Single-fuel or dual-fuel methanol engines for tugs?

Single-fuel methanol engines may hold the key to a greener future for tugs, marine propulsion systems manufacturer Rolls-Royce Power Solutions (RRPS) and Maersk’s towage operator Svitzer said in a joint paper.

Illustration; Image credit Svitzer

The main driver behind the development of dual-fuel engines is fuel flexibility, allowing vessels to operate even with limited methanol availability. However, such engines come with limitations that need to be carefully considered. RRPS has evaluated several dual-fuel concepts and compared them to single-fuel methanol engines. While dual-fuel engines offer some advantages, they fall short in achieving high diesel substitution rates, which is a primary goal of using methanol as a fuel.

Concept A involves upgrading a currently produced diesel engine with a methanol port fuel injection system. However, this concept has limitations in both high and low power outputs, resulting in a moderate average diesel substitution rate of 37%. Concept B addresses the low diesel substitution issue by replacing the diesel injector with a smaller one. Although it increases the diesel substitution rate to 79%, it also limits the total engine power and offers no benefit over a single fuel engine.

To overcome the limitations of Concepts A and B, Concept C combines both concepts, allowing for a higher diesel substitution rate of 74%. However, it requires three different injectors and increases the complexity and maintenance costs. Concept D aims to achieve a high diesel substitution rate at maximum power but requires intensive combustion and fuel injector development.

In contrast, RRPS has assessed a 100% methanol combustion engine and initiated the “MeOHmare” technology development project in collaboration with industry partners. The project aims to develop a methanol injection system and advance the knowledge required for a methanol combustion engine. Testing with a single-cylinder engine has shown positive results in terms of emissions, meeting IMO Tier III standards without exhaust aftertreatment. Additionally, single-fuel methanol engines offer significant reductions in CO2 emissions, especially when powered by green methanol. These engines also simplify vessel design and fuel logistics, as only methanol needs to be bunkered and stored onboard.

“Therefore, it seems to be a realistic target for RRPS to equip tugs in the range up to 80t bollard pull with S4000 single fuel methanol combustion engines allowing CO2 neutral operation (if green methanol is used) with low NOx and especially low particulate matter (PM) emissions,” the paper said.

Comparing single fuel and dual-fuel methanol engines, both have advantages and disadvantages. Single fuel engines completely substitute diesel with methanol, resulting in low emissions. On the other hand, dual-fuel engines provide fuel flexibility but have limitations in achieving high diesel substitution rates. From a commercial perspective, single fuel engines may be cheaper to operate, especially if green methanol is used.

Operators of tugs prioritize safety, fuel availability, substitution rates, endurance, power, and performance. While fuel availability cannot be influenced by engine manufacturers, dual-fuel engines may be initially preferred to ensure operational readiness in case of methanol supply disruptions. However, as the market for green methanol matures and long-term supply is secured, single fuel systems become an attractive option due to their compactness, higher substitution rates, and simplified tank systems.

In terms of the fuel market, there is no single replacement for marine diesel oil that matches its energy density, availability, and low cost. Operators must explore various fuel options in the short term while preparing for a future with multiple fuel choices. Methanol, ammonia, and hydrogen are leading alternative fuel technologies, and early adoption of these fuels will be crucial for the tug sector’s decarbonization efforts.

For Maersk Group and Svitzer, methanol is being explored as the most cost-effective option that can bring benefits today.

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“Svitzer and RRPS both are convinced that methanol will play a big role in the decarbonisation of shipping. In order to contribute to reduced GHG Emissions it is essential to start as soon as possible. However, many factors are not given at the moment, such as the availability of green methanol. This makes it hard to take investment decisions as the future is not clear,” the duo argues.

“To avoid stranded assets it is essential to either source fuel in advance or choose equipment which allows for fuel flexibility, such as dual-fuel engines. However dual-fuel engines are a compromise which will not be the best option when the situation arises that green methanol becomes widely available and price competitive. Therefore, we see strong benefits in single-fuel engines in the long term to provide completely CO2 neutral equipment and simpler vessel designs.”