Hydrogen is currently produced primarily from fossil fuels and is mainly used in oil refining and chemical production. When it is produced with zero or low emissions, it has the potential to play a role in the energy transition, particularly in sectors that are difficult to electrify such as maritime shipping or steel production. Clean hydrogen could also be used as a feedstock in the production of certain chemicals or synthetic hydrocarbons.
The EU’s current ambition for clean hydrogen under its RePowerEU programme includes plans to produce 10 megatonnes (Mt) and import a further 10Mt by 2030. The scale of such plans raises important questions over how best to transport hydrogen. Transporting it as a compressed gas or liquid comes with challenges including embrittlement of equipment and pipelines as well as leakage and boil-off.
Another option that is being explored is the use of liquid hydrogen carriers, meaning molecules used to bind and release hydrogen. These molecules can be organic (LOHC) or inorganic (LIHC) depending on whether they have carbon in their molecular structure.
Typically, hydrogen safety focuses on issues such as fires and explosions. However a study by the Dutch National Institute for Public Health and the Environment (RIVM) published earlier this year highlighted a little-known risk that LOHCs could be substances of very high concern.
‘Little to no attention’ to SVHC risk
Marino Marinkovic, co-author of the report, says that hydrogen’s role in the energy transition could lead to large quantities being transported globally, however, safety assessments of carrier substances has focused primarily on characterising risks of accidents. He said that the presence of SVHCs has received "little to no attention" in these technologies.
SVHCs are regulated under REACH. However, the Netherlands has a broader list (ZZS) of these substances.
"SVHCs are substances that may have serious effects on human health and the environment because they are, for example, carcinogenic, toxic to reproduction, or accumulate in the environment and food chains," says Marinkovic.
"Our research shows that there are liquid hydrogen carriers where one of the carriers is a SVHC, meaning substantial SVHC volumes would be handled. We also found that other candidates, while themselves not being SVHC, they do form one or more byproducts with SVHC properties, which is also not desirable. In fact, Dutch policy is to minimise the presence of ZZS in the living environment by preferably not introducing new ZZS uses wherever possible. We also note that relevant hazard data could not be found for all candidates, limiting their assessment.
"We hope our report will raise awareness on the SVHC aspect in the hydrogen arena, on one hand encouraging industry to follow a safe and sustainable by design approach where SVCH properties of substances are considered early on in the design process, while at the same time reminding authorities to keep the SVHC aspect on their radar. In that way, the energy transition can be shaped safely and sustainably."
Trading one environmental problem for another?
NGOs welcomed the report, with Anna Lennquist, senior toxicologist at ChemSec, calling it "very important work done by the RIVM".
"It is crucial that the solutions put forward for the green transition are truly green before scaling up. It makes no sense to trade one environmental problem for the other, but this is a clear risk as long as solutions for climate are discussed separately from the discussion on chemical pollution," Lennquist adds.
Geert Decock, energy manager at clean energy and transport advocate, Transport and Environment (T&E), says: "While T&E has not yet developed any in-house expertise on liquid hydrogen carriers, we agree that the fact that most of these carriers are SVHCs is an important aspect to highlight in discussions on long-distance trade of hydrogen. Most focus on hydrogen export and import discussions is about the low cost of producing hydrogen in countries with great solar and wind potential. Much less focus is dedicated to the transport costs and the risks involved in transporting hydrogen over long distances. LOHC or LIHC are often mentioned as potential solutions, but there is limited awareness about the risks involved."
Advantages and disadvantages
The RIVM study selected ten substances and their byproducts (see box) to assess for hazardous properties, which it says represent the breadth of the field and include the most promising candidates. It described the current state of development for each substance, and gave examples of companies currently performing industrial trials of LOHCs and LIHCs.
Chemical Watch News & Insight approached several of the companies mentioned in the report for their views on the results.
A company spokesperson at Hydrogenious LOHC Technologies, which is developing a LOHC based on benzyltoluene, said: "The RIVM publication effectively summarises the current state of knowledge but does not present new findings. Without coming to a definite conclusion, RIVM instead points out that each hydrogen carrier has its own advantages and disadvantages.
"There is no single substance for hydrogen transport that is superior in all respects. However, we are convinced that the LOHC-BT carrier system all in all offers numerous advantages over other carrier systems – it is liquid at ambient conditions but not acutely toxic, explosive, or flammable.
"In terms of mitigation measures, we are very conscious of the risk of handling chemicals. It is always a priority for us. Emissions are minimised and controlled, as is the industry standard for chemical process plants, and our carrier is not released; remaining part of a closed loop system."
Honeywell and Chiyoda Corporation, which are developing similar systems with methylcyclohexane and toluene, did not respond to a request for comment.
The RIVM study said it was unable to draw conclusions on silicon hydride derivatives as LOHCs due to insufficient information available. It said that it is unclear which substances are used as hydrogen carriers and whether solvents are being used. HSL Technologies, which is developing a silicon-based system known as HydroSil, said: "From the tests we already performed, we proved that HydroSil is non-toxic for humans and non-eco-toxic for the environment – tests performed in laboratory, following specific test protocol."
Substances investigated in the report
The ten carrier substances and their breakdown byproducts after hydrogen has been released are:
- methylcyclohexane and toluene;
- perhydrodibenzyltoluene and dibenzyltoluene;
- perhydrobenzyltoluene and benzyltoluene;
- decahydronaphthalene and naphthalene;
- dodecahydro-n-ethylcarbazole and n-ethylcarbazole;
- ethylene glycol and esters of ethylene glycol;
- formic acid and carbon dioxide;
- methanol and carbon dioxide;
- ammonia and nitrogen; and
- silicon hydride derivatives and silica/silicate.
Concerns over ammonia
Ammonia is the only one of the ten substances analysed that had no SVHC properties. However the RIVM report says ammonia "is not an obvious choice" as an energy carrier due to other hazardous properties.
Paul Martin, a chemical engineer and co-founder of the Hydrogen Science Coalition, an independent group of academics, scientists and engineers, agrees. "It is a very toxic and corrosive gas – one that kills people every single year even in closed system uses such as for refrigeration in ice skating rinks. Moving around and using vastly larger quantities of a poisonous liquefied gas as an energy carrier or, worse still, as an open system fuel such as in ship's engines, is a terrifying prospect for any chemical engineer worth their salt. If this is done, it absolutely will come at the cost of human lives which could otherwise have been saved."
However T&E’s Decock says ammonia is a "more promising way of transporting hydrogen over long distances" because the typical production process does not require carbon and that there is no need to convert the ammonia back to hydrogen if it is being used directly, such as for fertiliser production or as a shipping fuel.