PCDD/Fs from a large-scale municipal solid waste incinerator under transient operations: Insight formation pathways and optimal reduction strategies

Abstract

Polychlorinated dibenzo-p-dioxin and dibenzofurans (PCDD/Fs) emissions from the transient operation of municipal solid waste incinerators can reach up to 690 ng/Nm3, as measured in this study. To control the extreme emissions to meet the national standard, the formation pathways of PCDD/F were investigated under transient operations (cold start-up, hot start-up, and after start-up) and normal operations. Compared with normal operations, transient operations facilitate the formation of low-chlorinated congeners rather than highly chlorinated congeners. Statistically, for transient operations, strong correlations were found among tetrachlorodibenzo-p-dioxin or tetrachlorodibenzofuran isomers. An abundant carbon matrix is an important carbon source for PCDD formation. Moreover, the comprehensive study revealed that the oxidation of deposited soot is the main source of PCDD/F emissions, relative to de novo synthesis, chlorobenzene-route synthesis, chlorophenol-route synthesis, and chlorination of dibenzo-p-dioxin/dibenzofuran. In addition, the optimal start-up procedure was constructed by analyzing main formation pathways and operating conditions. The relationship between the international toxic quantity (I-TEQ) values (CI-TEQ) and the reaction time can be assigned as CI-TEQ = 11.72t−0.65 (R2 = 0.97) for the circulating fluidized bed. The relationship of CI-TEQ = 4.61t−0.59 (R2 = 0.85) was also proven on the dataset with a grate furnace. Then, the optimal feeding rate of activated carbon was further proposed by the relationship between the reaction time and I-TEQ, and the semi-empirical equation for PCDD/Fs adsorption. Finally, the PCDD/Fs emissions can be reduced to 0.1 ng I-TEQ/Nm3 under transient operations according to the time since start-up.