LNG – the Pros & Cons
Aricle presents the arguments for and against the use of liquefied natural gas (LNG). First, Rick McArthur presents Wärtsilä’s case for dual-fuel propulsion with gas combustion technology as the low-emission, economical alternative to conventional fuels.
Luxury vessels such as superyachts are a visible expression of wealth. For owners and operators who want to assert their environmental credentials – vessel operation without smoke or particulate emissions married with the lowest possible levels of carbon dioxide and sulphur oxide – dual-fuel propulsion with gas combustion technology offers the most comprehensive solution.
Three approaches can be employed to reduce the emissions generated by vessels powered by fossil fuels: using conventional liquid fuel and adding emission-reduction solutions to the engines; using LNG as the only on-board fuel; or specifying dual-fuel technology, which allows operation using the most economical option – marine diesel oil (MDO) or LNG – in all circumstances.
The strength of dual-fuel technology is its ‘fuel flexibility’: dual-fuel solutions make it possible to use MDO in transfers and LNG when operating in port, close to shore or in an emissions control area (ECA). Using LNG as fuel can also reduce a vessel’s operating costs as owners and operators are free to opt for the most suitable fuel.
SIMPLE & EFFECTIVE
Using LNG with combustion technology means that no exhaust gas treatment is required in order to comply with NOx emissions requirements, as the emission levels fall well within the parameters. SOx levels are practically zero, no particulates (soot) are generated and CO2 emissions are also significantly reduced. More than 100 vessels in merchant, offshore and ferry applications are already successfully operating on gas.
Natural gas consists primarily of methane, a potent greenhouse gas (GHG). “In overall terms, dualfuel propulsion solutions, applying lean-burn technology, produce approximately 10 per cent less GHG emissions than a diesel engine of equivalent output running on MDO,” said Tomas Aminoff, director of product management at Wärtsilä Ship Power.
From a design viewpoint, the biggest challenge when using LNG is providing the on-board space required for fuel storage. Specific bunkering arrangements also have to be provided. In dual-fuel configurations, as LNG will only be used in port and when operating in ECAs, the storage tanks can be smaller than if it was the only onboard fuel.
MAXIMISING COMFORT & CONVENIENCE
The design for an LNG-powered superyacht already exists. In a joint project, Fincantieri Yachts, Wärtsilä and Stefano Pastrovich of Pastrovich Studio have developed a new concept in which Wärtsilä’s dual-fuel technology is fully integrated into a world first – the 99m, dual-fuel, IMO Tier III-compliant XVintage motoryacht (see issue 131 of TSR, pages 64-73). The XVintage power and propulsion system is based on Wärtsilä 20DF dual-fuel engines, compact units, which provide all the advantages of fuel flexibility. Fully compliant with upcoming IMO Tier III regulations when operating in gas mode, they can be switched between fuels without any loss in speed or power output.
The dual-fuel gensets in XVintage are equipped with alternators mounted on a common bedframe and utilise double-mounting arrangements, meeting the most stringent standards for levels of noise and vibration.
BUNKERING & FUEL STORAGE
All fuels require safe handling arrangements and LNG is no exception. Pumped from one location to another through pipelines in gaseous form or transported by sea in liquid form, the transformation phase from gas to liquid requires cryogenic temperatures.
THE AVAILABILITY OF LNG
LNG carriers move natural gas from liquefaction terminals to re-gasification terminals all over the world, and LNG is available at all these shore-based facilities. Marine LNG import and export terminals are to be found almost everywhere, so LNG is basically available anywhere in the world, and several new terminals are scheduled to come on stream in future years.
What is missing at this time is the infrastructure to transfer LNG from these terminals to the vessels that require it. The availability of LNG bunkering for vessels utilising LNG as marine fuel is also spreading widely – mainly in environmentally sensitive regions or in locations where the same gas price makes this propulsion alternative particularly appealing.
Emissions performance: Dual-fuel engines in gas mode (based on Wärtsilä models)
NOx – lower than IMO Tier III levels
SOx – practically zero
Particulates – negligible
CO2 emissions – approximately 25 per cent lower than with liquid fuels