Effects of coal types on ash fragmentation and coalescence behaviors in pulverized coal combustion

It is important to be aware of the particle size distribution and composition of ash in coal combustion for prediction and control of ash deposition behavior, which results in the degradation of boiler performance via fouling and slagging. To elucidate fragmentation and coalescence behavior of mineral particles in a pulverized coal combustion process, char particles during the combustion reaction were sampled and analyzed for their particle size distribution and composition using a drop tube furnace. CCSEM analysis was conducted to identify included and excluded minerals in sampled char particles. Three coal samples having different char structures and ash melting temperatures were tested to estimate the effect of the physical properties of ash on fragmentation and coalescence behaviors of mineral particles. With the progress of combustion, the number of included mineral particles decreased due to their separation from char particles, leading to the increase of the number of excluded mineral particles. The total number of particles decreased with the progress of combustion. Some of particles were agglomerated, and some of them were fused to each other. These behaviors were less obvious for coal which experienced network structure of char particles during its combustion process. On the other hand, minerals in coal characterized by low melting point ash were rounded off during the coal combustion process. Fractions of included mineral were higher at smaller particle sizes, and they decreased with the progress of combustion. Elemental compositions of fused particles in coal characterized by low melting point ash are attributed to that of aluminosilicate. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.