Characterization of Methane Emissions from a Natural Gas-Fuelled Marine Vessel under Transient Operation 2021-01-0631

Natural gas is an increasingly attractive fuel for marine applications due to its abundance, lower cost, and reduced CO₂, NO_x, SO_x, and particulate matter (PM) emissions relative to conventional fuels such as diesel. Methane in natural gas is a potent greenhouse gas (GHG) and must be monitored and controlled to minimize GHG emissions. In-use GHG emissions are commonly estimated from emission factors based on steady state engine operation, but these do not consider transient operation which has been noted to affect other pollutants including PM and NO_x. This study compares methane emissions from a coastal marine vessel during transient operation to those expected based on steady state emission factors.

The exhaust methane concentration from a diesel pilot-ignited, low pressure natural gas-fuelled engine was measured with a wavelength modulation spectroscopy system, during periods of increasing and decreasing engine load (between 3 and 90%). Methane concentration, methane emissions, and excess air ratio were compared to steady state conditions. Load increases resulted in similar exhaust methane concentrations relative to steady state values (within 8%). In contrast, decreasing engine load increased the exhaust methane concentration up to 1.9-times relative to equivalent steady-load values and showed total methane emissions up to 43% higher over the transient duration. However, the marine vessel considered here, operated at steady load approximately 91% of the time and the transient CH₄ emissions will have only a negligible impact on the total in-use GHG emission (1.7% increase). This indicates that while transient operation does affect CH₄ emissions, the uncertainty introduced in using steady state emissions factors for the considered load cycles is negligible for ships that mostly operate at steady load. Moreover, the load transitions during which CH₄ concentration was measured were more aggressive than what is experienced routinely during sailings. The transient measurements indicated the need for CH₄ emission reduction control strategies, particularly for transients with load reductions.