Despite its many virtues as a marine fuel, LNG has an Achilles heel – methane slip
Greenhouse gases (GHG) or nitrogen oxides? In the fight to reduce emissions, it appears that shipping companies are being forced to make a choice to avoid one or the other, but not both.
A series of recent studies show that LNG-fuelled engines give off less CO2 than diesel-powered units because LNG contains less carbon and more energy per mass unit. However, the fuel’s widely acknowledged Achilles heel is methane slip, the loss of a GHG into the air.
Class society Bureau Veritas explains in a white paper issued in September entitled In Engines as well as Cargo Tanks, LNG is Powering Ahead: “Caused by incomplete LNG combustion in the ship engine, methane – a potent GHG – is emitted into the atmosphere, partly offsetting the CO2 emissions reduction.” The paper continues: “This is a key area for technological development, as experts are working to improve the design of engines to limit methane slip.”
Nor is there any time to lose, particularly for the deep-sea fleet which has fewer propulsion options than short-sea vessels, which can resort to alternative forms of propulsion such as batteries. As the LNG fleet expands – the global order book for 2019 lists 17% of LNG-powered newbuilds in terms of gross tonnage – the pressure is on engine makers to cut methane slip to negligible levels.
IMO is also on the case, tasking working groups in 2020 to come up with “concrete proposals to reduce methane slip and emissions of volatile organic compounds.” In time this may lead to engine-makers being handed regulations that will force them to reduce methane slip in dual-fuel and gas engines.
“For various reasons there is high uncertainty about which fuels will be used in the marine sector in the future,” pointed out the director of research and development for injection systems specialist Woodward L’Orange, Clemens Senghaas, at the Cimac conference in 2019. “Ignoring methane emissions would lead to a dangerously wrong choice of technology.”
Methane slip comes down largely to the efficiency of the combustion cycle. Citing recent studies with high-pressure engines, class society DNV GL explains that they can be tuned to slash tank-to-propeller GHG emissions by between 20-40% compared to marine gas oil-fuelled propulsion. By comparison,n low-pressure engines reduce such emissions by between nothing and 18%.
Enginebuilders are now toiling to reduce the amount of pollutants escaping from the engineroom, whether methane or carbon. For instance, MAN Energy Solutions, which has been working on methane slip for years, says its towering ME-GI engine using high-pressure gas injection “dispenses with the need for power derating and eliminates the significant problem of methane slip and resulting CO2 emissions.”
Yet while the transition from heavy fuel oil continues to pose challenges, many see it as a long-term environmental benefit. “Moving away from burning high-sulphur fuels in marine propulsion engines yields an opportunity to develop novel technologies for both the combustion chamber and the design of lubricating systems,” enthused Switzerland-headquartered WinGD’s director of research and development, Konrad Räss, citing improved resistance to corrosion among other important consequences that will be welcomed in the engineroom.
In short, the emissions regulations offer a window of opportunity which WinGD believes will encourage new technologies that deliver savings in terms of capital and operating expenditure for its two-stroke engines.
In the fight to reduce methane slip, the choice of fuel is a vital factor. And here LPG has its supporters. Like LNG, methanol and ethane, LPG burns cleanly with low emissions while releasing relatively little methane. The global warming potential value – an important number for regulators – of propane and butane, basic elements of LPG, is relatively low.
The fuel has other virtues, such as being plentiful on global markets. Unlike LNG it does not have to be shipped at cryogenic temperatures. It is more easily stored, is a long-established fuel with an extensive global infrastructure and it has lately become cheaper to buy because of a steady increase in available volumes.
Alfa Laval’s Robert Comelli, business manager for fuel conditioning systems in the marine division, is a supporter of LPG. He told the Cimac conference in a paper entitled Innovative LPG fuel supply system for MAN B&W LGP-P engines: Design challenges and performance results: “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.”
Alfa Laval has adopted a multi-pronged solution to methane slip in LPG. Its engineers developed a new pumping technology, a high-pressure heat exchanger, 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.
However, no fossil fuel ticks all the boxes in methane slip. As Mr Comelli also pointed out, LPG may contain contaminants, for instance particles of sand and dust, that can clog up fuel lines and lead the vessel to stop and vent fuel.
Not resting on its laurels, in a joint project with MAN Energy Solutions, Alfa Laval is working on sea-going engines that will run on even cleaner fuels than LPG. “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”, he said.
Encouragingly, progress is being made. Woodward L’Orange has developed a family of high-pressure dual-fuel injectors for high- and medium-speed gas and diesel-powered engines that, the company believes, reduces methane slip to “an absolute minimum”. As Mr Senghaas told the Cimac conference in a paper called New injector family for high-pressure gas and low-caloric liquid fuels, Woodward L’Orange has high hopes for the technology in the post-2020 world: “The proposed injector family is [designed] to inject several low-energy or gaseous fuels. It is ready to provide customers [with] a broad range of new combustion options.”
But does methane slip matter that much? Most of the debate about methane slip refers to LNG-fuelled vessels, but advocates argue that LNG is only a bridge – or interim – fuel on the way to a zero-emissions future and that methane slip is a small price to pay for its other virtues. Unsurprisingly, the general manager of the lobby group Society for Gas as a Marine Fuel, Mark Bell, agrees with this assessment: “It would be totally unfair to hang LNG on methane slip.”
The operative issue though is how much methane slip is there? According to a study compiled by the Society for Gas as a Marine Fuel and Sea/LNG entitled Life Cycle GHG Emission Study on the Use of LNG as a Marine Fuel released in April 2019, there is not a lot. Slow-speed dual-fuel engines showed methane emissions of 0.14 grams per kwh, which adds up to less than 1% of the total well-to-wake GHG emissions of the engine.
However, the numbers for the low-pressure injection system and combustion of Otto-cycle engines are higher. The dual-fuel power unit released methane emissions of 2.1 grams per kwh, adding up to 11% of total GHG emission from well-to-wake. To confuse the issue further, another paper presented at Cimac entitled Methane Slip Summarised: Lab v. Field Data suggested that methane slip may be higher in sea-going rather than test-bed conditions.
Meantime, what is the likelihood of the emergence of fuels that do not release any GHG?
As Bureau Veritas points out, biofuels offer a highly desirable carbon-neutral solution that is increasingly available as a marine fuel, although in small quantities. In fact, biofuels are in such short supply that not even kerosene-burning airlines or diesel-fuelled trucks can get enough of them, let alone shipping companies. However, volumes are growing; Mediterranean Shipping Company announced in mid-December that it is following up successful trials by loading up its cruise ships with 30% blends from bunkers in Rotterdam.
Although highly attractive, synthetic alternatives lie far over the horizon. Volumes of synthetic methane and bio-methane are extremely small and expensive. Perhaps closer, although not by much is ammonia, a ‘clean’ fuel based on one of the world’s most widely used chemicals.
Between now and 2030, when IMO’s tougher decarbonisation regulations will apply, the race is on to produce cleaner fuels in big volumes. As Wärtsilä’s director for marine in East Asia and China, Osten Lindell pointed out ahead of early December’s Marintec China 2019: “The single biggest contributor to lowering emissions is fuel choice.” Citing bio-LNG, synthetic LNG, ammonia and hydrogen as examples, he added: “It is clear that the industry needs to adopt greener fuels.”
Meantime, engine-builders are pulling out all the stops in the fight to reduce methane slip before regulators vote to get tougher.
The combustion engine’s problem child
Methane is slipping everywhere, not just from ships’ engines. It is escaping from stationary gas-powered industrial engines in wastewater treatment plants, for example, as well as from internal combustion engines in all kinds of vehicles.
“The problem child in engine exhaust emissions is total hydrocarbons (THC), consisting substantially of methane,” explains Caterpillar’s Heinrich Baas in a paper released in July 2019 entitled Emission Reduction for Combined Heat and Power Plants. Scientifically speaking, THC represent methane that has passed through the combustion process unburned and then into the exhaust gas flow. “This is a thorn in the side of development engineers because ideally this methane should burn in the combustion chamber, further increasing efficiency.”
Over the years, the amount of slip has been reduced by engineering modifications, for instance to the geometry of the combustion chamber. “However, from an environmental perspective this is not good enough as the GHG potential of methane is about 23 times that of CO2,” Mr. Baas notes.
In Europe alone, Caterpillar is engaged in a range of intensive projects the purpose of which is to find catalysts that slash methane losses, but so far these have not survived in industrial applications. “This area needs further research,” he notes.
And time is clearly running out.