There’s a sector that isn’t included in most discussions about decarbonization: shipping. The global shipping industry is currently responsible for 3% of global emissions, and vessels primarily rely on heavy fuel oil (HFO) for propulsion. Heavy fuel oil (also called bunker fuel) is a low-grade carbon-based fuel by-product of the crude oil refining processes, and the shipping industry consumes 3.5 million barrels of HFO each year. It’s a dirty fuel, and ships burning HFO emit huge amounts of atmospheric emissions and pollutants. The international community acknowledges it needs to be phased out in order to fight global climate change.
When it comes to moving away from heavy fuel oil, there are a few niche fuels and lower-carbon options (such as liquified petroleum gas and liquified natural gas) that are expected to see increased use over the coming decade, but all of them still result in some amount of emissions. There are, however, several truly clean options to choose from, each possessing its own advantages and disadvantages, namely hydrogen, methanol, ammonia, and batteries.
Hydrogen has been a growing part of the transportation discussion for about two decades, and it’s finally making headlines. It’s been positioned as a solution for “hard-to-decarbonize” sectors, and in just the past five years more than 30 countries have developed or started to prepare national hydrogen strategies.
There are a few different types of hydrogen, which are classified by how each is produced. “Gray” hydrogen, currently constituting 95% of global hydrogen production, is produced from fossil fuels through steam reforming. “Green” hydrogen is produced by electrolysis, which splits water into hydrogen and oxygen using electricity, and is the only way to produce hydrogen with renewable energy and without emissions.
Based on the requirements for how each is produced today, green hydrogen is significantly more expensive – up to 4x – than gray hydrogen. The cost of hydrogen from natural gas can range from $1.5 to $2.50 per MMBtu, which already loses to heavy fuel oil’s global average of $0.39 per MMBtu. Green hydrogen ranges from $4 to $6 per MMBtu.
Hydrogen is also less energy dense than heavy fuel oil at 1.2-2.4 kWh/L to HFO’s 10-11 kWh/L, and shipowners would potentially need to dedicate more cargo room to fuel tanks to achieve the same range. Some reports suggest the additional room needed would be minor, as low as 5 percent, if operators would simply be willing to refuel more frequently.
Safety is a major issue for hydrogen, especially in liquid form. Hydrogen is extremely flammable and has a larger ignition range than other traditional fuels, meaning it will burn at both low and high concentrations when combined with oxygen. Hydrogen has to be cooled to -253°C and stored in insulated tanks to maintain this low temperature and minimize evaporation. System designers must incorporate robust safety and protection features in practically all cases.
Many experts in the shipping world will tell you ammonia holds a lot of promise. It’s easier and cheaper to store than hydrogen, and has been transported as cargo for other commercial uses for decades.
However, there are a lot of technical hurdles that need to be addressed before it can be used widely, or at all. Not a single ship is currently equipped to use ammonia as a primary fuel, and the regulatory framework is still being developed to allow its use more widely due to the extensive safety risks.
Despite having a reasonable energy density of 3.5 kWh/L, using ammonia in an internal combustion engine is difficult as ammonia doesn’t burn well, and may need to be mixed with diesel, which increases the emissions profile. Another option—using ammonia in a fuel cell—is years away from maturity and currently isn’t demonstrating enough power capacity for ships. Shipowners won’t change from internal combustion engines to fuel cells until significant technological improvements are made.
Expanding on the safety concerns, ammonia is toxic, corrosive, and if even 0.5 percent of the air you breathe consists of it, it will kill you. High concentrations of ammonia in confined spaces, such as in a ship engine room, are a fire and explosion hazard; a new report by classification society Bureau Veritas and oil giant TotalEnergies confirms that ammonia’s toxicity poses serious health risks for crews.
The Wall Street Journal recently claimed methanol to be an early non-fossil contender to supplant heavy fuel oil. It has a higher energy density than hydrogen and ammonia at 4.33 kWh/L and is a liquid at room temperature, which means it doesn’t need to be stored in cryogenic chambers like hydrogen does.
Methanol can be produced through a variety of feedstocks, including natural gas, coal, and biomass, as well as renewable sources. Yet as DNV reports, market introduction of green methanol isn’t expected until 2024/2025, and will begin in small quantities. With limited production its price, on average, is 50% to 100% higher than bunker fuel.
Methanol is far less toxic than ammonia and therefore safer to transport, according to Recharge News. Thanks to its higher safety profile, the IMO has already provided a solid regulatory basis for designing, building and operating ships powered by methanol per DNV, which reported that thirty-five methanol powered ships have been ordered in their recent Maritime Forecast to 2050, up from eleven in operation. This greatly outnumbers the three hydrogen ships on order and zero ammonia ships combined.
Batteries are the simple answer to ship decarbonization. However, battery-powered vessels are few and far between aside from a handful of fully-electric demonstration ships. Batteries are primarily used in hybrid ship designs alongside fossil-based internal combustion engines if they’re used at all.
Battery cost, safety, and performance are the main reasons ship electrification has not hit the mainstream up until now. Lithium-ion battery systems onboard marine vessels typically cost $500/kWh, with the higher-than-average cost being attributed to additional packaging and systems required for marine environments. Lithium-ion batteries are in short supply as well which contributes to the high price, with key materials used in them being fought over between the EV, stationary storage, maritime, and other commercial sectors.
Safety depends on the chemistry as well. Lithium-ion batteries are inherently flammable, and while battery fires onboard marine vessels are rare, when they do happen they can be catastrophic, as seen with the February 2022 fire that ultimately sank the Felicity Ace. Lithium-ion battery fires burn hot and intensely, and release toxic fumes in the process. Safer battery alternatives are being developed that leverage non-flammable and non-toxic designs, which are expected to pour water on these issues.
Lithium-ion batteries have lower energy density than the other fuel alternatives at 0.5 kWh/L, which makes them ill-suited for primary propulsion on long-haul voyages. However, with increasing performance, declining battery costs and creative ship design, the prospects of all-electric interregional container shipping are accelerating, according to Nature.
Batteries will always be more efficient than hydrogen, ammonia, and methanol. By this we mean a lot less energy is lost capturing energy for use onboard ships than for the others, which will always take more energy to produce than they give in return.