Pilot-scale investigation of coal ash transformations under staged combustion

Fly ash formation during staged combustion was investigated using a pilot-scale laboratory combustor and two coals, Black Thunder and Pittsburgh #8. Fly ash particles were collected under conventional and staged combustion conditions and analyzed by computer-controlled scanning electron microscopy (CCSEM). Two new CCSEM techniques, the nearest phase classification algorithm and a char/ash liberation algorithm, were developed and applied to the ash.; A small shift in the composition of Black Thunder ash due to greater coalescence of the ash particles derived from the organically-bound inorganic constituents was observed under staged combustion. The shift was most evident in the nearest phase classification algorithm results for the particles with diameter less than 10 microns. The increased coalescence was explained by slower char oxidation under staged combustion.; The char/ash associations algorithm showed that included pyrite particles in Pittsburgh #8 released sulfur at a lower rate than excluded particles under both conventional and staged combustion. The initial reducing conditions encountered during staged combustion served to decrease the rate of sulfur release from included pyrite particles but did not significantly affect the excluded particles. After the introduction of the staged air, included particles quickly resumed releasing sulfur and behaved as they would have during conventional combustion. There were no shifts in the extent of coalescence of minerals associated with staged combustion or the altered pyrite behavior in the reducing zone.