Adsorbent assisted drying of fine coal

Abstract

Coal fines are generated by the increase in mechanization on coal mines, and have the ability to retain large amounts of moisture due to its inherently large surface areas (Reddick et al., 2007). The bulk of the moisture is retained by fine (-1mm+0.1mm) and ultra-fine (-0.15mm+0.1mm) coal fractions, which constitutes about 11% of the nominal product (SANEDI, 2011). This poses a problem for utility companies as moisture retention lowers the effective heating value of the coal. Coal fines are often dewatered, and combined with the coarse coal stream; however, as the coal fines rarely meet the desirable moisture levels, the quality of the final coal product stream suffers (Reddick, 2006). Commonly, fine coal dewatered by the best mechanised methods is still directed to the coarse coal circuit containing approximately 18%wt moisture (Mohanty and Akbari, 2012). Mohanty and Akbari (2012) remarked that by reducing the initial moisture levels of the fine coal by 50%, the overall turnover of a typically colliery can potentially be improved by ±6%. In the past, whenever mechanical dewatering techniques failed to deliver contract specifications, the solution has leaned towards thermal drying, which is the most effective and expensive drying technique (Bratton et al., 2012). Therefore, searches for innovative, cost efficient, and eco-friendly drying techniques have intensified (Bratton et al., 2012). One such advanced dewatering process that was developed employs drying media to adsorb remaining surface moisture from the coal fines after mechanical dewatering. This investigation was primarily focussed on successfully, and feasibly employing adsorbent material to lower the surface moisture content of coal fines. A surface moisture content of 8%wt or 0.08 g(moisture)/g(coal and moisture) was targeted, as dust problems and blending prospects were also considered. During this study, adsorbent assisted drying with integrated fixed-bed and cascading-bed drying techniques were employed to dry mechanically dewatered fine coal. Cascading-bed drying employed motion whilst the fixed-bed drying was operated in motionless state. Laboratory-scale experiments were conducted with various operating parameters and it was found that the surface moisture levels of the fine coal was effectively reduced from ±0.30 g(moisture)/g(coal and moisture) to well-below 0.08 g(moisture)/g(coal and moisture) within 10 minutes. The cascading-bed drying technique proved to be considerably less time consuming than the fixed-bed drying technique, which proves motion is of paramount importance when employing adsorbent assisted drying. The best performing adsorbent to coal mass ratio was found to be 2:1, while the -2mm+1mm fine coal delivered the lowest product moisture levels and the -1mm+0.5mm produced the highest overall initial desorption rates during the cascading-bed drying technique. In addition, it was found that alumina and silica-based adsorbent yielded similar drying performances, whereas the 3mm adsorbents proved to have increased desorption rates over the 5mm adsorbents. From an additional set of experiments, it was concluded that the adsorbent material could be reused without regeneration for six sequential cycles, while consistently lowering the moisture content of the fine coal below 0.10 g(moisture)/g(coal and moisture). Both alumina-and silica-based adsorbents were regenerated with ease by employing drying air, conditioned at 25°C and 40%RH. The moisture load of the adsorbents were reduced to between 0.08-0.10 g(moisture)/g(adsorbents and moisture) within 10 minutes, irrespective of particle size. In addition, it was found that adsorbent condition did not influence the final moisture content and drying rates of the fine coal. From an average initial moisture of 0.25 g(moisture)/g(coal and moisture), about 74%, 75% and 72% surface moisture was removed from -1mm+0.5mm coal fines, by the unused, used and air dried (regenerated) 3mm alumina-based adsorbents, respectively. Although it was concluded that the 2:1 adsorbent to coal mass ratio delivered the best drying performance, the adsorbent to coal mass ratio of 1:1 was selected for investigation of industrial application, as less adsorbent material was required, while respectable drying performances was still reached. The -1mm+0.5mm coal and 3mm alumina-based adsorbents yielded the best drying performance, based on initial desorption rate and final moisture content, at this mass ratio. Therefore, these operating parameters were further investigated for industrial application and energy considerations. The total amount of energy required to dry the -1mm+0.5mm coal fines (to 0.08 g(moisture)/g(coal and moisture)) with 3mm alumina-based adsorbents was 1637.75 and 1633.33kJ, thereby yielding an energy improvement of 6527.05 and 6531.47kJ/kg at 30%RH and 80%RH (average room temperature and pressure range recorded for South Africa), respectively. In conclusion, it was determined that the minimum and maximum energy required to remove 1kg of moisture from the -1mm+0.5mm coal, by adsorbent assisted drying, was 1012kJ/kg H2O and 1015kJ/kg H2O, respectively, which was comparatively lower than existing drying techniques.