16.1 Introduction

Some solid materials are susceptible to spontaneous heating; that is, they may increase their temperature without assistance from an external heat source. Usually, spontaneous or self-heating arises naturally as an outcome of low temperature (< 100 C) exothermic oxidation reactions, or condensation of moisture. Both phenomena can occur on pore surfaces, in the case of porous materials, or on the outside surfaces of micron-sized solid particles. Once the rate of heat generation exceeds the rate of heat dissipation to the environment, self-heating leads to self-ignition (or auto-ignition), and eventually to flaming or smouldering combustion. For this reason, we denote spontaneous combustion to include all three stages of this process, that is: (i) the initial self-heating; (ii) auto-ignition (i.e., in the absence of pilots and external heat sources); and, (iii) flaming or non-flaming combustion.

In nature, some materials may display the propensity to self-heating and auto-ignition, even at temperatures close to ambient. These materials are denoted as pyrophoric. Haystacks were perhaps the first materials observed to ignite spontaneously by early farmers, as noted by Cuzzillo (1997) in his literature survey. It is now appreciated that a variety of materials display similar pyrophoric properties, including coal, charcoal, carbon black, wood, grass, tree leaves, rags, and some other agricultural products such as cotton and grain. Some chemical products, such as alkali metals, white phosphorous, calcium oxide or quicklime acids, zinc powder, organic...