Page 41 - Advances in Eco-Fuels for a Sustainable Environment

18 Advances in Eco-Fuels for a Sustainable Environment

biodiversity reduction, spread of crop disease and pests, and sustainability issues, such
as low energy and/or cultivation yields (i.e., for corn, sugar cane, soybeans). Some
crops such as soybeans even presented an overall negative energy balance [9]. Sugar
beets, rapeseed, peanuts, and other foodcrops have also, at one point or another, served
as feedstock for first-generation biofuel. The main technology for obtaining first-
generation biofuels was fermentation, with preliminary mechanical and occasional
thermal preprocessing. Table 2.2 summarizes first-generation biofuels and relative
feedstocks [9].
All first-generation feedstock suffered from similar drawbacks and, ultimately,
they gave way to second- (and, later, third-) generation fuels. Though some first-
generation feedstock will still provide biofuel for some time, their importance is
diminishing as better sustainable alternatives are adopted. Second-generation
biofuels, or advanced biofuels, differentiate from first generation by the fact that feed-
stocks are not food crops, but instead are based on forest or waste-based cellulosic and
lignocellulosic biomass, extensively used in bioethanol production, or on other waste
biomasses. To qualify as “second-generation” feedstock, a source must not be suitable
for human consumption; some feedstocks, however, can be considered both first- and
second-generation depending on provenance. For example, “virgin” vegetable oil is a
first-generation feedstock; however, after it has been used and is no longer fit as a
cooking oil, it becomes a second-generation one. Therefore, the only way foodcrops
can be considered second generation is if they have fulfilled their food purpose
already. Also, although not a legal requirement, second-generation feedstock should
preferably grow on marginal land (land that cannot be used for growing food). The
implicit assumption is that second-generation feedstock, in addition to no food value,
should have minimal environmental impact, not requiring great amounts of water or
fertilizers, a fact that led to problems with several second-generation crops. Table 2.3
summarizes these biofuels and their feedstocks.
Different technologies are generally used to extract energy from second-generation
feedstock. This is the case for lignocellulose feedstock, which may require multiple
processing steps prior to fermentation into ethanol. Several biological and chemical
processes have been adapted for the production of second-generation biofuels: fer-
mentation with genetically modified bacteria is popular for second-generation feed-
stocks such as landfill gas and municipal waste. Thermochemical processes are also
adopted; one such processing route is gasification, used on conventional fossil fuels
for many years, even though second-generation gasification technologies have been
adapted to accommodate new feedstocks. Through gasification, organic carbon mate-
rial is converted to CO, H 2 , and CO 2 in an oxygen-limited environment. The product
gas (synthesis gas or “syngas”) is used to produce energy or heat, usually in CHP
(combined heat and power) facilities. A second thermochemical process is pyrolysis,
also used with fossil fuels in the past. Pyrolysis occurs in the absence of oxygen, some-
times in the presence of inert gases, and the fuel is generally converted into three prod-
ucts: tars, syngas, and char (or biochar, depending on its carbon content). Pyrolysis can
be achieved thermally or, more recently, via a microwave–assisted process (MAP)
that allows better control on its final products [10]. From the energy-recovery point
of view, tars are the most interesting product of pyrolysis, a liquid product with