Environmental Engineering Reference
In-Depth Information
increased in the early 1980s due to the increase in oil prices during the 1970s, and
the process garnered a renewed interest over the last 10 years [25]. All of the bac-
teria that catalyze this process are heterotrophic, that is, they utilize pre-made
carbohydrates as substrates. The production of butanol starts with the conversion
of glucose into pyruvate via glycolysis. The pyruvate is then transformed into
butanol via a five-step process involving acetyl-Coenzyme A [25]. The
Clostridium
species that catalyze this process go through three distinct growth phases: an
exponential phase during which acids are produced; a stationary phase during
which the acids are transformed into solvents; and third, a phase in which spores
are formed. Two of the stoichiometric reactions catalyzed by
C. acetobutylicum
are presented below [25]:
1. 1 glucose → 1 butanol + 2 CO
2
+ 2 ATP
2. 1 glucose → 0.3 acetone + 0.6 butanol + 0.2 ethanol + 2.3 CO
2
+ 1.2 H
2
Reaction (1) above is the simpler, but reaction (2) occurs more often in bacteria.
Recently, a novel pathway for the production of butanol and isobutanol was identi-
fied in
Saccharomyces cerevisiae
[26]. The substrate in this case was glycerol and
it was therefore suggested that mixtures of amino acids from side-stream processes
could be used to produce this biofuel. Interestingly, the bacterium
Escherichia coli
has also been considered a candidate for this purpose and Atsumi
et al.
[27] have
shown it to be capable of butanol production after genetic modification.
7.3.5
Microbiological Ethanol Production
At present, the vast majority of feedstocks for ethanol production comes from
grain or agricultural sources. However, it has been suggested that it may be uneth-
ical to use feedstocks that could be used for food production to generate biofuels.
There is therefore great interest in developing processes for ethanol production
that use lignocellulosic biomass.
7.3.5.1
Lignocellulosic Feedstocks for Microbiological Ethanol Production
Lignocellulosic or woody biomass consists of carbohydrate polymers such as cel-
lulose, hemicellulose, lignin and, to a much lesser extent, extractives, acids, salts,
and minerals.
Cellulose and hemicellulose can be hydrolyzed to sugars and fermented to
ethanol. Cellulose is a polymer of glucose-glucose dimers, and the linkages
between these dimers as well as the hydrogen bonds between polymeric glucose
strands make it very difficult to break up. The process of saccharification involves
the addition of water to break these bonds and liberate glucose for ethanol
production.
Hemicellulose consists of short, highly branched chains of different sugars
such as 5-carbon sugars xylose and arabinose and 6-carbon sugars mannose,
