Abstract
Microwave drying is a promising and effective way to drying and upgrading lignite. The influence of temperature (100–140 °C) and microwave power levels (500–800 W) on thin-layer drying characteristics of Zhaotong lignite under microwave irradiation were investigated. Fourteen thin-layer drying models were used to analyze the microwave drying process while six thin-layer drying models were used to analyze the hot-air drying process. The microwave drying processes at all temperature (100–140 °C) or low microwave power levels (500–700 W) exhibited four periods: a warm-up period, a short constant period, the first and second falling rate period, while one falling rate period was found during hot-air drying. The effective diffusion coefficient of lignite were calculated and it increases with increasing temperature and microwave power levels. During microwave drying, the two-term exponential model is the most suitable model for all applied conditions, while the Modified Page model is the most suitable model to describe the hot-air drying experiments. The apparent activation energy were determined from Arrhenius equation and the values for the first and second falling rate period are 3.349 and 20.808 kJ·mol−1 at different temperatures, while they are 13.455 and 19.580 W·g−1 at different microwave power levels. This implies the apparent activation energy is higher during the second falling rate period, which suggest that the dewatering of absorbed water is more difficult than capillary water. The value of apparent activation energy in hot-air drying is between the first and second falling rate period of microwave drying. Results indicate that microwave drying is more suitable to dewatering free water and capillary water of lignite.
Highlights
With the consumption of energy demands increasing, the storage of high-rank coal has decreased quickly
The constant rate period was 1260 s at 100 °C, while it was 900 s and 660 s as the temperature rose to 120 °C and 140 °C.The detected at all different temperature drying experiment which can be ascribe to the temperature of drying time decreases by about 29% and 48%, respectively
The mass change in these two periods was mainly ascribed to the evaporation of bulk microwave power level is 800 W, an obviously constant rate period appears, which is similar to the water, which mainly exists in the surface of the sample
Summary
With the consumption of energy demands increasing, the storage of high-rank coal has decreased quickly. Moisture in the lignite can be heated through direct interaction between microwaves and moisture and a more uniform temperature distribution can be achieved due to volumetric heating [17] Owing to these distinctive characteristics, numerous heating technologies based on microwaves have been developed to apply in various fields, such as the food processing industry [18], biological industry [15], agriculture industry [19], and mineral processing [20]. It should be an effective drying method to upgrade lignite. The observation of this work can provide directions and support for deep processing and further utilization of lignite
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