A new project is measuring methane slip on board a roro ferry and devising practical strategies to better manage it
Methane has a much higher global warming potential than CO2. Studies show that above a threshold of7% methane slip, clean burning natural gas produces exhaust gas with the highest global warming potential among most fossil fuels, defeating one of the advantages of the fuel – greater efficiency. While NOx emission limits and sulphur caps have been mandated, IMO is actively considering the phenomenon of methane slip from gas engines for possible regulation.
The experience of a roro ferry in Canada burning natural gas indicates that thoughtful load management and optimising combustion through closed-loop control for part-load operations can help to keep unburned methane levels low, while maintaining NOx limits. Engine operation needs to be a balance between keeping NOx under control and methane slip low, since the two place conflicting demands on the engine.
Part-load operations were a big worry on the ferry since the two engines on it were low NOx, lean-burn. Cutting out cylinders as well as operating on one engine reduced methane slip.
At hotel loads (10% of full load), engine CH4 emissions were significantly higher than propulsion loads. During port stay, when only hotel loads were on, the two engines were switched off and shore power used.
The inferences were presented in two papers: “Characterization and Reduction of In-Use CH4 Emissions from a Dual Fuel Marine Engine Using Wavelength Modulation Spectroscopy” and “Characterization and Reduction Strategies for GHG Emission from an LNG-Powered Coastal Vessel”. **
The ferry under study was on a coastal route. It had two engines, each driving generators. The propulsion was electric. The engines were four-stroke, nine cylinders, 4.5 MW with port injection for gas and pilot injection for diesel fueling.
Emissions were measured across 4% load to 92% full load (4,000 kW). At low loads, the engines were operating near the lean flammability limit of methane, which could lead to marked methane slip. The excess air ratio that was slightly above 2 at 90% load climbed sharply to around 2.3 at 60% and 2.5 at 30% load.
A 6.5 times increase in methane slip was observed when load reduced from 85% to 25%. The high excess air in the premixed gas charge at low loads reduced the speed of the flame front propagating from the diesel pilot, giving rise to this phenomenon.
Two adjustments were made in the engine software to reduce unburnt CH4 emissions. At low loads, cylinder deactivation was used to decrease the effective excess air in the fired cylinders, and at low and medium loads combustion timing was adjusted using closed-loop control to optimise the ignition of the gas charge and reduce unburnt CH4.
Engine 1 used an open-loop control (OLC) strategy for the diesel pilot injection mass and timing. Engine 2 used a closed-loop control (CLC) strategy for the pilot injection, where the pilot injection (timing and mass) and intake air parameters (temperature and pressure) were adjusted in real time, based on cylinder pressure. CLC was more effective, especially at low loads. During sailing, depending on the load, one engine was used instead of two, which reduced methane emissions by half.
The implementation of cylinder deactivation reduced the global warming potential of the exhaust gases from the ship by 11%, and cylinder deactivation and CLC combined offered a 15% reduction in total GWP when one engine was used for sailing and hotel loads. Overall, minimising low load operation (single engine, shore power) and applying CLC and cylinder deactivation resulted in 42% GWP reduction.
* David Sommer et. Al, Environmental science & technology 53.5 (2019): 2892-2899.
** David Sommer and Patrick Kirchen, University of British Columbia, and Harly Penner, Seaspan Ferries