Combustion in Oxygen-Enriched Environments

Oxygen enrichment occurs when oxygen concentration rises above its normal atmospheric level of about 21 percent. Oxygen is an oxidiser, not a fuel, but its increased availability allows fuels to ignite more readily and burn more intensely. The effect becomes especially dangerous where an enclosure permits oxygen to accumulate.

How enrichment changes combustion

Combustion depends on fuel, heat and oxygen. Increasing oxygen concentration accelerates oxidation and reduces the diluting effect of atmospheric nitrogen.

• The flammability range of many gases and vapours widens, allowing combustion at concentrations that may not burn in ordinary air.
• The minimum ignition energy can decrease, so a small spark, hot surface or static discharge may initiate fire.
• Flame propagation and the heat-release rate can increase, causing fire to grow more rapidly.

Effects on materials

Material classifications based on behaviour in normal air may become unreliable in an oxygen-rich atmosphere.

• Materials that are normally difficult to ignite or self-extinguishing can burn vigorously under oxygen-enriched conditions.
• Clothing, bedding, plastics and other porous or combustible materials can support rapid flame spread, increasing burn risk.
• Oils, greases and other hydrocarbon contaminants are especially hazardous in contact with concentrated oxygen systems.

Why enclosed spaces are especially hazardous

Poor ventilation allows an oxygen leak to raise the overall concentration or create local pockets. Oxygen is colourless and odourless, so enrichment cannot be reliably detected by human senses.

• Under OSHA confined-space criteria, an atmosphere containing more than 23.5 percent oxygen is hazardous.
• Rapid fire development can sharply reduce evacuation time and increase exposure to heat, smoke and toxic combustion products.
• Control requires leak prevention, adequate ventilation, oxygen-compatible equipment, ignition control and continuous atmospheric monitoring where enrichment is possible.