In natural gas combustion, what is the acceptable range for gas content in the air-gas mixture?

• A. 4% to 14% gas in the mixture
• B. 0% to 4%
• C. 14% to 24%
• D. 86% to 96%

Answer: (A)

The question tests the flammability range of natural gas in air. Natural gas (primarily methane) will burn only if its volume fraction in air falls between the lower and upper explosive limits. For methane in air, that range is roughly 4% to 14% by volume. If the mixture has less than about 4%, it’s too lean to ignite; if it has more than about 14%, it’s too rich and there isn’t enough oxygen to sustain the flame. So the acceptable range for the air-gas mixture is about 4% to 14% gas by volume. This is the range that allows reliable ignition and stable combustion.