Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
1The Best LNG Carrier for the Environment
SAYAENDO UST:Capable of Gas-Only Combustion in All Operations
KENJI TSUMURA*1

TOSHINORI ISHIDA*1

KAZUYOSHI HIRAOKA*1 KAZUSHI KUWAHATA*2

AKINORI HAMAJIMA*2

In response to the recent trend towards progressively more stringent environmental
regulations for exhaust emissions from vessels, liquefied natural gas (LNG)-fueled vessels havestarted to emerge. As LNG carrier vessels fitted with steam turbine propulsion plant produces alow level of nitrogen oxide (NOx), they can stay ahead of planned NOx and sulfur oxide (SOx)emission regulations by using the LNG cargo as the fuel source. However, such LNG carriers
currently adopt the dual-fuel combustion method (gas and fuel oil) in order to achieve a stable load
following while maneuvering operation, where significant load fluctuations occur. This article
describes new gas-only combustion technology that enables safe maneuvering operation withsuperior economic efficiency.

|1. Introduction
As shown in Figure 1, environmental regulations for exhaust emissions from vessels arebecoming more stringent. Particularly strict restrictions have been adopted in the Emission Control
Areas (ECA) designated by each country. These controls focus mainly on SOx and NOx. To adhere
to SOx emission rules, use of low-sulfur fuels or installation of desulfurization equipment is being
considered. With LNG carriers, however, SOx-related regulatory compliance is possible by using
the LNG cargo as the fuel. NOx emission control strategies require installation of NOx removing
equipment and use of exhaust recirculation systems for conventional vessels with diesel engine
propulsion. Meanwhile, NOx emissions generated in the main boilers of LNG vessels with turbine
propulsion plants are significantly low and thus have not been made subject to NOx regulations.

Figure 1

Trends in environmental regulations

Furthermore, LNG vessels fitted with turbine propulsion plants can also meet SOx-related
regulations by using the LNG cargo as the gas-only fuel source, thus achieving overall gas
emission compliance.
Conventional LNG vessels use boil-off gas (BOG) generated in cargo tanks as the main fuelsource, which creates a slow response in fuel supply. In harbors, these vessels operate using thedual-fuel combustion method (gas and fuel oil) as operating on gas-only combustion causes the*1 Chief Staff Manager, Ship & Ocean Engineering Division, Shipbuilding & Ocean Development*2 Nagasaki Ship & Ocean Engineering Department, Ship & Ocean Engineering Division, Ship & Ocean Engineering DivisionMitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)2

inner pressure to drop, thereby creating operational difficulty during significant load fluctuations.
In order to meet more stringent exhaust gas requirements and to optimize the use of surplus BOG,
the Low-Load Gas Mode (LLGM) has been developed to achieve gas-only combustion while
maneuvering operation. The LLGM's gas-only capability ensures a smooth transition from gas-only
to dual-fuel combustion mode in the event of any abnormalities as specified in the International
Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code).
The LLGM system manages such a swift transition by continuously maintaining a minimumpressure in the gas header. This capability, however, constantly uses more gas fuel than necessary
to support the load required by the turbine plant. Mitsubishi Heavy Industries, Ltd. (MHI) has
developed the system called New Low-Load Gas Mode (NLLGM) that, while achieving the same
swift transition to dual-fuel combustion mode, does not use more BOG than necessary forcombustion. To do so, the NLLGM is equipped with an Low duty gas compressor circulation
system that utilizes cargo lines. The system instantly restores gas header pressure to a point above
the minimum level in the event of an abnormality.

|2. Gas-only combustion for conventional vessels
LNG vessels fitted with turbine propulsion plants can meet tighter SOx regulations by using
the LNG cargo as the fuel gas. In maneuvering operation, however, conventional vessels use thedual-fuel combustion method (gas and fuel oil) due to the aforementioned issue regarding load
following capability. Therefore, the current system is not capable of meeting stricter SOx
restrictions by using the dual-fuel mode while operating in harbors, requiring instead use of
low-sulfur fuel oil.

|3. Recent developments
maneuvering operations

ingas-only

combustion

duringTo maintain gas-only combustion during maneuvering operations, the system must have abackup capability for safe transition to dual-fuel (gas and fuel oil) combustion mode as specified inthe IGC Code. Consequently, the LLGM designed specifically for gas mode has been developed(Figure 2). The LLGM allows continuous gas-only combustion by maintaining the minimum gas
header pressure needed to properly ignite the fuel oil. This capability, however, uses more gas fuelthan necessary to support the load required by the turbine plant. As such, the surplus steam isprocessed by steam dumping as shown in Figure 3.

Figure 2

Overview of Low Load Gas Mode (LLGM)

Figure 3 Comparisons: Boiler load and fuel mode

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
3|4. New cost-efficient gas-only combustion system
As described in Section 3, the LLGM uses more gas than the turbine plant demands duringlow-load operation mode such as while maneuvering operation. To facilitate use of gas thatproperly corresponds to the demand, we propose the NLLGM shown in Figure 4. In currently usedsystems, including the LLGM, a fuel gas compressor controls the flow rate in accordance withboiler load changes. In the NLLGM, the compressor controls the discharge pressure only up to theentrance to the gas flow control valve (FCV), and the FCV alone controls the flow rate to theboiler. The pressure at the entrance of the FCV, therefore, is the pressure that is required to recover
appropriate header pressure for safe and instant injection of fuel oil via opening of the FCV. Thus,
safe boiler combustion that consistently uses as much fuel gas as required by the plant is achievedwithout consuming excess fuel gas.

Figure 4

Overview of New Low Load Gas Mode (NLLGM)|5. Advantages of NLLGM
Comparisons for gas consumption were made between the LLGM and NLLGM operations.
Using a 153,000m3 SAYAENDO as a model vessel, the consumption during cargo discharge andloading in a harbor was calculated (Figures 5 and 6).
While with LLGM the gas header pressure must be continuously maintained at a level
enabling a switch to fuel oil throughout the operation, the NLLGM is capable of various header
pressures in accordance with load changes. This difference resulted in gas consumption with theNLLGM 31.3 tons lower at discharge and 24.4 tons lower at loading compared to the LLGM. Thus,a 153,000m3-class LNG vessel with NLLGM can save a total of 55.7 tons in gas consumption pervoyage over a vessel utilizing the LLGM system.

Figure 5

LLGM vs. NLLGM performance comparison: unloadingMitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
4Figure 6

LLGM vs. NLLGM performance comparison: loading|6. Conclusion
With the recent trend towards more stringent environmental regulations governing exhaustemissions, marine vessel propulsion systems are at a major turning point. Although not subject toNOx regulations, conventional turbine plants need to address SOx regulations. As the SOx
regulations are based on the sulfur content of fuel oils, regular diesel-powered vessels requirepost-processing systems such as an exhaust gas treatment device or use of fuel oils with low sulfur
concentration. For LNG vessels with a turbine plant, the LLGM using natural gas, which is morecost effective than low-sulfur fuel oils, has been developed in order to adhere to the SOxregulations. In this article, a more advanced system, the NLLGM, which facilitates gas supply
corresponding to turbine plant demand, has been proposed. The NLLGM enables compliance withmore stringent exhaust gas regulations while achieving cost-efficient operation.