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 NH? 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 NH? to NO. Although increasing NH? concentration was advantageous to NOx reduction, the existence of CaO led to the opposite result at 750-900 °C. Adding H?O can suppress the negative effect of CaO on NOx reduction. Decreasing O? 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 NH?, H?O, O?, and CO on the effect of CaO can be attributed to the impacts of the gas compositions on gas-phase NH? conversion, gas-solid catalytic NH? 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.