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The effect of surface properties of fine coal on the bulk coal handleability Holuszko, Maria Ewelina

Abstract

Handleability of the coal, which describes the ability of coal to pass through the handling system without causing major obstructions, has been studied for over 50 years. In the extensive research projects, a number of factors have been found to influence coal handleability. Among them the amount of fines (-0.5 mm) and moisture content were shown to be the most critical. Ash content in coal, size distribution of the bulk sample (spectrum of sizes), bulk density and content of clay materials were also found important. All these factors related only some physical properties of coal samples to the behavior of coal particles [i.e. behaviour of coal particles] in the presence of water. None of the studies addressed the effect of surface properties of fine coal in spite of the fact that the behavior of fine particles in the presence of water is dictated by the wettability characteristics of these particles. In this project, the effect of surface properties of coal on its handleability was studied with the emphasis on coal wettability. Characterization of surface properties, in terms of specific surface area and porosity, was carried out to supplement coal wettability studies. Furthermore, pelletization was used as a method to test behavior of particles in presence of water, as this process involves movement of particles and to some extent imitates possible handling conditions for coal samples. Moreover, the ability of fine particles to aggregate was found to have a deteriorating effect on the handleability of the bulk coal. The pellet’s strength was used as a measure of interparticle forces. The wettability-aggregation model was proposed to link the wettability characteristics with the ability of particles to aggregate. It was found that the strength of pellets produced from hydrophobic coals is only dependant on the ash content, while the strength of the pellets made of hydrophilic coal particles is related to the porosity, microporosity, total surface area and wettability of these particles. The more hydrophilic particles are, the easier they are to pelletize. Therefore, easy to pelletize fines tend to aggregate and thus become more difficult to handle. The relationship between the pelletization behavior of coal fines and the bulk coal handleability was examined using Durham Cone method. It was found that the effect of wettability of fine coal on the bulk sample handleability was significant. For hydrophobic coals, only the mineral matter affects the handleability of these coals; the flow rates deteriorate in presence of high amount of clays. An increase in moisture content affects flow rates only to a certain level, and even at high moisture contents, these coals do not cease to flow. It was concluded that because the aggregation of hydrophobic coal particles does not strongly affect the flow, the bridges that are built by these particles are breakable, even at high moisture content these samples continue to flow. This was confirmed by pelletization results of these coals. For hydrophilic coals, the pattern of flow rates change with the moisture increase was shown to be quite different; past the equilibrium moisture, the handleability of these coals drastically deteriorates, leading to non-flow conditions. The bulk tests confirmed further that the amount of fines (-0.5 mm) is a significant factor. The critical moisture as derived in this study, turned out to be a very important parameter, indicating the moisture content at which handling characteristics of the bulk sample sharply deteriorate. The validity of this finding is confirmed by the correlation between the surface moisture and ash fraction of fines (AF₀.₅₀), which was found to have best fit at the critical moisture for tested samples. The surface moisture, which is the amount of moisture in excess of equilibrium moisture, along with the ash content is the cause for particles’ aggregation, as predicted from the wettability-aggregation model. Bulk density measurements were carried out to supplement the handleability studies. It was found that for hydrophobic coals, the effect of increasing moisture on the bulk density was less pronounced than for hydrophilic coals; additionally, the range between the critical moisture and lowest bulk density moisture (LBD) was much narrower for hydrophilic coals, confirming that these coals reach non-flowing conditions much more quickly than hydrophobic ones. The fact that the moisture content at which the lowest bulk density LBD is reached coincides with the critical moisture prompted to develop a simplified procedure for predicting handleability behavior of a bulk coal from the bulk density tests. The comparisons between the Handleability Monitor and Durham Cone method were made, providing information on how these methods are influenced by changing conditions and possibly how they can be improved. The results of testing the same coal samples using both methods have shown that the effects of wettability on coal handleability can be used to explain behavior of coals with different characteristics.

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