2.2 The Nature of Biofuels, Technologies, and Production Pathways
29
there, yet economic viability and other sustainability concerns remain (Prussi et al.
2019).
2.3 Biofuels and the Environment
2.3.1 Climate Change Mitigation
Most scenarios in line with the 1.5 °C or 2 °C climate targets foresee substantive
reliance on biomass (Daioglou et al. 2017), but does the substitution of fossil fuels for
biofuels indeed reduce greenhouse gas (GHG) emissions? This question has been
central to many biofuel sustainability assessments, but it has no easy answer. In
principle, all biofuels provide emissions reductions because the carbon they emit
when burnt equals what they absorbed from the atmosphere during plant growth—
or what has been referred to as the carbon uptake credit (Searchinger et al. 2008).
This cycling would make biofuels, in theory, carbon neutral. However, the reality is
that at least three significant factors may alter that picture, putting into question the
“cleanliness” of large-scale biofuel production.
First, most biofuel production relies on chemical-intensive monocultures that have
large carbon footprints. Heavy utilization of pesticides and nitrogen fertilizers (which
use fossil energy in their production), in addition to transportation and processing,
entail substantial GHG emissions (McElroy 2006). In practice, the emissions savings
from fossil fuel replacement may be much smaller than in principle. Although there is
variation depending on the calculation methodology used, most assessments suggest
that corn-grain ethanol, for example, achieves only minor emissions reductions (12–
19%) due to fossil-energy use in its production chain (Hill et al. 2006; Farrell et al.
2006; Groom et al. 2008). Sugarcane-based ethanol, in turn, appears to be more
efficient, reducing emissions by 70% or more (Hill et al. 2006; Farrell et al. 2006;
Groom et al. 2008). More advanced biofuels such as grass-based cellulosic ethanol
could also achieve good results, particularly if little or no chemical inputs are used
(see Tilman et al. 2006). In some cellulosic-ethanol cases, GHG emissions reductions
may exceed 80%, but this can vary depending on the adopted land management
practices (Qin et al. 2018).
Second, climate impact assessments must include other GHGs such as methane
(CH 4 ) and nitrous oxide (N 2 O) emitted from feedstock cultivation. These gases are
far more potent than CO 2 in their contribution to the greenhouse effect and are
a crucial concern related to industrial agriculture and livestock farming (Lesschen
et al. 2011; Reay et al. 2012). N 2 O emissions, in particular, seem very much linked to
the widespread utilization of N-fertilizers in agriculture (Park et al. 2012). As such,
some authors have argued that an increase in other GHG emissions can hamper or
even negate savings from replacing fossil fuels (Melillo et al. 2009; Crutzen et al.
2007). These other GHGs have shown to be important not only for biofuels based
on agriculture but also for microalgae biodiesel’s life cycle (Frank et al. 2012).
29
there, yet economic viability and other sustainability concerns remain (Prussi et al.
2019).
2.3 Biofuels and the Environment
2.3.1 Climate Change Mitigation
Most scenarios in line with the 1.5 °C or 2 °C climate targets foresee substantive
reliance on biomass (Daioglou et al. 2017), but does the substitution of fossil fuels for
biofuels indeed reduce greenhouse gas (GHG) emissions? This question has been
central to many biofuel sustainability assessments, but it has no easy answer. In
principle, all biofuels provide emissions reductions because the carbon they emit
when burnt equals what they absorbed from the atmosphere during plant growth—
or what has been referred to as the carbon uptake credit (Searchinger et al. 2008).
This cycling would make biofuels, in theory, carbon neutral. However, the reality is
that at least three significant factors may alter that picture, putting into question the
“cleanliness” of large-scale biofuel production.
First, most biofuel production relies on chemical-intensive monocultures that have
large carbon footprints. Heavy utilization of pesticides and nitrogen fertilizers (which
use fossil energy in their production), in addition to transportation and processing,
entail substantial GHG emissions (McElroy 2006). In practice, the emissions savings
from fossil fuel replacement may be much smaller than in principle. Although there is
variation depending on the calculation methodology used, most assessments suggest
that corn-grain ethanol, for example, achieves only minor emissions reductions (12–
19%) due to fossil-energy use in its production chain (Hill et al. 2006; Farrell et al.
2006; Groom et al. 2008). Sugarcane-based ethanol, in turn, appears to be more
efficient, reducing emissions by 70% or more (Hill et al. 2006; Farrell et al. 2006;
Groom et al. 2008). More advanced biofuels such as grass-based cellulosic ethanol
could also achieve good results, particularly if little or no chemical inputs are used
(see Tilman et al. 2006). In some cellulosic-ethanol cases, GHG emissions reductions
may exceed 80%, but this can vary depending on the adopted land management
practices (Qin et al. 2018).
Second, climate impact assessments must include other GHGs such as methane
(CH 4 ) and nitrous oxide (N 2 O) emitted from feedstock cultivation. These gases are
far more potent than CO 2 in their contribution to the greenhouse effect and are
a crucial concern related to industrial agriculture and livestock farming (Lesschen
et al. 2011; Reay et al. 2012). N 2 O emissions, in particular, seem very much linked to
the widespread utilization of N-fertilizers in agriculture (Park et al. 2012). As such,
some authors have argued that an increase in other GHG emissions can hamper or
even negate savings from replacing fossil fuels (Melillo et al. 2009; Crutzen et al.
2007). These other GHGs have shown to be important not only for biofuels based
on agriculture but also for microalgae biodiesel’s life cycle (Frank et al. 2012).