Water transport mechanisms in coal stockpiles

Abstract

One of the key indicators of coal product quality is the moisture content. Excess moisture can have a negative effect as it results in handling problems and a reduction in coal value by decreasing the heating value. The moisture content of the coal can be regulated by way of the natural drainage and evaporation which occurs within stockpiles. Various factors influence the redistribution of moisture within a stockpile and it was the objective of this study to understand and quantify the influence which each of these factors has on the movement of water. For this investigation, the mechanisms (runoff, infiltration, evaporation and drainage) by which water moves within a stockpile was simulated separately and then compared to the results obtained from an experimental 9 tonne coal stockpile. Results showed that there exists a strong linear correlation between the rainfall intensity and the amount of surface runoff water. Once the surface of the stockpile became saturated, the infiltration rate was constant; thus meaning that any increase in rainfall intensity will result in an increased amount of surface runoff. Small interparticulate void spaces (either as a result of compaction or high fines content) inhibited infiltration, leading to an increased runoff proportion. An increased stockpile slope decreased the coal-water contact time, which increased the amount of runoff. Erosion occurred more readily at high slopes and high fines content. The natural drainage of a stockpile was simulated through the use of drainage columns. It was found that the fines content (especially the -0.5 mm fraction) had a large influence on the de-watering efficiency of drainage. Two coals were used during this investigation. The high-ash coal showed greater retention of moisture, even though the low-ash coal had a larger fines proportion. This was attributed to the large content of the clay mineral kaolinite in the high-ash coal. Moisture profiles were determined for each coal sample and it was found that moisture gradually migrated towards the bottom of the sample. Evaporation proved to be significantly more effective than drainage at drying coal stockpiles, but the effect was only seen up to a certain depth. While the fine coal bed had a large surface area, the small interparticulate voids had a negative influence on the rate of evaporation. It was found that the surface of a fine coal stockpile will evaporate faster than that of a coarse stockpile, but that a coarse stockpile will experience evaporation more effectively as a result of its porous structure. Results showed that a fine coal bed will only experience evaporation on its surface, while coarse coal beds showed evidence of evaporation up to 0.4 m below the surface. Weather conditions such as temperature, relative humidity, and wind speed influenced the rate of evaporation. The coal beds showed cyclic behaviour through adsorbing moisture during the night, and de-sorbing moisture during the day as part of the evaporation process. This is a result of the coal particles attempting to remain in equilibrium with the atmosphere. Results from the 9 tonne experimental stockpile supported the finding that the rate of infiltration (or by definition the final moisture content) was independent of the rainfall intensity. The final moisture samples confirmed that the redistribution of moisture took place by means of drainage and evaporation. Similar to the coal beds used in the small-scale evaporation experiment, the stockpile showed signs of cyclic behaviour – although to a lesser extent. All small-scale experiments – with the exception of the experiment which investigated the depth of evaporation – were representative of the way by which moisture migrated or were retained in a coal stockpile. The inherent moisture content of the coal particles remained constant for all experiments, which proved that the moisture of the greatest importance in the de-watering of coal is the surface moisture content.