The first stage
When the pulverized coal starts to burn, the volatile matter having a low vaporization temperature firstly escapes from the gap between the mineral and the solid carbon, so that the fly ash becomes a porous carbon particle. At this time, the coal ash, the state of the particles substantially maintains the irregular crumb shape of the raw coal powder, but the surface area thereof is made larger due to the porous type.
second stage
Along with the complete combustion of the organic matter in the porous carbon particles and the increase in temperature, the minerals therein will also be dehydrated, decomposed, and oxidized into inorganic oxides, at which time the coal ash particles become porous glass bodies, although their morphology is substantially It remains the same as the porous carbon particles, but the specific surface area is significantly smaller than that of the porous carbon particles.
The third phase
As the combustion progresses, the porous glass body gradually shrinks and forms particles, its porosity is continuously reduced, the roundness is continuously increased, the particle size is continuously reduced, and finally the porous glass is transformed into a dense sphere having a higher density and a smaller particle diameter. The specific surface area of ​​the particles is reduced to a minimum. Ash particles of different particle sizes and densities have significant chemical and mineralogical differences, and small particles are generally more vitreous and chemically active than large particles.
The finally formed fly ash (80% to 90% of which is fly ash and 10% to 20% of bottom ash) is a complex and variable multiphase material with similar appearance and fine and uneven grains. Fly ash is the finest part of the dust entering the flue gas. The bottom ash is the relatively coarse particles separated or slag. These things have enough weight and the burning belt runs to the bottom of the stove.
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