Energy Fuels 2010, 24, 6428–6436 Published on Web 11/01/2010
: DOI:10.1021/ef101198k
Experimental and Numerical Investigation of Convective Drying of Single Coarse Lignite Particles Kai Zhang and Changfu You* Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China Received September 5, 2010. Revised Manuscript Received October 12, 2010
The mechanism of convective drying of single coarse lignite particles has important theoretical and practical significance for the development of the lignite drying technologies. The drying kinetics of single lignite particles in hot air were investigated experimentally with particles diameters of 20 and 30 mm. The experiments were performed at hot air velocities of 0.7 and 1.5 m/s and hot air temperatures of 100 and 140 C. The results from these experiments show that the lignite particle size, the hot air temperature, and the velocity all significantly influence the drying. For the range of operating conditions investigated in this study, the drying time was reduced by increasing the temperature and velocity of the hot air flow and decreasing the particle size. The constant drying rate period was not obvious with the lignite characterized by a long decreasing drying rate. Both types of lignite had a good linear relationship between the drying rate and the dry-basis moisture content. A mathematical model for the drying of single particles was developed assuming local thermodynamic equilibrium to describe the multiphase flow in the porous particle with the energy and mass conservation equations to describe the heat and mass transfer during the drying. The drying model included convection of the free water, diffusion of the bound water, and convection and diffusion of the gas mixture in the lignite particle. The numerical results agree well with the experimental data verifying that the mathematical model can evaluate the drying performance of porous lignite particles. The effects of the drying conditions such as the particle size, the temperature, the absolute humidity, and the velocity of the drying air on the drying process were evaluated using the numerical model.
air, combustion gases, or superheated steam may be used as the heating medium. The dryers can be categorized into direct or indirect dryers, fixed and fluidized beds, rotary kilns, etc. Lignite is highly reactive and more susceptible to fire and explosion hazards due to spontaneous combustion. Hence, the oxygen content in the drying gas should not be too high to avoid fire and explosion hazards. Since large amounts of lignite need to be dried, a moving bed dryer design was selected with flue gas at low temperature (
