Comparison of high-unburned-carbon fly ashes from different combustor types and their steam activated products

Fly ashes of high-unburned-carbon content derived from coal-fired combustors are an increasing problem for the utility industry, since they cannot be marketed as a cement extender, and therefore, have to be disposed. A series of six unburned carbon samples from different combustors was collected and characterized by elemental analysis, petrographic composition, nitrogen adsorption isotherms, and thermogravimetric analysis. The elemental analyses show that all the unburned carbon samples consist mainly of carbon with very little hydrogen, nitrogen, sulfur, and oxygen. There appears to be a correlation between the C/H atomic ratio and the porous texture properties, where the lowest C/H ratio corresponds to the highest specific surface area and pore volume, and the smallest pore size. In addition, the potential use of unburned carbon as a precursor for activated carbon (AC) was investigated. Activated carbons with specific surface area up to 1270 m(2)/g were produced from the unburned carbon. The porosity of the resultant activated carbons was related to the properties of the unburned carbon feedstock and the activation conditions used. It was found that not all the unburned carbon samples are equally suited for activation, and furthermore, their potential as AC precursors could be inferred from their physical and chemical properties. The developed porosity of the activated carbon was a function of the oxygen content, porosity, and H/C ratio of the parent unburned carbon feedstock. It was observed that extended activation times and high activation temperatures increased the porosity of the produced activated carbon at the expense of the solid yield.