How do thermal explosions differ from chain explosions?

In the context of combustion, explosions are categorized by the mechanism that causes the reaction to accelerate out of control. The primary difference lies in whether the acceleration is driven by heat accumulation or by the proliferation of reactive intermediates.12

Thermal Explosions

Thermal explosion theory posits that an explosion occurs when the rate of heat released by a chemical reaction exceeds the rate at which heat is lost to the surrounding environment. As progressive heating occurs, the reaction rate increases until it reaches a critical point defined by the mixture's composition and pressure, leading to an explosion at a specific ignition temperature.3

Chain Explosions

Chain explosions are driven by branched-chain reactions, where the number of reactive intermediates—known as chain carriers—increases rapidly. For example, in a hydrogen-oxygen reaction, a single chain carrier can produce multiple new carriers, causing the reaction to auto-accelerate. A chain explosion occurs when the probability of chain branching exceeds the probability of chain termination.452

In practice, many explosions are of a "chain-thermal" nature, meaning both heat accumulation and chain auto-acceleration contribute to the event.2