Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)37

Environmentally Superior LNG-Fueled Vessels*1

TAKASHI UNSEKI

With increasing demand for environmental protection and improvements in marine vessel
operations, the International Maritime Organization (IMO) has adopted the Energy Efficiency
Design Index (EEDI) for the control of emissions of nitrogen oxide (NOX) and sulfur oxide (SOX),
as well as carbon dioxide (CO2) per ton mile. As one of the promising solutions for the reduction ofexhaust emissions from vessels, switching from conventional heavy fuel oils to liquefied natural gas(LNG) is being studied and pursued. This article describes concept vessels that use LNG fuels toreduce environmental impact, as developed by Mitsubishi Heavy Industries, Ltd. (MHI).

|1. Introduction
The IMO Tier III limits on NOX, which come into effect in 2016, are considered difficult toachieve even with an 80-percent reduction from the Tier I standard and further technologicalimprovements in inner engine combustion. As such, the adoption of a combination of various
technologies or a different approach is required. Furthermore, the EEDI requirements on CO2conflict with NOX-reduction technology. In terms of SOX regulations, the sulfur content of fuel oil
within Emission Control Areas (ECAs) will be reduced to no more than 0.1 percent, effective in2015, followed by a global 0.5 percent fuel sulfur limit coming into force in 2020. This will require
technological innovation such as a transition to low-sulfur fuels and the implementation ofdesulfurization equipment.
As one effective solution to meeting such reduction limits, switching from conventional
heavy fuel oils to LNG can potentially enable a significant reduction in NOX emissions. In addition,
LNG does not contain sulfur and is thus free of SOX emissions. LNG fuels are also very promising
due to the fact that CO2 emissions during combustion are becoming quite low. After reviewing theissues surrounding the use of LNG fuels in vessels, MHI has developed LNG-fueled ROPAX (rollon/roll off passenger) and VLCC (very large crude carrier) vessels that significantly reduce
environmental impact.

|2. IMO environmental regulations
2.1ECAAt the 53rd session of the IMO Marine Environment Protection Committee (MEPC) in 2005,portions of the Baltic Sea and the North Sea were designated as ECAs. At MEPC 59 in 2009, areas
within 200 nautical miles of U.S. and Canadian shores were also included. The ECA designation
will be further extended and the phased introduction of tighter standards is being implemented. In
particular, Tier III NOX limits taking effect in 2016 will target an 80-percent reduction from theTier I standard within ECAs (Figure 1). In terms of SOX regulations, the sulfur content of fuel oil
within ECAs will be reduced to no more than 0.1 percent, effective 2015 (Figure 2).

*1

Senior Manager , Shipbuilding & Ocean Development, Ship & Ocean Engineering DivisionMitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
38

Figure 1

IMO NOX standards

Figure 2

Fuel sulfur standards of IMO and CARB

2.2

ECA compliance through use of fuel oil

Several fuel oil-related methods are being examined by engine manufacturers toaccommodate the ECA standards. SOX limits can be addressed through the use of low-sulfur fueloils. Also being pursued and put to use is a device that cleans the exhaust gas with sea water or
fresh water (injected with neutralizing agents) and thus reduces the SOX content to the regulated
level (Figure 3).
Meanwhile, NOX emission reduction involves a wide range of issues and approaches
depending on the engine model. Technologies being examined include exhaust gas denitrationequipment (Figure 4) and an exhaust gas re-circulation system (Figure 5).

Figure 3 Exhaust gas
desulfurizationequipment

Figure 4 Exhaust gas
denitration equipment

Figure 5 Exhaust gas
re-circulation system

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
392.3

ECA compliance through use of LNG fuelSwitching from fuel oils to LNG fuels, especially in a 4-cycle engine, has proven not only toachieve compliance with the most stringent regulations, but also to result in environmental
performance that far exceeds such standards. In spark-ignition gas engines in particular, SOX andparticulate matter (PM) in the exhaust gas can be reduced by almost 100 percent, while also
reducing NOX and CO2 by more than 90 percent and 20 percent, respectively (Figure 6). A major
issue with current 4-cycle engines, however, is methane slip of 1-2 percent (emissions of unburned
methane, which has a Global Warming Potential (GWP) 25 times that of CO2). The development ofmethane slip reduction technology is ongoing.

Figure 6 4-Cycle engine emissions comparison between marine diesel oil(MDO) and LNG|3. Issues in LNG-fueled vessels
3.1

LNG tank system

Tanks are categorized in five shapes and types as shown in Table 1. Each has advantages anddisadvantages and requires widely different ancillary facilities. As such, an appropriate tank system
must be selected based on multiple requirements such as mode of operation, fuel consumption andLNG tank space.
Table 1

LNG tank system

Prismatic tankSpherical tankB

B or C

Cylindrical tank

Tank truck

Tank type

IMO type
Heat insulation

External

C
External

VacuumVacuumMax. pressure

0.7 bar

1 bar

10 Bar

10 BarSpace efficiency

HighLow

MediumLow/Medium

Gas delivery
Design cost

Pumping OutHigh

BOG treatment

Pressure Built-Up TypeMedium

Low

Necessary

Low-Not Necessary

Suitable cap.

>5,000m3>5,000m3

30-1,000m3

30-1,000m3<100m3CostHighHigh

MediumMediumLow/MediumMitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
40

3.2

LNG-fueled propulsion plant

LNG-fueled engines include gas-only, dual-fuel (2-cycle low-speed and 4-cycle mid-speed),and steam turbine systems as shown in Table 2. In general commercial vessels, the use of a turbine
plant is unlikely. When fitted with a gas-only engine system, the IMO International Code for theConstruction and Equipment of Ships Carrying Liquefied Gases in Bulk Code (IGF Code) requires
the installation of a backup LNG tank and propulsion system, neither of which is required with adual-fuel engine system. Figure 7 shows a comparison of the environmental impact of DF
low-speed and mid-speed engines and LNG-fueled re-heat steam turbines versus a heavy oil-fueled
low-speed diesel engine, which is the most fuel-efficient system currently available. The
comparison indicates that the fuel-efficient DF low-speed diesel engine performs best in terms of
CO2 emissions. By contrast, emissions from the steam turbine are near zero, with the exception of aslightly elevated CO2 Emissions level.
Table 2 LNG fueled engine
Pilot fuel
LNG Tank required

Gas Engine
4 CycleSpark igniter2-tanks or more

Dual Fuel diesel
4 CycleMicro pilot fuelNOX-Tier III

MeetMeetStand-by propulsion

Needs
Change over tostand-bypropulsion
Methane slip
1~2%

--

Dual Fuel diesel
2 CycleMicro pilot fuel1-TankAdditionaltreatment unit--

Change over to
H.F.O.

Change over to
H.F.O.

Change over to
H.F.O.

Methane slip
1~2%

Gas burning>20% load

Limited operator

Emergency
Remarks

Steam Turbine-Meet--Figure 7 LNG-fueled engine emissions comparison|4. Concept design of LNG-fueled vessel: ROPAX
The concept design of the LNG-fueled ROPAX vessel is described as follows:
Passenger capacity: 600/ Trailer capacity: 150/ Normal speed: 23 knots (Cruising range: 450
nautical miles)

4.1

Selection of LNG tank system

Due to the mode of operation, fuel consumption and restricted LNG tank space of a coastal
ROPAX vessel, vacuum-insulated pressure vessels have been adopted for the LNG tank system.
The pressure vessel design enables LNG fuels to be supplied to the main engine via a pressurebuild-up unit. In addition, as the system does not require a boil-off gas (BOG) treatment device, theoverall design is simpler compared to other tank systems. From a safety perspective, the desirable
location for the tank system is the exposed deck on the stern, away from the passenger area. On thecoastal ROPAX vessel used this time, however, the total weight of the LNG tanks and the electric
propulsion system (described in the next section) is around several hundred tons. Therefore, inorder to maintain stability and trim and to minimize the impact on propulsion performance, the tank

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)41

system is fitted below deck in the area adjacent to the forward side of the engine room at the middleof the ship (Figure 8).

Figure 8 ROPAX: Arrangenment of LNG- fueled propulsion plant

4.2

LNG-fueled propulsion plant

In order to accommodate the ROPAX vessel's engine room location requirements and to
avoid knocking due to engine load fluctuations while cruising, as well the risk of gas trip, the plantcombines a dual-fuel 4-cycle mid-speed engine and a dual-shaft electric propulsion system (Figure9). Table 3 shows a comparison of the environmental impact of the concept vessel and an existing
ROPAX vessel. By switching from the heavy-oil fueled, mid-speed direct-drive diesel dual-shaft
propulsion plant to the LNG-fueled dual-fuel mid-speed diesel dual-shaft electric propulsion plant,a 25-percent CO2 reduction, an 87-percent NOX reduction, and a 98-percent SOX reduction wereachieved, even with the decrease in fuel efficiency due to electric conversion loss. These reductions
are far below the IMO limits, thus indicating superior environmental performance.

Figure 9 ROPAX: LNG-fueled propulsion plant
Table 3

ROPAX emission comparison

ROPAX emission calculation(HFO.:3% S, MDO:0.5%)
4Cyc. DFDE with
LNG as fuel
PAX
600
12m Trailers
155
Trailers / Cars
Cars
50-Vessel speed
23 ktsMain propulsion eingine
12,000 kw x 2sets-Main generator-8,400 kw x 2sets
Auxiliary generator
1,270 kw x 3sets
5,400 kw x 2sets
Operation hours / Voyage
19.5 hrs
Voyage days / year
300 days
Fuel consumption /FO.
(k-ton/y)260.2year
LNG(k-ton/y)-23
CO2(k-ton/y)82
62-25%
EmissionNOX(ton/y)
1,398
233
-83%
SOX
(ton/y)
1,432
28
-98%
Tier III NOX and SOX regulations all satisfied with use of LNG fuelConventional twin
CPP diesel driven

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)42|5. Concept design of LNG-fueled vessel: VLCC
The concept design of the LNG-fueled VLCC is described as follows (Figure 10).

Figure 10

5.1

LNG-Fueled VLCC vessel general arrangement

Selection of LNG tank system

Based on a Japan-Persian Gulf roundtrip, VLCC vessels require large LNG tanks with acapacity exceeding 8,000m3, thus the vacuum insulated system is not applicable. As the exposed
upper deck has no particular space restrictions for the tanks, horizontally-placed cylindrical Type-Ctanks with exterior insulation were selected.

5.2

LNG-fueled propulsion plant

Based on the propulsion power, fuel efficiency and waste heat recovery power plant requiredfor VLCC vessels, the DF low-speed diesel engine was selected. The LNG is pressurized and raised
to the normal temperature in high-pressure pumps before being supplied as fuel. The auxiliary
boiler that supplies steam for turbine-driven cargo pumps is run on dual-fuels. The BOG from theLNG tanks is supplied via the gas heater and burned in the boiler. The turbine generator then uses
steam generated by this process to run and generate power, which is recovered as auxiliary
propulsion drive via the shaft drive motor (Figure 11).

Figure 11

VLCC: LNG-Fueled Propulsion PlantTable 4 shows a comparison of the environmental impact of the concept vessel and an
existing VLCC vessel. By switching from the heavy-oil fueled, low-speed diesel propulsion plant
to the LNG-fueled propulsion plant (Plant-A), a 14.4-percent CO2 reduction, a 12.6-percent NOX
reduction and a 92-percent SOX reduction were achieved. In addition, when incorporating the latest
propulsion capability, the concept vessel is able to further improve its environmental performancewith a 29.5-percent CO2 reduction, a 19.1-percent NOX reduction and a 92.6-percent SOX reduction(Plant-B). Other technologies such as EGR must also be combined in order to meet the IMO Tier
III NOX limits.

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
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Table 4

VLCC Emission Comparison

VLCC Emission calculation
H.F.O.:3% S, MDO:0.5% S
DF Slow-speed Diesel DF Slow-speed Diesel
Slow-speed Diesel
Main Propulsion
(LNG)
(LNG)
(H.F.O)
Plant-A
Plant-B
Vessel speed
15.5 kt
Propusion power
27,000 kw x 76 rpm
25,000 kw x 63 rpm
T/G
1,100 kw x 1set
2,000 kw×1set-Electric.
STG--2,000 kw x 1set
Gene.
D/G
1,100 kw x 2sets
1,400 kw x 2sets
1,400 kw x 2sets
SGM
1,000 kw x 1set
1,000 kw x 1set
Cargo capacity
355,000
326,000
326,000
Voyagedays / year
300 days
FuelFO.
(ton/day)
90
5.2
5.5
consumption LNG (ton/day)-81.8
66
CO2 (k-ton/y)84,60572,401-14.4％
59,649-29.5％
Emission
NOX (ton/y)
1,913
1,672
-12.6％
1,548-19.1％SOX (ton/y)
1,811
145
-92.0％
135-92.6％
SSD-Gi will required additional treatment unit to meet NOX Tier-III|6. Conclusion
The switch to LNG as a fuel source involves more than just changes in vessel specifications.
It is a long process that requires a significant investment in areas such as infrastructuredevelopment to transport and supply LNG fuels. Meanwhile, fuel oils continue to face stricter
environmental requirements, thus adding value to low-sulfur fuels. Combined with expected CO2surcharges, a significant price increase in fuel oils is predicted. As such, when considering theswitch to LNG fuels, price trend predictions for fuel oils and LNG fuels become a key deciding
factor. So far, the price of gas has followed a similar fluctuation to that of crude oil, while
maintaining a certain price gap. In the future, the addition of even cheaper shale gas as another fueloption is likely to further promote the wider adoption of more popular LNG fuels in place of crude
oil.