Why does an LEL not burn?

• A. It is too rich to burn
• B. It cannot burn because there is no oxygen
• C. It will burn only at high temperatures
• D. To lean to burn

Answer: (D)

The concept tested is the lower explosive limit, which defines the lean boundary of the flammable fuel-air range. A mixture can burn only if the fuel concentration is within this range; below the lower limit there isn’t enough fuel to sustain the flame, even though oxygen is present. The chemistry needs a certain amount of fuel to form enough reactive intermediates to keep combustion going, and when the mix is too lean, those conditions aren’t met. So, it won’t burn because it’s too lean to burn. (The other ideas—being too rich, needing high temperature, or lacking oxygen altogether—don’t describe the Lean side of the flammable range.)