Convective Drying of D Coal

Resumo

The energy value of fossil fuels may be improved by changing their excess moisture content. In studying the development of heat and mass transfer in the heating of coal samples at a rate of 5°C/min in air, with the plotting of a complete differential scanning calorimetry (DSC) curve, it is found that initial heating of the coal sample is accompanied by the removal of physical moisture up to 180°C and increase in its heat content (period 1). At ~340°C, oxidation of carbon develops, with the release of excess heat (period 2). The intensity of heat emission may be judged from the change in sample mass at 460–676°C. Heating period 3 (up to 740°C) is associated with endothermal decomposition of the coal components and culminates in combustion of the combustible mass. On the basis of the kinetics of moisture removal from a weighed sample on isothermal heating at 250°C in a 2-m/s air flux, the total drying time is found to be 229 min, with increase in the rate of moisture removal for 12 min and steady drying for 20 min. According to the laws governing the motion of friable materials in a drum-type dryer, the drying time for coal with a mean moisture content of 9% is 20–28 min, with 20–25% mean filling of the working space. To prevent restoration of the moisture content of a dried coal sample in air, the total drying time cannot exceed 64–88 min, while drying must end with a residual moisture content of at least 7%.