Modeling the formation of PCDD/F in solid waste incinerators.

Area of Science:

• Environmental Chemistry
• Combustion Science
• Pollutant Formation Modeling

Background:

• Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) are toxic byproducts of waste incineration, particularly from chlorine and metal-rich wastes like MSW and medical waste.
• PCDD/F formation occurs via gas-phase reactions above 600°C and surface reactions on flyash between 400-225°C, involving both de novo synthesis from carbon and precursor pathways.

Purpose of the Study:

• To develop an extended global model for PCDD/F formation during waste incineration.
• To incorporate both precursor and de novo formation mechanisms, as well as gas and solid phase contributions.

Main Methods:

• Extension of an existing empirical de novo formation model to include precursor pathways and gas-phase formation.
• Development of separate rate expressions for PCDD and PCDF formation.
• Validation of the extended model against experimental data from pilot and commercial incinerators.

Main Results:

• The enhanced model accurately describes PCDD/F formation, distinguishing between gas and solid phase contributions to the international toxic equivalent (I-TEQ).
• Homogeneous PCDD formation is primarily influenced by chlorophenols, while PCDF formation depends on chlorophenols and chlorobenzenes.
• Adsorption of gaseous PCDD/F onto flyash during cooling is minimal in incinerator gas ducts.

Conclusions:

• The developed model provides a comprehensive system for predicting PCDD/F emissions from MSW and medical waste incineration.
• Accurate prediction of PCDD/F emissions is possible using the model, given knowledge of ash properties and temperature-time profiles.
• This modeling capability can aid in the control of hazardous emissions from incineration processes.