Hydrogen as an energy carrier: properties and risks

So far, we have taken a qualitative look at the properties of hydrogen and the associated risks. Before we tackle additional hazards – in particular, the risk of fire and explosion and how to prevent them – it is worth going into a bit more detail and considering its physical characteristics from a professional perspective.

Gaseous hydrogen

Hydrogen is light. To be precise, with a density of 84 g/m³ at 15 °C and 1 bar, it is the lightest of all gases and is 14 times lighter than air.  It is therefore well worth ensuring that sufficient ventilation is present all rooms where hydrogen could collect. This should be the primary step for preventing explosions.

Hydrogen is extremely flammable, which means it reacts with oxygen.  The key figures are

• Firstly the minimum ignition energy of hydrogen is 20µJ, which is extremely low. Only the minimum ignition energy of acetylene and carbon disulfide is similarly low.  The
• Another important variable is the ignition temperature. In air, this is 585 °C. This is higher than the ignition temperature of methane (540 °C), the main component of natural gas. The ignition temperature of wood is around 280 °C and that of coal is between 240 and 280 °C. That sounds much less dangerous.
• As a general rule, hydrogen can only be ignited when a certain amount of oxygen is also nearby. There is a minimum amount of oxygen and a maximum amount of oxygen necessary for ignition to occur. Between these limits, an explosive mixture is present. This explosive range is extremely wide for hydrogen. It stretches from 4 vol% hydrogen content in air (known as the lower explosive limit or LEL) to 77 vol% (the upper explosive limit or UEL). If pure hydrogen or a hydrogen-air mixture containing less than 23 vol% of air is present, therefore, no explosion will occur. Even very small quantities of hydrogen in the air do not pose a hazard, which is why the explosion hazard can be eliminated by ensuring that sufficient ventilation is present and thereby "diluting" the air containing hydrogen, as mentioned earlier. However, if air enters a container that holds gaseous hydrogen, the UEL can be reached very quickly. This must be prevented at all costs.

Hydrogen also reacts with other elements, such as gaseous chlorine or fluorine. However, these two substances are not normally present in significant quantities when working with hydrogen as an energy source; as a result, we will not discuss them further here.

To summarise, a hydrogen-air mixture with a hydrogen concentration between 4 vol% and 77 vol% can be ignited and therefore explode. However, this requires energy – known as activation energy. At room temperature, with no additional source of ignition, nothing will happen. But even an extremely low amount of energy, just 0.02 mJ, can ignite the gas mixture. For comparison, the minimum ignition energy of methane is 0.28 mJ. This does not have to be energy from a flame. Even in the Stone Age, humans were aware that energy is produced when a stone hits metal or due to friction (e.g. a wood drill). A single spark is enough to ignite a hydrogen-oxygen mixture. However, it also requires a very high temperature, specifically 585 °C. This is because hydrogen is a good conductor of heat, which means that it transports away the heat acting on it very quickly. A surface needs to be extraordinarily hot before it can ignite an explosive hydrogen-air mixture. However, most sources of ignition will cause ignition in practice. Just one rust particle that is carried away by a stream of hydrogen can collide with metal and generate a sufficiently hot spark.

Another critical property – if hydrogen does burn, it produces an almost colourless flame, which is why fires can go unnoticed under certain circumstances in daylight.

Certain gas-air mixtures can also ignite as soon as they expand, since the temperature increases during this process. This is not the case for hydrogen. Its temperature only increases from, for example, 20 °C to 25 °C if hydrogen under high pressure (e.g. 175 bar) is brought to normal pressure (1 bar) quickly. This is far below the ignition temperature.

Liquid hydrogen

Liquid hydrogen (LH₂) poses unique explosion hazards. At -253 °C, it is so cold that any impurities it contains (except helium) freeze solid. This applies, in particular, to air that enters a container of liquid hydrogen. A mixture of solid air and LH₂ acts like an explosive. In addition, the low temperatures mean that air condenses on the external surfaces of uninsulated pipes or system parts. Little by little, nitrogen evaporates from this liquid air, meaning that the concentration of liquid oxygen in the mixture increases. If it comes into contact with combustible substances, for instance if the liquid oxygen drips onto wood, this can also lead to an explosion. If this occurs near a pipe containing hydrogen, the consequences could be devastating.