The authors applied a total fuel cycle analysis (TFCA) methodology to evaluate “well-to-hull” emissions for vessel operations. The analysis evaluated emissions along the entire fuel pathway, including extraction, processing, distribution, and use of particular fuels in vessels. The team used updated data on leakage rates in the natural gas fuel cycle to compare emissions from liquefied natural gas (LNG) or compressed natural gas (CNG) to petroleum marine fuels.

They modeled three vessel types (large ocean-going vessel [OGV], inland tug/tow, and coastwise OGV) as traveling typical fixed routes using natural gas and emissions control area (ECA)-compliant distillate fuels meeting 2012 and 2015 standards (that is, 10,000 ppm sulfur [S] and 1,000 ppm S, respectively).

The OGV was modeled on two West Coast routes, one from Los Angeles/Long Beach (LA/LB) to Shanghai, China, and the other from LA/LB to Honolulu, HI. The inland tug/tow vessel was evaluated transiting the Mississippi River between Peoria, IL and New Orleans, LA. The coastwise OGV was evaluated transiting the East Coast of the United States (US) between the Port Authority of New York and New Jersey (PANYNJ) and Jacksonville, FL.

In all cases, they evaluated the use of LNG as an alternative to distillate fuels. For the inland tug/tow, they also evaluated the use of CNG.

The results, they cautioned, do not provide a complete uncertainty analysis of all possible upstream scenarios. Further uncertainty analysis would provide a more comprehensive analysis on the nature of key factors that may affect results.

When comparing emissions produced using natural gas or traditional diesel fuel, results were mixed as to which would produce the fewest emissions. The total energy needed to make the trip is higher in the natural gas scenarios, as is the amount of CH₄, N₂O, and NO_x produced. In the all-diesel scenarios (both diesel main and auxiliary engines) more CO₂, PM₁₀, and SO_x are produced. Additionally, when taking into account the global warming potential of CH₄, N₂O, and CO₂, LNG fuel is found in coastal scenarios (both West Coast and East Coast) to produce less overall GHG emissions (as measured in CO₂ equivalent units) than diesel fuel scenarios using either low-sulfur distillate (depending upon pathway) or high-sulfur residual fuel.

Diesel in inland river scenarios results in less overall GHG emissions (as measured in CO₂ equivalent units). Diesel in the West Coast scenario results in more overall GHG emissions (as measured in CO₂ equivalent units) under either low-sulfur distillate (depending on pathway) or high-sulfur residual fuels (across all natural gas pathways). Diesel in the East Coast scenario also results in more overall GHG emissions (as measured in CO₂ equivalent units) under either low-sulfur distillate (depending on pathway) or high-sulfur residual fuels (across all natural gas pathways). While this analysis does not include an assessment of impacts resulting from each of those pollutants, one can consider the fact that the IMO deemed NO_x and SO_x both important enough pollutants to regulate.

This is an important consideration. Natural gas is considered by many to be a win-win-win marine fuel: i) economically attractive; ii) low-emitting for key air quality pollutants; and iii) lower GHGs (primarily lower CO₂). However, natural gas may achieve some goals better than others. … This study did find a small but positive GHG benefit along with economic and local/regional air quality benefits.

—Corbett et al.

The study was conducted as a part of MARAD’s Maritime Environmental & Technology Assistance program, which focuses efforts on emerging marine transportation and environmental issues.

Resources

• James J. Corbett, Heather Thomson and James J. Winebrake (2014) “Natural Gas for Waterborne Freight Transport: A Life Cycle Emissions Assessment with Case Studies”