The Flammable Envelope
Every combustible gas has a characteristic concentration range in air where flame propagation is possible. Below the lower flammability limit (LFL), the mixture is too lean — too few fuel molecules to sustain chain reactions. Above the upper flammability limit (UFL), the mixture is too rich — insufficient oxygen. Between these bounds lies the flammable zone, where any competent ignition source triggers combustion.
Temperature and Pressure Dependence
Flammability limits are not fixed constants — they depend on initial conditions. Raising temperature widens the flammable range because less external energy is needed to initiate reactions. The empirical Zabetakis correlation shows LFL decreasing linearly with temperature. Pressure effects are more complex: LFL is relatively insensitive, but UFL can increase dramatically above 5 atm as high-pressure chemistry enables additional reaction pathways.
Inerting and Explosion Prevention
The most reliable way to prevent explosions is to keep fuel concentrations outside the flammable range or to reduce oxygen below the limiting oxygen concentration (LOC) by adding inert gas. Process safety engineers design ventilation, inerting, and monitoring systems using flammability data. The flammability diagram — a triangular plot of fuel, oxygen, and inert — maps safe and hazardous regions for any mixture composition.
Mixture Rules and Applications
Real industrial gases are often mixtures. Le Chatelier's rule provides a simple but effective way to estimate the flammability limits of multi-fuel mixtures by weighted combination of individual limits. This simulation visualizes the flammable envelope and lets you explore how temperature, pressure, and diluent concentration shift the boundaries — essential knowledge for chemical plant design and hazard assessment.