Page 145 - Materials Chemistry, Second Edition

132 S. H. Gheewala

emissions from the combustion of biofuels in car engines are counterbalanced by
the atmospheric carbon dioxide sequestered during growth of biomass; this is
supposed to yield substantial benefits toward climate change mitigation. Also, their
biomass origin makes them renewable (if sustainably managed), which is a major
advantage over the non-renewable oil-based fuels. The substitution of oil-based
fuels by biofuels is of particular interest to a large number of countries that do not
have large resources of the former and thus have to rely on expensive imports from
the few countries that do. Energy security is a strong driving force for the pro-
motion of biofuels, particularly in the developing world. Socioeconomic benefits,
particularly for the agricultural community, are also an important consideration
(Daniel et al. 2010; ERIA 2008).
This chapter focuses particularly on the environmental sustainability assess-
ment of biofuels. As mentioned earlier, there are perceived environmental benefits
due to the ‘‘biogenic’’ greenhouse gas (GHG) emissions during use phase as well
as the renewable nature of biofuels. However, environmental sustainability of
biofuels cannot be evaluated only on the basis of carbon dioxide emissions in the
use phase. A broader perspective based on the entire life cycle of the biofuel is
imperative. In this case then, the carbon neutrality of biofuels does not hold as
there are GHG emissions associated with the cultivation as well as processing of
feedstock which are not balanced by the uptake of atmospheric carbon dioxide
during plant growth. Rigorous tools, based on the entire life cycle, are thus
required for the proper assessment of the environmental sustainability of biofuels
(Nguyen and Gheewala 2008a, b).

2 Tools for Environmental Sustainability Assessment

As mentioned earlier, the most commonly used tools are based on the entire life
cycle of the biofuels. They are distinguished into three broad categories: (1) net
energy balance and ratio, (2) renewability, and (3) life cycle assessment. The first
two are based on energy (particularly the first law of thermodynamics), and the
third one is for evaluating potential environmental impacts.

2.1 Net Energy Balance and Net Energy Ratio

As biofuels are energy carriers, two indicators that are absolutely essential in their
initial evaluation are the net energy balance and net energy ratio (Shapouri et al.
2006; Nguyen et al. 2007; Prueksakorn and Gheewala 2008). These are pre-
liminary indicators based on the first law of thermodynamics. The net energy
balance or NEB is the difference of the total energy output and the total energy
input over the entire life cycle of the biofuel. Intuitively, the NEB of the system
must be positive or there must be a net energy gain; else, it does not make sense to