Chemistry Reference
In-Depth Information
downstream of the SCR catalyst may be used to eliminate the possibility
of “ammonia slip”. The following equations describe the different reactions in
these systems:
CO (NH
2
Þ
2
!
NH
3
þ
ð
Þ
HNCO
thermolysis of urea
(5)
HNCO
þ
H
2
O
!
NH
3
þ
CO
2
ð
hydrolysis
Þ
(6)
NO
þ
NO
2
þ
2NH
3
!
2N
2
þ
3H
2
O
ð
reduction
Þ
(7)
Typical SCR systems may achieve on-road NO
x
conversion efficiencies of
60-70% [
18
]. However, the thermolysis of urea is an endothermic reaction that is
favoured at high temperatures; SCR is inefficient at temperatures below around
200
C[
19
]. Hence, NO
x
emissions from SCR-equipped vehicles can often increase
during urban driving where traffic conditions result in low exhaust temperatures
[
20
]. Supplementary systems, such as EGR, are required to maintain acceptable
emission-control performance under such conditions.
3.2.6 Lean NO
x
Trap
A lean NO
x
trap (LNT) (or NO
x
adsorber) is similar to a three-way catalyst.
However, part of the catalyst contains some sorbent components which can store
NO
x
. Unlike catalysts, which involve continuous conversion, a trap stores NO and
(primarily) NO
2
under lean exhaust conditions and releases and catalytically
reduces them to nitrogen under rich conditions. The shift from lean to rich combus-
tion, and vice versa, is achieved by a dedicated fuel control strategy. Typical
sorbents include barium and rare earth metals (e.g. yttrium). An LNT does not
require a separate reagent (urea) for NO
x
reduction and hence has an advantage
over SCR. However, the urea infrastructure has now developed in Europe and USA,
and SCR has become the system of choice for diesel vehicles because of its easier
control and better long-term performance compared with LNT. NO
x
adsorbers
have, however, found application in GDI engines where lower NO
x
-reduction
efficiencies are required, and the switch between the lean and rich modes for
regeneration is easier to achieve.
3.2.7 Diesel Particulate Filter
Diesel particle filters are a very efficient means of reducing PM mass emissions
from diesel vehicles, but do not directly target NO
x
. However, most diesel particu-
late filter (DPF) systems contain catalytic materials which assist in the DPF “regen-
eration” (the combustion of PM accumulated on the filter to clean the DPF and
prevent blockage). Such catalytic materials can have a similar impact to DOCs
