Hydrogen Tech World April 2025 | Page 44

[ oxygen ]

ignition source, such as a spark or flame. This phenomenon occurs when the temperature of the material is high enough to overcome the oxidation activation energy level and sustain combustion.

Higher oxygen concentrations can lower the autoignition temperature of many materials. The impact of oxygen concentration on AIT is shown in Table 2.

This change in AIT suggests that the likelihood of a successful collision increases as oxygen concentration rises. The Maxwell-Boltzmann distribution of molecules can help explain this phenomenon, as it describes the distribution of classical material particles across various energy states( see Figure 2).³

As this is a statistical plot, the shape of the graph will not change( provided the temperature remains constant) as the number of molecules increases( due to rising concentration), but the number of molecules above the activation energy level will increase.

Effect of oxygen pressure

As with temperature, elevated pressures and concentrations in oxygen systems allow materials to burn far more readily and rapidly. Commercial electrolysers commonly operate at up to 30 barg, introducing an additional risk factor. Table 3 shows that, under the right circumstances, materials not

Fig. 2. Maxwell-Boltzmann distribution with activation energy limit shown ³

typically considered fuels – such as stainless steel and aluminium – can sustain combustion.

It also highlights that some materials, which might otherwise be considered suitable for use in the construction of process equipment, such as aluminium 4043, are flammable at near-atmospheric pressures in an oxygen-enriched environment.

Increased system pressure functions in a similar way to increased oxygen concentration, as a higher number of oxygen molecules are present within a given volume, increasing the likelihood of a successful reaction collision per unit of time. Increasing the pressure does not alter the speed of the molecules and, therefore does not change the shape of the Maxwell-Boltzmann distribution curve.

Effect of oxygen temperature

The effects of temperature on combustible materials, with very few exceptions, are similar

Table 2. Effect of oxygen concentration on AIT 4 AIT Test Results

Material

Average AIT(° C) a

21 % O 2 34 % O 2

45 % O 2 100 % O 2

44 Hydrogen Tech World | Issue 21 | April 2025