Though the Hydrogen per is not corrosive, it can assist in the propagation of corrosion fatigue cracks and can also cause sulphide stress corrosion cracking in ferritic and martensitic steels, including the stainless grades. This is called Hydrogen embrittlement, also known as Hydrogen-assisted cracking or Hydrogen-induced cracking, is a reduction in the ductility of a metal due to absorbed Hydrogen since the Hydrogen atoms are small and can permeate solid metals. Thus, Bulk Hydrogen storage also needs vessel made of specially treated high-strength steels instead of regular steel.
Hydrogen flames have low radiant heat because its combustion primarily produces heat and water. Due to the absence of carbon and the presence of heat absorbing water vapour which is created when Hydrogen burns, a Hydrogen fire has significantly less radiant heat compared to a hydrocarbon fire. Since the flame emits low levels of heat near the flame (the flame itself is just as hot), the risk of secondary fires is lower. This fact has a significant impact for the public and rescue workers.
Like any flammable fuel, Hydrogen can combust. But Hydrogen’s buoyancy, diffusivity and small molecular size make it difficult to contain and create a combustible situation. In order for a Hydrogen fire to occur, an adequate concentration of Hydrogen, the presence of an ignition source and the right amount of oxidizer (like oxygen) must be present at the same time. Hydrogen has a wide flammability range (4~74% in air) and the energy required to ignite Hydrogen (0.02mJ) can be very low. However, at low concentrations (below 10%) the energy required to ignite Hydrogen is high – similar to the energy required to ignite natural gas and gasoline in their respective flammability ranges – making Hydrogen realistically more difficult to ignite near the lower flammability limit. On the other hand, if conditions exist where the Hydrogen concentration increased toward the stoichiometric (most easily ignited) mixture of 29% hydrogen (in air), the ignition energy drops to about one fifteenth of that required to ignite natural gas (or one tenth for gasoline).
The good part of Hydrogen storage is that no explosion can occur in its tank at any contained location without an oxidizer (i.e. oxygen) which must be present with certain level of concentration (at least 10% pure oxygen or 41% air). Hydrogen can be explosive at concentrations of 18.3%~59% and although the range is wide, it is important to remember that gasoline can present a more dangerous potential than Hydrogen since the potential for explosion occurs with gasoline at much lower concentrations, 1.1%~3.3%. Furthermore, there is very little likelihood that Hydrogen will explode in open air, due to its tendency to rise quickly. This is the opposite of what we find for heavier gases such as propane or gasoline fumes, which hover near the ground, creating a greater danger for explosion.
With the exception of oxygen, any gas can cause asphyxiation. In most scenarios, Hydrogen’s buoyancy and diffusivity make Hydrogen unlikely to be confined where asphyxiation might occur. Hydrogen is non-toxic and non-poisonous. It will not contaminate groundwater (it’s a gas under normal atmospheric conditions), nor will a release of Hydrogen contribute to atmospheric pollution. Hydrogen does not create “fumes.”
Hydrogen has a flame velocity which is seven times faster than that of natural gas or gasoline but the detonation of Hydrogen in the open atmosphere is highly unlikely, because of its higher stoichiometric ration at 29.53% against a value of 2% for Gasoline vapors & 9.46% Natural Gas). In order to explode, Hydrogen would first have to get accumulated to reach a minimum of 13% concentration level in a closed space and only then an ignition source, if triggered, can cause an explosion. Should an explosion occur, Hydrogen has the lowest explosive energy per unit stored volume, and a given volume of Hydrogen would have 22 times less explosive energy than the same volume filled with gasoline vapour.
Epilogue:
Pure Hydrogen in nature exists as a H2 molecule in gaseous form which is a highly combustible diatomic gas. The enthalpy of the H-H bond in H2 (436 kJ/mol) is huge, much larger than the respective values for the molecules Li2 (110.2 kJ/mol) and Cl2 (242.6 kJ/mol), and rather close to that of the strongly bound O2 molecule (498.4 kJ/mol), thus extracting the Hydrogen by electrolysis process requires a large amount of electricity yet on reverse side, the burning of Hydrogen gas (in fuel cell) also releases an similar amount of energy as shown below.
The energy in 1 kilogram of Hydrogen gas is about the same as the energy in 2.91 kilograms of gasoline (Considering the Avg Calorific values of Hydrogen as 142.50 MJ/kg vs. 49 MJ/kg of Gasoline), which is about 3 times more, however since the Hydrogen has a low volumetric energy density, it needs to be stored onboard a vehicle as a compressed gas to achieve the driving range of vehicles. However, considering the cost comparison it must be kept in mind that the 1 liter of Petrol contains about 30 MJ of energy which at present rate costs about Rs. 100, to obtain same energy approximately 126.50 (=30,000,000/237,160) moles of Hydrogen gas must be burned. The weight of 126.50 moles of Hydrogen is about 253 grams – the cost of which must be less than or at least equal to Rs. 100 for its economic viability, which in turn means that the price of Hydrogen must be approximately Rs. 39.53 per kg. Unfortunately, as per he estimates of KPMG for India, the current cost of green Hydrogen production is in anywhere between 320 and330 per kg (which is expected to fall in the range of about 160-170 per kg by 2030) while the cost of grey Hydrogen is about160-200 per kg. This cost comparison makes if very apparent that though the Hydrogen is a “dream” energy source because of, first its highest combustion energy release per unit of its weight compared to any commonly occurring fuel; second, its reaction product is pure water. These two properties make it the ideal fuel of choice, however, it is not easy to organize the Hydrogen fuel infrastructure. There are numerous obstacles which needs to be scaled up before Hydrogen could be produced in bulk and made as affordable, convenient alternative to either the present day hydrocarbon fuels powering ICEVs or the electricity powering the upcoming EVs.
Lastly, always remember no energy can be said to be 100% green if evaluated over its complete life cycle as even to harvest, store, & distribute the greenest energy, mankind needs means & methods, many of which can surely, not be created by 100% green mechanisms and also the second law of thermodynamics prevails.
Concluded
References:
11. Hydrogen: A Renewable Energy Perspective September 2019 Report Prepared For The 2nd Hydrogen Energy Ministerial Meeting In Tokyo, Japan (IRENA)
12. Hydrogen Production And Storage: R&D Priorities and Gaps (IEA)
Prabhat Khare holds BE (Electrical) & a Gold Medalist from IIT, Roorkee. He is an Automotive (EV) & Engineering Consultant, as well as a Technology Article Writer. He is a Certified Energy Manager (BEE) & Lead Assessor for ISO 9K, 14K, 45K & 50K. He can be reached at LinkedIn: https://www.linkedin.com/in/prabhatkhare2/.
