The case for liquid organic hydrogen carriers
With the shipping industry under increasing pressure to cut the emission of greenhouse gases, attention has turned to accessing the power of hydrogen.
Given the problematic and costly storage requirement and low volumetric energy density of liquid hydrogen, the leading fuel contenders for alternative and sustainable shipping fuels are currently ammonia and methanol.
However, there are significant safety and cost concerns associated with their toxic, corrosive nature, and that large quantities of carbon dioxide (CO2) are produced during their manufacture; therefore, there is growing interest in the use of liquid organic hydrogen carriers (LOHCs) such as dibenzyltoluene (DBT) or benzyltoluene (BT) with hydrogen fuel cells to power ships.
This was demonstrated ably by the EU-funded Ship-aH2oy project and its plan to instal a 1MW LOHC power system in the commissioning service operation vessel (CSOV), Edda Breeze, by 2027, or another similar vessel in the fleet.
The supporting argument for LOHCs are wide-ranging:
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They are much safer by virtue of their very low flammability, non-toxic, non-explosive nature.
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They are stable at ambient temperatures and pressures and totally compatible with existing fuel infrastructure.
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Purified DBT is used as a heat transfer oil in industry and widely available.
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They are reusable and since the weight of hydrogen in hydrogenated LOHC is very small, the stability of the vessel remains virtually unchanged during the voyage thereby reducing the need to take on ballast water when not carrying cargo.
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In the case of short sea shipping (SSS) fitted with additional fuel tanks to compensate for the low volumetric energy density, LOHCs may well remove the need for a ballast water treatment system altogether which represents a major cost saving.
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Polymer electrolyte membrane (PEM) hydrogen fuel cell technology is well established and with current development focused on the direct conversion of chemically-bound hydrogen in the fuel cell using a single, platinum-based catalyst, it avoids any issues associated with the release of free hydrogen.
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Using LOHCs to transport liquid hydrogen offers an attractive alternative to the only dedicated liquid hydrogen tanker (LH2) currently available, the Suiso Frontier.
How to make LOHCs financially viable
In general, there needs to be a significant reduction in the cost of producing green hydrogen, along with a steady, reliable supply of green electricity, plus an efficient distribution and collection system for hydrogenated and dehydrogenated LOHC.
Until recently, the only commercially available means of producing green hydrogen has been the wind, solar, nuclear or hydro-powered electrolysis of water which is very inefficient with a 20% loss of energy.
However, there is now a viable alternative in the form of plasma methane pyrolysis with the Monolith Olive Creek plant already operating at a commercial level of readiness. In addition, the by-product (pure carbon) can be used to produce a range of graphene and carbyne products for which there is a growing demand.
Logical locations in the UK for building LOHC hydrogenation and plasma methane pyrolysis plants are those already designated government-backed ‘hydrogen hubs’, and those currently being used to either produce hydrogen or import liquid natural gas (LNG).
To ensure a continuous supply of green electricity, the best solution is to use electricity generated by an existing nuclear power station and consideration should be given to bringing Aquind’s controversial ‘Portsmouth-Normandy electricity link’ ashore at Fawley, Hampshire.
Wind and solar-generated electricity should ideally be produced in an area with long hours of sunshine and where trade winds blow virtually continuously throughout the year. It is therefore logical that consideration be given to utilising the electricity that will be provided by the Xlinks Morocco-uk Power Project.
And finally, given the low volumetric energy density of LOHCs, it would be logical to focus on ships undertaking short voyages, namely SSS vessels, and with EU waters in mind, it is suggested that a fleet of hydrogen-powered coastal tankers, operating out of Southampton and Milford Haven, could supply European ports with hydrogenated LOHCs and concurrently collect the discharged dehydrogenated LOHCs.
This represents the views of the author, rather than IMarEST.
Image: The commissioning service operation vessel Edda Breeze in the port of Emden, Germany; credit: Shutterstock.
