Chemical forms of the fluorine and carbon in fly ashes recovered from electrostatic precipitators of pulverized coal-fired plants

Abstract

The functionalities of the fluorine and carbon present in fly ashes formed in pulverized coal combustion have been studied with X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) techniques. The ash samples include 20–130 μg/g-dry and 0.4–4.1 mass%-dry of fluorine and carbon elements, respectively, and these components are enriched at the outermost layer of the ash surface. The F consists of both inorganic and organic functionalities, and the proportion of the latter is as high as 84–98 mol%. The C has different types of surface oxygen species, such as carboxyl, lactone/acid anhydride and phenolic groups, and most of these groups decompose to CO 2 or CO up to 700 °C to yield carbon active sites. When the amount of the O-functional forms increases, the content of organic C–F forms tends to increase almost linearly. On the basis of the above results, it may be speculated as one possibility that the formation of covalent C–F bonds takes place mainly through secondary reactions between gaseous F-containing compounds (HF and/or F 2) in flue gas and carbon active sites produced below 700 °C downstream of coal-fired boilers.