Experimental study on freeze drying of Loy Yang lignite and inhibiting water re-adsorption of dried lignite

Abstract

Loy Yang lignite with a high water content was dewatered by freeze drying (FD). The drying kinetics of FD was calculated using thin-layer drying models given in the literature. Residual water content, re-adsorption or desorption behaviors, and pore size distributions of all of the samples were investigated. Furthermore, coating with different amounts of kerosene by direct mixing or adsorption methods to restrain water re-adsorption was also investigated. The results showed that FD dewatering contained three steps: a fast dewatering period (∼2h), followed by a reduced drying-rate period (2–3h) and an apparently falling-rate period (>3h). The Midilli-Kucuk model described the drying process perfectly and could be employed to predict residual water content in the sample at any time during the FD dewatering process. The moisture holding capacity (MHC) of the FD-treated samples was lower than that of raw lignite, which is because of the effect of the water–lignite bond strength and the diffusion adsorption or desorption force. Moreover, adding kerosene by either adsorption or direct-mixing methods can both decrease MHC because kerosene is coated on the surface and in the pores of the lignite. These prevent water re-adsorption. The adsorption method is better than the direct-mixing method because it consumes less kerosene.