Decarbonisation of Transport Myth Busting

Fact checking – How Safe is Hydrogen as a Transport Fuel?

Hydrogen is often discussed as fundamental in helping to decarbonise the transport sector, especially where energy density requirements, duty cycles or refuelling times are important, such as maritime, aviation, rail, and long-distance road freight. Interest in hydrogen continues to grow particularly in response to the Government’s net zero targets and this has resulted in the release of the UK’s first ever Hydrogen Plan, which outlines the steps that need to be taken over the coming years to deliver the benefits that hydrogen can bring to help us reach Net Zero by 2050.

Why is Hydrogen often considered unsafe?

Despite its potential, there are many who believe that hydrogen is too dangerous to base our low carbon future on. References are often made to the infamous Hindenburg airship disaster as an example of where, in the past, hydrogen use in transport has tragically gone wrong. So just how safe is hydrogen as a transport fuel?

Hydrogen can be used as a fuel in both liquid and gaseous form. While each option has its own strengths and weaknesses, one of the most significant differences is that liquid hydrogen has a higher energy density and can therefore be used in situations where longer distances and additional cargo space is desired, for example in long distance HGVs or long-haul aviation. However, due to its very low boiling point, liquid hydrogen (LH2) must be stored cryogenically, meaning that there are additional hazards relating to exposure to extremely low temperatures, if LH2 is to be used as a fuel. This article focuses on the safety aspects of storing and using hydrogen in gaseous form, however many of the points discussed are also relevant to LH2.

It is true that hydrogen-air mixtures can combust or explode in the presence of a spark, but this is the case for most other fuels in use today. The high amount of stored chemical energy is what make fuels useful for energy intensive applications such as transport, generating electricity, manufacturing and heating our homes.

As such, hydrogen, like many other fuels, is considered a dangerous substance as stated in the Dangerous Substances and Explosive Atmospheres Regulations 2002. However, in many cases, hydrogen’s characteristics differ from other gaseous fuels labelled as dangerous substances such as natural gas and liquified petroleum gases (LPG). Does this necessarily make it more dangerous to produce, store and use than other gaseous fuels?

Hydrogen is lighter than air

Where hydrogen is different to other fuels is in its high buoyancy properties (hydrogen is less dense and lighter than air) and its very high diffusivity, which allows hydrogen to quickly dissipate. This means that, for any given leak, the chance of the hydrogen/air mixture reaching flammable levels is reduced provided

that the leak is not in an enclosed space. When this is combined with informed design (e.g., large open spaces), this can lead to a considerable reduction in the risk from fire and explosion and these properties can simplify some aspects of the safety case compared with other fuels. Some examples of good practice when designing structures containing hydrogen components include the following:

  • As hydrogen is lighter than air, inlet openings for ventilation systems should be located close to floor level. The outlet opening should be located at the high point of the room in the roof or exterior walls
  • Ventilation rates should be sufficient to reduce hydrogen leaks to 25% of the lower flammable limit (LFL) of hydrogen in air
  • Components or structures which might accumulate hydrogen such as ceilings, covers or inverted pockets should be avoided or adequately ventilated.

On the other hand, LPG and petrol both form vapours which are denser than air, meaning that they concentrate on the ground and increase the chance of ignition by taking longer to dissipate. This comparison is important, particularly in the case of refuelling stations, as this is where there is likely to be the greatest public interaction with hydrogen. In the event of a hydrogen leak occurring at a suitably designed refuelling station, the hydrogen will quickly dissipate away from the refuelling station, mitigating the chance of ignition and danger to the public. In the event of a similar leak of petrol, the vapour will remain near the source of the leak (in this case, the person refuelling) for longer, increasing both the chance of ignition and risk of harm as ignition would be close to ground level.

Of course, there are parts of the transport network where enclosed spaces are unavoidable, such as tunnels. Work is underway to investigate the specifics of hydrogen safety within tunnels the HyTunnel project is an example of this.

But hydrogen is more flammable than other fuels?

This is partly true, Hydrogen/air mixtures are flammable over a wide range of concentrations, and this does intuitively suggest that hydrogen is more likely to ignite or explode than other fuels. However, many argue that the lower flammable limit (LFL) – the lowest concentration of a flammable mixture of gas or vapour in air that can be ignited at a given temperature and pressure – is actually the key parameter rather than the flammable range when determining the fire and explosion risk. The higher the LFL the lower the risk of ignition from external sources. This is because in the event of a leak, there are only rare occasions where the ignition comes significantly after the fuel/air mixture is well established. It is more likely that, following a leak, ignition will occur when the front of the flammable cloud (where the concentration is equal to the LFL) reaches an ignition source.

When comparing the LFL values of several fuels in table 1, it could be argued that in some cases, hydrogen is actually less flammable than some commonly used fuels (e.g., petrol and propane) because it has a greater LFL.

The LFL also dictates the size of a flammable cloud. All else being equal, a fuel with a greater LFL (e.g. hydrogen or methane) will have a smaller flammable cloud because the vapour released by a leak will be flammable for a shorter distance. Beyond this distance the concentration is diluted such that it is no longer flammable (see figure 1a). This is important because a smaller flammable cloud mitigates the fire/explosion risk by reducing the likelihood of a fuel/air mixture reaching an ignition source before the fuel concentration returns to a safe level (figure 1b).

While this already sounds promising for hydrogen’s safety case, we must also consider its low density and high diffusivity discussed in earlier sections. Provided that the leak does not occur in enclosed spaces, these dissipation effects make it even less likely for the LFL concentration to be reached, further reducing the fire/explosion risk for a given leak of hydrogen when compared to other fuels such as petrol and LPG. This can be explained by the following mechanisms:

  • High dissipation increases the rate of reduction of the concentration of hydrogen as you move further from the source of the leak. This in turn means that the fuel/air mixture will reduce to a level below the LFL at a point closer to the leak, leading to a smaller flammable cloud.
  • Hydrogen’s dissipative properties reduces the time that a given release of hydrogen will remain flammable (above the LFL) before it dissipates.

Crucially the dissipation effects reduce the chances of a flammable mixture of hydrogen in air (where the concentration is above the LFL) from reaching an ignition source and igniting/exploding before the concentrations return to safe levels.

What happens if hydrogen does ignite?

Of course, when ignited, like any fuel, hydrogen burns or explodes (depending on the environment) which presents safety concerns. That said, one interesting characteristic of hydrogen that may work in its favour is that it burns fast (approximately 8 times faster than natural gas or propane in air). Although this will make hydrogen fires more difficult to contain, the rapid rate of burning means that hydrogen fires give less heat to the surroundings than for other gaseous fuel fires. This reduces the chances of creating a secondary fire in surrounding materials which, you could argue, makes containing fires started by hydrogen easier than for other fuels, as they are less likely to ignite other objects and cause the fire to spread beyond the hydrogen fuel source.

Once ignited, hydrogen’s properties can present a higher risk in certain conditions when compared to other fuels. For example, hydrogen’s faster burning rate also leads to a greater explosion pressures and greater rates of pressure rises for a given scenario. However, as mentioned previously, hydrogen’s high rate of dispersal means that this is only likely to occur in confined spaces meaning the risk can be mitigated through informed design decisions.

How do we fight a hydrogen fire if burning hydrogen is almost invisible?

Except for large hydrogen fires where a surrounding heat haze is visible, hydrogen fires are nearly invisible in daylight which make them different to deal with compared to fires from other fuels. As a result, hydrogen fires need to be approached with this characteristic in mind. The harm that an invisible gas fire can pose is clear, and along with the low heat that hydrogen burns at, the risk of individuals approaching the fire without knowledge of its presence must be addressed. The most effective way to fight a hydrogen fire is to stop the flow of fuel, and detection of hydrogen leaks may mean the supply of fuel stops before a fire or explosion is triggered. There are several different methods that can be used to be used to identify a hydrogen leak. For example, fixed gas detection systems consisting of gas detection sensors linked to a gas control panel, are designed to continuously monitor for hazardous gases such as hydrogen. These systems can be designed with automatic shut-off capabilities where the supply of hydrogen is immediately shut down if the sensors detect concentrations at dangerous levels. Performance is dependent on the placement of the gas detectors so care should be taken to ensure these sensors are placed in suitable positions as close to a potential gas leak source as possible (leaks are most likely to arise from components such as pumps valves, flanges, joints and shut-off devices). Due to hydrogen’s extremely light weight, hydrogen detectors should be placed above a potential leak source.

Additionally, there are systems which can monitor the internal piping pressures and/or flow rates. A change in these values might suggest a leak is present in the system and this can notify relevant personnel to investigate and take appropriate action to identify and repair any leaks.

For situations where the leak is not detected, trained personnel and hydrogen fire-fighting procedures are key. Thermal imaging can be used to see the burning hydrogen, and by shutting off the hydrogen source and preventing it from spreading by cooling surrounding equipment, the fire will quickly burn out. Hydrogen combustion is faster than that of other fuels, a hydrogen-air gas cloud will burn in a matter of seconds, meaning that large hydrogen fires burning for an extended period of time are likely to be limited to industrial settings where significant quantities of hydrogen are present.

Hydrogen can be safe, but only if we use it correctly

It is important to understand that hydrogen, like other fuels, is flammable and presents significant risks if it is not handled correctly. But many of these risks can be suitably mitigated in the same way that we have for petrol, diesel, LPG and other fuels that are commonly used in today’s transport system. There are certain risks which are more specific to hydrogen due to its unique characteristics, but these peculiarities are part of what could enable hydrogen to be safer than what currently powers our transport system, at least under certain conditions.

Hydrogen is only more dangerous than the fuels we currently use, if we handle it as if it behaves in the same way. By understanding hydrogen’s unique characteristics, such as its low density and fast burning rate, we can design hydrogen systems accordingly (avoiding confined space etc.) and update fire-fighting processes and procedures to mitigate as much risk as possible and get to a point where it is at least as safe to use as current fuels such as petrol or LPG.

As hydrogen use becomes widespread, people will become more familiar with how it is used and the systems will become even safer through learned behaviours as well as data driven safety improvements informed by initial use cases such as trials, demonstrations and other early adopters. This will enable the benefits of hydrogen to be delivered safely as systems are improved iteratively, and any initial issues are ironed out before wider societal adoption.

Hydrogen may be a hazardous substance. But this risk hasn’t stopped the use of other dangerous fuels. System design and appropriate processes make the risk as low as reasonably possible, and this is the approach we must also take with hydrogen. By doing this, we will be able to safely deliver the benefits of a society fuelled, at least in part, by low carbon hydrogen.

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