What is Hydrogen?

Hydrogen is atomic number one in the periodic table of the elements, which means it is the simplest and lightest. It is also the most abundant element, making up 75% of the mass of the entire universe. Because of its reactivity, it usually forms a diatomic molecule (H2) or compounds with other elements. At room temperature and pressure, it exists as an odourless, colourless molecule with a density one fourteenth that of air.

This low density enables it to disperse quickly when released to the air. For hydrogen to exist as a liquid, the temperature must be reduced to 20.3 K. It has one of the highest energy densities per unit mass (between 120 and 142 MJ kg1 ), and as it has a higher combustion energy per unit mass than other fuels, it has become of interest to the renewable energy community.

One of hydrogen’s greatest attractions is its presence in water (H2O), which covers around 71% of the Earth’s surface. If hydrogen can be released from water economically, it will be the energy provider of the future and will significantly reduce the Earth’s greenhouse emissions. One of its greatest deterrents is its flammability (which, on the positive side, means it is a good fuel). While the flammable limits for a fuel such as natural gas is in the range of 5.0%e15% (gas to air volume ratio), hydrogen has a range from 4.0% to 75.0%. It has a relatively low ignition energy (0.02 mJ).

Associated Hazards

Hydrogen Leaks – 

Due to the extremely small size of hydrogen molecules, minor gaseous hydrogen leaks are quite common. In well-designed systems, these small leaks pose no significant danger since the minute amount of released hydrogen is insufficient to create a flammable mixture when combined with air.

The risk of a flammable mixture or asphyxiation only arises when hydrogen gas accumulates in a confined space over time.

Detecting small gaseous hydrogen leaks can be challenging for humans because hydrogen is both colorless and odorless. When hydrogen accumulates in a confined area at substantial concentrations, it behaves like other gases, except oxygen, as an asphyxiant.

Leaked hydrogen tends to rise and disperse rapidly in the air due to its low density, making it highly buoyant (14 times less dense than air). Hydrogen readily mixes with air, forming an ignitable mixture, which is of particular concern in situations where hydrogen can gather in an enclosed space.

Methods for detecting hydrogen leaks include:

• Listening for the sound of high-pressure gas escaping (characterised by a loud hissing noise).
• Utilising portable hydrogen detectors.
• Employing permanently installed hydrogen detectors that are connected to local or facility-wide audible or visual alarms.

In the case of liquid hydrogen (LH2), which is a cryogenic liquid, leaks behave differently than gaseous hydrogen leaks and may be more easily detected. Even in arid climates, a leak of liquid hydrogen will create a visible white cloud of condensed water vapor due to the cryogenic temperatures causing humidity in the surrounding air to condense. This low-temperature water vapor is denser than air, so the cloud remains localized and may drift horizontally. As the hydrogen warms, it dissipates and rises quickly.

Hydrogen Flames – 

Hydrogen exhibits flammability within a wide concentration range of 4% to 75% in air, which is considerably broader compared to other commonly used fuels (refer to the “Hydrogen Compared to Other Fuels” section for a comparison).

In the absence of adequate ventilation, a minor leak in an enclosed space can easily elevate the hydrogen concentration to the lower flammability limit (4%). Conversely, in outdoor settings, a small leak is likely to ascend rapidly and disperse.

It’s important to note that combustion cannot take place in a container or pipeline containing only hydrogen. Combustion requires the presence of oxygen (or air) and an ignition source.

Hydrogen-air mixtures are highly susceptible to ignition, and in documented incidents, the source of ignition is often unknown. Possible sources of ignition include:

• Electrical equipment generating sparks
• Electrostatic discharge sparks
• Mechanical impacts
• Open flames
• Hot surfaces (e.g., an exhaust manifold)

Hydrogen combustion produces a nearly invisible pale blue flame, which may appear yellow if impurities like dust or sodium are present in the air. A pure hydrogen flame does not produce smoke and emits low radiant heat. In fact, you may not feel any heat until you are in close proximity to the flame.

Given these characteristics, the most effective way to detect hydrogen flames is to utilise portable flame detection equipment such as a thermal imaging camera. Alternatively, a simple flammable “indicator” tool, like a broom, can be employed to check for the presence of hydrogen flames. When the broom contacts a hydrogen flame, it will ignite and produce a visible flame.

In cases where flame detection equipment is unavailable, listen for the sound of venting hydrogen and observe for thermal waves, which may indicate the presence of a heat source. It’s noteworthy that igniting vented gaseous hydrogen from vent stacks is a common occurrence, and these stacks are designed to manage such ignitions safely.

The release of hydrogen in significant quantities can lead to potentially harmful overpressure scenarios, which can result in both direct and indirect hazards, including structural damage or the dispersion of debris. Overpressures can manifest in two ways: as a consequence of unignited releases of pressurized gas or as a result of the ignition of a cloud of flammable gas.

Hydrogen Explosion – 

Overpressure Due to Unignited Releases When any cryogenic liquid undergoes a phase change from a liquid to a gaseous state, it expands considerably, occupying significantly more space. For instance, liquid hydrogen expands to approximately 850 times its volume during this transition. Consequently, confined vessels, pipelines, or sealed spaces have the potential to become over-pressurised.

Various factors, including unintended operations, exposure to heat such as in the case of a fire, or other causes, can lead to the over-pressurization of gas containers. To prevent overpressure events, pressure-relief devices (PRDs) like rupture disks or relief valves must be installed in all hydrogen equipment. These PRDs should be vented to a safe location.

Overpressure Due to Ignited Releases In addition to overpressure associated with stored gas, flammable gases like hydrogen can undergo combustion. If a released cloud of gas in the atmosphere is ignited, the rapid combustion of hydrogen can generate overpressure or even cause an explosion.

To mitigate the risk of ignition and subsequent overpressure, precautions should be taken to eliminate potential ignition sources from areas where vented or released hydrogen can accumulate in a concentration sufficient for ignition. Ignition sources may include electrical equipment generating sparks and electrostatic discharges.

Areas where hydrogen clouds can form are often designated as ‘exclusion zones,’ which may have defined separation distances specifying how far structures, vehicles, and equipment should be located to minimise the risk.

Two types of hydrogen combustion are typically distinguished:

• Deflagrations: These are combustion explosions characterised by subsonic flame propagation through the hydrogen-oxidant mixture, usually hydrogen-air.
• Detonations: In detonations, combustion explosions involve supersonic flame propagation through the hydrogen-oxidant mixture, leading to the generation of shock waves. Detonations are often more destructive than deflagrations.

Deflagrations can sometimes undergo acceleration, such as when flames propagate across repeated small obstacles or through lengthy pipes, potentially transitioning into detonations.

This transition, known as deflagration-to-detonation transition (DDT), is less likely to occur in hydrogen concentrations near the flammable limits. It is more commonly observed in large equipment or piping or in instances of very large hydrogen releases within partially confined spaces.

It’s essential to note that deflagration venting, as outlined in NFPA 68’s guidelines for explosion protection, is not an effective measure when DDT occurs. However, it remains effective for deflagrations. To address detonations and related pressure loads in gas mixtures within piping systems, NFPA 67’s guidelines for Explosion Protection for Gaseous Mixtures in Pipe Systems offer relevant information. For bystanders, both deflagrations and detonations are likely to be perceived as explosions.