Biomass-Based Energy Production - Introduction to Chemicals from Biomass - page 259

This equation is based on the use of a 6-carbon compound, but a wide range of
compounds can be used under natural conditions. Heterotrophic bacteria such as
Clostridium sp., Enterobacter sp., Ralstonia sp., Rhodobacter sp., and Frankia sp.
cannot perform photosynthesis and therefore require a supply of carbohydrates to
produce hydrogen. Organisms of this sort occur naturally in ecosystems all over
the world. Various industrial processes have been developed that use these natural
systems and attempt to mimic their capacity for hydrogen production. One strain
of Frankia , designated R43, was demonstrated to produce large quantities of
hydrogen under anaerobic conditions using the carbon source propionate [20]. It
was subsequently proven that its hydrogen-producing enzyme is present in both
its hyphae and its vesicles [20].
There is a wide range of organic waste products generated by various industries
that could potentially be used as carbon sources for large-scale heterotrophic
hydrogen production. The abundance of different types of waste were recently
reviewed by Redondas et al. [21]. The preferred substrates for such processes are
carbohydrates because their conversion is thermodynamically superior to that of
alternative substrates. Glucose is a carbohydrate that is present in most industrial
flows. The bioconversion of 1 mol of glucose will theoretically yield 12 mol of
hydrogen gas. After accounting for the reaction's stoichiometry, the bioconver-
sion of 1 mol of glucose into acetate would produce 4 mol of hydrogen gas per
mole of glucose. It has also been demonstrated that sulfate-reducing bacteria can
produce large amounts of hydrogen, with 100% efficiency using formate as their
substrate [22].

7.3.3.4 Coupling Heterotrophic Hydrogen Production to Photoproduction

The bioconversion of biomass by heterotrophic bacteria typically yields hydrogen
gas and acetate because the organic material is not completely oxidized. Dark
fermentation therefore produces both hydrogen and other carbohydrates that must
be removed because they have adverse effects on the energy balance of the hydro-
gen-producing organism. This can be achieved by coupling one of the products of
dark fermentation, acetate, to an additional container in which a second fermenta-
tion occurs. This second step is a photoheterotrophic process in which the organic
acids produced in the first reaction are converted into hydrogen and carbon diox-
ide [23]. It has been shown that if dark and photofermentation processes are per-
formed equally in this way, the theoretical yield of 12 mol H 2 per mole of glucose
can be achieved [24].

7.3.4 Butanol Production Using Bacteria as Biocatalysts

Butanol is an advanced biofuel as well as an important industrial solvent. Butanol
production occurs naturally in several species of Clostridium sp. that are capable
of synthesizing it from carbohydrates. Interest in biological butanol production