2015 Volume 41 Issue 2 Pages 140-147
The world’s coal resources amount to about 900 billion tons, but about the half of this is sub-bituminous coal and brown coal. Brown coal often contains more than 50% moisture (wet basis) and its pre-drying in large volume with low energy consumption is a key technology for its efficient utilization. A fluidized bed dryer using superheated steam for fluidizing gas is suitable for this purpose. In a steam fluidized bed dryer, heat transfer tubes are installed in fluidized bed and brown coal particles are heated indirectly by saturated steam under pressure of around 0.5 MPa (saturation temperature 150°C) in these tubes. Since brown coal particles have a wide particle size distribution and are cohesive due to their high moisture content, it is important to estimate the heat transfer coefficient outside of heat transfer tubes in the design of steam fluidized bed dryers.
In this study, basic cold tests using nitrogen as fluidizing gas were carried out with two kinds of brown coal with different moisture content. Influence of gas velocity and density of heat transfer tubes on the heat transfer coefficient was evaluated. In order to increase the heat transfer area, square fins were attached to the heat transfer tubes and their pitch was varied. Experimental results of heat transfer coefficient agreed well with the value estimated by the equation of Andeen and Glicksman by giving particle size and density as functions of the moisture content of brown coal. The heat transfer coefficient decreased with the increase in density of heat transfer tubes for high-moisture sticky particles, whereas it increased for dry particles. The fin effectiveness was smaller than the value calculated assuming the same heat transfer coefficient as bare tubes.