Many substances being considered to transport liquid hydrogen have properties that could be harmful to human health and the environment, according to a report from the Dutch National Institute for Public Health and the Environment (RIVM).
The study highlights an overlooked aspect of a proposed transport method in which chemicals are used to bind and release the gas. When in liquid form, they are known as liquid hydrogen carriers (see box).
Thus far, RIVM said safety considerations regarding them have focused mainly on the risk of accidents. Hazards to health and environment have "hardly been on the radar" and should be taken into account, it said in a report published on 6 February.
Dutch ports are likely to handle a large proportion of the estimated 10 megatonnes of renewable hydrogen expected to be imported into the EU by 2030, it added.
RIVM’s report looked at the hazardous properties of proposed liquid hydrogen carriers to determine if they are substances of very high concern (SVHC) under REACH or on the broader Dutch ZZS list of such substances.
It found that nine out of ten substances analysed were either on the ZZS list or had byproducts of them, while the tenth – ammonia – has other safety concerns, such as toxicity, and is "not an obvious choice" as an energy carrier.
Call to use SSbD approach, screen for SVHCs
RIVM recommended that industry screen for ZZS-listed substances when developing liquid hydrogen carriers and that REACH registrations be kept up to date.
Identifying listed substances should be considered early in the process of developing the technology alongside other hazards like explosion risks, RIVM said.
It also encouraged the use of safe and sustainable by design, considering all properties of carrier substances, as well as economic and practical aspects to ensure that the EU’s energy transition meets SSbD criteria.
In addition, RIVM said REACH registrations should be updated "when the application of a substance is altered", such as, use of heat transfer oil under closed conditions as a large-scale liquid hydrogen carrier.
Government authorities should ensure developers, financiers and policymakers are aware of this type of safety analysis, it said.
Liquid hydrogen carriers
Hydrogen can be transported in gaseous or liquid form. But the report said that dedicated infrastructure is needed before it can be done so in bulk.
Hydrogen carriers are molecules that can be used to bind and release the chemical element. Its uptake is called hydrogenation, an exothermic reaction that produces energy as heat; the reverse is dehydrogenation which results in energy loss.
The carriers are liquid at ambient environmental conditions and more likely suited to transport and storage infrastructure already in place for fossil fuels.
These substances can either have carbon or no carbon in their molecular structure. They are known as liquid organic hydrogen carriers (LOHC) and liquid inorganic hydrogen carriers (LIHC) respectively.
Analysis
RIVM investigated ten carrier substances representing the breadth of chemicals used in the field as well as the most promising candidates.
The ten substances and their breakdown byproducts after dehydrogenation 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.
Even if the carrier substance itself is not a ZZS, the byproducts can be, RIVM said.
Using them for bulk transport of liquid hydrogen "would appear especially troublesome" because it would mean that large quantities of high-concern substances would have to be handled and shipped, it said in the study.
It also noted the potential for environmental impacts and energy use from some of the carriers.
Two of the substances – formic acid and methanol – can release carbon dioxide during the dehydrogenation process, RIVM said. If this is captured before hydrogenation, then there is no net increase in the atmosphere. However, carbon capture itself is a challenging process, it added.
The analysis also found varying properties to the substances, for instance, some need a high temperature for dehydrogenation, making for an energy intensive process. Other substances have limited hydrogen release or require additional steps such as use of a solvent.
RIVM said it only considered the ZZS properties of hydrogen carriers in this report; and not potential exposure to people and the environment. These risk assessments could be addressed in follow-up research, it said.