Page 176 - Biofuels Refining and Performance

Fuel and Physical Properties of Biodiesel Components 159

5.6 Lubricity
With the advent of low-sulfur petroleum-based DFs, the issue of DF lubric-
ity is becoming increasingly important. Desulfurization of petrodiesel
reduces or eliminates the inherent lubricity of this fuel, which is essen-
tial for proper functioning of vital engine components such as fuel pumps
and injectors. Several studies [10, 11, 67–82] on the lubricity of biodiesel
or fatty compounds have shown a beneficial effect of these materials on
the lubricity of petrodiesel, particularly low-sulfur petrodiesel fuel.
Adding biodiesel at low levels (1–2%) restores the lubricity to low-sulfur
petroleum-derived DFs. However, the lubricity-enhancing effect of
biodiesel at low blend levels is mainly caused by minor components of
biodiesel such as free fatty acids and monoacylglycerols [83], which
have free COOH and OH groups. Other studies [84, 85] also point out
the beneficial effect of minor components on biodiesel lubricity, but these
studies do not fully agree on the responsible species [83–85]. Thus,
biodiesel is required at 1–2% levels in low-lubricity petrodiesel, in order
for the minor components to be effective lubricity enhancers [83]. At
higher blend levels, such as 5%, the esters are sufficiently effective
without the presence of minor components.
While the length of a fatty acid chain does not significantly affect
lubricity, unsaturation enhances lubricity slightly; thus an ester such
as methyl linoleate or methyl linolenate improves lubricity more than
methyl stearate [80, 83]. In accordance with the above observation on
the effect of free OH groups on lubricity, castor oil displayed better
lubricity than other vegetable oil esters [75, 80, 81]. Ethyl esters have
improved lubricity compared to methyl esters [75].
Standards for testing DF lubricity use the scuffing load ball-on-cylinder
lubricity evaluator (SLBOCLE) (ASTM D6078) or the high-frequency
reciprocating rig (HFRR) (ASTM D6079; ISO 12156). Lubricity has not
been included in biodiesel standards despite the definite advantage of
biodiesel over petrodiesel with respect to this fuel property. However, the
HFRR method has been included in the petrodiesel standards ASTM
D975 and EN 590.

5.7 Outlook
The fuel properties of biodiesel are strongly influenced by the proper-
ties of the individual fatty esters as well as those of some minor com-
ponents. Both moieties, the fatty acid and alcohol, have considerable
influence on fuel properties such as CN, with relation to combustion and
exhaust emissions, cold flow, oxidative stability, viscosity, and lubricity.
It therefore appears reasonable to enrich (a) certain fatty ester(s) with
desirable properties in the fuel, in order to improve the properties of the
whole fuel. For example, from the presently available data, it appears