Hydrogen for energy storage: Measures to prevent fires and explosions

Preventing fires and explosions is a key element of making hydrogen socially acceptable as a green energy source. The primary aim is to prevent explosive mixtures of hydrogen and oxygen from forming in the first place. This is known as primary explosion protection. When constructing systems and equipment, designers must take into account the specific properties of the gas, which changes its state from gaseous to liquid above a boiling point of -253 °C. Additional design measures, such as automatic shut-off valves at suitable positions within a system, are intended to prevent the accidental release of hydrogen. Venting equipment can also be used to prevent overpressures. The precise design of this equipment is highly dependent on whether the hydrogen system is located in a public area (e.g. a filling station) or on closed premises to which only trained personnel have access.

In both scenarios, electrical explosion protection measures are often – but not always – required. If a hydrogen system is located outdoors and reliably guaranteed to be leak-tight, then these measures may, under certain circumstances, not be required. The conditions for this are summarised in explosion protection guidelines (ATEX), which we will examine in more detail later.

If it cannot be ensured that no hydrogen will escape, a range of secondary explosion protection measures must be implemented. These all have one thing in common: They prevent an ignitable mixture from actually being ignited. In other worse, they ensure that no sources of ignition, such as sparks, open flames or hot surfaces, are present.

Among other things, this means that in the vicinity of a hydrogen plant:

• No smoking is permitted and open flames must be avoided
• Fire protection is a particular concern and can be implemented using a protective barrier or sprinkler system, for instance
• Lightning strikes must be prevented, e.g. by using effective lightning conductors
• Work involving flames (e.g. welding) must only be performed after a measurement has been taken to ensure that no hazardous atmosphere is present in the work area
• It must be ensured that all parts that could become electrostatically charged are earthed – this applies in particular to protective shoes and gloves, which must be electrostatically dissipative
• Only suitable, approved, explosion-protected electrical devices may be used

In reality, the range of explosion-protected devices that can be used near hydrogen plants is quite wide. After all, working with hydrogen has been common practice for many decades, such as in the chemical industry. There, hydrogen is used in a number of ways, including the automatic production of ammonia in large systems.

The specific form that explosion protection takes in each application – in a chemical plant or electrolyzer, in transport pipelines or tanks, and even fuel cells – is regulated by international standards. Experts will know these as IEC 60079 and IEC 80079.

There are a range of technical options when designing explosion-protected electrical devices. For hydrogen, the following types of device are suitable:

1. Devices that are in flameproof enclosures ("d" type of protection). This involves the use of an enclosure that can withstand the pressure of an explosion. If hydrogen enters the enclosure and is ignited by a discharge of energy in the device, the explosion is not transmitted outwards. It is nipped in the bud, so to speak – all the hydrogen in the enclosure burns away. Enclosures of this kind have flameproof joints, which must not fall below or exceed a specific length and width. It is also important that the external wall of the enclosure remains cold enough to prevent igniting a hydrogen-oxygen mixture outside the enclosure, even if an explosion occurs inside the enclosure. This means that it must remain under the hydrogen ignition temperature of 585 °C.
2. Devices that are designed to be intrinsically safe. This type of protection, represented by the "i" symbol, takes advantage of the fact that a certain amount of energy is required to ignite an explosive mixture. The electrical device is therefore designed from the very beginning so that specific current and voltage values are not exceeded inside the device. These values must be low enough that no excessively high temperatures, sparks or arcs can form. Measuring and control devices can be designed in this way, in particular.

The devices used in and near hydrogen plants vary significantly – they include light fittings, as well as electrically controlled valves, sensors, gas warning devices, HMIs and compressors. The builder or operator of a system must determine whether each individual device needs to be used in an explosion-protected version. To do so, they perform a risk assessment, considering all potential sources of hazards.

 Next, they perform zone classification; in the EU, this must be in line with the ATEX (explosive atmospheres) guideline:

• Zone 0, the zone with the highest risk of explosion, is present inside tanks in hydrogen-powered vehicles and in the immediate surroundings of their exhaust openings, as well as inside equipment and pipelines, for instance. Here, a hazardous explosive atmosphere is often present.
• Zone 1 is the zone in which a hazardous explosive hydrogen-oxygen mixture may occasionally form; it is present around the drain pipe in a hydrogen-powered vehicle or in the immediate vicinity of a compressor with a manual condensate separator.
• Zone 2 is, by definition, an area in which a hazardous explosive atmosphere is not normally present, or is present only for a short period. In a room with an otherwise leak-tight hydrogen system (e.g. an electrolyzer), this may be the case immediately under the ceiling, if no gas warning system has been installed.

Of course, there are also areas that do not belong to any of these zones, perhaps because hydrogen is only present in quantities that are small enough to ensure that no ignitable mixture can ever form. This is the case, for instance, in electrolysis systems that work at a maximum overpressure of 50 mbar.

Once zone planning is complete, the required protective measures, as well as suitable devices and components, can be selected. To do so, all devices that are approved for Ex zones are divided into device groups and categories. In the EU, the following rules apply. For hydrogen systems in Zone 0, only devices with an extremely high level of protection may be used, specifically those in category 1G. In Zone 1, devices in category 2G may also be used, and in Zone 2, devices in category 3G can also be used. Furthermore, the devices and equipment are also assigned to temperature classes (T1 = 450 °C to T6 = 85 °C); these specify the maximum surface temperature that they can reach.

Similarly, the combustible gases are also categorised in groups – explosion groups. Hydrogen is among the gases assigned to the highest risk level, group IIC.

As you can see, selecting the correct device for each application in a specific zone is a science in itself. Fortunately, available devices feature an Ex code on their type plate, which clearly specifies whether they are suitable and approved. This code is the result of ATEX approval in the EU. It contains the device group and category, the type of protection (such as "i" intrinsically safe or "d" in flameproof enclosure, as described above), the explosion group, and the temperature class. In addition, it also contains the characteristic Ex symbol – the letters "Ex" in a hexagon.

Even a seemingly simple torch or hand lamp that a member of staff wishes to use when performing maintenance on a hydrogen plant must be approved for this purpose. When it comes to portable devices, the safest option is always to select a device that is suitable for Zone 0. This eliminates the risk of accidentally taking a device that is only approved for Zone 1 into Zone 0.

However, most devices are installed in a fixed location and only need to be suitable for the zone in which they will be permanently used.

Following the applicable explosion protection guidelines, which differ by region, enables operators to ensure that their hydrogen plants are equipped in such a way that the installed devices cannot cause explosions. The guidelines in other regions, such as North America and Russia, are similar – for instance, the NEC and UL standard, NEMA in the USA, CEC in Canada, EAC in the Eurasian Economic Union, and previously GOST in Russia.