Page 170 - Biofuels Refining and Performance

Fuel and Physical Properties of Biodiesel Components 153

intercooled diesel engine with exhaust gas recirculation (EGR), no chain
length effect has been observed for NO exhaust emissions, although
x
the level of saturation still played a significant role [19]. PM exhaust
emissions were reduced to levels close to the US 2007 regulations
required for ultra-low-sulfur petrodiesel fuel. Also, PM levels were lower
than those for neat hydrocarbons which would be enriched in “clean”
petrodiesel fuel [19]. In both studies [18, 19], NO emissions of the sat-
x
urated esters were slightly below those of the reference petrodiesel fuel.
For petrodiesel fuel, higher CNs have been correlated with reduced
exhaust emissions [20]. This correlation has led to efforts to improve
NO x
the CN of biodiesel fuels by using additives known as cetane improvers
[8]. Despite the inherent relatively high CNs of fatty compounds, NO x
exhaust emissions usually increase slightly when operating a diesel
engine on biodiesel, as mentioned above. The relationship between the
CN and engine emissions is complicated by many factors, including the
technology level of the engine. Older, lower-injection pressure engines
are generally very sensitive to CN, with increased CN causing signifi-
emissions, due to shorter ID times and the result-
cant reductions in NO x
ing lower average combustion temperatures. More modern engines that
are equipped with injection systems that control the rate of injection are
not very sensitive to CN [21–23].
Historically, the first CN tests were carried out on palm oil ethyl
esters [24, 25], which have a high CN, a result confirmed by later stud-
ies on many other vegetable oil-based DFs and individual fatty com-
pounds. The influence of the compound structure on CNs of fatty
compounds has been discussed in more recent literature [26], with the
predictions made in that paper being confirmed by practical cetane tests
[7–9, 13]. CNs of neat fatty compounds are given in Table 5.1. In sum-
mary, the results show that CNs decrease with increasing unsaturation
moi-
and increase with increasing chain length, i.e., uninterrupted CH 2
eties. However, branched esters derived from alcohols such as iso-
propanol have CNs competitive with methyl or other straight-chain
alkyl esters [9, 27]. Thus, one long, straight chain suffices to impart a
high CN, even if the other moiety is branched. Branched esters are of
interest because they exhibit improved low-temperature properties.
Recently, cetane studies on fatty compounds have been conducted
using the Ignition Quality Tester™ (IQT) [9]. The IQT is a further,
automated development of a constant volume combustion apparatus
(CVCA) [28, 29]. The CVCA was originally developed for determining
CNs more rapidly with greater experimental ease, better reproducibil-
ity, reduced use of fuel, and therefore less cost than the ASTM D613
method utilizing a cetane engine. The IQT method, which is the basis
of ASTM D6890, was shown to be reproducible and the results compet-
itive with those derived from ASTM D613. Some results from the IQT