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 w ere 63-125 mu m (SJ) and 36-75 mu m (SB). These coal particles were se parated, using a float-sink procedure, into six fractions with densiti es ranging from 1.35 to 1.85 g cm(-3). Ash contents within the particl es range from 3.5 to 23 wt %. Combustion chars of intermediate burn-of f were prepared from coal particles in a linear flow furnace in an atm osphere of nitrogen and oxygen at 1500 degrees C. Organic and associat ed mineral matter were characterized using optical microscopy, XRD, FT IR, and SEM-EDX. These chars were used to study the effect of mineral matter on further conversion. Results show that char reactivities at 5 00 degrees C (thermogravimetry) and combustibilities at 1200 degrees C (entrained flow) increase with increasing mineral matter content of t he char, effects being more pronounced at the lower temperature. Surfa ce combustion temperatures of coal particles at 1200 degrees C did not vary with ash content. The increase in reactivity is not attributed t o catalytic effects of mineral matter or to differences in internal su rface areas of the chars as measured from carbon dioxide isotherms. It is considered that the enhancement in rate is associated with an enha nced macroporosity within chars in association with the decomposed cla y mineral illite, having a platelike morphology. The presence of the i llite 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.