Safety, risks, and hazards of hydrogen
The safe use and distribution of natural gas are fully understood, but a similar level of technical understanding is now required for the key properties of hydrogen.
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There is an emerging consensus that low-carbon and renewable hydrogen will play an important role in a future decarbonised energy system. How prominent a role remains uncertain, but various estimates point to hydrogen being anything from 10-20% of global energy use in a future low-carbon energy system. DNV’s Pathway to Net Zero has hydrogen at 13% of a net-zero energy mix by 2050 (DNV, 2021).
Scaling global hydrogen use is beset by a range of challenges: availability, costs, acceptability, safety, efficiency, and purity. While it is widely understood that urgent upscaling of global hydrogen use is needed to reach the Paris Agreement, the present pace of development is far too slow and nowhere near the acceleration we see in renewables, power grid, and battery storage installations. Nevertheless, there is a great deal of interest among a range of stakeholders and the media in the promise of hydrogen. Yet some commentators are taking a careful, dispassionate look at the details behind hydrogen’s likely global growth pathway, including safety.
Hydrogen is the most abundant element in the universe. However, on Earth it is found only as part of a compound, most commonly together with oxygen in the form of water but also in hydrocarbons. Hydrogen is the simplest of all the elements, but processes to produce it in pure form are not so simple; they are energy intensive and involve large energy losses, have significant costs, and can produce their own carbon emissions.
Safety, risks, and hazards
Hydrogen is not new to society; it has been produced and used in large quantities for more than a century. However, this has mostly been in industrial environments where there is a good degree of control, and where facilities are managed by people who have a clear understanding of the potential hazards. The forecasted significant market growth of hydrogen as an energy carrier will introduce many new hydrogen facilities that are very different from those we have had in the past. Moreover, some of the facilities will be in much closer proximity to the public. They will be built and operated by new entrants who may not have relevant experience in hydrogen safety.
Risk perception will be an important factor in acceptance of hydrogen use. Accidents involving hydrogen are likely to receive more media attention than comparable events with conventional fuels (at least initially). This could induce public resistance and prompt a more restrictive regulatory environment. The sensitivities to risk and risk perception will likely vary among sectors. However, they will be highest where the public is near the actual use of hydrogen, such as in aviation and domestic heating, and less so in more industrial-type applications, such as hydrogen storage.
Safety represents a significant business risk to investors and developers. There have already been examples where incidents at hydrogen refuelling stations have halted hydrogen use in vehicles for significant periods.
The industry has tried-and-tested methods for managing the safety of flammable gases that have been used for decades and these come with some very important, hard-won lessons:
υ Safety must be based on an understanding of how the properties of hydrogen and hydrogen derivatives affect the potential hazards ϖ It is by far most effective (in terms of both safety and cost) if appropriate risk-reduction measures are added early in the design stage. In many instances, if addressed early, these measures can be incorporated at little (and at times no) extra cost and can result in inherently safer designs ω The design intent needs to be maintained through the full lifecycle: safety measures should not degrade.
Achieving all this requires an understanding of the key properties of hydrogen (and its derivatives) that affect the hazards. As hydrogen is very different to its derivatives, we need to consider those separately.
Hydrogen is a flammable, non-toxic gas in ambient conditions. The effect of its properties on hazards and hazard management is probably best understood by reference to another flammable, non-toxic gas that is widely accepted by society: natural gas (or its primary component, methane).
So how do the properties of hydrogen change the potential hazards? For hydrogen, as with natural gas, ignition of accidental releases can result in fires and explosions. Research is very active in these areas, and DNV is engaged in large-scale experimental studies at our Research & Testing facility in Cumbria, UK. Although our understanding is still developing, we know enough to recognise where to concentrate efforts with hydrogen. Table 1 summarises the differences between hydrogen and natural gas/methane in both gaseous and liquid forms.
Ignition of a flammable gas cloud does not always result in an explosion. Pressure is generated when either the gas cloud is confined within an enclosure or the flame accelerates to high speed (or both). This could occur in a wide range of possible scenarios, from low-pressure leaks in domestic properties, medium-pressure leaks in hydrogen production facilities or marine applications, to high-pressure leaks from storage facilities.
The severity of an explosion will depend on many factors, but in general, the more ‘reactive’ the fuel, the worse the explosion. Reactivity, in this sense, relates to how fast a flame moves through a flammable cloud. At its worst, hydrogen flames can burn about an order of magnitude faster than natural gas and much faster than most commonly used hydrocarbons.
To add to this, when a flame travels very fast, going supersonic, the explosion can transition to a detonation. A detonation is a self-sustaining explosion process with a leading shock of 20 bar that compresses the gas to the point of autoignition. The subsequent combustion provides the energy to maintain the shockwave.
Detonability varies from fuel to fuel, and detonations would not occur in any realistic situation with natural gas but are entirely credible for hydrogen. It is also notable that current explosion simulation methods used by industry are not able to model the transition to detonation but only indicate when it might occur, though there is still considerable uncertainty in this area.
This sounds like bad news for hydrogen facilities, yet we know that these properties depend on the concentration of the fuel in air. If concentrations are kept below about 15% hydrogen in air, it is no worse than methane at similar concentrations. The implication is that a key element of managing hydrogen safety is the control of gas dispersion and build-up to prevent the concentration of hydrogen in air from exceeding 15% as far as is practicable. This is a particular challenge where dispersal space is constrained – for example, on board ships. Gas detection and rapid isolation of hydrogen inventories will be key measures. Consideration of ventilation rates and ventilation patterns is also critical. Importantly, current simulation methods can model gas dispersion and build-up with reasonable confidence.
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