Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 201

present a broader range of carbon numbers, such as the palm kernel (Fig. 11.10h), which is the
seed of the palm fruit, coconut (Fig. 11.10i), and babassu palm (Fig. 11.10j). For hydrotreatment
of vegetable oil into renewable jet fuel, it is advantageous to have a carbon distribution that is
heavily weighted in the C10-C14 range, which is closer to conventional jet fuel. In this case,
the biorefining process might be able to avoid the energy-intensive hydrocracking stage to break
down long chain-length molecules, as is needed for C16-C18. However, most of the feedstocks
shown in Figure 11.10 are not desirable feedstocks because they may be used for food and/or
have limited availability. In addition, cultivation techniques, such as that used to create palm
plantations, have been linked to deforestation.
For sustainability, we would like to avoid competition with food crops or nutritional supple-
ments, so much of the emphasis in feedstock development has shifted to other oil-producing
crops that can be grown on marginal land which is unsuited for farming. The state of Montana in
the US has instituted a program to grow camelina in intercropping, i.e., in crop rotations when
the land would otherwise have been left fallow. Camelina is related to canola, has 37-45% lipid
content ( 3 fatty acid

and vitamin E. It is one of the feedstocks that have been successfully used to create HEFA, and it
has been tested extensively in civilian and military aircraft. However, it has already been approved
as an animal feed supplement in the US, and it may be approved for human consumption, which
will limit its desirability as a biofuel feedstock. Two inedible beans that have generated much
interest are Jatropha curcus (Fig. 11.7f) and castor beans.
A decade ago, Jatropha curcas appeared to be the perfect answer to sustainable biofuel produc-
tion. Jatropha is a small tree (5-7m tall) that produces fruit after its first year and matures at 3-5
years, with a lifetime of about 50 years. It is native to Mexico, Central America and parts of South
America. Jatropha seeds are oil-rich but inedible. It was believed that the trees were pest-, disease-
and drought-resistant and could thrive without irrigation on marginal land. Its large central tap
root and shallow lateral roots are still widely believed to protect against wind and water erosion.
Aggressive campaigns to increase land devoted to jatropha production were instituted in India,
Africa and elsewhere.
Admittedly, jatropha was known to have some undesirable properties that needed to be
addressed. Some jatropha varieties are quite toxic, and their seeds contain toxins, such as phorbol
esters, curcin, trypsin inhibitors, lectins and phylates. To mitigate this drawback, some have tested
out the concept that the leftover meal after oil extraction could be detoxified with heat treatment
and used for animal feed (Xiao et al. , 2011) or the seed/kernel cake could be used as a fertilizer
or as biomass in an anaerobic digester. Except for its role as biomass, the other uses have not yet
been proven in a commercial setting.
The original optimism has been tarnished somewhat. Since it has not yet been domesticated,
crop production is subject to wide variability. In a compilation of data from 1-9 year-old plants in
South America, India, and Africa, the annual yields ranged widely from 313 to 12,000 kg oil/ha
(Achten et al. , 2008). In general, trees produce seeds at a rate of 0.2-2 kg per tree (Achten et al. ,
2008; Yang et al. , 2010), although there are reports of higher values. The oil content of the seeds
is in the range of 27-44% oil by mass (Achten et al. , 2007). Under controlled conditions for 2
year-old trees developed from wild varieties throughout southern China, the maximum oil yield
per tree per year was 15 times higher than that of the minimum oil yield (Yang et al ., 2010). (Note
that it is not appropriate to extrapolate from biomass yield per tree to oil yield per hectare; the
biomass yield depends on plant spacing, canopy management and other production parameters,
and the conversion between biomass and oil yield can vary substantially.)
Yields have not been impressive on marginal lands, and are highly dependent on rainfall, soil
type, soil fertility, genetics, plant age, spacing and management methods. While jatropha can
grow on a wide range of soils, for best biomass production, it requires an infusion of nitrogen and
phosphorus as fertilizer (Foidl et al. , 1996) and water (Achten et al. , 2008; Yang et al. , 2010).
Seeds do not mature all at once, which makes harvesting a labor-intensive process. Damage due
to pests or disease have been noted in continuous monocultures in India (Achten et al. , 2008).
In non-native localities, jatropha may be an invasive species, and it has already escaped into the

∼ 40% monounsaturated, ∼ 50% polyunsaturated) and is a good source of ω