Hydrogen Fundamentals
With global warming like never seen before and climate change joining hands, technological developments have become crucial to achieving decarbonization. On the other hand, our principal fossil energy sources are expiring, so there has been an increasing demand, and humanity is forced towards renewable energy sources. Though the primary source of renewable energy is the sun, wind, and hydropower, they are still not sustainable. Hydrogen energy plays a vital role in satisfying the energy demands and achieving the environment-friendly goal of sustainability. There are still technological demands that should be satisfied. This could only be solved with interest, financial aid, and time.
Hydrogen as an element is the lightest, most abundant in the universe existing almost everywhere- in the sun, in water, space, and air, but in compound form and rarely occurs naturally. All the fossil fuels we use today are based on hydrogen in the bound form known as hydrocarbons. However, extraction of hydrogen from hydrocarbons would lead to the emission of greenhouse gases which again does not support our clean energy goal. Hence, the best renewable way to extract hydrogen is from water. The following discusses the physical and chemical properties of hydrogen
Physical Properties of Hydrogen
• The first element in the periodic table.
• The most common isotopes of hydrogen are deuterium and tritium.
• Hydrogen is colorless, odorless, non-toxic, non-corrosive, and a non-metallic diatomic gas at STP
• Hydrogen has a very low density, so it needs to be either liquefied or compressed.
• Above a temperature of 22 K, H2 is highly buoyant, i.e., it rises on the top of a liquid or gas and is highly diffusive. On release to the atmosphere, it rapidly mixes with the ambient air. The diffusion coefficient of H2 in water at 298K is 4.50x10-5cm/s, whereas air is 2.5X10-5cm/s. The diffusion rates of H2 in the air are four times greater than air in the air.
• Hydrogen also has very low viscosity, but it varies with temperature and pressure.
• H2 gas also penetrates adjoining vessel materials at elevated temperatures and pressures, causing decarburization and embrittlement mainly in mild steel. Therefore, proper material selection is needed while storing or transporting hydrogen under pressure.
• Liquid hydrogen (LH2) is the cleanest and more economical way of storing hydrogen. A further decrease in temperature below the boiling point leads to slush hydrogen (SH2), a solid and liquid hydrogen mixture.
• Hydrogen becomes a conductor above a specific critical breakdown voltage but is an insulator in the gaseous and liquid state.
• Based on the temperature, H2 coexists in two forms- Ortho and Para. At normal temperature, it is 75% ortho and 25% para. In a temperature range, less than 80K para-hydrogen is more stable. At 20K, H2 is 99.82% para and 0.179% ortho.
Chemical Properties of Hydrogen
• The electronegativity of hydrogen is 2.20 (Pauling scale). Hence it reacts both with non-metals and metals to form ionic or covalent bonds. e.g., H2O
• The energy content of hydrogen by mass is very high compared to coal and natural gas as 1Kg of H2 contains 132.5 MJ, approximately two times more than natural gas and five times more than coal.
• The energy content of H2 is 242 KJ/mol LHV or 286 KJ/mol HHV.
• Hydrogen reacts readily with oxygen making highly flammable mixtures over a wide range of concentrations. A completely burned hydrogen-air stoichiometric mixture contains about 29.5 vol% of H2 as the combustion product is water vapor.
• The burning of hydrogen results in a non-luminous flame in an almost pale blue color and result in flame temperatures of up to 2403 K, i.e., 2130°C. H2 ignites in a wide range of concentrations in air (4% to 75%), and the probability of the mixture exploding is very high over the range from 15% to 59% at STP.
• Cryogenic hydrogen can ignite at all temperatures above the boiling point of 20K as hydrogen does not have a flashpoint in its gaseous state. However, the auto-ignition temperature is high in the range of 800K to 1000K. A hydrogen-air mixture requires minimum ignition energy of 0.02 MJ, which further decreases with increasing temperature, pressure, and the presence of oxygen.
• Due to high diffusivity, the burning velocity of hydrogen in air is the highest compared to another hydrocarbon fuel-air mixture. At stoichiometric ambient conditions, the flame velocity ranges from 2.55 m/s to a maximum of 3.2 m/s and could also reach 11.75 m/s in pure oxygen. Hence more safety measures are needed as the transition from deflagration to detonation will be faster due to higher flame velocity.
• The detonation velocity of hydrogen could reach 2000 m/s in air and up to 3500 m/s in pure oxygen.