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