Characteristics of fly ash from waste-to-energy plants adopting grate-type or circulating fluidized bed incinerators: a comparative study

Although the pressure of waste siege was reduced by the development of waste-to-energy (WTE) plants, every year million tons of toxic municipal solid waste incineration (MSWI) fly ash is produced which is regarded as a hazardous waste. In this paper, ten MSWI fly ash samples were collected to analyze the composition, physical properties, leachability of heavy metals, and toxicity of dioxins to obtain a comprehensive evaluation, which has not been discussed in the previous studies. The analysis and comparison of fly ash is based on the main type of incinerators applied nowadays. Due to differences in the incinerator types, there were significant differences observed in the size distribution, microstructure, element content, and crystalline structure of fly ash, which are related to the heavy metal leachability and dioxin toxicity. The heavy metal leaching toxicity of nine fly ash samples failed to meet the requirements of the landfill regulations. The main heavy metals above an unacceptable level are Pb (4.271 mg/L) in the grate-type fly ash and Cd (1.035 mg/L) in the circulating fluidized bed (CFB) fly ash. Furthermore, the I-TEQ concentrations of dioxins of two CFB fly ash (3.207 and 3.212 ng I-TEQ/g) exceeded the limit value. The main contribution to the dioxin concentrations of the fly ash is from OCDD (18.7-64.3%), and the main contributor to the I-TEQ (international toxicity equivalent quantity) value is 23478-PeCDF (range 33.0 to 43.2%). In addition, correlation between compositional analysis and toxicity indexes was discussed carefully. The findings in this work can provide directions for the disposal and utilization of the fly ash from different incinerators, which can be beneficial for MSWI fly ash management.