Drying characteristics and kinetics analyses for Yimin lignite at various temperatures

Lignite drying technology is an effective method to enhance lignite utilization efficiency. Combustible gases may be produced during drying, leading to decreased lignite utilization efficiency. Therefore, the drying characteristics and kinetics below the threshold temperature for combustible gas production are key points for dryer designs and lignite industry practice. In this paper, Yimin lignite, an attractive resource with abundant application, is used as a sample to study characteristics of combustible gas production, drying behavior, and kinetics. Results show that at low temperatures, only H₂O and CO₂ were produced. Combustible gases, including CO and CH₄, can be produced when drying temperature exceeds ∼250 °C. This threshold temperature increases with increasing heating rate. Below the threshold temperature for combustible gas production, changes in functional groups and surface micromorphology were observed at various drying temperatures. Analyses of drying behaviors of Yimin lignite reveal four drying stages, namely, preheating, constant-rate drying, falling-rate drying, and 2nd falling-rate drying. Residual moisture cannot be completely eliminated even if the drying temperature reaches 250 °C. The kinetic parameters during drying under Kissinger–Akahira–Sunose (KAS) method are established. Activation energy first decreases and then increases with increasing conversion ratio from 0.1 to 0.9. Oxygen in the air increased the activation energy during drying and the ratio of activation energies in oxygen to that in nitrogen decreased from 1.45 to 1.07 as the conversion ratio increases from 0.1 to 0.9.