Experimental study and CFD modeling of NOx reduction and reductive gas formation in deep reburning of cement precalciner

Abstract

A concept involving deep coal reburning in precalciner is proposed to reduce the amount of nitrous oxide (NOx) emitted from the rotary kiln, and less than 50 mg/m3 of NOx is emitted from the deep reburning zone. An experiment involving deep coal reburning of the flue gas emitted from a rotary kiln using an electric-heated down-firing furnace (E-DFF) was used to study the NOx reduction and the formation of reductive gases (CO, H2). The stoichiometry ratio (SR) of the reburning zone was in the range of 0.3–0.52, and the temperature was 900–1200 °C, with a residence time of 0–3.7 s. The results demonstrated that the NOx emitted from the rotary kiln can be reduced to <50 mg/m3 (10 vol% O2) by utilizing deep reburning technology under the following conditions: a lower SR (<0.3) and a residence time of 3 s. The concentrations of CO and H2 increased with decreasing stoichiometry ratio, and the CO concentration increased from 0.12% under SR = 0.52 to 3.43 vol% under SR = 0.3 at 1200 °C. Char gasification resulted in an increase in the reductive gases of CO and H2, and NO reduction could be enhanced by the presence of CO and H2. The gasification kinetics of char inside the deep reburning zone were obtained by a computational (CFD) optimization algorithm and were utilized in CFD modeling to predict the CO and H2 profiles along the E-DFF. A semiempirical model of homogeneous reduction of NO was built, and the relationship between the NOx reduction and CO + H2 concentration in the deep reburning zone was developed. An integrated NOx prediction model was utilized in a CFD simulation to predict the reduction in NOx in deep coal reburning technology. The results proved that the new model could accurately predict different NOx evolution characteristics under various conditions. These findings can provide reference data and CFD modeling methods to help in the design and optimization of low-NOx combustion in cement precalciners.