Question

A mixture of a flammable gas and air can ignite ONLY if

• A. the gas concentration is below the limiting oxygen concentration
• B. the gas concentration is above the upper flammable limit
• C. the gas concentration is between the lower and upper flammable limits
• D. the gas concentration is below the lower flammable limit

Hint:

Remember: Combustion needs the right fuel–air ratio. Too little gas (below LFL) or too much gas (above UFL) prevents ignition. Always check whether the concentration lies within the flammability limits for safety analysis.

Answer 1

The Correct Option is C

- For any flammable gas mixed with air, ignition is only possible within a certain concentration range. This range is defined by two important parameters: the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL).
- If the gas concentration is below the LFL, the mixture is too lean (not enough fuel) to ignite, as there is insufficient gas present to sustain combustion.
- If the gas concentration is above the UFL, the mixture is too rich (too much fuel and insufficient oxygen), which also prevents ignition.
- Ignition and sustained combustion occur only when the gas concentration lies between the LFL and UFL. Within this range, there is an adequate balance between fuel and oxygen to initiate and propagate a flame.
- Therefore, the mixture of a flammable gas and air can ignite only if the gas concentration is between the lower and upper flammable limits.