Low calorific value of lignite, mostly attributed to high moisture content, undermines usability of this low rank coal. Upgrading the quality of this energy carrier, which benefits coal-fired power plant's thermal efficiency, can be effectively realized by means of drying. The kinetics of superheated steam drying was studied for 30 mm spherical samples from Belchatow lignite mine in Poland. The experiment featured simultaneous and continuous measurements of weight and temperature. The drying kinetics, described by curves of moisture content, drying rate and temperature profile, were evaluated using the data acquired. The appearance of the sample throughout the process was video-recorded. Widespread cracking and shrinkage, which differ depending on the conditions, along with series of droplets typical for larger sample, were observed. The examined particles were suitable for predictions of thermodynamically derived drying rate and time, as proposed in the previous study for 5 and 10 mm particles from the same coal deposit. However, in comparison they exhibited more uniform density distribution. On the basis of experimental results and material properties, the numerical model of drying process was applied. Simulated drying behavior was consistent with empirical observations. Extensive investigation, revealing features of variously-sized samples as well as diameter dependence on drying process, is required for purposes of designing industrial drying system dedicated to specific type of coal.
2016 The Japan Society of Mechanical Engineers