[ oxygen ]
( e. g., wood, petrol, paper), under elevated oxygen conditions, the range of materials classified as fuels expands significantly. In fact, if exhaustive lists were compiled, it would be shorter to list the materials that will not burn in a 100 % oxygen environment. Most metals, plastics, and common electrolyser construction materials can readily combust in enriched oxygen environments.
Oxygen Oxygen, whether directly present or provided by an oxidiser, is required to fuel a fire, which is an oxidation reaction. The higher the oxygen concentration, the higher the flame temperature, the intensity of combustion, and the destructive capability of the fire.
Effect of oxygen concentration
An elevated( or enriched) oxygen environment is almost universally defined as one where the oxygen concentration exceeds 23.5 %.¹ Hydrogen electrolysers produce nearly pure oxygen at their anodes, presenting a serious risk factor that must be accounted for. Although this oxygen is fully saturated with water, it still constitutes an elevated oxygen environment.
Combustible materials, with very few exceptions, burn far more readily and rapidly in environments with higher oxygen concentrations. This results in more energy being released in a shorter period of time. The fact that most materials burn at an accelerated rate in oxygen-enriched conditions can be explained by Collision Theory, which states that for a chemical reaction to occur, the reacting particles must collide with one another.
Most materials have an autoignition temperature, or autogenous ignition temperature( AIT), which is the minimum temperature at which a substance will spontaneously ignite without an external
Table 1. Increase in burn rate at elevated oxygen concentrations ² Flammability of Non-Metals by Oxygen Concentration a, b, c
Material |
Oxygen Concentration |
|
|
Sample Burn Length |
Burn Rate( in / s) [ mm / s ] |
Polyester-Based Foam |
23 % 25 % |
1.3”( 33 mm) 12”( 305 mm) |
0.07( 1.8) 0.67( 17.0) |
Polyurethane and Epoxy |
21 % 26 % |
1”( 25.4 mm) 12”( 305 mm) |
0.05( 1.3) 0.33( 8.4) |
Polyurethane Foam |
21 % 23 % |
1”( 25.4 mm) 9”( 228 mm) |
0.05( 1.3) 0.27( 6.9) |
Polyisocyanurate Foam |
21 % 30 % |
2”( 50.8 mm) 12”( 305 mm) |
0.07( 1.8) 0.13( 3.3) |
Silicone RTV
21 % 24 % 30 %
0.5”( 12.7 mm) 2.6”( 66.0 mm) 12”( 305 mm)
0.01( 0.3) 0.02( 0.5) 0.02( 0.5) a
Data for oxygen concentration effect comparison only; not to be considered standard values for the listed materials. b
Data based on specimens tested per ISO14624-1, with specimen dimensions of 12”( 305 mm) in length and 2.5”( 64 mm) in width. c
Data derived from testing at NASA’ s George C. Marshall Space Flight Centre.
Hydrogen Tech World | Issue 21 | April 2025 43