When does the peak Heat Release Rate occur in a fire?

The peak Heat Release Rate in a fire typically occurs when flames spread across the entire surface of a fuel item. This is the point at which the maximum amount of fuel is actively participating in the combustion process, generating the most heat and resulting in the highest release rate of energy.

During this phase, the fire is often characterized by a rapid confinement of heat and combustion gases, allowing for an accelerated reaction between the fuel and the oxidizer (usually oxygen from the air). When flames cover a fuel surface completely, all of that fuel is being consumed in the combustion reaction, which enhances the efficiency and intensity of the fire, leading to the peak heat output.

The other phases of a fire, such as immediately after it starts or during the smoldering phase, do not achieve this maximum intensity. After full development, while the fire is very intense, the peak heat release rate has already occurred as the flames spread. Therefore, it is during the spreading phase across the fuel surface that the maximum heat is generated.