What notable phenomenon occurs in Stage 4 pyrolysis?

In Stage 4 of pyrolysis, the notable phenomenon is free burning. This stage is characterized by a significant release of energy and combustion of the gaseous products that have been generated during the earlier stages of pyrolysis. At this point, any solid material remaining has typically been converted into gases or has reached a state where it combusts freely, leading to a rapid, uncontrolled fire behavior.

In a fire behavior context, this is an important stage because it reflects the transition from smoldering or pyrolytic decomposition to an active combustion phase, whereby the heat, combustion gases, and available fuel contribute to a more intense fire. Understanding this stage is crucial for fire safety officers as it impacts their safety protocols and responses during firefighting operations.

Other options do not align with the characteristics associated with Stage 4. For instance, while charring and the formation of charcoal refer to earlier processes in pyrolysis, the rapid release of combustible compounds may occur throughout various stages, but free burning specifically denotes the transition to an open flame scenario.