Dual fuel ticks all the boxes, says engine designer
Wärtsilä believes that the dual-fuel option for gas-fuelled ships offers economic benefits as well as practical and environmental advantages.
The main driver, says the company, behind the rapid adoption of gas as a ship fuel, is emission regulation. 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 shipowners presently using heavy fuel oils (HFO) have to opt for one of three solutions: to switch to distillate fuels such as marine diesel oil (MDO), to fit exhaust gas cleaning equipment (scrubbers) or to use alternative fuels, e.g. gas. 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 large ships which spend most of their time in ocean waters; but the equipment is large, heavy, expensive and could be difficult or not cost-effective to fit on older ships. 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.
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 Thomas 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 25% reduction in GHG compared to conventional fuels. As gas engine technology advances, methane slip should be reduced, which will have a still more positive effect on EEDI and SEEMP, as well as whatever market-based measures (fuel levies, carbon trading or similar) might be introduced in the near future to limit shipping’s carbon footprint.
Wärtsilä’s experience of large gas-fuelled engines dates back to 1987, when it introduced the gas-diesel engine for land-based stationary applications. This was followed by a pure gas, spark-ignition, engine in 1992, and shortly after, in 1995, the dual-fuel engine, capable of running on either Diesel fuels or liquefied natural gas (LNG) was launched in both the power generation and marine markets. The company’s current reference list contains around 180 installations across five market segments – power plants, gas carrier ships, offshore vessels, ferries and coastal patrol craft, with some 5 million running hours clocked up so far. In the large-engine (>3.5MW) gas-fuelled power plant market, Wärtsilä estimates a market share of at least 70%, while at sea, thanks to the popularity of the Wärtsilä 50DF engine in the gas carrier segment, that particular engine type enjoys a 31% share of the market for all medium-speed marine engines, regardless of fuel type.
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. 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.
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 inherent flexibility in the Wärtsilä dual fuel engine, says Mr Aminoff, means that it retains all the efficiency and environmental benefits of a pure gas engine, but can run on a wide variety of fuels, depending on how it is optimised, including low-quality well gas, LNG, MDO and HFO, and, in the case of the 50DF, even on a mixture of fuels (fuel sharing). 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 superior to pure gas engines for ship propulsion. Taking a typical gas-fuelled installation, such as that on an offshore supply vessel, with twin dual fuel 34DF main engines and a 20DF auxiliary, 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.
A pure gas installation is, theoretically, simpler, in that only one type of fuel needs to be catered for onboard. However, redundancy considerations mean that more than one gas tank, not to mention dual pump rooms and dual piping, are needed: “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, it is highly possible that 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 requires at least 48% more gas capacity. And because gas tanks are very costly, and liquid fuel systems comparatively cheap, the total cost of a dual-fuel installation will always work out significantly less than that of a gas-only system, despite the dual fuel engines being more costly. There are other considerations too – extra gas tank capacity and pump rooms can impact on payload, reducing revenue-earning capability, whereas liquid fuel tanks can be accommodated in otherwise unused spaces. 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.
Another criticism that has been levelled at dual-fuel engines is that they perform less well in gas mode than a pure gas engine. Although it is true that an engine optimised for a single fuel will have the edge over another that is, in effect, a compromise, Wärtsilä says that current dual-fuel engines can be optimised for either gas or liquid operation, or a balance between the two, giving flexibility benefits. Development work continues on further optimisation of dual-fuel engines, which will reduce the gap still further.
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 80s, compared with 30s in diesel mode. However, most gearboxes and thrusters will not reliably accommodate such a rapid increase, so in normal operation 100% power is reached in about 300s. These figures compare well with the W32 diesel engine. In terms of actual loading figures, when running at constant speed the 34DF will, in gas mode, reach 100% load in about 30s.
Independent third-party simulations of a crash stop on a ferry showed that a 6L34DF engine actually performed better than either an MDO-fuelled engine or another maker’s pure gas engine, slowing from 16 knots to zero in 76s, compared with 86s for MDO and 92s for pure gas, representing an advantage in distance terms of about two vessel lengths.
Tests we witnessed at Vaasa demonstrated that the 34DF was 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 the medium-speed dual-fuel field will be concentrated on reducing methane slip and further engine optimisation. Currently methane slip on the 34DF engine in gas mode is running at around 6g/kWh at normal load, though as mentioned previously, this still represents about 25% reduction in GHG emissions compared with a diesel engine. This has been achieved through software tuning, and further optimisation, through skip firing (cutting out some cylinders at low load) and further optimisation for gas-running is expected to reduce methane slip to around 4.5g/kWh for the 2013 model year. One drawback is that the further gas optimisation might compromise the engine’s ability to run on HFO, but this is not seen as a major disadvantage in most 34DF applications. The 50DF engine currently exhibits about 5g/kWh methane slip, and this is expected to be cut to under 4g/kWh, even at 25% load. It should not be forgotten that even a pure diesel engine can produce small quantities of methane, around 0.5%.
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 Wartsila’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 sever 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.
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