Research On Drying Mechanism Of Coal Under Low Temperature In The Coal-fired Power Plant

Drying is one of the essential aspects in the process of coal ulitization.Coal drying is highly energy-intensive process so that drying cost must be taken into consideration. One of the most effective ways to improve the efficiency of coal-fired power units is to use the waste heat obtained from boiler exhaust gases or cooling water. In order to find the way to fully, high effectively use the waste heat, it is important to explore the interaction between coal and water, the drying characteristcs at low temperatures.The theoretical analysis and experiment were launched to rsearch the interaction between coal and water, the drying characteristcs at low temperatures. Coal surface molecular fragments models of four different rank was built by using quantum chemical method. On the basis of B3LYP/6-311G++(d,p) optimized geometries, the electrostatic potential surface and the interaction energies were corrected with BSSE were calculated using the Gaussian 09 program. In order to further understand this type of interaction, graphical analyses for these systems were performed using the reduced density gradient(RDG) color-filled isosurface map. Using thermobalance, four kinds of coal were dried under non-isothermal and isothermal conditions, the kinetic parameters were calculated using thermogravimetry and thin-layer models.Through the theoretical analysis of the interaction between coal and water, it has been showed that the area near the oxygen-containing functional groups in the model had a high chemical activity by the surface electrostatic potential analysis. The absorption of H2 O onto coal surface molecular fragments models was physical adsorption, and the mainly kind of adsorption was due to hydrogen bond and the other was mainly due to the van der Waals interactions. The H2 O adsorption sites of coal models mainly located in the side chains. The experimental results of drying of four kinds of coal showed that the non-isothermal drying process followed model with three-dimensional diffusion, the apparent activation energy of non-isothermal drying process increases with the increase of moisture content and heating rate. Isothermal drying rate curves were divided into three stages: pre-heat drying stage, constant rate drying stage and the falling rate drying stage. The Yagcioglu model was chosen to be the optimal isothermal thin layer drying model. The drying rate constants k increased with coal moisture content decreased, with increasing temperature increased.