Suppressing the formation of chlorinated aromatics by inhibitor sodium thiocyanate in solid waste incineration process

Suppressing the formation of chlorinated aromatics (Cl-aromatics) by chemical inhibitors is an important measure to reduce dioxin emission from the solid waste incineration plants. In this study, we first investigated the reduction effect of a novel inhibitor sodium thiocyanate (NaSCN) on the emission of dioxins in 2 full-scale solid waste incineration systems. Injection of NaSCN solution into the higher temperature flue gas resulted in about 60% reduction in the concentration of total tetra- to octa-chlorinated dibenzo-p-dioxins and dibenzofurans in stack flue gas. The suppression effect was further verified by a laboratory study on the chlorination of naphthalene over model fly ashes with or without NaSCN addition. By characterizing the reaction products between NaSCN and key catalysts Cu and Fe chlorides, two main suppression mechanisms were proposed: (i) reduction of highly active cupric chloride (CuCl₂) and ferric chloride (FeCl₃) to less active cuprous chloride (CuCl) and ferrous chloride (FeCl₂), (ii) sulfidation of Cu chlorides. The laboratory study indicated that the unreacted NaSCN in the combustion flue gas could be mainly decomposed into Na₂S, C₃N₄, Na₂S₂O₃, NaS₂, Na₂SO₄, CO₂, SO_2, NO₂ and COS. These decomposition products are low toxic or can be effectively removed by the air pollution control devices. CAPSULE: NaSCN suppressed the formation of chlorinated aromatics in combustion flue gas mainly through inducing the reduction of highly active Cu (II) and Fe (III) chlorides.

Application of inert gas-mediated ionization for qualitative screening of chlorinated aromatics in house dust by comprehensive two-dimensional gas chromatography-high-resolution time-of-flight mass spectrometry

The development of highly selective and sensitive analytical methods for the nontarget screening of persistent organic pollutants such as halogenated compounds in environmental samples is a challenging task. Soft ionization mass spectrometry has emerged as a powerful technique for obtaining essential molecular information, and it is expected to reveal compounds that remain hidden with conventional fragmentation techniques such as electron ionization (EI). In this study, a soft ionization method based on electron capture negative ionization using an inert gas was developed for the nontarget screening of chlorinated aromatics in environmental samples and was applied to comprehensive two-dimensional gas chromatography-high-resolution time-of-flight mass spectrometry (GC × GC-HRToFMS). In particular, argon (Ar) and helium (He) were evaluated as inert moderating gases, and were compared against the conventional methane (CH₄). The optimal ionization conditions, including the flow rate and ion source temperature, were investigated based on the molecular ion intensities of highly chlorinated aromatics decachlorobiphenyl and octachlorodibenzofuran. Ar-mediated soft ionization provided the best sensitivity to molecular ions among the three gases at a low flow rate (0.1 mL min^-1) and low ion source temperature, and more selective detection of molecular ions (i.e., less fragmentation) was obtained with the inert gases than with CH₄. This method is also applicable to other chlorinated aromatics such as tetra- to nonachlorobiphenyls, tetra- to heptachlorinated dibenzofurans, pentachlorobenzene, and hexachlorobenzene. To demonstrate the applicability of the proposed method to a wide range of chlorinated aromatics in environmental samples, both Ar-mediated soft ionization and conventional EI were applied to GC × GC-HRToFMS for analysis of a crude extract of house dust. Soft ionization enabled the selective and sensitive detection of molecular ions for minor amounts of chlorinated aromatics, even in complex matrices. Furthermore, the extracted ion chromatograms of halide anions (Cl^- or Br^-) were useful for screening other chlorinated or brominated compounds in the environmental samples. The results suggest that combining information on halide anions obtained by soft ionization and the structural information provided by EI would constitute a powerful approach for the comprehensive identification of chlorinated aromatics.

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.

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 (K_OA) 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 Cl_3-8DDs, Cl_3-7DFs, Cl_5-10Bs and Cl_4-8Ns 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.

Synergetic inhibition of thermochemical formation of chlorinated aromatics by sulfur and nitrogen derived from thiourea: Multielement characterizations

Nitrogen and sulfur (N/S)-containing compounds inhibit the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and furans (PCDFs) in thermal processes. However, few studies have examined the inhibition mechanisms of N/S-containing compounds. In the present study, we focused on thiourea [(NH2)2CS] as such a compound and investigated its inhibition effects and mechanisms. The production of PCDD/Fs, polychlorinated biphenyls (PCBs), and chlorobenzenes (CBzs) were inhibited by >99% in the model fly ash in the presence of 1.0% thiourea after heating at 300 °C. Experimental results using real fly ash series were indicative of the thermal destruction of these chlorinated aromatics by thiourea. Multielement characterization using K-edge X-ray absorption fine structures of copper, chlorine, sulfur, nitrogen, and carbon revealed three possible inhibition paths, namely, (a) sulfidization of the copper catalyst to CuS, Cu2S, and CuSO4; (b) blocking the chlorination of carbon via the reaction of chlorine with N-containing compounds to generate ammonium chloride and other minor compounds; and (c) changing the carbon frame involved in attacking the carbon matrix by sulfur and nitrogen. Thus, thiourea plays a role as a sulfur and nitrogen donor to achieve multiple and synergistic inhibition of chlorinated aromatics. Our results suggest that other N/S-containing inhibitors function based on similar mechanisms.

Mechanism of unintentionally produced persistent organic pollutant formation in iron ore sintering

Effects of temperature, carbon content and copper additive on formation of chlorobenzenes (CBzs) and polychlorinated biphenyls (PCBs) in iron ore sintering were investigated. By heating simulated fly ash (SFA) at a temperature range of 250-500°C, the yield of both CBzs and PCBs presented two peaks of 637ng/g-fly ash at 350°C and 1.5×10(5)ng/g-fly ash at 450°C for CBzs, and 74ng/g-fly ash at 300°C and 53ng/g-fly ash at 500°C. Additionally, in the thermal treatment of real fly ash (RFA), yield of PCBs displayed two peak values at 350°C and 500°C, however, yield of CBzs showed only one peak at 400°C. In the thermal treatment of SFA with a carbon content range of 0-20wt% at 300°C, both CBzs and PCBs obtained the maximum productions of 883ng/g-fly ash for CBzs and 127ng/g-fly ash for PCBs at a 5wt% carbon content. Copper additives also affected chlorinated aromatic formation. The catalytic activity of different copper additives followed the orders: CuCl2∙2H2O>>Cu2O>Cu>CuSO4>CuO for CBzs, and CuCl2∙2H2O>>Cu2O>CuO>Cu>CuSO4 for PCBs.