ABSTRACT: High-concentration CaO particles and gas compositions have a significant influence on NOx reduction by selective non-catalytic reduction (SNCR) in cement precalciners. The effect of gas composition on NOx reduction by SNCR with NH₃ was studied in a cement precalciner atmosphere with and without CaO at 700-1100 °C. It was found that CaO significantly lowers NOx reduction efficiency between 750 °C and 1000 °C, which is attributed to the catalytic oxidation of NH₃ to NO. Although increasing NH₃ concentration was advantageous to NOx reduction, the existence of CaO led to the opposite result at 750-900 °C. Adding H₂O can suppress the negative effect of CaO on NOx reduction. Decreasing O₂ content from 10% to 1% shifts the temperature range in which CaO has a significant effect from 750-1000 °C to 800-1050 °C. CO has a variety of influences on the CaO effect under different experimental conditions. The influences of NH₃, H₂O, O₂, and CO on the effect of CaO can be attributed to the impacts of the gas compositions on gas-phase NH₃ conversion, gas-solid catalytic NH₃ oxidation, or both processes. A proposed pathway for the effect of gas compositions on NOx reduction in CaO-containing SNCR process was developed that well predicted the CaO-containing SNCR process.