Much has been said lately about the cost of hydrogen for heating in people's homes, with 32 independent studies all concluding that it would be far more expensive than all-electric options such as heat pumps.

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But there is another issue that would be of perhaps even greater importance to households — whether burning hydrogen in their homes would actually be safe.

The simple answer is that hydrogen is inherently less safe than natural gas from a fire and explosion standpoint. It has a much broader explosive range, it is much more prone to leak, a much lower ignition energy level and can produce higher blast overpressures in explosions.

In its favour is its buoyancy; it will disperse more easily than natural gas and there is no carbon monoxide emission risk with hydrogen combustion. Another downside to hydrogen, however, is it produces more NOx — a greenhouse gas 265 times more damaging to the planet than CO2 over a 100-year period — than natural gas when combusted.

The UK government investigated the safety of hydrogen in its Hy4Heat programme (under Work Package 7) — the aim of which was to demonstrate that hydrogen for household heat can be made as safe as natural gas from a fire and safety viewpoint.

To demonstrate comparable risk — ie, a combination of probability of the event happening and the consequence of the event — a Quantified Risk Assessment (QRA) was undertaken, led by engineering firm Arup.

Arup found that there would be many more fire and explosion events with hydrogen — 39 a year (across the UK population) compared to nine for natural gas. When taking into account the consequences of those event, it gave a predicted number of individuals injured annually of 65 for hydrogen, compared to 17 for natural gas.

Clearly, unmoderated hydrogen for domestic heating is much less safe from a fire and explosion standpoint than natural gas.

Bringing the overall risk down to the same level as natural gas would require the installation of two excess flow valves (EFV) in series. So if a hydrogen leak occurs in a household, the EFVs are there to reduce the size of the leak so the consequence of a fire and/or explosion will be reduced.

These EFVs would bring the overall risk down to 16 injuries a year for hydrogen, says the Arup report. However, the frequency of fires/explosions remains three times higher for hydrogen than of natural gas — 26 compared to nine.

In other words, there will be three times as many fires and explosions with hydrogen, but because the EFVs have reduced the size of the hydrogen gas leak, an individual is less likely to suffer injury.

The Hy4Heat QRA project concludes: “This assessment indicates that the use of 100% hydrogen can be made as safe as natural gas is when used for heating and cooking in certain types of houses (detached, semi-detached and terraced houses of standard construction), that were studied.”

I have worked with hazardous substances for over 40 years and taught process safety at university. To my mind, the Arup QRA is not a convincing demonstration of the domestic safety of hydrogen. Would you buy a car if the salesman told you it will crash more often but because of the safety features you will be just as safe?

Here is another take. The risk to an individual in the QRA is made up from occupancy in two locations; the kitchen and the whole downstairs. If you look only at the kitchen (a place where I spend much of my time), the frequency of an EFV-protected hydrogen explosion is 19 times a year across the UK, compared to six for natural gas. The overall number of injuries in the EFV-protected kitchen is eight for hydrogen and six for natural gas. So the kitchen is now a less safe place.

Let’s extend that argument to two adjacent chemical plants — A and B. Modifications are made that means the individual risk in plant A increases but reduces in plant B. I work in plant A, do I believe that my increased risk is acceptable because other colleagues are safer?

The Royal Academy of Engineering’s recent report, The Role of Hydrogen in a Net Zero System, cites Hy4Heat and comments: “There are limitations of the Hy4Heat assessment that future projects and demonstrations will need to address, such as flats and multi-occupancy buildings, housing that lacks natural ventilation, and the safety of supply through gas networks to homes.”

It adds: "The [UK] Health and Safety Executive [HSE] independently reviewed the evidence and was satisfied that the assessment provides an adequate basis for gas network operators to design and risk-assess future hydrogen trials.”

In a letter of assistance to the UK government's energy department, the HSE refers to the Hy4Heat programme and states: “... [The] HSE is satisfied that this provides an adequate basis, if applied appropriately by the relevant dutyholder, for: Designing the scope of future hydrogen trials, Assessment of the risks arising from those trials, and Management of those risks in accordance with a suitable and sufficient risk assessment for those trials.”

In other words, the HSE is clearly not in a position to recommend that hydrogen can be made safe in a domestic setting. Much more work is needed.

I, however, am unequivocal — there is sufficient knowledge and evidence available now to dismiss hydrogen for domestic heat.

When I worked for ICI Petrochemicals I was fortunate enough to meet Trevor Kletz and attend his training courses. Kletz is one of a small grouping of chemical engineers that the Institution of Chemical Engineers have labelled as having “changed the world”. His contribution to process safety is immense and he is viewed as the father of inherent safety. I use much of his teachings in my own safety lectures.

Kletz structured inherent safety principles as follows — and none of them sit well with domestic hydrogen:

Intensification: Use small amounts of hazardous materials so the consequences of accidents arising from the escape of materials are much reduced.

Substitution: Use a less hazardous material – less flammable or less toxic.

Attenuation: If a hazardous material must be used, use it a) under less hazardous conditions or b) in the least hazardous form.

Limitation of effects: Limit the effects of failures by changing the design or conditions of use rather than by adding protective equipment that may fail or be neglected.

I wonder what Trevor would have thought about household hydrogen using excess flow valves to mitigate the risk of fire and explosion?

I think he might have said: “Don’t be silly.”

Tom Baxter, FIChemE, is an independent energy consultant, visiting professor at the University of Strathclyde and a founding member of the Hydrogen Science Coalition. Before semi-retiring, he was technical director at Genesis Oil and Gas Consultants and a senior chemical engineering lecturer at the University of Aberdeen.