Non-equilibrium thermodynamics approach for the coupled heat and mass transfer in microwave drying of compressed lignite sphere

The microwave drying of materials is a typical non-equilibrium process. Based on the non-equilibrium thermodynamics fundamentals, a transport model related to the thermal and moisture migration, and molecular polarization coupling transmission mechanism was developed. The microwave drying behavior for the compressed lignite sphere (20 mm in diameter) was simulated numerically in the lignite of the proposed model at the microwave power levels of 231-700 W. The calculated values for the drying and temperature profiles of the sample fitted well the experimental ones with the average deviation of 10% and 16%, respectively. The pumping phenomenon occurred in the sample during the microwave drying according to the variation of the water saturation at the sphere surface. The maximum water saturation at the sample surface was 0.56, which implied no water drops or films formed on the sample surface. The heat flux driven by polarization relaxation could weaken the heat transfer during the whole drying process. The electric-thermal effect would decrease with the increasing of the sample temperature, and the decreasing of the dielectric properties and the microwave energy intensity.