Size Dependence of the Drying Characteristics of Single Lignite Particles in Superheated Steam

Abstract

Loy Yang lignite particles 10, 5, and 2.5 mm in diameter were dried with superheated steam at temperatures ranging from 383 K to 443 K (110 °C to 170 °C) under atmospheric pressure. The drying rates were obtained by measuring their weights with an electronic balance, while temperatures of the particles were monitored. The following typical drying characteristics were observed: (i) Temperature of lignite rose to 373 K (100 °C) accompanied by condensation of steam on the surface. (ii) A constant drying rate period was then observed, while temperature of the overall particle was maintained at 373 K (100 °C). (iii) A decreasing drying rate period began accompanied by a rise in surface temperature, and eventually the overall particle reached the temperature of the superheated steam. Based on these results, the evaporation rate of water was expressed as a function of particle diameter. A numerical model for simulating the drying process of lignite, which was developed for a large particle in a previous study, was modified to make it applicable to small particles. The model is applicable for simulations of drying behaviors of lignite with size distribution in various dryers when an appropriate heat transfer coefficient is given.