CHARACTERIZATION OF SYNTHETIC-COAL CHAR PARTICLES USING FRACTAL DIMENSION ANALYSIS

The development and change of surface ruggedness in chars was studied at conditions typical in a pulverized coal furnace. The fractal dimens ion, a measure of surface ruggedness, of chars was measured using phys isorption techniques. By adjusting the temperature encountered (1173 t o 1773 K) and residence time (0.1 to 1.5 s) of the synthetic coal (siz ed to 46-106 mum diameter), chars at different stages of combustion we re prepared in a laminar flow (drop-tube) furnace. The particles were quickly cooled and quenched in an inert atmosphere. The samples were e xamined using a scanning electron microprobe, and their fractal dimens ions were determined using gas physisorption. The adsorption data were used to test if the char surface was fractal on a molecular scale, to determine the fractal dimension, and to quantify changes in the fract al dimension during combustion. The fractal dimension of the unburned synthetic coal was approximately 2. The fractal dimension increased as high as 2.85 as the carbon matrix burned away and exposed mineral moi eties. However, as combustion continued the carbon burned completely a way leaving a mineral fly ash particle with a fractal dimension as low as 2.47.