Page 146 - Materials Chemistry, Second Edition

Environmental Sustainability Assessment of Ethanol 133

produce the biofuel. Of course, the first law of thermodynamics does not allow for
‘‘creation of energy.’’ The positive NEB is possible for biofuels only because the
solar energy input during biomass growth is not accounted for in the calculation.
Similar in approach to NEB, the net energy ratio or NER is the ratio of the total
energy output to the total energy input over the entire life cycle of the biofuel. This
ratio must be greater than one for the biofuel production to be meaningful. It must
be emphasized that having a positive NEB or an NER greater than one is not
sufficient to establish the environmental sustainability of biofuels. However, they
give a very good first check; if the NEB of a biofuel is negative or NER is less than
one, then it probably does not make sense to produce the biofuel as we are
investing more energy into the production of the fuel than we actually get back
from its use. Of course in certain special circumstances, we may still go ahead with
the biofuel production even in such a case if the energy carriers used as input to the
biofuel system are cheap and easily available, but cannot themselves be used as
transportation fuel substitutes.

2.2 Renewability

Another energy-based indicator, similar to NER, is the renewability ratio which is
defined as the ratio of the total energy output to the total fossil energy input. This
ratio distinguishes between the types of energy carriers that are input to the biofuel
cycle; if there is more (renewable) energy output than the (non-renewable) fossil
energy input, i.e., renewability ratio higher than one, it indicates that the invest-
ment of fossil energy into biofuel system has yielded a higher amount of renewable
energy.

2.3 Life Cycle Assessment

Life cycle assessment (LCA) is a tool for environmental sustainability assessment
that is particularly suited and hence widely used for assessment of biofuels. This
tool evaluates the environmental impacts of a product (or service) over the entire
period of its life, starting from raw materials extraction (or production) and
including materials processing, distribution, use, and waste disposal at the end of
life. The principles and criteria of LCA are covered by the ISO 14040:2006 and
ISO 14044:2006 standards. The environmental impact categories commonly of
interest for evaluating biofuel systems are climate change (global warming),
acidification, nutrient enrichment (eutrophication), human and ecotoxicity, land
use, and biodiversity. Very often, biofuel studies are limited to climate change
partly due to its global nature and importance in many scientific discussions and
partly because of the perceived benefits of biofuels toward reducing GHG emis-
sions as compared to their fossil counterparts which must be scientifically verified.