Supercritical hydrothermal staged combustion: study on extinction characteristics and combustion mechanism

Supercritical hydrothermal combustion is an efficient and clean advanced combustion technology. However, the high flow velocity and low flame propagation speed result in poor flame stability, severely limiting its application. This work investigated the extinction characteristics and combustion mechanism of supercritical hydrothermal flames under staged combustion. The results showed that both primary and secondary fuels with flow rates of 72 kg·h⁻¹ and 1080 kg·h⁻¹, respectively, could be injected into the reactor at 25 °C and maintain a stable flame during staged combustion. The high-temperature oxygen-rich flame generated by the primary fuel continuously heats the secondary fuel, increasing its flame propagation speed and allowing it to remain stable at high flow rates and low extinction temperature. The secondary fuel undergoes premixed combustion in the oxygen-rich zone rather than diffusion combustion. The large amount of OH· radicals required for the reaction is generated by the reaction of H₂O and HO₂· radicals, which are produced by the primary fuel, rather than through the reaction pathway O₂ → HO₂·→H₂O₂ → OH·. As the thermal power input from the primary fuel increases, the minimum thermal power ratio R required to maintain flame stability decreases exponentially. This work preliminarily provided a matching relationship between the primary and secondary fuels, providing a perspective for achieving flame stability under high flow load and high flow velocity conditions while avoiding fuel and energy waste.