ABSTRACT
Self ignition is defined as thermal runaway due to internal exothermic reactions.
Thomas' classic analysis of self heating to ignition led to laboratory-scale test methods
identifying conditions under which spontaneous combustion is possible. These experimental
techniques, as described e.g. in Beever’s chapter in the second edition of the SFPE Handbook
of Fire Protection Engineering, have considerable utility in fire-hazard assessment. The
NFPA Fire Protection Handbook Table A-10 “Materials Subject to Spontaneous Heating”
is incorrect and should be abandoned. As a practical example, the SFPE technique is applied
here to the question: How large a pile of charcoal briquets is required for self heating to
ignition? Correction factors for the Frank-Kamenetskii approximations are examined in
detail. The data show that the largest commercially-available bag of charcoal briquets, 9 kg
(20 lb.), cannot self ignite at an ambient temperature below 394 K (121°C or 250°F). All
tested variations: size, different formulations, addition of water or dry wood, aging, and
different bag configurations, raised this critical temperature even higher. At 25°C (77°F )
these data show a bag of charcoal briquets would have to exceed the size of a typical house
(>103 m3 ) to self ignite. Self ignition at ambient temperatures of bagged charcoal briquets in
commercially available sizes is impossible.

John Lentini, CFI, D-ABC
Fire Investigation Consultant
Florida Keys
[www.firescientist.com]