions [82]:

Stage I: Dark fermentation (facultative anaerobes)

C H O + 2 H O → 2 CH COOH + 2 CO + 4 H

(4.18)

6 12 6 2 3 2 2