Environmental Engineering Reference
In-Depth Information
standpoint that sources of food should not be used as sources of fuel [although this discussion
does not seem to affect other nonfood uses such as lubricants, polymers, etc. (FAO 2009) and the
carbon footprint of some feedstocks]. The food-versus-fuel issue has caused some shifting toward
emphasizing feedstocks that yield inedible oils. On the other hand, soybeans have historically been
grown for their protein content so that if production is increased for the sake of the oil, the protein,
used as animal feed, should become more plentiful and therefore less expensive, reducing the cost
of feeding animals. The carbon footprint issue, incorporating the issue of land-use change, currently
affects largely commodity oils. For palm oil, concern has been voiced over the clearing of tropical
rainforest for the sake of new palm plantations, but greenhouse gas mitigation by using palm-based
biodiesel has also been discussed (May 2005). Although similar concerns have been raised over the
increased cultivation of soybeans, especially in tropical areas, biodiesel from soybean oil possesses
a positive energy balance (Hill et al. 2006). For rapeseed/canola (but other feedstocks as well),
concerns exist over the use of fertilizers and nitrous oxide (N
2
O) emissions (Crutzen et al. 2007),
runoff, and the carbon footprint of producing fertilizers. It is not clear how some of these issues will
in the future affect feedstocks, which are discussed in this chapter because of their current nascent
status because of which many issues have not yet been addressed.
In some cases, other distinct aspects play a role. For example, the major background for the
investigation of coffee oil as a biodiesel feedstock was to possibly find a use for damaged coffee
beans to minimize the resulting economic losses (Oliveira et al. 2008). No significant differences
in the fatty acid profiles of healthy and damaged coffee beans were observed (Oliveira et al. 2006).
Another report (Mariod et al. 2006) discusses oils obtained from bugs, one of them a pest of
watermelons, and both oils are used for cooking and medicinal applications. Although formally not
vegetable oils, for the sake of interest they are included in this chapter.
In any case, with the increasing interest in these issues, the number of publications concerned
with feedstock variety has grown considerably. A significant number, if not the majority, of the oils
briefly discussed in this chapter can be obtained from plants found most commonly in tropical or
subtropical climates because many of these oils are of limited physiological use and the issue of
energy supply is becoming more pressing in the developing countries located in these regions of
the world.
33.1.1 f
atty
a
cid
p
rofilE
/f
uEl
p
ropErtiES
An important issue that does not always find the interest it deserves when the above issues are
discussed is that of the composition of the fuels being discussed. Ultimately, it is the composition of
the fuels and the properties resulting from this composition that determine if a feedstock is viable
as a source of fuel. Therefore, a brief discussion of fuel properties imparted by the fatty acid profile
of a vegetable oil is presented here with reference to the corresponding specifications in biodiesel
standards. Furthermore, most classical commodity oils and most oils discussed in this chapter
largely contain the same five major fatty acids in their profiles: palmitic (hexadecanoic; C16:0),
stearic (octadecanoic; C18:0), oleic [9(
Z
)-octadecenoic; C18:1], linoleic [9(
Z
),12(
Z
)-octadecadienoic;
C18:2], and linolenic [9(
Z
),12(
Z
),15(
Z
)-octadecadienoic; C18:3].
Cetane number, kinematic viscosity, oxidative stability, and cold-flow specifications in biodiesel
standards are those most directly affected by the fatty acid profile. Minor components of biodiesel
(e.g., mono- and di-acylglycerols formed during the transesterification reaction, sterol glucosides,
and antioxidants) can also influence these properties, especially oxidative stability and cold flow,
but they will not be discussed here.
The cetane number is a dimensionless descriptor of the ignition quality of a diesel fuel. It is
related to the ignition delay time that a fuel experiences upon injection into the combustion chamber
of a diesel engine. Hexadecane (trivial name: cetane) is the high-quality reference compound on the
cetane scale and has been assigned a cetane number of 100. The shorter the ignition delay time,
the higher the cetane number and vice versa. Generally, higher cetane numbers are more desirable.
