A comparison of coal char reactivity determined from thermogravimetric and laminar flow reactor experiments

The reactivity of nine different coals ranking from subbituminous to low-volatile bituminous has been studied by thermogravimetric analysis (TGA). At a standard set of conditions a qualitative fuel reactivity classification (ranking) with respect to one of the coals, Cerrejon, is presented. Particle reaction rates per unit external surface area and a normalized reactivity index based on raw experimental data were used as reactivity parameters to compare the fuels. The TGA chars were prepared at 900 degrees C with 15 min holding time and then combusted in a 20 mol % O-2 environment at several temperatures in the range 450-650 degrees C. TGA reaction rate data were adequately interpreted by a random pore model. However, at 650 degrees C it is believed that particle ignition gave rise to a char reaction rate behavior that the model was incapable of describing properly. Except for two Southern Hemisphere coals, the reactivity ranking obtained with the TGA apparatus at a combustion temperature of 550 degrees C agrees well with a corresponding classification based on experiments carried out in another study with a laminar flow reactor (LFR) at similar to 1400 degrees C. A possible explanation for this is a more dense structure of the Southern Hemisphere coals compared to the Northern Hemisphere coal Cerrejon in the high-temperature combustion regime, where char morphology and thereby mass transfer effects such as internal pore diffusion are reaction rate determining. The maximum difference in reaction rates based on external surface area between the coal chars in the low-temperature TGA experiments was 1 order of magnitude higher than in the high-temperature LFR experiments, due to the increasing effect of pore diffusion and thermal annealing of the coal chars in the LFR tests. The similarity in the reactivity ranking obtained for the Northern Hemisphere coals from both reactor systems indicates that a ranking can be performed by thermogravimetric analysis. This provides a simple means for determining a fuel reactivity ranking that could be applied to full scale suspension fired plants.