PCDD/F catalysis by metal chlorides and oxides

Incineration-source fingerprints and emission spectrums of dioxins with diagnostic application

Despite increasing waste-to-energy (WtE) capacities, there remain deficiencies in comprehension of 136 kinds of tetra- through octa-chlorinated dibenzo-p-dioxin and dibenzofurans (136 PCDD/Fs) originating from incineration sources. Samples from twenty typical WtE plants, encompassing coal-fired power plants (CPP), grate incinerators (GI), fluidized bed incinerators (FBI), and rotary kilns (RK), yielded extensive PCDD/F datasets. Research was conducted on fingerprint mapping, formation pathways, emission profiles, and diagnostic analysis of PCDD/Fs in WtE plants. Fingerprints revealed a prevalence of TCDF, followed by PeCDF, while CPP and RK respectively generated more PCDD and HxCDD. De novo synthesis was the predominant formation pathway except one plant, where CP-route dominated. DD/DF chlorination also facilitated PCDD/F formation, showing general trends of FBI > GI > CPP > RK. The PCDD/F emission intensities emitted in air pollution control system inlet (APCSI) and outlet (APCSO) followed the statistical sequence of RK > FBI > GI > CPP, with the average I-TEQ concentrations in APCSO reaching 0.18, 0.08, 0.11, and 0.04 ng I-TEQ·Nm-3. Emission spectrum were accordingly formed. Four clusters were segmented for diagnosis analysis, where PCDD/Fs in GI and FBI were similar, grouped as a single cluster. PCDD/Fs in CPP and RK demonstrated distinctive features in TCDD, HxCDD, and HxCDF. The WtE plants exceeding the limit value tended to generate and retain fewer TCDD and TCDF yet had higher fractions of HxCDD and HxCDF. The failure of APCS coupled with the intrinsic source strength of PCDD/Fs directly led to exceedance, highlighting safe operational practices. This study motivated source tracing and precise evaluation of 136 PCDD/Fs based on the revealed fingerprint profiles for WtE processes.

Influence of different loads on PCDD/F removal by SCR during municipal solid waste incineration

This study was conducted on a full-scale (500 t/d) municipal solid waste incinerator (MSWI), investigating the influence of different loads on the emission of polychlorinated dibenzodioxins and polychlorinated dibenzofurans (PCDD/Fs) and their removal by selective catalytic reduction (SCR) system. The total concentration of PCDD/Fs at the SCR inlet under 100% load was higher than that under 80% load. The changing loads caused different distribution characteristics of PCDD/Fs at the SCR inlet, and the dominant congeners changed from high-chlorinated PCDDs (80% load) to low-chlorinated PCDFs (100% load). Moreover, the increased load enhanced the removal efficiency of PCDD/Fs by SCR from 17.3% to 64.2%, which was influenced by the inlet PCDD/F distribution and the moisture content. The high-chlorinated PCDD/Fs with the more stable structure were more difficult to be deteriorated and the high moisture content can weaken the catalytic activity of SCR catalysts. Correlation analysis was used to study the relationship between major air pollutants and PCDD/F emissions. The results showed that HCl positively correlated with PCDD/F emission concentrations, while NOx and SO2 negatively correlated. The results of this study can provide a reference for MSWI to operate properly under variable loads.

Formation pathways, gas-solid partitioning, and reaction kinetics of PCDD/Fs associated with baghouse filters operated at high temperatures: A case study

The application of the 3T method during combustion (i.e., a Temperature > 850 °C, a residence Time > 2 s, and sufficient Turbulence) can lead to elevated operating temperature in the baghouse filter for the municipal solid waste incineration (MSWI) systems without sufficient heat exchange capacity, which is potentially detrimental to the emission control of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Herein, a field study focusing on the distribution and variation of PCDD/Fs in gaseous and solid phases in a baghouse filter with high operating temperature (225-230 °C) was carried out. The concentration of PCDD/Fs in gases at the outlet of the baghouse filter was around 1 order of magnitude higher than that in inlet gases (i.e., noticeable memory effect of PCDD/Fs), because of the significant PCDD/Fs formation in filter fly ash (primarily contributed by the precursor pathway) followed by PCDD/Fs desorption. In addition, the mechanisms and factors resulting in the memory effect of PCDD/Fs were identified based on a laboratory study that carefully investigated the formation and desorption of PCDD/Fs at potential operating temperatures of baghouse filters (i.e., 180, 200, and 225 °C). The temperature was identified as the key factor inducing the memory effect of PCDD/Fs, because: i) PCDD/Fs memory effect was not observed for baghouse filters with low operating temperatures of ~150 °C in previous studies; ii) both the formation and desorption of PCDD/Fs were noticeably favored by rising temperature from 180 to 225 °C; iii) increasing temperature appeared to facilitate the transformation from inorganic Cl to organic Cl and the conversion from aliphatic carbon to aromatic carbon or unsaturated hydrocarbons, both of which were favorable to PCDD/Fs formation; and iv) the release rate of PCDD/Fs from fly ash was exponentially dependent on temperature based on the modeling results of reaction kinetics.

Assessment of PCDD/Fs Emission during Industrial-Organic-Solid-Waste Incineration Process in a Fluidized-Bed Incinerator

This study was conducted in a fluidized-bed incineration plant, evaluating the formation, emission and flux of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from industrial-organic-solid-waste (IW) incineration. The results revealed that both the total (or I-TEQ) concentrations of toxic and 136 total PCDD/Fs in flue gas (FG), fly ash (FA) and bottom ash (BA)were ramped up to a higher level than those during municipal-solid-waste (MSW) incineration. A possible explanation was the chlorine (Cl) content of IW. However, the emitted PCDD/Fs in FG (FA/BA) still fulfilled the criteria. Subsequently, similar distribution patterns of PCDD/F isomers were observed in subsystems, indicating a unified formation-pathway. De novo synthesis was detected as the dominant formation-pathway of PCDD/Fs, while high-temperature and precursor syntheses were excluded. DD/DF chlorination formed PCDD/Fs to some extent. Furthermore, the mass flow chart indicated that PCDD/Fs output in primary FG was significantly strengthened (>1000 times) by de novo synthesis, from 1.25 μg I-TEQ/h to 1.67 mg I-TEQ/h. With effective cleaning by APCS, 99.6% of PCDD/Fs in FG were purified. PCDD/Fs in the gas phase were finally emitted at a discharge rate of 7.25 μg I-TEQ/h. However, accumulated FA took most PCDD/Fs into the environment (>99%), reaching 3.56 mg I-TEQ/h.

Influence of different kinds of incinerators on PCDD/Fs: a case study of emission and formation pathway

Few studies focused on the emission of polychlorinated-ρ-dibenzodioxins and dibenzofurans (PCDD/F) from different kinds of waste incinerators. This study was conducted in a full-scale MSW incineration plant to investigate the influence of different incinerator types on PCDD/F. Experimental results indicated that the 2,3,7,8-PCDD/F concentration in the inlet gas of the air pollution control system (APCS) in the studied fluidized bed was higher (2.03 ng I-TEQ/Nm³) than that of the grate (0.77 ng I-TEQ/Nm³). But gas in the outlet of APCS from both incinerators had an approximate concentration, lower than the Chinese emission limit of 0.1 ng I-TEQ/Nm³. Similar distribution patterns were observed for 2,3,7,8-PCDD/Fs, as well as 136 PCDD/F congeners. Specifically, OCDD and 1,2,3,4,6,7,8-HpCDD were major isomer constituents for 2,3,7,8-PCDD/F isomers. In terms of formation pathways, a similar formation mechanism was observed based on fingerprint characteristics of 136 PCDD/F congeners. De novo synthesis was the dominating formation pathway for both incinerators. Meanwhile, DD/DF chlorination was another contributor to PCDD/F formation, which in the fluidized bed was higher. In addition, little correlation (0.009 < R² < 0.533) between conventional pollutants (HCl, CO, PM) and PCDD/Fs was found, suggesting little high-temperature synthesis observed and verifying the dominance of de novo synthesis.

Disposal technology and new progress for dioxins and heavy metals in fly ash from municipal solid waste incineration: A critical review

Incineration has gradually become the most effective way to deal with MSW due to its obvious volume reduction and weight reduction effects. However, since heavy metals and organic pollutants carried by municipal solid waste incinerator fly ash (MSWI FA) pose a serious threat to the ecological environment and human health, they need to be handled carefully. In this study, the current status of MSWI FA disposal was first reviewed, and the harmless and resourceful disposal technologies of heavy metals and organic pollutants in MSWI FA are summarized as well. A summary of the advantages and disadvantages of each technology, including sintering, melting/vitrification, hydrothermal treatment, mechanochemistry, solidification/stabilization of MSWI FA, is compared. Finally, the research work that needs to be strengthened in the future (such as codisposal of multiple wastes, long-term stability research of disposal products, etc.) was proposed. Through comprehensive analysis, some reasonable and feasible suggestions were provided for the effective and safe disposal of MSWI FA in the future.

Variation in the formation characteristics of PBDD/F, brominated PAH, and PBDE congeners along the secondary copper smelting processes

Simultaneous determination of 58 congeners of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs), brominated polycyclic aromatic hydrocarbons (Br-PAHs), and polybrominated diphenyl ethers (PBDEs) from multiple stages of industrial-scale secondary copper smelting plants was conducted with the aim of understanding their variations and control. In addition to the historical manufacture of PBDEs as brominated flame retardants, this study confirmed that PBDEs can be unintentionally produced and released from the secondary copper industry. The average mass emission factors of PBDD/Fs, PBDEs, and Br-PAHs from different sources were 10.0, 5.21 × 10³, and 7.24 × 10³ μg t^-1, respectively. Therefore, the emission of brominated persistent organic pollutants (POPs) in the secondary copper industry should be of concern. The concentration of brominated POPs increased from the gas cooling stage to stack outlet due to the possible "memory effect" and the regenerated POPs were mainly low-brominated homologs. A comparison of brominated POPs with corresponding chlorinated analogs in the same process indicated that the formation pathway of Br-PAHs was consistent with that of chlorinated PAHs. However, unlike chlorinated dioxins and furans, PBDD/Fs can also be formed from PBDEs as precursors, leading to obvious increases in highly brominated furans. Therefore, inhibiting the unintentional formation of PBDEs is important for controlling brominated POPs emissions.

Abatement of chlorobenzene by plasma catalysis: Parameters optimization through response surface methodology (RSM), degradation mechanism and PCDD/Fs formation

Dielectric barrier discharge coupled with 10 wt% Co/γ-Al₂O₃ catalyst was developed to degrade chlorobenzene in this study. The effects of experimental parameters including applied voltage, flow rate, initial chlorobenzene concentration, and their interactions on the chlorobenzene degradation performance were investigated by the response surface methodology integrated with a central composite design. Results indicated that applied voltage was the most significant parameter affecting the mineralization rate and the concentration of ozone generated, while energy yield was mainly determined by initial chlorobenzene concentration. As a key precursor of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorophenols were found during the identification of the intermediates produced during chlorobenzene degradation through GC-MS. Furthermore, HRGC-HRMS was used to detect the remaining byproducts on the catalyst surface after 3 and 10 h discharge time, and three types of PCDD/Fs (2,3,7,8-TCDF, 1,2,3,4,6,7,8-HCDF and OCDD) were detected after 10 h of discharge. The degradation mechanism of chlorobenzene was analyzed based on these detected intermediates, and the possible formation mechanisms of the three PCDD/Fs were proposed for the first time in plasma catalytic degradation of chlorobenzene.

Dioxin Formation in Biomass Gasification: A Review

The amount of PCDD/F emissions produced by gasification operations is often within standard limits set by national and international laws (<0.1 ng TEQ/Nm3). However, a recent assessment of the literature indicates that gasification cannot always reduce PCDD/Fs emissions to acceptable levels, and thus a common belief on the replacement of incineration with gasification in order to reduce PCDD/Fs emissions seems overly simplistic. A review that summarizes the evidence on when gasification would likely result in environmentally benign emissions with PCDD/F below legal limits, and when not, would be of scientific and practical interest. Moreover, there are no reviews on dioxin formation in gasification. This review discusses the available data on the levels of dioxins formed by gasifying different waste streams, such as municipal solid wastes, plastics, wood waste, animal manure, and sewage sludge, from the existing experimental work. The PCDD/Fs formation in gasification and the operational parameters that can be controlled during the process to minimize PCDD/Fs formation are reviewed.

PCDD/Fs, dioxin-like, and non-dioxin like PCBs in the sediments of high Arctic fjords, Svalbard

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) are highly toxic organic compounds, and very few studies on their presence in polar environments have been conducted. This study assessed the concentration and distribution of PCDD/Fs, dioxin-like polychlorinated biphenyls (DL-PCBs), and non-dioxin-like polychlorinated biphenyls in selected fjords of the Svalbard archipelago in Norway. The ∑PCDD/Fs observed for Raudfjorden, Smeerenburgfjorden, Magdalenefjorden, and Kongsfjorden were 22.80 pg/g, 25.65 pg/g, 18.27 pg/g, 33.50 pg/g, and 21.69 pg/g, respectively. The WHO's toxic equivalents values of both ∑PCDD/Fs and ∑DL-PCBs were comparatively higher than those reported in other polar regions. Of the four fjords studied, the sediments from Kongsfjorden exhibited the presence of the most toxic materials, including PCB-126 and PCB-169, of DL-PCBs. More than 80% of the total analysed PCDD/Fs were comprised of highly chlorinated congeners (hexa-to-octa forms). More studies are required to understand the destination and transport of these hazardous pollutants in high Arctic sediments.

Effect of calcium-based sorbents on the reduction of chlorinated contaminants during municipal solid waste thermal treatment

Chlorinated contaminants are a cause of significant concern in the development of municipal solid waste (MSW) thermal treatment techniques. This study investigates the efficacy of two calcium (Ca)-based in-furnace additives, calcium oxide (CaO), and calcined dolomite (CD), at reducing the levels of chlorinated contaminants during MSW thermal treatment. The results reveal that Ca-based additives could effectively reduce the chlorine (Cl) content by more than 76.8% and 37.3% in the gas and tar phases, respectively. The total concentration and the international total equivalent (I-TEQ) value of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzo-p-furans (PCDD/Fs) were significantly higher under the incineration condition than pyrolysis and gasification conditions. Adding CaO could reduce the total concentration and the I-TEQ value of PCDD/Fs by more than 43.4% and 36.7%, respectively. The reduction effect on PCDD/Fs was more significant in the gaseous phase and the tar phase than the solid phase. CD was more effective than CaO at reducing the chlorinated contaminants, including hydrogen chloride, Cl in the tar phase, and PCDD/Fs. Thus, adding Ca-based sorbents in the furnace during MSW pyrolysis and gasification can effectively reduce PCDD/Fs generation. Based on the experimental results, the mechanism of Ca-based sorbents on the high-temperature homogeneous reaction of PCDD/Fs formation was analysed.

Evolution of fusion and PCDD/F-signatures of boiler ash from a mechanical grate municipal solid waste incinerator

Boiler ash formed at different temperature ranges in a typical mechanical grate incinerator is collected and systemically studied, with the aim of providing a reference for ash disposal and revealing the formation routes and distribution of polychlorinated ρ-dibenzodioxins and dibenzofurans (PCDD/Fs). Key physical and chemical properties are carefully analyzed, including chemical component, ash fusion temperatures (AFTs), crystalline phases, chemical species, and PCDD/Fs. Several fouling and slagging indices are introduced and their relationships with AFT are revealed. The fouling index (F_u) and a slagging index (R_b/a×Na) are well fitted with ash flow temperatures, with correlation coefficient (R²) of 0.82 and 0.82, respectively; these could be better potential indices for disposal applications of municipal solid waste incineration fly ash. CC/C-C/C-H (69.25-80.93%) and inorganic chlorine (94.23-98.68%) are the dominant carbon and chlorine species, respectively. The increasing AFT is mainly attributed to the changing components, the increasing proportions of crystalline CaSO₄, NaCl and KCl and the decreasing crystallinity and content of SiO₂. Twice as much PCDD/Fs is generated by the low-temperature heterogeneous reaction (6.71-19.22 ng/g) than by the high-temperature homogeneous reaction (0.59-6.71 ng/g). The proportions of highly chlorinated homologues increase and gradually become the main component. Principal component analysis reveals that PCDD/Fs is positively correlated with Cl, Cu, Pb, Sn, Sb, Zn and CC/C-C/C-H but negatively correlated with less volatile elements, e.g., Ni, Mn, Al, Ti, Si, and Cr. These results can benefit further research on boiler ash disposal and PCDD/F formation routes in the post-combustion area of incinerators.

Formation pathways of polychlorinated dibenzo-p-dioxins and dibenzofurans from burning simulated PVC-coated cable wires

Open burning of PVC-coated cables is a major source of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F). In the present study, the formation characteristics of PCDD/F from burning of PVC-based samples with and without metallic copper were evaluated over the dioxin formation temperature window (200-500 °C). This temperature range also inevitably occurs under open burning conditions. The PCDD/F yield from PVC added with Cu increased by factors of 1390 (300 °C), 65 (400 °C) and 17 (500 °C) compared with that from PVC alone, confirming the stimulatory effect of metallic Cu on PCDD/F production. For the first time, a relatively complete isomer-specific analysis is established for PVC acting as source of PCDD/F. Formation pathways of PCDD/F and the reaction mechanisms were investigated using a combined analysis of PCDD/F isomer signatures, thermogravimetric results and Cl K-edge X-ray absorption spectra. De novo synthesis is the major pathway leading to massive production of PCDD/F. Copper extends the temperature range for the concurrence of de(hydro)chlorination of PVC with cross-linking and aromatisation of polyenes and then stimulates cracking of the chlorine-embedded carbon network. Together, these processes contribute to the strongly enhanced formation of PCDD/F via de novo synthesis.

Inhibition of chlorobenzenes formation by calcium oxide during solid waste incineration

Solid waste incineration is a major emission source of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). The injection of N- and S-containing compounds is an effective way to suppress the formation of PCDD/Fs, but this approach is still shortcoming because additional pollutants such as NH₃ and SO_x are emitted. To avoid the secondary pollutions, a de novo synthesis inhibition mechanism in the presence of CaO was postulated to transform CuCl₂ to CuO and deplete Cl₂ and HCl. Chlorobenzenes (CBzs), which are indicators and precursors of PCDD/Fs, were adopted to prove the inhibitory effect of CaO at 400 °C, using both simulated synthetic ash and extracted air pollution control residues. As the molar ratio of CaO to CuCl₂ exceeded 3, the residual carbon increased, and the inhibition efficiency of CBzs exceeded 93 %. This performance is superior to the corresponding performance of NH₄H₂PO₄, which has been proved to be a potential inhibitor. Furthermore, with CaO, chlorides remained in the solid phase and had inactive catalytic performance; and they were the major products rather than HCl, Cl₂ and Cu₂OCl₂. The addition of CaO during waste incineration therefore can facilitate the abatement of PCDD/Fs contamination and reduce the emissions of acid gas simultaneously.

Insights into PCDD/Fs and PAHs in Biomass Boilers Envisaging Risks of Ash Use as Fertilizers

Since ashes are a possible source of Persistent Organic Pollutants (POPs) contamination, their application in soils must be subject to more study and control. In this scope, feed residual forest biomasses and biomass ashes, collected along one year in four biomass power stations, were characterized mainly for their polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) and Polycyclic Aromatic Hydrocarbons (PAHs) contents. The biomasses present concerning levels of Cl (0.04–0.28%) that may lead to PCDD/Fs formation. The biomasses also contain OCDD (29–260 ng/kg) and 1,2,3,4,6,7,8-HpCDD (35 ng/kg) that may contribute to increased Toxic Equivalents (TEQs) of ashes, possibly involving dechlorination and ash enrichment mechanisms. While the WHO2005-TEQs in bottom ashes (14–20 ng TEQ/kg) reaches the proposed limit (20 ng TEQ/kg) for ash use as fertilizers, in fly ashes (35–1139 ng TEQ/kg) the limit is exceeded. PAHs are below 0.02 mg/kg in bottom ashes and 1.5–2.5 mg/kg in fly ashes, complying with the proposed limit of 6 mg/kg. As bottom and fly ash streams may contain different ash flows, a clear definition of ash mixes is required. Correlations between unburned carbon (C), PAHs and PCDD/Fs were not found, which highlights the need for compulsory PCDD/Fs analysis in ashes, independently of their origin, burnout degree or levels of other contaminants. A sensitivity analysis was performed to evaluate the impact of handling non-detected values, which showed more impact for TEQs values close to the proposed regulatory limit of PCDD/Fs. These findings highlight the need to define reporting protocols of analytical results for risk assessments and conformity evaluation.

Decomposition and reformation pathways of PCDD/Fs during thermal treatment of municipal solid waste incineration fly ash

Thermal treatment of municipal solid waste incineration (MSWI) fly ash (FA) allows heavy metals solidification, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) decomposition, and environmentally stable materials production, but lacking advanced insights into PCDD/Fs dramatically limits its development. In this study, the decomposition and reformation of PCDD/Fs during thermal treatment of two typical fly ashes (loading 0.294 and 0.594 ng I-TEQ/g PCDD/Fs, respectively) are systematically investigated, under conditions of three heating temperature (500, 800, and 1100 °C) and two atmospheres (oxidative and inert). Over 95 % of PCDD/Fs in FA are effectively decomposed for all tests mainly via cyclic skeleton destruction accompanied by dechlorination, but reformation predominantly through de novo synthesis in off-gases significantly reduces the overall elimination efficiency. Moreover, both de novo synthesis and chlorination are enhanced as temperature increase promoting migrations of catalytic metals and chlorine, yet are weakened at the absence of oxygen, both of which are revealed by PCDD/F-signatures evolution. Catalytic metal is identified as the most critical factor accounting for PCDD/Fs reformation, which is also evidenced by kinetic models of de novo synthesis. Finally, the decomposition and reformation pathways during thermal treatment of fly ashes are proposed. The results pave the way for controlling PCDD/Fs reformation and improving the thermal treatment of fly ashes.

Thermochemical formation of dioxins promoted by chromium chloride: In situ Cr- and Cl-XAFS analysis

Chromium is commonly found in the flue gases and ashes of Municipal Solid Waste Incineration. It has been reported as an active catalyst for the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) during de novo tests, yet its specific mode of action has remained unclear. This study aims to identify the effects of chromium chloride on the formation of PCDD/F and other chloro-aromatics and to elucidate the underlying reaction mechanisms. A series of de novo tests, conducted over a wide range of temperature (from 250 to 550 °C) and for four different oxygen contents (0, 5, 10, 20 %), confirmed the promoting effect of CrCl₃ on the PCDD/F formation. In situ X-ray Absorption Fine Structure (XAFS) spectroscopy was applied to investigate the behavior of CrCl₃ during heating, describing the entire picture of CrCl₃-promoted formation pathways of dioxins. The effect of oxygen was studied by measuring XAFS spectra on samples heated at different oxygen concentrations. According to these spectra, chromium compounds play two key roles during dioxins formation: (a) chlorinating carbon, using chlorine derived from conversion of CrCl₃ into Cr₂O₃, and further oxidation to Cr(VI), and (b) facilitating oxidative destruction of the carbon matrix, while reducing Cr(VI) to Cr₂O₃.

Emissions of dioxins and furans during steam gasification of Automotive Shredder residue; experiences from the Chalmers 2-4-MW indirect gasifier

Emissions of dioxins and furans during the gasification of Automotive Shredder Residue (ASR) were investigated. The experimental work was carried out in a Dual Fluidized Bed (DFB) system, which consists of a 2-4-MW_th gasifier that is fluidized with steam, and an interconnected fluidized bed combustor that is fluidized with air. Two different ASR fractions with higher and lower contents of plastic were tested. Measurements were carried out in the flue gas stream exiting the combustion side of the DFB, as well as in the raw gas stream exiting the gasifier side. A calcium (lime) coat was applied to the flue gas filter to ensure compliance with the emissions regulations regarding the retention of HCl and dioxins. The results showed lower emissions of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/PCDF) in the flue gas when the raw gas derived from the ASR gasification was combusted, as compared to the direct combustion of ASR. The level of polychlorinated compounds in the flue gas before the lime-coated filter was 0.11 ng/m³_{N dry gas} (I-TEQ) when gasification was used as a pre-step, as compared to 0.27 ng/m³_{N dry gas} (I-TEQ) when the ASR was directly combusted. The raw gas produced by gasification contained very low levels of PCDD/PCDF, whereby the toxicity per kg of ASR was 0.17 ng/kg_ASR, as compared to 3.44 ng/kg_ASR after passage through the combustion and cooling sections and 0.34 ng/kg_ASR at the outlet after the lime-coated filter. A higher content of plastic in the ASR led to an increase in the levels of dioxins and furans in the raw gas, with the highest yield seen for highly chlorinated compounds, while higher temperature in the gasifier is shown to be beneficial in reducing dioxin formation.

The bibliometric analysis and review of dioxin in waste incineration and steel sintering

Facing the common treatment problems of dioxin whose major sources come from waste incineration and steel sintering, we handled a massive literature dataset from the Web of Science database and analyzed the research hotspot and development trend in this field in the past 40 years by bibliometric method. The result indicates that the field of dioxins generated from waste incineration and steel sintering has entered a stage of rapid development since 1990. China occupies a leading position in terms of comprehensive strength with the largest publications output as well as a greater influence in recent years. The most productive institutions and journals are Zhejiang University and Chemosphere, respectively. In addition, the most commonly used keywords in statistical analysis are "fly ash," "emission control," "risk assessment," "congener profile," "formation mechanisms," "sources," "catalysis," and "inhibition," which reflects the current main research direction in this field. The similarities and differences of dioxins generated in waste incineration and steel sintering are reviewed in this paper, which will provide guidance for the future research.

Characterization of PBDD/F emissions from simulated polystyrene insulation foam via lab-scale programmed thermal treatment testing

Accidental fires and open combustion are regarded as major potential contributors to the environmental release of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs). The characterization of PBDD/Fs emitted from thermal treatment of simulated polystyrene (PS) foam is reported in this study. PS, hexabromocyclododecane (HBCDD) and metals (or metallic compounds) composed the test samples, which imitated real extruded (XPS) and expanded (EPS) polystyrene thermal insulation foams. Test samples were subjected to thermal treatments under different experimental conditions. This study shows that the temperature, metal (metallic compound) content, and type of atmosphere are the key factors in the formation of congeners and PBDD/Fs during thermal processes. The total yield of polybrominated dibenzofurans (PBDFs) was greater than that of the polybrominated dibenzo-p-dioxins (PBDDs) during the test, and 1,2,3,7,8-PeBDF and 2,3,7,8-TBDF were the predominant congeners emitted during the thermal treatment experiments.

Formation pathways of PCDD/Fs during the Co-combustion of municipal solid waste and coal

The co-combustion of simulated municipal solid waste (SMSW) and the coal in a drop-tube furnace is studied in five test cases. The concentration and signature evolution of polychlorinated dibenzo-p-dioxins (PCDD) and -furans (PCDF) in both flue gases and fly ashes are monitored at the level of individual congeners, using statistical methods. Special attention is paid to chlorophenol (CP)-route congeners, 2,3,7,8-substitution, and 1,9-substitution, to reveal the formation pathways of PCDD/Fs and the interaction between SMSW and coal. It is identified that the increase of SMSW proportion alters the major formation pathways from CP-route to chlorophenols/chlorobenzenes condensation and de novo synthesis. The coal-induced carbon enhances the adsorption capacity of fly ash particles for PCDD/Fs, yet facilitates the generation of carbon matrixes and polycyclic aromatic hydrocarbons, both of which will significantly boost the de novo synthesis with the increase of SMSW-induced chlorine and catalytic metals. Further investigations about restricting the participation of chlorine in PCDD/Fs synthesis are essential to increase the treatment capacity of MSW and to reduce the PCDD/Fs emission.

Evolution of PCDD/F-signatures during mechanochemical degradation in municipal solid waste incineration filter ash

Mechanochemical degradation (MCD) is employed for the dechlorination of polychlorinated dibenzo-p-dioxins (PCDD) and -furans (PCDF) in filter ashes from municipal solid waste incinerators, respectively with the assist of six additive systems. The evolution of PCDD/F-signatures in all eleven samples are systematically monitored and studied at the level of individual congeners, and special attention is paid to CP-route congeners, 2,3,7,8-substitution, 1,9-substitution, and 4,6-PCDF. The PCDD/F-isomers distribution follows an analogous pattern, indicating the similar acting mechanism for all additives: additives transfer electrons to attack the CCl bond and then expulse chlorine. MC dechlorination is not favored for the chlorine on β-position (2,3,7,8-position). The oxygen with stronger electronegativity in PCDD/Fs negatively influences CCl bond to accept donated electrons, hindering the removal of chlorine on 1,9-position for PCDD, and chlroine on 4,6-position for PCDF. Finally, two fair dechlorination pathways for PCDD and PCDF are respectively proposed based on the detailed analysis of CP-route congeners. The evolution of PCDD-signatures is clear, yet obscure for PCDF-signatures, which still requires further investigations.

Characterising boiler ash from a circulating fluidised bed municipal solid waste incinerator and distribution of PCDD/F and PCB

In this study, ash samples were collected from five locations situated in the boiler of a circulating fluidised bed municipal solid waste incinerator (high- and low-temperature superheater, evaporator tubes and upper and lower economiser). These samples represent a huge range of flue gas temperatures and were characterised for their particle size distribution, surface characteristics, elemental composition, chemical forms of carbon and chlorine and distribution of polychlorinated dibenzo-p-dioxins (PCDD), dibenzofurans (PCDF) and biphenyls (PCB). Enrichment of chlorine, one of the main elements of organochlorinated pollutants, and copper, zinc and lead, major catalytic metals for dioxin-like compounds, was observed in lower-temperature ash deposits. The speciation of carbon and chlorine on ash surfaces was established, showing a positive correlation between organic chlorine and oxygen-containing carbon functional groups. The load of PCDD/F and PCB (especially dioxin-like PCB) tends to rise rapidly with falling temperature of flue gas, reaching their highest value in economiser ashes. The formation of PCDD/F congeners through the chlorophenol precursor route apparently was enhanced downstream the boiler. Principal component analysis (PCA) was applied to study the links between the ash characteristics and distribution of chloro-aromatics. The primary purpose of this study is improving the understanding of any links between the characteristics of ash from waste heat systems and its potential to form PCDD/F and PCB. The question is raised whether further characterisation of fly ash may assist to establish a diagnosis of poor plant operation, inclusive the generation, destruction and eventual emission of persistent organic pollutants (POPs).

Synergy of iron and copper oxides in the catalytic formation of PCDD/Fs from 2-monochlorophenol

Transition metal oxides present in waste incineration systems have the ability to catalyze the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) through surface reactions involving organic dioxin precursors. However, studies have concentrated on the catalytic effects of individual transition metal oxides, while the complex elemental composition of fly ash introduces the possibility of synergistic or inhibiting effects between multiple, catalytically active components. In this study, we have tested fly ash surrogates containing different ratios (by weight) of iron (III) oxide and copper (II) oxide. Such Fe2O3/CuO mixed-oxide surrogates (in the Fe:Cu ratio of 3.5, 0.9 and 0.2 ) were used to study the cooperative effects between two transition metals that are present in high concentrations in most combustion systems and are known to individually catalyze the formation of PCDD/Fs. The presence of both iron and copper oxides increased the oxidative power of the fly ash surrogates in oxygen rich conditions and led to extremely high PCDD/F yields under pyrolytic conditions (up to >5% yield) from 2-monochlorophenol precursor. PCDD/F congener profiles from the mixed oxide samples are similar to results obtained from only CuO, however the total PCDD/F yield increases with increasing Fe2O3 content. Careful analysis of the reaction products and changes to the oxidation states of active metals indicate the CuO surface sites are centers for reaction while the Fe2O3 is affecting the bonds in CuO and increasing the ability of copper centers to form surface-bound radicals that are precursors to PCDD/Fs.

Effects of the addition of municipal solid waste incineration fly ash on the behavior of polychlorinated dibenzo-p-dioxins and furans in the iron ore sintering process

Raw materials were co-sintered with municipal solid waste incineration (MSWI) fly ash through iron ore sintering to promote the safe treatment and utilization of MSWI fly ash. To assess the feasibility of this co-sintering method, in this study, the effects of the addition of MSWI fly ash on the formation and emission of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) were estimated via iron ore sintering pot experiments. During co-sintering, most of the PCDD/Fs in the added MSWI fly ash were decomposed and transformed into PCDD/Fs associated with iron sintering, and the concentrations of lower- and mid-chlorinated congeners increased. As there was a sufficient chlorine source and the sintering bed permeability was decreased by the addition of MSWI fly ash, the PCDD/F concentration in the exhaust gas increased. The mass emission of PCDD/Fs decreased; however, the emission of toxic PCDD/Fs increased beyond the total emissions from the independent MSW incineration and iron ore sintering processes due to the transformation of PCDD/F congeners. The co-sintering may be an important solution after technological improvements in the flue gas cleaning system and PCDD/F formation inhibition procedures.

Molten salt oxidation of organic hazardous waste with high salt content

Organic hazardous waste often contains some salt, owing to the widespread use of alkali salts during industrial manufacturing processes. These salts cause complications during the treatment of this type of waste. Molten salt oxidation is a flameless, robust thermal process, with inherent capability of destroying the organic constituents of wastes, while retaining the inorganic ingredients in the molten salt. In the present study, molten salt oxidation is employed for treating a typical organic hazardous waste with a high content of alkali salts. The hazardous waste derives from the production of thiotriazinone. Molten salt oxidation experiments have been conducted using a lab-scale molten salt oxidation reactor, and the emissions of CO, NO, SO₂, HCl and dioxins are studied. Impacts are investigated from the composition of the molten salts, the types of feeding tube, the temperature of molten carbonates and the air factor. Results show that the waste can be oxidised effectively in a molten salt bath. Temperature of molten carbonates plays the most important role. With the temperature rising from 600 °C to 750 °C, the oxidation efficiency increases from 91.1% to 98.3%. Compared with the temperature, air factor has but a minor effect, as well as the composition of the molten salts and the type of feeding tube. The molten carbonates retain chlorine with an efficiency higher than 99.9% and the emissions of dioxins are below 8 pg TEQ g^-1 sample. The present study shows that molten salt oxidation is a promising alternative for the disposal of organic hazardous wastes containing a high salt content.

Catalytic oxidation of PCDD/F on a V2O5-WO3/TiO2 catalyst: Effect of chlorinated benzenes and chlorinated phenols

Catalytic oxidation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) is a well-proven technique, applied in a rising number of Municipal Solid Waste Incineration plants, yet the simultaneous and possibly competitive co-oxidation of other compounds, such as chlorinated benzenes (CBz) or phenols (CP), is still poorly documented. In this study, a grinded commercial catalyst (vanadium-tungsten supported on titanium dioxide) was submitted to exploratory testing: the PCDD/F present in a gas test flow were catalytically oxidised (200°C, 10,000h^-1), either as such or in the presence of benzene (Bz), monochlorobenzene (MCBz), and 1,2-dichlorobenzene (DCBz) and the effect of these additions on the catalytic destruction of PCDD/F was verified experimentally. Both removal efficiency (RE) and destruction efficiency (DE) declined during the exploratory testing and, importantly, some DCBz even converted into supplemental PCDD/F. Also, the occurrence of carbon deposition negatively influenced catalytic oxidation activity. Regeneration with oxygen or air allowed to remove the deposited carbon and the original catalytic activity was largely restored after calcination. In a second part of this study, the PCDD/F-formation from DCBz, hexachlorobenzene (HCBz), o-monochlorophenol (o-MCP) and pentachlorophenol (PeCP) was demonstrated and tentatively explored. To prepare for further elucidation of the reaction mechanism, a complete isomer-specific analysis was prepared.

Iron chloride catalysed PCDD/F-formation: Experiments and PCDD/F-signatures

Iron chloride is often cited as catalyst of PCDD/F-formation, together with copper chloride. Conversely, iron chloride catalysis has been less studied during de novo tests. This paper presents such de novo test data, derived from model fly ash incorporating iron (III) chloride and established over a vast range of temperature and oxygen concentration in the gas phase. Both PCDD/F-output and its signature are extensively characterised, including homologue and congener profiles. For the first time, a complete isomer-specific analysis is systematically established, for all samples. Special attention is paid to the chlorophenols route PCDD/F, to the 2,3,7,8-substituted congeners, and to their relationship and antagonism.

Dioxins degradation and reformation during mechanochemical treatment

Mechanochemical dechlorination and destruction of polychlorinated dibenzo-p-dioxins and -furans (PCDD/F) on fly ash from Municipal Solid Waste Incineration was tested with and without additives (CaO and CaO/aluminium powder). The first results disappointed because of obvious PCDD/F-reformation and a second test series was conducted after removing soluble salts (NaCl, KCl …) by thorough two-stage water washing. This second test series was successful and demonstrated good destruction results, especially with combined CaO/aluminium powder as additive. In a third test series salt was again added to the water-washed fly ash, and the first, poor results were largely reconstituted. For all tests a fairly complete (94 out of 136 congeners) isomer-specific analysis was conducted and analysed, allowing to differentiate between, e.g., 2,3,7,8-substituted PCDD/F and congeners formed following the chlorophenol route. The first became more important in the samples series Fly Ash, Milled Fly Ash, milling with added CaO, and milling with CaO/aluminium-addition. The second follow the opposite trend. This isomer-specific analysis will form the basis for further study using Principal Component Analysis.

De novo formation of dioxins from milled model fly ash

Municipal solid waste incineration (MSWI) fly ash has been classified as hazardous waste and needs treatment in an environmentally safe manner. Mechanochemical (MC) treatment is such a detoxification method, since it destroys dioxins and solidifies heavy metals. Milling, however, also introduces supplemental metals (Fe, Ni, Cr, Mn…), following wear of both steel balls and housing. Milling moreover reduces the particle size of fly ash and disperses catalytic metal, potentially rising the reactivity of fly ash to form and destroy 'dioxins', i.e. polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD + PCDF or PCDD/F). To test this issue, model fly ash (MFA) samples were composed by mixing of silica, sodium chloride, and activated carbon, and doped with CuCl₂. Then, these samples were first finely milled without any additives for 0 h (original sample), 1 h and 8 h, and the effect of milling time (and hence particle size) was investigated on the formation of polycyclic aromatic hydrocarbons (PAHs), and of polychlorinated phenols (CP), benzenes (CBz), biphenyls (PCB) and dioxins (PCDD + PCDF) during de novo tests at 300 °C for 1 h, thus simulating the conditions prevailing in the post-combustion zone of an incinerator, where dioxins are formed and destroyed. These compounds are all characterized by their rate of generation (ng/g MFA) and their signature, i.e. internal distribution over congeners as a means of gathering mechanistic indications. PAH and CBz total yield did not decrease in MC treated MFA with milling time, while total pentachlorophenol (PeCP), PCB and PCDD/F yield decreased up to 86, 94 and 97%, respectively. International Toxic Equivalents (I-TEQ) concentration decreased more than 90%, while degree of chlorination varied inconsistently for PCB and PCDD/F, and average congener patterns of PCDD/F do not vary considerably with milling time for both gas and solid phase.