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Proceedings of the International Symposium on Marine Engineering
(ISME) October 17-21, 2011, Kobe, Japan Summary or Paper-ISME586
Tier III EGR FOR LARGE 2-STROKE MAN B&W DIESEL ENGINES
Johan Kaltoft
MAN Diesel & Turbo, Teglholmsgade 41, 2450 Copenhagen SV, Denmark
Abstract The IMO Tier III NOx regulations that will come into force in 2016 means that NOx emissions from
large two-stroke diesel engines must not exceed a cycle value of 3.4 g/kWh, and NOx emission must not exceed
5.1 g/kWh at individual load points of the load cycle. To comply with the Tier III requirements, MAN Diesel &
Turbo (MDT) is involved in targeting development of Exhaust Gas Recirculation (EGR). This paper describes
the EGR principle, the investigation of EGR on two-stroke diesel engines as well as service test experience and
test results. Test on MAN Diesel & Turbo´s two-stroke diesel engine in Copenhagen has proved that EGR is a
compliant IMO Tier III NOx technology and service tests are currently ongoing in order to investigate long term
influence on engine components and the EGR system.
Keywords: IMO Tier III, Exhaust Gas Recirculation (EGR), MAN B&W engines.
1. INTRODUCTION
The worlds marine engine manufacturers have since the
ratification of the IMO Tier III criteria for NOx emission in
Emission Controlled Areas (ECA´s) from large marine
diesel engines, been challenged to develop new measures in
order to reduce NOx. The extend of the necessary measures
for NOx reduction up to 80% for meeting the IMO NOx
criteria from January the 1st 2016, is beyond well known
adjustments of the combustion process in two-stroke diesel
engines. NOx reduction in his magnitude on two-stroke
diesel engines, requires “add-on” technologies like Exhaust
Gas Recirculation (EGR) or Selective Catalytic Reduction
(SCR) as described in the ISME paper number 587 (2011).
Back in 2004, MAN Diesel & Turbo started the first test
program with EGR on the large 4T50ME-X two-stroke
diesel test engine in Copenhagen, in order to verify the
effect of EGR. The effect of EGR on smaller four-stroke
diesel engines used in the automotive sector has been
known since the 1970’ies as a very efficient means to
reduce NOx in combustion engines. The HFO burned in
large marine engines is a challenge when using EGR, due to
the presence of high sulphur content and high content of
solids thus a wet scrubber was introduced in the EGR
system.
In parallel with the EGR investigation on the 4T50ME-
X test engine, MAN Diesel & Turbo planned to make a
service test on a ship in order to investigate long term
effects on the engine components. In March 2010, a retrofit
EGR system was installed on a 10MW 7S50MC Mk-6
engine onboard A.P. Moeller Maersk 1100 TEU container
vessel Alexander Maersk.
The recent EGR investigation and service test is a part of
the large European development project named
HERCULES-B with focus on high engine efficiency and
low emissions.
This paper describes the investigation and testing which
MAN Diesel and Turbo have completed with EGR on large
two-stroke diesel engines.
2. EXHAUST GAS RECIRCULATION
The principle of EGR is based on exchange of the in-
cylinder oxygen (O2) with carbon dioxide (CO2) from the
exhaust gas which is re-circulated into the scavenge air. The
exchange of O2 with CO2 leads to a decrease of combustion
speed, resulting in lower peak temperatures during
combustion. Besides the exchange of O2 with CO2 results in
a higher in-cylinder heat capacity of the gas, which also
lowers the combustion temperature. Lower combustion
temperatures and especially lower peak temperatures result
in lower formation of thermal NOx during the combustion
process.
There are different ways to utilise EGR on a two-stroke
diesel engine:
A) Internal EGR (primary methods)
- Poor scavenging of the combustion chamber.
- Internal trapping of combustion gas in the
combustion chamber.
B) External EGR (secondary methods)
- Low pressure EGR on the exhaust side of the
turbocharger, downstream the turbine.
- High pressure EGR on the engine side of the
turbocharger, upstream the turbine.
3. THE EGR SYSTEM FOR A MAN B&W ENGINE
The reasons why the high pressure EGR in MAN Diesel
& Turbo´s large two-stroke diesel engines is chosen as the
preferred EGR solution are:
- The system is compact compared to the low
pressure EGR system and is an “on engine”
system.
- The turbo compressor is not exposed to additional
sulphur, particles and water droplets than
conventional engines. - Only the rugged engine components from the
scavenge air receiver to the exhaust gas funnel is
affected. The components from air intake to the
scavenge air receiver, i.e. the main engine cooler,
is not exposed to additional sulphur, particles and
water droplets than in conventional engines.
Figure 1: Basic EGR system layout diagram
The MAN Diesel & Turbo EGR system basically
consists of three flow loops:
- EGR gas loop.
- Scrubber water loop.
- Water cleaning loop.
As seen from Figure 1, the EGR system comprises the
following main components:
EGR blower for creating a flow from the exhaust
receiver to the scavenge air receiver of up to around 40% of
the total exhaust gas amount. The pressure difference
between the receivers is around 0.3 bar at 100% engine load
and the EGR blower has to overcome this pressure
difference as well as the pressure loss through the scrubber,
cooler, pipes etc., which is approx. 0,2-0,3 bar. The EGR
blower speed is controlled by a frequency converter control
of the blower motor.
Pre scrubber for removal of SO2 and for energy
conversion by humidification, in order to precondition the
exhaust gas before the gas enters the EGR scrubber. The
exhaust gas is washed with re-circulated fresh water with
addition of sodium hydroxide (NaOH). NaOH neutralizes
the sulphuric acid that is formed in the scrubber water.
Around 95% of the scrubber is continuously re-circulated.
The temperature of the exhaust gas entering the Pre
scrubber can vary from 200-500ºC over the engine load
range.
Scrubber for removal of particles and residual SO2 in
the exhaust gas before it is introduced into the scavenge air
receiver and further into the combustion chamber. The gas
is washed with re-circulated scrubber water supplied from
the same pipe that feeds the Pre scrubber. The temperature
of the EGR gas entering the EGR Scrubber varies from 50-
100°C over the load range.
Drainers for separation of the scrubber water and the
EGR gas before collecting the scrubber water in a buffer
tank. The drainer’s ensure that only scrubber water is
discharged from the scrubber to the buffer tank without
leakage of EGR gas.
EGR cooler for cooling of the EGR gas and conversion
of the enthalpy in the EGR gas to the cooling water. The
water evaporated in the pre scrubber is condensed in the
EGR cooler. The EGR cooler cools the scrubbed gas down
to approx. scavenge air temperature between 35-40ºC.
Shut Down valve (S/D valve) for switching on and off
the EGR system. The valve is gas tight and ensures no flow
of scavenge air into the exhaust system, while the EGR
system is not operated. The valve is “on/off” controlled.
Change over valve (C/O Valve) for control of the EGR
gas amount in cooperation with the EGR blower speed. The
valve is operated as a throttle valve with variable
positioning.
Oxygen sensor for measurement of the oxygen content
in the scavenge air receiver. The EGR gas amount is
controlled by the oxygen content in the scavenge air
receiver, which varies from 16-21 % v/v.
Water Treatment System (WTS) for handling of the
scrubber water in the system. The system controls the water
supply to the scrubber (quality and amount) dependant on
engine load. The quality (pH, turbidity and poly aromatic
hydrocarbons) of the scrubber water, for discharge to the
sea, is also controlled by the WTS. The quality of the
discharge water to the sea is hereby ensured to be within the
IMO criteria’s for scrubber water discharge criteria’s. Alfa
Laval and MAN Diesel & Turbo are via continuous
cooperation developing a WTS solution on a unit base, for
installation in an adjacent room close to the engine.
Buffer tank is a part of the WTS.
NaOH tank for bunkering and storage of the NaOH.
The size of the NaOH tank is depending on the engine size,
sailing patterns, bunkering facilities and NaOH
concentration. A guiding size for a container vessel is
approx. 2.0 m3/MW (installed engine power).
Sludge tank for collection of sludge from the WTS
system. The Sludge tank size is depending on engine size,
sailing patterns and disposal facilities. A guiding size for a
container vessel is approx. 1.5 m3/MW (installed engine
power).
EGR control system for starting and stopping the EGR
system and for controlling of the amount of EGR gas
reintroduced to the scavenge air receiver, depended on
engine load and engine mode. The safety of the EGR
system is covered by the EGR control system via
appropriate alarms and an EGR shut down sequence if
necessary. Input is O2 concentration in the scavenge air and
controlled components are the S/D valve, C/O valve, EGR
Blower and the WTS system.
The design of the EGR system is currently being
matured and the target for MAN Diesel and Turbo is to
integrate the engine related EGR components on the engine
to an extent that is viable in order to keep the engine
installation as simple as possible and with a minimum need
for additional space around the engine.
The system components described above is the current
system design for engines with only one turbo charger. For
engines with more than one turbo charger, other system
layouts will be relevant utilizing turbocharger cut valves for
compensating the reduced exhaust gas during EGR
operation, see Figure 2 below.
Figure 2: Design of 6S80ME-C9 engine with EGR and two
turbochargers.
4. INVESTIGATION OF EGR ON 4T50ME-X TEST
ENGINE
4.1 Objective of the EGR test programme
During 2009 and 2010 EGR was thoroughly
investigated on MAN Diesel & Turbo’s 7MW 4T50ME-X
test engine in Copenhagen.
The objective of the test programme was to examine
how IMO Tier III NOx compliance could be achieved by
using high pressure EGR on a large two-stroke diesel
engine. The investigation covered influence of variations on
different engine parameters; maximum pressure (Pmax),
compression pressure (Pcomp), scavenge air pressure
(Pscav), scavenge air temperature (Tscav), hydraulic
injection pressure (Phyd) etc. The effect on NOx, Specific
Fuel Oil Consumption (SFOC), Particulate Mass (PM),
carbon monoxide (CO) and hydro-carbons (HC) were
studied.
Moreover, both IMO Tier III (ECA) operation and Tier
II (Non ECA) operation and the switch between the two
modes were tested as well as different control strategies.
Also the EGR scrubber was tested during the test
programme in order to verify the performance with regard
to particle trapping and SO2 removal.
4.2 Test results from EGR test on 4T50ME-X engine
The study of engine parameter variations during EGR
operation revealed the following effects on SFOC and
emissions as also seen from Table 1:
- Increased Pcomp/Pscav ratio has a positive impact
on the SFOC penalty.
- Increased Phyd has a positive impact on the SFOC
penalty.
- Increased Pscav has a positive impact on the
SFOC penalty.
- Increased Tscav has a negative impact on the
SFOC penalty.
- Increased Phyd has a positive impact on CO and
hereby also visible smoke.
Table 1: Test results from engine parameter variations at
75% engine load (auxiliary power for EGR blower,
separator and pumps is not included in dSFOC).
NOx
(g/kWh)
dSFOC
(g/kWh)
CO
(g/kWh)
Pmax
(bara)
EGR rate
(%)
O2
(vol. %)
No EGR 17.8 0 0.65 152 0 - Max.
EGR 2.3 +4.9 4.17 151 39 16.0
EGR ref. 3.7 +3.0 2.57 151 36 16.8 Incr.
Pcomp/
Pscav
4.0 +2.5 2.18 156 36 16.8
Incr. Phyd 4.2 +2.8 1.83 151 37 16.6 Incr.
Pscav 3.6 +1.9 2.12 156 37 16.6
Incr.
Tscav 3.9 +3.6 2.82 156 34 16.8
Tier III
setup 3.4 +0.6 1.34 157 41 16.2
The reduced energy to the turbine side of the turbo
charger, up to around 40%, when operating the EGR
system, results in reduced scavenge air pressure and hereby
negative effects on the SFOC. This highlights the need for
compensating means. Both variable turbine geometry and
cylinder bypass has been tested and seems to be able to
compensate the decrease in scavenge air pressure. Figure 3
shows the two very different operating areas for the
compressor running with and without EGR, corresponding
to utilisation of a turbocharger cut out solution.
Figure 3: Turbocharger compressor maps running the
engine with and without EGR.
As seen from Figure 4, the heat release is only slightly
affected by EGR. Increased hydraulic injection pressure can
compensate for reduced heat release in the early part of the
combustion.
Figure 4: Heat release running with and without EGR.
The scrubber performance was also measured during the
EGR test programme and showed a particle trapping
efficiency of around 70% according to ISO 8178 standard
for PM measurements. The SO2 removal efficiency could
more or less be controlled by the amount of added NaOH in
the scrubber water.
The investigation on the 4T50ME-X test engine has
showed that IMO Tier III NOx compliance is achievable by
use of High Pressure EGR solely. A cycle value below
3.4g/kWh of NOx was obtained and also the Not To Exceed
(NTE) level of 5.1 g/kWh of NOx at each engine load point
25, 50, 75 and 100% were proved during the test, see
Figure 5 below.
Figure 5: NOx emission at different engine loads as a
function of oxygen content in the scavenging air.
5. EGR service test
5.1 Objectives of the EGR service test
The main objective of the service test, which still is
ongoing, is mainly to investigate the long term impact on
the engine during EGR operation.
The more detailed objectives is outlined below:
- Investigate impact of EGR operation on engine
components: cylinder liner, piston, piston rings,
piston rod, cylinder cover, exhaust valve etc, when
burning HFO with high content of sulphur and
solids.
- Reduce NOx with 50% during the test.
- Investigate impact on the EGR components.
- Hand over operation of the EGR system to the ship
crew in order to get feedback on operation of the
system, in order to adjust the system for easy,
reliable and safe operation.
5.2 Preparation for EGR service test
During the summer 2008, A. P. Moeller Maersk and
MAN Diesel and Turbo agreed on testing a High Pressure
EGR system on one of A. P. Moeller Maersk’s smaller
container vessels. The vessel pointed out was Alexander
Maersk a 1092 TEU container feeder operating in the
Mediterranean. Alexander Maersk is installed with a 10,126
kW, 127 rpm 7S50MC Mk 6 engine produced by Hitachi
Zosen Cooperation.
A retrofit EGR system was designed by MAN Diesel
and Turbo from August 2008 to March 2009. Components
were manufactured and the main EGR components were
installed in August 2009 at Lisnave shipyard in Lissabon in
Portugal.
Figure 6: Arrangement of the EGR system on Alexander
Maersk.
It was necessary to exchange the two existing turbo
chargers on the 7S50MC engine with one new larger
turbocharger with variable turbine geometry in order to
compensate for the reduced exhaust gas amount in EGR
running mode. An ABB A175L VTG turbocharger was
chosen for this purpose.
The EGR scrubber, EGR blower and the EGR cooler is
integrated in one unit mounted on the fore end of the engine
as seen from Figure 6 and 7.
On Alexander Maersk, the EGR gas is introduced before
the main engine coolers in order to ensure good mixing of
the EGR gas and the compressed air from the turbo
compressor. Besides, it was of interest to investigate how
the main engine coolers were affected by the sulphur and
particles in the EGR gas.
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
24.0
15.0 15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5 20.0 20.5 21.0 21.5 22.0
Sp
eci
fic
NO
x (
g/k
Wh
)
Oxygen conc. in Scav.Rec (wet, vol %)
100% load
75% load
50% load
25% load
NTE NOx
IMO Tier 3 cycle value
Figure 7: EGR unit on board Alexander Maersk
Below in Table 2, the capacities and material of some
selected specified EGR components for Alexander Maersk
are listed:
Table 2: List of capacities and materials for selected
components specified for Alexander Maersk.
Component
s
Capacity Material
EGR Blower Flow: 4.5 kg/s
dp: 0.6 bar
Power: 240
kW
Wheel: Corten steel.
Housing: coated mild
steel.
EGR
Scrubber
Gas flow:
4.5 kg/s
Stainless steel 316L.
EGR Cooler Heat transfer:
2300 kW
Copper tubes and nano
coated copper fins.
Housing: coated mild
steel.
NaOH tank 4 m3
Stainless steel 316L
NaOH pump Flow: 60 l/min Teflon, 316L
Sludge tank Approx. 20 m3 Coated mild steel
WTS
separator
30 m3/h Rotating parts is stainless
steel and housing is
coated mild steel.
In order to ensure an efficient WTS solution for the EGR
system, different separators from Alfa Laval have been
tested at the 4T50ME-X test engine.
Figure 8: Left - Alfa Laval testing a separator at 4T50ME-X
test engine. Right – scrubber water samples before and after
test of discharge scrubber water.
5.3 EGR service test results
Currently the EGR system onboard Alexander Maersk
has been in operation close to 500 hours with the engine
running on HFO with 3% sulphur. The EGR system is
currently operated by the ship crew. The EGR system is a
push bottom system controlled from the engine control
room, except for the separator in the WTS system, which
has to be started up on-site by the crew.
Figure 9: Measurements of NOx reduction on board
Alexander Maersk during a performance test.
The thermo dynamical performance of the EGR
components was successfully tested and the EGR
components fulfilled the expected performance.
Commissioning of the EGR system in automatic mode was
also successfully completed.
Until now, the combustion chamber components and the
exhaust gas path are not negatively affected by EGR
operation. Figure 10 shows the piston rings before and after
approx. 300 running hours in EGR operation.
Figure 10: Piston rings before and after approx. 300
running hours in EGR operation on HFO.
The service test, which still is ongoing, has been quite
challenging due to HFO operation with high sulphur and
solids content. The challenges have mainly been related to
the following issues:
- Corrosion of non stainless components, e.g. due to
insufficient coatings. Heavy corrosion has been
experienced on the EGR cooler housing, EGR
cooler element, EGR blower wheel, drainers, EGR
pipe and separator in the WTS system.
- Difficulties with controlling the dosing of the
correct amount of NaOH.
- Water carry over from the scrubber system,
resulting in heavy deposits in the EGR system.
Figure 10: Left – deposits of sodium sulphate, iron sulphate
and soot on main engine cooler top, caused by water carry
over from the scrubber system. Right – almost no deposits
when water carry over from EGR scrubber system is
avoided.
In order to deal with corrosion challenges, the following
components have been exchanged with stainless steel: EGR
blower wheel, drainers and some valves in the WTS
system. The EGR cooler element will be exchanged with a
stainless steel element. In addition, a comprehensive repair
of the EGR cooler housing and the EGR pipe from the
blower to the connection on the charge air pipe have been
completed due to insufficient coatings.
The service test has gained a lot of important learning
and information on what the challenges are when running
EGR on a HFO burning two-stroke marine diesel engine.
Corrosion of EGR components and deposits in the EGR
system is important to target. Until this state of the service
test the engine components is not affected by high pressure
EGR operation.
8. CONCLUSIONS
The comprehensive investigation in High Pressure EGR
on large two-stroke marine diesel engines carried out at
MAN Diesel & Turbo’s test centre in Copenhagen have
proven that the IMO Tier III NOx emission limits coming in
to force by January 1st 2016 are possible to meet with
EGR. A cycle value below 3.4g/kWh of NOx was obtained
and also the NTE level of 5.1 g/kWh of NOx at each engine
load point 25, 50, 75 and 100%, were proved during tests.
The tests have also revealed which parameters to adjust in
order to obtain the optimal trade off between SFOC, NOx
CO and HC.
The EGR service test on Alexander Maersk has until
now, and will be in the future, an important test platform for
knowledge on how the engine and the EGR components are
affected by EGR and for identifying by which means
reliable and safe operation can be ensured. The test has
after approx. 500 running hours with EGR on HFO with 3%
sulphur showed no negative impact on vital engine
components. The EGR system is operated in fully
automated mode by the ship crew. The challenges have so
far been related to corrosion of EGR components, deposits
in the EGR system and deposits on the main engine coolers.
Furthermore, water carry over from the EGR scrubber
system, NaOH dosing and scrubber water quality control
are equally important parameters to control.
Regarding compliance on the scrubber water discharge,
the development in separator designs for this purpose, have
successfully shown that the discharge criteria’s can be met.
At current state, the overall conclusion is that High
Pressure EGR on large two-stroke marine diesel engines
burning HFO is a very promising measure for IMO Tier III
NOx compliance, although there still is a need for further
investigation and testing of the technology.
NOMENCLATURE
EGR : Exhaust Gas Recirculation
ECA : Emission Controlled Area
SCR : Selective Catalytic Reduction
IMO : International Maritime Organisation
HFO : Heavy Fuel Oil
NTE : Not To Exceed
SFOC : Specific fuel oil consumption [g/kWh]
TEU : Twenty-foot equivalent units
WTS : Water Treatment System
S/D : Shut Down
C/O : Change Over
WMC : Water Mist Catcher
Pscav : Scavenge air pressure [bara]
Pmax : Maximum pressure [bara]
Pcomp : Compression pressure [bara]
Phyd : Hydraulic Injection Pressure [bara]
Tscav : Scavenge Air Temperature [°C]
PM : Particulate Mass
ISO : International Organization for
Standardization
NOx : Nitrogen Oxides
CO : Carbon Monoxide
CO2 : Carbon Dioxide
SO2 : Sulphur Dioxide
HC : Hydro Carbon
DISCLAIMER
All data provided in this document is non-binding. This
data serves informational purposes only and is especially
not guaranteed in any way. Depending on the subsequent
specific individual projects, the relevant data may be
subject to changes and will be assessed and determined
individually for each project. This will depend on the
particular characteristics of each individual project,
especially specific site and operational conditions.