Environmental Engineering Reference
In-Depth Information
transportation use is in direct competition with food production in most countries,
keeping ethanol price relatively high and discouraging its use as a motor fuel
except where it is strongly subsidized, such as in Brazil and USA. From this point
of view, ethanol produced from lignocellulosic feedstocks (woody and herbaceous
biomass) could represent a more suitable source of this renewable fuel.
Biodiesel is a renewable Diesel fuel substitute. Generally, it can be considered
as a Diesel fuel derived from any biomass; however, its current production method
is based on the transesterification reaction (or alcoholysis) of triglycerides by
methyl alcohol (methanol), to obtain a product known as fatty acid methyl ester
(FAME). Triglycerides can be contained in different fatty biomaterials, such as
vegetable oils and animal fats. In the European Union, world leader in production
and consumption of biodiesel, it is mainly derived from rapeseed and sunflowers,
while in USA, the predominant source is the soybean oil. The European Union is
expected to continue to be the main producer of biodiesel, thanks to the intro-
duction in 2003 of European Directive 2003/30/CE, which requires that 5.75% of
conventional fossil fuels will have to be substituted by biofuel. Triglycerides are
esters of three fatty acids and one glycerol, with the type of fatty acid depending
on the nature of the bio-source. FAME is regarded as ''first generation'' biofuel as
it is produced by only a small fraction of the crop (the oil extracted from the sees
of the plant), while second and third-generation biofuels will use the whole crop
thanks to new technology under development. In principle, vegetable oils could
be directly used in Diesel engines; however, some of their characteristics would
cause several problems in vehicle applications. In particular, the viscosity of
vegetable oils is about 10-20 times higher than that of petroleum Diesel fuel,
which implies very different injection and atomization characteristics. As fuel
injection systems of modern Diesel engines are very sensitive to viscosity change,
the problem deriving by the direct use of vegetable oils has been avoided by the
transesterification with methyl alcohol. However, the use of methyl alcohol
implies the introduction into the production process of an additional fossil energy
(methyl alcohol is produced by natural gas via syngas), which partially compro-
mises the closed-loop carbon dioxide circle regarded as the main environmental
advantage of using plants as energy sources. Biodiesel can be used in Diesel
engines neat or blended with petroleum Diesel fuel, and its beneficial properties
with respect to the conventional fuel include higher cetane number, zero sulfur and
aromatics content, lower particulate matter, CO and polycyclic aromatic hydro-
carbons (PAH) emissions (higher emissions of aldehydes can be controlled by
exhaust catalytic converters). While the use of FAME as a component of blended
Diesel fuels is commercially increasing (blend B20, containing 20% of biodiesel),
the neat FAME biodiesel is present only in niche market. Similarly to any other
fuel derived from plants (i.e. bioethanol) the worldwide limited landscape surface
for cultivation, with the consequent competition with food production, is a key
concern which strongly hinders the biomass fuel diffusion. Although the second
and third-generation biofuels promise to utilize the whole crop of non-edible
plants, land exploitation issues will remain of fundamental importance for all fuels
derived from biomass, especially if the very low efficiency (\1%) of the natural
