Comparison of Superheated Steam and Air Fluidized-Bed Drying Characteristics of Victorian Brown Coals

Abstract

Brown coals represent a large resource for Victoria, with reserves of over 500 years at the current rate of consumption. However, its high moisture content (∼60%) impedes its utilization in an efficient manner. Information on drying kinetics of these coals is still scarce, affecting the development of economic and efficient drying technology. This experimental and modeling study presents a comparison of the drying kinetics between steam fluidized-bed drying and air fluidized-bed drying of three Victorian brown coals: Loy Yang, Yallourn, and Morwell. The effects of gas temperature (100−200 °C), gas velocity (0.32−0.61 m/s), and particle size (0.5−1.7 mm) on the drying kinetics have been examined. Both air and steam fluidized bed drying has been found to result in similar trends, with air drying having shorter drying times compared to steam drying. Increase in temperature and velocity, and a reduced particle size decreases the length of time required for complete drying to occur. Comparing air to steam fluidized bed drying, the relative drying ratios for a similar conditions (such as 130−170 °C) remains consistent, with the exception of particle size. The resultant 130 °C dried coal was used to analyze the moisture readsorption properties of the coals. The data show that steam fluidized-bed dried coals readsorb less moisture than air fluidized-bed dried coal, regardless of the coal type, with an average difference in moisture of 1.6%. The physical and chemical characterization of dried coal shows that moisture readsorption is a function of the oxygen functional group content. Several drying models available in the literature were compared against the experimental data acquired during this study. The results indicate that the Midilli− Kucuk model accurately describes the drying kinetics of the three investigated Victorian brown coals.

Drying kinetics of coarse lignite particles in a fixed bed

“…Therefore, modeling studies in the literature similar to this study can be compared. Tahmasebi [22][23][24]. Midilli model fitted well to their results.…”

“…Thin-layer drying models have been adopted to coal drying in a few studies [21][22][23][24]. Pickles et al investigated microwave drying of a lowrank sub-bituminous coal [21].…”

“…They used three different thin-layer drying models to describe drying kinetics, and Midilli model fitted well to their experimental results. Stokie et al compared drying characteristics of Victorian Brown Coals in superheated steam and air fluidized-bed dryers [24]. They investigated the effects of gas temperature and velocity and particle size.…”

Size Dependence of the Drying Characteristics of Single Lignite Particles in Superheated Steam

“…A steam fluidized bed dryer (SFBD) on a commercial scale has been developed recently in Germany, [2] and a pilot dryer using pressurized steam also has been constructed. [3] While the drying characteristics of lignite in an air fluidized bed dryer have been studied, [4][5][6][7] available information on steam fluidized bed drying is limited despite continuing dryer research and development. Additionally, the behavior of single lignite particles, which is fundamental to comprehending the drying processes, has been reported only for particles 10 mm or larger, [8,9] although sizes are assumed to be less than 10 mm in an actual process.…”

Evaporative Drying of Low-Rank Coal

“…However, there is not much study on this issue. The thin-layer drying models and the neural network methods were applied to estimate the drying curve [23][24][25][26][27][28][29]. The performance of the used models and methods seem so satisfactory.…”