An increasing number of ships are being built to run on liquified natural gas (LNG), which emits about 25% less carbon dioxide than conventional fossil fuels in terms of the same amount of propulsion power. However, it is questionable whether it is a solution for decarbonization or reduction of GHG emissions in that LNG, which consists of almost entirely methane (CH4), the simplest hydrocarbon compound, is a potent greenhouse gas (GHG) that traps atmospheric heat 87 more effectively than CO2. The climate conversation is focusing on the emissions of carbon dioxide, which is the main culprit in heating the planet while it is not the only gas driving global warming. Unlike carbon dioxide, which is more stable and has a high global warming potential (GWP) value, methane is short-lived in the atmosphere.
Concerns over LNG’s Carbon Footprint caused by the production and use of LNG
Methane is a simple gas, a single carbon atom with four arms of hydrogen atoms.
Methane released into the atmosphere traps a large amount of heat in the first decade when most methane emitted has reacted with ozone to form carbon dioxide, which, in turn, exacerbates the elevated temperature of the atmosphere for more than hundreds of years to come. Some climatologists argue that emitting methane will be more detrimental than spewing the same quantity of carbon dioxide, regardless of the time scale on the ground that methane heats the climate by 28 times more than carbon dioxide when averaged over a century and 84 times more when calculated over two decades. While methane comes from many sources, including natural and human-related activities, about 60 percent of methane emissions are human-caused sources, of which the global and gas industry contributes to at least 25 % of today’s global warming. Therefore, undervaluing the impact of methane will make our planet end up facing a clear risk.
As the transition to alternative fuels that contain less carbon or zero carbon from conventional marine fuels accelerates after the IMO adopted an initial GHG strategy in 2018, more ships than ever are now using liquefied natural gas. While LNG represented less than 3% of ship fuel consumption from 2013 to 2015, there were 756 LNG-powered ships. LNG is becoming popular since it contains a very tiny quantity of sulfur. Also, the LNG engine emits low nitrogen oxide (NOx) emissions -even though it is at the cost of higher methane emissions- and incorporates NOx reduction technologies, including exhaust gas recirculation (EGR) or selective catalytic reduction (SCR). The decrease of SOx and NOx emissions renders LNG an attractive fuel for ships operating in Emission Control Areas (ECAs). Furthermore, some shipowners find that operating LNG propelled vessels makes economic sense as LNG has been less expensive than marine gas oil (MGO), heavy fuel oil (HFO) with a scrubber, or very low sulfur fuel oil (VLSFO). The IMO initial GHG strategy to reduce the carbon intensity of shipping by about 40% by 2030 and absolute GHG emissions by about 50% by 2050 makes LNG viewed as a key bridge fuel for meeting the goal. However, there is methane leakage and methane slip from LNG that needs to be recognized.
Methane Slip from LNG fueled engines
As when LNG is burned as fuel, most of the methane is consumed but the unburned methane escapes into the atmosphere, which is referred to as methane slip, addressing ‘the methane slip’ is critical to LNG’s viability as a key bridge fuel. Even when a tiny quantity of methane leaks into the atmosphere without being burned, these emissions will outrun LNG’s lower carbon dioxide emissions. The LNG engine most widely used by the shipping industry allowed 3.7 percent of methane to pass unburned through its engine and into the atmosphere. The ship engine designs include an open crankcase that vents a small amount of unburned methane. It is reported that the 3.7 percent of methane emitted from vessels is a higher percentage of leakage than the rest of the natural gas sector combined.
Optimizing LNG-fuel engines and the combustion process can cut the methane slip. Unburned methane primarily from incomplete combustion and fuel concealed in crevices in the com combustion during compression. Four types of engines are currently available in the market to be used in gas-propelled vessels. Medium Speed 4-Stoke Lean Burn Spark Ignition (LBSI) engines run on only natural gas based on the Otto cycle, which is an idealized thermodynamic engine that describes the functioning of a typical spark ignition piston engine. This type of engine has an efficiency of about 42% and a power output ranging from 316kW to 9.7 MW. Rolls-Royce Marine/Bergen, Mitsubishi, and Hyundai are manufacturers of these engines. Medium Speed 4-Stroke Low-Pressure Dual-Fuel (MS-LPDF) engines also operate on the Otto cycle, requiring a lower compression ratio than diesel engines of the same size to prevent pre-ignition or knocking. These engines have an efficiency of about 44%, providing flexibility to use different fuels depending on fuel price or availability. They were designed initially for LNG bulk carriers but they have been successfully deployed in ferries, service vessels, and several other vessel types. Wärtsilä, MAN, and MAK are manufacturers of these engines. Low-Speed 2-Stroke Low-Pressure Dual-Fuel (LS-LPDF) engines operate on a similar mechanism, but when in gas mode, gas under low pressure is injected into the cylinder before the compression stroke. The efficiency of these engines is about 51%. The manufacturers of these engines are Win GD and their power range is 4.5MW to 56MW. Low-Speed 2-stroke High-Pressure Dual-Fuel (LS_HPDF) engines operate on the diesel cycle, unlike the other three engines. Natural gas at high pressure is injected into the cylinder near the top of the compression stroke and these engines provide similar performance to diesel engines with no power loss, although NOx emissions are higher than Otto cycle engines due to higher combustion chamber temperatures. Their efficiency is about 50%. Marine LS-HPDF engines are manufactured under license from MAN and provide a power output of 42.7 MW.
One of the main issues with LBSI and LPDF engines is methane slip, which occurs when methane from the fuel enters the engine exhaust while unburned, the primary cause of which is due to incorrect air-fuel mixtures or gas getting trapped in crevices in the combustion chamber. These undesired emissions from LBSI and LPDF engines continue to reduce the GHG benefits of natural gas-fueled ships since methane is a potent GHG and has a global warming potential (GWP) of 30 to 85 times greater than CO2. In contrast, LS-HPDF engines have been identified to have almost no methane slip ( about 0.01%). However, since their NOx emissions are higher than the other engine types, HPDF engines should require exhaust gas recirculation (EGR) or selective catalytic reduction (SCR) to reduce NOx emissions.
Questions arise on using LNG as a marine fuel, despite its merits
Natural gas fuel is now a feasible option for all vessel types and sizes thanks to the development of large low-speed gas engines. Smaller vessels powered by medium or high-speed engines, and the alternative natural gas engines emitted higher levels of GHG compared to their heavy fuel oil (HFO) counterparts because of high methane slip from these engines. Therefore, LNG cannot count as a robust means of reducing GHG emissions, though LNG as an alternative to HFO for marine shipping has the potential to reduce NOx, SOx, and particulate matter (PM) emissions. To use LNG as a bridge fuel to green energy sources, the technologies to minimize methane slip from LBSI and LPDF engines need to be more advanced.