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

Abstract

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 CO2 up to 700 °C to provide active carbon sites. The results of the TPD also indicate that the ashes have surface CaCO3, and most of this species can be converted to CaO and CO2 around 600-700 °C. Interestingly, there is a significant correlation between organic fluorine concentrations and carboxyl/lactone/acid anhydride groups or surface CaCO3 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 F2) in flue gas, active carbon sites and surface Ca species produced around 600-700 °C downstream of coal-fired boilers.

Raman Spectroscopy as a Versatile Tool for Investigating Thermochemical Processing of Coal, Biomass, and Wastes: Recent Advances and Future Perspectives

“…There are difficulties in detecting the structure of unburned carbon by common techniques like XRF and XRD. 266,285 As mentioned in section 2, RS is a highly sensitive technique to characterize the presence and structures of carbon. Thus, it can also be a useful tool for studying the unburned carbon in fly ash.…”

“…The Raman results revealed that the degrees of carbon graphitization of the carbon concentrates from fly ash were similar to those of cokes and carbon blacks and higher than those of typical coals. Tsubouchi et al 285 used RS as a supplement to temperatureprogrammed desorption (TPD)/temperature-programmed oxidation (TPO) methods in structural analysis of unburned carbon. As the rate profiles for CO 2 and CO formation in the TPO run were similar between six samples, they further used RS to investigate the proportion of amorphous and crystallized carbon by the normalized intensity of bands at 1355 and 1575 cm −1 in the ashes, respectively.…”

“…Besides, the presence of unburned carbon may affect the performance of fly ash in concrete materials and limit its utilization. , In most circumstances, the bulk content of unburned carbon in coal fly ash below 5 wt %. There are difficulties in detecting the structure of unburned carbon by common techniques like XRF and XRD. , As mentioned in section , RS is a highly sensitive technique to characterize the presence and structures of carbon. Thus, it can also be a useful tool for studying the unburned carbon in fly ash.…”

“…The Raman results revealed that the degrees of carbon graphitization of the carbon concentrates from fly ash were similar to those of cokes and carbon blacks and higher than those of typical coals. Tsubouchi et al . used RS as a supplement to temperature-programmed desorption (TPD)/temperature-programmed oxidation (TPO) methods in structural analysis of unburned carbon.…”