Recent progress and prospects of hydrogen combustion chemistry in the gas phase

Hydrogen is emerging as a clean and renewable energy source indispensable to the realization of a carbon neutral society. Inspired by the prospect of sustainable and carbon-free energy supplies, hydrogen has been widely utilized in various combustion engines. This review article highlights recent progress in understanding hydrogen combustion chemistry in the gas phase. At first, the explosion limits of hydrogen-oxygen mixtures are discussed to demonstrate the intrinsically nonmonotonic kinetic behavior. Fundamental experiments of hydrogen combustion in terms of ignition delay times, laminar flame speeds and speciation are systematically summarized, and the value of the reported data is discussed. Furthermore, effective strategies towards more accurate experimental diagnostics are outlined. The current status of detailed and simplified kinetic model development is then appraised, followed by a critical discussion on the rate constants of important elementary reactions that are still in dispute. The essential importance of the comprehensiveness of chemical fidelity for mechanisms at the detailed and reduced levels is emphasized. Subsequently, the knowledge of ozone-assisted oxidation of hydrogen is overviewed. The effects of ozone addition on the characteristics of hydrogen oxidation are analyzed, including ignition temperature, flame burning velocity and flame structure. The ozone sub-mechanism and associated reaction rates are also carefully assessed. Finally, concluding comments and an outlook towards future research on gas-phase hydrogen combustion chemistry are presented.