Hygroscopicity of coal powders with different ranks

[Display omitted] •Hygroscopicity of coal powders with different ranks was compared and investigated.•Reasons behind varying hygroscopicity of coal powders were analyzed and explained.•Variation of hygroscopicity of coal powder with particle size was studied.•Effects of ambient temperature and humidity on the hygroscopicity were examined. The hygroscopicity of coal powder is one of the crucial factors that determine its moisture content, which significantly affects its adhesion, flowability, charge properties, and combustion behavior. In this study, the effects of coal particle size, ambient temperature, and relative humidity on the hygroscopicity of coal powders with different ranks were investigated through experimental methods. The results indicated that an increase in coal particle size resulted in a moderate reduction in the maximum moisture adsorption capacity and a substantial increase in the effective moisture adsorption time of the coal powder. As the coal particle size increased from 19 to 21 μm to 6310–6941 μm, the maximum moisture adsorption capacities of the coal powders decreased by 0.28–0.54 times, while their effective moisture adsorption times increased by 1.47–1.68 times. The maximum moisture adsorption capacities of the coal powders increased dramatically with increasing ambient relative humidity, whereas the impact of temperature on their maximum moisture adsorption capacities was not significant. The hygroscopicity of coal powders varied significantly among different ranks. Low-rank coal such as lignite and subbituminous showed strong moisture adsorption ability, while high-rank coal such as anthracite exhibited moderate moisture adsorption ability, and middle-rank coal such as medium volatile bituminous showed the weakest moisture adsorption ability. FTIR spectra analysis and pore testing results revealed that the content of oxygen-containing functional groups and the distribution characteristics of internal pores were critical factors causing the difference in hygroscopicity of the coal powders. The practical value of this study lies in its potential to determine the moisture adsorption rate and the maximum moisture adsorption capacity of coal powder under specific environmental conditions, which is essential for evaluating the moisture status, transportation efficiency and combustion performance of coal powder.