2.2 The Nature of Biofuels, Technologies, and Production Pathways
27
importantly, it allows the use of feedstocks that are not used for food production,
such as grasses or wood pellets, thereby reducing food vs. fuel competition. However,
these more advanced technologies still need to become more cost-effective before
they can be further scaled up and mainstreamed.
Once produced, ethanol can be blended with gasoline or even replace it entirely,
depending on the engine. A 10% ethanol blend with gasoline is often used, as
this generally does not require engine changes (IEA 2006, p. 389). In that form,
ethanol replaces not only a fraction of gasoline but also the additives commonly used
to enhance its performance (e.g., antiknock agents, octane enhancers, oxygenates)
(Solomon et al. 2007; Sagar and Kartha 2007).
2.2.5 Liquid Biofuels: Biodiesel (FAME and HVO)
Biodiesel is conventionally produced through the chemical conversion (transesterification) of animal fats or vegetable oils into compounds called fatty acid methyl esters
(FAME), which receive the commercial name of “biodiesel” due to their chemical
resemblance with petroleum-based diesel.
These methyl esters can be blended with or replace conventional diesel completely
in various applications, such as vehicles and stationary engines used for heat or
electricity generation. Most commonly, plant oils are (chemically or mechanically)
extracted from seeds and then mixed with an alcohol and a catalyst for the reaction,
resulting in biodiesel and glycerin. After a cleaning process, biodiesel can be utilized
with very little or no modification in conventional combustion engines (Agarwal
2007). Various lipid feedstocks can be used and mixed, but fuel quality may vary.
Usually, producers seek specific standards of viscosity and concentration of certain
compounds (e.g., iodine) for the sake of engine performance and durability. For
example, biodiesel produced purely from animal fats may revert to its denser form at
low temperatures and compromise engine function. Therefore, feedstock mixing is
sometimes a way to achieve “ideal” physicochemical standards (see Karmakar et al.
2010).
Over the past years, a different chemical compound, hydrotreated vegetable oil
(HVO), has been increasingly used due to closer similarity with fossil diesel and
superior fuel performance. Its (costlier) processing consists of treating oily feedstocks
with hydrogen to remove oxygen and synthesize a diesel analog (Chiong et al. 2018).
Chemically distinct from FAME, HVO is at times referred to as “green diesel” to
distinguish it from conventional biodiesel, but its biological origins and end-uses are
the same. (This book refers to both as biodiesel, as is frequently done, e.g., REN21
2019. The specific type is pointed out only when a distinction may be needed.)
As of 2020, most biodiesel production (of either type) came from edible vegetable
oils (e.g., rapeseed, sunflower, soybean, and palm oil). However, non-edible vegetable
oil crops (e.g., castor bean, Jatropha curcas) have also been used to a lesser extent,
and so have microalgae. Microalgae can grow rapidly, and many of them have very
high oil content (Chisti 2007). They can potentially grow in non-potable or industrial
27
importantly, it allows the use of feedstocks that are not used for food production,
such as grasses or wood pellets, thereby reducing food vs. fuel competition. However,
these more advanced technologies still need to become more cost-effective before
they can be further scaled up and mainstreamed.
Once produced, ethanol can be blended with gasoline or even replace it entirely,
depending on the engine. A 10% ethanol blend with gasoline is often used, as
this generally does not require engine changes (IEA 2006, p. 389). In that form,
ethanol replaces not only a fraction of gasoline but also the additives commonly used
to enhance its performance (e.g., antiknock agents, octane enhancers, oxygenates)
(Solomon et al. 2007; Sagar and Kartha 2007).
2.2.5 Liquid Biofuels: Biodiesel (FAME and HVO)
Biodiesel is conventionally produced through the chemical conversion (transesterification) of animal fats or vegetable oils into compounds called fatty acid methyl esters
(FAME), which receive the commercial name of “biodiesel” due to their chemical
resemblance with petroleum-based diesel.
These methyl esters can be blended with or replace conventional diesel completely
in various applications, such as vehicles and stationary engines used for heat or
electricity generation. Most commonly, plant oils are (chemically or mechanically)
extracted from seeds and then mixed with an alcohol and a catalyst for the reaction,
resulting in biodiesel and glycerin. After a cleaning process, biodiesel can be utilized
with very little or no modification in conventional combustion engines (Agarwal
2007). Various lipid feedstocks can be used and mixed, but fuel quality may vary.
Usually, producers seek specific standards of viscosity and concentration of certain
compounds (e.g., iodine) for the sake of engine performance and durability. For
example, biodiesel produced purely from animal fats may revert to its denser form at
low temperatures and compromise engine function. Therefore, feedstock mixing is
sometimes a way to achieve “ideal” physicochemical standards (see Karmakar et al.
2010).
Over the past years, a different chemical compound, hydrotreated vegetable oil
(HVO), has been increasingly used due to closer similarity with fossil diesel and
superior fuel performance. Its (costlier) processing consists of treating oily feedstocks
with hydrogen to remove oxygen and synthesize a diesel analog (Chiong et al. 2018).
Chemically distinct from FAME, HVO is at times referred to as “green diesel” to
distinguish it from conventional biodiesel, but its biological origins and end-uses are
the same. (This book refers to both as biodiesel, as is frequently done, e.g., REN21
2019. The specific type is pointed out only when a distinction may be needed.)
As of 2020, most biodiesel production (of either type) came from edible vegetable
oils (e.g., rapeseed, sunflower, soybean, and palm oil). However, non-edible vegetable
oil crops (e.g., castor bean, Jatropha curcas) have also been used to a lesser extent,
and so have microalgae. Microalgae can grow rapidly, and many of them have very
high oil content (Chisti 2007). They can potentially grow in non-potable or industrial