Solids, that do not melt, undergo the pyrolysis.
Pyrolysis or thermal decomposition, is an irreversible chemical decomposition of a given substance as a result of the effect of heat.
Reaching a certain temperature (different for individual substances, but generally in the range of 150-250°C), causes the destruction of bonds between the particles and the conversion of the fuel in the form of gas.
It is a fuel that has a huge potential of energy because it has not been burnt yet. By enriching the smoke layer, it can lead to the combustion of smoke and fire gases with the release of large amounts of heat.
"Pyrolysis smoke", i.e. the products of thermal decomposition, is most often of a light colour, because the relatively low temperature, slight convection currents and the relatively intact structure of the thermally decomposed static body cause a small amount of soot to enter the gaseous products, staining smoke dark.

During compartment fires, pyrolisis proces is a key one. Products of pyrolisis, which are contain in smoke, can reach their temperature of ignition or autoignition and rapidly spread the fire.

1.8. Stages of solids combustion