As a chemist with a background in fuels and combustion, I was deeply interested in the article by E. G. Senkbeil and T. P. Gonnella, "Combustion Demonstration Using Updated Flame Tornado" in the November 2000 issue of this Journal. One point they made is particularly striking, namely, the statement (p 1450), with reference to the behavior of nitrocellulose in their apparatus, that "having both fuel and oxidizer in the same structure significantly increases the rate of combustion."

I believe that in tertiary teaching of combustion science this point, made on the basis of the simple experimental observations, is very helpful. TNT and dynamite, like nitrocellulose, rely on intramolecular oxygen for reaction, and in my own lecturing to fourth-year students in Fire and Explosions I present them with the statement (1) "There is more energy in a candle than in a stick of dynamite [of the same weight]", or sometimes I substitute "TNT" for "dynamite".

This statement is of course true. Hydrocarbon wax has a heat of combustion of around 40 MJ kg^-1; TNT, a heat of combustion of 15 MJ kg^-1. Moving from hydrocarbon wax to propane gas, heat of combustion 50 MJ kg^-1, we can adapt the expression quoted to "kilo for kilo, there is more energy in propane than in TNT"--in fact, by a factor of 50/15 or 3.3. Yet materials such as TNT and dynamite, while having smaller heats of combustion than hydrocarbons, obviously have greater blast potential ("brisance"). This is because about 28% of the combustion energy of TNT becomes blast energy, or about 4.2 MJ of blast energy per kilogram of TNT reacted. By contrast (1), when propane ignites only something of the order of 5% of the heat of combustion becomes blast energy, or 2.5 MJ of blast energy per kilogram. This can be linked to the statement by Senkbeil and Gonella quoted above. High explosives have "fuel and oxidizer in the same structure" as already noted, and the consequent enhancement of reaction rate and its effects on the post-combustion gases are the origin of the powerfully explosive behavior of such substances.

Literature Cited

1. See for example, Marshall, V. C., Major Chemical Hazards; Wiley: New York, 1987; p 255.