Conversion Technologies 93
Acetyl CoA may undergo a two-step conversion into ethanol or condensation followed
by saturation to form butyryl-CoA. These two substrates may form their respective alco-
hols in two analogous reactions:
CoA-S-C 2 H 3 O+NADH↔CoA-SH+C 3 H 4 O+NAD+
C 2 H 4 O+NADH+H+↔C 2 H 5 OH+NAD+
and
CoA-S-C 4 H 7 O+NADH↔CoA-SH+C 4 H 8 O+NAD+
C 4 H 8 O+NADH+H+↔C 4 H 9 OH+NAD+
The nature of ABE fermentation results in the formation of multiple products
acetone–butanol–ethanol in the ratio of 3:6:1, respectively. ABE fermentation was
one of the leading industrial processes at the beginning of the twentieth century with
impacts well beyond the fields of science and technology and deep into military and
politics. Although historically acetone was the most important of the three, currently
most emphasis is placed on butanol, and efforts are being made to maximise its
productivity. Butanol is an important industrial commodity with a primary use as a
solvent and an important precursor of numerous compounds such as acrylate, and
methacrylate esters, glycol ethers, butyl acetate, butylamines, and amino resins [37].
Its physicochemical properties such as higher energy content, unrestricted miscibility
with traditional fuels and low vapour pressure make it a promising bio-fuel with better
characteristics than ethanol [37]. ABE fermentation is a strictly anaerobic process
that can use various feedstocks depending on the producer strain. Traditionally,
starchy feedstocks were preferred due to amylolytic enzymes secreted by solvengenic
Clostridia.Inanefforttodecreasethecostofthefeedstocks,firstsugarmolassesand
recently lignocellulose feedstocks have been utilised. The conversion of lignocellulose to
butanol currently enters commercial phase. ABE fermentation is usually carried out in
batch and is generally more complex than ethanol fermentation with yeast. Solvengenic
Clostridiaare strictly anaerobic organisms, sensitive to oxygen and rather slow growers.
The yields of this fermentation have been traditionally rather low, and total solvent
concentration rarely exceeds 20 g l−^1. Additionally, at about 2% concentration butanol
is highly toxic to the cells and fermentation efficiency drops even further. Another
bottleneck is the separation of diluted solvents from large volume of the fermentation
broth, which makes the process of butanol purification expensive. It is envisaged that
advances in fermentation technology and strain improvement may be required before
the second wave of fermentatively produced butanol comes back to the market as fuel
and platform chemical.
3.4.9.3 Biohydrogen
There are many biological routes to produce hydrogen such as direct biophotolysis,
indirect biophotolysis, photofermentation and dark fermentation [38, 39]. Photolysis
routes although very promising, suffer from low productivities and major engineering
challenges in product separation. Fermentative routes and especially dark fermenta-
tion looks like the most technologically feasible option at the moment [38]. Among
fermentative routes, hydrogen production by hyperthermophilic bacteria of the order
