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A pilot-plant study for destruction of PCBs in contaminated soils using fluidized bed combustion technology
Abstract
Destruction of polychlorinated biphenyls (PCBs) in contaminated soils and wastes using circulating fluidized bed combustion (CFBC) technology was studied using a pilot plant and simulated waste material. The results show that the technology is effective and particularly promising for treatment of PCB-containing materials like the toxic sludge from a large contaminated site. Destruction of the toxics in the gas phase appears to be very fast, and over 99.9999% destruction and removal efficiency can be achieved in the temperature range 875-880 degrees C. Heat transfer in the fluidized bed also appears adequate. Toxic residues in treated soil can be reduced to very low levels. Rate-controlling factors of the decontamination process are analyzed, and key issues for determination of the process conditions are discussed.
... Thermal treatment, if properly performed, is an effective way of destroying organic compounds (Marani et al., 2003;Desai et al., 2007), while impact on co-present inorganic contaminants varies depending on the element. Previous studies show that soil pH increases after thermal treatment and so does the mobility of oxyanions of arsenic (As) and chromium (Cr), while leaching of copper (Cu) decreases (Nordmark et al., 2011). ...
Thermal treatment, if properly performed, is an effective way of destroying organic compounds in contaminated soil, while impact on co-present inorganic contaminants varies depending on the element. Leaching of trace elements in thermally treated soil can be altered by co-combusting different types of materials. This study aimed at assessing changes in mobility of As, Cr and Cu in thermally treated soil as affected by addition of industrial by-products prior to soil combustion. Contaminated soil was mixed with either waste of gypsum boards, a steel processing residue (Fe3O4), fly ash from wood and coal combustion or a steel abrasive (96.5% Fe(0)). The mixes and unamended soil were thermally treated at 800 °C and divided into a fine fraction <0.125 mm and a coarse fraction >0.125 mm to simulate particle separation occurring in thermal treatment plants. The impact of the treatment on element behaviour was assessed by a batch leaching test, X-ray absorption spectroscopy and dispersive X-ray spectrometry. The results suggest that thermal treatment is highly unfavourable for As contaminated soils as it increased both the As leaching in the fine particle size fraction and the mass of the fines (up to 92%). Soil amendment with Fe-containing compounds prior to the thermal treatment reduced As leaching to the levels acceptable for hazardous waste landfills, but only in the coarse fraction, which does not justify the usefulness of such treatment. Among the amendments used, gypsum most effectively reduced leaching of Cr and Cu in thermally treated soil and could be recommended for soils that do not contain As. Fly ash was the least effective amendment as it increased leaching of both Cr and As in majority of samples.
... Thermal treatment technologies such as thermal desorption, incineration, pyrolysis, gasification, vitrification, supercritical water oxidation, and wet-air oxidation have been used for soil remediation [14][15][16]. Application of incineration technology can effectively destroy hazardous organic substances and immobilize heavy metals [17][18][19]. Fluidized bed incinerators are considered as a beneficial treatment of low-calorific value materials such as sewage sludge and contaminated soil [20][21][22]. The high degree of heat and mass transfers in fluidized bed is associated with rapid solid mixing and uniform combustion [23,24]. ...
This study presents the application of a low-temperature two-stage fluidized bed incinerator to remediate contaminants in the soil. The system was designed to control emissions of both gaseous pollutants and heavy metals during combustion. Soil co-contaminated with lube oil and heavy metals such as cadmium, chromium, copper, and lead was examined. Experiments were conducted by estimating various parameters such as operating temperature in the first-stage reactor (500-700 °C), ratio of sand bed height/diameter in the second-stage reactor (H/D: 3, 4, 6), and gas velocity (0.21-0.29 m/s). Heavy metal and gaseous pollutant emissions were also investigated during contaminated soil co-firing with polyethylene. The experimental results indicated that the destruction and removal efficiency of lube oil in treated soil products ranged from 98.27 to 99.93%. On the other hand, leaching tests of bottom ashes illustrated that heavy metals such as chromium, copper, and lead in leachates were complied with the regulations. For gaseous emissions, carbon monoxide concentrations decreased apparently with increasing ratio of sand bed height/diameter in the second-stage reactor. The increase of gas velocity had significant potential to generate the lowest carbon monoxide and particulate matter emissions. Nevertheless, during co-firing with polyethylene, emissions of organic pollutants such as benzene, toluene, ethylbenzene, and xylene and polycyclic aromatic hydrocarbons decrease by using the low-temperature two-stage fluidized bed incineration system.
... The fluidized bed incineration was used to study the remediation of soils contaminated with either heavy metals or hydrocarbons (Corella and Toledo 2000;Desai et al. 2007;Helena Lopes et al. 2003;Hu et al. 2013;Lee et al. 1998;Rink et al. 1993;Shao et al. 2008). However, the remediation of soils contaminated with high levels of mixed contaminants has not been clarified. ...
This paper aims to investigate the emission characteristics during the thermal treatment of co-contaminated soils containing lube oil and heavy metals (Cd, Cr, Cu, and Pb). Experiments were conducted using a laboratory-scale bubbling fluidized bed combustor under various fluidization parameters including temperature (500-800°C), air velocity (11.5-17.9 cm/s), and sand bed particle size (545-920 μm). The emission products from the system, including flue gas, fly ash, and bottom ash, were sampled, and the concentrations of heavy metals and organic pollutants (such as benzene, toluene, ethylbenzene, xylene, and polycyclic aromatic hydrocarbons) along with the metal leachability were determined. The average emissions were lowest at 800°C, air velocities were in the range of 13.7-17.9 cm/s, and sand bed particle diameters were 770-920 μm. Removal of more than 99% and 48-85% of lube oil and heavy metals, respectively, was observed. Portions of the heavy metals were vaporized and attached to the fly ash, while the residual heavy metal fractions in bottom ash were supposed the effect of particle aggregated during thermal treatment. The experimental results showed that all heavy metals were concentrated in the bottom ash due to particle agglomeration; therefore, the metals leachability was in accordance with the standards.
... PCBs are one of the most dangerous pollutants for both human health and the environment. PCBs are a family of man-made chemicals that contain 209 individual compounds with various toxicity levels and have been widely used as coolants and lubricants in capacitors and other electrical equipment because of their insulating and non-flammable properties [1,2]. Actually, at European level, waste-containing PCBs are carefully monitored and specific regulations are applied. ...
The paper presents results of laboratory experiments focused on the efficiency of polychlorinated biphenyls (PCBs) removal by electroremediation, pyrolysis and incineration decontamination methods. The experiments were carried out using soil contaminated with heavy metals and PCBs. Different process parameters were changed during the experiments in order to achieve maximum removal efficiency. Concerning the thermal methods (pyrolysis and incineration), the operating temperature and retention time were the two key parameters affecting the processes that were assessed. With regards to the electroremediation alternative for soil decontamination, the redox potential (ORP), pH, electric field and temperature were continuously monitored. Results showed that thermal treatments are the most indicated methods for removing PCBs from contaminated soils. Pyrolysis and incineration remediation methods ensured, with some exceptions, an efficiency of over 99%. The same efficiency level was attended just in case of applying the electrochemical method for removing PCB126.
... These findings indicate that input of PCB due to glacier melting was lower than that of DDT homologues. Actually, PCBs were used in Italy only in closed systems and, as in other parts of the world, they en- tered the environment accidentally or as a consequence of release from polluted soils and landfills, or from waste discharged by treatment plants (Desai et al., 2007). Since tourism and agriculture are the prevailing activities in the Alpine and sub-alpine area, it is very likely that local sources of PCB pollution were not as significant as those of DDT, which was used for fruit-tree pest control in an area very close to the glaciers' zone. ...
A sharp increase in 2005 of pp'DDT and its metabolites was observed in mussels and fish from lakes Como and Iseo, the main glacier-fed southern Alpine lakes. DDTs in zebra mussels (Dreissena polymorpha) were more than 150 times higher than levels in 2003, and concentrations in pelagic fish (0.12 mgkg(-1) w.w.) exceeded the Italian safety threshold for human consumption (0.05 mgkg(-1) w.w.). The histological examination of the ovaries revealed many mussels with oocyte degeneration throughout the studied period. Prior to being banned in Italy in 1978, DDT was used in large amounts for fruit-tree treatment from the 1950s to 1970s in valleys just below the glaciers. Since glacier volume was increasing at that time and then continuously retreated, meltwater should be the main cause of the pollution peak recently observed in biota of downstream lakes. PCBs did not peak in biota tissues to a comparable extent probably because local sources were not as important as for DDTs.
Mit den thermischen Verfahren können Böden über ein breites Korngrößenspektrum behandelt werden. Ein Sanierungserfolg wird vor allem bei organischen Schadstoffen erzielt, während es für die meisten Schwermetalle nicht einsetzbar ist. Bei der thermischen Bodenbehandlung gibt es sowohl in‐situals auch ex‐situ‐Verfahren, wobei den Ex‐situ‐Verfahren eine größere Bedeutung zukommt. Zu den wichtigsten thermischen Verfahren gehören: Destillation oder thermische Desorption, Pyrolyse und Verbrennung. Sie unterscheiden in dem angewendeten Temperaturbereich und den Redox‐Bedingungen.
In view of the problems existing in catalytic degradation, such as low degradation efficiency, poor dechlorination efficiency and large amount of exhaust gas production, three iron-based bimetallic catalysts were developed for the dechlorination of polychlorinated biphenyls (PCBs) from thermal desorption off-gas, among which manganese-doped iron-based bimetallic catalyst showed the highest degradation efficiency (93.6%) and dechlorination efficiency (92.2%). Specifically, the effect of dechlorination under the air conditions was studied. The GC–MS analysis of intermediate products suggested the presence of three synergistic degradation pathways, namely, hydrodechlorination (HDC), preliminary oxidation, and deep oxidation over manganese-doped iron-based bimetallic catalyst. Through the hydrodechlorination, the chlorine atoms of PCBs were successively removed to obtain the final product biphenyl. During the oxidation stage including preliminary oxidation and deep oxidation, PCBs and intermediates were oxidized by reactive oxygen species (•OH and O2–•). This study provides an efficient and safe dechlorination technology of PCBs in the presence of oxygen with little exhaust gas generated.
Many industries use fluidization of solid particles for energy efficiency or environmental friendly process development, and this paper introduces research techniques developed for investigating gas-particle systems At present there is plenty of room for refining gas-particle fluidization process. With the rapidly rising application of mathematical modelling, real time visualization of processes will be widely used for validation of those models in the near future. In presented research, photogrammetry, as a part of close range vision metrology, has been expanded to allow dynamic space and time analysis of the phase concentration distribution inside fluidization devices. A novel videogrammetry method was created with additional stochastic process analysis for detailed frequency and amplitude characteristics. Videogrammetry was used for the assessment of flow regimes, which were held in various types of fluidization apparatuses. Classic bubbling, jet-spouted and fast circulating fluidization processes were explored under the investigation. Videogrammetry is non-invasive flow regime recognition method, which enables detailed research of gas-particle fluidization phenomena. Until now, there were no comparative studies for three different types of fluidization processes with the use of one complex approach. Developed videogrammetric method consists of the flow structure visualization and dynamic image analysis. The analysed feature is the grey level of the image in time domain, and grey level signals were analysed with the use of autocorrelation function and power density function. The results are presented as images, plots and a flow map. Efficiency of the method was tested by comparison of real observed flow structures to the reconstructed flow structures and the recognition accuracy reached 92%.
This paper reviews the removal of contaminants including nutrients, metals and organic pollutants by vegetations in aquatic environments. The removal efficiencies are considered with respect to 16, 19 and 14 kinds of different aquatic plants, respectively in three tables. Due to different characteristics, the removal effects of plants on contaminants from the overlying water differ greatly. The vegetation can improve the water quality mainly through two ways: (1) to adsorb and absorb pollutants from water, (2) to prevent pollutants from releasing from sediment. The contaminant removal mechanisms of vegetations and related physical, chemical and biological effects are discussed. The effects of vegetations on the contaminant removal are found to depend on the environmental conditions, the number and the type of plants, the nature and the chemical structure of the pollutants. In addition, the contaminant release and removal by vegetations under hydrodynamic conditions is specially addressed. Further research directions are suggested.
The goal of this work was to evaluate a washing process that uses organic solutions for polychlorinated biphenyl (PCB)-contaminated soil, and includes an ultrasound pre-treatment step to reduce operational times and organic solvent losses. In a preliminary trial, the suitability of 10 washing solutions of different polarities were tested, from which three n-hexane-based solutions were selected for further evaluation. A second set of experiments was designed using a three-level Taguchi L27 orthogonal array to model the desorption processes of seven different PCB congeners in terms of the variability of their PCB concentration levels, polarity of the washing solution, sonication time, the ratio washing solution/soil, number of extraction steps and total washing time. Linear models were developed for the desorption processes of all congeners. These models provide a good fit with the results obtained. Moreover, statistically significant outcomes were achieved from the analysis of variance tests carried out. It was determined that sonication time and ratio of washing solution/soil were the most influential process parameters. For this reason they were studied in a third set of experiments, constructed as a full factorial design. The process was eventually optimized, achieving desorption rates of more than 90% for all congeners, thus obtaining concentrations lower than 5 ppb in all cases. The use of an ultrasound-assisted soil washing process for PCB-contaminated soils that uses organic solvents seems therefore to be a viable option, especially with the incorporation of an extra step in the sonication process relating to temperature control, which is intended to prevent the loss of the lighter congeners.
Tar pond wastes from Sydney, Nova Scotia, containing 50 ppm or more of polychlorinated biphenyls (PCBs) were treated in a pilot-scale rotary kiln. In order to use the existing feed system attached to the rotary kiln, the wastes were first oven-dried. Stack gas sampling was conducted during the test, which included measurement of volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), semi-volatile organic compounds (SVOCs), HCl, and metals. The purpose of this study was to determine emissions from treatment of the tar pond waste using rotary kiln technology. It was found that the dried sludge could sustain combustion in the kiln without any supporting fuel. The emissions of polychlorinated dibenzodioxins/furans (PCDD/Fs) were higher than the Canadian Council of Ministers of the Environment (CCME) air emissions guidelines, and the reasons for this are discussed. © 2008 Wiley Periodicals, Inc.
PCBs exhibit a wide range of toxic effects, and they are very stable compounds and do not degrade readily. Although they had been banned in the 1970s; however; it is still urgent to investigate and develop a financially viable, environmentally benign and safe technology to treat the soils contaminated by PCBs. This study investigated the feasibility of coupling of supercritical fluid carbon dioxide (ScCO(2)) extraction with polymer-stabilized palladium nanoparticles for the destruction of pentachlorinated biphenyl (2,2',4,5,5'-PCB) from contaminated sand or soil samples. The extracted 2,2',4,5,5'-PCB can be converted into non-chlorinated products by hydrodechlorination catalyzed by palladium (Pd) nanoparticles, which were stabilized in high-density polyethylene (HDPE) beads. Nearly all 2,2',4,5,5'-PCB was removed quantitatively from solid matrices at 200atm and different temperatures. The final product was proved to be biphenyl and cyclohexylbenzene. The polymer-stabilized palladium nanoparticle catalyst, which does not contact the contaminated matrix directly, can be reused without losing the high catalytic activity inherent by nanometer-sized particles. Deactivation factors such as leaching of metal particles from support, agglomeration and sintering are minimized in this catalyst system due to the unique plastic matrix environment. A combination of supercritical fluid extraction and an on-line catalytic reaction system utilizing the plastic catalysts may have great advantages over other processes for destroying toxic chlorinated compounds in environmental samples.
The town of Sydney, located on the north coast of Nova Scotia, is Canada's most contaminated community. The local tidal estuary, called the tar ponds, was used as a receptacle for industrial waste from a century of coke production and steel making and is estimated to contain more than 700,000 metric tons of polycyclic aromatic hydrocarbons, 50,000 metric tons of polychlorinated biphenyls, and many other residuals including arsenic, naphthalene, and toluene. Many residents have expressed consternation over the potential for exposure and subsequent health effects from the ponds. Recent epidemiological studies estimate a 30 to 40 percent increased incidence in several types of cancer within the community. This paper examines the claims and responses made by a variety of interested parties about the chemical contamination in Sydney. It also considers how those claims, in addition to a number of other mediating factors, may have influenced the local community in the mobilization of a response to the contamination.
The emissions of NO, N2O, SO2 and CO from a 165 MWth atmospheric-pressure circulating fluidized-bed boiler were measured for operating conditions involving changes of bed temperature, excess-air ratio and limestone feed rate. The investigation covered four fuels (two coals, peat and wood) and two types of limestone. In addition, the emissions from the 165 MWth boiler were compared with results from a 40 MWth circulating fluidized-bed boiler in which the same coal and limestone were used. The results show that a sulfur capture of 80 to 90% can be attained with a limestone addition corresponding to a molar ratio Ca:S = 2. At the same time, the NO emission was low (below 50 ppm). The sulfur capture was slightly better in the 40 MWth boiler, but NO emissions were twice as high. The emission of N2O was very low for wood (about 20 ppm) but 60 to 160 ppm for the other fuels. It was similar in the two boilers.
Yields, compositions, and global release rates of major products from heating a synthetic soil matrix under conditions pertinent to current and emerging thermal remediation technologies are reported. The soil specimens were mixtures prepared for the U.S. EPA to reflect certain attributes of soils from U.S. Superfund sites. Because this study focuses on determining how soil decomposition itself may affect thermal cleanup, exogenous contaminants were not added to the soil. The experiments involved heating 49−54 mg batches of 63−125 μm neat soil particles, thinly spread on a 0.001 in. thick stainless steel foil, at nominal rates of 1000 °C/s, to temperatures in the 350−1050 °C range, under 3 psig of helium. Above 500−600 °C the total weight loss increased strongly with increasing temperature. Devolatilization exceeded 21 wt % when soil was heated to 1033 °C and then maintained at this temperature for 5 s before cooling. For this heating regimen, CO2, CO, and tars were the major volatile products, material and carbon balances were good, but oxygen balances were only fair and hydrogen balances were poor, possibly because yields of chemical water were not quantified. A single first-order reaction kinetics model well describes the global release rates of most gaseous volatiles except CO2, which, because of apparent step-like evolution, is better fitted by a model consisting of two independent parallel first-order reactions. A multiple independent parallel first-order reaction kinetics model provides a good fit to the soil weight loss data.
A series of tests to burn mixtures of tar pond sludge and coal was carried out using a mini-circulating fluidized bed combustor (mini-CFBC). During the tests, carbon dioxide, oxygen, carbon monoxide, sulfur dioxide, and nitrogen oxides in the flue gas were monitored continuously. Stack gas sampling was carried out for hydrochloric acid, metals, particulate matter, volatile organic compounds (VOCs), total hydrocarbons, semivolatile organic compounds (SVOCs), dioxins and furans (PCDD/Fs), and polychlorinated biphenyls (PCBs). Results showed that hydrochloric acid, mercury, particulate matter, PCDD/F, and metal concentrations were all below both the current limits and the gas-release limits to be implemented in 2008 in Canada. The new 2008 emissions limits will reduce the maximum allowable concentrations of most pollutants by half. Thus, the maximum concentration for particulate matter will be 5 mg/m3 (from the current maximum concentration of 10 mg/m3);the maximum concentration for hydrochloric acid will be 5 mg/m3 (from 10 mg/m3); and the-maximum concentration for dioxins and furans will be 0.032 ng/m toxic equivalent (from 0.08 ng/mcurrently). Sulfur capture efficiency was 89–91 percent. The percentage of fuel nitrogen converted to nitrogen oxides was of the order of 4.7 to 6.1, which is significantly lower than that of conventional pulverized coal-fired boilers and well within the normal range for fluidized bed combustors (FBCs). PCB and polycyclic aromatic hydrocarbon (PAH) emissions levels were comparable or lower than levels reported in the literature for industrial-scale FBCs. VOC concentrations were low except for benzene, for which the concentration was higher than that reported for pulverized coal-fired utility boilers. In addition, carbon monoxide concentration was high at 1,200 to 2,200 parts per million. However, these carbon monoxide concentrations are typical of the mini-CFBC firing coal. The trials showed that for 10 percent by weight tar pond sludge mixed with 90 percent by weight coal, the combustion was both stable and efficient. The tests demonstrated that CFBC technology is an environmentally sound option for eliminating tar pond waste sludge. © 2005 Wiley Periodicals, Inc.
The objective of this work was to demonstrate the technical feasibility of a fluidised bed as a clean technology for burning high moisture municipal solid waste (MSW). For co-firing high moisture MSW with coal, the combustion efficiency and the flue gas composition were investigated and compared to burning 100% coal. Different waste materials were burnt: a simulated MSW, vegetable waste and olive oil waste. The maximum moisture content of the simulated MSW that could be burnt was 20% because, at any higher value, the bed temperature was lower than 600 °C, and combustion could not be sustained. The different high moisture content wastes were co-fired with coal to study the effect of moisture and ash content on combustion efficiency. The 60% moisture content simulated MSW at 10%, 15% and 20% concentration by mass were co-fired with coal and it was found that the combustion efficiency decreased ≈12% compared to burning 100% coal (80% and 92%, respectively). For co-firing vegetable waste up to 30% concentration by mass with coal, the combustion efficiencies were ≈90%. It was concluded that the effect of ash content is more pronounced than the moisture content when comparing the combustion efficiency of combustion of simulated MSW and coal to that of vegetable waste and coal. Both the vegetable waste and simulated MSW have the same moisture content (60%), but the ash content of the vegetable waste is lower than that of the simulated MSW, 5% ash content compared to 26% ash content. This conclusion is also supported by the results of co-firing of olive oil waste with coal. The combustion efficiencies were the same as those of combustion of vegetable waste and coal, 90% combustion efficiency. Again, both olive oil waste and vegetable waste have a similar moisture content and ash content, 60% and 5%, respectively.
Flame chemistry in incineration systems involves the formation of many organic products of incomplete combustion, including chlorinated species such as polychlorinated biphenyls (PCB), polychlorinated dibenzo-p-dioxins (PCDD), and polychlorinated dibenzofurans (PCDF). Because the latter are of environmental concern, a great deal of research has been expended on understanding their formation. There are two temperature windows in which they can form: the homogeneous route between 500 and 800��C and the heterogeneous one at 200 to 400��C. Homogeneous reactions, which are the result of the pyrolytic rearrangement of chlorinated precursors, such as chlorophenols and chlorobenzenes in the gas phase, have not been researched as extensively as the heterogeneous mechanism. Heterogeneous formation is a catalysed reaction, which takes place on the ash or soot particles present in combustion systems. There are conflicting views regarding the relative amounts of PCDD/F formed from precursors such as chl
Contenido: Introducción; Estequiometría; Selección de temas de procesos de combustión; Caracterización de los desechos; Sistemas de combustión cerrados y recuperación de calor; Flujo de fluidos aplicados a la incineración; Preparación y manejo de materiales; Sistemas de incineración para desechos sólidos municipales; Sistemas de incineración de sedimentos fangosos; Sistemas de incineración para desechos líquidos y gaseosos; Sistemas de incineración para desechos peligrosos; Otros sistemas de incineración para desechos sólidos; Aspectos de la contaminación del aire por sistemas de incineración; Control de contaminación del aire en sistemas de incineración; Aproximación a la selección y diseño de incineradores.
In order to determine the temporal variation in the levels of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in the vicinity of an old municipal solid waste incinerator (MSWI) (S. Adrià del Besòs, Barcelona, Spain), 24 soil and vegetation samples were collected at the same sampling points in which samples had been taken 1 year before. Each sample was analyzed for PCDDs and PCDFs by high-resolution gas chromatography/high-resolution mass spectrometry. While in the previous study PCDD/F concentrations in soil ranged from 1.22 to 34. 28 ng I-TEQ/kg (median and mean values: 9.06 and 12.24 ng I-TEQ/kg), in the present study, PCDD/F levels ranged from 1.33 to 54.23 ng I-TEQ/kg (median and mean values: 11.85 and 14.41 ng I-TEQ/kg). On the other hand, in the previous study, PCDD/F levels in vegetation ranged from 0.33 to 1.98 ng I-TEQ/kg (median and mean values: 0.58 and 0.70 ng I-TEQ/kg), whereas in the present study, PCDD/F levels ranged from 0.32 to 2.52 ng I-TEQ/kg (median and mean values: 0.82 and 0.97 ng I-TEQ/kg). During the last 12 months, PCDD/F levels increased in 16 of the 24 soil samples and in 17 of the 24 vegetation samples analyzed. However, no significant differences in the median I-TEQ concentrations of both studies were found either in soil or vegetation samples.
Conversion of benzene to chlorobenzenes and monochlorophenols by reaction with chlorine radicals (Cl*) in the cool-down zone of a plug-flow combustor has been studied, and a mechanistic analysis of the initial steps of the oxy-chlorination process is proposed. Superequilibrium concentrations of Cl* are formed during combustion of chlorocarbon species and persist at significant concentration levels even after a substantial reduction in the flue gas temperature (T = 500-700 degrees C). At these temperatures, Cl* attack on benzene present in trace concentrations (initial benzene concentration of 300 ppmv or 1080 ppmv were used for the experiments) in the post-flame gas is shown to result in stable chlorinated products (chlorobenzenes and chlorophenols) and loss of benzene. These results suggest that Cl* attack on trace level aromatics and possibly other organic species may be the initial step in the formation of a broad class of chlorinated and oxy-chlorinated pollutants in the post combustion zone.
Polychlorinated dibenzodioxins/furans (PCDD/F) were formed in substantial quantities in a pilot-scale fluidized bed combustor burning salt-laden waste wood, a common fuel for Canadian coastal pulp and paper mills. Formation of PCDD/F increased with increasing chloride content in the feed, and appeared to correlate with the chlorine content in the fly ash. It took a very long time for the ash chlorine content to stabilize, suggesting that chlorine transferred slowly from the flue gas to the ash. The baghouse may contribute largely to formation of the PCDD/F, owing to its temperature range and the potentially long residence time for ash particles. Controlling the baghouse temperature to reduce the PCDD/F formation in the baghouse should be effective in reducing the total emission level. While sulphur addition was found to reduce the emission level by as much as 90%, the emission level was still above the regulated level for the mills burning salt-laden wood under the conditions of the present study. No relation between the emission level and CO concentration in the flue gas was observed.
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