When coal is combusted in air it burns in a two step process. In the first step gases are driven out of the coal structure leaving behind a carbon char that burns in the second step. These chars play a critical role in combustion in that they must burn up in the reaction zone of the furnace or be carried out of the furnace as unburnt carbon in fly ash. This unburnt carbon represents an inefficiency as well as an economic loss because the energy in the unburnt carbon is not being used. Excess unburnt carbon also destroys the ability of the fly ash to be use as a cement in a variety of applications.
In modern combustion systems coal is usually ground into a fine powder (-200 mesh or - 74 micometers) that features many single maceral particles. This is significant in that the various macerals tend to have different reactivities and therefore burn at different rates. Because the different maceral groups form chars with different morphologies, it is possible to analyze coal combustion chars to gain information about the nature and reactivity of coals being combusted.
The vitrinite macrerals form chars that take the form of hollow spheres, centispheres. Semifusinite macerals form centispheres with a lacey or honeycomb structure, and fusinite maceral char come through the combustion process unfused.
Coal Combustion Char Classification
|Tenuisphere||Fused or partially fused hollow spherical or angular char with walls less than 10 micrometers and porosity greater than 85 %|
|Crassisphere||Fused to partially fused hollow spherical or angular char with walls thicker than 10 micrometers and porosity greater than 75 %|
|Tenuinetwork||Partly fused, thin-wall char with internal network structure and porosity greater than 75%|
|Mesophere||Partly fused, thin-wall char with internal network structure and porosity 40-60%|
|Inertoid||Unfused particle with a rectangular to irregular shape and low porosity of 5-40%|
|Solid||Unfused particle with a rectangular to irregular shape and no porosity|
|Fusinoid||Unfused particle resembling fusinite with original plant cell structure|
|Mixed Porous||Mixed particle showing both fused and unfused sections with fused porous section dominant|
|Mixed Dense||Mixed particle showing both fused and unfused sections with unfused porous section dominant|
|Skeletal||Unfused, angular but highly burnt out char, still resembling fusinite|
|Mineraloid||Char with over 50% mineral matter|
Bailey, J. G., Tate, A., Diessel, C. F. K., and Wall, T. F., 1990, A char classification system with applications to coal combustion: Fuel, v. 69, no 2, p. 225-239.