SULFUR RETENTION BY ASH DURING FLUIDIZED-BED COMBUSTION OF BITUMINOUS COALS

The present work examines SO2 retention by coal ashes during the fluidized bed combustion of four bituminous coals with high ash content (> 47 wt%) and moderate sulphur content (almost-equal-to 1 wt%). The combustion was carried out in a 0.14 m i.d. fluidized bed reactor, in a range of temperatures between 700 and 900-degrees-C. A dependence of the emissions on combustion temperature was not observed. The ashes resulting from combustion were examined by X-ray diffraction and scanning electron microscopy with energy dispersive X-ray analysis. The study shows that the sulphur retention at low temperature (700-degrees-C) takes place by direct sulphation of the calcite. When the temperature is increased, the phenomena observed are calcination and breakage of the calcite particles. This results in an increase in the calcium content and consequently in the retained sulphur, in the smallest particles (< 63-mu-m). In consequence, most of the calcium and the retained sulphur are carried away in the elutriated material.