Formation Of PCDD Research Articles

Effect of injection of di- and tricyclic aromatic compounds on post-combustion formation of polychlorinated dibenzo-p-dioxins and dibenzofurans

The formation of mono- to octachlorinated dibenzo-p-dioxins (PC1–8DD) and dibenzofurans (PC1–8DF) was studied using a model waste in a laboratory-scale combustion reactor with simultaneous collection of flue gas at three different temperatures (450°C, 300°C, and 200°C) in the post-combustion zone. To investigate the influence of chlorination reactions and the effects of carbon backbone-containing compounds present in the flue gases, five aromatic compounds were injected into the flue gas, namely dibenzofuran (DF), biphenyl (BP), naphthalene, phenanthrene and fluorene. The injection of DF induced a reduction in the concentration of PC3–5DD, but did not significantly influence the concentration of PCDF. A reduction in the concentration of PC3–5DD was also observed during the injection of fluorene, which is structurally very similar to DF. The injection of biphenyl, naphthalene and phenanthrene had less pronounced effects on the formation of PCDD and PCDF.A possible explanation of the observed changes during injection of DF and fluorene, based on homologue profiles and affected congeners, involves formation of radical species from fluorene and/or dibenzofuran. The fluorene radical is stabilized by the delocalization of electrons across the aromatic ring structure and has the propensity to react with highly abundant hydrogen chloride, whereas the molecular species would require reaction with Cl2 or chlorine radicals.

Formation of PCDD and PCDF in the thermal treatment of footwear leather wastes

The leather waste generated by the footwear industry is considered dangerous due to the presence of trivalent chromium, derived from the salt utilized to tan hides. In Brazil, the majority of this waste is disposed on landfills and only about 3% are recycled. The thermal treatment is an alternative method for purification of such residues. By using this technique it is possible to generate energy and recover the chromium present in the ash for the production of basic chromium sulfate (tanning industry), high carbon ferrochromium or carbon-free ferrochromium (steel industry). In the last 10 years, the gasification and combustion of footwear leather waste have been intensively studied at the Federal University of Rio Grande do Sul. The research experiment for characterization of the emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) were carried out in a semi-pilot unit (350 kW th). From new investments the thermal capacity of the unit will increase to 600 kW th. The unit will produce power from the heat generated in the combustion. The experimental results indicated that during the thermal treatment of footwear leather wastes, the formation mechanism of PCDD/F is the de novo synthesis. Most of PCDD/F were found in the particulate phase (>95%). A kinetic model was used for discussion of the achieved experimental results. The model is based in the carbon gasification, PCDD/F formation, desorption and degradation. From the conclusions obtained in this work will be possible minimize the PCDD/F formation in process of combustion of footwear leather wastes.

Formation of PCDD/F from the pyrolysis of 2-chlorophenol on the surface of dispersed copper oxide particles

The formation of polychlorinated dibenzo- p -dioxins and polychlorinated dibenzofurans (PCDD/Fs) from2-chlorophenol on supported copper oxide catalyst was studied over the temperature range 200–500°C and a contact time of 0.17–0.5 s under pyrolytic conditions. It was found that despite the pyrolytic conditions, the majority of 2-chlorophenol undergoes oxidation to CO and CO 2 . Approximately 0.5% of 2-chlorophenol was converted to PCDD/F. The results are consistent with a Langmuir-Hinshelwood mechanism for PCDF formation and an Eley-Rideal mechanism of formation of PCDD. Only one PCDF isomer was observed, 4,6-dichlorodibenzofuran, while substantial concentrations of highly chlorinated PCDD were detected. This observation can be explained by a mechanism of formation involving a surface-bound PCDD intermediate that can be further chlorinated. PCDFs are desorbed upon formation and are consequently not chlorinated.

Effects of oxygen on formation of PCB and PCDD/F on extracted fly ash in the presence of carbon and cupric salt

The effect of oxygen–nitrogen atmosphere (N 2+10%O 2,N 2+1%O 2 and 99.999% N 2) on the formation of PCB, PCDD and PCDF by the de novo synthetic reactions in the system consisting of extracted fly ash (from municipal waste incinerators – MWI), activated carbon, CuCl 2 · 2 H 2 O and NaCl at 340°C was studied. The content of PCDD/F for systems with 10% O 2,1%O 2 and 99.999% N 2 was decreasing and corresponded to 17304, 5544, and 1437 ng/sample. In all studied systems the isomer OCDD/F was prevailing. The content of PCBs in the same system was also decreasing from 1214 to 166 ng/g. Formation of nonortho PCB was relatively high compared to the system where only nitrogen was present. The possible mechanism of formation is outlined.

Reactions of 2,4,6-trichlorophenol on model fly ash: oxidation to CO and CO 2, condensation to PCDD/F and conversion into related compounds

Thermal treatment of 2,4,6-trichlorophenol on a magnesium silicate-based model fly ash in the temperature range between 250°C and 400°C leads predominantly to carbon monoxide and carbon dioxide. The fraction of 2,4,6-trichlorophenol which is oxidized to CO and CO 2 increases from 3% at 250°C to 75% at 400°C. Further products are polychlorinated benzenes, dibenzo- p-dioxins, dibenzofurans and phenols. The homologue and isomer patterns of the chlorobenzenes suggest chlorination in the ipso-position of the trichlorophenol. The formation of PCDD from 2,4,6-trichlorophenol and 2,3,4,6-tetrachlorophenol on municipal solid waste incinerator fly ashes and model fly ash were compared and the reaction order calculated.

Soxhlet extraction parameters’ influence on the recovery of polychlorinated dibenzo‐p‐dioxins and dibenzofurans (PCDD/F) from a sediment, and artifact formation of PCDD

Soxhlet extraction of polychlorinated dibenzo‐p‐dioxins/dibenzofurans (PCDD/F) from sediment was studied using a designed experimental set‐up. Three variables were studied: three solvent combinations n‐hexane/methanol, dichloromethane/acetone and toluene/methanol; two extraction times, 8 and 16 h, and addition of bulk substances (sodium carbonate). The extraction efficiency was almost identical in all extractions, with the exception of extraction where toluene/ methanol was combined with sodium carbonate. In these samples a major effect was observed, due to formation of Hx‐, Hp‐ and OCDD isomers. Composition of formed PCDD was similar to PCDD isomers found in pentachlorophenol products.

The Influence of Fly Ash Load and Particle Size On the Formation of Pcdd, Pcdf, PCBz and Pcb in a Pilot Incinerator

A laboratory-scale, fluidized bed reactor fuelled by a synthetic fuel was used to study the influence of fly ash load, particle size, temperature and residence time on the low-temperature formation of chlorinated aromatic compounds. Large fly ash particles were removed from the flue gases by means of a cyclone at the entrance to the cooling section of the reactor. The experimental variables were varied according to an experimental plan of full factorial design. Polychlorinated dibenzo- p-dioxins, dibenzo-furans, polychlorinated benzenes and biphenyls were analysed in the collected flue gas samples. Despite the fact that most of the fly ash load was removed by the cyclone, formation of chlorinated aromatics occurred to the same extent as in earlier experiments, without the cyclone. These results demonstrate the importance of small fly ash particles in the post-combustion formation of chlorinated aromatics.

The influence of fly ash load and particle size on the formation of PCDD, PCDF, PCBz and PCB in a pilot incinerator

A laboratory-scale, fluidized bed reactor fuelled by a synthetic fuel was used to study the influence of fly ash load, particle size, temperature and residence time on the low-temperature formation of chlorinated aromatic compounds. Large fly ash particles were removed from the flue gases by means of a cyclone at the entrance to the cooling section of the reactor. The experimental variables were varied according to an experimental plan of full factorial design. Polychlorinated dibenzo- p-dioxins, dibenzo-furans, polychlorinated benzenes and biphenyls were analysed in the collected flue gas samples. Despite the fact that most of the fly ash load was removed by the cyclone, formation of chlorinated aromatics occurred to the same extent as in earlier experiments, without the cyclone. These results demonstrate the importance of small fly ash particles in the post-combustion formation of chlorinated aromatics.

The Effect of the Electric Field on the Formation of PCDD in Electrostatic Precipitators

Sufficient evidence exists to suggest that there is some PCDD formation in electrostatic precipitators attached to municipal solid waste incinerators. This paper deals with the possible effect of the corona discharge in an electrostatic precipitator on PCDD formation from chlorophenols in the gas phase. The corona discharge may cause an increase in O and OH present in the system which in turn may lead to an increase in the concentration of chlorophenoxyl radicals. Preliminary modeling studies indicate that increases in PCDD can be explained by electric field effects.

Development of the Ammonia Injection Technology (AIT) for the Control of PCDD/PCDF and Acid Gases from Municipal Solid Waste Incinerators

This article describes the development of the Ammonia Injection Technology (AIT), a technology for the simultaneous control of the emissions of PCDD/PCDF (polychlorinated p-dibenzodioxins and polychlorinated dibenzofurans), HCI, SO2 and NOX from municipal solid waste incinerators. It briefly reviews the theoretical basis of the technology and the bench-scale and pilot-scale experiments. It describes the results of the pilot-scale experiment in detail and reports on the finding that the formation of PCDD and PCOF takes place in different regions of the boiler system. Finally, a concept is introduced for the treatment of emission control residues which could lead to the recovery of chlorine from waste products and its “recycle” to the chlorine manufacturing process (“closed chlorine life cycle“ concept).

Thermally induced formation of PCDD and PCDF from tri- and tetrachlorobenzene in dielectric fluids

The thermal behavior of dilute (5000-50 ppm) solutions of tri- and tetrachlorobenzene in mineral oil, silicone oil, and tetrachloroethylene (TCE) has been investigated. Although pyrolyses or combustion of neat chlorobenzenes readily forms significant yields of PCDDs/PCDFs, corresponding treatment of dilute solutions was generally non-productive. This result is consistent with the proposition that, under the conditions studied, the rate-limiting step for PCDD/PCDF formation is bimolecular in chlorobenzene. In contrast, earlier studies of PCBs implied a rate-limiting step that was unimolecular in chloroaromatics. Combustion of dilute solutions of chlorobenzenes in tetrachloroethylene did produce detectable concentrations of PCDF; this result reflects combination of chlorobenzene and TCE- or TCE-derived species.

Kinetic aspects from the laboratory simulation of the formation and destruction of PCDD/PCDF in fly ash from municipal waste incinerators

The formation of PCDD and PCDF congeners in fly ash from MSW incinerators was interpreted. This was done from laboratory simulation data in air at 300°C. The whole process was treated as a series of independent simultaneous reactions of the pseudo-first order and the kinetic constants were calculated. Furthermore the data for the destruction reaction under an oxygen deficient atmosphere were interpolated using a general kinetic equation. It was shown that the destruction reaction can also operate in oxygen flow if the reactant sources and/or catalytic sites are no longer available. The temperature effects are discussed.

Investigation of the effect of water, acids, and bases in the gas stream in the catalytic formation of PCDD and PCDF over MSW fly ash

Municipal solid waste (MSW) incinerator fly ash has previously been implicated in the formation of PCDD and PCDF during MSW incineration. This paper describes a series of experiments conducted to further investigate the role of the fly ash using carrier gas components that may inhibit or enhance PCDD/F formation. Model compounds were used to probe the formation of dioxins and furans. The role of water in the gas stream was investigated and it was found that at the typical moisture levels found in incinerators (approximately 15%) water in the gas stream can enhance the catalytic activity of fly ash. Ammonium hydroxide permanently inhibits the formation of PCDD from pentachlorophenol over MSW fly ash but does not inhibit the formation of PCDF from pentachlorodiphenylether. It was also found that some acidic components could not only enhance the catalytic activity of fly ash but also regenerate catalytic sites in fly ash which had lost its activity.

Response surface models for the formation of PCDD and PCDF in a pilot plant combustion of MSW

Experimental data on PCDD and PCDF emissions in raw gases from municipal solid waste combustion in a pilot plant were processed by Response Surface models to gain information on process parameter values which minimize both the total PCDD/PCDF concentrations and those of a selected congener group.

Post furnace formation and progressive chlorination of PCDD and PCDF in municipal waste incinerator

The changes in isomer pattern of chlorobenzene, chlorophenol and chlorinated dioxin (PCDD) were determined for flue gas samples from a municipal waste incinerator. The isomer specific analysis for PCDDs in flue gas samples collected from different locations in the incinerator revealed a correspondence to the isomer patterns in precursor molecules such as those of chlorophenols. The chlorodioxins substituted in positions 1,3,6,8 and 1,3,7,9 predominated in place of isomers substituted in positions 1,3,7,8, 1,4,7,8 and 1,2,7,8 downstream of the incinerator. This change corresponds closely to the post furnace increase in chlorophenol substituted in positions 2,4,6.

Effect of temperature, carrier gas and precursor structure on PCDD and PCDF formed from precursors by catalytic activity of MSW incinerator fly ash

The difference in products formed from precursors by catalytic activity of fly ash under an inert atmosphere of nitrogen and an oxygen-rich atmosphere of high purity air were examined. Replacement of nitrogen with air affected a shift to the higher chlorinated isomers being formed and also resulted in a greater overall yield of both PCDD and PCDF. A series of diphenylethers with different levels of chlorine substitution at varying positions were studied in the catalytic formation of PCDD and PCDF. All the precursors formed at least some PCDD and PCDF congeners, but the yields and isomer distributions varied with the structure of the precursor. The temperature dependance of product yield and congener distribution at low temperatures (200–350°C) is also presented.

Formation and fate of PCDD and PCDF from combustion processes

Formation and fate of PCDD and PCDF from combustion processes