Deposit formation mechanisms in a pulverized coal fired grate for hematite pellet production

- Combustion of pulverized coal had a significant influence on deposit formation.
- Complete combustion of coal did not occur and residual carbon was generated.
- Hematite in pellet powder was reduced to FeO by residual carbon in coal ash.
- The deposit formation process was analyzed using the FactSage software.

Pulverized coal combustion was adopted at the end of the grate to provide sufficiently high preheating temperature in the grate when hematite was roasted to produce hematite pellets. Unfortunately, serious deposit which stuck to the walls at the end of the grate was found after using pulverized coal combustion. In the present study, chemical analysis methods were used to detect the chemical compositions of the preheated pellet powder, deposit sample and coal ash. In addition, the major phases and microstructure of the deposit sample were determined using X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) analysis, respectively. Furthermore, a thermodynamic software (FactSage 7.0) was adopted to analyze the deposit formation process. The chemical and mineralogical analyses indicated that the deposits contained high amounts of iron, particularly of silicate, aluminum, calcium and magnesium and, to lesser extent, of alkali metals. The deposit formation process in the grate was found to be caused by the mixing of hematite pellet fines and unburnt carbon from the coal ash. The hematite was reduced to wustite by the presence of residual carbon, which generated low melting phases forming sticky deposits on the grate walls.