A study released this summer has concluded that using natural gas as a marine propulsion fuel has the upper hand when compared to conventional fuels not just in improving air quality, but in overall greenhouse gas (GHG) impact as well.
The study, commissioned by the US Department of Transportation Maritime Administration (MARAD), evaluated total fuel cycle emissions, or "well-to-hull" emissions for vessel operations for natural gas versus conventional marine fuels.
It assessed how fuels compare in terms of producing pollutants contributing to climate change, mainly carbon dioxide (CO2) and methane (CH4), and pollutants that contribute to poor air quality such as sulphur oxides (SOx), participate matter (PM)and nitrous oxide (NOx). SOx and NOx also have a climate impact, which the study took into account.
Proponents of liquefied natural gas (LNG) as fuel have long been pointing to its advantages over oil-derived conventional fuels because it virtually eliminates emissions of SOx and PM, and reduces NOx emissions significantly. Because of this, using LNG as bunker fuel has the potential to meet all emission control area (ECA) standards, including ECAs where the strictest NOx Tier III standards apply.
But the green credentials of LNG have been questioned because of methane slip occurring during ship operations or at other points in the production and supply chain. Methane is a potent GHG said to have about 21 times the global warming potential of CO2.
Unsurprisingly, the study concluded that in the total fuel cycle analysis, natural gas fuels "reduce air quality pollutants substantially."
But it also concluded that using LNG or compressed natural gas (CNG) instead of marine distillates for propulsion reduce major GHG emissions "slightly" overall.
"We also find that the upstream configuration for natural gas supply matters in terms of minimizing GHG emissions on a total fuel cycle basis, and current infrastructure for marine fuels may produce fewer GHGs," the study said.
The researchers, including James J. Corbett from the University of Delaware and James J. Winebrake from the Rochester Institute of Technology, analysed the life cycle for fuel production and use "along the entire fuel pathway, including extractions, processing, distribution, and use of particular fuel in vessels." They used "best available data reflecting recent research" on methane leakage.
The total fuel cycle energy consumption were assessed for three vessel types; large ocean-going vessels (OGVs), inland tug/tow and coastal OGVs. They were modelled for use of natural gas, and for using distillate fuels complying with ECA sulphur standards.
The overall study concluded that continued improvements to reduce methane emissions during vessel engine operations would contribute to lower GHG emissions from marine applications.
"This is important because growing supplies of natural gas can provide a feasible and economic alternative fuel to improve air quality in and near populated regions of the world," the study noted.
The study was part of MARAD's Maritime Environmental & Technology Assistance program, and looked at use of LNG or CNG for vessel operations in US waters.
LNG as marine fuel has been attracting growing interest in the United States as there are ample supplies of natural gas and it could offer ships a reasonable alternative to meeting ECA standards.
The study is available on the MARAD website, at http://www.marad.dot.gov/documents/Total_Fuel_Cycle_Analysis_for_LNG.pdf