Spraying polyacrylamide solution to improve the removal of particle-phase dioxins by bag filter in a full-scale municipal solid waste incineration system.

A field experiment was conducted in a modern municipal solid waste (MSW) incineration power plant to explore the feasibility of using chemical agglomeration agent anionic polyacrylamide (PAM) to reduce the atmospheric emission of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Spraying PAM solution into the cooling tower caused an obvious decrease in the volume fraction of ultrafine and fine particles with diameter of 0.3-30 µm in BF fly ash, and a significant reduction in dust content in stack gas. The increased agglomeration of particles promoted the removal of particle-phase PCDD/Fs by BF, and thus resulted in a lower atmospheric emission of PCDD/Fs. The calculated removal efficiency of PCDD/Fs by BF was almost positively proportional to the concentration of PAM solution, while inversely proportional to the average content of dusk in stack gas. Compared with the control treatment, the spraying of 0.1 g/L PAM solution enhanced the removal efficiency of total tetra-to octa-CDD/Fs (∑PCDD/Fs) from 93.8% to 97.8% by BF, and resulted in a decrease of 47.0% in the concentration of international toxicity equivalent (I-TEQ) in stack gas. During the experiment of 2 d, the spraying of PAM solution did not induce a significant change in the differential pressure of BF, and did not essentially affect the partitioning behaviors of PCDD/F homologues between flue gas and BF fly ash. In view of technical safety and low cost, PAM application is recommended for reducing the atmospheric emission of PCDD/Fs from MSW incineration system.

Electrophilic chlorination of dibenzo-p-dioxin and dibenzofuran over composite copper and iron chlorides and oxides in combustion flue gas.

Dibenzo-p-dioxin (DD) and dibenzofuran (DF) chlorination mediated by Cu and Fe chlorides can make a direct contribution to the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in combustion flue gas. In this study, a kind of composite Cu and Fe chlorides and oxides (CuFe9O9.5Cl10) was prepared by impregnating oxides with HCl solution to imitate the coexistence status of Cu and Fe species in combustion flue gas. Composite CuFe9O9.5Cl10 was active in promoting the electrophilic chlorination of DD/DF at 150-300 °C, with the highest activity at 200 °C. DD/DF chlorination could occur under inert atmosphere, and 5% O2 atmosphere was most favorable for DD/DF chlorination. Electrophilic chlorination of DD/DF primarily favored at 2,3,7,8 positions. Hybridization of Cu and Fe chlorides and oxides not only decreased the starting temperature and activation energy of DD/DF chlorination, but also induced a synergistic effect for accelerating the chlorination of DD/DF. The measured activities of composite CuFe9O9.5Cl10 for promoting the chlorination of DD/DF were near to those of composite Cu chloride and oxide (CuO0.2Cl1.6), whereas 2 orders of magnitude higher than those of composite Fe chloride and oxide (FeO0.3Cl2.4). Comparison of PCDD/F congener distribution patterns indicated that DD/DF chlorination should be a main source of Cl1-3DFs and Cl1-2DDs in combustion flue gases.

Partitioning and removal behaviors of PCDD/Fs, PCBs and PCNs in a modern municipal solid waste incineration system.

An extensive evaluation on a modern full-scale municipal solid waste incineration system was conducted for characterizing the distribution of highly toxic chlorinated aromatics, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs) and polychlorinated naphthalenes (PCNs), and their corresponding mass fluxes in post combustion zone. It was found that the flue gas/fly ash partitioning behaviors of chlorinated aromatics could be essentially described by their octanol-air partition coefficients (KOA) and strongly affected by the flue gas temperature. Above 93% of chlorinated aromatics formed in boiler section was partitioned into the flue gas and transported into the subsequent flue gas cleaning system, in which above 92% of Cl3-8DDs, Cl3-7DFs, Cl5-10Bs and Cl4-8Ns in flue gas was removed by the discharge of fly ash. The results of mass flux calculation indicated that the memory effect in flue gas cleaning system remarkably elevated the emission levels of chlorinated aromatics, especially the less chlorinated ones. The memory effect should mainly result from the direct chlorination mechanism mediated by the deposited particles. In addition, activated carbon injection could cause an obvious increase in PCBs emission. The obtained results provided some important implications for further reducing the emission of highly toxic chlorinated aromatics.