Hydrogen Safety Properties
As hydrogen gains more importance every day as an energy carrier, questions arise concerning the safety of this gas. What are the safety concerns with hydrogen? Can we use the existing infrastructure with hydrogen? How safe is hydrogen compared to the fuels we utilize nowadays?
Research on hydrogen safety has started many years ago. The importance of this concept arose when many tragic accidents occurred and caused immense casualties. For example, we can refer to accidents in the aerospace and power plant sectors. Although hydrogen is known as a very hazardous gas, with proper care, it is no less or more dangerous than other fuels we use nowadays. But it is important to note that hydrogen, has very specific properties and behavior. Therefore special safety measures need to be taken into account. Below a few of these specific properties are discussed.
Odor, color, and taste
Hydrogen is odorless, colorless and tasteless. So it is not easily detectable by human senses. Also, by the tendency of hydrogen to rise quickly, a leakage in a confined space would result in a hydrogen accumulation beneath the roof and away from humans’ sense range. Comparing with natural gas, both fuels are odorless, colorless, and tasteless, but in the case of natural gas, it is possible to add a sulfur-containing odorant, called mercaptan, to make it detectable by human senses. The use of such odorants in hydrogen would result in the contamination of fuel cells and the PEM (Proton Exchange Membrane) therefore making it not possible.
Density and diffusivity
Hydrogen spreads about 3.8 times faster than natural gas, which means that when released, it spreads quickly into the room and does not easily form flammable clouds thanks to the low fuel concentration and the high diffusion rate. In order to form a flammable cloud, you need to reach a volumetric ratio of hydrogen to air of at least 4%.
Also hydrogen has a density of 0.0838 kg/m3 at natural temperature and pressure (NTP) which is far below the density of air (1.205 kg/m3 ) at equal conditions. Therefore this gas is positively buoyant over almost the whole temperature range of its gaseous state. This spontaneous rise by physic laws gives us an advantage compared to other fuels in discarding the unwanted gas. By designing a proper roof or ventilation we can prevent the accumulation of hydrogen very easily. Since hydrogen is the lightest element in the universe it is very hard to confine hydrogen. Industry uses these properties in the design of hydrogen labs to help hydrogen escape up and away from the user in case of an unexpected release.
But caution should be taken in working with hydrogen in cryogenic temperatures. Hydrogen vapors in this temperature are denser than air at normal temperature and pressure. The low-temperature hydrogen will cause the humidity in the air to condense and add water to the mixture cloud making it visible and augmenting the molecular mass of the mixture even more.
Expansion ratio
The expansion of hydrogen with the addition of heat at the normal boiling point is about 23 times the expansion of water at ambient conditions. The safety concern is when liquid hydrogen stored in a vessel hasn’t got enough space to expand and results in an over-pressurization of the tank or a leakage. When hydrogen changes phase from liquid to gas a great volume change occurs. Also as it warms up from natural boiling point to normal temperature and pressure (20 centigrades and 1 atmosphere) another instance of volume growth follows. The ratio of the final to the initial volume of liquid hydrogen heated from NBP to NTP is 847. This volume change can cause a pressure rise of about 177MPa (starting from 0.101 MPa) if the liquid is in a closed vessel. This can lead to large explosions.
Flame Heat Radiation
Hydrogen compared to hydrocarbon fuels has lower radiant heat, meaning that the heat emitted from the flame to the surrounding is much less than carbon-based fuels. The flame from burning hydrogen is just as hot as hydrocarbon fuels but has lower heat emission due to the absence of carbon and the presence of heat-absorbing water vapor. This greatly reduces the risk of initiating secondary fires and has a significant impact on the public and rescue workers.
Combustion
Hydrogen’s buoyancy, diffusivity, and small molecular size make it difficult to create a combustible situation. For combustion to occur, an adequate concentration of hydrogen, the right amount of oxidizer, and a source of ignition energy must be present. Compared to other fuels, hydrogen has a wider flammability range (4 – 74 %). For example, the flammability limits for natural gas are 5.3 – 15%). However, at low concentrations (below 10%) the ignition energy for hydrogen is very high, making hydrogen practically harder to ignite close to the lower flammability limit. If the hydrogen concentration augments to the stoichiometric (most easily ignited in air) mixture of 29% the ignition energy drops much lower than natural gas (about 1/15) making the mixture very easily combustible [2]
Fires produced from hydrogen are safer than gasoline fires. Since hydrogen tends to move upwards quickly, if the reservoir of a fuel cell car catches on fire, the flames tend to move up and away from the car. But in a gasoline car, the fire will quickly take over the whole vehicle.
The energy released from burning hydrogen is more than gasoline, but this energy is released in a shorter period of time. A reservoir of liquid hydrogen will burn with a velocity of about 3-6 cm/min and a reservoir of liquid methane will burn with a velocity of about 0.3-1.2 cm/min and gasoline will burn with a velocity of about 0.2-0.9 cm/min. Hydrogen will burn much faster than other fuels.
Explosion
If a tank contains only hydrogen (without any oxidizer) explosion can’t occur. An oxidizer such as oxygen (at least 10%) or air (at least 41%) must be present. Hydrogen can cause an explosion in a concentration range of 18.3-59%. Although this range is wide, it is still less dangerous than gasoline. Because gasoline’s explosive concentration ranges from 1.1-3.3% making it potential for explosions at low concentrations. Also, because of its tendency to rise quickly, hydrogen is less likely to form a combustible cloud in an open atmosphere. In contrast, heavier gasses such as propane and gasoline hover over the ground creating a greater danger of explosion.
Embrittlement
Hydrogen easily can deteriorate metals. Even though hydrogen is non-corrosive, if absorbed by steel it will brittle the metal and cause failure. This effect is called “Hydrogen Embrittlement” and it involves many factors such as the environment temperature and pressure, the purity, concentration, and exposure time of hydrogen. Other factors include the stress points, physical and mechanical properties, microstructure, surface conditions, and etc. Most of the problems arise when welding (by causing thermal stress points) and improper material is used.
Material Selection depends on the mechanical properties of the material. The selected material must have a minimum value for each of the mentioned properties over the whole operating temperature. Also, we must consider emergency situations where the temperature and pressure rise or drop out of the operation range. The material must be resistant to phase change in the crystalline structure so that with repeated thermal cycles or time it stays stable.
This article was an extract of my bachelor of science thesis on the safety of hydrogen laboratories. The goal was to familiarize the reader with the concerns and benefits of hydrogen and compare it to other existing fuels. You can read the full thesis here. All references are documented in the mentioned file.
