Invisible Fuel Hydrogen: Unveiling Properties
Hydrogen: A Promising Clean Energy Solution
Introduction to Hydrogen
History of Hydrogen Discovery & Naming
Hydrogen was first discovered in the 16th century.
This was when Pencillus accidentally poured iron fillings onto sulfuric acid. However, it was Henry Cavendish who identified this as a separate element in the 1780s.
Anton Lavoisier then gave it the name hydrogen.
The origin of the name hydrogen comes from the Greek words "hydro" meaning water and "genes" meaning to produce, so literally "water-producing."
Abundance of Hydrogen
Hydrogen is the most abundant element in the universe, about 75 percent by mass it makes up 90 percent by volume.
However, in Earth's atmosphere, the quantity of hydrogen is very small. This is because of the very high diffusivity of hydrogen.
Hydrogen as a Clean Fuel
Hydrogen is richest in terms of energy per unit mass.
When burned in air, it produces a clean exhaust. The byproduct obtained is water
It can be produced from a wide variety of sources, including renewable resources like solar and wind power.
Properties of Hydrogen
Simplest Element
Hydrogen is the simplest element and the first one in the periodic table.
Lightest Element
It is the lightest element, having one proton and one electron in its atomic form. Due to this arrangement, it is very reactive and not found as atomic hydrogen, rather it combines with various other elements forming either water or different hydrocarbons or organic compounds.
Atomic hydrogen does not exist; it exists in the form of a diatomic molecule, and this molecule, if dissociated to form atomic hydrogen, requires a high dissociation energy of 435 kilojoule per mole.
Basic Properties
- Hydrogen itself is a colorless, odorless, tasteless gas.
- It is flammable, non-corrosive, and non-toxic in nature. However, it can act as an asphyxiant.
- Hydrogen can displace oxygen if it is in confined spaces.
Low Density
- Hydrogen has a very low density of 0.08 kg per meter cube.
- It is 14 times lighter than air.
- It has very high diffusivity and buoyancy.
Isotopes
Hydrogen exists in three different isotopic forms
Protium
This is the most common form, wherein there is one proton and its mass is 1.008. It makes up a relative abundance of about 99.98%.
Deuterium
It has a mass of 2.014 and an abundance of 0.02%.
Tritium
Having a mass of 3.016, it occurs in extremely small amounts, negligible amounts, and can be produced during nuclear reactions.
Solubility
Hydrogen has very low solubility in solvents but very high solubility in metals.
Phase Diagram
If we look at the phase diagram of hydrogen, there are different regions where hydrogen exists in the form of solid, liquid, and gas.
The triple point of hydrogen, wherein all three phases coexist, is at 13.8 K (minus 259 degrees Celsius) and a pressure of 7.2 kPa, which is very low.
Boiling Point
Hydrogen's boiling point is at minus 253 degrees Celsius (20 K) at atmospheric pressure.
However, this boiling point can be increased.
Hydrogen can still be liquefied at higher pressures, up to a maximum of minus 240 degrees Celsius (33.2 K) at a pressure of 1.3 MPa, which is the critical point beyond which it cannot be liquefied simply by increasing the pressure.
Hydrogen can be obtained in solid form at a temperature of minus 259.2 degrees Celsius (14 K) at atmospheric pressure.
Liquid Hydrogen
Under normal conditions, liquid hydrogen is a mixture of ortho- and para-hydrogen at NTP.
This is a mix of 75% ortho-hydrogen and 25% para-hydrogen. The difference between these two stages of hydrogen is the nucleus spins: parallel in ortho-hydrogen and anti-parallel in para-hydrogen.
When hydrogen is to be liquefied, the 75% of ortho needs to be converted into para-hydrogen because that is a lower energy state, and that process is an exothermic (slightly exothermic) reaction and a slow process, as such requiring catalysts.
Ideal Gas Relationship
If we look at hydrogen, the ideal gas relationship holds still at a particular pressure under ambient temperature, say up to 100 bars.
However, deviation from the ideal gas behavior occurs, and this can be accounted for by the use of a factor which is known as the compressibility factor.
Chemical Properties of Hydrogen
Hydrogen is a powerful reducing agent and can react with a large number of salts, oxides, chlorides, their nitrates, nitrites, and cyanides to convert them into free metal.
It can react with most of the elements in the periodic table, both metals and non-metals, to form hydrides.
It can react with various oxidizers, producing a lot of heat. It reacts violently with these oxidizers.
Fuel Properties of Hydrogen
Diffusivity
Hydrogen has a very high diffusivity in air.
If we look at these numbers, hydrogen diffusivity is 0.63 centimeter squared per second.
This is three times that of natural gas and an order of magnitude higher than that of gasoline vapor.
This diffusivity has both advantages and disadvantages. Since the molecule of hydrogen is very small compared to any of the other gases, it can diffuse very fast.
It can diffuse through airtight or impermeable materials as well, and as such, this becomes very difficult to contain hydrogen unlike other gases.
But this high diffusivity along with high buoyancy has an advantage in the sense that if there is a hydrogen leak, then because of high buoyancy, it rises and diffuses very fast so that it dilutes very quickly along with air.
Density:
The density of hydrogen is very low under normal conditions.
For hydrogen at NTP, this is 0.0899 kg per meter cube. the density of hydrogen is 7% of that of air.
However, liquid hydrogen, which has a density of 70.8 kg per meter cube, is 7% of that of water.
Energy Content
If we look at the numbers for hydrogen, the higher heating value is 141.8 megajoule per kg, and the lower heating value is 120 megajoule per kg. as compared to gasoline, which is 48.6 megajoule per kg meand hydrogen contains approximately three times that much energy per unit weight as that carried by gasoline or diesel means it has the highest energy content per unit mass basis.
When we consider the volume basis, the amount of energy contained in a given volume of fuel (energy density) is very low for hydrogen. For hydrogen at a temperature of 15 degrees Celsius and 1 atmosphere, this value is 10.05 megajoule per meter cube. However, for gasoline, this is 31,150 megajoule per meter cube.
Liquid hydrogen has a higher density (8,491 megajoule per meter cube), but is very low numbers compared to conventional fossil fuels. So, that means the density (the volume) that will be required to store hydrogen will be very high unless they are compressed to very high pressures.
Even when it is compressed to pressures like 690 bar, the energy density is still 4500 megajoule per meter cube, which is lower than that of gasoline.
Flammability Limit
The flammability limit refers to the concentration range where a combustible mixture can sustain a self-propagating flame upon ignition.
It includes the lower flammability limit (LFL), which is the minimum concentration that can support combustion, and the upper flammability limit (UFL), which is the maximum concentration that can support combustion.
For hydrogen, the flammability range is wide, spanning from four to seventy-five percent by volume.
Explosive limits
Explosive limits indicate the concentration range that, when mixed with air and ignited, can trigger an explosion. Unlike fire, which can happen in an open environment, an explosion necessitates confinement.
When a combustible mixture within the explosive limit is contained and ignited, the subsequent rise in temperature and pressure can forcefully release from confinement, resulting in an explosion. Hydrogen's explosive range spans from 15 to 59 percent.
Equivalence Ratio
When discussing engines, we refer to the equivalence ratio, which for hydrogen spans from 0.1 to 7.1. This ratio is the actual fuel-to-air ratio divided by the stoichiometric fuel-to-air ratio. In contrast, for gasoline, it ranges from 0.7 to 4. This wide range for hydrogen has both benefits and drawbacks.
On the positive side, using hydrogen in engines allows even dilute mixtures to combust, leading to stable operations, easy starting, and improved combustion with lower emissions.
However, this wide flammability limit also poses a risk. In case of a leak, both very dilute and concentrated hydrogen mixtures can ignite, potentially causing a fire or explosion.
Ignition Energy
Ignition Energy refers to the minimum energy needed to ignite a mixture of hydrogen and oxidant under specific conditions.
For hydrogen, this ignition energy is exceptionally low at 0.02 millijoule, significantly lower than that of gasoline, which stands at 0.24 millijoule.
This indicates that only a small amount of energy is necessary for ignition, whether it's from a spark, flame, or electrical short circuit in an electrical device.
Due to hydrogen's poor electrical conductivity, any flow or agitation in gaseous or liquid hydrogen can generate electrostatic charges, leading to sparks and potential ignition.
Auto-ignition Temperature
On the flip side, the auto-ignition temperature is the lowest temperature necessary to start self-sustained combustion in a combustible fuel mixture without an external ignition source.
Hydrogen requires an auto-ignition temperature of 585 degrees Celsius, while gasoline's range falls between 240 to 460 degrees Celsius.
The higher auto-ignition temperature of hydrogen means it's less likely to ignite from heat alone, thus requiring an external ignition source or spark.
This characteristic of hydrogen is advantageous in engines requiring higher compression ratios for improved efficiency. However, it also means that even under high compression, hydrogen won't ignite on its own, requiring an external ignition source to initiate combustion.
Flame Speed
The flame speed refers to how far a flame travels in a given time when the fuel-air mixture is in the stoichiometric ratio. Hydrogen's flame speed is notably fast at 3.46 meters per second, which is significantly faster than gasoline's 0.42 meters per second. This rapid flame speed enables hydrogen engines to operate efficiently at higher engine speeds.
However, this high flame speed also presents challenges in combustion control, particularly in larger engines.
Burning Characteristics
Hydrogen burns with a colorless flame, which can be difficult to see in daylight.
This can be a safety concern, as a hydrogen leak may not be readily visible.
Nitrogen Oxides (NOx) Emissions
When hydrogen undergoes combustion with air, the primary product is water vapor (H2O), unlike fossil fuels that produce carbon dioxide (CO2), a significant greenhouse gas.
However, the combustion process at high temperatures can lead to the formation of nitrogen oxides (NOx), which contribute to smog and air quality issues.
There is ongoing research to develop strategies aimed at reducing NOx emissions from hydrogen combustion engines.
