Effects of combustion and operating conditions on PCDD/PCDF emissions from power boilers burning salt-laden wood waste

This paper discusses the effects of combustion conditions on PCDD/PCDF emissions from pulp and paper power boilers burning salt-laden wood waste. We found no correlation between PCDD/PCDF emissions and carbon monoxide emissions. A good correlation was, however, observed between PCDD/PCDF emissions and the concentration of stack polynuclear aromatic hydrocarbons (PAHs) in the absence of TDF addition. Thus, poor combustion conditions responsible for the formation of products of incomplete combustion (PICs), such as PAHs and PCDD/PCDF precursors, increase PCDD/PCDF emissions. PAH concentrations increased with higher boiler load and/or low oxygen concentrations at the boiler exit, probably because of lower available residence times and insufficient excess air. Our findings are consistent with the current understanding that high ash carbon content generally favours heterogeneous reactions leading to either de novo synthesis of PCDD/PCDFs or their direct formation from precursors. We also found that, in grate-fired boilers, a linear increase in the grate/lower furnace temperature produces an exponential decrease in PCDD/PCDF emissions. Although the extent of this effect appears to be mill-specific, particularly at low temperatures, the results indicate that increasing the combustion temperature may decrease PCDD/PCDF emissions. It must be noted, however, that there are other variables, such as elevated ESP and stack temperatures, a high hog salt content, the presence of large amounts of PICs and a high Cl/S ratio, which contribute to higher PCDD/PCDFs emissions. Therefore, higher combustion temperatures, by themselves, will not necessarily result in low PCDD/PCDFs emissions.     

The formation and emission of dioxins in large scale thermal processes

The paper assesses extensive data of PCDD/F measurements on flue gas emissions from thermal processes, including, e.g. municipal solid waste incinerators (MSWIs), combustors of wood and industrial waste, coal fired powerplants and boilers, ferro and non-ferro processes. Numerous investigators have conducted laboratory experiments to assess the formation mechanisms of PCDD/F. The results, obtained from fixed-bed experiments, have been critically evaluated and indicate that de novo synthesis is the dominant mechanism in actual thermal processes where conditions that favour the precursor formation are not experienced. The analysis of PCDD/F profiles from the large scale thermal processes in general, and MSWIs in particular, supports the dominant role of the de novo synthesis, irrespective of the type of thermal process considered. The PCDF/PCDD ratio exceeds 1 and the degree of chlorination points towards the dominant presence of HpCDD and OCDD within the dioxin group, and of PeCDF, HxCDF and HpCDF within the furan group. Since real-time measurement of PCDD/F is impossible, the correlation of PCDD/F emissions with operating parameters and/or emission levels of other more easily measured pollutants could be a tool in predicting the PCDD/F formation levels. Data of Flemish MSWIs were used to statistically assess such correlations. From an evaluation of the data at a given operating temperature, misleading conclusions can be drawn. Only the effect of temperature is evident. After converting all data at a reference temperature of, e.g. 230 degrees C, PCDD/F concentrations achieve nearly constant values, irrespective of the values of other parameters, thus stressing that the major controlling parameter for the PCDD/F emission is the temperature of the ESP. The PCDD/F concentrations increase with temperature in the range up to 280 degrees C. The ESP temperature should be kept preferably between 180 degrees C and 200 degrees C, where de novo synthesis is reduced and where PCDD/Fs are increasingly adsorbed on the fly ash, in line with the standard temperature dependence of adsorption isotherms.     

Assessment of emissions and removal of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) at start-up periods in a hazardous waste incinerator

A study was conducted to observe the changes in polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) levels and congener profiles in the flue gas of a hazardous waste incinerator during two start-up periods. Flue gas samplings were performed simultaneously through Air Pollution Control Devices (APCDs) (including boiler outlet, electrostatic precipitator (ESP) outlet, wet scrubbers (WS) outlet, and activated carbon (AC) filter outlet) in different combustion temperatures during a planned cold (long) start-up and an unplanned warm (short) start-up. The results showed that PCDD/F concentrations could be elevated during the start-up periods up to levels 3–4 times higher than those observed in the normal operation. Especially lower combustion temperatures in the short start-ups may cause high PCDD/F concentrations in the raw flue gas. Assessment of combustion temperatures and Furans/Dioxins values indicated that surface-catalyzed de novo synthesis was the dominant pathway in the formation of PCDD/Fs in the combustion units. PCDD/F removal efficiencies of Air Pollution Control Devices suggested that formation by de novo synthesis existed in ESP also when in operation, leading to increase of gaseous phase PCDD/Fs in ESP. Particle-bound PCDD/Fs were removed mainly by ESP and WS, while gaseous phase PCDD/Fs were removed by WS, and more efficiently by AC filter.

Implications: This paper evaluates PCDD/F emissions and removal performances of APCDs (ESP, wet scrubbers, and activated carbon) during two start-up periods in an incinerator. The main implications are the following: (1) start-up periods increase PCDD/F emissions up to 2–3 times in the incinerator; (2) low combustion temperatures in start-ups cause high PCDD/F emissions in raw gas; (3) formation of PCDD/Fs by de novo synthesis occurs in ESP; (4) AC is efficient in removing gaseous PCDD/Fs, but may increase particle-bound ones; and (5) scrubbers remove both gaseous and particle-bound PCDD/Fs efficiently.     

Establishing the propensity for dioxin formation using a plume temperature model for medical waste incinerator emissions in developing countries

Air pollution control devices (APCDs) are not compulsory for medical waste incinerators (MWIs) in developing countries. In South Africa, combustion gases are usually vented directly to the atmosphere at temperatures greater than the formation temperature of dioxin. The possibility of dioxin formation outside the incinerator stack has been hypothesized. A plume model has been developed and tested in the wind tunnel with a scale model of an incinerator stack. The plume temperature and trajectory predictions of the plume model were verified within a +/- 3% experimental accuracy. Using South African data, the plume model predicts that the residence time of gases in the temperature range of 150-450 degrees C in a plume is 1.3 sec on average for 5% of a year (18 days) at meteorological conditions resulting in wind speeds of less than 1 m/sec. Two published dioxin formation models were used to assess the probability of dioxin formation in the plume. The formation models predict that the average polychlorinated dibenzodioxins/furans (PCDD/Fs) formed in the plume will exceed the stack emission regulations in South Africa of 0.2 ng/Nm3 toxic equivalent quotient (TEQ) by between 2 and 40 times. The calculated concentrations do not include additional gaseous PCDD/F compounds that may be formed at high-temperature post-combustion zones through pyrosynthesis mechanisms.     

Formation of polychlorinated dibenzo-p-dioxins/dibenzofurans from residual carbon on municipal solid waste incinerator fly ash using Na37Cl

Na37Cl was used to study the role of chlorine in the formation of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) from carbon. Adding Na37Cl to fly ash showed that this compound was a (relatively) poor chloride source; chlorine naturally present on the ash - which could include both chlorine in residual carbon and (metal) chlorides - was found to be ca. 17x more reactive. When both Na37Cl and CuCl2 were added to aqueous extracted fly ash, the percentage of 37Cl from Na37Cl included in PCDD/F increased, compared to the combination of Na37Cl/fly ash. When Na37Cl and CuCl2 were exchanged in water, followed by evaporation of the solvent, and mixed with aqueous extracted fly ash, the percentage of 37Cl included in PCDD/F was much higher. Apparently, direct transfer of 37Cl from CuCl2 to carbon and PCDD/F was much faster than transfer of 37Cl- from Na37Cl via a metal chloride (such as CuCl2) to carbon and PCDD/F. In addition to chlorine in PCDD/F originating from exchanged NaCl/CuCl2, chloride left on the fly ash after aqueous extraction and chlorine present in residual carbon could also have been incorporated in PCDD/F.     

Inhibition of PCDD/F by adding sulphur compounds to the feed of a hazardous waste incinerator

Sulphur compounds, including (NH₄)₂SO₄ and pyrite, were tested as suppressants in a hazardous waste incineration facility. The test results suggested that adding sulphur compounds only slightly reduced PCDD/F stack emissions; this restricted effect was attributed to the release of fly ash in large amounts during the sulphur adding experiments, i.e., it was due to a malfunctioning of the baghouse filter. Nevertheless, for the combined flow of flue gas + fly ash a reduction of more than 50% was achieved for the total PCDD/F concentrations and the total toxic concentrations, and an even higher inhibition capability was observed for PCDD. Also, a simulation of the thermodynamic equilibrium conditions by sulphur dioxide was conducted in the domain of experimental interest. Deactivation of catalysts, which promote PCDD/F formation, was found to be the dominant inhibition mechanism in low temperature PCDD/F formation. SO₂ could also inhibit the formation of molecular Cl₂ via the Deacon reaction, but that was not the main reason for inhibition.     

Correlation of PCDD/F emissions with operating parameters of municipal solid waste incinerators

Municipal solid waste incinerator (MSWI) operators frequently have expressed the need for an indirect indication of the expected production of PCDD/F and associated abatement procedures through a possible correlation with other real-time data, through using marker pollutants (e.g., penta/hexachlorinated aromatics), or through other indications. Since real-time measurement of PCDD/F is impossible, a correlation of PCDD/F emissions with other operating parameters could help in achieving these goals. Literature data of large-scale MSWIs concerning these correlations are scarce and seldom conclusive. Extensive data of Flemish MSWIs are used to statistically assess possible correlations of PCDD/F and other operating parameters. The effect of temperature in the electrostatic precipitator (ESP) is of major importance. When converting values measured between 180 and 270 degrees C to a reference temperature of, for example, 230 degrees C, PCDD/F concentrations achieve nearly constant values, stressing that the major correlating parameter is the temperature of the ESP. High temperatures that enhance de novo synthesis should be avoided.     

Formation of aromatic chlorinated compounds catalyzed by copper and iron

This study shows the catalyzing effects of iron and copper on the formation of chlorinated compounds such as chlorobenzenes (ClBzs), chlorophenols (CIPhs), polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). Both total concentrations and congener distributions have been studied. The parameters and conditions varied during the combustion tests were the complete and incomplete combustion and the metal and chlorine addition. The incomplete combustion promoted the formation of organic chlorinated compounds in flue gas particles. Highly chlorinated congeners of PCDD/F were dominant in the flue gas particles, whereas the importance of lower chlorinated congener were increased in the gas phase. In the complete combustion conditions the concentrations of PCDD/Fs increased when the degree of chlorination were high, nevertheless the concentrations of tetra and penta PCDD/Fs were higher in the gas phase than the concentrations in the fly ash particles. Organic chlorine promoted the formation of chlorinated compounds more effectively than inorganic chlorine, which instead promoted the formation of PCDD/Fs in the gas phase, especially with copper catalyst. Different concentration levels of chlorinated compounds were observed in the gas phase and in particles when the chlorine source and combustion conditions were varied from incomplete to optimum conditions. Both copper and iron seem to have a catalytic effect on PCDD/F formation.     

Correlation of PCDD/F Emissions with Operating Parameters of Municipal Solid Waste Incinerators

ABSTRACT

Municipal solid waste incinerator (MSWI) operators frequently have expressed the need for an indirect indication of the expected production of PCDD/F and associated abatement procedures through a possible correlation with other real-time data, through using marker pollutants (e.g., penta/hexachlorinated aromatics), or through other indications. Since real-time measurement of PCDD/F is impossible, a correlation of PCDD/F emissions with other operating parameters could help in achieving these goals. Literature data of large-scale MSWIs concerning these correlations are scarce and seldom conclusive. Extensive data of Flemish MSWIs are used to statistically assess possible correlations of PCDD/F and other operating parameters.

The effect of temperature in the electrostatic precipita-tor (ESP) is of major importance. When converting values measured between 180 and 270 °C to a reference temperature of, for example, 230 °C, PCDD/F concentrations achieve nearly constant values, stressing that the major correlating parameter is the temperature of the ESP. High temperatures that enhance de novo synthesis should be avoided.     

The effect of inorganic and organic chlorine on formation of highly chlorinated organic compounds during incineration: Laboratory pilot study

Aliphatic liquid as a basic fuel was incinerated in a laboratory scale pilot plant. Inorganic chlorine and organic chlorine mixed with basic fuel were used as additive chemicals. Sodium chloride (NaCl) and tetrachloroethylene (C₂Cl₄) were used as the sources of inorganic and organic chlorine. Combustion parameters were adjusted for optimum combustion and, consequently, the amount of particles in flue gases was low. The concentrations of chlorine in flue gases were high enough for possible formation reactions of organic chlorinated compounds in all of the chlorine input tests. An increase in chlorine input did not significantly increase the amounts of highly chlorinated organic compounds, like PCDD/Fs. The main result was that chlorophenol concentrations increased in parallel with organic chlorine input. Comparing organic chlorine to inorganic chlorine tests showed that more highly substituted PCDD/F congeners were formed when organic chlorine was the additive chlorine source. The formation of highly chlorinated organic compounds such as PCDD/Fs requires not only chlorine and aliphatic fuel to be formed, but some catalysts are also needed.     

A municipal solid waste incinerator as the single dominant point source of PCDD/Fs in an area of increased non-Hodgkin's lymphoma incidence

Since 1971, a municipal solid waste incinerator (MSWI) with high dioxin emission levels has been in operation in Besan?on, France. We recently found a 2.3-fold risk of non-Hodgkin's lymphoma in the highest exposure zone using a Gaussian-type dispersion model as a proxy for dioxin exposure. However, the sources of PCDD/Fs in this area are a matter of controversy. The aim of this survey was therefore to examine the nature of the PCDD/F soil contamination in the surroundings of the MSWI to characterize whether more than one potential emission source could explain the presence of the PCDD/Fs. PCDD/F congener profiles were determined in 75 soil samples collected in the vicinity of the MSWI. They were compared according to the most environmentally impacted zones and to various spatial contrasts. PCDD/F concentrations ranged from 0.25 to 28.06 pg WHO-TEQ g(-1). Two different clustering algorithms identified the same main cluster (consisting of 73 samples). The remaining two soil samples composed either one, or two clusters. All clusters showed similar congener profiles. Moreover, no contrast was observed for congener distributions between complex and simple topographies, inside and outside the city boundary, the two most and the two least exposed areas, reflecting a common fingerprint. Congener profiles indicate that the area under influence of the MSWI is not subject to other point sources of PCDD/Fs. Since, the most polluting combustion chambers were recently shut down and replaced by a new one with up-to-date pollution control, slowly decreasing dioxin concentrations in the soils are to be expected.     

Formation of chlorinated and brominated dioxins and other organohalogen compounds at the pilot incineration plant VERONA

Experiments were conducted at the VERONA pilot plant, an incineration plant with stationary grate and separate post-combustion chamber, using wood and propane as basic combustible materials and with controlled dosage of various bromine-, chlorine- and copper-containing compounds. The behaviour of the following compounds was studied in the combustion chamber, after the post-combustion chamber and after the heat exchanger: polychlorinated phenols (PCPh), polybrominated phenols (PBrPh), polychlorinated benzenes (PCBz), polybrominated benzenes (PBrBz), polychlorinated dioxins and furans (PCDD/F) and polybrominated dioxins and furans (PBDD/F). The bromine co-incineration leaded to very high bromophenol concentrations after the post-combustion chamber. The formation of brominated and mixed-halogenated phenols and the further reaction to halogenated dioxins is apparently a relevant reaction mechanism for dioxin formation in processes involving bromine. This assumption is supported by the high formation rates of PBDD/F found in the heat exchanger, which were 4-20 times higher than those of PCDD/F. Moreover, the strong correlations found between the formation rates of PCPh, PCBz and PCDD/F in the heat exchanger indicate that in addition considerable new formation of dioxins takes place through de novo synthesis. Experiments involving the variation of primary operational parameters and fuel properties have shown that the quality of post-combustion plays a much greater role than the other parameters. Furthermore, it became apparent that the congeners of the chlorophenols and of the chlorobenzenes, measured in various incineration stages, do not correlate closely with the dioxin concentrations after the heat exchanger.     

Changes in PCDD/PCDF formation processes during instationary phases of combustor operation--exemplified by the use of Cl4DD isomer patterns

In this paper results of various measurement campaigns at different municipal waste incineration (MWI) plants concerning the change of the PCDD/PCDF isomer distribution in the crude gas during transiently impaired combustion conditions are presented. The focus is on the Cl4DD isomer distributions exemplarily for all other homologue groups to demonstrate the change in PCDD/PCDF formation mechanism at transient combustion conditions. Additionally to crude gas samples, at one plant filter and boiler ash were investigated simultaneously to determine if there is any difference in the isomer distribution between the matrices. For the ash from an electrostatic precipitator (ESP ash), the boiler ash and the corresponding crude gas sample, nearly identical changes in the Cl4DD isomer distribution under transient combustion conditions in relation to the normal operation process could be detected. By comparing the Cl4DD isomer distributions from different incineration plants (two municipal waste incinerators and one little incinerator burning wood chips for heating domestic household) under transient combustion conditions, in all cases the 1,3,6,8- and 1,3,7,9-Cl4DD were dominating the isomer distribution, whereas under normal operation other isomers were predominant. Obviously PCDD/PCDF formation mechanisms under transient combustion conditions are independent from the type of incinerator and of the burned fuel, respectively. Data sets were analyzed with respect to the possible reaction mechanism via chlorophenols and a good correlation of 2,4,6-trichlorophenol during the second phase of a start-up process and during a CO experiment was found. To get more detailed information about possible formation mechanisms, at one plant the dependence of the PCDD/PCDF isomer distribution on the different matrices was studied. Separate analysis of fly ash collected at the boiler exit, subsequent gas phase, ESP ash and boiler ash under normal operation conditions showed that, apart from the fly ash, the Cl4DD isomer distributions are nearly the same in the different matrices. Surprisingly, the Cl4DD isomer distribution of the fly ash was more similar to the distributions found under transient combustion conditions.     

Dioxins from thermal and metallurgical processes: recent studies for the iron and steel industry

In thermal metallurgical processes such as iron ore sintering and metal smelting operations, large flows of off-gases are generated. Mainly due to residue recycling in such processes, chlorine and volatile organics are always present in the feed. As a consequence of "de novo" formation, the off-gases from such processes typically contain dioxins in the range 0.3-30 ng I-TEQ/Nm3. So far there are only very few studies about the mechanisms of dioxin formation and destruction in these metallurgical processes. In an European Union (EU) research project "Minimization of dioxins in thermal industrial processes: mechanisms, monitoring and abatement (MINIDIP)", integrated iron and steel plant has been selected as one of the industrial sectors for further investigation. A large number of particulate samples (feed, belt siftings, electrofilter) were collected from the iron ore sintering installations from various steel plants and analyzed for their organochlorocompound contents. Measurable amounts of PCDD/F, PCBz, PCB were found for all samples. The various parameters influencing their de novo synthesis activity were also evaluated in laboratory experiments, and such activity was found to be moderate for samples from the ore sinter belt, but extremely high for some ESP dusts. Fine dust is active in a wide range of temperatures starting at 200 degrees C and declining above 450 degrees C; the optimal temperature for de novo synthesis was found to be around 350 degrees C; some inhibitors, such as triethanolamine, may reduce de novo activity by 50%, and lowering the O2 concentration in the gas stream leads to a much lower amount of PCDD/F formation. On the basis of their relative mass, typical operating conditions and specific activity of the different samples, the regions in the sintering plant where de novo synthesis may take place were tentatively established.     

Evaluation of the emission characteristics of PCDD/Fs from electric arc furnaces

Distribution of PCDD/F (polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran) congeners at two electric arc furnaces (EAFs) in Taiwan is evaluated via intensive stack sampling and analysis. Two kinds of exhaust system in EAFs including stack system and shutter system are selected for measuring dioxin emissions. In addition, dioxin emissions during oxidation and reduction stages at EAF-A were characterized. Results indicate that the PCDD/F concentration of stack gas in EAF-A was 4.39 ng/N m³ while total Toxic Equivalent Quantity (TEQ) concentration was 0.35 ng I-TEQ/N m³. The PCDD/F concentration of stack gas in EAF-B was 2.20 ng/N m³ and the TEQ concentration was 0.14 ng I-TEQ/N m³. 1,2,3,4,6,7,8-H_pCDF, OCDD and OCDF are the major contributors of the dioxin concentrations for two EAFs investigated and the percentage of PCDD/F in particulate phase increases as the chlorination level of the PCDD/F congener increases. The results obtained on gas/particulate partitioning of PCDD/Fs in flue gases prior to the APCD in EAFs indicate that more than 90% exists in particulate phase. In EAF-A, the PCDD/F concentration during oxidation stage is slightly higher than that measured during reduction stage, including the sampling points of CO converter outlet, prior to bag filter and stack. Majority of PCDD/Fs emitted from steel-making processes exists in particulate-phase (about 60–70%) at both EAFs investigated.     

The influence of level and chlorine source on the formation of mono- to octa-chlorinated dibenzo-p-dioxins, dibenzofurans and coplanar polychlorinated biphenyls during combustion of an artificial municipal waste

The formation of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and the three coplanar polychlorinated biphenyls (pPCBs) was studied during labscale fludized bed combustion of eight artificial municipal solid waste (MSW) fuel mixtures. The level of chlorine as well as the chlorine source varies within the different fuel mixtures. Four different chlorine sources were studied, viz, an inorgnaic (NaCl) and three organic sources, pure PVC plastic and two products (floor and cable) and the total chlorine level varies between 0.28% and 1.1%. The experiments were performed in a 5 kW laboratory scale fluidized bed reactor. A correlation between the total chlorine in the fuel and the formation of the hepta- and octa-chlorinated PCDD/F homologues was found. However, the most important variable for changes in the PCDDs/Fs and pPCBs formation was disturbance in the combustion condition and not the variation in chlorine content of the fuel. Furthermore, no differences in formation between the chlorine sources could be seen.     

Sources of dioxins in the United Kingdom: the steel industry and other sources

Several countries have compiled national inventories of dioxin (polychlorinated dibenzo-p-dioxin [PCDD] and polychlorinated dibenzofuran [PCDF]) releases that detail annual mass emission estimates for regulated sources. High temperature processes, such as commercial waste incineration and iron ore sintering used in the production of iron and steel, have been identified as point sources of dioxins. Other important releases of dioxins are from various diffuse sources such as bonfire burning and domestic heating. The PCDD/F inventory for emissions to air in the UK has decreased significantly from 1995 to 1998 because of reduced emissions from waste incinerators which now generally operate at waste gas stack emissions of 1 ng I-TEQ/Nm3 or below. The iron ore sintering process is the only noteworthy source of PCDD/Fs at integrated iron and steelworks operated by Corus (formerly British Steel plc) in the UK. The mean waste gas stack PCDD/F concentration for this process is 1,2 ng I-TEQ/Nm3 based on 94 measurements and it has been estimated that this results in an annual mass release of approximately 38 g I-TEQ per annum. Diffuse sources now form a major contribution to the UK inventory as PCDD/Fs from regulated sources have decreased, for example, the annual celebration of Bonfire Night on 5th November in the UK causes an estimated release of 30 g I-TEQ, similar to that emitted by five sinter plants in the UK.     

Improvements in dioxin abatement strategies at a municipal waste management plant in Barcelona

This study presents the results of a dioxin abatement programme undertaken in the municipal waste incineration plant of Montcada i Reixac (Barcelona, Spain) after the replacement of an obsolete air cleaning device by a new flue gas treatment system. A number of sampling campaigns were conducted with the aim of characterising stack gas emission levels of polychlorinated dibenzo-p-dioxins (PCDDs)/polychlorinated dibenzofurans (PCDFs) and to evaluate initial specifications of dioxin stack gas emission values below 0.1 ng I-TEQ/Nm(3). Preliminary results revealed levels between 44 and 111 ng I-TEQ/Nm(3) when the gas-cleaning system consisted only of an old electrostatic precipitator (ESP). Decreased levels around 15 ng I-TEQ/Nm(3) were observed when the semi-dry scrubber began to operate and the ESP was switched off. Again, remarkable dioxin removal was observed after the installation of the fabric filter and levels around 0.3-0.4 ng I-TEQ/Nm(3) were soon achieved. Nevertheless, the limit of 0.1 ng I-TEQ/Nm(3) was reached by additional injection of activated carbon which helped to lower PCDD/PCDF levels to around 0.036 ng I-TEQ/Nm(3). The results also demonstrated a significant change in the dioxin distribution present in combustion-derived materials (stack gas emission, bottom ash and solid waste from gas treatment). The major dioxin fraction was found in gaseous matrices before the flue gas control system was upgraded. After this step, the major dioxin fraction content was observed in solid waste from gas treatment.     

Formation and mitigation of PCDD/Fs in iron ore sintering

The sintering of iron ore is presently a significant industrial source of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) worldwide owing to the fundamental requirement of the operation of a high temperature process to pre-treat fines and to recycle plant by-products arising from the integrated iron and steelworks. The process is a noteworthy contributor of PCDD/F indirectly due to decreasing PCDD/F releases from municipal solid waste incineration. Commonly PCDD/F formation from the process is associated with the addition of oily mill scales although raw material containing a combination of C, Cl and specific metal catalyst has been shown to drastically increase PCDD/F formation in the process. The degenerate graphitic structure of carbon present in coke fuel and soot formed and the chemistry of catalytic metals and Cl are important factors. A review on PCDD/F emission in this process has been carried out, including examination of its formation mechanism, congener distribution, contributing factors and mitigation strategies, with emphasis on the use of inhibiting compound to achieve suppression. A detailed analysis of the de novo and precursor theories of formation and the contributing factor is given since the subject of PCDD/F formation is still controversial. The de novo formation pathway identified in sinter plants and controlled studies performed in the laboratory as well as at pilot-scale are discussed; where appropriate, a comparison is drawn between sintering and other thermal processes emitting PCDD/Fs. Summary of the latest developments in PCDD/F downstream abatement strategies presently employed in full scale industrial plants is provided. Potential inhibiting compounds are discussed in terms of their mode of action and merits under sintering conditions.     

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 (N2 + 10%O2, N2 + 1%O2 and 99.999% N2) 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, CuCl2 x 2H2O and NaCl at 340 degrees C was studied. The content of PCDD/F for systems with 10%O2, 1%O2 and 99.999% N2 was decreasing and corresponded to 17,304, 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.