METHANE OXIDATION: HOW TO HELP LNG KEEP ITS PROMISES

Originally, the promise of LNG for the maritime industry had been the same performance for a reduced greenhouse gas contribution of 80%

8 Modern lowpressure and high-pressure dual-fuel lowspeed engines have seen significant reductions in their methane emissions since the first engines entered the market

As well as a specific reduction in CO2 emissions, liquefied methane burned very cleanly, in stark contrast to heavy fuel oil, with vanishingly little particulate matter (PM), and sulphur emissions. Even the more difficult to address nitrogen oxides (NOx), arising not from any particular impurity in the fuel but through incomplete combustion of atmospheric nitrogen, were lower with LNG.

So it came as a shock to those who had not realised -- or had pretended not to realise – when the IMO Fourth GHG Study 2020 revealed that methane engines leaked a whole lot. As a gas with a much higher global warming potential (GWP) than carbon dioxide, a comparatively small amount of leaked methane in ship exhaust, or ‘slip’, could quickly mitigate much of that 20% reduction in CO2 emissions.

SEA-LNG, an interest group whose membership includes at least nine fossil fuel companies, proposes that bio-LNG, captured from biogenic sources of methane such as municipal sewage and compost, may one day take over from fossil methane, and that LNG-fuelled ships are a crucial “pathway” to these circular-economy fuels.

SEA-LNG has criticised what it has called “sensationalist claims” regarding methane slip, as “a transparent attempt to distract the industry from investing in LNG.” The group cited one study which showed a diesel-cycle LNG engine was able to reduce greenhouse gas emissions – whether CO2 or LNG – by 23%, compared with a diesel VLSFO equivalent. Another interesting finding was that the difference between an LNG dual-fuel diesel engine and an otto-cycle engine was substantial; the latter experienced higher methane slip.

In fact, MAN Energy Solutions (MAN) has suggested that the addition of direct gas injection, as seen in its range of MEGI engines, can amount to a reduction of as much as 90%, with a recorded maximum methane slip of 0.2%.

Despite this finding, manufacturers of LNG-powered engines have been unsatisfied with their level of methane slip, and have sought to devise solutions. Drastic reductions have been achieved by tweaking combustion variables, and a certain amount of progress has also been attributed to exhaust gas recirculation (EGR), which has been a reliable method for finishing off combustion in other cases, such as with NOx in conventional liquid-fuel engines.

This is the preferred method for Winterthur Gas & Diesel (WinGD), using its intelligent control by exhaust recycling (iCER) on its X-DF series of two-stroke engines. Introduced in 2020, the system, which cools and recirculates exhaust gases, allows for a reduction in methane slip of as much as 50%, WinGD has claimed.

Steve Esau, Chief Operating Officer at SEA-LNG, told The

Motorship that methane slip is “effectively solved” in twostroke diesels, “which make up more than half of the LNGfuelled vessels on order today.”

MAN, meanwhile, claims that it has halved methane slip from its engines over the last 15 years. Like SEA-LNG, MAN notes that substantial reductions are available on four-stroke engines by switching from otto to diesel cycle with the addition of direct gas injection. This is because a long period spent around the inlet and exhaust valves in the otto process leaves abundant time for gas fuel to evade combustion.

Nipped in the bud

But it all adds up. What is needed, then, is a strategy to deal with the emissions after they leave the engine, and for this, the engine makers are reviving an old technology, methane oxidation. “Methane oxidation technologies are currently being lab tested by OEM’s where they are showing promising results,” said Esau.

MAN embarked on a project to improve LNG dual-fuel engine emissions with methane oxidation. The premise is to use a catalyst, and heat in excess of 500°C, to displace the hydrogen from methane (CH4), turning it into carbon (CO2) and water (H2O).

Some progress has been made. “The exhaust gas temperatures from MAN’s four-stroke SI and DF engines are in a range that supports the oxidation of methane,” Alexander Knafl, Vice President and Head of R&D Four-Stroke, MAN energy Solutions, told The Motorship

There are a number of precious metals which can be used as catalysts for oxidation: palladium, rhodium, and platinum. The choice of which is best suited to the application is a function of the heat being applied, with palladium offering the best characteristics at low temperature. At 500°C – the temperature being regarded as a minimum by engine manufacturers – theoretical conversion efficiency with a palladium catalyst reaches close to 100%; for platinum, it is closer to 40%.

Because 500°C is the minimum exhaust temperature required, methane oxidation or ‘oxycat’ technology is being touted only for four-stroke engines. “We started developing oxicats as a countermeasure to methane slip in 2017, and we’ve made great progress,” explained Knafl

According to the consensus among engine manufacturers, a methane slip reduction of 70% ought to be possible using methane oxidation technologies. Knafl declared that MAN had managed to meet with this expectation. “In the laboratory we have achieved our 70% methane slip reduction target.”

MAN will put oxicat into operation later this year, Knafl said. “The first pilot installation aboard ship in commercial operation will start in 2023. We anticipate this technology will be available as a commercial solution for methane slip reduction around 2025.”

The future of LNG

A new idea would eliminate methane slip at the source – and a number of other nasties, too. A company called Rotoboost has developed a similar process to methane oxidation, but designed to be applied to LNG fuel before it reaches the engine, rather than after.

Approved in Principle by classification society ABS in late 2022, and Lloyd’s Register as of March this year, the ‘Rotobox’ process uses a combination of heat and catalyst – in a similar vein to oxycat – to separate liquefied methane into hydrogen gas and solid carbon, rich in graphite. Being solid, the carbon is straightforward to store, requiring between four and six times less volume than CO2 gas. Trivial to offload from a vessel, the graphite can even be sold as a commodity, used to make electrodes and even fuel cells.

Though it would not do anything to stem the demand for fossil methane, the Rotobox would enable existing vessels with LNG fuel tanks and lines to convert to hydrogen operation – and zero emission -- at reasonably low cost, and overcome issues relating to hydrogen’s low volumetric energy density.

The system would, LR Chief Commercial Officer Andy McKeran said, “…solv[e] the methane emissions perception in the industry through technology and evidence,” and would “enable LNG to become a future fuel that is readily available today, subject to affordability – which ranks higher than any other alleged lower emissions fuel available today.”

Despite its problems, SEA-LNG argues that LNG is a frontrunner thanks to the ease with which it can be replaced by biogenically-sourced alternatives. The methane given off by the decomposition of organic matter is practically the same as that which comes out of the seabed in the Gulf of Mexico.

“Bio-LNG is pure methane, so is identical to the highest quality LNG (in terms of methane number),” said Esau. “It can be used in existing LNG-fuelled engines, storage tanks and gas systems without any modification.”

However, if there is to be a bio-LNG revolution, it will suffer from the same issues on a tank-to-wake basis as LNG does now, if not addressed, he explains. “In terms of methane slip bio-LNG will be identical to LNG as methane slip depends on the engine technology and not the form of LNG used -- i.e.