MAN Diesel’s 51/60 DF dual-fuel engine on the test stand at the company’s Augsburg factory. As well as LNG carriers, the new engine targets a full range of low-emissions vessel applications.
Fuel costs, environmental concerns and available technology drive consideration for increased use of gaseous fuels
The increasing availability of high-powered low-emission gas-powered engines, in particular dual-fuel types, from the main engine builders is providing a strong focus of interest for marine engineers. This stems not just from the pronounced environmental advantages of gas as a fuel, but also from the soaring price of oil. Gas is also in plentiful supply throughout the world: at current rates of consumption it is estimated that there is enough natural gas to last us for at least a century and a half.
The announcement by MAN Diesel late in 2007 that it had obtained the first major order for its new, 1000 kW/cyl, 51/60DF, dual-fuel engine represented something of a landmark for the company. The contract covered five inline, eight-cylinder, L51/60DF engines each rated 8000 kW at 514 r/min. These will provide the main propulsion for the largest LNG carrier ever to be commissioned with electric propulsion based on dual-fuel engines.
The 51/60DF is offered in inline versions of six, seven, eight and nine cylinders, as well as Vee-configuration models with 12, 14, 16 and 18 cylinders. In gaseousfuel mode, the gas is ignited by micro-pilot injection of marine diesel oil (MDO) and a liquid-fuel mode in which the engine can operate on either 100% MDO or heavy fuel oil (HFO).
In a review of the potential for gas as a marine fuel, Dipl.-Ing Oliver Bille, MAN Diesel’s sales manager marine medium-speed, points out that while initially targeted at LNG carriers and stationary gen-sets, the 51/60DF engine’s combination of high thermal efficiency as well as high power output in both gas and oil modes is creating a great deal of interest in other vessel sectors. A special attraction to ship owners is that in the gas mode, the NOx emissions
are able to meet the proposed IMO Tier 3 limits.
“Prime factors driving this enthusiasm are, of course, emissions reduction and the availability of gaseous fuels in specific areas of the world,” noted Bille. “But political decisions are also important. In Norway, for instance, there is a desire to use local resources, while in areas like Japan and California strict emissions regulations covering coastal waters, harbors and inland waterways often exceed IMO standards.”
Bille emphasized that gaseous fuels are already being employed in marine propulsion and onboard power generation based on smaller displacement high-speed gas engines with spark ignition. Typical applications include small passenger and car ferries, i.e., vessels operating fairly close to a shore base having a fuel supply. Good examples are the coastal ferries in Norway.
“Looking dispassionately at the overall potential for gaseous fuels, it must be accepted that they are not likely to replace liquid fuels like HFO and MDO or marine gas oil (MGO) with their ease of transport and storage, universal availability and, in the case of HFO, traditionally favorable pricing,” he continued. “Rather, gaseous fuels have the potential to take a limited, but economically viable share of the marine propulsion and onboard power generation market in the form of inroads into specific market niches.
“It follows that applications being considered for gaseous operation are mostly emissions-sensitive applications and/or vessels having a short radius of operation including cruise
ships, passenger or RoPax ferries, small container feeder vessels, offshore supply ships, patrol vessels and other special applications.”
The case for cruise vessels is a controversial one since there is a body of opinion that gaseous fuel may not be suitable for cruise vessels on safety grounds. However, Bille noted that several papers and presentations have been published by classification societies, state authorities, engine builders and oil and gas companies on the subject. The technologies of LNG gas as a marine fuel have been widely proven in LNG carriers with dual-fuel engines and the derivatives of these technologies are economically feasible. Likewise, safety measures like the MAN Diesel Dual-Fuel Safety Concept already exist and have classification society approvals in principle.
Shown is the technical data of the 51/60 DF engine. It can burn gaseous fuels ignited by a distillate-fuel micro-pilot and distillate or heavy fuel oil in liquid fuel (100% diesel) operatingmode. (Right) Shown is a typical external gas supply system layout for an LNG-fueled vessel.
Bille emphasizes that — unlike applications on LNG carriers — in cruise ships or ferries, the LNG would be stored in well-insulated pressure vessels at up to around 10 bar. “Considering the various types of gaseous fuels, LNG has distinct advantages,” said Bille. “Its main constituent is methane, which is the simplest combination of carbon and hydrogen in the hydrocarbon series (CH4) and hence intrinsically clean burning. Dual-fuel engines readily achieve emissions that meet Germany’s TA Luft clean-air regulations and the World Bank Pollution Prevention and Abatement Handbook.
Moreover, LNG is well suited to storage aboard ships. When liquefied by subcooling to around -160°C, LNG occupies only 1/600 of its volume at normal ambient temperatures and can be stored in special insulated tanks at atmospheric pressure or in insulated vessels with safe working pressure below 10 bar.” Bille points out that, in a cruise or ferry vessel, the gas container would take up more room than a traditional oil tank in the ship’s hull. But this would partly be offset in applications using only marine diesel or marine gas oil by the elimination of the HFO day tank, centrifugal separators, filters and other fuel conditioning equipment. The safe working pressure of the pressure vessel would allow sufficient quantities of natural boiloff
gas to be stored safely for short periods during engine downtime when the cruise ship is in port, for example.
With the ship under way, natural boil-off gas, due to the gradual heating of the fuel in its insulated vessel and the movement of the ship, would need to be supplemented by forced vaporization of the LNG to create the quantity of fuel needed by the engines. As the pressure in the LNG tank reduces, compression of the boil-off gas to the required fuel gas inlet pressure would be required. Again, these basic technologies are well established.
It is worth noting here that, given the clean burning characteristics of natural gas, the necessity for “cold ironing” — shutting down onboard oil-fueled gensets while in harbor and connecting to a land-based electrical supply — would no longer be necessary. Indeed, one scenario calls for LNG to be supplied to the ship from a shore-based supply. This alternative is in fact currently under investigation for medium- and large-sized container vessels. The main technical features of MAN Diesel’s 51/60 DF dual-fuel engine have been covered in these pages thoroughly in the past.
The new engine is effectively the company’s response to the need for a dual-fuel engine for both LNG carriers and other vessels. Its design draws heavily from the 1200 kW/cyl 48/60B HFO engine, which has an excellent record of high availability since its introduction in 2002. More than 150 48/60B types have been installed to date with over 330 000 total running hours achieved. The contract to supply eight 51/60 DF models for the Korean LNG carrier being built for a European owner is its first step in the marine field following the engine’s approval by seven major Classification Societies in September 2007.
At the time of writing,MAN Diesel has announced the first retrofit of its dual-fuel technology to a 12-cylinder 48/60A engine, which had previously operated in HFO in an industrial cogeneration plant at Vila Nova de Famalicão in Portugal. Stephan Mey, head of MAN Diesel’s Power Plant business unit, noted that the availability of natural gas is increasing all over the world, along with a need in many areas to conform to cleanair legislation.
Converting liquid fuel to gas offers opportunities for meeting emissions requirements. While dualfuel conversion of a 48/60 diesel engine to 48/60 DF status is possible, in this case the customer chose full conversion to 12V 51/60 DF specification since the cylinder liners of both models are interchangeable. It is expected that many more such conversions will be requested over the coming years.