THE NAVAL ENGINEER

11
processes can be summarised as
follows [34]:
3 O 2
+ e – → 2 O 3
+ e – (7)
NO + O 3
→ NO 2
+ O 2
(8)
HC + O 3
→ OHC + O 2
(9)
NOx + OHC + catalyst →
N 2
+ H 2
O + CO 2
+ catalyst
(10)
Complex reactions of OHCs with
exhaust gases also result in the
formation of N 2
O and NH 3
. N 2
O is
a greenhouse gas with a 100-year
global warming potential 298 times
greater than CO 2
. [46] NH 3
is a
human irritant and is dangerous to
the environment [47].
An alternative approach is to use
a two-stage HC-SCR and NH 3
-
SCR. The first stage is optimised
to produce NH 3
, which is then
used with a conventional platinum
catalyst (as used with urea-SCR)
to further reduce NO x
[48]. This has
achieved promising results using
an exhaust stream NO:NO 2
ratio of
1:1; because real diesel exhaust
streams contain much more NO,
it is likely that an NTP device
would be needed upstream of the
two-stage catalyst to achieve a
satisfactory system performance.
Additional fuel consumption
here is less predictable than the
reductant consumption of urea-
SCR systems. To avoid a reduction
in the ship’s range, the additional
fuel requirement could be between
half (hypothesised potential) and
twice (for a current test system) the
urea tank volume for a urea-SCR
system.
Where an external fuel reformer
or NTP device is used the system
(excluding reductant) will have a
greater volume than urea-SCR.
Volumetric gas flow to catalyst
volume ratios of between 1.8 and
8.3 s-1 for 182 to 200 ppm NO x
feeds have been quoted, the high
end of which is similar to urea-SCR
[34] [48]. These fuel reformers
operate with diesel fuel at between
140°C and 160°C and at these
temperatures the fuel vapour will
ignite readily [34], posing significant
safety challenges for RN ships.
Noble metal catalysts are not
required so the catalyst matrix in
the exhaust stream is likely to be
less expensive than with urea-
SCR [34]. Where NTP is not used,
higher temperatures of 375°C to
425°C are required for adequate
conversion rates [48]. In this case,
it may be necessary to heat the
exhaust gases to facilitate efficient
operation. This will system increase
size, power consumption and IR
signature when the system is in
use.
Practical marine HC-SCR
systems are at an early stage
of development and constitute
a high level of technical risk.
The moderate system size and
complexity is likely to result in a
moderate acquisition cost and
the low to moderate reductant
(diesel) consumption penalty and
possible requirement for catalyst
replacements will lead to moderate
operational costs.
Dissolution in a Liquid Medium
(Figure 6)
Scrubbers are a common method of
removing SO x
from diesel exhausts.
These work by dissolving sulphur
oxides in seawater [10]. NO has
a low solubility in seawater so
this approach is largely ineffective
at reducing NO x
concentrations.
However, NO 2
readily hydrolyses in
water to give nitrous
and nitric acids [44]:
2 NO 2
+ H 2
O →
HNO 2
+ HNO 3
(11)
By first converting
NO to NO 2
, NO x
can
be removed from the
exhaust stream. The
conversion is most
effectively conducted
using NTP and this
overall process is
termed ‘Wet NTP’ [11].
The process of
NO 2
hydrolysis
is accelerated by
the presence of a
base such as NH 3
or sodium sulphite
in solution, which
prevents saturation
Seawater in
Discharge
overboard or
treatment and
recirculation
Diesel Engine
of the removal liquid by nitrate
(NO 3–
) and nitrite (NO 2–
) ions [44].
This requires a significant volume
of base to be carried on board.
Much of the recent research has
focussed on the use of these,
especially where a combined NTP
and dissolution device is used [38]
[39] [44].
For ships it is more practical to
avoid acid saturation of the removal
liquid by increasing the volume
flow rate of sea water. This method
has been shown to be effective:
Ecospec [28] market a scrubber
that removes around 66% of NO x
whilst Balon and Clark [49] project
that 90% NO x
removal is feasible
by scaling up their system. Both of
these used a two-stage oxidation
and dissolution process.
These scrubber systems also
remove SO x
, PM, HCs and some
CO 2
from the exhaust. All of these
processes result in significant
acidification of the operating
liquid, and pumping this directly
overboard poses significant local
marine pollution and global ocean
acidification problems. Storing the
liquid on board is impractical for a
densely packed warship and the
usual approach is to neutralise it
with an alkaline injector or dilute it
with additional seawater until it is
has a pH greater than 6.5 [15].
Strainer
Filtration
plant
Scrubbing
tower
Strainer
Contaminants to
sludge tank
Oxidation
catalyst or NTP
device
20:1 1:20
Exhaust
Seawater
sprays
Exhaust gas NO:NO 2
Ratio
Figure 6: Dissolution in a Liquid Medium
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