Why dual fuel is the best answer for gas-fuelled ships
The dual-fuel option for gas-fuelled ships offers economic benefits as well as practical and environmental advantages.
The main driver behind the rapid adoption of gas as a ship fuel is emission regulation, while the likely increasingly attractive price of LNG provides another important incentive. The need to lower sulphur levels, first to 0.1% in emission control areas (ECAs) from 2015, and then globally to 0.5% from 2020, means that ship owners presently using heavy fuel oils (HFO) have to opt for one of three solutions, says Wärtsilä. The options are to use alternative fuels, e.g. gas , to fit exhaust gas cleaning equipment (scrubbers) or to switch to distillate fuels such as marine diesel oil (MDO) . Which is the most appropriate will depend on many factors, but will be principally influenced by the relative costs of HFO, MDO and gas fuels, and the time spent in areas where sulphur emissions are severely limited.
Demand for distillate fuels is likely to be very high, which will force up the cost relative to heavy fuels, which, consisting mainly of the residual oils remaining after the distillation process for other petroleum products, are likely to remain comparatively inexpensive. That should make the scrubber solution attractive for a number of vessels.
The third option is to switch to LNG and dual-fuel engines. As the price of gaseous fuels is likely to be more stable, and could even come down in relative terms as new extraction methods come on stream, that option looks attractive, at least in areas where the supply infrastructure already exists or is planned.
The major difference between these new sulphur emission limits and previous regulations are that they will apply to all ships, not just newer vessels. Wärtsilä can offer solutions based on all three alternatives. Engines in the current portfolio can run on either residual or distillate fuels without modification, though ancillary systems such as fuel pumps may need to be changed, and particular attention must be paid to lubrication when running on low-sulphur fuels. The company is able to supply scrubber equipment, and can provide either its own freshwater-based system or, following the Hamworthy acquisition, the Krystallon-designed seawater system. And Wärtsilä has long experience in gas- and dual-fuelled engines.
Proven benefits of gas
Running on natural gas provides great savings – around 85% - in emissions of nitrogen oxides (NOx) as well as cutting sulphur oxides (SOx) by some 99%, thus providing a useful means towards meeting not only the forthcoming ECA limits but also IMO Tier III. Wärtsilä Ship Power product management director Tomas Aminoff says that he expects particulate matter (PM) emissions to become the next major focus – comparatively little attention has been paid to these so far, but they will come under the spotlight as a result of tighter US Environmental Protection Agency (EPA) limits. Here again, with natural gas fuel, PM emissions are negligible.
The energy efficiency design index (EEDI) and ship energy efficiency management plan (SEEMP) regulations entering into force in 2013 will seek to regulate greenhouse gas (GHG) emissions. Because methane gas – the major component of most gaseous fuels – is a particularly potent greenhouse gas, the existence of unburnt gas in the exhaust (methane slip) means that the difference in GHG emissions when using gas fuel rather than Diesel fuels in a dual-fuel engine is less marked than the reductions in NOx, SOx and PM. However, running on gas still provides a significant benefit, with gas-fuelled ships typically producing a clear reduction in GHG compared to conventional fuels.
Wärtsilä’s experience of large gas-fuelled engines dates back to 1987, when it introduced the gas-diesel engine (GD) for land-based stationary applications. This was followed by a pure gas, spark-ignition (SG), engine in 1992, and shortly after, in 1995, the dual-fuel engine (DF), capable of running on either Diesel fuels or liquefied natural gas (LNG).
Both the pure gas spark ignition and dual fuel engine types run on the Otto cycle (similar to a gasoline-fuelled vehicle engine) while the gas-diesel engine runs on the Diesel cycle. This means, says Mr Aminoff, that only pure gas and dual fuel engines are able to meet IMO Tier III emission limits without after treatment. The gas-diesel option also operates on high gas pressures, making it less suitable for ship power, while the pure gas engine lacks the fuel flexibility and redundancy inherent in the dual-fuel approach. Therefore Wärtsilä’s choice for ship power is dual fuel. Wärtsilä dual-fuel engines have proven their reliability by reaching 5 million running hours by 2012, which clearly indicates the company’s leading position in this field.
Dual-fuel brings flexibility, redundancy, economy
A major benefit of fuel flexibility is that it allows ships to choose fuel according to operational patterns and economic factors. For example, a vessel such as an FPSO may be optimised to operate on low-quality unrefined liquid fuels or low-quality gas; a gas carrier will have the engine tuned for a balance between liquid and gas, using gas when readily available (e.g. as boil-off from cargo), when cheaper, or in ECAs, and conventional fuel at other times; and a tug, offshore support vessel or short-sea ferry will be optimised for running on gas as primary fuel, with a capability to use liquid fuels for out-of routine operations, redundancy and re-positioning.
The dual fuel engines can start and stop in gas mode, can idle for up to 8 hours in gas mode, and can change between gas and liquid fuels, and vice versa, on the go.
And this flexibility provides the clue to why the company believes the dual fuel option to be a better fit compared to pure gas engines for ship propulsion. Taking a typical gas-fuelled installation, such as that on an offshore supply vessel, with a combination of two Wärtsilä 34DF and one Wärtsilä 20DF based generating sets, the engines are set up with both MDO and LNG supplies to all three engines, and in normal operation all engines will run on LNG from a single gas supply such as Wärtsilä’s LNG-Pac. In the case of a gas system failure to one engine, that engine can be simply switched to MDO operation. Similarly, should a problem be encountered with the main gas supply, all engines can be switched to MDO. So, full redundancy can be simply achieved.
To achieve the same redundancy level on a pure gas installation requires a greater investment on the gas handling side: “And the gas tank is one of the single most expensive components,” says Mr Aminoff. Additionally, the full redundancy required under some class rules can mean the added complication of power take-in arrangements on each main shaft, and maybe, in some applications, even a second auxiliary running on MDO – thus negating the main advantage of a single-fuel gas installation.
Looking at the size of gas tanks needed, taking a typical autonomy of 14 days at 50m3/day, with a 10% safety margin, a tank capacity of 770m3 will be the minimum required. However, some missions (say one in 20) may be up to 18 days, and a further one in 20 may be during bad weather, which will require a 15% power increase, with a corresponding increase in gas consumption. A dual-fuel installation will be able to switch to MDO to accommodate longer missions and poor weather, so the 770m3 gas capacity will suffice. In contrast, the pure gas installation will require two systems, with a total capacity of at least 1,139m3 (50 x 18 x 1.15 x 1.1).
Therefore, a pure gas vessel could require 48% more gas capacity to achieve an identical operational flexibility. And because gas tanks are costly, and liquid fuel systems comparatively cheap, the total cost of a dual-fuel installation will work out significantly less than that of a gas-only system. And a dual-fuel installation allows vessels to be easily moved from one operational area to another, which may not be possible because of the limited supply infrastructure for LNG.
One area where the pure gas engine is sometimes claimed to be superior to a dual-fuel engine in gas mode is in transient load performance. However, Wärtsilä’s trials – witnessed by us on a visit to the Vaasa plant – indicate that the disadvantage is far less than suggested. Successive loading figures show that the 6L34DF engine in Wärtsilä’s test laboratory can reach 100% power from zero in about 30s while still remaining well within the 5% variation in RPM as required by the regulations. These figures compare well with equal size diesel engines like the Wärtsilä 32.
The tests conducted in Delivery Centre Vaasa demonstrate that the 34DF is able to switch instantaneously and seamlessly from gas to diesel – as might be required in case of a problem with the gas system. The only noticeable change was that the smoother running characteristic of the Otto cycle gave way to the typically slight increase in vibrations with the Diesel cycle. Changing back from MDO to gas was also simply accomplished, thanks to the fully automated systems.
Future developments in medium and low speed
Future developments in the medium-speed dual-fuel field will be concentrated on reducing methane slip and further engine optimisation. This can be achieved through software tuning in the control system, and further optimisation of the hardware.
Also currently in progress is the development of Wärtsilä’s dual-fuel low speed engine, capable of running on LNG, MDO and HFO. Unlike other low-speed gas engines, it uses low gas feed pressure in conjunction with about 1% pilot fuel at 100% load. Gas feed pressure is below 10bar at all points, in all conditions, which allows the system to be fed by the handling system’s own in-built pressure. More importantly, the adoption of pre-mixed lean burn combustion, and the Otto cycle, allows the engine to meet IMO Tier III and 0.1% sulphur emission limits with no secondary equipment.
Availability of gas
Whatever the advantages of LNG as a fuel, availability of gas is seen as a key issue – if ships cannot bunker LNG where and when it is needed, there will be no incentive to take up this option. Some experts draw an analogy with the advent of the motor ship, which needed Diesel fuel rather than the bunker fuels used by steam ships (only later were combustion engines developed to the stage when they could burn boiler fuels). That did little to halt the rise of the Diesel engine for ship power.
LNG bunkering is seen by most experts as being a classic ‘chicken and egg’ situation. If the fuel is not available, there will be little demand, but without demand it is unlikely to be available. Shipowners surveyed by Lloyd’s Register proved serious and upbeat about adopting LNG fuel, but regarded it as a long-term measure.
Lloyd’s Register’s study indicates that many of the major bunkering locations for oil fuels are conveniently close to LNG terminals, and some, like Rotterdam, are in ECAs. So the physical aspects of LNG supply may not be as much of a problem as some fear.
On the other hand, some experts see too wide a difference between the large-scale LNG supply industry, as exemplified by terminal operators, and the smaller-scale operations needed for bunker supply. LNG Brokers, in a paper given to the 2011 Gas Fuelled Ships conference, said that a logical contact point needs to be established between the large scale and small scale LNG supply worlds, probably within the large regasification terminals.
Dual fuel engines provide an interim – and probably a long-term – solution to this dilemma. Wärtsilä points to the feasibility of a dual-fuel engined container ship, for example, running between Asia and Europe, burning HFO outside ECAs and switching to LNG for ECA operations. Gas can, most likely, be bunkered in ports such as Rotterdam, but one solution would be to use portable LNG tanks, the same size as a standard forty foot equivalent container, several of which could be loaded onto the ship at a container port, and when empty, exchanged for full containers at another port.
Successful conversion reference
In terms of actual operational experience, Mr Aminoff points to the success of the Bit Viking project. This tanker racked up a number of ‘firsts’ – it was the first ship to be converted to LNG operation, the first to use gas fuel in conjunction with a mechanical drive (others are electrically powered), the first gas fuelled installation to gain ‘single main engine’ class approval, the first installation of Wärtsilä’s own gas handling system, and the first cargo-carrying merchant vessel other than a gas carrier to use LNG as fuel. It has proved its ability to run continuously and reliably in gas mode, even in the severe weather conditions and sea states encountered in winter around the Norwegian coast which impose considerable loads on propeller sand engines. The ship has accumulated well over 3,000 running hours in gas mode, with 99% availability. It as bunkered every two weeks, at a transfer rate of 430m3/h, which allows sufficient gas for two weeks of operation to be bunkered in about 2.5h.
Wärtsilä 20DF engine