Using bio-diesel onboard vessels

Alternative marine fuels can be delivered from various alternative feedstocks and production processes. Bio-diesels and bio-oils are liquid biofuels made from sustainable biomass and biowaste with lower climate impacts than traditional fossil fuels. They may provide direct, low-emission replacements for conventional marine fuels or within blends. They are also potential pilot fuels for dual-fuel internal combustion engines (ICEs) using other alternative fuels like methane, methanol, and ammonia.

Bio-oils offer greater long-term viability and more potential impact on decarbonization than bio-diesels as they can utilize a larger variety of feedstocks, increasing their potential availability. As a result, the Mærsk Mc-Kinney Møller Center for Zero Carbon Shipping (MMMCZCS) only considers bio-oils as a scalable alternative fuel pathway. However, although bio-diesels are already available and in use in the shipping industry, bio-oils are not yet available in sufficient quantities for onboard use and testing.

While bio-diesels and bio-oils do not have the same chemical composition, performance, or qualities as each other, they present similar challenges for onboard use. As a result, we established a dedicated working group to study the potential challenges of using biodiesel onboard vessels, which we expect to provide foundational knowledge of the challenges associated with the use of bio-oils onboard.

This paper presents the results from the working group, including an overview of regulatory drivers, onboard vessel bunkering, handling, and storage considerations. We also include experimental results from our investigations of fuel stability and NO_X emissions measurement tests. Furthermore, we provide our recommended step-by-step processes for bunkering and onboard handling.

Based on our investigations, the working group made the following conclusions:

• Recent regulatory developments have lowered the barriers to using biofuels onboard vessels. However, work is still needed on fuel standards and sustainability life-cycle certification.
• The Unified Interpretation (UI) approved at MEPC 78 in June 2022 allows for a more streamlined regulatory scenario for the use of biofuels onboard vessels, especially if flag states adopt the UI and classification societies and engine makers confirm no changes to the engine’s NO_X
  critical components or settings/operating values.
• With the lack of applicable fuel standards, it is presently up to the individual shipowner or operator to build knowledge and experience about proper fuel parameters and handling.
• While some sustainability certification schemes exist, they must be standardized and aligned with the current and upcoming global and regional regulations.
• Due to the possibility of varying stability, acidity, and corrosion characteristics of bio-diesels, they must be handled and stored correctly onboard using operational measures with limited technical changes.
• We recommend pre-bunkering steps including determining the blend composition, laboratory testing, and confirming compliance with International Maritime Organization (IMO) safety regulations and machinery.
• Onboard handling requires bunker storage tank cleaning (subject to the sludge and sediment content in the bunker tanks) and adjustment of storage and transfer temperatures.
• Fuel sample analyses can be used to indicate acceptable oxidation stability levels for typical fuel use periods. If required, readditization of antioxidants can prevent fuel oxidation, as seen from studies done in the automotive industry. Such trials are yet to be conducted in the maritime industry onboard vessels.
• Results from three emissions measurement tests showed that NO_X emission levels of bio-diesel and bio-diesel blends combusted in two-stroke slow speed marine engines are comparable with conventional fossil fuels, including heavy fuel oil (HFO), very low sulfur fuel oil (VLSFO), and marine gas oil (MGO).
• These results also indicate that there is no need for special emission reduction technologies or solutions to reduce NO_X emissions further when using bio-diesels. However, if NO_X emission reduction is necessary, commercially available technologies and solutions exist.

The conclusions presented in this paper are based solely on tests of fatty acid methyl esters (FAME)-based bio-diesel fuels. The NO_X emission levels presented here form a benchmark for comparison with fast pyrolysis and hydrothermal liquefaction bio-oils when their technology matures, and they are available in larger quantities.

To increase the viability of the liquid biofuel pathways, we encourage flag states, classification societies, and engine makers to jointly leverage the approved UI to remove the regulatory barriers for the use of biodiesels and bio-oils onboard vessels and support the development of fuel standards for blends with higher percentages of bio-diesel and bio-oil.