Mechanisms of NOx Removal from Flue Gas by ZeroValent Iron

摘要

Chemical reaction between nitrogen monoxide (NO) and zero valent iron (ZVI) was studiedrnin a packed bed column process with high temperatures based on ZVI strong reducingrnabilities. For six controlled temperatures of 523 K-773K and 400 ppm of NO (typical flue gasrntemperature and concentration), under short empty bed contact time (EBCT) (0.0225 sec torn0.0679 sec), NO was completely removed for temperature of 573 K-773 K, but not for 523 K.rnExhaustion test was conducted for the five working temperatures and the results indicated NOrnreductions by ZVI (mg NO/g ZVI) were varied from 1.0 mg NO/g ZVI (low temperatures andrnshort detention time) to 11.1 mg NO/g ZVI (high temperatures and long detention time). XRDrn(X-Ray Diffraction) and ESCA (Electron Spectroscopy for Chemical Analysis) were conductedrnto analyze the crystal structure and oxidation state of the reacted ZVI. Four layers of ironrnspecies were detected by XRD: Feo(ZVI),FeO, Fe3O4 and Fe2O3. ZVI was the most prevalentrnspecies, Fe3O4 and Fe2O3 were less, and there was only slight peak for FeO from the XRDrnanalysis. By ESCA, the oxidation state on the reacted ZVI surface was determined and thernspecies was identified as Fe2O3, which is the most oxidizing species for iron. Therefore, fourrnlayers from the ZVI core to the ZVI surface can be identified: ZVI, FeO, Fe3O4 and Fe2O3.rnCombining the results from XRD and ESCA, the mechanisms for ZVI and NO can bernproposed as three consecutive reactions from lower oxidation state (ZVI) in the core to higherrnoxidation state on the iron surface (Fe2O3) (1) 2Fe+2NO←(hight temperature)→high temperatue 2FeO+N2(2) 6FeO+2NO←(hight temperature)→high temperatue 2Fe3O4+N2 (3) 4Fe3O4+2NO←(hight temperature)→high temperatue 6Fe2O3+N2.rnSince there was only less than 2 % ZVI used to remove NO comparing to theoretical ZVIrnused based on the proposed stoichiometry, it can be concluded that the heterogeneous reactionrnwas only occurred on the ZVI surface instead of bulk of the ZVI.