Environmental Engineering Reference
In-Depth Information
A review of material compatibility of diesel with respect to biodiesel (Fazal et al. , 2011) has
identified some common themes among biodiesels: although they have better inherent lubricity
than diesel (Holser and Harry-O'Kuru, 2006; Knothe and Steidley, 2005), they are hygroscopic
and tend to absorb any moisture that may be present in their surroundings. This can promote
the growth of microbes that produce corrosion-producing metabolic products, as well as render
electrochemical corrosion more likely upon exposure to certain metals. In field-testing for auto-
motive applications, biodiesel generally produces similar or less wear than conventional diesel.
However, elemental testing suggests that certain metals should not be paired with other particular
metals. Biodiesel is, in general, particularly corrosive to copper compounds as compared to fer-
rous compounds, but corrosion is dependent on both the parent feedstock and the type of metal
(Fazal et al. , 2011).
Figure 11.9d shows that melting point increases with chain length andwith increasing saturation
for cis -type compounds. However, trans -type compounds exhibit reduced melting point relative to
saturated compounds, but the melting point is substantially higher than their sister cis -type esters.
Note that, for most pure substances, the melting point is nearly equivalent to the freeze point, i.e.,
the temperature at which solids and liquids co-exist. Some substances can be supercooled below
this value when there are no nucleation sites, so the use of “melting point” may be considered
more precise by some than “freeze point”. Moreover, some substances like agar do have different
solid-to-liquid and liquid-to-solid transition temperatures. For the methyl esters discussed here,
the freeze point and the melting point are nearly equivalent.
In any event, the high melting/freezing points of methyl esters become problematic for creating
an adequate jet fuel blend, since the jet fuel specifications call for a freeze point of
40 Cor
below. We have seen that the freeze point of a fuel is a function of its composition. Unlike pure
hydrocarbons, some FAAEs may not exhibit a simple, linear dependence on blend ratio, making
it more difficult to predict the freeze point of the final blend.
Examination of the melting point data points out another issue with respect to jet fuel suitability.
Jet fuel specifications use a reference temperature of 20 C for measurement of kinematic
viscosity. At this temperature, Figure 11.9d shows that the saturated methyl esters from C10:0
and up are solids. Most vegetable oils are heavily weighted toward compounds in the range of
C16-C18, and consist of both saturated and unsaturated components. The cis -type unsaturated
compounds at C16-C18 may exist naturally as liquids at 20 C in their pure state, but the
blends that are typical of vegetable oil will include solid components at this temperature. Even
at 12.5 C, solids were were identified in three commercial biodiesels due to freezing methyl
palmitate (C16:0) and methyl stearate (C20:0) (Coutinho et al. , 2010). Unsurprisingly, most
measurements of viscosity as a function of temperature for FAAE-based biodiesel stop short of
-20 C, but they tend to exceed the 8mm 2 /s upper bound for jet fuel well above this temperature
(see e.g., Freitas et al. , 2011).
The Oxidative Stability Index (OSI), measured in hours, is used to quantify susceptibility
to fuel degradation through exposure to oxygen. Increased OSI indicates increased oxidative
stability (Fig. 11.6e). The OSI increases with chain length and decreases with the presence
of double bonds. Finally, the cetane number (Fig. 11.6f), which increases as ignition delay time
decreases, exhibits an increase with carbon number and a decrease with level of saturation. See
Moser (2009) for further discussion.
The monounsaturated compounds (C x :1) thus present a marked advantage in terms of reduced
viscosity and melting point with little or no penalty in terms of energy content. However, the
polyunsaturated compounds (C x :2 and C x :3), while they may be beneficial in terms of reduced
viscosity, may increase the density to undesirable levels, as shown in Figures 11.9a and 11.8a
for methyl linolenate (C18:3). Polyunsaturated esters are also less stable with a reduced storage
stability (Ramos et al. , 2009).
For further reading on the effects of structural composition on biofuel properties, see Knothe
(2005, 2008a), Moser (2009), and Yuan et al. (2003). For a good description of the analytical
methods used for evaluating biodiesel composition, see Chapter 5 of Knothe et al. (2005), also
van der Westhuizen et al. (2011).
Search WWH ::




Custom Search