Combined effects of additives and power levels on microwave drying performance of lignite thin layer

Abstract

The microwave drying performance of lignite thin layer in a bench-scale setup was highlighted in terms of three additives with 10% dosage. The dielectric loss for dried lignite, raw lignite, lignite/coal fly ash, lignite/Na 2 SO 4 and lignite/Na 2 CO 3 at 2450 MHz were 0.06, 0.13, 0.14, 0.15 and 0.18. In comparison with raw lignite, the average temperature rising of the thin layer at 385 W for lignite blending with sodium carbonate, sodium sulfate and coal fly ash was about 10, 7 and 2 °C. The apparent activation energies of both falling rate periods for lignite blending with three additives were less than that of raw lignite. Sodium carbonate among three additives could be preferable one, followed by sodium sulfate and coal fly ash. The energy efficiency increased with the addition of sodium carbonate, sodium sulfate and coal fly ash. The required electricity energy for lignite/Na 2 CO 3 blend at 385 W was reduced by about one half in compared with the raw lignite.

Effect of Temperature and Microwave Power Levels on Microwave Drying Kinetics of Zhaotong Lignite

“…Consequently, the drying force increases gradually. Similar results were reported by Fu et al [12] at the microwave power of 119-700 W. An interesting phenomenon shows that the effective diffusion coefficient at different temperatures are higher than that at different power levels. (8) where m and P are the mass of sample (g) and the microwave power levels, respectively.…”

“…This is due to the high dielectric constant and loss factor of the water [26]. The drying rate is a short constant in the (B) period, which could be attributed to the sufficient supply of bulk water and the balance between the water evaporation energy and the thermal energy generated by microwave energy [12]. However, the constant rate period was detected at all different temperature drying experiment which can be ascribe to the temperature of the sample.…”

“…In addition, traditional methods will lead to heating inhomogeneity, which is not beneficial for lignite upgrade. Among these dehydration technologies, microwave drying of low-rank coal is a very promising method due to its unique heating features.Microwaves are electromagnetic waves with frequencies that range from 300 MHz to 300 GHz [12,13]. Microwave heating is a type of dielectric heating in which microwave energy is converted directly within material into thermal energy in the form of molecular friction or dielectric loss.…”

“…The apparent activation energy of MWC and RC is 28.590 kJ·mol −1 and 24.250 kJ·mol −1 , respectively. Fu et al [12] evaluated the influence of additives on apparent activation energy and energy efficiency of lignite, which were increased with the addition of Na 2 SO 4 , Na 2 CO 3 and coal fly ash of lignite. Fu et al [27] also examined the microwave energy and temperature distribution of compressed lignite Processes 2019, 7, 74 3 of 18 Processes 2019, 7, 74 4 of 18 Processes 2018, 6, x FOR PEER REVIEW 4 of 20…”

“…Microwaves are electromagnetic waves with frequencies that range from 300 MHz to 300 GHz [12,13]. Microwave heating is a type of dielectric heating in which microwave energy is converted directly within material into thermal energy in the form of molecular friction or dielectric loss.…”