Breakthrough paves way for broader use of LPG

When Alfa Laval’s engineers devised a fuel supply system for MAN Energy Solutions’ methanol-fuelled marine engines in 2014, it was a breakthrough for low-flashpoint fuels and the power units were quickly put to work on chemical tankers. But the Swedish specialist in fluid transference has not been resting on its laurels in the years since.

Building on the 2014 system – technically known as a fuel conditioning module – Alfa Laval has now adapted it for LPG-fuelled two-stroke engines in yet another advance that promises to turn LPG into an important alternative for the maritime industry in the low-emission era.

Technically, the adaptation presented a considerable challenge. Unlike methanol, LPG must be pumped to the engine at a higher supply pressure in a way that avoids phase changes. The system must be able to process a broad spectrum of propane and butane that may contain high levels of ethane, plus smaller amounts of nitrogen. Finally, compared to other fuels, LPG demands a more complex strategy for recovery during stops and purging sequences. Although not illegal at present, uncontrolled venting into the atmosphere is frowned on by IMO.

As Alfa Laval’s Roberto Comelli, business manager for fuel conditioning systems in the marine division, told a conference of the International Council on Combustion Engines (Cimac) in Vancouver in June, the project required a multi-pronged solution. The company’s engineers had to come up with a new pumping technology, an effective high-pressure heat exchange, and an automated control system that can pump the LPG at a rate that is constantly proportional to the fluctuating engine load that would be expected to occur in heavy weather. Alfa Laval presented the research at Cimac in a paper titled Innovative LPG fuel supply system for MAN B&W LGP-P engines: Design challenges and performance results, one of a number of presentations that included original case studies of advances in gas and diesel engines, fuels and lubricants.

But why LPG? Quite apart from having the cleaner, low-emission properties shared with LNG, methanol and ethane, LPG has lately become cheaper to buy. The fuel’s price has steadily decoupled from the general price of crude, in part because of the USA’s fast-rising exports of LPG in the last few years, now up to half a million barrels a day. Thus, LPG has become more plentiful on global markets. The fuel is now cheaper than marine diesel oil, heavy fuel oil and even LNG. Additionally, unlike the latter, LPG does not have to be shipped at cryogenic temperatures.

Another much-cited merit of LPG is that there is an existing global infrastructure. As Mr Comelli told the conference: “There is already a huge network of LPG import and export terminals in use worldwide. These are generally already built for a wide range of vessel sizes, making it relatively easy for local storage providers to add bunkering equipment in their facilities and [so] capitalise on the supply and sale of LPG as a fuel.” This is clearly not the case for LNG, methane and ethanol, for which the essential infrastructure is being built as quickly as possible.

From an emissions perspective, LPG may also be more attractive than rival fuels. As Mr Comelli explains: “LPG is favourable from a climate-change perspective. Whereas LNG is a greenhouse gas when discharged into the environment, LPG is not classified as such.” He adds that methane has a global warming potential value – a key number in the battle against emissions – as high as 28, while the value of propane and butane, basic elements of LPG, is 10 times lower. Even better, suggests Mr Comelli, LPG also scores over competing fuels in terms of easier storage. For all vessels other than dedicated LPG carriers, the fuel will be bunkered and contained in fully pressurised Type C tanks, while an LPG carrier carries the fuel in either the main cargo tanks or an existing deck tank.

Different compositions

From the outset of testing, Alfa Laval designed its system to handle the wide range of LPG mixtures on the market, all of which may react differently under combustion. Technically speaking, as the paper explains: “Each composition has a different saturated pressure curve [also known as a bubble curve] and, at equilibrium and ambient temperature, a different storage pressure. It is possible to expect different [flashing] behaviours from the fuel depending on its composition.”

However, one of the recurring downsides that LPG also shares with LNG and ethane is the presence of contaminants that can cause engine damage or even seizure. Typically, LPG may contain particles of sand and dust among other debris that would impair an engine’s performance. “Solid contaminants are even more common in LPG than in LNG and ethane, meaning the demands on fuel line systems will be greater as well,” notes the paper. “The result will be repeated starting and stopping that leads to increased venting and loss of LPG.”

To forestall these unwanted events, Alfa Laval’s system cleans the LPG before it is used as a fuel through a special filtration system that removes contaminants before it reaches the combustion phase.

Starting in early 2019, a series of rigorous tests at MAN’s facility in Copenhagen put the system through its paces with a prototype two-stroke, the first marine engine to run on LPG, according to Alfa Laval. The results proved so encouraging that Alfa Laval and the engine-designer are moving on to the next phase of development – that is, from the testbed to sea-going engines that will be run on even cleaner fuels than LPG in another important step forward in greener steaming. “The Alfa Laval technology developed thus far and the experience gained in the testing process will be invaluable when testing other zero-carbon alternatives, such as ammonia and other hydrogen mixtures,” the paper concludes.

An injection of technology

In another advance in the quest for lower emissions, German injection systems specialist, Woodward L’Orange, has developed a high-pressure family of dual-fuel injectors designed for high-speed marine engines that, the company says, works equally well for gas and diesel power plants. The system also works with medium-speed engines. Thousands of hours of dry-land testing suggest that the injectors not only reduce greenhouse gas-producing methane slip – a bugbear of gas-powered engines – to almost negligible levels, but can also be adapted to a whole new wave of sustainably produced fuels, such as methanol and even ammonia, that are now being brought to industrial scale.

A versatile and clever technology, these next-generation injectors work with common rail diesel systems that require pressures up to 2,200 bar as well as with gas pressures that typically reach 500 bar. Until now, the variation in the pressures on the diesel and gas side of the combustion system has been a recurring problem. As a bonus, although it might be expected that these injectors would be bigger than the current technology, because they have to work on both sides of the fence, as it were, Woodward L’Orange has managed to make them smaller and more compact. As the group’s director of research and development, Clemens Senghaas, told the Cimac conference, citing a paper called New injector family for high-pressure gas and low-caloric liquid fuels: “The proposed injector family is [designed] to inject several low-energy or gaseous fuels. It is ready to provide customers a broad range of new combustion options.”

From the outset, Woodward L’Orange was focussed on a versatile technology that could handle whatever fuels are thrown at it. As Mr Senghaas points out: “For various reasons there is high uncertainty about which fuels will be used in the marine sector in the future.” He adds that one of the big future question marks is greenhouse gas emissions, rather than just carbon dioxide emissions, which are the main current preoccupation of the industry: “Ignoring methane emissions would lead to a dangerously wrong choice of technology.

“Unburned methane originates from incomplete combustion on the cold cylinder walls and combustion chamber surfaces and from the combustion chamber crevices,” he explains. So far though, Woodward L’Orange’s engineers have been able to reduce methane slip “to an absolute minimum”.