Hydrogen is one of the zero-carbon fuels being considered for use in marine applications of the future. The other zero-carbon fuel is ammonia, and the production pathway of the two are directly linked. Hydrogen can be produced from a variety of sources, utilising conventional or renewable energy, which determine the cost of the fuel to the end-user as well as its lifecycle carbon footprint.

Hydrogen can be extracted from fossil fuels and biomass, from water, or from a combination of the two. The total energy used worldwide hydrogen production in 2019 was about 275mn t oe, which corresponds to 2pc of total energy demand, according to the IEA.

Reducing the cost of green hydrogen to $2/kg would make it competitive for use in the marine sector

Natural gas is the primary source of hydrogen production and is used widely in the ammonia and methanol industries, with this ‘grey hydrogen’ accounting for 75pc of global output. The second source of hydrogen production is coal; this ‘brown hydrogen’ is the dominant form in China and accounts for 23pc of global production. The remaining 2pc is based on oil and electric power.

The strong dependence on gas and coal means that the production of hydrogen is very carbon intensive, ranging between 10t CO₂/1t H₂ for gas to 19t CO₂/1t H₂ for coal, but these emissions can be reduced with the use of carbon capture and sequestration technology.

However, the most interesting option for the future is the production of green hydrogen through electrolysis of water using fully renewable energy.

Production methods

The extraction of hydrogen from gas is accomplished through reformation using three established methods: steam reforming, which uses water as an oxidant and a source of hydrogen; partial oxidation, which uses the oxygen in air in the presence of a catalyst; and autothermal reforming, which is a combination of the first two.

In all cases, syngas (CO + H₂) is formed and then converted to hydrogen and CO₂ through the water-gas shift reaction. However, to reduce the carbon intensity of hydrogen production, biomass can be used for production of syngas though gasification, or renewable electric power can be used to electrolyse water.

275mn t oe - Energy used to create hydrogen globally

Once produced, hydrogen can be stored as a gas or liquid, depending on the amount, the storage time and the required discharge rate. Hydrogen use can range from small-scale mobile and stationary applications to large-scale intercontinental trade. Therefore, different applications create different storage needs.

The availability and low cost of coal and gas make the production of hydrogen more economical than using renewable energy, which is reflected in the cost of the finished fuel. The cost of brown and grey hydrogen ranges from $1-4/kg, whereas the price of green hydrogen ranges from $6-8/kg.

However, the cost of producing green hydrogen has halved since 2015, and this trend is expected to continue in the coming decade as the number of projects focused on deploying renewable energy for hydrogen production increase. Reducing the cost of green hydrogen to $2/kg would make it competitive for use in the marine sector.

Marine applications

The heating value of hydrogen is the highest among all candidate marine fuels, at 120mn J/kg. However, its energy density per unit of volume, even when liquefied, is significantly lower than that of distillates. Compressed hydrogen at 700 bar has only c.15pc the energy density of diesel, so storing the same amount of energy onboard requires tanks about seven times larger.

This means that compressed or liquefied storage of pure hydrogen may be practical only for small ships that have frequent access to bunkering stations. The deep-sea fleet may need a different medium as a hydrogen carrier, such as ammonia or liquid organic hydrogen carriers, to significantly limit the loss of cargo space. Ammonia has higher energy density than hydrogen, which reduces the need for larger tanks, but its advantages need to be balanced against the energy losses and additional equipment required for conversion to hydrogen before it is used in the engines or fuel cells, according to a 2019 IEA report.

Alternatively, ammonia could be used directly as a liquid fuel in engines rather than as a hydrogen carrier. Reducing the size of the tanks needed for hydrogen storage is an active research topic. In addition, hydrogen storage in solid-state materials such as metal and chemical hydrides is in the early stages of development, but it can enable a higher density of hydrogen to be stored at atmospheric pressure.

Bunkering facilities

The cost of hydrogen bunkering facilities is expected to be higher than the cost of such facilities for LNG, primarily because of the greater cryogenic storage requirement of liquid hydrogen and the materials needed for tanks, pipes and seals. The main cost components are the storage and bunker vessels, which need to be scaled based on the number of ships serviced. On-site availability of hydrogen would be needed for small ports given the lower flows and high cost of dedicated hydrogen pipelines.

However, ship and infrastructure costs are a relatively small fraction of total shipping costs over a 15-20 year lifespan, with the fuel cost being the primary factor, according to the IEA.

Developing the hydrogen economy has been seen in the energy and transport sectors as the potential long-term objective to provide a sustainable and clean future. This would require the production of hydrogen from clean renewable sources and the commercialisation of fuel cells. Fuel supplied directly from hydrogen sources, rather than through the reforming of other hydrogen carriers, is the preferred option. 

By Sotirios Mamalis, Manager, Sustainability, Fuels and Technology, ABS

---

Author: Sotirios Mamalis