Hydrogen is fast becoming an essential part of the worldwide energy mix in transport and stationary applications (such as a Hydro Genesis system). 

In 2015, a European consortium funded by European Union Fuel Cells and Hydrogen Joint Undertaking published a detailed report titled 'Hyindoor' on the basics of hydrogen safety. The report has been adopted and disseminated by various international bodies such as the ISO (International Organization for Standardization) and country-specific regulatory institutions, such as the Health and Safety Executive in the UK. 

The report details many risks and raises awareness of why strict adherence to hydrogen safety is important. The main conveyed message is that simple processes can be followed to make the handling of hydrogen gas safe; points relevant for stationary storage can be summarised in the extracted points below:

• Consider whether it is necessary to house the hydrogen system within a room or enclosure, or whether it could be relocated outdoors, where an accidental leak would less likely lead to accumulation of hydrogen gas in flammable concentrations due to better ventilation
• Reduction of hydrogen supply pipeline diameter and operational pressure to the minimum is required to satisfy technological requirements for mass flow rate
• Install various levels of ventilation to prevent buildup in high-level areas.

Hydrogen gas has many properties that assist in the safety and prevention of danger, and this article will attempt to summarise those reasons. The report advises that the use of hydrogen in a confined environment requires a detailed assessment of hazards and associated risks, including potential risk prevention and mitigation features. 

In short, the above three points detail that as long as the Hydrogen gas is stored with precaution, with acceptable pressure limitations and key mitigation and monitoring, the safety of hydrogen can be managed to a mass-acceptable and, more importantly, mass-adoptable degree.

 

Hydrogen Gas Disaster

When people think about hydrogen, among the first thoughts is usually the 1937 Hindenburg disaster. This disaster imprinted a visual memory on the world that hydrogen gas is dangerous and explosive, and it is in certain situations.

Hydrogen was used in Hindenburg as it is the lightest gas in the entire Universe (though the airship was initially designed to be fueled by the more inert helium, that is a different story). One litre of hydrogen weighs only 90 milligrams under normal atmospheric pressure, meaning it is eleven times lighter than the air we breathe. If released in unrestricted air, hydrogen disperses vertically at 50m/s (112 mph); a good way to visualise this speed is by thinking of a golf ball being driven from a tee.

Explosions and fire are not possible if pure hydrogen is properly contained in pressure vessels as hydrogen can only ignite around a flame, and if there is a greater than 4% v/v hydrogen, which is the lower explosive limit (LEL), up to as much as 75% v/v, which is the upper explosive limit (UEL).

Hindenburg’s hydrogen was unpressurised, so it followed the development of a ‘slow leak’ that prevented the gas from escaping rapidly. A leak from a pressurised vessel would have caused the gas to escape away and not allow the hydrogen to mix with oxygen within the container, which is required for combustion.

 

Safety Precautions for Hydrogen gas

Due to its lightness, hydrogen gas will concentrate in elevated regions of an enclosed space, whereas other well-used gases, dependent upon their relative mass, will concentrate at ground level or at low elevation (such as LPG or Natural Gas). 

The general advice for hydrogen systems is to maintain a good level of ventilation (if outside storage is not possible) and by locating all potential ignition sources lower than the equipment level from which hydrogen may leak and accumulate whilst ensuring adequate ventilation and safe discharge.

Above is a picture of the "Safety Square" or "Fire Diamond" used by emergency teams to quickly and easily identify the risks posed by hazardous materials during the initial stages of an emergency response.

The top three coloured spaces have a number ranging from 0 to 4 (0 being of no concern to 4 being extremely high risk), identifying the flammability (red), health hazard (blue), and reactivity (yellow) with the white space reserved for corrosivity or special toxicity listings other materials.

The left diamond above shows the rating for petroleum, the middle natural gas and the right hydrogen; hydrogen has the highest possible flammability rating and the lowest possible rating for health hazard and reactivity; petrol and natural gas are both rated as more dangerous.

Hydrogen is a colourless, odourless gas undetectable by our natural human senses. Adding an odorant, as is done with natural gas in the national grid to help us identify leaks, has been widely considered but deemed unsuitable for most hydrogen systems.

This is due to the damage the additives can cause to fuel cell membrane catalysts. Also, the size of the odorant molecules is much larger than the hydrogen molecules, so it will not be detectable in the slow and most dangerous hydrogen leaks.

The pressure at which hydrogen is stored is a critical safety factor that needs careful mitigation and safety systems. 1kg of hydrogen will require a space of approximately 11 sq. M if uncompressed in atmospheric conditions. For a seasonal stationary storage application that requires approx. 25kg for a domestic system and >100kg+ for a commercial setting, the gas needs to be compressed to make the application viable.

There are several storage solutions currently on the market and in development for hydrogen with pressure ratings from 35 bar to 1000 bar. These storage solutions need to be specialised as hydrogen can cause embrittlement of high strength steels, titanium alloys and aluminium alloys with cracking and catastrophic failure of the metals at low-stress levels.

With a low to zero risk of explosion due to containing pure hydrogen, the high pressure becomes the primary safety concern with hydrogen cylinders. A vessel releasing pressure through a small opening can act like a missile. Imagine a balloon being let go, except it is not a piece of rubber flying through the air.

This is where hydrogen safety standards, discussed in the Hyindoor report, such as the minimum length of high-pressure pipework, from the pressure source to the regulator, as well as several pressure release valves at key interfaces to vent hydrogen, when necessary, into a safe, open space, become key. Standard and well-understood physics are needed to eliminate blast pressure-related incidents, but they should not be trivialised.

 

Key Characteristics of Hydrogen when Looking from a Safety Perspective

Hydrogen gas has a very low viscosity, creating a challenge in leak prevention. Hydrogen-suitable sealing interfaces with welded, not threaded joints, appropriate hydrogen-rated components and limited interference with the system once commissioned will significantly reduce the leak potential.

The energy necessary to initiate a hydrogen/air explosion is very small. The ignition energy for a 2:1 hydrogen/oxygen mixture is only about 0.02 mJ. This is less than one-tenth of other fuels such as methane, LPG or petrol.

Even tiny sparks, such as those produced by wearing certain types of clothing, can ignite hydrogen/air mixtures and cause an explosion. Therefore, detection devices, active ventilation systems and shut-off valves are necessary to remove any risk of ignition.

Hydrogen gas burns with an invisible flame making it difficult to detect a hydrogen fire. This low emissivity of hydrogen flames reduces the heat transfer by radiation to other objects in proximity, thus reducing the risks of secondary ignition.

Similarly, the speed at which hydrogen burns means that surrounding objects and materials are exposed for less time when compared to similar volumes of other flammable gases; so, again, there is less risk of secondary ignition.

Hydrogen may have been tarnished early on in its life as an energy vector by the Zeppelin airships. Still, the benefits it brings to a world demanding clean fuel, when coupled with risk reduction and mitigation means the hydrogen-fueled Earth is coming.

Hydrogen is less dangerous than natural gas and petrol. Still, due to our familiarity and strict safety codes when working with them, the general world population are comfortable using and operating machines combusting them.

Volume concentration of a gas is expressed as % v/v, which stands for volume per volume.

 

This article was contributed by our expert Ben Sayers

 

Frequently Asked Questions Answered by Ben Sayers

Q1. What is hydrogen gas used for?

Hydrogen is used in many industrial sectors, including the chemical industry, textiles manufacturing, electronics and glass making. It can also be used to fuel stationary power and transportation systems. Very early in the 20th century, the Haber process was invented as a way of producing ammonia (NH3) on industrial scales and is today used to generate significant amounts of fertiliser.

 

Q2. Why is hydrogen important in industry?

Hydrogen is essential as it is a clean fuel-producing only water when reacted to create electricity or when combusted to produce heat.

 

Q3. How does hydrogen help the economy?

Hydrogen is an important gas for the future energy of the world. By harnessing hydrogen to power our houses and our vehicles, we can reduce the global warming associated with carbon emissions associated with fossil fuels.

 

Q4. How was the hydrogen safety issue resolved in fuel cell cars?

Hydrogen can be used in fuel cell cars by developing Type IV cylinders. These are completely non-metal and consist of a polymer lining supported by a complete carbon fibre resin wrap. They are significantly lighter than any of the cylinders made from metal, meaning the vehicle’s range is not adversely affected by the weight of the tanks.

 

Q5. What is the safe limit of hydrogen gas?

Explosions and fire are not possible if pure hydrogen is properly contained in pressurised cylinders as hydrogen can only ignite around a flame, and if there is a greater than 4% v/v hydrogen, which is the lower explosive limit (LEL), up to as much as 75% v/v, which is the upper explosive limit (UEL). 

 

Q6. How do you safely vent hydrogen gas?

Due to its lightness, hydrogen will concentrate in elevated regions of an enclosed space, whereas other well-used gases, dependent upon their relative mass, will concentrate at ground level or at low elevation (such as LPG or Natural Gas).

 

Q7. How do you store hydrogen gas cylinders?

Hydrogen gas cylinders need to be looked after. They need to be stored away from sources of ignition and away from areas where there is a risk of damage both accidentally and maliciously. As hydrogen is the lightest element, it compresses very poorly, which requires a high pressure (up to 1000 Bar), and therefore there are dangers associated with high pressures.

The pipework and cylinders should be in cages to protect them, and they should be inspected periodically to test integrity and assess signs of any embrittlement. However, on an annual storage cycle, this shouldn’t be an issue.