Environmental Engineering Reference
In-Depth Information
Paper 2007). It should be noted that the national standards for biodiesel often reflect the national
regulatory requirements for maximum sulfur content in fossil diesel (White Paper 2007).
9.2.5 c
oppEr
S
trip
c
orroSion
The copper strip corrosion parameter of a biodiesel sample indicates its tendency to cause corrosion
to copper, zinc, and bronze parts of the engine, the storage tank (Prankl et al. 2004; White Paper
2007), and the fuel system (Rilett and Gagnon 2008). The presence of acids or sulfur-containing
compounds can accelerate the aging process (McGill et al. 2008) and tarnish the copper strip (Foon
et al. 2005; Rilett and Gagnon 2008). From this oxidation, sediments will be created (WWFC 2006;
DEWHA 2008; McGill et al. 2008) that may plug fuel filters (WWFC 2006). Additionally, even
small quantities of copper can lead to significant injector fouling and as a result cause power loss of
the engine and increase exhaust gas PM (WWFC 2006). Thus, at the transitioning from diesel fuel
to biodiesel blends, fuel system parts must be specially chosen for their compatibility with biodiesel
properties (WWFC 2006; DEWHA 2008).
More specifically, copper, as a dissolved metal, contributes as an oxidation catalyst to the conver-
sion of precursors to species of higher molecular weight, more often nitrogen- and sulfur-containing
compounds, organic acids, and reactive olefins (Bacha et al. 2007). The sulfur and acid compounds
of biodiesel accelerate this oxidation. As a result, the copper strip corrosion parameter can be used
as an indicator of the acid number of the biodiesel (White Paper 2007). The use of metal deactiva-
tors can tie up (chelate) the copper and neutralize its catalytic effect (Bacha et al. 2007).
The effects noted during copper strip corrosion demonstrate the need for an adequate limit with
regard to corrosion (Foon et al. 2005; Rilett and Gagnon 2008). However, the current limits of this
parameter [i.e., Class 1 for EU (EN ISO 2160) and Class 3 for the United States (ASTM D130)] are
under discussion because the results are unlikely to give a rating higher than Class 1. Hence, a steel
strip corrosion test would be more realistic for the present fuel systems (White Paper 2007).
9.2.6 c
EtanE
n
umBEr
The cetane number measures the ignition quality of the fuel (WWFC 2006; Bacha et al. 2007;
Rilett and Gagnon 2008; Crown and Warfield 2009). It is defined as the volume percent of
n
-hexa-
decane in a blend of
n
-hexadecane and isocetane that gives the same ignition delay period as the
test sample; a high value represents fuels that readily ignite (WWFC 2006). From this definition,
it is demonstrated that the cetane number depends on engine design, size, nature of speed and load
variations, and on starting and atmospheric conditions (Rilett and Gagnon 2008).
Moreover, the cetane number of a biodiesel sample depends on the molecular properties of the
fatty acid esters of the fuel (Mittelbach 1996; Wörgetter et al. 1998). The cetane number increases
with the length of both the fatty acid chain and the ester group and decreases with the number of
double bonds (Wörgetter et al. 1998; White Paper 2007); that is, saturated fatty acids such as those
delivered from animal fats and used vegetable oils have a higher cetane number than unsaturated
acids (Singh 2005; McGill et al. 2008; Table 9.4).
In general, blends of biodiesel score a better cetane number than their base diesels* (Prankl
et al. 2004; White Paper 2007; Rilett and Gagnon 2008). At the same time, the ethyl esters have a
cetane number slightly higher than methyl esters (Knothe et al. 2003). A high cetane number indi-
cates a short ignition delay period, and as a result good cold-start behavior (reduces white smoke on
startup) and smooth combustion [reduces oxides of nitrogen (NO
x
) and PM emissions as well as fuel
consumption] (WWFC 2006; Bacha et al. 2007; Rilett and Gagnon 2008). In contrast, a low cetane
number in a fuel indicates an incomplete combustion, which results in engine knock and increased
*
Most of the biodiesel in the United States has cetane numbers higher than 47, compared with a low of 40 for highway
diesel fuel (average for United States is 42-44; in Europe this is 51 min) (McGill et al. 2008).
