Experimental study on moisture re-adsorption characteristics of dried coal

Recently, Korean coal-fired power plants have used sub-bituminous coals with moisture and volatile contents higher than design coal of those plants. This causes a decrease in the efficiency of boilers that are especially designed for high-quality bituminous coals. Pre-drying processes for low-rank coals have been developed and employed for some of these boilers. However, dried coal can re-absorb atmospheric moisture when it is stored in coal yards or is being transported. The moisture re-adsorption characteristics need to be investigated for evaluating the pre-drying process for coals with a high moisture content. In this study, water re-adsorption characteristics of dried coal were investigated experimentally. Sub-bituminous coals from Indonesia were used in the study, and these were subjected to a coal drying process involving a fluidized bed type reactor with superheated steam as the heat carrier. Moisture contents of the coal before and after drying are about 37% and 20%, respectively. The experiment was performed in a chamber where temperature and humidity were maintained constant at, respectively, 20-40 degrees C considering the temperature in Korean coal yards and 60% and 90% (corresponding to autumn and summer humidity values, respectively). To analyze the re-adsorption of dried coal, water was added to raw coal and dried coal. The drying rate was analyzed in the humidity chamber and outdoor. Amount of moisture re-adsorbed by dried coal was found to be lower than that by raw coal. Also, the higher the coal moisture content, the greater the drying tendency. Coal dried with superheated steam under humid conditions showed lower moisture re-adsorption (by 16.1%) than raw coal, which indicates that structural changes occur in the coal drying process. The BET surface area, porosity, and FT-IR spectrum were analyzed to determine the effects of thermal drying. The results show that the difference in the moisture re-adsorption characteristics between dried coal and raw coal was caused by effects of pore generation/destruction and changes in the functional groups during the drying process. Among these, the change in the functional group removal of -OH during the drying process is dominant, as determined from FTIR analysis results.