[curiouscat]

I was a little surprised to know that the auto-ignition temperature of Hydrogen is above 500 C. Any reason why so?

My intuitive feeling for H2 being so easily flammable is at odds with H2 being able to be heated so hot without igniting. Even gasoline and diesel have much lower auto-ignition temperatures.

PS. As an aside, how do they measure this auto-ignition temperature of a gas? Does this mean that if I put a 50-50 mix of H2 and air (Oxygen?) inside a cylinder it won't ignite till 500 C so long as no spark / flame enters the cylinder?

[Enthalpy]

Hi Curiouscat,

I confirm the high autoignition temperature of hydrogen, higher than most hydrocarbons.

Yes, autoignition is measured with  a mix. Often given as a function of the proportion; generally with air, but sometimes with oxygen, where hydrocarbons ignite just a few 10K earlier. The proportion easiest to light is generally near the stoechio in air, that is, making CO₂, and tends to be the chosen condition if not told. And yes, the mix catches fire when reaching this autoignition temperature (air heated by compression in a Diesel engine), or if ignited by some other means (spark in a gasoline engine).

Application: in a gasoline engine, the premix shall not ignite before the spark, so you want a high autoignition temperature, related with a high octane number; highly branched alkanes like isooctane achieve it, aromatics too. In a Diesel engine, the oil shall light easily when injected in the cylinder's hot air, so you want a low autoignition temperature, related with a high cetene number; long straight alkanes achieve it. And with a similar shape, longer alkanes are less volatile (determines much the flash point) but have a lower autoignition, so flame stability in a jet engine results from a mix of molecule sizes, kerosene being one good compromise (bringing also a low melting point and some more).

I haven't checked the detailed reasons for hydrogen's autoignition temperature, but:

• H-H holds with 436kJ/mol, versus variable 400 for C-H and variable 350 for C-C;
• Shocks with the too light H₂ could be inefficient.
• Possibly (I'd need to think more at that one) longer molecules with more degrees of freedom can concentrate from time to time more heat energy in few atoms.

[curiouscat]

Thanks @Enthalphy

I guess the non-intuitive part to me is how something like gasoline has a high autoignition temp. but very low flash point.

Is there a qualitative explanation for this?

My attempt: Autoignition depends on heat alone creating enough energy to transcend an activation energy barrier. So the height of the combustion activation energy barrier is what matters.

In flash points the energy needed for activation is gurranteed via the spark / flame. So the governing constraint is developing a mixture that will sustain a flame without dampning out. So that becomes a volatility issue / a stoichiometry of combustion issue.

Comments?

[Enthalpy]

Yes.

The flash point only provides enough vapour for the mix with air (rarely oxygen) to be flammable; a separate efficient means is provided for ignition.

Hydrogen is a more extreme case, with a flash point below 20K since <1atm partial pressure suffices. Common practice doesn't even tell a flash point for cryogenic gases, since everyone knows he better keeps the matches away.

Some rough models (Laurent Catoire and Valérie Naudet) have been proposed to predict flash points to ±15K accuracy, by similarities within compound families. Also fun: while a flash point is normally below the boiling point, some compounds have their flash point below the melting point; hexamethylene tetramine actually burns without visibly melting.

[snorkack]

The flash point only provides enough vapour for the mix with air (rarely oxygen) to be flammable; a separate efficient means is provided for ignition.

Hydrogen is a more extreme case, with a flash point below 20K since <1atm partial pressure suffices.

Um, that I´m suspicious about. What is the partial pressure of solid oxygen at 20 K?
(I think oxygen, nitrogen and argon do not mix in their respective solid states. If so, the vapour pressure of air below freezing point should be the sum of the vapour pressures of solid components. Right?)

Also fun: while a flash point is normally below the boiling point, some compounds have their flash point below the melting point; hexamethylene tetramine actually burns without visibly melting.

Flammable gases with high triple point pressure, in this case actually above atmospheric, include ethyne.

[Enthalpy]

And what relation do you see with the triple point?

[snorkack]

And what relation do you see with the triple point?

a) At flash point, saturated vapour pressure gives lower explosive limit.
b) Lower explosive limit is below 100 % concentration
c) Therefore, the vapour pressure at flash point is necessarily below ambient pressure, and flash point is necessarily below boiling point
d) For a substance with high triple point pressure, vapour pressure of the solid below triple point may exceed the lower explosive limit and thus allow flash point below melting point.
e) One example of a flammable substance with high triple point pressure is ethyne.

Listing other examples, what are the triple point pressure and flash point temperature of neopentane?

[Enthalpy]

Triple points too go better when I sit at the computer during daytime.