Fire Safety Science Digital Archive

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 briquettes 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 briquettes, 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 briquettes would have to exceed the size of a typical house (>103 m³) to self ignite. Self ignition at ambient temperatures of bagged charcoal briquettes in commercially available sizes is impossible.