Question:
What is the difference between an alcohol fire and a hydrocarbon fire? and what is the impact of such difference on detection of fire by IR flame detectors?


ANSWER:
If the burning alcohol is methanol (wood alcohol), no soot particles are created in the flame. All hydrocarbon flames (and all other alcohol flames) produce soot; if there is enough oxygen present, the soot is burned inside the flame, and does not escape. This process of soot burning is responsible for the orange-yellow glow of flames. What is happening is that the burning, solid soot particles are heated to incandescence (glowing) and emit a lot of infrared by so-called "black body radiation".
Consequently, soot-producing flames will emit more IR (and visible light, as
well) than flames that do not produce soot. A methanol flame will be harder
for an IR detector to "see" than a hydrocarbon flame.
Richard E. Barrans Jr., Ph.D.
Assistant Director, PG Research Foundation
Darien, IL USA