Safety Aspects of Hydrogen

Given the role of hydrogen in electrochemical processes of all kinds, it is critical to treat it safely, both in the laboratory and in commercial or industrial processes [3].Since hydrogen has been a valuable product in the chemical industry for over a century, there is a wealth of expertise in treating, including storing and transportation [3]. Additionally , all the sites that store liquefied hydrogen must have a placard that indicates danger as it is shown on Figure 1.

Introduction

The use of hydrogen as an energy carrier has a bright and competitive future, whereas the cryogenic liquid state is thought to be economical and reliable in terms of storage and transportation [1].It is important to be mentioned that hydrogen liquefaction is an energy -intensive operation , which amounts to approximately 30 % of the hydrogen lower heating value (LHV) [2]. Even so, it seems that the liquid process is the only way to transfer vast amounts of hydrogen over long distances or to store it aboard massive hydrogen-powered vessels such as submarines [2] .

Figure 1: Warning Placard (Source : https://h2tools.org/bestpractices/warning-placards )

Liquid hydrogen was not widely used in industry until after the mid-twentieth century [3]. A significant explanation for this was the space race, when hydrogen is the best fuel for rockets [3]. In addition to gas at all practical pressures, liquid hydrogen is an appealing choice for automotive applications due to its high density [3].

Dangers of Hydrogen

Hydrogen has the second lowest boiling and points of any element , just after helium[4].Hydrogen becomes a liquid at temperatures below the boiling point of 20 K [4].Undoubtedly, this is a very low temperature [4]. Cryogenic temperatures are described as temperatures below 200K, and cryogenic liquids are defined as liquids at these temperatures [4]. A fuel’s boiling point is an important parameter since it determines the temperatures at which it must be cooled in order to be stored and used as liquid [4].

Due to the low boiling point of cryogen and the flammability of hydrogen cloud, a rupture of a storage tank or pipeline during the storage and shipment of liquid hydrogen may result in a leak of liquid hydrogen, posing a significant danger to the surroundings [1].

Figure 2 :Illustration of the fire triangle of Hydrogen ( Source : https://wha-international.com/hydrogen-fire-risk-management/ )

Corrosiveness, Embrittlement and Materials

Hydrogen in liquid form is non-corrosive. Corrosion protection would not necessitate the use of special building materials. Nevertheless, due to the intense coldness of the environment, equipment must be constructed and made from materials that are ideal for very low temperature operation.

Low temperatures can cause material embrittlement [3]. As liquid hydrogen or cold gas is used in a system, the components used in its construction must maintain a certain degree of elasticity at operating temperatures if the pieces constructed from them are to survive the existing stresses or pressures [3].

Most polymers are ineligible for use because they have a glass transition temperature well above 20 K, at which they lose all elasticity [3]. Hydrogen has a high compatibility with organic products [3].Diffusion through them is not dangerous and therefore does not pose a safety risk [3]. Austenitic steels, as well as aluminum and brass, are ideal [3]. Thermal expansion and deformation, but also changes in other structural properties as a result of temperature, must all be taken into account [3].

A material's susceptibility to hydrogen embrittlement is determined by a number of factors [3].As a consequence, the possibility of hydrogen embrittlement must be considered in both material selection and system design [3].

Storage

It is not feasible to store hydrogen completely safely [3].Hydrogen contains energy, and improper storage or other mishandling of it, as with natural gas, oil, or uranium, can result in the accidental release of energy [3]. The pressure at which liquid hydrogen is held is less than 1 bar , which is much lower than the pressure at which compressed gas is kept [3].The extreme temperature differential with respect to the atmosphere is a safety-relevant function of this storage mode [3]. Tanks need extensive heat insulation to reduce heat influx to the liquid, which would otherwise result in rapid evaporation; at the same time, the insulation prevents the tank's atmosphere from the cold temperatures inside.[3] With that being said , each storage system entails its own set of risks.

Despite these precautions, it is almost difficult to prevent minor traces of air from entering the tank[3] .Since nitrogen and oxygen have much higher boiling points than hydrogen, they are more likely to condense and freeze in the liquid, eventually falling to the bottom of the tank [3].If sufficient ignition energy is given, this mixture of air and hydrogen can explode [3].

Consequently, the tank slump should be purged at frequent intervals, or another approach to prevent or manage air accumulation should be used [3].Helium is the only gas that can efficiently purge a cryogenic hydrogen device until it is warmed up [3].This is due to the fact that helium, with a boiling point of 4.2 K and a critical point of 5.2 K, remains a gas while hydrogen is in liquid state [3].

Release of Liquid Hydrogen

Not only is liquid hydrogen very cold, but the gas produced by evaporation is often cold enough to condense air on the exterior of exposed pipes and tubes [3]. Since oxygen has a higher boiling point than nitrogen, it has a propensity to concentrate as air condenses, potentially increasing the fire risk significantly [3].Liquid oxygen dripping from insufficient insulated pipes or cryogenic liquid vessels can aid in the combustion of materials such as tar and bitumen, which do not normally pose a safety risk [3].Touching cold sections with bare skin can result in burn-like effects (cold burns) [3].

The release of a liquefied gas typically results in the deposition and creation of a liquid pool on the field, which spreads radially away from the releasing site, depending on the amount spilled and the rate of release, and which often instantly begins to vaporize [5]. The heat supply from the external conditions , and, in the situation of a burning fire, radiation heat from the blaze, decide the pool's equilibrium state [5].

Accidentally leaked liquid hydrogen evaporates rapidly, forming a transparent cloud of condensed water [3].It is a general rule of thumb that the explosive portion of the cloud coincides with the visible part, although this is not a valid protection guideline [3].When the release is stopped, the cloud will quickly steam, expand, and dissolve in a matter of minutes [3].If the rate of release is fast enough to compete with evaporation, puddles or pools of cryogenic liquid can form[3].The heat of evaporation per unit mass of liquid hydrogen is comparable to that of methane or propane, but much lower per unit liquid volume [3].

Because of this element, as well as the greater temperature difference with the atmosphere, pools of liquid hydrogen can evaporate faster than those of the other liquids [3] .If such a hydrogen reservoir is lit, the fire will have a much smaller range which will be much shorter in duration than all the other gases [3].

Additionally, another advantage that might be considered , is that the evaporation of liquid hydrogen accelerates the upward flow of the cloud [3]. As the liquid mass fraction is increased from 30% to 80%, the downwind distance and height increase by 10.8 and 114.3 percent, respectively, while the detachment distance decreases by 42.6 % [1]. So if placed in highest levels or open spaces the potential of explosion is eliminated .

Autoignition and Explosions

There are several claims that hydrogen release from a leak instantly ignites [3]. This is often referred to as 'autoignition' [3]. This is undoubtedly incorrect; as ‘autoignition' indicates that a material reacts naturally with oxygen when it is present at a sufficient concentration, under normal conditions, and without the addition of additional ignition energy [3].

This probability can be easily ruled out since the autoignition temperature for hydrogen/air mixtures at atmospheric pressure is greater than 800 K [3]. As a result, the reaction mentioned above will almost definitely be triggered not by an autoignition, but by a spontaneous combustion caused by natural effects rather than human interference [3].

There are sufficiently accurate sources to confirm that such ignitions do exist, but the occurrence is rare, difficult to replicate, and therefore not well studied scientifically [3]. One possible theory is that dust and other contaminants in the gas flow can generate static electrical charge and serve as ignition sources until enough oxygen is present to cause a reaction to occur [3].

Hydrogen cannot be considered as an explosive [3]. This is often misunderstood as hydrogen mixtures with air become explosive , however pure hydrogen is not [3]. A hydrogen explosion often necessitates the presence of oxygen or any oxidizing gas in a sufficient concentration [3]. It is vulnerable to ignition as the flammability range is between 4 to 75 % [3].

With regards to safety issues, it is worth mentioning that H2 as a molecule ignites with very low energy, but it is very flammable when it meets air mixture , as mentioned p. Therefore, by storing it in liquid form the risk of explosion is mitigated since the pressure needed is 1bar whereas at gaseous hydrogen the essential pressure is 520bar [6].

Lastly , when a liquefied gas vessel is used, one of the unusual accident situations that can occur is a boiling liquid expanding vapour explosion (BLEVE) [2]. BLEVE is a natural explosion since it is caused by a sudden phase shift and extension rather than chemical reactions [2].

Toxicity and Asphyxiation

Although hydrogen is non-toxic ,asphyxiation and hypoxia may occur because hydrogen can displace oxygen [15]. For humans , oxygen levels below 19.5 %are biologically inactive [4] . For instance, to minimize the chance of explosions, air aboard a submarine is held at lower oxygen concentrations (about 19.5 percent) than in the natural environment [15].

Rapid breathing, reduced mental alertness , lack of muscle control, defective judgement , cognitive dysfunction , depression of all sensations , and exhaustion are all symptoms of oxygen deprivation [4].

Dizziness, prostration, vomiting, nausea, and loss of consciousness can occur as asphyxiation progress , ultimately leading to seizures , coma and even death [4].

Without any previous warning signs, instantaneous unconsciousness may occur at concentrations below 12% [4]. Small leaks pose little risk of asphyxiation in a confined space, but massive leaks can be a major concern since hydrogen diffuses rapidly to fill the volume [4].Owing to hydrogen's high buoyancy and diffusivity, the risk of asphyxiation in open areas is almost non-existent [4].

Measurements to achieve safety

When treating pressure vessels and flammable gases, the general principles of Good Laboratory Practice as well as the relevant legislation for handling pressure vessels and flammable gases should be followed [3]. The properties of hydrogen do not necessitate extreme precautions or protective measures [3].

When hydrogen is being stored in extensive amounts and used regularly , as in the case of Offsh2re Shetland ; daily , the following suggestions are applicable :

Pipes , trucks and any kind of transportation should be insulated very carefully [3].
Make-and-break connections should be avoided , when possible or at least reduced to a minimum level [3] .
Appropriate , passive ventilation should be provided [3].
Gaseous H2 is transparent , tasteless, and odourless and as a result it is very difficult to detect any leakage in case of a failure condition [6]. Nevertheless, for liquid H2 there is the capability of utilization of proper sensors and alarms to detect any leakages, based on LNG’s philosophy [6]. LNG’s characteristics can be set as a benchmark for liquid H2, since they are quite identical and their fundamental difference is the temperature that are stored at 1 bar (LNG at -162℃ and liquid H2 at -252.87℃) [6]. So , if sensors and alarms are used, they should be placed at the top of the room and near areas where hydrogen escape is most possible.
It is a necessity , placards to be placed around the area that hydrogen is stored to indicate danger and avoid the risk of potential accidents .
All safety precautions should be as passive as possible [3]. They should be able to operate without an additional energy source or, at the very least, continue to function if the main energy supply fails [3].
The avoidance of unintentional mixing must be considered at all costs . By preventing an unintended release and internal leaks in the hydrogen tanks, the stored hydrogen that persists in a non-flammable concentration range, posing no fire danger [7],[8],[9],[10].
The detailed design and material selection are also crucial for stable hydrogen systems [7],[8],[9],[10].
The anticipate of ignition mechanism is essential . With such a low minimum ignition energy requirement, ignition will occur from a wide range of sources[7],[8],[9],[10] .While caution should be exercised to avoid ignition, the facility occupants (e.g. employees , engineers) should presume an ignition source exists where a combustible hydrogen-oxidizer mixture has formed [7], [8],[9],[10].
The risk of a potential hydrogen fire can be significantly minimized by adopting best practices for construction, testing, assembly, service, repair, and safety systems [7], [8],[9],[10].
Special training for the facility employees about the appropriate handling of hydrogen should be provided [7],[8],[9],[10]. When liquid hydrogen is used or transported , or a container is transferred , two people at least should be present at all times [14]. This does not occur when liquid hydrogen is handled by specially qualified staff of the liquid hydrogen provider on a regular basis [14]. Withdrawing liquid from a tanker or liquid container necessitates the use of a closed system with appropriate protective relief equipment that can be evacuated or/and purged to prevent the formation of a flammable atmosphere or volatile mixture of liquid air and liquid hydrogen [14].
Until transferring liquid, all devices must be electrically grounded and bonded [14].

Figure 3 :The fire triangle after the measurements (Source: https://wha-international.com/hydrogen-fire-risk-management/ )

Simulations of pipelines

The simulation of pipelines was initially done for the gaseous H2 before the decision making. The distance from the production area to the port is almost 1 km, which explains the value of the pipelines’ length during HyRAM (Hydrogen Risk Assessment Models) simulations. The whole procedure was conducted only for research purposes, considering that the produced gaseous hydrogen could be transported via pipes. However, we ended up choosing liquid hydrogen over gaseous for the reasons mentioned in other sections.

At the following paragraphs, several details regarding each input are analysed.

Characteristics of the pipes

Research today focuses on overcoming technical concerns related to pipeline transmission, including:

The potential for hydrogen to embrittle , control hydrogen permeation and leaks and
The need for reliable, and more durable technology.

Potential solutions include using fiber reinforced polymer (FRP) pipelines for hydrogen distribution. The installation costs for FRP pipelines are about 20% less than that of steel pipelines because the FRP can be obtained in sections that are much longer than steel, minimizing welding requirements.

From the pipes that were compared Soluforce Reinforced Thermoplastic industrial Piping system (RTP, also known as FCP) ,seemed ;Figure 4 , to be the best option as it :

Is approved for hydrogen applications at operating pressures up to 42 bar [13].
Is environmentally friendly because it is maintenance , scaling, and corrosion, free, and has a life expectancy of 20 years when built and 50 years when buried [13] .
There is no hydrogen embrittlement [13].
Production and manufacturing are complying with API and ISO [13].

Before carrying out the risk assessment , Offsh2re Shetland came into contact with the manufactures of SoluFace who provided both the dimensions and informed the team that the flow rate is compliable with this type of pipeline.

Figure 4: Hydrogen Pipe ( Source: https://www.soluforce.com/product-overview/pipe-types/hydrogen-tight.html )

Table 2 : Characteristics of the pipes simulated in HyRam

* This number were provided by the manufacturer of the pipes.

As inputs for the simulation , were taken into account the characteristics that are shown on Table 2. After adding those inputs both for the pipeline and for the state of the hydrogen (gas), HyRam provided results that present the risk assessment of the transportation of Hydrogen .

On Figure 5 , when there is a 000.01% of release of the gaseous hydrogen detected the risk contribution is negligent . And when there is a 100% release , the risk contribution show a 22 % of jet fire and a 3% of explosion scenario outcome . Although both of those cases are most likely to be avoided as the amount of the events that may occur during the year are close to 0 .

Figure 5 Scenario Ranking (Source : HyRam)

And on Figure 6 the risk metrics showed for the number of fatalities per system/year (PLL) , number of fatalities per million exposed hours (FAR) and the number of fatalities per exposed individual (AIR) , the probability of having fatal accidents is negligent

Figure 6 Risk Metrics (Source: HyRam)

Furthermore , plots tab generate visual representations of the Radiative Heat Flux that system occupants may experience while in the presence of varying hydrogen leak sizes. The Radiative Heat Flux sustained by the occupants may serve as an indication of potential harm associated with the scenario. The parameters of the generated plots were altered through the HyRAM QRA mode inputs . The assumption that was taking was , that there are going to be 9 occupants constantly close to the pipeline.

On Figure 7 ,the blue square in the corner of the generated facility plot represents the leak (0.00356m) that may happen at our 122.0 mm hydrogen pipe while the blue line on the bottom, on the x-axis, is the jet centerline of the hydrogen leak. The dots represent locations of the 9 facility occupants and respective dot colors indicate the radiative heat flux (in kW/m2) that those facility occupants would experience relative to their locations to the hydrogen leak.

Figure 7 Plot Tab Output (Source: HyRam)

Lastly , the scenario of jet flame was investigated as it is shown on Figure 8 .The Flame Temperature/Trajectory window contains the variables that calculate behavior of a jet flame, including flame temperature, direction, and heat flux.

Figure 8 Flame Temperature/Trajectory Output (Source: HyRam)

Conclusion

After taking into consideration , all the aspects that were mentioned previously the outcome is that :

· Hydrogen is contained in pressure tanks, whether as a compressed gas temperature or as a cryogenic liquid. The pressure is much lower in the latter case, but additional thermal insulation is a necessity. Impacts and other natural forces are not a problem with any kind of vessel.

· Hydrogen and the combustion properties are not considered toxic.

· Hydrogen has no adverse biochemical effects.

· Hydrogen can be considered as an asphyxiant substance , as if there is a certain concentration , it can displace oxygen .

· Hydrogen is not explosive. This is often misunderstood because hydrogen-air mixtures are volatile, but pure hydrogen is not. An explosion involving hydrogen often necessitates the presence of oxygen or another oxidizing gas in high quantity.

· Hydrogen is preferred to be stored on open space rather than a closed room , as it in case of a leakage it can evaporates easier minimizing the risk. If a hydrogen spill occurs in an outdoor environment, the quick dispersal of the gas ensures that a high enough accumulation of the gas to pose a respiratory danger is impossible.

· Hydrogen is not auto-ignited .To make hydrogen react, there must still be enough energy available to initiate the reaction.

· Hydrogen is non-corrosive, so it's a great alternative. Corrosion prevention does not necessitate the use of special building materials. However, due to the intense cold of liquid hydrogen, devices must be engineered and built using materials that are appropriate for very low temperatures.

Hydrogen is a new , developing technology ;even though a series of accidents made the industry sceptical about it , with the proper treat, handle and respect to the several and different properties of it , it can be considered as one of the technologies of the future that can saves us from the vortex of climate change.

Safety Aspects

References

[1] Liu,Y.,Liu,Z.,Wei,J.,Lan,Y.,Yang,S.,Jin,T.,(2021) ‘Spread characteristics of hydrogen vapor cloud for liquid hydrogen spill under different source conditions’, International Journal of Hydrogen Energy,46(5),pp.4606-4613 . DOI : https://doi.org/10.1016/j.ijhydene.2020.10.165

[2] Ustolin,F.,Paltrinieri,N.,Landussi,G.,(2020) ‘An innovative and comprehensive approach for the consequence analysis of liquid hydrogen vessel explosions’, Journal of Loss Prevention in the Process Industries , 68.DOI: https://doi.org/10.1016/j.jlp.2020.104323

[3] Schmidtchen,U.,(2009) ‘FUELS-SAFETΥ |Ηydrogen: Overview’, Encyclopedia of Electrochemical Power , pp.519-527 . DOI : https://doi.org/10.1016/B978-044452745-5.00842-X

[4] (2001)’Hydrogen Fuel Cell Engines and Related Technologies’

[5] Verfondern,K.,Dienhart,B.,(2007) ’Pool spreading and vaporization of liquid hydrogen’ , International Journal of Hydrogen Energy, 32(2) ,pp. 256-267 . DOI : https://doi.org/10.1016/j.ijhydene.2006.01.016

[6] Shell Wasser Wasserstoffstudie, (2017) ‘Energy of The Future – Sustainable Mobility through Fuel Cells and H2’, Shell Hydrogen Study. Available at : https://www.shell.de/medien/shell-publikationen/shell-hydrogen-study.html#vanity-aHR0cHM6Ly93d3cuc2hlbGwuZGUvaDJzdHVkeS5odG1s (Accessed : May 2021)

[7] WHA International, Inc. (2020) ‘What’s Hydrogen Fire Risk Management Philosophy’ . Available at :https://wha-international.com/hydrogen-fire-risk-management/ (Accessed : May 2021)

[8] ISO/TR 15916:2015, (2015) ‘Basic considerations for the safety of hydrogen systems’, ICS:71.020 Production in the chemical industry. Available at : https://www.iso.org/standard/56546.html (Accessed : May 2021)

[9] ARC, (2018) ‘Guide to Safety of Hydrogen and Hydrogen Systems ( ANSI/AIAA G- 095 -2017)’, American Institute of Aeronautics and Astronautics. Available at: https://arc.aiaa.org/doi/full/10.2514/4.105197.001 (Accessed : May 2021)

[10] Woods, S., Lee, Jonathan., (2016) ‘Hydrogen Embrittlement’, Metals and Metallic Materials. Available at : https://ntrs.nasa.gov/citations/20160005654 (Accessed : May 2021)

[11] ANSI Webstore, (2021) ‘Hydrogen – 8th Edition’, American National Standards Institute (ANSI). Available : https://webstore.ansi.org/standards/cga/cga2017-1668762 (Accessed : May 2021)

[12] National Technology and Engineering Solutions of Sandria,LLC (2021) ‘Hydrogen Risk Assessment Model (HyRAM) ’. Available at : https://energy.sandia.gov/programs/sustainable-transportation/hydrogen/hydrogen-safety-codes-and-standards/hydrogen-risk-assessment-model-hyram/ (Accessed : May 2021)

[13] SoluForce (2021) ‘SoluForce Hydrogen Tight (H2T) . Available at : https://www.soluforce.com/product-overview/pipe-types/hydrogen-tight.html (Accessed : May 2021)

[14] University of Florida, (2016) ‘Liquid Hydrogen’, Environmental Health and Safety – Safety In The Workplace. Available at : https://www.ehs.ufl.edu/programs/lab/cryogens/hydrogen/ ( Accessed : May 2021)

[15] National Research Council. (2008) ‘Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants vol:2’, The National Academies. (2). DOI: https://doi.org/10.17226/12032