PARTITIONING OF NITROGENOUS SPECIES IN THE FUEL-RICH STAGE OF REBURNING

The effect of reburing fuel type on the partitioning of fixed-nitrogen species in the fuel-rich stage of reburning was studied in a bench scale flow reactor. Methane, hexane, benzene, bituminous coal, and lignite were used as reburning fuels. A simulated flue gas consisting of CO2 (16.8%), O2 (1.95%), NO (1000 ppm), and helium was used for experiments at 1100-degrees-C and ca. 0.2 s residence time. The total fixed nitrogen (TFN) speciation was found to depend strongly on fuel type and stoichiometric ratio (SR). Rich stoichiometries promoted conversion of NO to HCN, whereas conversion of HCN to N2 was promoted by lean stoichiometries. In this simulated reburning stage, the optimum stoichiometry of TFN production for gaseous fuels was ca. SR = 0.95. For bituminous coal and lignite, the optimum stoichiometry was lower (ca. SR = 0.85). For the gaseous fuels, the HCN contribution to TFN increased as the C/H ratio of the fuel increased. The dominant TFN species for bituminous coal was HCN, but for lignite NH3 dominated. Lignite reburning results were strongly influenced by heterogeneous conversion of NO to N2 on char and catalytic conversion of HCN to NH3 on fly ash. Due to these effects, which overcome kinetic barriers in reburning, lignite produced the lowest gas-phase TFN of any of the fuels tested. Consequently, enhanced reburning, by injection of lignite char/ash in the reburning zone, has been proposed.