Chemical Characterization of Unburned Carbon in Coal Fly Ashes by Use of TPD/TPO and LRS Methods

摘要

Functional forms of the unburned carbon present in six kinds of coal fly ashes have been examined mainly by the temperature-programmed desorption (TPD)/temperature-programmed oxidation (TPO) and laser Raman spectroscopy (LRS) methods. The carbon contents of the ash samples range from 0.4 to 4.1 mass%. The LRS analysis shows that the C consists of both amorphous and crystallized forms, and the proportion of the former is as large as 50-65 C%. Further, the TPD measurement exhibits that the C contains several types of surface oxygen species, such as carboxyl and lactone/ acid anhydride groups, which can readily be decomposed into CO_2 up to 700 ℃ to provide active carbon sites. The results of the TPD also indicate that the ashes have surface CaCO_3, and most of this species can be converted to CaO and CO_2 around 600-700 ℃. Interestingly, there is a significant correlation between organic fluorine concentrations and carboxyl/lactone/acid anhydride groups or surface CaCO_3 contents in the ash samples. It might thus be possible that the formation of organic F forms proceeds through gas-solid-solid interactions among HF (and/or F_2) in flue gas, active carbon sites and surface Ca species produced around 600-700 ℃ downstream of coal-fired boilers.