Hydrogen - overcoming safety challenges to achieve a cleaner future

In this blog I would like to highlight some of the safety challenges associated with the increasingly wide use of hydrogen, as one of the key fuels that can replace fossil fuels and is necessary to prevent wide spread and unacceptable global warming.

Hydrogen is one of the most reactive fuels available. The maximum laminar burning velocity of any hydrogen-air mixture is 2.89 m/s, a factor of 8 faster than that of methane. The laminar burning velocity is the speed of the reaction front (flame) compared to the speed of the unburned mixture just ahead of the reaction front, and is an important property when predicting the possible consequences of hydrogen-air explosions. If a hydrogen-air cloud of this optimum mixture exploded out in the open, i.e. not inside an enclosure, the generated overpressures would be relatively low. Large-scale tests performed with optimum mixtures of hydrogen and air in hemispherical balloons of 40 m diameter resulted in a maximum overpressure of 57 mbar. This pressure is only slightly higher than one would expect just on the basis of the high laminar burning velocity. Flame acceleration occurs due to flame instabilities, causing the flame surface area to increase and exposing the hot flame to more unburnt fuel-air mixture, thereby spreading faster.

Flame acceleration can also occur due to turbulence generated around obstructions present in the flammable cloud, caused by the expanding combustion products. As an example: tests performed in a geometry with obstructions consisting of repeated horizontal pipes, caused hydrogen flames to quickly accelerate over a distance of 1.4 m generating a pressure of 1.17 bar.

The figure below shows a comparison between the overpressures generated by different gases in the same rig, highlights the difference in reactivity.

Figure 1 Maximum explosion overpressure generated by explosions of different fuels in a 3m x 3m x 2m volume obstructed by a number of metal grids (from Shirvill et al, 2019)

The high reactivity of hydrogen is also demonstrated by the overpressures generated when igniting a free turbulent jet escaping from a 50 mm hole in a 60 bar reservoir (initial release rate 8 kg/s). The violent explosion caused an overpressure of 0.43 bar at 10 m distance (Jallais et al., 2017) which can be compared to the maximum pressure generated by the explosion of free natural gas jet of 100-120 kg/s of approximately 0.2 bar (measured in the jet) (Hisken et al, 2021).

Among currently used protective measures of rooms and vessels against the consequences of accidental explosions, explosion venting is the most common method- a deliberately weak element of the enclosure designed to fail early during the explosion. The created opening allows gases to vent from the enclosure, preventing the explosion and increased pressure from building up and exceeding the failure pressure of the room or vessel. However, it is not as easily applicable for hydrogen, assuming a relatively weak cubic enclosure of 20 m3 without any internal obstructions, it can be calculated that the opening should be 2 m2 for methane and 13.2 m2 for hydrogen. Whereas, the former implies an opening with an area equal to 27% of one of the walls at 13.2 m2 , hydrogen is almost equal to two full walls of the enclosure, indicating the difficulty of using the method for relatively weak structures for hydrogen applications.

The high reactivity makes a hydrogen-air mixtures very prone for a transition to detonation (DDT=deflagration-detonation-transition). In case of a detonation, the reaction front propagates with a velocity of 2000 m/s or more, resulting in pressures of close to 16 bar. A detonation can propagate through a big part of the explosive region of a hydrogen-air cloud, generating high pressures without the need of turbulence-generating obstructions. The transition to detonation has been seen in vented enclosure explosions where flames have run into turbulence generated by a fan and after the flames have accelerated due to obstacles.

Another challenging property of hydrogen is its very low minimum ignition energy. The minimum ignition energy describes the energy of the smallest electric spark needed to start a self-sustained reaction in a mixture of hydrogen and air. This energy is only 0.017 mJ for hydrogen compared to 0.26 mJ for methane. Hydrogen is flammable over a very wide range of concentrations (4% - 75 % hydrogen-air). The minimum ignition energy is concentration-dependent and lower than the value for methane in the range of 8% - 64% hydrogen-air. The low ignition energy puts strong demands on electric equipment when used in areas where hydrogen-air mixtures may arise to avoid ignition by this equipment. The same applies to electrostatic sparks and discharges. Hydrogen can be ignited by so-called corona discharges which occur when sharp-pointed objects are in the vicinity of a charged piece of equipment. Laboratory experiments indicate that hydrogen-air concentrations with a minimum ignition energy of < 0.1 mJ can be ignited by corona discharges (implying hydrogen-air mixtures of 13% - 65% hydrogen-air). Strong focuses should therefore be on:

• earthing conductive parts of equipment

• avoiding charging of non-conductive parts of equipment

• avoiding sharp-edged objects

Corona discharges may arise at sharp objects due to atmospheric electricity as well. This is especially important for vent stacks, where hydrogen released to the atmosphere may ignite unless the vent is specifically designed to prevent corona discharges.

Personnel/people can easily become an ignition source, especially when conditions are favorable for electrostatic charging, for example when the air humidity is low. It is important to ensure that personnel/people are earthed by using dissipative footwear and flooring.

The possibility of charging should be considered for non-electrical rotating equipment. Mechanical friction resulting in hot surfaces and consequently sparks, can be incendiary. Experiments show that hydrogen can be ignited by single impact sparks (even low energy ones), frictional sparks (especially lean hydrogen-air mixtures) and hot surfaces. The material combinations involved in the mechanical friction play an important role when considering the incendivity.

Figure 2 Ignition by mechanical friction: comparison of ignition at steel pin pressed against rotating wheel: mild steel vs. stainless steel. Variation of rotating velocity of wheel and force with which pin is pressed against wheel. All mild steel ignitions are caused by mechanical sparks. All stainless steel ignitions are caused by hot surfaces generated at the pin surface (from Welzel et al, 2011).

Significant attention has been given to the so-called spontaneous ignition of hydrogen. This is related to the sudden release using a bursting disc of hydrogen from a pressurised reservoir into a vent stack. Currently, a bursting disc and the presence of the stack can cause this phenomenon, which is related to generation of shock waves and their reflection onto the stack walls.

Since hydrogen has a very low density, different solutions are used to store and transport it as it can be compressed to very high pressures (750 – 1000 bar), can be liquified (temperature of liquid hydrogen is -253°C) or can use a carrier (such as ammonia). Each of these solutions imply new challenges. A leak from a high pressure reservoir can result in a strong turbulent jet which upon ignition can lead to the aforementioned high pressures, but also to a strong jet flame as its length is dependent on hole size and reservoir pressure. Hydrogen jet flames are shorter than those involving hydrocarbon fuels and radiate less due to the absence of CO2, CO and soot. As a result, hydrogen flames are practically invisible in daylight.

Liquified hydrogen implies some new, partly unknown, hazards including:

• possibility of BLEVEs (Boiling Liquid Expanding Vapour Explosions) involving liquified hydrogen storage vessels

• consequences of releases of liquified hydrogen onto water or under water

• the co-existence of liquified hydrogen and solid/liquid oxygen upon a release

Research has been ongoing into several aspects of the consequences of the release of liquified hydrogen.

The use of hydrogen carriers may lead to hazards and while ammonia appears to be the most likely carrier to be used, it is a highly toxic material and in the event of a leakage can lead to personnel being harmed. The concentration at which ammonia is immediately harmful to life or health (IDLH) is 300 ppm (0.03 %).

At atmospheric conditions, hydrogen is considerably lighter than air (density is 0.090 kg/m3 and 1.22 kg/m3 respectively). As a result, there is a strong tendency for hydrogen to move upwards in case of jet releases at some distance from the point of release where the momentum of the release has decayed. This is very positive for installations in the open, but inside buildings the gas would accumulate against the roof of the building. To avoid accumulation of flammable hydrogen-air mixtures inside buildings, the ventilation is important considering possible release locations, inlet and outlet and effect of larger obstructions.

In my next blog I will explore these areas in more detail but please contact me if you would like to discuss any area.