BIOCHEMICAL CONVERSION: BIOFUELS BY INDUSTRIAL FERMENTATION - Biomass as a Sustainable Energy Source for the Future - page 429

The last point remains the most challenging one. Wild-type organisms may tolerate
butanol up to aqueous concentrations of only ~15 g.L -1 , while engineered organisms
are still limited to ~30 g.L -1 . In contrast, a tolerance of 170 g.L -1 has been achieved in
some ethanol fermentations. Ethanol is less hydrophobic and has less tendency to
interfere with the lipid bilayer of the cell membrane.
The low final butanol concentration leads to relatively short batch fermentations
and relatively long downtimes after each batch and thus to inefficient fermentor
use and high investment cost. Moreover, for both batch and continuous fermentation,
the recovery of butanol from dilute aqueous solutions is energy intensive, which is
expensive and also energetically unfavorable if the butanol is to be used as fuel.
The default recovery method is distillation.
Because of these issues, 1-butanol does not yet seem to be competitive with ethanol
as biofuel, despite its much better properties. Nevertheless, significant commercial
fermentative 1-butanol production has started again in China and is planned at several
other locations. This butanol seems to be used as chemical rather than as fuel.
Similarly, isobutanol production using recombinant bacteria is reaching large-scale
commercialization.

13.5 DIESEL-LIKE PRODUCTS

Current biofuels, namely, first-generation ethanol and biodiesel, supply 2% of the
global transport energy demand. This number is expected to increase to 27% by
2050 (EIA, 2011), in particular due to the need for replacement of liquid fuels for
planes, marine vessels, and other heavy transport vehicles that are not suited for other
renewable energy sources such as electricity.
Biodiesel is a mixture of fatty acid alkyl esters (FAAE) obtained from free fatty
acids and triglycerides (see also Chapter 18). Currently, biodiesel is being produced
from vegetable and animal oils or fats and cooking oil waste, and it is mostly used in
conventional diesel engines in blends of up to 20 vol.%. Alternative feedstocks and
routes are being evaluated in the search for products that resemble current petroleum
fuels and, hence, can make use of existing infrastructure. These are also referred to as
advanced or drop-in biofuels. In this section, we look at the routes involving industrial
fermentation. At the time of writing, most of these routes were still in research or
pilot-scale stage.

13.5.1 Routes Using Oleaginous Microorganisms

All microorganisms have the ability to synthesize lipids as a fundamental constituent
of the cell membranes. However, a few genera of bacteria, yeasts, molds, and algae —

referred to as oleaginous microorganisms are also able to accumulate lipids to more

than 20% of their dry cell mass. In such microorganisms, lipid accumulation starts
when a carbon source is present in excess and an element in the growth medium
becomes limiting. The microorganisms are not able to grow and hence convert the
excess carbon into lipids, storing them as energy reserve in a vacuole inside the cell.

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