Environmental Engineering Reference
In-Depth Information
To avoid low oxidation stability because of the presence of acids in biofuel, most specifications
set the maximum limit at 0.50 mg KOH/g of FAME. However, the methods on which most stan-
dards are based, namely EN 14104* and the ASTM D664 , present several differences. As a result,
the comparison between the different limits is not feasible (White Paper 2007).
9.2.10 t otal and f rEE g lycErin and m ono -, d i -, and t riglycEridES
Glycerin, or else glycerol, is formed as a by-product in transesterification (van Walwijk 2005;
Nylund et al. 2008) during the production of biodiesel. The level of glycerin in the fuel, includ-
ing free and bound glycerin, is measured by the total glycerin method (Bacha et al. 2007; White
Paper 2007; Rilett and Gagnon 2008). This measurement can ensure the conversion of the fat
or oil to monoalkyl esters (Prankl et al. 2004; Singh 2005); that is, a low level of total glycerol
ensures that high conversion has taken place (Bacha et al. 2007; Rilett and Gagnon 2008). The
level of free glycerin alone in the fuel is estimated by the free glycerin method (Rilett and
Gagnon 2008) and can ensure the removal of glycerin from the fuel (Prankl et al. 2004; Singh
2005). Additionally, the level of glycerin bound in the form of mono-, di-, and triglycerides
is measured by the relative method. The contents of mono-, di-, and triglycerides indicate the
degree of conversion from oil to methyl esters or completion of the esterification/transesterifica-
tion process (Foon et al. 2005).
The content of glycerin in biodiesel is process dependent (White Paper 2007; Rilett and Gagnon
2008). High values of free glycerin may result from the insufficient separation or washing of the
ester product (Prankl et al. 2004; White Paper 2007), which causes the glycerin to separate during
storage once the solvent (methanol) has evaporated (White Paper 2007). Alternatively, glycerin may
also be formed because of hydrolysis of the remaining mono-, di-, and triglycerides in stored fuel
(Mittelbach 1996).
Glycerin is a clear, viscous, nontoxic, sweet-tasting liquid used in cosmetics, soaps, food produc-
tion, and pharmaceuticals. However, when it is mixed with fuel, it can harm the engine. Fuels out
of specifications for total glycerin with respect to these parameters are prone to coking and may
thus cause the formation of deposits on injector nozzles, pistons, and valves (Mittelbach et al. 1983;
Bacha et al. 2007; White Paper 2007).
High levels of free glycerin can cause problems during storage by settling to the bottom of
storage tanks or damage the fuel injection system by clogging fuel systems and injector depos-
its (Mittelbach 1996; Prankl et al. 2004; Bacha et al. 2007; Rilett and Gagnon 2008). Moreover,
high levels of mono-, di-, and triglycerides can cause injector deposits and may adversely affect
cold weather operation and filter plugging (Bacha et al. 2007). This is particularly true for high-
saturated monoglycerides (Rilett and Gagnon 2008). For these reasons free glycerin is limited in
the specifications.
A maximum content of 0.02% m/m of free glycerin is consistently defined in all of the standards
discussed. The European standard limits the amounts of mono-, di-, and triglycerides to 0.80, 0.20,
and 0.20% m/m, respectively, and defines a maximum amount of 0.25% m/m for total glycerin (EN
ISO 14105). The United States, Brazil, Australia, and India are the only countries with national
standards that do not have explicit limits for the contents of partial glycerides. However, they have
a limit for total glycerin, set at approximately 0.25% m/m , with the exception of Brazil, which is at
0.38% m/m (ABNT NBR 15344/EN 14105/ASTM D6584). Brazil is planning to reduce this limit,
but first a new method for the measurement of total glycerin has to be developed that will also be
suitable for castor oil (White Paper 2007).
* EN 14104.
ASTM D664.
Only the United States has this limit to 0.24% (m/m) for total glycerol. However, this difference is not important.
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