The down side of dual fuel, by Rolls-Royce

Speaking to an international marine press party in Ulsteinvik, Norway, Oddbjorn Eliassen, Rolls-Royce Marine president, merchant, disagreed with the views presented in favour of the dual-fuel option by most of the speakers at the recent Mercator Media Gas Fuelled Ships conference in Rotterdam.

Eliassen said that the gas-only engine would always be a better engineering solution than competitors’ combined dual-fuel engines, which cannot be optimised for either gas or diesel. A gas-only engine can be significantly more efficient, with the Bergen lean-burn option recording figures of 50.3% efficiency, compared with a figure of 45% to 46% for diesels, less for dual-fuel versions.

Apart from the superior specific fuel consumption, the gas-only solution avoids having to provide both gas and diesel tanks and fuel systems on board, which incur extra capital cost and reduce potential cargo volume.

Another problem with dual-fuel engines is the poor transient load performance, taking some 2 minutes to respond at part load in gas mode, which, according to Rolls-Royce makes them unsuitable for use in harbour, which is exactly when the emissions benefit of gas fuel is most needed.

Pure gas also, says Eliassen, produces less methane slip than dual-fuel engines in gas mode. As methane is a powerful greenhouse gas, some 21% to 25% more aggressive than CO₂, this is another significant consideration.