Hydrogen infrastructure—Efficient risk assessment and design optimization approach to ensure safe and practical solutions

Highlights

• Hydrogen risk assessments and safety assessments.

• Consequence tools – screening models and CFD.

• Liquid and compressed hydrogen.

• Assessment of explosion incident.

• DDT and detonation.

Abstract

With the ambition to cut emissions from transport hydrogen fuelled vehicles and marine vessels are now being introduced several places in the society. To support this development there is a need for infrastructure to produce and transport gaseous and liquid hydrogen. The properties and safety challenges related to the use of hydrogen are very different from those of conventional fuels, thus safe design may require unconventional solutions. Hydrogen has extreme properties in many ways. It is buoyant when in gas phase while a liquid hydrogen spray will develop a dense plume. The reactivity is higher, flammable range wider and the ignition energy lower than for conventional fuels. Flames may be invisible, and radiation is low. When performing risk assessments for land planning purposes, bunkering assessments or passenger and crew safety these aspects must be reflected. Properties like the positive buoyancy, strong dilution for sonic releases into air, and a low reactivity and energy content for concentrations below 10% must be exploited during design to ensure acceptable risk levels. In this article a two-level risk assessment and design optimization approach is presented in which risk screening with rapid consequence calculations and frequency assessments for release, dispersion, fire and explosion can be performed during concept selection phase with indicative hazard distances estimated. Possible risks of concern are in this way identified, and design can be adjusted, or mitigation measures introduced. For final design risk assessment CFD calculations can be performed for more precise consequence estimates. The risk assessment approach is described with illustrating examples. The focus is not only to ensure safety, but to do so in a cost-efficient and practical way.

Introduction

In the years 2005–2010 there were numerous R&D initiatives looking into the use of hydrogen as a zero-emission fuel for cars and buses, and safety aspects were evaluated in projects like HySafe and in IEA Hydrogen Implementation Agreement expert groups. Some risk assessment studies were performed e.g. for hydrogen refuelling stations (Ham et al., 2011), infrastructure (Hansen and Middha, 2008) and for tunnel scenarios (Middha and Hansen, 2009). Most projects in this period had the character of strongly subsidised demonstration activities, and many players had doubts whether hydrogen would have a role in the future low emission society due to challenges with storage density and price. A stronger dedication from authorities for emission cuts and a drive towards offering competitive zero emission solutions seem to have sparked a new hydrogen wave from 2015 and onwards, this time many of the projects seem to have a more commercial character, and developments are seen to facilitate energy demanding transportation on land (trucks and trains) and sea (fast passenger vessels, car ferries and smaller cruise ships), as well as green and carbon neutral hydrogen production facilities and distribution. In order to support this development a good understanding of the safety aspects of hydrogen is required, including the ability to efficiently and accurately estimate possible risks and give recommendations for risk reduction measures

Being the first element hydrogen properties are in many ways extreme, see Table 1. To optimize safety while ensuring cost efficient solutions it is important to exploit inherently safe properties of hydrogen, like strong buoyancy and low reactivity and energy at concentrations less than 10–15%, while preventing accumulation of hydrogen at more reactive concentrations within congestion or confinement. For the best results safe design should be in focus from the start.

Some important implications of the differences include:

• For significant hydrogen releases inside confined volumes a very high reactivity mixture can be expected, while for methane releases both the likelihood for a flammable mixture and the worst-case reactivity will be much lower.

• For methane, explosion proof equipment may be a robust barrier against explosions, while for hydrogen the minimum ignition energy is much lower with a significant likelihood that a release will self-ignite for some reason.

• Inerting of hydrogen mixtures is more challenging than for methane, with N₂ dilution a minimum O₂ concentration below 5% is required to stop flame propagation (Zabetakis, 1965) while methane becomes non-flammable below 12% O₂ at ambient temperature. If N₂-based solutions are used for inerting the atmosphere could be breathable for methane (37% added N₂ or 0.6 parts N₂ per air volume), but not for hydrogen (73% added N₂, or 2.7 parts N₂ per air volume).

• Deflagration to Detonation Transition (DDT) is a real threat for hydrogen explosions at nearly any scale while for methane (natural gas) DDT has only been reported in major coal mine accidents and selected full scale experiments, see e.g. Hansen and Johnson (2015). In a detonating gas cloud flames are supersonic and would not be influenced by traditional mitigation methods like explosion venting, DDT could even be triggered by vents. Blast energy from a detonation outdoors will normally be significantly higher and sent in all directions while a deflagration to a greater extent will direct blast energy along the flame propagation direction.

• Liquid hydrogen (LH2) with a boiling point of 20.4 K will not co-exist with air and will immediately evaporate while condensing and freezing surrounding air. For large LH2 releases the cold hydrogen-air vapour cloud may be denser than air and have a higher propensity to develop explosive clouds than compressed gas releases with similar release rates. The explosion potential of the cold hydrogen clouds can be very high, due to the higher energy density at low temperatures. Combined with the generally high reactivity of hydrogen, the maximum explosion pressures can be 30–40% higher than for explosions at ambient temperature. Deposits of solidified air may also be a challenge, due to preferential melting and boiling of O₂ and N₂, respectively, situations with O₂ enriched air or O₂-enriched liquid/frozen air can appear. A.D. Little (1960) demonstrated that the latter could detonate when mixed with LH2.