Is Hydrogen Flammable? (A Comprehensive Overview)

Is Hydrogen Flammable?

Hydrogen gas (H2) is extremely flammable and explosive. Hydrogen gas is classified by the National Fire Protection Association standards with a flammability degree of 4 (the highest in this system).

The flammability range of hydrogen  in air, that is the volumetric ratio range within which it can easily catch on fire, has a minimum of 4 and a maximum of 74%.

Furthermore, a mixture of hydrogen and air requires a relatively very small amount of energy to ignite (catch on fire), about 7% the energy needed to ignite natural gas.

Many fire related incidents over the past century have been related to hydrogen gas. One of the earliest incidents happened in 1937 in the United States; the zeppelin Hindenburg was consumed by a fire involving hydrogen cells.

Despite the hazards related to fires and explosions, hydrogen is a very important substance for its extremely useful applications. Hydrogen applications include spacecraft fuel and raw material for the production of ammonia (NH3).

What Is The Difference Between The Hydrogen element And Hydrogen Gas?

The hydrogen element is the chemical element with an atomic number of one (that is, the atomic nucleus has one proton). The hydrogen element is present in many well known substances such as water (H2O), natural gas (CH4), DNA, gasoline, and alcohol.

Often, the hydrogen name refers to molecular hydrogen (also known as dihydrogen, hydrogen gas or by its chemical formula H2). Molecular hydrogen is a molecule composed of two hydrogen atoms chemically bonded to each other (H2).

Some Facts About The Hydrogen element (H)

Hydrogen is the most common chemical element in the known universe accounting for around 75% in abundance. Most stars are composed mainly of hydrogen in the plasma state.

A hydrogen atom usually has no neutron in its atomic nucleus, when there is on neutron hydrogen is called instead deuterium (D).

An isolated hydrogen atom has one electron and it can be called a radical hydrogen (H.). Acids such as sulfuric acid or hydrochloric acid have their chemical behavior attributed to the ability of ‘releasing’ a hydrogen atom without its electron (H+, also called a proton).

A hydride is a hydrogen atom possessing one extra electron in its electron shell and it can be represented as H. Hydrides are very strong bases.

Some Facts About The Hydrogen Gas (H2)

Hydrogen is a colorless, tasteless and odorless gas above -253 ºC (-423 ºF) at ordinary pressures. While in the gas phase, hydrogen has high buoyancy and elevated capacity to crack metals.

Hydrogen is a transparent liquid within the temperature range of -259 ºC to -253 ºC (-434 ºF to -423 ºF) at 1 atm of pressure. Below -259 ºC (-434 ºF) hydrogen is a transparent solid. In the Sun and most other stars, hydrogen is in the plasma state.

Appearancecolorless gas
Molecular formulaH2
Molecular weight2 g/mol
Boiling point-253 ºC (-423 ºF)
Melting point-259 ºC (-434 ºF)
Flash pointnot applicable
Autoignition temperature~585 ºC (1085 ºF) 

Molecular deuterium (²H2 or D2) and hydrogen deuterium (HD), both have pretty much the same behavior as hydrogen gas in terms of its fire-related character.

Hydrogen Gas Fire Hazards

Hydrogen gas is a few orders of magnitude easier to ignite than most flammable liquids and gasses such as those present in gasoline, natural gas, or benzene.

The minimum ignition energy (MIE) of hydrogen gas in regular air is 0.000019 Joule, while methane has an MIE of around 0.000140 J (the lower the value of .MIE the more easily the given material ignites).

This means that hydrogen can burst into flames at relatively low amounts of energy, even.sunlight, small electrical discharges or metal friction can cause hydrogen to ignite which in turn can lead to a fire or an explosion.

Hydrogen Combustion With Oxygen

When hydrogen gas catches on fire  in the air, the combustion reaction taking place is a reaction between hydrogen gas (H2) and oxygen gas (O2), in a 2 to 1 ratio, to form water (one molecule of water per molecule of hydrogen gas and half molecule of oxygen gas).

This chemical reaction releases a lot of energy in the form of heat (around 141 million Joules per kilogram of hydrogen gas). This amount of energy (heat) gives rise to flames.

The flames produced by the combustion of hydrogen by oxygen gas are nearly invisible unless they are in a dark place. If other chemicals are in contact with the flames they may be more visible.

Can Hydrogen Catch On Fire Without Oxygen?

Yes, hydrogen can undergo combustion to produce fire without oxygen as long as there is another strong oxidant such as chlorine gas (Cl2) ro fluorine gas (F2) in certain proportions.

Mixing hydrogen and chlorine can cause a fire and a  severe explosion. The chemical reaction between H2 and Cl2 forms HCl (hydrochloric acid) along with a large amount of heat (which is the source of the fire produced).

The explosive reaction between hydrogen and chlorine can be initiated only by light without the need of adding heat to the mixture.

Examples Of Chemical Reactions That Form Hydrogen Gas

Hydrogen can be formed by a series of types of chemical reactions, which include:

  • Water electrolysis (‘splitting’ of a water molecule into H2 and O2 by electrolytic means).
  • Methane pyrolysis (heating methane to very high temperatures in the absence of an oxidizer to break methane into H2 and C).
  • Partial oxidation of hydrocarbons (e.g. oxidation of methane into carbon monoxide and hydrogen).
  • Metals like iron (Fe0) and zinc (Zn0), when treated with acids like sulfuric acid or nitric acid (HNO3), produce hydrogen gas along with metallic salts.
  • Anaerobic metabolism by some microorganisms.

Is Hydrogen Toxic?

Hydrogen gas can cause asphyxiation if it reaches high concentrations in the ambient atmosphere.

Liquid hydrogen can cause frostbite if in contact with skin.

What Is Hydrogen Used For?

The main method for the production of ammonia (NH3) is by reacting hydrogen (H2) with nitrogen (N2). Ammonia in turn is used in a wide variety of further transformation to obtain fertilizers,

As a fuel in fuel cells or internal combustion engines. Nonetheless, there are no natural sources of hydrogen gas in appreciable amounts and so hydrogen must be produced by some methods.

Other important applications of hydrogen include:

  • Chemical reduction of compounds containing sulfur or nitrogen in the refining of petroleum.
  • Production of methanol (CH3OH) by the reaction between hydrogen gas and carbon monoxide (CO).
  • Conversion of unsaturated fats into saturated fats (e.g. to produce margarine).
  • Purification process of metals.

Conclusion

Hydrogen gas is a substance of very  high importance in industry, technology, research, and energy fields. At the same time, hydrogen can lead to catastrophic incidents if not handled with proper safety measures.

Frequently Asked Questions (FAQ): Is Hydrogen Flammable?

Is hydrogen flammable without oxygen?

Hydrogen gas can catch on fire without any oxygen if there is another strong oxidizer in contact with hydrogen. But, if flammable is defined as being easily ignitable in the presence of air (and oxygen) then nothing can be flammable without oxygen. That doesn’t mean that fire requires oxygen.

Why is hydrogen gas so flammable?

Hydrogen needs very little energy to ignite and onc it ignites it produces a large amount of energy. That means that hydrogen easily catches on fire, and that the fire easily sustains itself. As for why hydrogen gas has these particular properties, a quick answer is that both hydrogen and oxygen are somewhat chemically unstable and when  they react with each other to form a fire they combine to form water which in turn is a relatively stable molecule. The instability of the reactants (hydrogen and oxygen) makes it so they don’t need a lot to react, while the stability of the product (water) in relation to the reactants leads to a large amount of energy being released in the process.

Is hydrogen a safe fuel?

Hydrogen ignites and explodes easier than gasoline, natural gas or ethanol. Hydrogen storage is a lot more demanding than most other fuels. On the other hand, hydrogen quickly dissipates when released on the atmosphere, given its low density. Hydrogen gas also has almost no toxic effects.

Does pure hydrogen explode?

A recipient containing pure hydrogen can explode if there is contact with an oxidizer such as oxygen. If hydrogen is pure and it is completely isolated from other substances it can not explode.

Can hydrogen tanks explode?

Yes, if a hydrogen tank is damaged and hydrogen leaks the release of hydrogen can have the potential to cause an explosion. If the leak is mild but it allows for hydrogen build up, ignition can occur if a source of ignition such as heat, light or an electrical spark is  in contact with a mixture of hydrogen and air. Heating can also cause the explosion of a hydrogen tank as hats increase the inner pressure in the tank.

Can you light hydrogen on fire?

Hydrogen gas can be set on fire very easily. The fire from burning hydrogen emits mostly ultraviolet light and is almost invisible.

References

https://h2tools.org/lessons/fire-hydrogen-fueling-station (Accessed August 07th, 2022)

https://h2tools.org/lessons/small-fire-fuel-cell-test-stand (Accessed August 07th, 2022)

https://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/h2_safety_fsheet.pdf (Accessed August 07th, 2022)

https://ntrs.nasa.gov/citations/19970033338 (Accessed August 08th, 2022)

Ryo Ono, Masaharu Nifuku, Shuzo Fujiwara, Sadashige Horiguchi, Tetsuji Oda,

Minimum ignition energy of hydrogen–air mixture: Effects of humidity and spark duration,

Journal of Electrostatics, Volume 65, Issue 2, 2007, Pages 87-93. 

https://doi.org/10.1016/j.elstat.2006.07.004

https://edu.rsc.org/exhibition-chemistry/hydrogen-and-chlorine-a-photosensitive-free-radical-reaction/2020059.article (Accessed August 09th, 2022)

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