Coal-derived unburned carbons in fly ash: A review

Unburned carbon (UC) in fly ash indicates inefficiency in combustion and may be an impediment to the beneficial use of fly ash or ash products in a variety of applications. The characteristics of the coal-derived UC are a function of the rank and type of the coal, as well as the size of the feed coal and the combustion conditions. At any coal rank, inertinite macerals are inherently more difficult to combust than the associated vitrinite, and some will have a tendency to appear in the fly ash more or less unchanged from their appearance in the feed coal. The nature of UCs resulting from vitrinite is dependent upon the coal rank. Low-rank huminite/vitrinite will tend to form an isotropic char; bituminous vitrinite will appear as isotropic and anisotropic cokes; and anthracite vitrinite, naturally anisotropic, is observed as partially combusted vitrinite fragments in the ash. The absorption of air entraining agents by UCs limits the use of high-UC fly ashes as a Portland cement substitute, with both standards organizations and regulatory bodies imposing limits on the acceptable UC concentrations. UC in fly ash can be used to adsorb organic compounds (such as phenols, dyes, herbicides, polychlorinated biphenyls, and petroleum constituents) and to capture trace elements (particularly Hg) from flue gas. UCs can also be used as sources of activated carbons, manufacture of graphite, and cokes in the metallurgical industry, as well as a source of carbon to feed back into the boiler. Beneficiation of fly ash to segregate relatively UC-free or UC-rich splits for beneficial re-use can be done by size classification, electrostatic separation, and froth flotation, as well as density separation, acid digestion, and incipient fluidization. Thermal processing may also be used to burn off the UC, leaving a relatively UC-free fly ash as the product.