A life-cycle assessment of emissions from domestic and imported liquefied natural gas (LNG), and heavy-fuel oil (HFO) for marine shipping by a team from the University of British Columbia has found that only high-pressure dual-fuel (HPDF) engines robustly reduce well-to-wake GHG emissions by 10% compared with their HFO-fueled counterparts.

Low speed 2-stroke high pressure dual-fuel (LS-HPDF) engines—unlike the other major three marine engine types—operate on the diesel cycle. Natural gas at high pressure is injected into the cylinder near the top of the compression stroke. The gas is ignited through an injection of liquid pilot fuel. These dual-fueled engines provide similar performance to diesel engines with no power loss, although with higher NO_x emissions. The direct gas injection system assures lower methane emissions from the tailpipe exhaust.

The efficiency of these engines is the same as the low-speed diesel engines from which they are derived: about 50%. This engine technology is only available for large low-speed engines used in ocean-going vessels (OGVs).

The medium-speed low-pressure dual-fuel (MS-LPDF) and lean burn spark ignition (LBSI) gas engines currently used in smaller vessels, such as ferries, cannot reliably reduce GHG emissions, the researchers found. This is primarily due to the high levels of methane slip from these engines.

The paper is published in the journal Energy Policy.

WTW GHG emissions from LNG- and HFO-fueled engines. Sharafian et al.

For reducing criteria air pollutants, however, gas engines were found to be an effective means of reducing nitrogen oxides, sulfur oxides and, particulate matter without any additional engine aftertreatment.

Natural gas engines emit 95–98% lower sulfur oxides and 97–98% lower particulate matter than heavy-fuel oil engines.

The HPDF engines, however, need aftertreatment or exhaust gas recirculation to meet the International Maritime Organization Tier III regulations.

Sulfur controls, such as the 2020 act, move to limit sulphur to 0.5% globally. However, this will increase the cost of the HFO used by most OGVs, enhancing the economic case for natural gas fuel.

Resources

• Amir Sharafian, Paul Blomerus, Walter Mérida (2019) “Natural gas as a ship fuel: Assessment of greenhouse gas and air pollutant reduction potential,” Energy Policy, Volume 131, Pages 332-346 doi: 10.1016/j.enpol.2019.05.015.