Heat capacity and thermal conductivity considerations for coal particles during the early stages of rapid heating

Abstract

Heating and cooling transients for a number of individual coal particles in the 100-μm size range were measured under rapid heating conditions (10 4–10 5 K/s heating rate). In addition to temperature measurements, each particle was fully characterized with respect to external surface area, volume, mass, and density prior to heating. Measured temperatures were compared with model predictions and a sensitivity analysis was performed to critically evaluate model assumptions regarding particle thermal properties. Simulations using temperature-dependent heat capacity and thermal conductivity correlations routinely applied to coal severely under predicted the particle temperature rise during the early stages of heating. Simulations using constant room temperature values for heat capacity and thermal conductivity showed excellent agreement with measurements during the early stages of heating. Increases in coal heat capacity and thermal conductivity reported in the literature are observed under slow heating conditions and result from bond breaking and structural changes which lead to an increase in vibrational modes of freedom in the coal structure. Results of the present study suggest that under rapid heating conditions the coal structure is frozen and that these vibrational modes only become accessible at higher temperatures or longer soak times. These considerations are important if one desires to accurately model the combustion behavior of coals.