Jan-2022

Evolution of marine fuels and move toward a sustainable future

LNG can play an important role in reducing GHG emissions of international shipping by at least 50% by 2050 and contribute to the IMO GHG reduction target.

Oliver Schuller and Bambi Majumdar
Sphera

Article Summary

The international shipping industry is under immense pressure to reduce emissions. Cargo ships are particularly significant sources of air pollution. Their fuel oil is made from the bottom of the barrel of the refining process and includes substances such as sulphur and heavy metals. When burned, sulphur produces toxic gases and fine particles that harm the environment and living beings. Moreover, the greenhouse gas (GHG) emissions released through the combustion of these conventional marine fuel oils are a major concern.

Therefore, the International Maritime Organization (IMO) is focused on reducing GHG emissions from international shipping by at least 50% by the year 2050 (compared with 2008 standards) to fight climate change.

This historic nonbinding agreement will spur investments in clean-ship technologies, including alternative fuels, fuel cells and advanced sail designs.
Liquefied natural gas (LNG), as one of the alternative marine fuels available on the market, seems to be a feasible solution to contribute to the IMO's GHG reduction target. The environmental benefits of LNG as an alternative marine fuel compared with fuel oils are obvious concerning local pollutants such as sulphur oxides (SOx), nitrogen oxide (NOx) and particulate matters (PM) up to 90-95% in providing the same amount of propulsion power. In terms of GHG emissions, LNG would help curb carbon dioxide emissions by 20-25% less than conventional marine fuel oils.

Different data, assumptions and methodologies used by different studies have made it hard to reach a consensus in the industry, government and the general public about the environmental performance of LNG. The data issue is especially of prime concern since assumptions widely differ about methane emissions in the LNG supply chain and methane slip in ship engines.

Assumptions differ because there have been major developments and improvements in reducing methane emissions over the past few years. Therefore, the question of using up-to-date primary data vs using outdated literature data is critical. Secondly, one must consider the complexity of the shipping sector when considering the data. The marine engine market, in contrast to the road transport market, for instance, consists of a multitude of different engine technologies for various shipping applications and power requirements. This results in different engines with two/four strokes, single/dual-fuel, combustion cycles, efficiencies, exhaust gas cleaning systems, along with the bunkering of different fuels, geographically specific supply chains, and so on.

New emissions study
SEA-LNG and SGMF commissioned Sphera to conduct a Life Cycle Assessment (LCA) study of marine fuels called Life Cycle GHG Emission Study on the Use of LNG as Marine Fuel. The study analyses the life cycle GHG emissions (from well-to-wake, WtW) of LNG as marine fuel compared with fuel oils.

The study was first published in 2019 and updated in April 2021. The second study provides an update of GHG emissions to reflect ongoing technology developments in fuel supply and marine propulsion systems. It includes the latest data for the fuel supply consumption mixes, and the latest fuel consumption and emissions data for the different ship engines. It focuses on the latest marine engine models, engine generations where at least one engine has been built and delivered.

The study confirms the first GHG study conclusion that LNG significantly improves air quality, particularly in ports and coastal areas. Beyond the benefits of reducing air pollutants, LNG reduces GHG emissions from international shipping and contributes to the IMO's GHG reduction ambition.

Results
As mentioned, the GHG impact of LNG- fuelled vessels cannot be summarised by one representative technology and propulsion and power provision system. Large container ships or bulk carriers, for instance, are used to transport goods from one continent to another, and hence mainly operate in deep-sea regions and mainly with a constant engine load after leaving the harbour. These applications typically use two-stroke slow-speed engines.  

In contrast, ferries or cruise ships mainly operate in coastal areas, may change engine load more frequently, and are typically equipped with four-stroke medium-speed engines. For smaller ships, such as support vessels and tugboats, the engine response with many engine load changes is crucial. Therefore, we cannot consider just one single ship engine as the operational parameter that will eventually affect the choice of engine technology.

Two-stroke slow-speed engines burn more than 70% of the fuel used in global shipping. Because of their high efficiency and high power, these engines are mainly used in large ocean-going cargo ships. LNG is used in two forms of engine technologies that differ in their underlying combustion cycle and gas injection system:

 The WtW GHG emissions of the two-stroke slow-speed diesel dual-fuel engine (high-pressure gas injection) are 533g CO2-eq/kWh when using LNG. This is 23% less than the same engine operating on very low sulphur fuel oil (VLSFO) (688g CO2-eq/kWh), as shown in Figure 1.

 The WtW GHG emissions of the two-stroke slow-speed Otto dual-fuel engine (low-pressure gas injection) are 594g CO2-eq/kWh when using LNG. This is a reduction of 14% compared with the VLSFO operation. For these LNG-fuelled engines, the GHG emissions of the supply chain contribute about 21-23% of the entire life cycle emissions (WtW). For oil-based fuels, the supply chain accounts for 15-16%.

Four-stroke medium-speed engines burn 18% of the fuel used in global shipping. They typically have lower engine power and are used in car and passenger ferries, cruise ships and short sea shipping. Both engines investigated in the study are Otto cycle engines and can be differentiated according to their ability to run on a single (SI) or dual fuel (DF):

 The WtW GHG emissions of the four-stroke medium-speed Otto-DF engine are 685g CO2-eq/kWh running on LNG. This is a 6% reduction compared with the operation on VLSFO (725g CO2- eq/kWh), as shown in Figure 2.

 The WtW GHG emissions of the four-stroke medium-speed Otto-SI engine, a single fuel, pure gas engine, are 624g CO2-eq/kWh. This is a 14% reduction compared with the same engine running on VLSFO.

For these LNG-fuelled engines, the WtW GHG emissions of the supply chain contribute about 20-22% of the entire life cycle emissions (WtW). For oil-based fuels, the supply chain accounts for 15-16%.