Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 199

In testing for automotive emissions, the presence of FAAE reduces all types of emissions,
except for NO x . However, one study found that the release of this pollutant can be controlled
by modification of injection timing (Krahl et al. , 2005). For aviation purposes, the presence of
FAAE or F-T fuel in a jet fuel blend reduced all categories of the emissions discussed here (Timko
et al. , 2011).
For a thorough description of the complex interplay by which gaseous and particulate emissions
influence the climate, see the excellent review by Lee et al. (2010). For a discussion of particulate
matter, see Kumar et al. (2010), and Timko et al. (2010); biofuel combustion chemistry and the
generation of toxic emissions Kohse-Hoinghaus et al. (2010), and Krahl et al. (2009); systems
analysis of emissions reduction strategies for Europe, see Dray et al. (2010).

11.4 BIOFUEL FEEDSTOCKS FOR AVIATION FUELS

There are three commonly used categories of biofuels that indicate their readiness for commer-
cialization. Although there is no universal agreement on the specifics, we will categorize them
by the sophistication of the conversion technology as follows:
•

First-generation biofuels : Easiest to bring to market using current technology. This category
uses fermentation processes to produce bioethanol. Commonly used feedstocks include corn
in the US and sugar cane in Brazil. The corn-based bioethanol in particular is widely criticized
for producing a larger environmental cost than petroleum fuels. The low energy density of
bioethanol is inadequate for aviation fuel (Hileman et al. , 2010).

• Second-generation biofuels : Requires process improvements in refining technology to

commercialize. This category uses biomass or coal as feedstocks in gasification/liquefaction
processes, such as Fischer-Tropsch, to produce biofuel. Biomass sources include switchgrass,
agricultural waste, wood chips and other forest residue.

• Third-generation biofuels : Requires cost-effective, sustainable means of producing the feed-

stock, as well as efficient harvesting, oil extraction, and conversion. This category includes
fuels from oils derived from vegetables and microbes, as well as greases from animal fat.
Second-generation biofuel processing is discussed elsewhere in this topic and in other sources.
See, e.g., for general background Kinsel (2010), Kreutz et al. (2008), Sims et al. (2010),
Sivakumar et al. (2010); for cellulosic genomics, see Rubin (2008); and for lignocellulosic
co-products, see Mtui (2009). In this section, we will focus on feedstock production for first- and
third-generation biofuels. The latter is an important area, since the high cost of feedstock has the
biggest impact on the fuel price. For sustainability, the feedstock choice may change from one
geographic region to another, depending on factors such as climate, land use, water availability,
and the location of the nearest processing facilities.

11.4.1 Crop production for oil from seeds
Many plants have oil-rich seeds that can be converted to fuel, such as soybeans, camelina, canola
(rapeseed), and sunflower. Other feedstocks with good oil content are cotton seed, babassu, palm,
and coconut. Much of the research to date on biodiesel has been performed on these crops, particu-
larly soybeans (see, e.g., Akbar et al. , 2009; Freitas et al. , 2011;Yuan et al. , 2005, 2009). The lipid
composition of soy methyl esters is quite consistent across different studies, with a preference for
C16-C18 range, and a high proportion of monounsaturated and polyunsaturated C18 compounds,
as shown in Figure 11.10a. (Yuan et al. , 2005) presented data that showed genetically modified
(GM) soy, which had a more desirable fatty acid profile with a higher concentration of monoun-
saturated methyl oleate (C18:1) (Fig. 11.10b). In Europe, canola has dominated the scene as a
biofuel feedstock. It has a high lipid content of up to 50%and a composition that is mostly in unsat-
urated methyl esters at C18 and saturated ME at C16 (Fig. 11.10c). Many vegetable oils exhibit a
preference for the C16-C18 range, such as cotton (Fig. 11.10d), jatropha (Fig. 11.10e), sunflower
(Fig. 11.10f), palm (Fig. 11.10g), milkweed, and camelina. There are also some vegetable oils that