ASH FORMATION DURING PULVERIZED SUBBITUMINOUS COAL COMBUSTION .1. CHARACTERIZATION OF COALS, AND INORGANIC TRANSFORMATIONS DURING EARLY STAGES OF BURNOUT

As part of a broad effort to develop the capability to predict ash behavior based on detailed analyses of the inorganic matter in coals, two subbituminous coals were tested in a new down-fired combustor of 745 MJ/(h.m3) capacity. This paper deals with changes in the inorganic matter during the first 30-50% burnout of combustibles, corresponding to 0.07 s residence time. The inorganic components reached temperatures of 1210-1220-degrees-C. Detailed analyses of coals and chars were performed by transmission and computer-controlled scanning electron microscopy and X-ray diffraction. Sodium, calcium, and magnesium decreased in concentration in the larger char particles, but increased in the smaller size particles, indicating volatilization of these elements at a rate more rapid than combustible burnout. The rate of sulfur release was the same as the rate of combustible burnout. There was little significant change in the overall size distributions of the mineral particles, chemical effects being limited to some interaction of calcium and iron with quartz and the aluminosilicates. Two subsequent papers in this series will examine the inorganic transformations during further stages of combustible burnout. There we will demonstrate that most volatilization and condensation processes are complete by 50% combustible burnout and are followed by coalescence interactions during later stages of combustion.