Gas-fuelled two-strokes come into play

by | 22nd September 2016 | News

Home News Gas-fuelled two-strokes come into play

Marine Power & Propulsion: October 2016

Recent vessel deliveries have raised the commercial profile of a new generation of low-speed propulsion engines with the capability to run on gas or liquefied gas.

MAN Diesel & Turbo sees significant opportunities arising for gas-fuelled machinery as concerns over both CO2. and SOx emissions increase, impacting on the regulatory environment.

Besides achieving significant reductions in CO2., NOx, SOx and PM, the company claims that its ME-GI gas-injected two-stroke, unlike competing engines, has only negligible, unburnt gas slip (‘methane slip’), consequently adding very little to the greenhouse effect. Moreover, the design’s use of the Diesel combustion principle leaves no formaldehyde emissions, a product of methane oxidation.

A milestone in the deepsea shipping industry’s take-up of gas-burning propulsion plant was signified by the entry into service this year of the world’s first LNG carrier newbuild specified with MAN electronically-controlled, dual-fuel ME-GI low-speed engines employing gas injection. Under five-year timecharter in the USA’s new LNG export traffic from the Sabine Pass LNG terminal in Louisiana, the 174,000m3– capacity Creole Spirit is the first of nine sisters contracted by Teekay LNG Partners from the Daewoo yard.

The vessel’s twin propulsion engines are five-cylinder models of the G70ME-C9.2-GI type, manufactured by licensee Hyundai Heavy Industries, and capable of burning heavy fuel oil (HFO) or marine gas oil (MGO) as well as natural gas. The auxiliary outfit is also based on dual-fuel prime movers, in the shape of four Wärtsilä 34DF engines.

The ME-GI technology is claimed to be 20% more efficient than the dual-fuel diesel-electric (DFDE) solution most widely favoured today for LNG tankers, and up to 30% more fuel-conservant relative to traditional steam turbine propulsion.

Teekay’s decision to opt for ME-GI powering was also motivated by through-life maintenance considerations. The two-stroke plant achieves the requisite power with substantially fewer engine cylinders than in a four-stroke engine-based DFDE installation. It also entails less drive train and electrical system complexity. 

The Burckhardt compressor system delivers fuel gas, derived from the boil-off gas (BOG) naturally produced through cargo evaporation during the voyage, to the ship’s dual-fuel engines.  Compression takes place at a pressure of 300bar for direct injection into the two ME-GI engines.

MAN’s yet more versatile ME-LGI concept has also made its seagoing debut this year through the initial deliveries in a programme of seven 50,000dwt product/chemical tankers chartered for methanol transport.

In contrast with the ME-GI engine where fuel is injected in its gaseous phase, the ME-LGI version provides a dual-fuel solution for low-flashpoint liquid fuels. The design overcomes the challenge of low-octane fuels with characteristically poor self-ignition qualities, such as methanol, by using the ME-GI principle of pilot injection of MGO or HFO. Fuel injection is accomplished by a fuel booster injection valve, using 300bar of hydraulic power to raise injection pressure to some 600bar.

The adoption of ME-LGI propulsion machinery that offers the flexibility to run on methanol cargo fuel, as well as HFO, MDO and MGO, is central to the series of seven newbuild tankers ordered by various owners from Hyundai Mipo Dockyard and Minaminippon Shipbuilding. The programme was implemented on the strength of long-term charters to methanol producer and supplier Methanex Corporation.

The initial three ships were commissioned in April, and are claimed to be the first ocean-going vessels installed with dual-fuel engines incorporating a low flashpoint liquid (LFL) fuel system. By using methanol, SOx emissions are reduced by about 95% and NOx is cut by some 30% compared with conventional marine diesel oil. The parties to the newbuild project consider that methanol could become one of the popular alternative marine fuels in the future as an environmentally friendly solution with lower fuel costs, easier handling, and lower installation and retrofit costs.

MAN is currently developing an ME-LGI engine version capable of running on LPG, and the expectation is that this will be ready for delivery by mid-2018.

Provision for operation on ethane is the latest phase in the evolution of the ME gas-fuelled series. The ethane gas-injected ME-GIE variant will have its debut application in the first of three 36,000m 3 liquefied ethylene gas (LEG) carriers ordered in China by German owner Hartmann Reederei.  As well as ethane, the ME-GIE engine will be able to operate on HFO, MDO and MGO.  Ethane fuel has a similar profile to methane, and contains negligible sulphur and comparatively lower CO2.  

In addition to the three 7G50ME-C9.5-GIE engines specified for the Hartmann ethylene gas tanker trio, MAN has logged orders for ethane-burning 6G60ME-GIE units to be installed in five 85,000m3 ethane carriers contracted by a Hartmann/Jaccar joint venture. These five newbuilds will be Tier III NOx-compliant through the use of MAN Diesel & Turbo’s proprietary exhaust gas recirculation (EGR) system.

Whereas high-pressure gas injection two-stroke engines operate on the Diesel cycle, the low-pressure Wärtsilä X-DF dual-fuel type works on the lean-burn Otto cycle when in gas mode. The requirement of the X-DF generation for only low pressure gas compression makes for greater simplicity and lower costs relative to alternative gas engine solutions, according to engine designer Winterthur Gas & Diesel (WinGD). High-pressure, electrically-driven compressors are not required.  Moreover, the technology allows stable operation on gas across the entire load range, obviating any need to switch to diesel at low loads.

Installed with the first dual-fuel, low-speed engine employing WinGD’s low-pressure X-DF gas admission technology, the 15,000dwt oil/chemical products carrier Ternsund entered the North European distributive traffic during August. The Danish-registered vessel leads a class of four ordered from China’s AVIC Dingheng Shipbuilding by Terntank Rederi.

Her Wärtsilä RT-flex50DF main engine provides a maximum output of 5,850kW. When the machinery is operating in gas mode, it is expected to yield negligible SOx and particle (PM) emissions, 85% less NOx than under IMO Tier II regulations, and 25% less CO2 than those of a conventional diesel engine. 

Ternsund can accordingly meet the 0.1% sulphur ‘cap’ applicable in IMO-designated Sulphur Emission Control Areas (SECAs), within which she will predominantly trade. In gas-fuelled operation, the X-DF main engine also facilitates compliance with the Tier III restrictions on NOx, without necessitating additional exhaust aftertreatment measures.

Furthermore, as LNG has a 10-15% better energy value than MGO, and in conjunction with an optimised underwater hull design, the tanker offers a daily fuel consumption of no more than 15tonnes per day, compared with 22tonnes for existing ships of similar size. The adoption of a large-diameter propeller in conjunction with a two-stroke main engine means that only 65% of the maximum power output will be needed to reach a service speed of 14.5knots.

The fleet development programme was implemented under the EU’s LNG fuel usage-promoting Baltic SO2lution project and has qualified for partial EU funding in accordance with the Trans-European Transport (TEN-T) initiative.

Mitsubishi Heavy Industries provides a source of home-grown technology in large, two-stroke propulsion machinery for Japanese shipyards and their customers, and serves as the sole competitor to MAN Diesel & Turbo and Winterthur Gas & Diesel in the global low-speed propulsion engine market. Now the company has raised the stakes by responding to the European licensors’ introduction of LNG dual-fuel two-stroke designs by developing its own solution, the UEC-LSGi type.  

The nascent UEC-LSGi has a high-pressure natural gas direct injection system, rather than the low-pressure, pre-mixed method. In a paper on advances in the UEC engine range, delivered to June’s CIMAC Congress, MHI’s Katsumi Imanaka explained that the direct injection solution was favoured as offering high combustion stability across the load range, being resilient to the fuel gas composition and ambient conditions, obviating methane slip, and promising equivalent performance in gas mode to a diesel engine.

It was acknowledged that the system required the arrangements to ensure a high gas supply pressure, in the order of 250-300bar, and that NOx reduction was not as great as in the premixed DF approach. However, these demerits have to be weighed against the perceived, claimed disadvantages of the latter as regards combustion sensitivities, lower efficiency in oil mode, methane slip incidence, and longer fuel switchover times. Full IMO Tier III regulatory compliance of the dual-fuel two-stroke will be assured by equipping the engine with selective catalytic reduction (SCR) or exhaust gas recirculation (EGR). 

The UEC-LSGi’s electrical control system has been derived from that of the latest UEC Eco engine generation, augmented by combustion monitoring.  By constantly monitoring combustion pressure, automatic change from gas to diesel can be effected in the event of abnormal events.

Development and evaluation of the LSGi has involved extensive use of MHI’s 4UE-X3 test platform and demonstrator. The company has a long track record in studying gas combustion technology for two-stroke plant, having undertaken evaluations with an 840mm-bore Sulzer engine 30 years ago.
MHI indicated last year that it would start marketing the UEC-LSGi in 2017, and that the first application could be a 600mm-bore engine in the 11,000-18,000kW power range.

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