Page 62 - Advances in Eco-Fuels for a Sustainable Environment

Ecofuel feedstocks and their prospects 39

Table 2.10 International market prices for ethanol,
n-butanol, and sugarcane in 2016 [45]

Product Price
Sugarcane US$ 27.26/t
Ethanol US$ 0.66/L
n-Butanol (chemical) US$ 1.34/L
n-Butanol (biofuel) US$ 0.83/L

commercialization in the chemical market collects slightly higher revenues than those
of an integrated first- and second-generation ethanol production. If butanol is sold in
the fuel market, earnings are not yet attractive for investors. More research has to be
done to improve production of n-butanol, and bring it to a competitive scale against
gasoline and ethanol in the fuel market.
Microalgae, the third-generation feedstock, can be used for direct energy produc-
tion by lipid extraction or fermentation with considerable advantages over other feed-
stocks due to high photosynthetic efficiency, faster growth rates, and the capacity of
providing valuable microalgal components (carbohydrates or glycerol) for fermenta-
tive bacteria. Algae offer tremendous benefits over most conventional biofuels: they
are easily manipulated using genetic engineering technology, allowing organisms to
be modified to produce more useful oil and require fewer nutrients. Second, they can
be grown in small facilities on marginal land. This means they are not a threat to the
food supply and, theoretically, could replace most fossil fuels. Microalgae are rich in
carbohydrates (>40%) that can be converted to simple sugars and used for fuel
production, and this makes them the ideal feedstock for biobutanol production via
fermentation [46]. Algal biomass is harvested, pretreated to release the monosaccha-
rides (microalgal sugars), and fed into the fermentation process. Cheng et al. [47]
observed that when using microalgae-based feedstock for butanol production,
C. acetobutilicum is capable of converting the residual solid matter present in the
medium to butanol, and that butanol yield is higher when using unfiltered hydrolysate
as a substrate (21.96mg/g residues versus 10.03mg/g residues of filtered hydrolysate).
Notwithstanding these promising characteristics, however, the main obstacle for bio-
butanol production from microalgal biomass and its commercialization is the current
high capital and operating costs for microalgae cultivation (400€/t biomass) [15].
Currently, there are only a few reports regarding biobutanol production using
microalgal feedstock cultivated from wastewater; the microalgae were pretreated to
release the component simple sugars and used as a substrate for ABE fermentation.
Algae, mentioned earlier, require large amounts of water, nitrogen, and phosphorus
to grow. While the growth of microalgal biomass from polluted water could lower
process costs, the quality of monocultural microalgae is more stable and may give
a higher carbohydrate content, making it more suitable for industrial application.
Only a few microbial processes are currently used for commercial production of
biofuels, but this will change with the enhanced production capabilities being sought
through microbial metabolic engineering. Research, recently, has in fact turned to the