EFFECTS OF CLAY-MINERALS ON CHAR TEXTURE AND COMBUSTION

This study reports on the influence of mineral matter in parent coals on the formation and combustion of chars from these coals. Two Spanish high mineral matter coals were used, San Jose (SJ), 1.85% reflectance, and Santa Barbara (SB), 1.04% reflectance. Sizes of coal particles were 63-125 mu m (SJ) and 36-75 mu m (SB). These coal particles were separated, using a float-sink procedure, into six fractions with densities ranging from 1.35 to 1.85 g cm(-3). Ash contents within the particles range from 3.5 to 23 wt %. Combustion chars of intermediate burn-off were prepared from coal particles in a linear flow furnace in an atmosphere of nitrogen and oxygen at 1500 degrees C. Organic and associated mineral matter were characterized using optical microscopy, XRD, FTIR, and SEM-EDX. These chars were used to study the effect of mineral matter on further conversion. Results show that char reactivities at 500 degrees C (thermogravimetry) and combustibilities at 1200 degrees C (entrained flow) increase with increasing mineral matter content of the char, effects being more pronounced at the lower temperature. Surface combustion temperatures of coal particles at 1200 degrees C did not vary with ash content. The increase in reactivity is not attributed to catalytic effects of mineral matter or to differences in internal surface areas of the chars as measured from carbon dioxide isotherms. It is considered that the enhancement in rate is associated with an enhanced macroporosity within chars in association with the decomposed clay mineral illite, having a platelike morphology. The presence of the illite facilitates the formation of parallel wide pores within the char matrix and these enhance mass-transfer of oxygen to the carbon and of reactants from the carbon. The development of such macropores is not detected by the adsorption isotherm.