A safe approach to mastering green hydrogen

The shift to renewable energy is in full swing. Hydrogen technology plays an important role in this phenomenon. However, the entire value creation chain – from the production of hydrogen, via storage and transport, to its use as an energy source – is fraught with challenges when it comes to safety. This blogpost will focus on the production of hydrogen to discuss what particular factors need to be considered with respect to explosion protection and what tasks international and national standardisation committees are yet to tackle.

We almost never encounter hydrogen in its elementary form in our everyday lives. It forms compounds with other elements quickly and spontaneously. Due to its high reactivity, it is classified as hazardous. Generations of school pupils have witnessed the oxyhydrogen test themselves, which shows just how much energy is released when hydrogen reacts with oxygen. However, its properties make hydrogen a key element in the energy revolution because hydrogen can be used to store energy.

Alongside nuclear energy, which will not be discussed in further detail here, renewable energy is being hailed as a new, climate-neutral source of energy. The use of typical renewables, such as hydrogen and biogenic raw materials (e.g. renewable raw materials like wood), has been almost exhausted in many regions. In contrast, the use of photovoltaics, solar thermal energy and wind power offer significant opportunity for growth. These do, however, also exhibit some serious disadvantages, in particular a low energy density and high volatility, i.e. periodic fluctuations in the supply.

In order to be able to power a national economy such as Germany reliably and continuously using predominantly renewable energy, significant energy storage capacity and high energy density are required. Assuming a basic capacity of 70 GW that must be buffered over long periods of time, a storage capacity corresponding to a gigantic pumped-storage power plant with a volume equal to Lake Constance and a fall height of 800 m would be required. The properties of hydrogen mean that it is a practical energy source and storage medium; additionally, it can compensate for the two main disadvantages of volatile renewable energy sources. Currently, electrical energy from wind power plants or solar power plants that is not required can be used to produce hydrogen. To do this, water is separated using electrolysis. If additional electrical energy is required, the stored hydrogen can be converted back into power (reconversion). There are, of course, other potential ways to use the chemical energy in hydrogen, which will not be discussed in more detail here.

Under normal conditions, hydrogen is a colourless, odourless gas. It is not toxic and causes no damage to the environment. At a temperature of 0 °C, it has a density of 0.089 g/l. Hydrogen therefore dissipates quickly in open air, as it is 14 times lighter than air, which has a density of 1.29 g/l.

However, hydrogen molecules are extremely small. This means that hydrogen has a high diffusivity, even through metallic materials. For instance, hydrogen atoms can lead to metal becoming brittle at grain boundaries or defects. This gives rise to unique challenges when it comes to the density of hydrogen equipment – albeit challenges that can be entirely overcome using technology.

Selected properties of hydrogen and methane (main component of natural gas).

As mentioned earlier, hydrogen and oxygen can form an explosive mixture (oxyhydrogen). From a safety perspective, the extremely wide explosive range of this mixture, from 4 vol.% (lower explosion limit, LEL) to 77 vol.% (upper explosion limit, UEL), is of particular interest. The minimum ignition energy of 0.02 mJ is among the lowest; there are only two other gases – acetylene and carbon disulphide – alongside hydrogen in the most hazardous ignition group, IIC. The relatively high ignition temperature of 585 °C appears to be inconsistent with the low ignition energy. This discrepancy is due to the hydrogen's high thermal conductivity. Since the net influx of heat is always critical for an explosion to be triggered, only very hot surfaces and long dwell times are able to transmit enough heat into a hydrogen-air mixture to cause ignition.

Additionally, a hydrogen-air mixture is also different from other explosive mixtures due to its extremely high flame speed, which is around eight times higher than that of a methane flame. When used deliberately, such as in rocket propulsion systems, this property generates huge momentum (mass multiplied by flame propagation speed). However, unwanted and uncontrolled hydrogen explosions have extreme destructive power.

Overcoming the risks associated with hydrogen has been part of safety technology for decades. Large-scale technological use of hydrogen began with the development of the Haber-Bosch process for synthesising ammonia at the start of the 20th century. Currently, over 100 million tons of hydrogen are produced every year. As well as the production of ammonia, this vast quantity of hydrogen is also used primarily in refinery processes and for producing methanol.

Explosion protection has functioned reliably and safely in all these applications for many decades. Internationally, it is governed by the IEC 60079 and IEC 80079 standards series, for which the IEC's TC 31 Technical Committee is responsible. In most countries and regions, these standards have been converted into virtually identical regional and national standards and are observed in this form. Two very common types of protection, "flameproof enclosure" and "intrinsic safety", account for the aforementioned properties of hydrogen by categorising it as belonging to explosion group IIC.

As part of the energy revolution, the use of hydrogen has noticeably expanded and become consolidated. This has given rise to new safety-related issues. The following sections explain the current situation using selected elements of the hydrogen economy.

Among the different methods for producing hydrogen, natural gas steam reformation is the most common, accounting for about 50% of production. Producing hydrogen from carbon is, as ever, very important, particularly in China. In the sustainable hydrogen cycles of the future, these conventional methods will not play as significant a role, due to the amount of carbon dioxide they release, or will have to be expanded to include technology that uses energy (Carbon Capture, Utilisation and Storage – CCUS – technology). Currently, the global proportion of hydrogen generated through electrolysis is fairly low, at less than 5%, primarily due to the high production costs. The production of "green" hydrogen (i.e. hydrogen produced using "green" energy) costs three to five times more than hydrogen production using natural gas steam reformation. The proportion of green hydrogen in the future will depend predominantly on how energy prices develop, the efficiency of the electrolysis process and the development of natural gas prices. Global, mainly politically launched, schemes to develop hydrogen infrastructures, such as the German "national hydrogen strategy" or the "Roadmap to a US Hydrogen Economy" should create suitable framework conditions for positive development.

The electrolysis of water involves splitting water into its constituent parts – hydrogen and oxygen – when subjected to direct current. In this process, membranes separate the two resulting gases so that no explosive mixture (crossover) can form. However, certain ions can pass through the membranes.

At present, there are four electrolysis methods, which differ according to the process temperature and type of membranes used. Alkaline electrolysis is the most common and most technically sophisticated method. In this process, liquid mixed with sodium hydroxide is responsible for transporting the ions (OH-) from the cathode to the anode. In both PEM (Proton Exchange Membrane) electrolysis (H+) and AEM (Anion Electrolysis Membrane) electrolysis, a solid is used as the membrane. All listed methods have process temperatures between 60 °C and 80 °C, meaning they are low-temperature electrolysis methods. The typical efficiency of these processes is between 65% and 82%. SOE (Solid Oxide Electrolysis) is the most important high-temperature electrolysis method. In this process, the membrane material is oxide ceramic. The process temperature is between 700 and 900 °C. Steam is split, rather than water in its liquid form. Although it takes more energy to reach the required process temperatures, the system achieves a high efficiency of 84%.

There are a range of international standards that regulate the safety concerns relating to the most important elements of the hydrogen value chain, including electrolysis. Technical Report ISO/TR 15916 proposes hazard prevention measures. This non-binding Technical Report is a good introduction to this complex topic. International standard ISO 22734 "Hydrogen generators using water electrolysis – Industrial, commercial and residential applications" (current version from 2019) governs the electrolysis of water. It sets out detailed requirements for the design, construction, safety and operation of electrolysis plants. It must be emphasised that manufacturers of plants of this kind are required to perform a risk assessment in order to systematically record potential hazards, determine the probability of occurrence, and implement suitable countermeasures. Accordingly, for explosion protection, zone classification using IEC 60079-10-1 or appropriate national standards must be performed. This can be used as a basis for implementing suitable primary (preventing the generation of explosive atmospheres) and secondary (avoiding ignition sources in accordance with the IEC 60079 standards series) explosion protection measures.

The primary explosion protection measures are intended to prevent the release of hydrogen, in particular, through the use of sufficiently leak-tight plant parts. This is reinforced by monitoring the immediate surroundings using gas measurement equipment. Leak-tightness is one specific aspect in most safety concepts for hydrogen plants. As mentioned above, hydrogen molecules are particularly small and can diffuse through metallic materials. As a result, it is imperative that international standards are expanded to include precise specifications for the specific implementation of leak-tight pipeline connections and equipment.

In Germany, the two categories "technically tight" and "permanently technically tight" have been successfully used for a number of years. The specific embodiments are defined in TRGS 722 "Preventing or restricting hazardous explosive atmospheres". At a European level, this requirement has been taken into account in the latest version of EN 1127-1 "Explosive atmospheres – Explosion protection – Part 1: Basic concepts and methodology" in the form of Appendix B: Leak-tightness of devices. In any case, it must be established that there are significant differences between the German national specifications in and European standard with regards to the concepts and some details. At IEC or ISO standard level, there is currently no adequate specification at all with specific reference to explosion protection. Particular requirements exist for high-temperature electrolysis due to the process temperature, which is far higher than the minimum ignition temperature of hydrogen.

At a national level, specifications for hydrogen plant explosion protection have existed for a long time. These include Section 1.2.7 "Plants for the production and use of hydrogen" within a collection of examples regarding explosion protection regulations (EX-RL) in DGUV Regulation 113-001. Here, too, the leak-tightness of the plant, combined with suitable ventilation and gas monitoring measures and supplemented by organisational measures, is the central element of the safety concept. For electrolysis plants in enclosed rooms and without supplementary technical and organisational measures, only the area under the ceiling is classified as Zone 2.

The storage, transport and reconversion of hydrogen involve equally strict safety requirements. However, just like for production, there are currently no existing international standards that are sufficient to ensure explosion protection in all processes. Section 1.2.7.2 of the collection of examples from German explosion protection regulations (EX-RL), in contrast, contains corresponding specifications for indoor and outdoor compression of hydrogen. The corresponding ISO 19880-1 international standard "Gaseous hydrogen – Fuelling stations – Part 1: General requirements" has applied to hydrogen filling stations since 2020. In Germany, TRGS 751 "Avoidance of fire, explosion and pressure hazards at service stations and filling systems for filling land-based vehicles" applies as a specific national regulation on explosion protection. In its current version, dated 2nd October 2020, it has been expanded to include aspects relating to hydrogen as a fuel.

Fuel cells are used to regain electrical energy from hydrogen as a storage medium. A process that is essentially a reversal of the electrolysis process described above takes place in these cells. At an international level, explosion protection is regulated by IEC Technical Committee TC 105. Since the start of 2021, eleven working groups have been focusing on standardisation projects regarding a range of applications, from micro-fuel cells to drive units for drones. In the meantime, a number of new standards have been finalised. Among others, the standards for the safety of stationary and portable fuel cell power systems, as well as for fuel cell modules are already available.