Why is the Autoignition Temperature of Hydrogen so High? 530 C

Hydrogen autoignition temperature and its flashpoint pose a stark difference which is particularly interesting. This article explores all of that.

Introduction:

Before we get into flashpoints and start to investigate the hydrogen autoignition temperature, it is necessary to understand the basics of fire. Back in school, if you were attentive enough in your chemistry classes, you might remember the word tetrahedron. The word explains the three requirements for an element to burn i.e. oxygen, heat, and fuel. When these three ingredients are allowed to be in a chemical reaction, you have a fire.

When it comes to calculating the metrics related to the burning of any elements, auto ignition, and its flashpoint temperatures are often misunderstood. Yet, they both are extremely important to understand to optimize different industrial processes especially when it comes to dealing with Class 3 Flammable Liquids. Such liquids are extremely hazardous if they are not handled properly and can result in uncontrolled fire leading to a risk of human life and equipment damages.

Auto Ignition Temperature

The term goes by different names: ignition temperatures, minimum ignition temperature, self-ignition temperature, spontaneous ignition temperature, or sometimes is termed as kindling point. This temperature defines lowest the value of temperature in degrees or kelvins at which an element ignites in a normal atmosphere pressure.

At this point, there should be no external source of ignition, such as a flame or spark to support the ignition process. It is important to note that these ignition temperature values depend on a simple significant factor: substances with more stable and stronger molecular structures are found to have higher ignition temperatures and vice versa.  

A mix with oxygen gas or air is often used to measure autoignition and expressed as a function of the percentage. In the absence of information, the condition that is most easily light-sensitive is typically chosen to be close to the air in terms of stoichiometry. Upon testing, this mixture catches flame when it reaches this autoignition temperature.

Flash Point：

A chemical's flash point can be used to identify if it is a combustible or non-combustible substance. To put a definition, it is the lowest temperature at which a chemical may catch fire in the presence of an ignition source. This property is usually studied for liquids and for its vapors.

Both flashpoints and auto ignition differ from each other and explain different aspects of a flammable substance. Autoignition tells us about the temperature at which the substance itself burns a certain temp without an external source while in flashpoint, external energy is present for ignition and is usually used for liquids and its vapors.

Hydrogen and its Flashpoint:

Hydrogen is extremely flammable and can ignite in the air when present between 4% to 75% by volume. Capable of being flammable with various mixes of gaseous, hydrogen can be extremely dangerous when dealing with industrial conditions. If exposed to an external energy source, one can consider the hydrogen flash point to be extremely low as compared to other gases in at room or near room temperature. It has a flash point below 20K at <1atm.

When measuring flash points, the spark or flame guarantees the energy required for activation of "flash". Hence, creating a combination that would maintain a flame without becoming dulled is the limiting factor in such measurement. Thus, turns into a problem with volatility or stoichiometry of combustion.

It is also important to note that considering flashpoint of hydrogen is not a traditional way to refer to its fallibility. In real practice, Lower Flammability Limit and the Upper Flammability Limit is taken into account.

Hydrogen Autoignition Temperature

Below we established that this is the temperature at which a gas ignites itself without a certain external source. Hydrogen has a very high auto-ignition temperature, taking more than 500 degrees when measured. In fact, it is entitled with highest autoignition temperatures in all the c hydrocarbons. Such high temperatures can be explained for its chemical nature associated with its simple molecular structure.

The reason behind this behavior is many. One, the amount of energy with which the H-H are being held is significantly higher, 436kJ/mol which is much higher as compared to 400 kJ/mol for C-H and 350 kJ/mol for C-C bonds. Next, hydrogen’s simple diatomic molecular structure also gives it a relatively stable nature in its molecular form as compared to other gases in their pure form.

This simple and smaller molecular structure also influences autoignition temperatures in another way; since they are small molecules, these H-H molecules have higher vibrational frequencies as compared to other gases with larger molecular radius. This makes it extremely challenging and hard to penetrate and supply enough energy to break the molecular bonds and start ignition.

Wrapping Up:

Hydrogen autoignition temperature is relatively higher as compared to other near similar gases. This is because of the stability of the molecular structure and the size of the molecules in H₂ gas. Moreover, the energy required with temperature increase to initiate the combustion process is extremely high to break through the bonding. But with ignition source, this gas is extremely flammable indicating a very low flashpoint.