With IMO mandates looming, engine makers may have to make trade-offs between meeting NOx and unburned hydrocarbon limits
Liquified natural gas (LNG) has emerged as an alternative fuel that can help the shipping industry meet sulphur, NOx and greenhouse gas (GHG) mandates. In 2014, the number of LNG-fueled vessels in operation stood at 56, according to the DNV GL Alternative Fuels Insight database. That number has now more than tripled to 170. Some 35 ships are on order and 112 have been categorised as ‘LNG-ready’ by the database; ‘LNG-ready’ means the vessel is capable of future retrofitting or LNG operation today, indicating keen interest among shipowners in the fuel. Along with this growth, the phenomenon of methane slip – unburned methane escaping through the exhaust of ships fueled by LNG – is becoming a key concern.
IMO has actively taken up the issue of methane slip. The terms of reference for related working groups meeting in November this year and early 2020 include “concrete proposals to reduce methane slip and emissions of volatile organic compounds”. Dual fuel and gas engine development may well have to factor in the reduction of methane slip among performance goals.
Advocates for the use of LNG as a marine fuel say it is a bridge fuel and it has proven benefits in reducing GHG emissions, even when methane slip data is factored in. They argue methane slip mandates will have to be realistic and take into account the overall scenario that less than 3% of the world’s GHG emissions come from shipping. They point out that it took several decades for the slow speed, two-stroke engines to achieve today’s performance levels on the dirtiest of fuels. “It would be totally unfair to hang LNG on methane slip,” says Society for Gas as a Marine Fuel’s (SGMF) general manager Mark Bell. “We have to be extremely careful and provide a balanced argument for all fuels before picking on one fuel.”
SGMF and Sea\LNG, another advocacy group for LNG, commissioned a study through consultancy thinkstep titled Life Cycle GHG Emission Study on the Use of LNG as Marine Fuel. The report, released in April 2019, considered a range of issues, among them methane slip.
The study found that slow speed dual-fuel engines showed methane emissions of 0.14 g/kWh, resulting in 4g CO2-eq/kWh, which is less than 1% of the total well-to-wake GHG emissions of the engine. Due to the low-pressure injection system and combustion in the Otto cycle, the Otto cycle dual-fuel engine showed methane emissions of 2.1 g/kWh, resulting in 63 g CO2-eq/kWh (11% of total GHG emissions from well-to-wake).
The study evaluated methane release across the entire lifecycle of the methane gas. For evaluating emissions in the tank-to-wake portion, it took extensive test-bed data provided by engine manufacturers.
Burning natural gas often imposes conflicting demands on an engine. The requirement of pushing down NOx is met by keeping temperatures as low as possible and uniform across the cylinder to tamp down thermally produced emissions. This is typically achieved by keeping the excess air-to-fuel ratio high so the air mixes well with the fuel, ensuring uniform combustion and the absence of hotspots in the combustion zone.
In such lean-burn engines, the air volume can become so high in some parts, especially at part loads, that some of the methane does not burn and leaves the engine as unburned fuel through the exhaust. NOx emissions increase with the load, while methane slip decreases with the fuel-air ratio increasing with load. These conflicts play out fully out at sea when the engines are faced with actual operating conditions.
The thinkstep study used test-bed data and noted that the emission profiles of the engines may well be different at sea because of different operational regimes, but said such changes could be observed in engines burning fuel oil, too. A paper presented at the CIMAC Congress 19 held at Vancouver this year titled, Methane Slip Summarized: Lab vs. Field Data*, showed that methane slip at sea could vary considerably from test data. Engines are often tuned and the paper reported disparities in methane slip between test and field data, which were collected largely from LNG ships whose owners were concerned that valuable fuel was being lost. Emission measurements from the field collected by SINTEF Ocean, as well as test-bed data from manufacturers, were used in the study.
Three types of gas engines are currently in the market: lean-burn spark ignited (LBSI) engines; low-pressure dual fuel (LPDF); and high-pressure dual fuel (HPDF).
The CIMAC paper reports that both the LBSI and LPDF engines, the two predominant marine engine types, operate at high excess air ratios which results in lower peak combustion temperatures and corresponding lower NOx emissions. Leaner conditions in such cases make it more difficult to provide stable ignition, which is solved by using a fuel-enriched pre-chamber and spark plug in case of LBSI engines, or by application of pilot diesel fuel in case of LPDF engines. Despite this, methane slip is not sufficiently addressed.
In one case, methane slip at 25% load from one of the engines studied was much higher in practice than in test-bed reports. The authors note that the engine was likely “overtuned” for NOx, leading to methane slip that was four times as high as test measurements.
The authors infer that combustion tuning performed in the yard does not account for actual conditions during sailing. “It is desirable to perform final adjustments in the engine’s control and regulation system after the engine has been installed onboard, with all tests performed at sea approaching real operational conditions,” noted the paper. Such tuning will need to ensure that meaningful trade-offs are made between methane slip and NOx in actual sea conditions.
In the case of LPDF engines, lean combustion conditions at low loads result in unstable combustion behavior, resulting in somewhat less complete fuel burn-out (with possible misfire) and corresponding higher methane slip. Here, additional factors, such as actual operational conditions, fuel methane number and the amount of pilot fuel used, play an important role in methane slip. LPDF engines produce somewhat higher levels of NOx due to the need for a pilot diffusion flame.
The paper notes that LBSI performance in methane slip has improved considerably in the last five years, while keeping NOx low and the results are consistent. LPDF engines show a slightly larger variation in results.
The experience of engines operating on LNG would serve as useful inputs for future engine development, in which trade-offs between the various demands on the engine will need to be made. NTNU marine technology Professor Sergey Ushakov, lead author of the CIMAC paper cited above, says that while the LBSI and the LPDF operate on the Otto cycle, the HPDF operates on the Seiliger cycle, which is a combination of the Otto and the Diesel cycle. “The engine speed (engine application) will determine the gas technology used and not all engine manufacturers are capable of delivering models for different speed ranges. Development of the gas engine is therefore very much an ongoing process,” says Prof Ushakov.
The Otto cycle, in principle, involves fuel-air mixture compression, which results in gas entering the crevice volumes such as between the piston and the liner, and the piston rings and the piston. Gas also gets absorbed by the oil film. This gas avoids combustion and eventually results in methane slip. “At low loads, additional unburned methane comes from bulk quenching, i.e., too lean mixtures and too low combustion temperatures,” adds Prof Ushakov.
The latter can be substantially reduced for some engine types by more optimal control, such as providing more pilot fuel for dual-fuel engines, while it is rather difficult to deal with gas caught in crevice volumes, explains Prof Ushakov. “The essential problem of these types of gas engines is that they are dependent on a certain fuel quality, i.e. methane number. Deviation from the ‘design’ methane number may increase methane emissions substantially,” he says. He advocates the use of online gas quality analysers as part of bunkering procedures.
In the HPDF approach, methane slip may not be a significant problem, since the fuel is delivered into the cylinder right before the piston reaches Top Dead Centre. As such, methane does not get into crevices, nor does it get absorbed by the lube oil film. Further, only air is delivered by the intake manifold and the volumetric efficiency can be high. For these reasons, combustion in HPDF can be more efficient and methane slip much less.
HPDF can handle wider variations in methane number, too. But NOx must be dealt with separately, through aftertreatment. More efficient combustion results in higher combustion temperatures, correspondingly high NOx emissions and low methane slip when there is enough excess air.
The HPDF is the opposite of the LBSI, in which NOx is kept under control due to lean-burn conditions. Methane slip is high in the LBSI. “It is possible to minimise either NOx or methane slip, but it is rather difficult to achieve low levels in both, keeping in mind that the engine should at the same time provide the required power and torque which are additional constraints for optimisation,” says Prof Ushakov. In his opinion, it would be best if engine makers focused on reducing methane slip from the engine itself and leave NOx reduction for aftertreatment devices.
*Sergey Ushakov, Norwegian University of Science and Technology, Dag Stenersen, SINTEF Ocean, and Per Magne Einang, SINTEF Ocean