No matter what it involves, the new and unknown always create uncertainty. The same seems to be true of hydrogen, which is already being talked about as the fuel of the future. Its everyday use as a fuel in transport or thermal power generation raises considerable doubts, mainly related to safety issues. However, if we dig deeper and understand the unique properties of hydrogen and learn how to handle it, it has the potential to become the best alternative to conventional fuels.
The future will be hydrogen
The Latin name for hydrogen is hydrogenium. However, the original name of the gas comes from the Greek words hydōr (water) and gignomai (creation, formation). As a unique gas, it was discovered by Henry Cavendish in 1766. Just seven years later, Antoine Lavoisier gave it the name “forming water,” thus proving that water is composed of two basic elements – hydrogen and oxygen. Few know, however, that hydrogen was first observed as early as 1671, long before Cavendish’s discovery, when British chemist Robert Boyle dissolved iron in dilute hydrochloric acid.
Hydrogen has been produced and used for industrial purposes for over a century. As a new energy vector, hydrogen shows many advantages over traditional hydrocarbon fuels. It is energy-efficient, environmentally friendly, and can be obtained from renewable sources. Potentially, it can solve many problems in the future on the grounds of ecology and environmental improvement as well as those related to energy security.
Basic physicochemical properties of hydrogen
The wider use of hydrogen technologies will require the creation of a new culture, innovative safety strategies, and specific engineering solutions.
To achieve this, engineers, designers, operations personnel, plant users, etc., should be aware of all the specific risks associated with the handling and use of H₂ systems. Interestingly, most of the risks associated with hydrogen stem directly from its properties. Therefore, it is important to have access to knowledge about its physical and chemical properties including flammability and explosivity.
Under normal conditions, hydrogen is a gas formed by binatomic molecules (composed of two hydrogen atoms) with the formula H2 (molecular weight 2.016 g/mol). The two hydrogen atoms form a single covalent bond. Due to the atomic arrangement of hydrogen, a single electron orbiting the nucleus is highly reactive. For this reason, hydrogen atoms can easily combine into pairs.
Hydrogen is the lightest and most abundant element in the universe, making up about 75% of its elemental mass. It is also the third most abundant element on Earth, after oxygen and silicon, but it is virtually nonexistent in the Earth’s atmosphere as a free element. In its free state, however, it is found in the Sun and stars.
Gaseous hydrogen – properties, characteristics
At standard temperature and pressure, hydrogen is a colorless, odorless, and tasteless gas.
There is a high risk associated with these properties, as leaks from the system are difficult to detect with the human senses. The low weight and small particle size of hydrogen also contribute to the high diffusivity of hydrogen gas and its tendency to leak through fittings, flanges, threads, gaskets, porous materials, etc. The low viscosity of hydrogen and the small particle size explain the relatively high outflow rates accompanying gas leaks.
When analyzing the issue of hydrogen odorlessness, we must remember that odorant compounds such as mercaptans (usually used as odorants to detect natural gas leaks) cannot be added to hydrogen systems because they contaminate (“poison”) the fuel cells. In addition, because of the smaller particle size of hydrogen compared to known odorants, it can migrate and leak through openings that are insufficient in size for odorants to pass through. Hydrogen tends to move away from the source of leakage faster than odorants due to its high dispersion coefficient.
Hydrogen is a non-toxic and non-corrosive element. It is a flammable gas and should be stored away from heat sources, open flames, and sparks. In a confined space, it can cause asphyxiation by diluting oxygen in the air below concentration levels necessary to sustain life.
It is the lightest of all known gases. Hydrogen gas (GH2) is 14 times lighter than air, which means it will float and diffuse quickly when released from a plant in an open environment. This is a major advantage of hydrogen from a safety standpoint because it will float and diffuse quickly if released. Thus, its concentration in a mixture with air will be diluted relatively quickly by the surrounding air below the lower flammable limit (LFL). Therefore, in many real-world situations, currently used hydrocarbon fuels may pose a greater fire and explosion hazard than hydrogen.
Although hydrogen is non-corrosive and non-reactive under standard conditions, it is capable of reducing the mechanical strength of some materials through a series of processes and interactions commonly referred to as hydrogen embrittlement.
Moreover, the thermal conductivity of hydrogen is much higher than that of other gases. In the Joule-Thomson process (the change in the temperature of a real gas during isenthalpic gas expansion) starting from the ambient temperature, the temperature of hydrogen will not decrease but increase. However, it should be noted that the temperature increase is not sufficient to cause ignition.
Liquid hydrogen – properties, characteristics
Hydrogen in the liquid state (LH2), in turn, is a clear liquid with a light blue hue. It is odorless, non-corrosive, and low in reactivity. It is a cryogenic liquid. The specific gravity of liquid hydrogen is 0.071. For comparison, the specific gravity of water is equal to 1. This means that liquid hydrogen has about 14 times less dense than water itself. As temperature increases, the volume of liquid hydrogen increases significantly. This property is indicated by the coefficient of thermal expansion, which is 23 times greater concerning the thermal expansion of water.
Hydrogen liquefaction is an exothermic process that is very slow and can take several days to complete a phase transformation. Interestingly, the liquefaction process can be accelerated by using a paramagnetic catalyst.
It is important to note that LH2 can boil rapidly and will turn into a gas if exposed or spilled into a normal temperature environment. Heating LH2 to ambient temperature can lead to very high pressures in confined spaces.
Liquid hydrogen vs. gaseous hydrogen – storage
The volume ratio of LH2 to GH2 is 1:848. LH2 expands approximately 850 times when converted to gas at normal temperature and pressure, so it is stored in double-walled, vacuum-insulated tanks equipped with several process safeguards. GH2 storage tanks also have appropriate process protection, including pressure relief devices (PRDs). It is important for safety reasons to provide a buffer space in the cryogenic storage tanks to accommodate the expansion of the gas volume with temperature changes (rise). Lack of buffer space can lead to an overpressure in the tank.
LH2 has the lowest density of any liquefied gas. Unlike propane, the compression of hydrogen gas does not liquefy it. Therefore, the LH2 phase is absent from compressed hydrogen storage tanks.
The continuous evaporation of LH2 in the vessel generates GH2, which must be discharged to a safe location. When hydrogen gas is heated from NBP to NTP, its volume increases, where:
1) NTP – Normal Temperature and Pressure (NTP): 293.15 K (20°C) and 101.325 kPa
2) NBP – Normal Boiling Point (NBP) at an absolute pressure of 101.325 kPa of hydrogen is 20.3 K (-252.85°C)
For fixed volume storage tanks, the phase change from LH2 to GH2 and the associated temperature increase (from NBP to NTP) will result in a pressure increase from 0.1 MPa to 177 MPa. This can lead to an overpressure in the tank or leakage of liquid hydrogen into the transfer and vent piping (this phenomenon must be taken into account during storage tank design).
Liquid hydrogen (in NBP) has a density of 70.78 kg/m3. The higher vapor density of saturated hydrogen at low temperatures can cause a cloud of liquid hydrogen to “flow” horizontally or even fall immediately upon release if a spill or leak of LH2 occurs.
Hydrogen combustion – characteristics of the phenomenon
At normal temperature, hydrogen is an unreactive substance unless it is activated in some way, such as by a suitable catalyst. The reaction of hydrogen with oxygen to form water at ambient temperature is extremely slow. However, if it is accelerated by said catalyst or spark, it proceeds rapidly and violently with the production of energy.
Hydrogen burns in a clear atmosphere with a pale blue, almost invisible flame* and does not emit visible light during the day (sunlight can dim the visibility of hydrogen flames) or smoke (it only produces water when it burns in the air). The situation changes when particles containing impurities get into it and burn along with the combustible mixture. Compared to other fuels, it has a higher adiabatic flame temperature for a stoichiometric mixture* in the air – it is ~2130°C.
*A stoichiometric mixture is a mixture in which both the fuel and oxidizer are fully consumed (i.e., complete combustion) with the formation of the product(s) of combustion. For example, two two-atom gases, hydrogen (H2) and oxygen (O2) can combine to form water as the only exothermic product of the reaction.
* Flame (hydrogen) – the zone of combustion of a gas from which light and heat are emitted.
Ignition of hydrogen in air occurs if its air content is below the Upper Flammable Limit (UFL) and above the Lower Flammable Limit (LFL) and if an effective ignition source is present. The flammability range of hydrogen is much wider compared to hydrocarbons, i.e. 4 to 77 vol% [according to ISO/TR 15916:2015(E)] in a mixture with air at NTP. The flammability range of hydrogen burned in pure oxygen is even wider, i.e. 4.1 to 94 vol% in a mixture with O2 at NTP [according to ISO/TR 15916:2015(E)].
Hydrogen is very easily ignited. Ignition sources include examples such as mechanical sparks from rapidly closing valves, electrostatic discharge, sparks from electrical equipment, catalyst particles, heating equipment, atmospheric discharge near a vent stack, etc. Therefore, ignition sources should be properly eliminated or isolated.
The auto-ignition temperature should also be mentioned at this point. This is the minimum temperature required to initiate the combustion reaction of a fuel-oxidizing mixture in the absence of an external ignition source. The standard auto-ignition temperature of hydrogen in air is from 584.85°C [according to ISO/TR 15916:2015(E)]. It is relatively high compared to long molecule hydrocarbons. Compared to hydrocarbon combustion, hydrogen flames also emit much less heat. Thus, the human physical sensation of this heat does not occur until there is direct contact with the flame. A hydrogen fire can go undetected and will spread despite direct human monitoring in areas where hydrogen can leak, spill, or accumulate and form potentially flammable mixtures.
Hydrogen is the fuel of the future
This is indicated by the above description of selected parameters. Hydrogen is no more or less dangerous than other currently used combustible fuels such as gasoline or natural gas. It has a unique set of characteristics that distinguish it from other fuels.
Some of its properties even provide safety advantages over other fuels used to date. But hydrogen, like all combustible fuels, must be handled responsibly. Like gasoline and natural gas, hydrogen is flammable and can create hazards under certain conditions. Understanding the properties of hydrogen and knowing its applications will therefore lead to the safe implementation of the gas as a new fuel.
Let us remember that the level of safety at the consumer interface with hydrogen must be similar or higher than with the fossil fuels used to date. Thus, the safety parameters of hydrogen products and fuel cells will directly determine their competitiveness in the marketplace. The expected growth of the hydrogen economy raises many questions about the safety of hydrogen production, transportation, storage, and end-use. These questions are answered by Hydrogen Safety Engineering, which can be defined as the application of scientific and engineering principles to protect life, property, and the environment from the adverse effects of incidents and emergencies involving hydrogen.
Literature:
ISO/TR 15916:2015(E) – Basic considerations for the safety of hydrogen systems
Lecture: Hydrogen properties relevant to safety, HyResponse, Grant agreement No: 325348, Compiled by S. Tretsiakova-McNally; reviewed by D. Makarov (Ulster University)
Molkov, V (2012). Fundamentals of hydrogen safety engineering, Part I and Part II.
NASA (1997). Safety standard for hydrogen and hydrogen systems. Guidelines for hydrogen system design, materials selection, operations, storage, and transportation. Technical report NSS 1740.16, Office of safety and mission assurance, Washington.
Baratov, AN, Korolchenko, AY and Kravchuk, GN (Eds.) (1990). Fire and explosion hazards of substances and materials. Moscow: Khimia. 496 p., ISBN 5-7245-0603-3 part 1, ISBN 5-7245-0408-1 part 2
Butler, MS, Moran, CW, Sunderland, PB and Axelbaum, RL (2009). Limits for hydrogen leaks that can support stable flames. International Journal of Hydrogen Energy, 34. pp. 5174-5182.
Sunderland, PB (2010). Hydrogen microflame hazards, Proceedings of the 8th International Short Course and Advanced Research Workshop in the series “Progress in Hydrogen Safety”, Hydrogen and Fuel Cell Early Market Applications, 11 – 15 October 2010, University of Ulster, Belfast.
Swain, MR and Swain, MN (1992). A comparison of H2, CH4, and C3H8 fuel leakage in residential settings. International Journal of Hydrogen Energy. Vol. 17, pp. 807-815.
