Steep rise projected for dual-fuel applications

30 Nov 2011
The Wärtsilä 50DF tri-fuel medium-speed engine can switch smoothly between natural gas, light fuel oil and heavy fuel oil during operation

The Wärtsilä 50DF tri-fuel medium-speed engine can switch smoothly between natural gas, light fuel oil and heavy fuel oil during operation

Implicit in the recent news that Wärtsilä dual-fuel, medium-speed engines have topped 3million running hours in marine and landside applications, the technology has attracted wide endorsement as a viable solution to environmental and economic challenges facing the respective markets. In fact, DF capability is spearheading the company’s technological drive in engine design.

David Tinsley writes: About half the total operating hours of the 470 engines of DF type in service are attributable to plant at sea. Most of the vessels powered to date by dual-fuel engines are offshore support ships, LNG carriers, and passenger ferries. Wärtsilä is convinced that within the next 10 years, the number of DF-powered vessels will have grown to more than 1,000, with the accelerated uptake driven by the expansion of designated Emission Control Areas (ECAs) coupled with operating cost considerations. When running on gas, Wärtsilä DF machinery already complies with IMO Tier III criteria governing NOx(nitrogen oxide) emissions.

The essential concept of DF technology is to provide the means of utilising natural gas as a fuel offering low levels of harmful emissions plus price competitiveness, while retaining the option to switch to liquid fuels, if required. The change between fuel types can be made automatically, and without loss of power.

In gas-burning mode, the engines operate on the lean-burn principle, whereby the mixture of air and gas in the cylinder has more air than is needed for complete combustion, reducing peak temperatures. The formation of NOx is accordingly greatly reduced. Furthermore, SOx (sulphur oxides) emissions are eliminated, as natural gas contains no sulphur, while CO2 formation is lowered because the gas contains less carbon per unit of energy than liquid fuels. Since natural gas has no residuals, the production of PM (particulate matter) is practically non-existent. NOx reduction in gas mode is in the order of 85%, while carbon-based greenhouse gas emissions are curbed by at least 15%.

The DF offering from Wärtsilä has been augmented by the 20DF series, with initial deliveries in 2011. The largest engine in the range, the 50DF, has been used in marine applications since 2006, fostering a step change in LNG carrier propulsion. Dual-fuel diesel-electric power based on Wärtsilä 50DF engines has effectively broken the longstanding supremacy of steam turbine plant in the LNGC newbuild sector.

A broader market is developing for the 50DF, however, as expressed in the nomination of the design for Viking Line’s 56,850gt newbuild Baltic ro-pax ferry, and in the modification of the two Wärtsilä 46 heavy fuel main engines in the 25,000dwt product tanker Bit Viking to become 50DF engines. Operated by Statoil along the Norwegian coast, the conversion will allow the tanker to qualify for lower NOx taxes under the Norwegian government’s NOx Fund scheme.

The 34DF series is most commonly applied in the offshore service and support sector. In a marine context, the main scope for the new 20DF type is seen as a mechanical-drive prime mover for powering tugs, ferries, and small cargo vessels operating in particularly strict emission regimes, and as a harbour genset drive for a wide range of vessels. The opening contract for the 20DF entails two six-cylinder models as the auxiliaries in an 89m newbuild platform supply ship ordered by Eidesvik Offshore of Norway. The main engines will be two six-cylinder examples of the 34DF in a gas-electric propulsion system.

Wärtsilä incorporation of advanced electronic combustion control has paved the way to improved performance and greater reliability in gas-burning engines. The flexibility to shift between natural gas, heavy fuel oil, light fuel oil and potentially other liquid fuels confers numerous practical benefits to vessel owners and operators. These advantages will be underscored not only by tightening environmental legislation but also by sustained volatility in oil prices.

While further technical advance in conjunction with regulatory pressures will promote growth in the use of DF powering solutions, the geographically limited LNG bunkering infrastructure imposes a constraint. Norway has provided a beacon for the industry with its development of a gas fuel supply network, and investment in distribution facilities serving shipping is increasing in certain other areas.

A recent cooperative agreement between Wärtsilä and Shell Oil promises significant improvements in the availability of LNG as a marine fuel. Under the pact, initially focusing on US Gulf Coast sourcing, Shell will provide supplies to operators of natural gas-fuelled, Wärtsilä-engined vessels. The arrangements will be subsequently extended to cover a wider geographic range.

The need for onboard LNG storage and handling systems has also been seen as a restricting influence on the adoption of gas fuel. Addressing this area of ship design and engineering, Wärtsilä devised the LNGPac integrated solution, encompassing the insulated LNG storage tanks, ship-to-shore bunkering connections, pressure control valves and associated automation systems.

The environmental sustainability of DF engine technology is not simply a factor of the comparative cleanliness of natural gas as a hydrocarbon fuel, but also the performance efficiency achieved during leaner combustion. This in turn leads to lower levels of exhaust emissions. Gas has to emerge as a mainstream fuel if the inter-related economic and environmental challenges that face both the marine and power generation industries are to be met, considers Wärtsilä.

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