Chemistry Reference
In-Depth Information
14
12
EGR
SCR
10
8
6
4
2
0
0
20
40
60
80
100
Speed [km/h]
Fig. 5 NO
x
emission factors for Euro V trucks equipped with SCR and EGR systems. The
example refers to articulated trucks in the 34-40 t gross vehicle weight range. Source: COPERT
[
29
], original data derived from Hausberger et al. [
30
]
sulphur content have been tightened in a stepwise fashion, and in the latest step
Directive 2003/17/EC required a full transition to “sulphur-free” petrol and diesel
(having less than 10 ppm of sulphur) by 2009. This should enable advanced
technologies - such as lean-burn engines, particle traps and regenerative NO
x
storage systems - to meet stringent exhaust emission limits.
In an effort to tackle climate change, biofuels such as fatty-acid methyl esters
(FAME or biodiesel), paraffinic fuels from biomass-to-liquid procedures (BTL) or
hydrotreated vegetable oil (HVO) and bioethanol have also been introduced into the
energy mix, mostly in blends with conventional fuels. Current European regulations
(Directive 2009/30/EC) permit up to 10% vol. and 7% vol. bioethanol and biodiesel
mixing in fossil fuels, respectively. Higher bioethanol blends (up to 85%) require
dedicated vehicle technology (so-called “flexi-fuel” vehicles) due to the need to
adjust combustion parameters to the new oxygenated fuel and to use materials that
can withstand the corrosive character of ethanol.
The impacts of biofuels on NO
x
emissions are variable, and depend heavily on
vehicle technology, operational conditions, blending ratio and biodiesel feedstock.
On average, low biodiesel blends (up to 10%) do not affect NO
x
emissions, or lead
to a slight increase (less than 5%), but they do reduce PM emissions [
31
-
33
].
Effects at the individual vehicle level may be higher [
34
]. All these studies propose
several possible explanations for the biodiesel impact on emissions, including the
effects of higher density and viscosity, and lower compressibility and heating value,
on fuel metering. Biodiesel also contains more oxygenated molecules than fossil
diesel, which may also affect the combustion chemistry. It has been shown that
HVO fuels can lead to reductions in both NO
x
(~10%) and PM (~30%) due to their
100% paraffinic composition, and the absence of aromatics and other trace elements
[
35
]. However, due to the unique character of these fuels (high cetane number, low
density) engine recalibration may be needed to maximise their benefits [
36
].
