The role of CuCl on the mechanism of dibenzo-p-dioxin formation from poly-chlorophenol precursors: A computational study

A Reactive Molecular Dynamics Study of Chlorinated Organic Compounds. Part II: A ChemTraYzer Study of Chlorinated Dibenzofuran Formation and Decomposition Processes

In our two-paper series, we first present the development of ReaxFF CHOCl parameters using the recently published ParAMS parametrization tool. In this second part, we update the reactive Molecular Dynamics - Quantum Mechanics coupling scheme ChemTraYzer and combine it with our new ReaxFF parameters from Part I to study formation and decomposition processes of chlorinated dibenzofurans. We introduce a self-learning method for recovering failed transition-state searches that improves the overall ChemTraYzer transition-state search success rate by 10 percentage points to a total of 48 %. With ChemTraYzer, we automatically find and quantify more than 500 reactions using transition state theory and DFT. Among the discovered chlorinated dibenzofuran reactions are numerous reactions that are new to the literature. In three case studies, we discuss the set of reactions that are most relevant to the dibenzofuran literature: (i) bimolecular reactions of the chlorinated-dibenzofuran precursors phenoxy radical and 1,3,5-trichlorobenzene, (ii) dibenzofuran chlorination and pyrolysis, and (iii) oxidation of chlorinated dibenzofurans.

Inhibition of PCDD/Fs in a full-scale hazardous waste incinerator by the quench tower coupled with inhibitors injection

The control of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from the flue gas in hazardous waste incinerators (HWIs) is an intractable problem. To figure out the formation mechanism of PCDD/Fs and reduce the emission, a field study was carried out in a full-scale HWI. Ca(OH)₂ & (NH₄)H₂PO₄ or CH₄N₂S & (NH₄)H₂PO₄ were injected into the quench tower, and the detailed inhibition effect on PCDD/Fs formation by the inhibitors coupled with quench tower was studied. Gas and ash samples were collected to analyze PCDD/Fs. XPS, EDS characterization and Principal component analysis were adopted to further analyze the de novo and precursors synthesis. The PCDD/Fs emissions reduced from 0.135 ng I-TEQ/Nm³ to 0.062 or 0.025 ng I-TEQ/Nm³ after the injection of Ca(OH)₂ & (NH₄)H₂PO₄ or CH₄N₂S & (NH₄)H₂PO₄, respectively. The quench tower was found mainly hindering de novo synthesis by reducing reaction time. CP-route was the dominant formation pathway of PCDD/Fs in quench tower ash. Ca(OH)₂ & (NH₄)H₂PO₄ effectively inhibit precursors synthesis and reduce proportions of organic chlorine from 4.11% to 2.86%. CH₄N₂S & (NH₄)H₂PO₄ show good control effects on both de novo and precursors synthesis by reducing chlorine content and inhibiting metal-catalysts. Sulfur-containing inhibitors can cooperate well with the quench tower to inhibit PCDD/Fs formation and will be effective to reduce dioxins formation in high chlorine flue gas. The results pave the way for further industrial application of inhibition to reduce PCDD/Fs emissions in the HWIs flue gas.

Analysis and discussion on formation and control of dioxins generated from municipal solid waste incineration process

Dioxins are a kind of persistent organic pollutants (POPs) with extremely toxic. Municipal solid waste incineration (MSWI) process has become one of the most dominant discharge sources of dioxins. A comprehensive discussion about dioxin formation mechanisms was reviewed in this paper, and the mechanisms of high-temperature gas-phase reaction and "de novo" synthesis were systematically illustrated in the form of diagrams. What's more, the effects of various influencing factors on the formation of PCDD/Fs were briefly analyzed in the form of a table. We believed that temperature, catalyst, chlorine source, carbon source, oxygen concentration and moisture were necessary factors for PCDD/Fs formation. Control technologies of dioxins in MSWI process were summarized subsequently from three stages: pre-combustion, in-combustion and post-combustion, and a device for synergistic removal of dioxins based on multi-field force coupling and technical routes for controlling dioxin emissions were proposed, so as to provide mechanisms and methods for effectively reducing the emission concentration of dioxins. An introduction was also conducted of dioxin control technologies in municipal solid waste incineration fly ash (MSWI-FA) in this paper, and their mechanisms, advantages, disadvantages and technical maturity were illustrated in the form of diagrams, which can provide theory and reference for in-depth research of follow-up scholars and industrial application of dioxin control technologies. Finally, current research hotspots, challenges and future research directions were proposed.Implications: In this paper, the main research contents and achievements are as follows: With the emphasis placed on the formation mechanism of dioxins and effects of various influencing factors on the formation of PCDD/Fs. The control technology of dioxins in MSWI process is summarized subsequently from three stages: pre-combustion, in-combustion and post-combustion.A device for synergistic removal of dioxins based on multi-field force coupling and technical routes for controlling dioxin emissions are proposed.A systematic review is conducted of the research progress on control technologies of dioxins in MSWI fly ash in the most recent years.The mechanisms, advantages, disadvantages and technical maturity of PCDD/Fs degradation technologies in MSWI fly ash are illustrated in the form of diagrams.Current research hotspots, challenges and future research directions are proposed.

Effect of urea on chlorinated aromatics formation mediated by copper and iron species in combustion flue gas

Urea ((NH₂)₂CO) is widely applied to the reduction of NO_X in modern full-scale solid waste incineration systems, but there is a lack of knowledge about how urea affects the formation and emission of Cl-aromatics. In this study, we investigated the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polychlorinated polychlorinated naphthalenes (PCNs) via electrophilic chlorination and precursor pathway mediated by model fly ashes containing Cu and Fe species with or without urea addition. The results indicated that the addition of urea promoted the direct chlorination of parent aromatics over Cu (Ⅱ) chlorides and the coupling reaction of chlorophenols over Fe species, while suppressed the catalytic chlorination of parent aromatics over Fe (Ⅲ) chlorides and the coupling reaction of chlorophenols over Cu species. The diverse effects should be mainly attributed to the formation of complex salts containing NH₃ and NH₄⁺. The formation of complex salts of Fe chlorides and NH₄Cl could hinder the oxidization of Fe chlorides, and thus maintain the high activity of Fe species for catalyzing the coupling reaction of chlorophenols. The formation of complex salts of Cu (Ⅱ) chloride and NH₃ could prevent the chemical sorption of phenoxyl groups, and thus suppress the coupling reaction of chlorophenols. NH₃ released from the thermal decomposition of urea could not only react with Cl₂ to suppress the catalytic chlorination of aromatics, but also neutralize HCl to accelerate the direct chlorination of aromatics. In general, urea should act as inhibitor for suppressing the formation of Cl-aromatics in solid waste incineration systems.