Environmental Engineering Reference
In-Depth Information
The finite quality and perceived short-term scarcity of fossil reserves,
aggravated by existing geopolitical tension, has induced energy instability
and high prices in recent years. Within this context, biomass has been
rediscovered for its biofuel and chemical production potential as an
alternative to fossil fuels. Both in the case of bioenergy and biorefineries,
advantages in the valorization of biomass are found. Producing bioenergy
from crops and agro residues is interesting for many reasons; perhaps the
most attractive for investors being that some of the resulting biofuels such as
bioethanol, biodiesel and biogas can be incorporated into the market using
the energy infrastructure already in place. Other important advantages are
its renewable character as long as vegetation is carefully managed, its
relatively easier accessibility as compared with fossil fuels and the fact that it
may be exploited using less capital-intensive technologies. In addition, the
decrease in greenhouse gas (GHG) emissions coming from the use of fossil
fuels can be a gain as they are replaced by carbon-neutral biofuels. The
savings in GHG emissions could become an appealing economic incentive,
particularly to less economically developed countries, as encouraged by the
Clean Development Mechanism. Furthermore, processing biomass can
provide a setting for industries to be brought into rural areas, which in turn
can potentially create jobs and return money into rural systems and give the
opportunity for local, regional and national energy self-sufficiency across
the globe. Finally, in many cases, use of biomass can contribute to solve
environmental problems, related for example to the inadequate management
of waste, or undesirable biomass growth caused by eutrophication.
However, the many advantages of biomass use for energy or chemicals
production holds true only as long as the harvesting of solar energy via
biomass is not performed at the expense of fossil fuel expenditure or
inappropriate land use changes. In this sense, use of energy crops as main
substrate should always be approached with caution, taking into
consideration the expected impacts in the environmental and social spheres,
because of the negative implications of intensive agricultural production and
the possible competition with alternative biomass uses such as food, feed or
soil conservation.
Within this context, anaerobic digestion (AD) is a very appealing
alternative to add value to liquid and (semi) solid biomass. AD is considered
a plain technology, converting chemically bound energy in organic (in)
soluble matter into an 'easy-to-get' energy-rich gaseous end product
(methane, CH
4
) and a nutrient-rich semi-liquid stabilized by-product (i.e.
digestate). Furthermore, AD is considered a very flexible technology,
accepting a wide range of different types of substrates, producing an energy
carrier with a very flexible end-use and with implementation scales varying
from very small to very big. AD can be used to convert agricultural (by)
products or energy crops into methane but it can also be part of different
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