The transformation and enrichment of Cd in fine particulate matter during coal combustion: The key roles of Ti-bearing components

Abstract

Fine particulate matter (PM) emitted from coal-fired power plants (CFPPs) has aroused widespread environmental and human health concerns. Cadmium (Cd), as one of the prime toxic air pollutants, which exists in fine PM intensifies its toxicity. In this research, three particle sizes of PM (PM1, PM2.5, and PM10) at the outlet of electrostatic precipitator (ESP), along with feed coal, bottom ash, and ESP ash, were collected from a 300-MW pulverized CFPP. Results showed that Cd was more enriched in the fly ash than in bottom ash. With the PM diameter decreasing, the relative enrichment index (REI) value of Cd increases from 2.30 to 7.84. The concentration of Cd was found to be intimately related to Ti in the PM. Abundant Ti-rich particles were identified in the PM1, having quite different chemical compositions as compared to particles larger than 2.5 μm. The major processes governing the formation of Ti-rich particles included melting of silicate-bound Ti and heterogeneous condensation of gaseous Ti from the decomposition of organic-associated Ti in coal. The transformation behavior of Cd to PM in a coal combustion process was discussed based on its occurrence mode in coal, thermodynamic property, and the possible interactions with ash particle compositions. The retention of Cd (g) by Ti-rich particle were probably dominated by chemisorption at high temperature; at lower temperature, physical adsorption or condensation may be the primary mechanisms for capturing CdCl2 (g).