Safety aspects of new energy carriers

Hydrogen explosion testing at Gexcon

Challenges of a CO₂ neutral future

Within the following decades, CO₂ emissions must be reduced drastically to reach the goals of the Paris Agreement and to limit global warming well below 2^oC. This is considered one of the most urgent missions today. Europe is targeting to be CO₂ neutral by 2050, which includes major changes for the industry, energy providers, consumers, and local authorities. In the meantime, governments must take action and define the corresponding legal framework.

Many different solutions and approaches are being developed to save energy and replace fossil fuels with alternative energy carriers.  Therefore, new ways of energy storage must be developed. These new energy storages also play a key role to balance the supply and demand for renewable energies from wind turbines and photovoltaics. Large differences between energy produced and consumed have to be bridged between day and night or summer and winter. Therefore, large amounts of energy will have to be stored temporarily.

New energy carriers

Today, two of the new energy carriers are hydrogen and ammonia. However, these materials are not new. They have already been used daily for many years in the industry. Production and storage of these materials at high pressures or liquefied at low temperatures is common practice. When used at a large scale during the energy transition process, the safe handling of these energy carriers plays a major role. At first instance, it seems to be a common perception that these new energy carriers are more dangerous than the already used ones.

Just like in 1825, when George Stephenson applied for the first railway line between Manchester and Liverpool, The British House of Commons asked for an opinion due to the concern of the ‘unusual speed’. In 1835, on the first railway line between Fürth and Nürnberg in Germany, passengers could travel at their own risk at a speed of 30km/h. However, a barrier of 6ft height had to be placed to protect the visitors – better safe than sorry and possibly an early example of external safety measures.  

Today, much more is known about the safety aspects of hydrogen and ammonia. Hydrogen has already been extensively used in the last century on a large scale. From the 1940s to the 1970s, gas distribution networks were installed, and town gas (also known as coal gas) was the main energy carrier for the industry and households. Town gas consists of up to 50% of hydrogen (besides other hydrocarbons and carbon monoxide). With the discovery and exploitation of natural gas reserves, town gas was replaced by natural gas.

Safety aspects related to the material properties

There is no doubt that new energy carriers might cause severe incidents when handled unsafely, just as any other energy carrier used today. Simply due to the fact they contain large amounts of energy.

One of the differences between hydrogen and other well-known energy carriers like natural gas is its specific weight. Natural gas is already lighter than air (approximately 45%). Nevertheless, it is usually stored at very low temperatures (as a refrigerated liquefied gas) or very high pressures. Upon its release, liquefied natural gas (i.e., LNG) may flash (when superheated) but will create a pool of rapidly evaporating cryogenic liquid resulting in a very cold vapour cloud. The low temperature and potential presence of liquid droplets in the cloud results in a higher cloud density compared with ambient air, even after air entrainment has taken place. Thus, dispersion takes place as a heavy gas, and it rarely shows buoyant behaviour.

Ammonia has a gas density comparable with natural gas, while hydrogen density is more than ten times lower than air. Due to this, hydrogen shows a buoyant dispersion behaviour even when stored at high pressures.

The air entrainment to the hydrogen jet exiting the high pressure vessel is usually not enough to increase its density to that of ambient air. As a result, lifting plumes are commonly observed in the case of hydrogen releases. This also applies to low momentum releases, which might stay at the ground for a while but finally lift off. Effect distances might be relatively short, but with a buoyant plume, safety issues might arise away from the ground when the plume wakes reach installations and structures.

For ammonia or LNG lifting releases, plumes can be observed as the final stage of their dense gas dispersion behaviour, depending on external conditions. These plumes are initially cold, thus denser than air, and can travel over warm surfaces such as road pavements and vertical walls at buildings and tunnel walls. The substantial heat transfer from these surfaces increases the cloud’s temperature without additional air entrainment. This can cause the plumes’ rising behaviour, especially at vertical surfaces.

Consequently, the low-density hydrogen has to be stored at very high pressure (300-900bar) or cooled (at -250^OC) as a liquid to obtain high energy densities during storage or transport.

Toxicity

Hydrogen is a non-toxic gas, whereas ammonia is highly toxic. Even small leakages of ammonia can harm the respiratory system and might result in chronic diseases or damage to the central nervous system in the long term. Therefore, safety aspects for large scale storage might have an enormous impact on the surrounding area.

Flammability

Hydrogen and ammonia are both flammable. Compared with other flammable substances, hydrogen can be ignited within a very wide fuel-to-air ratio. Hydrogen can be ignited between the lower (4% by volume) and upper flammability limit (75% by volume) and requires a very low amount of ignition energy. In comparison, natural gas can only be ignited between 5 and 15% and ammonia between 15 and 28%. For ammonia, the flammability limits are very narrow, leading to often neglectable flammability risks (it is hardly possible to ignite a pool of ammonia).

The ignition energy of hydrogen is approximately 17 times lower, and the laminar burning velocity is eight times faster than other typical hydrocarbons. As a result, the risk of a Vapor Cloud Explosion (VCE) of hydrogen is higher due to both a higher probability (chance ignition) and impact (overpressure consequence). When ignited, hydrogen burns with an almost invisible flame. Only impurities like particles or gases present in the combusted air make the flame visible. As a result, no smoke is generated from a hydrogen flame. Therefore, the radiation emitted by a hydrogen jet fire is much lower compared with a jet fire from hydrocarbons.

Further safety aspects

In particular, for hydrogen systems, gas leakage through the structural material of the pipeline but also through components like gaskets, connectors and valves creates potential risks. In addition, hydrogen can lead to embrittlement of the structural material, which can significantly decrease the fatigue limit.

Safety in the future

A lot has changed in the world since the steam train changed the world in the 1820s. People rely more and more on technology as a part of their life, but doubts remain until there is evidence. This holds in particular for new energy carriers, which imply new and different safety issues. These safety issues have to be addressed thoroughly beforehand. Better be safe than sorry.

At Gexcon, we want to assist in making the world a safer place. That is why we face the challenges of the future while continuously improving our software tools and conducting experimental tests. Recently we improved the hydrogen jet fire model. EFFECTS and RISKCURVES undergo significant improvements with emphasis on lighter-than-air conditions for the energy carriers of the future.