Environmental Engineering Reference
In-Depth Information
taBle 9.2
typical Properties of Biodiesel and ulsd
Property
Biodiesel
ulsd
130
60
Flash point, °C
Cetane number
55
44
Sulfur, ppm
<15
15
Relative density, 15°C
0.88
0.85
6.0
2.6
Kinematic viscosity at 40°C, mm
2
/s
Heating value, net, Btu/gal (kJ/kg)
128,000 (40,600)
130,000 (42,700)
Source:
Bacha, J., Freel, J., and Gibbs, A.,
Diesel Fuels Technical Review,
Chevron Corporation,
San Ramon, CA, 2007.
as an indicator of the efficiency of that process for residual methanol removal (Foon et al. 2005;
Rilett and Gagnon 2008).
To ensure that the manufacturers remove the excess methanol used in production, the European
Union (EU) sets a low limit for the flash point of biodiesel—120°C (EN 14214), whereas the United
States (ASTM D6571)
sets the same limit at 130°C* (Prankl et al. 2004; Foon et al. 2005; White
Paper 2007). At the same time, the Brazilian specification (NBR 14598
†
) defines a flash point mini-
mum of 100°C, which correlates with 0.2% v/v alcohol, because this limit value is compatible with
the NFPA nonhazardous category code (White Paper 2007). Although the limits of the EU and
United States were set at these values because of the high uncertainty of their test methods (EN
14110 and ASTM D93, respectively), the development of improved or alternative test methods will
allow them soon to reduce their flash point limit to 100°C (Bacha et al. 2007; White Paper 2007).
9.2.2 v
iScoSity
The injection of diesel or biodiesel fuel into a compression-ignition engine is controlled by positive
displacement (volumetrically) or by a solenoid valve. The viscosity is one of the fuel parameters that
determine these injection systems. A high or a low viscosity can influence the engine performance
and durability, as well as increase exhaust emissions (Prankl et al. 2004; WWFC 2006; McGill
et al. 2008). On one side, low viscosity can lead to loss of power and fuel economy (Bacha et al.
2007) because of injection pump and injector leakage (Rilett and Gagnon 2008); on the other side,
high viscosity is limited by the characteristics of the injection system (Rilett and Gagnon 2008) and
reduces fuel flow rates, resulting in inadequate fueling (WWFC 2006; White Paper 2007).
The viscosity of biodiesel is higher than that of fossil diesel (Prankl et al. 2004; see Table 9.2).
At the same time, the viscosity of biodiesel is much lower than that of the pure oil it is derived from
(Zhang et al. 2003; WWFC 2006). The viscosity is related more to molecular weight than to hydro-
carbon class [i.e., for a given carbon number, naphthenes generally have slightly higher viscosities
than paraffins or aromatics (Bacha et al. 2007)]. The high content of high molecular compounds
such as nonreacted glycerides and polymers can increase the viscosity, which is the case when
used frying oil is used as feedstock (Mittelbach 1996; Wörgetter et al. 1998; Prankl et al. 2004),
or palm oil for biodiesel (Bacha et al. 2007). In contrast, free methylesters (in polyolesters) can
reduce the viscosity of the fuel (Singh 2005), e.g., a high amount of mid-chain fatty acids in coconut
resulted in lower viscosity (Bacha et al. 2007). Additionally, the viscosity of biodiesel can be used
*
Only if methanol is not measured directly; otherwise, 93°C.
†
Based on ASTM D93, but considers ASTM D93 and EN ISO 3679.
