The advantages are comparable to those of post-IMO oil-based marine fuels, such as low-sulfur fuel oil (LSFO), or the use of HFO with scrubbers. Post-IMO 2020 gas-fueled two-stroke engines have advantages of 14 percent to 22 percent and four-stroke engines between 6 percent to 16 percent compared to current HFO.
If the entire global marine transport fleet for 2015 completely switched to LNG, it would reduce GHG emissions by 15 percent based upon engine technology, according to the analysis.
“Beyond the benefits associated with reducing air pollutants, LNG is a viable solution to reduce GHG emissions from international shipping and to contribute to the International Maritime Organization (IMO) GHG reduction targets,” the study reads. “However, methane emission from the supply chain and engine slip need to be reduced further to maximize the positive impact on both air quality and GHG emissions.”
Low-pressure two- and four-stroke Otto engines have 10 percent to 17 percent of their WtW GHG emissions coming from unburned methane in the combustion process. New engine generations, however, are expected to achieve methane emissions in the range of 50 percent lower than today’s levels, the study stated.
Methane oxidation catalysts could reduce methane slip rates between 60 and 80 percent. Depending on the engine technology, a 40 percent reduction of methane slip leads to a 4 to 6 percent WtW GHG reduction, which could be up to a 12 percent WtW GHG decrease for a four-stroke medium-speed diesel-fuel engine if a catalyst is used.
LNG also offers benefits on a tank-to-wake (TtW) basis. The TtW emission reduction for gas-fueled engines compared to HFO engines are between 18 percent and 28 percent for two-stroke slow speed engines and 12 percent and 22 percent for four-stroke medium-speed engines.
Synthetic LNG and bioLNG can provide up to 90 percent additional benefits in terms of WtW GHG intensity. A blend of 20 percent bioLNG as a drop-in fuel can reduce GHG emissions by an additional 13 percent compared with 100 percent fossil fuel LNG, according to the study.
“The study shows that biofuels like bioLNG and synthetic LNG have a potential to further reduce the GHG intensity,” the study’s conclusions states. “Improvements in the combustion process in the engine focusing on the reduction of methane emissions and the use of a specific after-treatment system can further decrease the overall WtW GHG emissions of LNG. New conversion technologies, such as fuel cells, may also penetrate the market in the upcoming years and may further help to reduce GHG intensity of LNG-fueled vessels.”
SEA/LNG is a U.K.-registered not-for-profit collaborative industry foundation that advocates for the use of LNG as maritime fuel. SGMF is a nongovernmental organization established to promote safety and best practices in the use of gas as a marine fuel.