Environmental Engineering Reference
In-Depth Information
9.2.13 o
xidation
S
taBility
The oxidation stability value of biodiesel measures the formation degree of undesirable breakdown
products (DEWHA 2008). Low oxidation stability (i.e., high concentration of acids and polymers
in biodiesel) affects the performance and durability of fuel system components (DEWHA 2008)
because it can cause fuel system deposits and lead to filter clogging and fuel system malfunctions
(Rilett and Gagnon 2008). However, these effects can be avoided with the use of additives (White
Paper 2007; Rilett and Gagnon 2008), either antioxidants or stabilizers (Bacha et al. 2007).
In general, biodiesels are more sensitive to oxidative degradation than diesel because of their
chemical composition (FAMEs) (Prankl et al. 2004). This oxidation sensitivity increases with the
length and the saturation of the fatty acids that constitute biodiesel; that is, a fuel composed primar-
ily of C18:3 is 100 times more unstable than a fuel made of C18:1 (McGill et al. 2008). Fuels highly
saturated are less chemically reactive with oxygen, so they have high oxidation stability (Rilett
and Gagnon 2008). Alternatively, the unsaturated esters of biodiesel are easily oxidized and form
insoluble sediments and gums (Prankl et al. 2004; White Paper 2007). These residues are associ-
ated with fuel filter plugging and deposit within the injection system and the combustion chamber
(Mittelbach and Gangl 2001).
The content of natural antioxidants, such as tocopherols and carotenes (Simkovsky 1997), in
biodiesel can improve the oxidation stability of the fuel (Prankl et al. 2004; McGill et al. 2008).
Although these antioxidants are removed during distillation (Prankl et al. 2004; Rilett and Gagnon
2008) or destroyed in the case of used frying oil, the use of synthetic antioxidants is equally efficient
(Mittelbach and Schober 2003). However, they have to be added in high concentrations to compen-
sate for those destroyed during fuel storage (Mittelbach and Schober 2003).
For the protection of biodiesel blends and engine and fuel injection equipment from the del-
eterious effects of poor oxidation stability, biodiesel standardizations have set a specific limit on
oxidation stability for B100 at 6 h (Rilett and Gagnon 2008) measured by the EN 14112 method.
Only the United States is using a 3-h limit; however, this is for the B20 blend (White Paper 2007),
whereas the EU is for B100 fuel. Another concern is where the standards must be met: At the
point of production or at the point of delivery? Consideration must be given that the oxidation
stability degrades with time (White Paper 2007). However, the induction period of oxidation sta-
bility is well correlated with other biodiesel quality parameters, such as kinematic viscosity, ester
content, acid value, and polymer content (Lacoste and Lagardere 2003; Soriano 2008). Hence, if
a biodiesel has low oxidation stability, then it will score out of the specification range and also
these parameters.
9.2.14 a
lkali
and
a
lkalinE
-E
arth
m
EtalS
Sodium and potassium (alkali metals) are common catalysts for the biodiesel reaction (Prankl et al.
2004; White Paper 2007; Rilett and Gagnon 2008) and are removed during processing; otherwise
sodium/potassium can form soap and cause adverse effects on new particulate trap technologies
being considered for future diesel engines (Rilett and Gagnon 2008).
Calcium and magnesium (alkaline-earth metals) may originate from the use of soft water in the
washing step of biodiesel production (Prankl et al. 2004; White Paper 2007; Rilett and Gagnon
2008). These can be detected in biodiesel as abrasive solids or soluble metallic soaps. If these com-
pounds are not removed, abrasive solids can wear the engine by damaging the injector, fuel pump,
piston, and ring and thereby contribute to engine deposits. From their side, soluble metallic soaps
can contribute to filter plugging and engine deposits. Moreover, a high calcium and magnesium
value may damage the vehicle exhaust system because these compounds are collected in exhaust
particulate removal devices and they are not removed during regeneration. In this way, the back-
pressure increases, thus reducing the time between service maintenance (Mittelbach 1996; Rilett
and Gagnon 2008).
