Experimental study on hydrogen–oxygen spontaneous ignition limits in supercritical water using a wall-cooled reactor

Hydrogen is a promising fuel for many industrial applications because of its carbon-free properties. The characteristics of hydrogen–oxygen reaction in supercritical water were thoroughly investigated using a newly constructed experimental facility. Three typical temperature profiles were obtained, representing different reaction states: mild oxidation, transition state, and intense combustion. The spontaneous ignition limit was mapped based on 42 experimental data points. The minimum ignition temperature decreases with increasing fuel concentration, reaching as low as 517 °C at a hydrogen concentration of 23.2 mol%. The reliability of the spontaneous ignition limit was confirmed by the first hydrogen hydrothermal flame images obtained. The initial reactor temperature and fuel concentration increase allows the hydrogen–oxygen reaction to transition from mild oxidation to intense combustion. The oxygen excess ratio does not affect the reaction states, but it affects the temperature profile and hydrogen conversion efficiency. The introduction of cooling water changes the concentration and temperature of the reactant mixture in the reactor, thus affecting the stability of the hydrothermal flame.