Formation of fly-ash carbon residue in a circulating fluidized bed boiler and its disposal

Circulating fluidized bed combustion techniques have been widely used in China due to its numerous merits. However, a universal problem in operation is that the carbon content of fly-ash is much higher than generally expected. The major factors influencing the carbon content of fly ash are: coal index, coal structure and coke reaction activity, fed-coal particle diameter and structure of the circulating fluidized bed as well as other operational parameters etc. At present, the methods for reducing fly-ash carbon content mainly include: fly ash recirculation, secondary air strength enhancement and pressure-drop adjustment for a circulating fluidized bed etc. The experiments performed by the authors indicate that under the condition of a low air speed, the carbon residue in fly ash can be fully burnt up. In addition, high voltage electrostatic separation and fly-ash water activated agglomeration can also provide a new approach for utilizing carbon residue in fly ash.