Improving the permittivity of Indonesian lignite with NaCl for the microwave dewatering enhancement of lignite with reduced fractal dimensions

To upgrade Indonesian lignite with high moisture content for generating quality gasification fuel, NaCl was used to improve the permittivity of this lignite, thus enhancing microwave dewatering rate. The dielectric loss factor of lignite with 2.34 wt% NaCl increased from 0.190 to 0.311 at 2.45 GHz; as a result, the conversion of electromagnetic energy to thermal energy increased by 64%. Furthermore, the peak dewatering rate and moisture diffusion coefficient of lignite with microwaves increased exponentially from 6.5%/min to 13.20%/min and from 1.61 m2/s to 4.02 m2/s, respectively, when NaCl content increased from 0 wt% to 4.67 wt%. The pore fractal dimension of dewatered lignite pores decreased from 2.40 to 2.32 with a microwave at 600 W for 8 min; this dimension then dropped further to 2.26 with the addition of NaCl. The promoting effect of Na+ on microwave dewatering was better than those of K+ and Ca2+, whilst the promoting effect of Cl− was better than that of SO42−. This phenomenon may be because Na+ and Cl− with small hydrated radii exhibit high movement speeds in electromagnetic fields.