Mechanistic insights into the effects of H2 and CO on NO removal by high-temperature reducing-zone ammonia injection

High-temperature reducing-zone ammonia injection has emerged as a promising strategy for NOx control due to its cost-effectiveness and compatibility with existing combustion systems. Nevertheless, the effects of reducing gases on NO removal under such high-temperature reducing atmospheres, as well as the underlying mechanisms, remain poorly understood. In this work, the roles of reducing gases (H 2 and CO) in the NO removal characteristics of NH 3 were systematically investigated through experiments and kinetic modeling. NH 3 injection into high-temperature, oxygen-deficient zone shifted the temperature window for NO removal toward higher temperatures, thereby extending the operating temperature window of conventional SNCR. Under oxygen-free conditions, H 2 and CO exhibited negligible effects on NO removal, as their conversion pathways did not promote radical accumulation. In contrast, in the presence of O 2 , H 2 oxidation pathways promote H radical generation, while CO suppressed H consumption via the chain-propagation reaction CO + OH ⇌ CO 2 + H, which converted one radical species (OH) into another (H) without net radical generation. Consequently, the addition of CO and H 2 facilitated rapid replenishment of the radical pool and shifted the temperature window toward lower temperatures. In particular, H 2 significantly broadened the temperature window for NO removal at 0.1 % O 2 but slightly narrowed it at higher O 2 concentrations. In contrast, CO slightly narrowed the temperature window and reduced the maximum NO removal efficiency. This work provides mechanistic insights into the roles of reducing gases in high-temperature NH 3 -based NO removal and offers guidance for process optimization in practical applications.