Experimental and statistical determination of indicator parameters for the evaluation of fly ash and boiler ash PCDD/PCDF concentration from municipal solid waste incinerators

On-line detectable indicator parameters in the flue gas of municipal solid waste incinerators (MSWI) such as chlorinated benzenes (PCBz) are well known surrogate compounds for gas-phase PCDD/PCDF concentration. In the here presented work derivation of indicators is broadened to the detection of fly and boiler ash fractions with increased PCDD/PCDF content. Subsequently these fractions could be subject to further treatment such as recirculation in the combustion chamber to destroy their PCDD/PCDF and other organic pollutants' content. Aim of this work was to detect suitable on-line detectable indicator parameters in the gas phase, which are well correlated to PCDD/PCDF concentration in the solid residues. For this, solid residues and gas-phase samples were taken at three MSWI plants in Bavaria. Analysis of the ash content from different plants yielded a broad variation range of PCDD/PCDF concentrations especially after disturbed combustion conditions. Even during normal operation conditions significantly increased PCDD/PCDF concentrations may occur after unanticipated disturbances. Statistical evaluation of gas phase and ash measurements was carried out by means of principal component analysis, uni- and multivariate correlation analysis. Surprisingly, well known indicators for gas-phase PCDD/PCDF concentration such as polychlorinated benzenes and phenols proved to be insufficiently correlated to PCDD/PCDF content of the solid residues. Moreover, no single parameter alone was found appropriate to describe the PCDD/PCDF content of fly and boiler ashes. On the other hand, multivariate fitting of three or four parameters yielded convenient correlation coefficients of at least r=0.8 for every investigated case. Thereby, comprehension of plant operation parameters such as temperatures and air flow alongside concentrations of inorganic compounds in the gas phase (HCl, CO, SO2, NOx) gave the best results. However, the suitable set of parameters suited best for estimation of PCDD/PCDF concentration in solid residues has to be derived anew for each individual plant and type of ash.     

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.     

Correlation of PCDD/PCDF and CO values in a MSW incinerator--indication of memory effects in the high temperature/cooling section

The correlation of PCDD/PCDF levels with the CO emissions in a full-scale municipal waste incinerator was assessed during a four-week measurement effort. PCDD/PCDF concentrations in fly ashes-containing more than 99% of the total PCDD/PCDF burden of the fluidized bed incinerator (FBI)-were measured and compared with the emitted CO concentrations. The CO concentration during the sampling time showed no significant correlation to the PCDD/ PCDF amount in fly ash (R2 = 0.078). However, a comparison of the time integrated CO concentration several hours before sampling lead to a correlation with the PCDD/PCDF burden. Maximum correlation was found for the time integrated CO values of 3 and 4 h before sampling (R2 = 0.467 and R2 = 0.457 respectively). This indicates a memory effect in the high temperature cooling section of several hours. Possible mechanisms leading to the memory effect are discussed. The correlation of PCDD/PCDF with CO concentration demonstrate that the combustion conditions play an important role for PCDD/PCDF formation in FBIs. However the variability in the correlation of CO to PCDD/PCDF levels show that other factors have a significant influence on PCDD/PCDF formation.     

CuCl2-catalyzed PCDD/F formation and congener patterns from phenols

Homologue and isomer patterns of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) in CuCl2-catalyzed formation were studied in an isothermal flow reactor using a distribution of 20 phenols as measured in municipal waste incinerator (MWI) exhaust gases. A mixture of 20 phenols was synthesized and used as reactants for this study because phenols are known to be key precursors in the formation of PCDD/F. Experiments were conducted at 400 degrees C. The 92% of nitrogen (N2) and 8% of oxygen (O2) were used as a carrier gas. PCDD/F homologue and isomer patterns with dibenzo-p-dioxin (DD) and dibenzofuran (DF) were obtained from a mixture of 20 phenols. DF+PCDF formation was favored over DD+PCDD formation. The major homologue groups formed were non-chlorinated DD and DF, and PCDD/F homologue fraction decreased with the degree of chlorination. PCDD/F homologue and isomer distributions were almost constant. Phenol and lower chlorinated phenols present in high amount played an important role in PCDD/F congener distributions. The results presented here can be used as characteristics or fingerprints for homologue and isomer patterns of PCDD/F formation attribution in CuCl2-catalyzed reaction from phenols.     

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.     

Dioxin contents in fly ash from large-scale MSW incinerators in Taiwan

In this study, fly ash samples were collected from three municipal waste incinerators (MWI) in Taiwan. These MWIs investigated are equipped with different air pollution control devices (APCDs). Preliminary results indicated that 2,3,7,8-PCDD/Fs homologue patterns of various types of fly ash were quite similar for all three MWIs. Concentrations of higher-chlorinated congeners of PCDDs and PCDFs were remarkably higher than those of lower-chlorinated congeners. In the case of MSW-A, the PCDD/PCDF ratios of ashes were found in the decreasing order for cyclone, boiler and baghouse. The PCDD/PCDF ratios in various types of fly ash of MWI-B was boiler-A < boiler-B < ESP < boiler-C. As for MSW-C, no obvious trend has been observed for PCDD/PCDF ratio. However, the ratio in boiler ash was higher than that in baghouse ash of MWI-A. The dioxin contents in fly ash would increase as the fly ash passed through APCD zones. In other words, the environmental conditions of APCD may actually cause the increase of the dioxin contents in fly ash. The trend for dioxin contents in fly ash collected from three MSW incinerators investigated was MSW-C < MSW-A < MSW-B.     

PCDD/PCDF Isomer patterns in waste incinerator flyash and desorbed into the gas phase in relation to temperature

The influence of temperature on the PCDD and PCDF isomer profiles of a municipal waste incinerator flyash and the PCDD/PCDF desorbed from the ash under an inert atmosphere was investigated using a bench-scale reactor. Exposure to temperatures of 250 degrees C and above resulted in significant changes in the distribution of isomers within most dioxin and furan congener groups. The PCDD content tended to become more evenly distributed across the range of isomers than was the case for the raw flyash, while the PCDF content became concentrated across a relatively lower number of isomers. In most cases the desorbed PCDD/PCDF were mainly mono- to tri-chlorinated, and hence had a relatively low I-TEQ value. The isomer profiles of the desorbed species were also influenced by temperature. There were some significant differences between the PCDF isomer profiles of the desorbed species and those of the treated flyashes under the same conditions, but no clear differences between the PCDD isomer profiles.     

Dechlorination and destruction of PCDD,PCDF and PCB on selected fly ash from municipal waste incineration

The potential of fly ash to dechlorinate and destroy PCDD, PCDF and PCB was tested under oxygen deficient conditions in the laboratory. Specifically, two types of fly ash were compared, originating either from a fluidized bed incinerator using Ca(OH)₂ spray (FA1), or a stoker incinerator without Ca(OH)₂ impact (FA2).

Results from the present study indicate that on FA2 type fly ash, the degradation processes of OCDD, OCDF and D₁₀CB occurred primarily via dechlorination/hydrogenation up to temperature settings of 340 °C. In contrast, FA1 type fly ash was found to effect both dechlorination and destruction of these compounds already at temperature settings of 260 °C.

The dechlorination velocity of PCDD and PCDF did not differ significantly. However, the first dechlorination step of OCDF in the 1,9-position occurred faster compared to the first dechlorination step of OCDD.

The isomer pattern resulting from the dechlorination processes was quite similar on both FA1 and FA2, indicating that differences in alkalinity or elemental composition of the two types of fly ashes do not have a significant influence on the position of dechlorination. PCDD and PCDF dechlorination of the 2,3,7,8-positions was not favoured over dechlorination of the 1,4,6,9-positions on either type of fly ash. In contrast, dechlorination of PCB occurred predominantly on the toxicological relevant 3- and 4-positions.

The dechlorination/destruction processes were completed on both types of fly ash at 380 °C within one hour, which correlates well with results obtained from actual plant operation practices.     

Correlation of polychlorinated naphthalenes with polychlorinated dibenzofurans formed from waste incineration

Isomer composition of polychlorinated naphthalenes (PCNs) was measured for municipal waste incinerator fly ash samples and for emission samples produced from soot and copper-deposit experiments conducted at the United States Environmental Protection Agency (US-EPA). Two types of PCN isomer patterns were identified. One pattern contained specific PCN isomers in which chlorine atoms are substituted as if the peri(alpha-) position were dechlorinated from the higher chlorinated PCNs one by one. In another pattern, the isomers had a tendency for the chlorine atoms to assume successive positions on the naphthalene ring, which may be caused by specifically oriented chlorination. Some of these isomers increased, together with several polychlorinated dibenzofuran (PCDF) and a few polychlorinated dibenzo-p-dioxin (PCDD) isomers. The ratios between some specific PCN, PCDF, and PCDD isomers measured for the fly ash samples agree with those obtained from the soot and copper-deposit experiments. The observations suggest that these isomers were formed possibly from de novo synthesis utilizing the carbon structure contained in soot under the catalytic effect of a copper compound. Typical isomers for PCNs and PCDFs produced from incineration emissions were identified.     

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.     

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.     

Simultaneous sampling of PCDD/PCDF inside the combustion chamber and on four boiler levels of a waste incineration plant

At a MSWI (municipal solid waste incinerator) plant PCDD/PCDF samples (gasphase and particulates) were taken simultaneously be a shock-freezing method in the incinerator combustion chamber at approx. 800°C and in four sampling sections in the boiler at about 490°C, 370°C, 330°C and 270°C. In this way PCDD/PCDF-formation in the flow through the boiler was determined. Two data sets were evaluated. A considerable PCDD/PCDF-formation had occurred already at boiler temperatures of about 490°C; the highest concentration, however, was found at the end of the boiler at about 300°C. The accompanying measuring program of plant parameters made the calculation of the PCDD/PCDF mass flows possible, which allowed the inclusion of the PCDD/PCDF-content in the ESP dust in the mass flow calculations.     

Matrix effects on the de novo synthesis of polychlorinated dibenzo-p-dioxins, dibenzofurans, biphenyls and benzenes

The formation of polychlorinated dibenzo-p-dioxins, dibenzofurans, biphenyls and benzenes in de novo synthesis experiments have been studied on model fly ashes with a wide range of matrices. The model fly ash consisted of 18 selected matrices with the addition of CuCl(2) x 2H(2)O, activated charcoal and NaCl. The studied matrices were not restricted to the commonly investigated matrices with defined chemical composition (silica gel, alumina, florisil) and industrially produced adsorbents with silicate structures (diatomaceous earths), but also included natural occurring matrices (clays, kaolin, bentonite and feldspars). In addition fly ashes from a hazardous waste incinerator were included in the study for comparison. Differences in the isomer composition (homologue profiles and isomer patterns) of the substances formed by de novo synthesis experiments are discussed in dependence on the chemical composition of the studied matrices. The de novo synthesis experiments on matrices with silicate structures resulted in high concentration of mainly perchlorinated aromatic compounds while for other matrices in particular alkaline matrices a homologue shift to lower chlorinated homologues and lower formation rates were found. The paper discusses the resulting PCDD/PCDF pattern and compares them to the PCDD/PCDF profile found in naturally occurring kaolin and ball clay (illite).     

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.     

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.     

Determination of polychlorinated dibenzo-p-dioxins and dibenzo-furans in solid residues from wood combustion by HRGC/HRMS

PCDD/PCDF were determined in solid samples from wood combustion. The samples included grate ashes, bottom ashes, furnace ashes as well as fly and cyclone ashes. The solid waste samples were classified into bottom and fly ash from native wood and bottom and fly ash from waste wood. For each of the four classes concentration distribution patterns from individual congeners, the sums of PCDD/PCDF and the international toxicity equivalents (I-TEQ) values are given. The I-TEQ levels of fly ash from waste wood burning can be approximately up to two thousand times higher than the values from fly ashes of natural wood. The I-TEQ levels in bottom ashes from waste wood combustion systems are as low as the corresponding ashes from the combustion of native wood. Grate ash samples from waste wood combustion systems with low carbon burnout show high levels of PCDD/PCDF.     

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.     

流化床垃圾焚烧炉产生的有毒二噁英同类物分布研究

测定了流化床垃圾焚烧炉焚烧产生的飞灰、烟尘和烟气中的2，3，7，8位氯取代二噁英同类物的含量及其毒性当量。结果表明，产生的二噁英主要存在于飞灰中，烟气中的含量很少。飞灰中二噁英总浓度和毒性当量分别为8.44ng／g和0．80ng／g，经过布袋除尘器后的烟尘和烟气中二噁英的浓度之和与毒性当量之和分别为0．34ng／m^3和0．02ng／m^3，而布袋除尘器前的烟尘和烟气中二噁英的浓度之和与毒性当量之和分别为40．78ng／m^3和3．0ng／m^3。飞灰和烟尘中2，3，7，8位氯取代二噁英同类物的分布相似，但是与烟气中2，3，7，8位氯取代二噁英同类物的分布差别较大。通过了解有毒二噁英同类物的分布，可以进一步优化流化床垃圾焚烧炉的焚烧条件，降低二噁英的排放量，减少垃圾焚烧对环境的污染。     

Interlaboratory sampling and analysis of PCDDS and PCDFS in emissions from urban incinerators

Three laboratories participated in a PCDD and PCDF sampling experiment on the stack of a municipal waste incinerator and six laboratories examined a fly ash sample for PCDD_S and PCDF_S in order to assess analytical precision.

The flue gas sampling results are in agreement for the three laboratories and show that the PCDD_S and PCDF_S are predominantly in the gas phase, whereas emitted particulate account for 20% of the total PCDD/PCDF concentration.     

Effect of reaction time on PCDD and PCDF formation by de novo synthetic reactions under oxygen deficient and rich atmosphere

The effect of reaction time on formation of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) was studied under laboratory conditions in the system containing municipal waste incineration fly ash, activated carbon and copper chloride dihydrate at 300 degrees C in 99.999% N2 and N2 + 10% O2 atmosphere. The concentrations of tetra- to octa-chlorinated isomers as well as I-TEQ concentrations of toxic congeners are reported. The mechanism of PCDD and PCDF formation from chlorophenols and chlorinated biphenyls is discussed in the light of the time changes of PCDD/PCDF ratios.