Biohydrogen Production from Anaerobic Fermentation - Biotechnology in China: Biofuels and Bioenergy - page 153

3.3 Strategies for Increasing Hydrogen Yield
Apart from sugars, a variety of toxic compounds such as acetic acid, furfural,
5-hydroxymethyl furfural, and aromatic and polyaromatic compounds are usually
generated during the pretreatment of lignocellulosic wastes. Because of their
inhibitory effect on the fermentation, these by-products may be a limiting
factor for hydrogen fermentation. Cao et al. [ 75 ] used Ca(OH) 2 to detoxicify
HCl-pretreated corn stover hydrolysate, and the hydrogen production from the
supernatant was improved after the precipitate had been removed. The choice of
detoxification methods depends on both the hydrolysates to be treated (regarding
the formation of harmful by-products) and the fermentation microorganisms
employed (regarding their stress tolerance).
On the other hand, as lignocellulosic hydrolysates contain various monosac-
charides, a single type of microorganism may not produce rapid and efficient
fermentation for hydrogen production. In this sense co-culture or mixed cultures
might be a choice for efficient fermentation of lignocellulosic wastes to expand the
utilization of carbon sources and improve substrate conversion rates. In addition,
the metabolic engineering technique may be an alternative approach to improve
hydrogen production from lignocellulosic wastes. Li et al. [ 64 ] knocked out the
gene encoding L -lactate dehydrogenase from Thermoanaerobacterium aotearoense
to redirect the electron flow, which resulted in twofold and 2.5-fold increases in the
hydrogen yield and production rate; the maximum hydrogen yields of the Dldh
mutant were 2.71, 1.45, and 2.28 mol H 2 /mol on glucose, xylose and a glucose-
xylose mixture, respectively.
Although biohydrogen production from lignocellulosic wastes is promising, it is
still not cost-effective, and more work is required to improving hydrogen produc-
tion, such as searching for highly efficient hydrogen-producing bacteria, developing
cost-efficient pretreatment technology, and optimizing the fermentation process.
4 Emerging Technologies for Biohydrogen Production
4.1 Limitation of Dark Fermentation
Theoretically, 4 mol H 2 can be generated per mole of glucose consumed in dark
hydrogen fermentation with 2 mol acetate as a by-product. However, in addition to
acetate, small-molecule organic compounds such as butyrate and ethanol are also
produced, and are termed a ''dead-end'' or ''fermentation barrier,'' and limit the
hydrogen yield to a maximum of 2-3 mol H 2 /mol glucose. Usually, no more than
one third of the total potential electrons in complex biomass can be transferred to
hydrogen, and the remaining two thirds end up in the forms of these fermentation
by-products. On the basis of the Gibbs free energy, additional energy is required
for hydrogen production from these ''dead-end'' compounds.