LNG is not dead. long live LNG
The terminology is significant. Up to this point, CO2 was the GHG recognized by the body. But LNG (liquefied natural gas)-powered ships suffer a ‘methane slip’—small leaks of unburned methane gas. Late last year, one engine maker showed us what to expect. “MAN Energy Solutions will, for the complete two-stroke program, be prepared to guarantee a maximum methane slip of 0.35g/kWh,” its 2020 Marine Engine Program noted.
Another engine builder, Wärtsilä, points out that in land-based powerplants using LNG engines, it has reduced methane slip to 1g/kWh. “Wärtsilä is confident that it will have marine engines operating at this level by 2023, as well as having retrofittable technologies available to enable similar performance in older engines,” it says.
Unfortunately, methane has a far greater global warming potential (GWP) than CO2—up to 84 times as much over a 20-year period. It eventually degrades into CO2, meaning that from a 50-year perspective, this figure falls to 28 times.
Even small degrees of methane slip could be a major environmental problem. However, this does not mean we are back to square one with LNG. There are ways of reducing methane slip and improving fuel efficiency at the same time. One has to do with the reason MAN Diesel & Turbo changed its name.
Batteries are big and heavy and are unlikely to have the capacity to power deep-sea ships by themselves until well after 2050. But for a hybrid system, the goal lies in decoupling engine RPM from propulsion load – which does not need a big battery.
For ship engines, the most efficient RPM or ‘load’ range is around 80%. Inefficiencies, on the other hand (including higher methane slip in LNG engines) tend to arise at lower loads, around 20-30%—the range ships use for maneuvering.
What’s more, as every driver knows, the engine is doing extra work whenever the driveshaft accelerates, and that energy is wasted when it slows down. Ideally, then, the engine would run continuously at 80% load.
Hybrids do exactly this. An electric drivetrain steps in to handle the ups and downs of propulsion demand; meanwhile, an engine, always running at 80% load, charges the battery. In its purest form, even a gearbox is not needed. “[This] will allow engines to be run more consistently at optimal, higher loads with the lowest possible greenhouse gas emissions,” Wärtsilä reports.
This is something modern hybrid Formula-1 cars can do, transferring as much as 50% of the energy in their fuel straight to the tarmac. Ship engines, fortunately, do not need to be as light as a Mercedes M08 EQ Power+, so there is more potential for supplementary waste-heat-recovery (WHR), turbocharging, and other technologies to eke out further fuel savings—and cut methane slip.
Some powerplants have enjoyed more than 60% fuel-efficiency for some time using gas turbines, which are in use on LNG carriers, as well. In 2015, Mitsubishi Heavy Industries (MHI) and GE announced the combined gas turbine electric and steam (COGES) system—a hybrid gas turbine system which might improve LNG propulsion efficiency even further.
But meeting with IMO 2050 targets may require even more outlandish solutions. Fuel cell technology is not yet practical on a large scale; however, it appears to be capable of efficiencies of up to 71.4% using methane. In 2018, DNV GL had this to say on the subject of fuel cells: “[They] are under development but will take time to reach a level of maturity sufficient for substituting main engines.”
Best of all, we could leave methane in the ground, recycling it from other sources instead: “…the same supply, storage, and combustion technologies can later be used with carbon-neutral bio- and synthetic gas as these become available,” Wärtsilä notes, adding: “…this pathway represents the simplest, fastest, and most cost-efficient way for shipping to reach its 2050 vision.”
LNG may be imperfect, then, but it is only the first step in the transition to newer and better fuels. There is a tendency to view emerging technologies more skeptically than everyday ones—but climate change will not reward us for this approach.
Source: SKF, Engineering at Sea