In the first stage, when pulverized coal starts to burn, the volatile matter with low gasification temperature first escapes from the gap between minerals and solid carbon, making pulverized coal become porous carbon particles. At this time, the particle state of coal ash basically maintains the irregular lump of original coal powder, but due to its porosity, its surface area is large.
In the second stage, with the complete combustion of organic matter in porous carbon particles and the increase of temperature, the minerals in them will also be dehydrated, decomposed and oxidized into inorganic oxides. At this time, coal ash particles will become porous glass body, although its morphology remains basically the same as porous carbon particles, but its specific surface area is obviously smaller than porous carbon particles.
In the third stage, with the progress of combustion, the porous glass gradually melts and shrinks to form particles, and its porosity decreases, roundness increases, and particle size decreases. Finally, the porous glass becomes a dense sphere with higher density and smaller particle size, and the specific surface area of particles decreases to the minimum. There are significant differences in chemical and mineralogical characteristics of ash particles with different particle sizes and densities. Small particles are usually more glassy and chemically active than large particles.
The final fly ash (80% ~ 90% of which is fly ash and 10% ~ 20% is bottom ash) is a complex and changeable multiphase substance with similar appearance and fine and uneven particles. Fly ash is the finest part of the dust entering the flue gas, and bottom ash is the separated coarse particles or slag. These things are heavy, and the burning zone extends to the bottom of the stove.