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ALTERNATIVE MARINE FUELS: A COMPREHENSIVE REVIEW OF PATHWAYS, CHALLENGES, AND PROSPECTS FOR DECARBONIZING SHIPPING
Introduction
The shipping industry accounts for approximately 80% of global commerce, producing around one billion tonnes of carbon dioxide per year [1, 2]. This high level of emissions is largely due to the scale of global trade and the widespread use of traditional marine fuels such as heavy fuel oil (HFO) and marine gas oil (MGO), which are renowned to be highly carbon intensive. While effi cient for transporting mass commerce, large diesel engines burn huge quantities of these fuels, resulting in signifi cant carbon dioxide emissions. As illustrated Figures 1 and 2, bulk carriers and container ships account for among the highest carbon dioxide emissions within the maritime sector, refl ecting their size, cargo capacity, and extensive fuel consumption.
Following the policies such as the IMO’s sulfur cap in 2020 under MARPOL Annex VI, LNG has emerged as one of the earliest and most prominent sources of alternative fuels to HFO and MGO. This immediate, unprecedented change in policy directly contributed to the adoption of alternative fuels, such as LNG and dual-fuel systems that can operate on both LNG and conventional marine fuels. Moreover, LNG is seen as a ‘band- aid’ or transition fuel while truly zero-carbon alternatives are developed. Although still fossil-fuel based, it burns much cleaner than conventional fuels and is viable for large-scale deployment, serving as a ‘bridge’ to renewable fuels.
Figures 1 & 2 Carbon Emission Intensities: AER (Annual Effi ciency Ratio) vs. dwt for Bulk Carriers and Cargo Ships, respectively [4].
Consequently, the International Maritime Organization (IMO) strives to reach net-zero emissions by 2050. In addition to carbon emission concerns, the IMO has enforced a global sulfur cap of 0.50% by mass. Compared to the limit of 3.50%, this represents an approximate 85.7% reduction in allowable sulfur content of marine fuels, aimed at reducing sulfur oxide emissions. Further, in emission control areas (ECAs), fuels must contain no more than 0.10% sulfur by weight [3]. In response to these environmental regulations, shipowners are being pushed to transition from conventional fuels toward low and zero-carbon alternatives. Over the past two years, signifi cant advancements have been made in marine fuels: options that were previously hypothetical, such as liquefi ed natural gas (LNG), methanol, hydrogen, ammonia, and biofuel/green variants, are being exposed to more experimentation, innovation, and even commercial usage. However, environmentally friendlier alternatives are still immature, as they often come with lower energy density, higher production costs, and limited global infrastructure.
Liquefi ed Natural Gas
LNG is primarily composed of methane, with small amounts of ethane, propane, and traces of other hydrocarbons. It takes a liquid state after being cooled to about -162 ℃; this process makes it much denser, enabling storage convenience and effi cient transportation. It is seen as a transition fuel for the maritime industry because it produces almost zero sulfur oxides, reduces nitrogen oxides by ~85%, and cuts carbon emissions by ~25% [5]. Generally, LNG burns cleaner compared to HFO, paving the pathway to net-zero emissions. However, there are signifi cant downsides to the current state of LNG technology. Aside from expensive infrastructure and lifecycle emissions, methane slip is a major fl aw. Methane slip refers to the unburned methane that escapes from the ship’s engine during combustion or fuel leakage, entering the atmosphere. This occurs because portions of methane are not completely oxidized to carbon dioxide and water.
As of 2025, a total of 1,369 LNG dual-fuel vessels is in operation or on order globally, supported by nearly 200 ports equipped with LNG bunkering facilities [7]. This supports LNG’s increasing prevalence and status as a popular transitory fuel; to deal with mentioned downsides, initiatives have been taken to advance LNG technology. For instance, MAN Energy Solutions introduced a new future-proof, cost effi cient, and methanol-compatible auxiliary engine to reduce emissions. The MAN 33/44DF CD incorporates features aimed at lowering greenhouse gas emissions for LNG container ships and carriers. Based on controlled and fi eld tests, the engine reduces methane slip by 85% compared to typical marine fuel standards [8]. Features include engine calibration, operating strategies, and advanced oxidation catalysts such as the IMOKAT II, which is a sulfur- resistant, precious metal-free catalyst for four-stroke engines. Further, this catalyst specifi cally aims to reduce 70% in methane slip at 100% load [9]. Figure 3 shows an image of the engine:
Figure 3: An image of the MAN Energy Solutions 35/44DF CD Engine [10].
Alongside other engine prototypes, the MAN 35/44DF CD passed the Type Approval Test in April 2025 at the STX Headquarters in South Korea, with expected upcoming commercial usage within 2025 [7]. Similarly, Technology group Wärtsilä has introduced the Wärtsilä 46TS-DF dual-fuel engine that operates
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