Double bind: an industry caught between issues
The maritime industry is currently trying to make sense of clashing emission requirements, writes Stevie Knight
Although LNG has helped bring down overall airborne emissions, four-stroke gas and dual fuel engines have been caught in “a trade off” between mitigation strategies says Dag Stenersen of SINTEF Ocean.
On one hand, nitrous oxides are typically associated with peak flame temperatures high enough to bind the combustion air’s nitrogen and oxygen. On the other, methane issues originates from a small amount of premixed gas and air hiding in nooks and crannies as well as a lower temperature combustion, says Stenersen.
It’s largely the result of an early focus on NOx emissions and efficiency, both of which helped to push the four-stroke market far toward lean-burn operation: in fact, this “is typically double” the stoichiometric ideal, the amount strictly necessary for complete combustion says Paolo Tremuli of ABB Turbocharging.
Tremuli adds that beyond NOx reduction, lean burn has related advantages: “The engine is less sensible to knocking and as the thermal loading of the components is lower, the power density of the engine can be increased.”
But now it seems there’s a price to pay. “While a lean mix reduces the NOx output by decreasing flame temperature, at low loads the combustion will meet cooler, quenching areas as it advances along the cylinder liner,” as a result, the temperatures simply don’t reach the 500C-plus temperatures necessary to burn the methane, explains Stenersen: These, along with those molecules hidden between the piston top and cylinder liner, around the gasket, or behind the anti-polishing ring “get pulled out during the exhaust stroke”.
Unfortunately, as Tremuli notes, despite the phenomenon being stronger at part loads and the slip – when it does occur - amounting to a tiny fraction of the total exhaust volume, “it has a greenhouse effect around 25 times that of CO2”, says Georg Wachmeister of KIT’s Institute of Internal Combustion Engines. Multiply the effects and it stands to erode those hard-won carbon gains.
Engine manufacturers have nibbled away at the issue but there are limits: firstly, to what eliminating troublesome dead volumes can achieve: as Stenersen points out, the engines may have reduced methane slip, but the amount that’s left is still “significant”. Further, although quenching characteristics can be reduced by enriching the mixture, there’s a danger of sliding too far in the other direction, raising NOx to unacceptable levels.
It isn’t straightforward to find a control strategy that answers these troubles – but having said that, there’s reason to dig deep for a sweet spot. As Wachmeister explains: “The correlation between the air-fuel ratio and emissions is not linear, for each engine there will be regions [varying with mix and load] where both NOx and methane reduce, then others where the NOx rises again.”
Therefore, says Stenersen, it primarily comes down to ignition control. While the low-pressure dual-fuel variety has the advantage of fuel flexibility, the issue is, he explains, that they face “a narrow window” of combustion as they ignite a gas and air mix in a diesel-like compression stroke with a few drops of pilot oil. “Too low on air and you have problems in ignition... but there’s the potential for knocking if the mixture gets too rich,” he adds.
Interestingly, Rolls-Royce subsidiary MTU has considered both low-pressure dual fuel and lean burn spark ignition (LBSI) alternatives along the way but Manuel Boog explains a similar choice drove the decision: “One of the original reasons for moving into low-pressure pure gas instead of dual fuel solutions was because there’s always a compromise between knocking and a high enough temperature to ignite the diesel, in the pure diesel mode, so we decided to optimize the LBSI which enables IMO3 NOx limits without SCR”.
By contrast, “a single fuel engine doesn’t have that issue, it can be optimised in any direction” points out Boog’s colleague, Peter Kunz.
That doesn’t mean it’s easy, just easier. Indeed, MTU spent a good few years bringing these divergent emissions concerns together in its latest Series 4000 mobile gas engines, the new 16 cylinder version coming out this year with the 8V model due in 2021/22.
Kunz argues for realism: “We have been looking deeply into a lean burn development that’s still optimised for as low as possible methane slip,” he says adding: “You can’t completely avoid it, but you can mitigate it.” He points out the emissions are “well under the only methane regulation in existence at the moment, the EU Stage V limit” of 6g per kWh.
There were, of course, related issues. While MTU has based these high-speed engines on existing stationary power technology, “we had to further develop it for the mobile market” says Kunz. That meant tackling something that’s traditionally been a tough call for gas engines: “One big goal was to make it as dynamic as possible – it had to follow the acceleration and deceleration curves you find in marine applications.”
To achieve this, the fuel input was redesigned, moving from single point to multipoint injection with a control valve sitting in front of each cylinder. “Our stationary gas engines have one valve upstream the turbocharger, but that’s a long way for the gas to travel,” he explains. “To keep up with variations in engine speed you need that distance to be much shorter.” Further, the valves on the S4000 engine support a flexible injection strategy, optimising mixture quality for combustion stability at each engine operating point. As a result, “the dynamics are now comparable to a standard diesel engine” says Kunz.
This should make a big difference to overall ship design “as it means you don’t have to move to a diesel-electric configuration for performance stability, you can retain the shaft drive,” says Boog. It’s good news for vessels such as the Lake Constance ferry which is installing the very first 8V version with “similar characteristics” for load response the diesel equivalent.
Shortening the run also helped with “the other big challenge”, making the engine gas safe. “We could have made our life easier with an emergency shut down design, but this way you have a more robust operation,” says Boog.
Methane can be oxidised to carbon dioxide and water - so, what about introducing a catalytic system to do just that?
Once again, the challenge is methane’s stability. As Addy Majewski of DieselNet explains: “Catalytic systems usually work on other hydrocarbon emissions because they have longer molecules: methane’s is particularly short, so it’s harder to break – and needs very high, 500C-plus temperatures to do it.”
Because of this, the most effective are still higher-temperature palladium-based chemistries, but they have an issue: “These are de-activated by just a few parts per million of sulphur,” says Majewski.
Put simply, reduced sulphur compounds (sulphides) exhibit an affinity for palladium and effectively clog it up by forming deposits and reducing the availability of the catalytic surface.
According to Wachmeister, the Institute’s recently released research suggests that there’s a plausible solution to the problem: “Right now we are looking at strategies to reactivate the catalyst while in use.” There are a number of techniques but the easiest to achieve in situ is an extra burst of heat to burn the sulphur off. He explains: “One way to do this is to have occasional gas-rich ‘pulses’ that change the fuel-air ratio for a few seconds.” Testbed trials show this seems to be a workable solution.
It’s not the only answer: “Palladium, with a little platinum, is the best we have at the moment but a lot of research – including ours - has been trying to find alternatives able to work at lower temperatures”, says Wachmeister, although both he and Majewski admit that this tantalising possibility still appears some way off.
There’s another, related challenge, says Boog: “A palladium-based catalyst often has to be positioned in advance of the turbo because the airflow after the turbine is too cool, requiring a huge surface area to do the same job.” However, positioning it in the hotspot can impair engine dynamics as “a pre-turbo catalyst slows the flow,” he adds.
Interestingly, this is where bigger turbochargers – over 500kW apiece - have an advantage: “The oxidation of methane increases the gas temperature which partially compensates for the pressure losses,” says Tremuli. “All in all, larger turbochargers can survive the catalyst with a small penalty in efficiency.”
Still, many installations could benefit from a little help and MTU aims to lift efficiency with an electrically supported unit, explains Boog. The idea originally came from G+L innotec, and like many good ideas, it’s fairly simple. An electric motor is fitted in front of the turbocharger compressor ensuring that extra air is available to feed the engine on ramp-up by building up boost pressure. As the turbocharger is no longer relying entirely on the energy coming out of the exhaust, it allows its operating point to be decoupled from the speed of the diesel engine.
The first MTU engines with electrically assisted turbocharging (E-ATL) should enter the market in 2021, although one advantage of the system is that it may be possible to retrofit existing installations with the technology without creating additional stresses on the turbo rotor.
Despite all this, low-pressure dual-fuel engines might have other tricks up their sleeves.
For example, Stenersen explains that a couple of engine manufacturers are looking at so-called ‘skip firing’.
Both Caterpillar and Wärtsilä have investigated this possibility: according to Niclas Liljenfeldt of Wärtsilä, active cylinder technology, cylinder deactivation or skip firing has already been utilised in various non-marine applications. In the automotive industry, its aim is mainly fuel economy, but there are also emission control benefits – including methane mitigation: “The main drivers to introduce active cylinder technology (ACT) for Wärtsilä gas engines [is] to reduce the unburned hydrocarbons as well as to improve the efficiency at part loads,” writes Liljenfeldt.
But, he adds that noise, vibration and harshness (NVH) always needs to be taken into consideration when ACT is introduced, with the manufacturer looking carefully at the impact on both the base frame and flexible couplings. In other words, one of the significant challenges is that it can present both the engine and close connections with a rough ride.
There’s also the potential for close-coupled SCR systems to play a part in handling the NOx side of the equation, leaving methane to be targeted more effectively.
Having said that, developments of this kind remain, for the time being, outside commercial application and a real advance on slip may demand more fundamental change - such as switching to a high-pressure dual fuel strategy.
Despite its other successes, it seems certain the industry will have to rethink some of its strategies in order to meet the physically small but pernicious methane issue. According to an analysis by Transport & Environment, ‘from the perspective of WTW (Well-to-wake) CO2 equivalent emissions, the change for better or worse compared to existing marine fuels depends on the level of methane leakage. It adds that with a rate of methane leakage of just 3.5%, ‘the savings are nullified’.
Therefore, if we are to meet the IMO target of halving our GHG emissions by 2050 then it seems sensible that new propulsion technologies should be able to demonstrate how they mitigate or avoid this considerable greenhouse threat altogether.
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