Chemical kinetic modeling of de novo synthesis of PCDD/F in municipal waste incinerators

A kinetic model is developed for de novo synthesis of PCDD/F from carbon in incinerator fly ash. The main mechanistic steps considered in the model are carbon gasification, PCDD/F formation, desorption and degradation. Rate equations are derived which can relate PCDD/F formation with process variables including carbon concentration of fly ash, partial pressure of oxygen, reaction temperature and time. The kinetic model has been verified using laboratory de novo synthesis data reported in the literature. When the model is applied to industrial incinerator conditions, PCDD/F formation levels of 0.1-0.5 microg/N m3 in the gas phase and 0.1-1.2 microg/g in the solid phase are calculated, and both are in good agreement with incinerator measurements.     

Kinetics of carbon degradation and PCDD/PCDF formation on MSWI fly ash

The native carbon oxidation and PolyChloroDibenzo-p-Dioxins and PolyChloroDibenzoFurans, PCDD/F, formation were simultaneously studied at different temperatures (230-350 degrees C) and times (0-1440 min) in order to establish a direct correlation between the disappearance of the reagent and the formation of the products. The kinetic runs were conducted in an experimental set up where conditions were chosen to gain information on the role of fly ash deposits in cold zones of municipal solid waste incinerators in PCDD/F formation reaction. The carbon oxidation measured as the decrease of total organic carbon of fly ash was in agreement with the carbon evolved as sum of CO and CO(2). The carbon mass balance indicated an increase in the efficiency of carbon conversion in CO and CO(2) with temperature. The CO and CO(2) formation was the result of two parallel pseudo first order reactions thus giving significant information about the reaction mechanism. PCDD/F formation as a function of temperature showed that the maximum formation was achieved in a narrow range around 280 degrees C; the time effect at 280 degrees C was a progressive formation increase at least up to 900 min. The PCDF:PCDD molar ratio increased with temperature and time, and the most abundant homologues were HxCDD, HpCDD, OCDD for PCDD, and HxCDF, HpCDF within PCDF. These experimental results supported the hypothesis that the formation mechanism was the de novo synthesis.     

Influence of residual carbon on the decomposition process of PCDD/Fs in MSWI fly ash

In heating treatment of fly ash to reduce PCDD/Fs, cooling process is important to inhibit de novo formation of PCDD/Fs. In this study, assuming that residual carbon is the dominant factor of de novo synthesis, the relation between PCDD/Fs and residual carbon was examined. Firstly, by using MSWI fly ash which was treated in an actually operated facility, both the content of PCDD/Fs and residual carbon were decreased as heating temperature increased. At temperatures higher than 400 degrees C, the reduction rate of residual carbon was higher than 20% and more than 95% of PCDD/Fs was decomposed. In order to simulate a heating treatment process, fly ash was heated at different temperatures and gas atmospheres, oxygen or nitrogen. Heated fly ash was placed for 2 h at 300 degrees C in oxygen to promote de novo synthesis, or cooled immediately. As a result, good correlation between PCDD/Fs and residual carbon was found, therefore it was shown experimentally that residual carbon was the main factor for PCDD/Fs formation by de novo synthesis in fly ash.     

The effects of temperature and oxygen content on the PCDD/PCDFs formation in MSW fly ash

In this study, the effects of the temperature, oxygen content in the gas stream and carbon content in ash particles on PCDD/Fs formation on the fly ash surface were investigated. The optimum temperatures for dioxin formation were found at 350 degrees C for boiler ash, 300 degrees C for cyclone ash and 250 degrees C for ESP ash, respectively. Preliminary results indicate that the optimum temperature will decrease as the particle size decreases. When the O2 concentration is varied between 0% and 100%, the optimum oxygen content for PCDD/Fs formation is found to be at 7.5% for cyclone ash, and the PCDD/PCDF ratio increases with the increase of oxygen content. Dioxin formation is observed even for the gas containing no oxygen passed through the fly ash. Hence, other reacted routes that do not need O2 for dioxin formation take place on fly ash. The carbon content in fly ash is varied between 0% and 20% in this study, and the results have indicated that the maximum dioxin formation is to be found at 5%. The precursors are not injected into the fly ash or gas stream in all formation experiments, however, dioxin is still formed in fly ash. Consequently, other chlorinated routes besides Deacon reactions may take place on the fly ash surface.     

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.     

Characterization of residual carbon influencing on de novo synthesis of PCDD/Fs in MSWI fly ash

Using 19 samples of fly ash collected from various MSW incineration facilities, residual carbon was characterized by gasifiable fraction at 450 degrees C (C450), and the correlations with de novo synthesis of PCDD/Fs were experimentally examined. Fly ashes were classified into three groups by the ratio of C450 to total residual carbon. By comparison of CO and CO2 generation patterns with those of reference materials, unburnt carbon of solid waste and activated carbon powder injected into flue gas were identified as a carbon source in fly ash. In the experiment of de novo synthesis of PCDD/Fs, the content of PCDD/F synthesis depended on C450 regardless of the origin of carbon. In addition, the model to predict the content of PCDD/F synthesis, DeltaPCDD/F=0.989.Cu.C450, fitted well with experimental values.     

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.     

Reactions of 2,4,6-trichlorophenol on model fly ash: oxidation to CO and CO2, 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 degrees C and 400 degrees C leads predominantly to carbon monoxide and carbon dioxide. The fraction of 2,4,6-trichlorophenol which is oxidized to CO and CO2 increases from 3% at 250 degrees C to 75% at 400 degrees 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.     

Effects of selected metal oxides on the dechlorination and destruction of PCDD and PCDF

OCDD and OCDF spiked silica/graphite based model fly ash containing various copper compounds and metal oxides were thermally treated under oxygen deficient conditions. All copper compounds tested showed a considerable dechlorination/hydrogenation reaction at 260 °C. After 30 min at 340 °C, less than 1% of the spiked OCDD and OCDF was recovered as T₄CDD/F to OCDD/F. Other compounds tested demonstrated a lower rate of dechlorination compared to the copper compounds. However, all other metal oxides showed a small dechlorination effect at 260 °C, which was considerably increased at 340 °C.

The model fly ash containing the different copper compounds or metal oxides showed comparable PCDD and PCDF isomer patterns after thermal treatment. However, small differences were observed among the different tested compounds. The PCDD and PCDF isomer patterns on the model fly ashes were similar to patterns found during dechlorination experiments on fly ashes from waste incineration processes.

Model fly ash containing Ca(OH)₂ exhibited the highest destruction potential, but a low dechlorination potential. In contrast, model fly ash containing any of the remaining compounds tested, was found to predominantly dechlorinate the spiked OCDD and OCDF.     

Laboratory studies on formation and minimisation of polychlorinated dibenzodioxins and -furans (PCDD/F) in secondary aluminium process

The objectives of this work were to study the formation mechanisms of polychlorinated dibenzo-p-dioxins/polychlorinated dibenzofurans (PCDD/F) in thermal aluminium recycling processes by use of laboratory experiments. The pattern of isomers of PCDD/F indicates that de novo synthesis is important in aluminium smeltery. The mechanisms of PCDD/F formation in aluminium smelting are similar to that of various incineration processes of waste material. The results of bioanalysis (EROD-test) confirms the existence of de novo synthesis of PCDD/F, but points out to the existence to some additional, toxic compounds of unknown structure. To reduce the amount of PCDD/F the input of carbon at the metal should be reduced; in addition the metal smeltery plants should be cleaned from fly ash particles. It is suggested to use good primary methods in the technical plants like constant feeding of the metal into the oven will minimise PCDD/F concentration. The biological EROD-bioassay is a good tool to estimate PCDD/F-TEQ values also for this technical process simulated in the laboratory.     

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.     

Chemical kinetic modelling of PCDD formation from chlorophenol catalysed by incinerator fly ash

A kinetic model is developed for PCDD formation from chlorophenol catalysed by incinerator fly ash. The key step in the model is a Langmuir-Hinshelwood type elementary step for the coupling of two adsorbed chlorophenol species to PCDD. Kinetic expression is derived which can relate PCDD formation rates with process variables including temperature, precursor concentration, fly ash loading and number of active sites in fly ash. Calculated PCDD formation rates based on this kinetic model are in good agreement with laboratory measurements reported in the literature. When the model is applied to industrial incinerator conditions, at maximum a PCDD yield of 10(-3) microg/N m3 is calculated.     

Comparison of 2,4,6-trichlorophenol conversion to PCDD/ PCDF on a MSWI-fly ash and a model fly ash

We performed experiments on two different matrices with 2,4,6-trichlorophenol as precursor to Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD)/F. A municipal solid waste incinerators (MSWI) and a model fly ash were spiked in two different ways. The experiments demonstrated a three times higher formation potential of the trichlorophenol to PCDD on MSWI fly ash compared with the model fly ash used. For both fly ashes the PCDD yield was higher when gaseous trichlorophenol was fed continuously compared to mixing the fly ashes prior to the experiments with the total amount of the precursor. Despite dilution of the fly ashes tenfold with an inactive matrix the conversion of the chlorophenol was very high.     

Formation and destruction of PCDD/F inside a grate furnace

Formation and destruction of polychlorinated dibenzo-p-dioxins and dibenzofurans PCDD/F during the combustion process was investigated experimentally in a pilot plant. All important process steps like the burnout of the fuel bed on the grate, the burnout of the flue gas inside the combustion chamber, the heat recovery in a boiler as well as influences of the fuel composition are described in detail.

High concentrations especially of PCDF are formed during the burnout of the fuel bed. The formation reaction is mainly influenced by the fuel composition and the burnout characteristic of the fuel bed. Fuels with low chlorine and low metal content (Cu) result only in negligible concentrations of PCDD/F.

Under stable combustion conditions characterized by an excellent flue gas burnout PCDD/F will almost be completely destroyed already inside the combustion chamber. “Cold strands” of unburned flue gas (high CO concentrations) caused by disturbed combustion conditions will result in high concentrations of PCDD and especially of PCDF in the raw gas.

A second place of PCDD/F formation is the well-known boiler section. Here fly ash deposits containing residual carbon (mainly soot particles) are the source for the formation reaction. Under stationary effective combustion conditions, they are dominant for PCDD/F concentrations in the raw gas over a very long period of time.

Stationary efficient flue gas burnout (especially soot) together with effective boiler cleaning will guaranty low concentrations of PCDD/F in the flue gas in front of the flue gas cleaning system.     

Dioxins and their fingerprint in size-classified fly ash fractions from municipal solid waste incinerators in China—Mechanical grate and fluidized bed units

The distribution of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs), in brief dioxins, has seldom been addressed systematically in fly ash from municipal solid waste incinerators (MSWIs). This study shows the amount and fingerprint of PCDD/Fs in fly ash from four different Chinese MSWIs, that is, three mechanical grate units and one circulating fluidized bed unit. In these fly ash samples, dioxins-related parameters (international toxic equivalent quantity, total amount of PCDD/Fs, individual isomer classes, and 17 toxic 2,3,7,8-substituted congeners) all tend to increase with decreasing particle size for mechanical grate incinerators, yet only for the finest fraction for fluidized bed units. Moreover, the fluidized bed incinerator seems superior to grate incineration in controlling dioxins, yet a comparison is hampered by internal differences in the sample, for example, the fluidized bed fly ash has much lower carbon and chlorine contents. In addition, the presence of sulfur from mixing coal as supplemental fuel to the MSW may poison the catalytic steps in dioxins formation and thus suppress the formation of dioxins. With more residual carbon and chlorine in the fly ash, it is easier to form dioxins during cooling. Nevertheless, there is no apparent relation between Fe, Cu, and Zn contents and that of dioxins in fly ash.

Implications This paper is of interest because it presents the amounts and distribution of PCDD/Fs in fly ash samples from some typical waste incineration plants in China, featuring distinct incinerator types, combustion conditions, fuel composition, or residual carbon, chloride, and heavy metal contents in fly ash.     

Dioxins and their fingerprint in size-classified fly ash fractions from municipal solid waste incinerators in China--mechanical grate and fluidized bed units

The distribution of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs), in brief dioxins, has seldom been addressed systematically in fly ash from municipal solid waste incinerators (MSWIs). This study shows the amount and fingerprint of PCDD/Fs in fly ash from four different Chinese MSWIs, that is, three mechanical grate units and one circulating fluidized bed unit. In these fly ash samples, dioxins-related parameters (international toxic equivalent quantity, total amount of PCDD/Fs, individual isomer classes, and 17 toxic 2,3,7,8-substituted congeners) all tend to increase with decreasing particle size for mechanical grate incinerators, yet only for the finest fraction for fluidized bed units. Moreover, the fluidized bed incinerator seems superior to grate incineration in controlling dioxins, yet a comparison is hampered by internal differences in the sample, for example, the fluidized bed fly ash has much lower carbon and chlorine contents. In addition, the presence of sulfur from mixing coal as supplemental fuel to the MSW may poison the catalytic steps in dioxins formation and thus suppress the formation of dioxins. With more residual carbon and chlorine in the fly ash, it is easier to form dioxins during cooling. Nevertheless, there is no apparent relation between Fe, Cu, and Zn contents and that of dioxins in fly ash.     

Generation of PCDD/F in fly ash from municipal solid waste incinerators

Fly ash from municipal solid waste incinerators (MSWIs) has been characterized in terms of polychlorinated dibenzyl-p-dioxin and polychlorinated dibenzofuran (PCDD/F) content. Increasing values of PCDD/Fs have been found to correlate with decreasing temperatures of sampling points in flue gas treatment lines of the plants, confirming other researchers' findings about temperature as the major controlling parameter for the PCDD/F formation. Measured PCDD/F ratios show that de novo synthesis is the dominant formation mechanism. The increasing trends of particulate-bound PCDD/Fs can be explained not only through the dominant de novo synthesis process but also considering the adsorption of gaseous PCDD/Fs on fly ash deposits, even outside the typical de novo synthesis temperature ranges. The effective role of a post-combustor unit, imposed by Italian law to destroy PCDD/ Fs, also needs to be carefully reconsidered.     

Mechanistic studies on the role of PAHs and related compounds in PCDD/F formation on model fly ashes

Model fly ashes containing Florisil, CuCl2.2H2O and PAHs with structures similar to dibenzo-p-dioxin or dibenzofuran were heated at 250 degrees C in He/O2 with regard to a supposed intramolecular reaction mechanism for oxygen incorporation. Highest reactivities in PCDF formation could be found for model compounds containing a biphenyl structure, while condensed pi-systems lead to a decrease in reactivity for such compounds. Biphenyl is almost completely converted to dibenzofuran. PCDD formation from six-membered rings like xanthene/9,10-dihydroanthracene is of minor importance. 18O-labeling of gaseous oxygen reveals no common reaction step for oxygen incorporation using 9-fluorenone, xanthene, diphenyl ether and diphenyl-2-carboxylic acid as model compounds. Pre-existing oxygen in reactants is a major source for ether groups in PCDD and PCDF. Determination of labeled and unlabeled CO and CO2 besides He/O2 reflects higher reactivities towards oxidation for model compounds containing ether groups than for compounds with carbonyl groups.     

Levels and characteristic homologue patterns of polychlorinated dibenzo-p-dioxins and dibenzofurans in various incinerator emissions and in air collected near an incinerator

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) were monitored in stack gas and fly ash of various Korean incinerators and in air samples collected near the facilities. Concentrations of PCDD/Fs in emissions were investigated, and characteristic PCDD/F homologue patterns were classified using statistical analyses. The PCDD/F emission levels in stack gas and fly ash samples from small incinerators (SIs) were higher than those from municipal solid waste incinerators (MSWIs). The PCDD/F concentrations ranged between 0.38 and 1.16 pg I-TEQ/m3 (21.2-75.2 pg/m3) in ambient air samples. The lower-chlorinated furans were the dominant components in most of the stack gas and fly ash samples from SIs, although this was not the case for fly ash from MSWIs. This homologue pattern is consistent with other studies reporting a high fraction of lower-chlorinated furans in most environmental samples affected by incinerator emissions, and it can be used as an indicator to assess the impact of such facilities on the surrounding environment.     

Adsorption of PCDD/F on MWI fly ash

The removal of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) from waste incinerator off-gas is a costly task, because a considerable part of the PCDD/F may exist in the gas phase (often 50-100% around 200 degrees C). The volatile fraction passes the particle filter and the subsequent gas cleaning equipment, so that an additional unit is needed to remove the gaseous PCDD/F from the flue gas. Moreover, dioxins and furans can accumulate in some parts of the equipment in a way that they can act as a latent source. In this work, we investigate the possibility to adsorb the PCDD/F at the fly ash particles and to remove them during the filtration. The gas/particle partitioning of the PCDD/F depends on the temperature, the vapor pressure, the particle size, the particle number density and on the physical and chemical properties of the particle surface. These relationships are investigated by model calculations and by pilot scale experiments (500 Nm3/h) which employ one selected hexachlorinated dioxin congener. At room temperature, approx. 90% of the HxCDD are found in the particulate phase, while at 135 degrees C that portion is only 10%. This means that at ambient temperatures, the gas/particle partitioning of the dioxin corresponds well to the sublimation equilibrium. At higher temperatures, it is much different from the sublimation equilibrium and the apparent adsorption enthalpy is smaller than the enthalpy of sublimation. This observation is in agreement with literature data. From the above experiments and from similar literature data, the efficiency of fly ash particles as a sink for PCDD/F can be evaluated. The data suggest that the adsorption rate is not the limiting factor for the transfer into the particulate phase. The important factors appear to be the chemical composition of the fly ash and the temperature.