Quantifying the Heats of Coal Devolatilization

An experimental apparatus was developed to measure the thermal properties of coal in order to determine the heats of reactions during devolatilization. Samples were continuously heated from room temperature to 1000°C at a constant heating rate. Measuring the temperature changes at two selected locations within the sample allowed the calculation of the apparent spec ific heat and thermal conductivity by employing an inverse numerical technique to solve the heat conduction equation. Five major reaction regions of coal devolatilization were detected: dehydration, transition, resolidification, secondary reactions and contraction reactions. The most complex region was the exothermic resolidification reaction, where coking coals exhibited significant differenc es from the thermal coals due to higher plasticity. Small endothermic reaction related to tar vaporization also appeared in this region, becoming stronger at higher heating rates. The exothermic contraction reactions ranging between 140 and 250 MJ/m3 appeared above 600°C as a result of the dehydrogenation and reformation of stronger C-C bonds. The effects of the heating rate wi thin the range of 5°C/min to 100°C/min were also examined, showing a shift of the reaction peaks to higher temperatures at increased heating rates. The thermal conductivity and diffusivity were almost constant until the decomposition point, rapidly increasing at higher temperatures.