Study on reduction characteristics of Fe species in coal ash under SNCR condition

Abstract To obtain the reduction characteristics of NH3 on Fe2O3 in coal ash, and verify that the injected ammonia-bearing reductant led to slagging during SNCR, the slag sample was collected at the flue gas temperature range of 720 °C–780 °C in a medium temperature separation circulating fluidized bed boiler (MTS-CFBB). The XRF, XRD were used to determine the elemental distribution and mineral composition. The reduction reaction of CO, NH3 with Fe2O3 was calculated by HSC Chemistry 6.0 to obtain the reduction ability. Moreover, the mineral transformation in coal ash was studied by the tube furnace under N2 and N2-NH3 atmosphere. The results indicated that the contents of Fe2O3 in the three layers of slag sample were 16.36%, 16.00%, and 12.63% respectively from outside to inside, which were higher than that in burned coal ash. The NH3 has stronger reduction ability as compared to CO reacting with Fe2O3 and Fe3O4 at the temperature range of 720 °C–780 °C. With the reacting temperature increasing, the added NH3 led to more Na and O release, lower decomposed temperature of CaSO4. The NH3 reduced the Fe3+ in coal ash to form the Fe3O4, FeO or other Fe2+ contained compounds at the high-temperature. The formed Fe2+ contained compounds with low melting temperature played a glue role to promote the slagging process. At last, the slagging mechanism caused by injected ammonia-bearing reductant under the SNCR condition was established.