Conditions for onset and sustained char oxidation

Abstract

Smouldering combustion of timber presents a significant fire safety concern, particularly in scenarios where heat retention enables sustained char oxidation. This study isolates char oxidation from other smouldering processes to investigate its onset and sustained reaction under close-to-critical incident heat fluxes. Experiments using pre-pyrolysed char samples provided direct measurements of CO and CO₂ generation, mass loss, and temperature evolution during char oxidation. The onset of char oxidation was characterised by a rapid increase in CO generation rate, occurring consistently at an external heat flux of 10 kW/m². Among the methods tested, CO mass flow rates proved to be the most reliable indicator of char oxidation onset, offering greater precision than traditional mass loss measurements or temperature data in determining the char oxidation onset time. Once initiated, oxidation led to sustained heat release, with in-depth temperatures exceeding 400 °C, peak heat release rates of ∼29 kW/m² and a mean effective heat of combustion of ∼30.3 kJ/g, close to the char's gross heat of combustion, at 10 kW/m². These findings improve the understanding of char oxidation kinetics and support the development of predictive models for smouldering in engineered timber, informing fire hazard assessment and mitigation strategies.