Hydrogen: Properties, Safety, Dangers
What are the characteristics of hydrogen?
In the case of hydrogen (H2) is a non-toxic gas that has neither colour nor odour. However, hydrogen is highly flammable. Therefore, it must not fall into the hands of children. Appropriate containers should be stored in a well-ventilated place. It is also important to keep hydrogen away from ignition sources and to take measures against electrostatic charge. Complications can occur if high concentrations are inhaled – but this then corresponds to the lack of oxygen (O2). The complications range from movement disorders to loss of consciousness to the risk of suffocation. Hydrogen also poses no cancer risk and is not self-flammable. With a density of 0.0899 g/l (0°C), hydrogen is about 14 times lighter than air. Due to its high diffusion speed, it spreads quickly in all directions and mixes quickly with air.
The density of liquid hydrogen is 70.99 g/l. In addition, hydrogen accounts for 11.2% of the weight of water. Its melting point is -259.125 °C and the boiling point is -252.882 °C. 2.8 kilograms of gasoline or 2.1 kilograms of natural gas contain as much energy as one kilogram of hydrogen, if you look at the lower calorific value. In other words, given its properties, hydrogen has the highest mass-related energy density among common fuels. The volume-related energy density of liquid hydrogen is only about 1/3 that of natural gas and 1/4 that of gasoline.
Is hydrogen environmentally friendly?
Hydrogen generally has environmentally friendly properties. If it is burned with air in combustion engines, emissions are produced that are very low or negligible if the combustion is properly directed. Hydrogen combustion increases emissions of nitrogen oxides (NOx) increases exponentially with the combustion temperature. At a low combustion temperature, nitrogen oxide emissions can therefore be significantly reduced with fuels obtained on the basis of natural gas or mineral oil. Pollutant emissions can be completely avoided if hydrogen is used in low-temperature fuel cells. These include, for example, polymer electrolyte membrane fuel cells (PEMFC). Electricity generation from hydrogen and oxygen produces only demineralized water as a reaction product.
Compared to conventional power plants, emissions are up to 100 times lower when hydrogen is used in fuel cells with a higher operating temperature. Furthermore, due to its status as a secondary energy source, hydrogen enables the flexible introduction of different renewable energies in the power and fuel sector. However, in order to assess how hydrogen specifically affects environmental quality, the entire fuel chain must be taken into account. This ranges from primary energy to end use.
Hydrogen: Focus on safety
With oxygen or air, hydrogen burns to form water (H2O). Therefore, hydrogen poses a certain danger, whereby this property gives hydrogen its suitability as a fuel. According to its physical and chemical properties, hydrogen is no more dangerous than conventional energy sources such as natural gas or oil. Nevertheless, high safety standards must apply when handling hydrogen, as there is a risk of dangers such as explosions or hydrogen embrittlement.
In principle, there is a risk of fire carpets forming at the site of an accident with liquid hydrogen. However, hydrogen rises very quickly into the air, which reduces the danger. This also applies to the mixture of hydrogen with air, which can therefore only come into contact with an ignition source for a short time. Due to intensive work with hydrogen, the industry has excellent and positive experience with its safety-related aspects. So there is a long tradition in dealing with hydrogen. Unexpected complications can occur as with any other fuel, but ultimately the use of hydrogen does not pose any greater dangers than with conventional energy sources. If you are looking for more information on the safety of hydrogen, the R and S phrases as well as publications on H2-Substance data (DIPPR, NIST) recommended.
Risk of explosion with hydrogen
How explosive is hydrogen? This question is often asked because hydrogen is associated with explosions due to the oxyhydrogen experiment from chemistry class and some well-known accidents from the history of technology. The fire on board the Hindenburg airship in particular is still often cited as an example of the risk of explosion of hydrogen. However, it has long been proven that there was no explosion at all and that the accident was not caused by hydrogen, but by an electrostatic spark. Above all, it is important to note that hydrogen does not explode per se. This requires other factors – an oxidizer (e.g. pure oxygen, air or chlorine) in a certain volume ratio to hydrogen and an ignition source such as the spark resulting from electronic charging. Pure hydrogen cannot burn.
If about 4% hydrogen is mixed into air under atmospheric pressure, this mixture can be ignited with an ignition source. However, there is no risk of explosion here yet. This is only given from a hydrogen concentration of 18%. As soon as around 75% hydrogen is present, ignition and thus explosions are no longer possible, as the amount of oxygen is not sufficient for this. Since hydrogen is 14 times lighter than air and thus volatilizes quickly outdoors, the risk of hydrogen explosion is further reduced. Ventilation is therefore a decisive factor, especially in closed rooms. When handling hydrogen, care should also always be taken to keep it at a distance from ignition sources, which also include electrostatic discharge ("electrostatic discharge" = ESD).
Know and avoid hydrogen embrittlement
Hydrogen embrittlement is a phenomenon that has been studied for a long time and is also one of the typical dangers of hydrogen. This occurs when ionized hydrogen penetrates the crystal lattice of a metal. Accordingly, metals or metal alloys are affected by hydrogen embrittlement. Accelerated crack growth or material failure can be caused by hydrogen embrittlement, especially with increased material stress. This is also referred to as "hydrogen-induced corrosion". Whether the respective material is susceptible to hydrogen embrittlement depends on several factors:
- Shape of the crystal lattice (e.g. space or surface centering)
- Metal surface quality (e.g. welds, fractures or defects)
- Load (e.g. temperature, pressure, voltage or alternating load)
Therefore, the effects of ageing caused by hydrogen should always be taken into account when selecting components. By choosing the right material, the risk of hydrogen embrittlement can be reduced or avoided altogether. Stainless steel has proven itself for this purpose. It is also important to know that hydrogen diffuses very quickly into other gases such as air. H+ ions can also be formed in pipelines and storage tanks on catalytically effective surfaces. This is ionized hydrogen: This is even smaller than the actual molecule, so it is able to diffuse easily into metals. For this specific reason, hydrogen embrittlement can occur in some types of steel and under special conditions.
Hydrogen embrittlement in pipelines
Especially in the case of pipeline pipes for hydrogen transport, resistance or resilience to the phenomenon of hydrogen embrittlement is of fundamental importance. This is the only way to avoid hydrogen embrittlement and corrosion. The active electron of hydrogen can also endanger the connections (welds) between the pipes. Official standards or regulations regarding safe hydrogen transport have yet to be published. The main challenge is that the existing natural gas pipeline infrastructure is to be used for hydrogen transport and not special hydrogen pipelines. Thus, an individual consideration and one's own experience or the involvement of an experienced expert are crucial in order to specifically rule out hydrogen embrittlement and other risks.
Hydrogen Hazards: Summary
How dangerous is hydrogen? This question must be considered in a differentiated way. Several aspects play a role here: On the one hand, the danger of hydrogen should be compared with that of established energy sources. On the other hand, the effort required to control these dangers and the risk-benefit assessment must be taken into account. Hydrogen can be explosive and spreads quickly with an appropriate mixing ratio with oxygen, but it also evaporates in a short time.
At the same time, the risk of hydrogen explosion should be taken seriously and reflected in appropriate safety precautions. A danger that should not be underestimated is the colorlessness and odorlessness of hydrogen. For this reason, hydrogen leaks often go unnoticed. These are also even riskier in closed rooms. Hydrogen embrittlement, which leads to cracking, is also one of the typical dangers of hydrogen. However, these risks can be counteracted – with sufficient ventilation in enclosed spaces and the right choice of materials to avoid hydrogen embrittlement.