Effect of gaseous inhibitors on PCDD/F formation (original) (raw)
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Environmental …, 2012
This study investigated dioxins and dioxinlike polychlorinated biphenyls in gasses emitted from waste incinerators and thermal processes in central and western parts of India. The concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/DFs) ranged from 0.0070 to 26.8140 ng toxicity equivalent (TEQ)/Nm 3 , and those of dioxin-like polychlorinated biphenyls (PCBs) ranged from 0.0001×10 −1 to 0.0295 ng TEQ/Nm 3 . The characteristics of mean PCDD/F I-TEQ concentration and congener profiles were studied over all the samples of air. In particular, a pattern consisting of a low proportion of dioxin-like PCBs and high proportion of PCDDs/ DFs was common for all the samples from incinerators and high-temperature processes.
Potential Role of Chlorination Pathways in PCDD/F Formation in a Municipal Waste Incinerator
Environmental Science & Technology, 2004
The role of chlorination reactions in the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) in a municipal waste incinerator was assessed by comparing predicted chlorination isomer patterns with incinerator flue gas measurements. Complete distributions of PCDD and PCDF congeners were obtained from a stokertype municipal waste incinerator operated under 13 test conditions. Samples were collected from the flue gas prior to the gas cleaning system. While total PCDD/F yields varied by a factor of 5 to 6, the distributions of congeners were similar. A conditional probability model, dependent only on the observed distribution of monochlorinated isomers, was developed to predict the distributions of polychlorinated isomers formed by chlorination of dibenzo-p-dioxin (DD) and dibenzofuran (DF). Agreement between predicted and measured PCDF isomer distributions was high for all homologues, supporting the hypothesis that DF chlorination can play an important role in the formation of PCDF byproducts. The PCDD isomer distributions, on the other hand, did not agree well with model predictions, suggesting that DD chlorination was not a dominant PCDD formation mechanism at this incinerator. This work demonstrates the use of PCDD/F isomer patterns for testing formation mechanism hypotheses, and the findings are consistent with those from other municipal waste combustion studies.
Chemical inhibition of PCDD/F formation in incineration processes
Science of The Total Environment, 2004
This review summarises results of our pilot-scale experiments to find suitable inhibitors for preventing the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDyF) during waste incineration and to specify the role of the main factors affecting the inhibition process, and is based on doctoral dissertation of . Results of previous experiments reported by other researchers are also presented and compared with ours. The detailed aims of our experiments were (1) to compare the effects of different inhibitors on PCDDyF formation during incineration in a pilot plant, (2) to investigate the role of the particle size distribution of the flue gas on the inhibition of PCDDyFs, and (3) to find the main parameters affecting PCDDyF inhibition in waste incineration. Prevention of the formation of PCDDyFs with chemical inhibitors and the effects of different supply points, feed temperatures and process parameters were studied in a pilot scale incinerator (50 kW) using light heating oil and refuse-derived fuel as test fuels. Various concentrations of the gaseous inhibitors (sulfur dioxide, ammonia, dimethylamine and methyl mercaptan) were sprayed into the flue gases after the furnace, in addition to which urea was dissolved in water and injected in at different concentrations. The residence time of the flue gas between the furnace and the PCDDyF sampling point was varied in the tests. In another set of urea tests, urea-water solutions at three concentrations were mixed with the RDF prior to incineration. PCDDyF and chlorophenol concentrations, together with other flue gas parameters (e.g. temperature, O , CO, CO and NO), were analysed in the cooling flue gases. The gaseous and liquid 2 2 inhibitors both notably reduced PCDDyF concentrations in the flue gas, the reductions achieved with the gaseous inhibitors varying from 50 to 78%, with dimethyl amine the most effective, while that produced with urea was up to 90%. The PCDDyF reductions were greater at increased inhibitor concentrations and with increased residence time of the flue gas between the furnace and the sampling point. PCDDyF concentrations in the particle phase decreased much more markedly than those in the gas phase. The urea inhibitor did not alter the particle size distribution of the PCDDyFs when the amount of inhibitor was adequate. Chemical inhibitors seem to offer a very promising technique for preventing the formation of PCDDyFs in waste incineration. The addition of urea to the fuel before combustion proved to be very effective approach and could be a useful technique even in the full-scale incinerators. ᮊ
Progress in Energy and Combustion Science, 2009
Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) constitute a group of persistent organic pollutants that form almost inexorably in all thermal and combustion operations. This review focuses on mechanisms that govern their formation, chlorination, dechlorination and destruction. As a consequence of their extreme toxicity and propensity to bioaccumulate, PCDD/Fs have been subjected to much scientific research, designed to understand mechanisms and conditions that govern their emission rates and congener distribution (fingerprints). Consensus of opinions in the literature points to heterogeneous pathways contributing substantially more in the total yield of PCDD/Fs in combustion systems than the gas phase pathway. However, in our view, a great complexity of both homogeneous and heterogeneous routes and uncertainties in many thermochemical and rate parameters enable no conclusive statement about the contribution of each route. Chlorination patterns of precursors appear to play a major role in final congener profiles of PCDD/F emissions. According to the most recent theoretical studies, these congener profiles seem consistent with thermodynamic stabilities of dioxins and furans produced in thermal processes, however, further theoretical investigations at more accurate levels are needed to clarify this matter further. Theoretical studies along with experimental findings reveal that the PCDD/PCDF ratio remains very sensitive to the operating conditions, with pyrolytic conditions favouring the formation of PCDFs. A number of reaction mechanisms has been proposed to answer many of the most intriguing questions about the formation of PCDD/Fs. These mechanisms include models of gaseous and heterogeneous reactions, often inferred from theoretical quantum chemical calculations studies, which incorporate steps responsible for formation, chlorination, dechlorination and destruction of dioxins and furans. The review identifies gaps in our present understanding of the reaction mechanisms and suggests that further progress in the field needs to be facilitated by development of reliable mechanistic models for (i) catalytic pathways, (ii) chlorination/ dechlorination reactions including the Deacon reaction and the direct transfer of chlorine from solid surfaces into the aromatic moieties, and (iii) formation of PCDD/Fs from precursors other than chlorophenols, especially pesticides.
Chemosphere, 2009
Formation of polychlorinated dibenzo-p-dioxins (PCDD), dibenzofurans (PCDF), benzenes (PCBz), and phenols (PCPh) was studied during combustion of an artificial municipal solid waste (MSW) in a laboratory-scale fluidized-bed reactor with simultaneous collection of flue gas samples at three different temperatures in the post-combustion zone (450°C, 300°C, and 200°C). PCDF, PCBz, and PCPh were predominantly formed at or above the first sampling point (450°C) with a dominance of the lower chlorinated homologues. PCDDs, on the other hand, were dominated by the intermediately chlorinated homologues with concentrations peaking at 300°C. The dominating PCPh congeners clearly displayed the ortho-para directionality, which is indicative of electrophilic aromatic substitution, as did the PCBz isomer distribution patterns to some extent. Comparison of the observed PCBz isomer distribution patterns to prior work may indicate coupling of aliphatic species in chlorobenzene formation. The PCDDs seemed to be largely influenced by chlorophenol condensation reactions and to some extent chlorination reactions, while the PCDFs displayed a chlorination-oriented pattern for the mono-to tri-chlorinated homologues and a PCPh condensation pattern for the higher chlorinated homologues. Injection of nonchlorinated dibenzo-p-dioxin at 650°C resulted in increased formation of Tri-HxCDD and a decrease in the dibenzofuran levels. The affected PCDD and PCDF congeners were not products expected to form from chlorine substitution, but instead are well known chlorophenol condensation products.
International journal of sustainable development and planning, 2024
Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) are considered the most toxic and most concerning volatile organic compounds in the industrial flue gases. They are formed in various processes involving the combustion of organic materials in the presence of chlorine compounds, including modern and domestic coal burning, waste incinerators, sinter plants, cement industry, and metallurgical processes. The primary objective of this study was to investigate the destruction of PCDD/Fs and polychlorinated biphenyl (PCBs) using effective combustion practices. Flue gas samples were collected from the medical waste incineration plant to analyze PCDD/Fs and PCBs destruction under different temperature intervals, O2 content, residence time, and exhaust gas recirculation. It has been found that, relying solely on good combustion practices is not sufficient to completely destruct PCDD/Fs and PCBs emissions. Even with the implementation of optimal combustion techniques, certain congeners such as 2,3,4,7,8-PeCDF and 1,2,3,7,8-PeCDD were still measured in high concentrations. Consequently, additional techniques such as dry sorbent injections, should be explored to meet the new stringent requirements of 0.1 ng I-TEQ/m 3 .
Aerosol and Air Quality Research, 2014
This study describes polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) behavior during the incineration of laboratory waste, including combustible laboratory solid waste (LSW), laboratory plastic waste (LPW), and organic laboratory liquid waste (LLW). Stack flue gas (SFG), input materials, bottom ash (BTA), first quenching tower ash (FQA), secondary quenching tower ash (SQA), and baghouse ash (BHA) were sampled and analyzed using high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS) assay and bioassay. The PCDD/F concentration of SFG met the standard in Taiwan. The Cl levels of LPW and LLW were roughly equivalent to that of municipal solid waste (MSW). Therefore, the SFG concentration, content of fly ash, and distribution behavior of PCDD/Fs are reasonably similar to those of MSW incinerators. The LSW had an extremely high Cl level (11.4%). The emission factor of the whole incineration system was 888 µg I-TEQ/ton-waste, which is 10-fold higher than that of MSW. The PCDD/F was mainly in BTA (31.6 wt.%) and fly ash (63.1 wt.%), resulting in higher PCDD/F level of ashes compared with that of MSW ashes. Both HRGC/HRMS analysis and bioassay results show similar PCDD/F emission characteristics during the incineration of LW. In addition, the linear regression between the values acquired using these two methods show a good relation (R 2 > 0.84), indicating that Ad-DR bioassay is a promising fast-screen method for determining PCDD/F levels.
Environmental Science & Technology, 2005
The role of phenol precursors in polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) formation in municipal waste incinerators is assessed on the basis of homologue and isomer patterns. Homologue and isomer patterns of PCDD and PCDF congeners formed from phenols both in the gas phase and via particle-mediated reactions were studied in an isothermal flow reactor. A mixture of unsubsitituted phenol and 19 chlorinated phenols in relative concentrations found in a municipal waste incinerator (MWI) stack gas was used for this study. PCDD and PCDF homologue and isomer patterns obtained from the phenol experiments were compared with those observed in MWI data. From the phenol experiments, gas-phase formation at 600-700°C favors PCDF formation whereas particle-mediated formation at 400°C favors PCDD formation. Unsubstituted phenol, which was present in high concentration, played a significant role in the formation of PCDD/F congeners under both sets of experimental conditions. PCDD/F distributions in MWI flue gas and fly ash samples were different from those observed in the phenol experiments, suggesting that direct phenol condensation was not the primary route of PCDD/F formation at the incinerators. Gas-phase phenol condensation is a source of dibenzofuran, with subsequent particle-mediated chlorination resulting in PCDF formation. In the case of PCDD formation, phenol condensation may be responsible for the formation of certain highly chlorinated congeners. In this paper we demonstrate the use of homologue and isomer patterns for PCDD/F formation mechanism attribution in municipal waste incinerators.