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.     

Release of polychlorinated dibenzo-p-dioxins and dibenzofurans by setting off fireworks.

Selected pyrotechnic articles were set off under laboratory conditions. Residues and vapors of smoke as well as unburnt charges were analyzed for polychlorinated dibenzo-p-dioxins, dibenzofurans (PCDD/F) and their precursors chlorinated benzenes and phenols. The contamination of the selected products with these organic xenobiotics proved to be very variable. Remains of fireworks contained octachlorinated dioxins and furans up to 142 ng I-TEQ/kg as well as hexachlorobenzene in the range of 0.05 to 1,400 mg/kg. The deflagration of detonating compositions usually resulted in a dispersion of contaminants, whereas continuously burning flare compositions partially led to a thermal decomposition of organic pollutants. A significant rate of formation of polychlorinated dioxins and furans was observed when setting off blue-lightning rockets and fountains. Further investigations revealed that even high temperatures during the deflagration of black powder charges could not suppress the formation of PCDD/F from appropriate precursors.     

PCDD/F inhibition by prior addition of urea to the solid fuel in laboratory experiments and results statistical evaluation

The objectives of this work were to study the effect of urea addition to the fuel before the combustion on the suppression of PCDD/F emissions, to examine the influence of urea to PCDD/F isomer patterns by employing statistical analysis techniques and to determine the most effective method of urea addition. Urea in the form of powder and as an aqueous solution was mixed with RDF and the fuel mixtures were incinerated in a lab-scale reactor at 1000 degrees C. PCDD/F emitted during combustion experiments were collected in a quartz wool filter downstream the reactor unit. Analysis and quantification of PCDD/F showed that urea significantly affected PCDD/F emissions. Combustion of RDF containing 10% w/w urea resulted in PCDD/F emissions lower than 8 I-TEQ ng/g RDF, while the corresponding average value from single RDF combustion was 17 I-TEQ ng/g RDF. The PCDD/F prevention capacity of urea was independent from the method of urea addition to the fuel, as similar results were obtained after the addition of urea in the solid and in the aqueous phases. Results assessment by statistical methods showed that isomer patterns remained almost stable and were not affected by the method of urea addition.     

Effects of burnings of wax apple stubble and rice straw on polychlorinated dibenzo-p-dioxin and dibenzofuran concentrations in air and soil

Measurements of the concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) were made in ambient air, ash, and soil impacted by the open burning of wax apple and rice straw residues. Measurements showed that the mean PCDD/F concentration (0.458 pg I-TEQ/Nm3; international toxicity equivalence) in air at two wax apple orchards during open burning increased markedly, -8.1 times higher than that (0.057 pg I-TEQ/Nm3); before open burning. In addition, the mean PCDD/F concentration (0.409 pg I-TEQ/Nm3) in ambient air at a rice straw field was 4.6 times higher than that (0.089 pg I-TEQ/Nm3) before open burning. After burning the residues of wax apple stubble and rice straw, the contents of PCDD/F in ashes were 1.393 and 1.568 ng I-TEQ/kg-ash, respectively, and the contents of PCDD/F in soil were 2.258 and 2.890 ng I-TEQ/kg-soil, respectively. Therefore, the turnover of soil with the ash after open burning over years will result in the accumulation of PCDD/Fs in farm soils.     

Influence of variation in the operating conditions on PCDD/F distribution in a full-scale MSW incinerator

Optimizing the operating parameters to minimize polychlorodibenzo-p-dioxins and polychlorodibenzofurans (PCDD/F) emission is the common interest of the municipal solid waste (MSW) incineration industry. In this study, we investigated the distribution of tetra- to octa-CDD/F along the flue gas line in a full-scale reciprocating grate incinerator and evaluated the effects of temperature control and O(2) level on PCDD/F formation. Six runs were laid out and all performed under sufficient burning conditions, in which the combustion efficiency of MSW was more than 99.9%. The total concentration of tetra- to octa-CDD/F measured at the boiler outlet showed an increasing tendency with the increase of boiler outlet temperature (T(B)) from 214 degrees C to 264 degrees C. When flue gas ran across the semi-dry scrubber and cyclone precipitator, in which the temperature varied from 264 degrees C to 162 degrees C, the concentrations of the lower chlorinated dioxins and furans were significantly raised, especially for the TCDF. Increasing O(2) supply from 6.0% to 10.5% essentially led to a higher yield of tetra- to octa-PCDD/F, suggested that under sufficient burning conditions the lower O(2) level was favorable for reducing PCDD/F formation and emission. The variation of O(2) level did not give rise to a systematical change of PCDD/F homologue pattern. For all measurements, the isomer distributions of tetra- to hepta-PCDD/F were more or less the same, nearly independent of variations in the operating conditions and sampling positions. Only the significant increase of the sum of 1,3,7,8-TCDF and 1,3,7,9-TCDF was found in the zone after the boiler section.     

In-field measurements of PCDD/F emissions from domestic heating appliances for solid fuels

Within this project the emissions into the atmosphere of polychlorinated dibenzo-p-dioxins and -furans (PCDD/F) of 30 domestic heating appliances in Austrian households were tested. The appliances were single stoves (kitchen stove, continuous burning stove and tiled stove) and central heating boilers for solid fuels up to a nominal heat input of 50 kW. A main objective of this survey was to determine the PCDD/F emissions of domestic heating units under routine conditions. Therefore, the habitual combustion conditions used by the operators were not influenced. The original fuels and lightning supports were used and the operation of the units was carried out by the householders according to their usual practice. The data obtained were used to calculate in-field PCDD/F-emission factors. Most of the appliances have shown PCDD/F emissions within a concentration range of 0.01-0.3 ng TEQ/MJ. Modern fan-assisted wood heating boilers with afterburning and units for continuously burning of wood chips and wood pellets had the lowest emissions. High emissions were caused by unsuitable heating habits such as combustion of wastes and inappropriate operation of the appliances. There were only small differences between single stoves and central heating boilers or between wood and coal-fired appliances. The emission factors calculated are higher than those cited in literature, which are mainly derived from trials on test stands under laboratory conditions.     

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.     

Influence of start-up on PCDD/F emission of incinerators

This study aims to evaluate the influence of start-up on polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) concentration in the stack flue gas of incinerators and its contributing PCDD/F emission. The PCDD/F emission of the first sample among three consecutive stack flue gas samples of five intermittent incinerators, which sampled at a stable combustion condition after start-up, is 2-3 times higher than the mean of the others. For verifying the PCDD/F characteristics of incinerators during start-up, one continuous MSWI was investigated for two years. The elevated PCDD/F emissions of the MSWI during start-up could reach 96.9 ng I-TEQN m(-3) and still maintained a high PCDD/F emission (40 times higher than the Taiwan emission limit) even 18 h after the injection of activated carbon, indicating the memory effect. Taking the MSWI for example, which consists of four incinerators, the estimated annual PCDD/F emission from normal operational conditions was 0.112 g I-TEQ. However, one start-up procedure can generate approximately 60% of the PCDD/F emissions for one whole year of normal operations. And the PCDD/F emission, which is the result of the start-ups of four incinerators, was at least two times larger than that of a whole year's normal operations, without consideration for the PCDD/F emission contributed by the long lasting memory effect.     

Emissions of PCDD/F from uncontrolled, domestic waste burning

Emissions of polychlorinated dibenzodioxin and dibenzofuran (PCDD/F) result from inefficiencies of combustion processes, most typically waste combustion. Uncontrolled combustion, such as occurs during so-called "backyard burning" of domestic waste, may therefore produce optimal conditions for formation and emission of PCDD/F. However, few assessments of PCDD/F emissions are available from these sources. This work describes the first known comprehensive assessment of PCDD/F emissions from uncontrolled, domestic waste burning. Emissions were copious, but highly variable, ranging over several orders of magnitude. The potential for emissions appears to be related primarily to combustion parameters and concentrations of various gas-phase species, the latter which may be affected by changes in waste composition, waste orientation, and/or combustion conditions.     

Thermodynamic analysis and kinetic modelling of dioxin formation and emissions from power boilers firing salt-laden hog fuel

Both organic chlorine (e.g. PVC) and inorganic chlorides (e.g. NaCl) can be significant chlorine sources for dioxin and furan (PCDD/F) formation in combustion processes. This paper presents a thermodynamic analysis of high temperature salt chemistry. Its influence on PCDD/F formation in power boilers burning salt-laden wood waste is examined through the relationships between Cl2, HCl, NaCl(g) and NaCl(c). These analyses show that while HCl is a product of combustion of PVC-laden municipal solid waste, NaCl can be converted to HCl in hog fuel boilers by reactions with SO2 or alumino-silicate materials. Cl2 is a strong chlorinating agent for PCDD/F formation. HCl can be oxidized to Cl2 by O2, and Cl2 can be reduced back to HCl by SO2. The presence of sulphur at low concentrations thus enhances PCDD/F formation by increasing HCl concentrations. At high concentrations, sulphur inhibits de novo formation of PCDD/Fs through Cl2 reduction by excess SO2. The effect of NH3, CO and NOx on PCDD/F formation is also discussed. A semi-empirical kinetic model is proposed. This model considers both precursor and de novo formation mechanisms. A simplified version is used as a stack emission model. The kinetic model indicates that stack dioxin emissions will increase linearly with decreasing electrostatic precipitator (ESP) efficiency and exponentially with increasing ESP temperature.     

Emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans from stack gases of electric arc furnaces and secondary aluminum smelters

This study investigates the emissions of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) from four electric arc furnaces (EAFs) and eight secondary aluminum smelters (secondary ALSs) in Taiwan. The mean PCDD/F International-Toxicity Equivalents (I-TEQ) concentrations in the stack gases of these EAFs and secondary ALSs are 0.28 ng I-TEQ/Nm3 (relative standard deviation [RSD]= 100%) and 3.3 ng I-TEQ/Nm3 (RSD = 260%), respectively. The high RSDs, especially for those obtained from secondary ALSs, could be caused by the intrinsic differences in their involved feeding materials, furnace operating conditions, and air pollution control devices. The mean I-TEQ emission factor of PCDD/Fs for EAFs (1.8 microg I-TEQ/tonne-feedstock) is lower than that for secondary ALSs (37 microg I-TEQ/tonne-feedstock). This result might be because the involved furnace temperatures for secondary ALSs (650-750 degrees C) are lower than those for EAFs (1600-1700 degrees C), resulting in the deterioration of the combustion condition, leading to the formation of PCDD/Fs during the industrial process. This study found that the total PCDD/F emissions from EAFs (20 g I-TEQ/yr) and secondary ALSs (18 g I-TEQ/yr) are approximately 27, 53, and approximately 24, 49 times higher than those from municipal solid waste incinerators (MSWIs; 0.74 g I-TEQ/yr) and medical waste incinerators (MWIs; 0.37 g I-TEQ/yr), respectively; while those are 44 and 40% of total PCDD/F emission from sinter plants (45 g I-TEQ/ yr), respectively. Considering a more stringent emission limit has been applied to waste incinerators (0.1 ng I-TEQ/Nm3) in Taiwan lately, the results suggest that the control of the emissions from metallurgical processes has become the most important issue for reducing the total PCDD/F emission from industrial sectors to the ambient environment.     

PCDD/F formation from oxy-PAH precursors in waste incinerator flyash

The yield of PCDD/F in relation to the presence of oxygenated PAH in model waste incinerator flyash has been investigated in a fixed bed laboratory scale reactor. Experiments were undertaken by thermal treatment of the model flyash at 250 and 350 °C under a simulated flue gas stream for 2 h. After reaction, the PCDD/F content of the reacted flyash and the PCDD/F released into the exhaust gas, and subsequently trapped by XAD-II resin in a down-stream condensation system were analyzed. The PAHs investigated were, dibenzofuran and benzo[b]naphtho[2,3-d]furan and were spiked onto the model flyash as reactant precursors for PCDD/F formation. The results showed significant formation of furans from both of the PAH investigated, however except from some highly chlorinated dioxin congeners, the formation of dioxins was not so common. Benzonaphthofuran was significantly more reactive than dibenzofuran in PCDD/F formation, in spite of the fact that dibenzofuran is structurally more similar to that of PCDD/F. Thus, there was no clear attribution between the chemical structure of PAH used and the formation of PCDD/F. There were considerable differences between the yields of PCDD/F congeners in the gaseous species and those in the reacted flyash under the same operational conditions. The concentration of PCDD/Fs was reduced at the higher reaction temperature of 350 °C; however, the higher temperature resulted in the majority of the PCDD/F formed on the flyash being released into the gas phase.     

Polychlorinated dibenzo-p-dioxins and dibenzofurans in soils and sediments from Daliao River Basin, China

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) were analyzed in surface sediments and top soils collected from 30 sites in Daliao River Basin. The concentrations of PCDD/F ranged from 0.28 to 29.01 ng TEQ kg(-1) dw (mean value, 7.45 ng TEQ kg(-1)dw) in sediments, and from 0.31 to 53.05 ng TEQ kg(-1)dw (mean value, 7.00 ng TEQ kg(-1)dw) in soils. PCDD/F pollution in sediments from the mid- and downstream sections of Hun River was found to be relatively heavy, and the levels of PCDD/F contamination in paddy soils were generally higher than those of upland soils. Using multivariate statistical analysis, the PCDD/F homologue and congener profiles of all soil and sediment samples were compared with those of suspected PCDD/F sources. The results showed that, PCDD/F contamination in most sediments of Hun River should mainly originated from the production of organochlorine chemicals, while metal smelting was the important potential source of PCDD/F in the drainage area of Taizi River. PCDD/F contamination in paddy soils should be simultaneously attributed to the polluted water irrigation and the organochlorine pesticide application.     

Polychlorinated dibenzo-p-dioxins and dibenzofurans emissions from open burning of crop residues in China between 1997 and 2004

Annual emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from open burning of crop residues in each province of China mainland between 1997 and 2004 were estimated to be ranged from 1.38x10(3) to 1.52x10(3) g I-TEQ/yr, with the average of 1.50+/-0.08x10(3) g I-TEQ/yr, which contributed to approximately 10% approximately 20% of the total emissions in China. The PCDD/F emissions mainly occurred in the largest crop-producing provinces, especially in those of higher economic levels. The major sources of PCDD/F emissions from open burning in China were found to be cereal residues (i.e. rice, wheat, and corn), which accounted about 70% of the total emissions. Moreover, the first-order one-variable grey differential equation model (GM (1,1) model) for annual emissions of PCDD/Fs was established based on grey system theory. The GM (1,1) model was proved to be robust to predict the annual PCDD/F emissions from crop residue field burning in forthcoming years.     

Open burning of household waste: Effect of experimental condition on combustion quality and emission of PCDD, PCDF and PCB

Open burning for waste disposal is, in many countries, the dominant source of polychlorinated dibenzodioxins, dibenzofurans and biphenyls (PCDD/PCDF/PCB) release to the environment. To generate emission factors for open burning, experimental pile burns of about 100 kg of household waste were conducted with emissions sampling. From these experiments and others conducted by the same authors it is found that less compaction of waste or active mixing during the fire - “stirring” - promotes better combustion (as evidenced by lower CO/CO₂ ratio) and reduces emissions of PCDD/PCDF/PCB; an intuitive but previously undemonstrated result. These experiments also support previous results suggesting PCDD/PCDF/PCB generation in open burning - while still highly variable - tends to be greater in the later (smoldering) phases of burning when the CO/CO₂ ratio increases.     

Assessment of current serum levels of PCDD/Fs, dl-PCBs and PBDEs in a Dutch cohort with known perinatal PCDD/F exposure

As part of a longitudinal cohort study, now in its second decade, we determined PCDDs/Fs dl-PCBs and PBDEs in serum of adolescents with known perinatal PCDD/F exposure. Of the original cohort, 33 adolescents aged 14-19 years, who had been studied previously during their neonatal (n=60), toddler and pre-pubertal period (n=41) agreed to participate in the current follow-up. PCDD/F-, dl-PCB- and PBDE congeners were measured using GC/MS. Current serum levels of PCDD/Fs determined in our cohort were relatively low (mean of 2.2pg/g) compared to the perinatal exposure. No correlation between perinatal exposure and current serum PCDD/F was found. Planar PCB TEQ levels were 2.2pg/g. Current summation operatorPBDE levels were 8.7ng/g lipid. There was one outlier with a summation operatorPBDE of 74ng/g lipid. The presence of this high value indicates that the exposure pathway is different from PCDD/F and PCB, most likely by dust and food contaminated with dust. Concluding we can say that current PCDD/F levels are quite low compared to the perinatal PCDD/F exposure of the cohort. PBDE levels however are relatively high compared to other European countries, more research on possible health effects of these levels, especially for subjects with outlier concentrations, should be performed.     

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.     

Monitoring of polychlorinated dibenzo-p-dioxins and dibenzofurans, dioxin-like PCBs and polycyclic aromatic hydrocarbons in food and feed samples from Ismailia city, Egypt

Concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyl (PCBs) and polycyclic aromatic hydrocarbons (PAHs) have been determined using GC/HRMS in food (butter, seafood and meat) and feed samples (chicken, cattle and fish) purchased from Ismailia city, Egypt. PCDD/F concentrations in food samples ranged between 0.12 and 3.35 pg WHO TEQ/g wet w, while those in feed samples were between 0.08 and 0.2 pg WHO TEQ/g dry w. Levels of PCB TEQ ranged from 0.14 to 3.2 pg/g wet w in the food samples. The feed samples have an average of 0.35 pg PCB TEQ /g dry w. In this study, butter samples showed the highest contamination levels of PCDD/Fs and PCBs. The PCBs contribution to the total TEQ was on average 63% in seafood and on average 49% for meat and butter. The highest contamination levels of PCDD/Fs and PCBs were found in butter samples. The butter TEQ content is several times higher than that reported in all EU countries and exceeded the EU limits, while the PCDD/F levels in seafood and the feed samples is far below the current EU limit. Generally, congener profiles in the food samples reflect the non-industrialized nature of the city and suggest solid waste burning as a significant source of emission. Nevertheless, the profiles for butter suggest an impact from various sources. In the case of the sum of 16 PAH contamination levels in food samples were in the range of 11.7-154.3 ng/g wet w and feed samples had a range of 116-393 ng/g dry w. Benzo(a)pyrene (BaP) has been detected in the range of 0.05-3.29 ng/g wet w in the food samples; butter showed the highest contamination which exceeded the EU standard set for fats and oil. Fingerprints of PAHs suggested both petrogenic and pyrolytic sources of contamination.     

Suppressing effect of goethite on PCDD/F and HCB emissions from plastic materials incineration

Polyethylene (PE) and polyvinyl chloride (PVC) are the leading plastics in total production in the world. The incineration of plastic-based materials forms many chlorinated compounds, such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). In this study the addition of goethite (alpha-FeOOH) was investigated to determine its suppressing effect on the emission of PCDD/Fs and hexachlorobenzene (HCB) during the combustion of wastes containing PE and PVC. Goethite was being considered since it acts as a dioxin-suppressing catalyst during incineration. Results showed that incorporation of goethite greatly reduced the generation of PCDD/Fs and HCB in the exhaust gas and fly ash. The concentration of PCDD/Fs in flue gas decreased by 45% for lab-scale and 52% for small incinerator combustion experiments, where the goethite ratios in feed samples were 0.54% and 0.34%, respectively. Under the same conditions, the concentration of HCB in flue gas decreased by 88% and 62%, respectively. The present study showed a possible mechanism of the suppressing effect of the goethite for PCDD/F formation. It is likely that iron chlorides react with particulate carbon to form organo-chlorine compounds and promote PCDD/F formation in the gas phase. XRD analysis of combustion ash revealed that the goethite was partially dehydrated and converted to alpha-Fe(2)O(3) and Fe(3)O(4) but no iron chlorides formation. Therefore the goethite impregnated plastics can contribute the reduction of PCDD/Fs and HCB in the exhaust gas during incineration of MSW.     

Innovative PCDD/F-containing gas stream generating system applied in catalytic decomposition of gaseous dioxins over V2O5-WO3/TiO2-based catalysts

Development of effective PCDD/F (polychlorinated dibenzo-p-dioxin and dibenzofuran) control technologies is essential for environmental engineers and researchers. In this study, a PCDD/F-containing gas stream generating system was developed to investigate the efficiency and effectiveness of innovative PCDD/F control technologies. The system designed and constructed can stably generate the gas stream with the PCDD/F concentration ranging from 1.0 to 100ng TEQ Nm(-3) while reproducibility test indicates that the PCDD/F recovery efficiencies are between 93% and 112%. This new PCDD/F-containing gas stream generating device is first applied in the investigation of the catalytic PCDD/F control technology. The catalytic decomposition of PCDD/Fs was evaluated with two types of commercial V(2)O(5)-WO(3)/TiO(2)-based catalysts (catalyst A and catalyst B) at controlled temperature, water vapor content, and space velocity. 84% and 91% PCDD/F destruction efficiencies are achieved with catalysts A and B, respectively, at 280 degrees C with the space velocity of 5000h(-1). The results also indicate that the presence of water vapor inhibits PCDD/F decomposition due to its competition with PCDD/F molecules for adsorption on the active vanadia sites for both catalysts. In addition, this study combined integral reaction and Mars-Van Krevelen model to calculate the activation energies of OCDD and OCDF decomposition. The activation energies of OCDD and OCDF decomposition via catalysis are calculated as 24.8kJmol(-1) and 25.2kJmol(-1), respectively.