Non-isothermal combustion behaviour of coal blends in a thermobalance as seen by optical microscopy

Abstract

Combustion at programmed temperature in a thermobalance is a common test for the rapid assessment of coal combustibility. In this study two series of blends (low rank/medium rank coal-AB and low rank/petroleum coke-AC) with the low rank coal in three different proportions (1/4, 2/4 and 3/4) have been tested. Samples have been ground and sieved to 20–75 μm after blend preparation. The combustion profiles indicated different behaviour for the two series of samples: the AB series showed wide curves with the presence of shoulders whereas the AC series showed two maxima corresponding to the component fuels. The comparison of the calculated and experimental curves indicated different effects of blending on the relevant temperatures and reactivity of the blends. In the AB blend most relevant parameters were similar than expected and only a slight reduction of burnout temperature was observed. In the AC series the difference between calculated and experimental values was large with higher initiation temperatures, lower burnout temperatures and higher reactivity in the experimental curves. This indicates synergistic effects for the combination of subbituminous/petroleum coke blends. In order to visualize the relative combustibility of the coals the reaction was stopped at 50% conversion and the samples were examined through the microscope. The combustion of the particles followed a shrinking core pattern in which the core of the low rank particles remained isotropic whereas anisotropy development was observed in the medium rank coal. The reflectance of the coals increased with increasing the temperature at which the reaction was stopped regardless the rank of the parent coal following a linear trend. The consumption rate of the low rank coal increased with temperature rise.