Heat transfer characteristics on lignite thin-layer during hot air forced convective drying

Heat transfer characteristics of the lignite thin-layer during the hot air forced convective drying were investigated experimentally as a function of hot air temperatures (100, 110, 120, 130, 140, 150, and 160 °C) and speeds (0.6, 1.4, and 2.0 m s−1). The average temperature and surface temperature of the thin-layer increased rapidly in the first falling rate period, whereas those rose slightly in the second falling rate period. The stabilized temperature of the thin layer at hot air temperature range of 100-160 °C, increased by about 4.2% to 14.8% when the wind speed rose from 0.6 m s−1 up to 2.0 m s−1. The average surface heat transfer coefficients in the first falling rate period were about 2-3 times of those in the second falling rate period. With an increase of the hot air temperature from 100 to 160 °C, the average surface heat transfer coefficients increased by about 71.9% to 80.9% in the first falling rate period and about 56.8% to 146.0% in the second falling rate period. The dimensionless surface heat transfer correlation of the lignite thin-layer was obtained for the first falling rate period.