A Model for Softening and Resolidification of Coals Heated at Different Rates.

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Abstract
Needle penetration and dilation characteristics were examined for cylindrical coal pellets upon heating. The net penetration depth obtained from the observed penetration and dilation curves was analyzed by an equation of motion to derive the apparent viscosity of coal. Mass loss due to volatiles release and yields of pyridine extract were also measured for the pellets. These experiments were carried out for a range of the heating rate from 1 to 50 K/min, the holding temperature from 773 to 823 K and the nitrogen gas pressure of 1.0 MPa and the results depended on the operating variables as well as coal nature. Changes of the apparent viscosity with heating were well explained by a simple model with the following assumptions: 1) Coal behaves instantaneously as a Newtonian fluid. 2) Coal consists of two reactive components; one converting to semicoke via an intermediate along with volatiles release and the other converting to volatiles directly. 3) The intermediate is represented by the pyridine extract and behaves as a liquid while the others, i.e., unreacted coal, semicoke and ash as solids. 4) Apparent viscosity change is described by Vand's equation for slurry combined with Andrade's equation for liquid viscosity.

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