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Influence of temperature and atmosphere on polychlorinated dibenzo-p-dioxins and dibenzofurans desorption from waste incineration fly ash a
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Jie Yang , Mi Yan , Xiaodong Li , Tong Chen , Shengyong Lu , Jianhua Yan & Alfons Buekens a
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, People's Republic of China b
College of Mechanical Engineering, Zhejiang University of Technology, Chaowang Road 18, Hangzhou 310014, People's Republic of China Accepted author version posted online: 22 Sep 2014.Published online: 03 Oct 2014.
Click for updates To cite this article: Jie Yang, Mi Yan, Xiaodong Li, Tong Chen, Shengyong Lu, Jianhua Yan & Alfons Buekens (2015) Influence of temperature and atmosphere on polychlorinated dibenzo-p-dioxins and dibenzofurans desorption from waste incineration fly ash, Environmental Technology, 36:6, 760-766, DOI: 10.1080/09593330.2014.960480 To link to this article: http://dx.doi.org/10.1080/09593330.2014.960480
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Environmental Technology, 2015 Vol. 36, No. 6, 760–766, http://dx.doi.org/10.1080/09593330.2014.960480
Influence of temperature and atmosphere on polychlorinated dibenzo-p-dioxins and dibenzofurans desorption from waste incineration fly ash Jie Yanga , Mi Yanb , Xiaodong Lia∗ , Tong Chena , Shengyong Lua , Jianhua Yana and Alfons Buekensa a State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, People’s Republic of China; b College of Mechanical Engineering, Zhejiang University of Technology, Chaowang Road 18, Hangzhou 310014, People’s Republic of China
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(Received 17 April 2014; final version received 27 August 2014 ) A fly ash sample was heated for 1 h to 200°C, 300°C and 400°C, in order to study the influence of temperature and gas phase composition on the removal of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from fly ash derived from municipal solid waste incineration. The tests were conducted by treating a fixed bed of fly ash both in an inert (nitrogen) and in a reducing (nitrogen + hydrogen) gas flow in a horizontal bench-scale quartz tubular reactor, heated by a surrounding tubular furnace. The results indicate that most of the PCDD/Fs in fly ash were removed by thermal treatment, especially when the temperature was higher than 300°C: the PCDD/Fs’ removal efficiency attained up to 96%. PCDD/Fs dechlorination and destruction were much more important than PCDD/Fs desorption, under either inert or reducing atmosphere. At 200°C and 300°C, the experiments with reducing atmosphere yielded slightly better results than those in nitrogen; yet, this tendency was reversed at 400°C. In general, both treatment modes can fully meet the requirements regarding the concentration of dioxins in fly ash to be sent for landfill in China. Keywords: fly ash; PCDD/Fs; temperature; atmosphere; desorption
Introduction With the rapid development of Chinese economy, also the production of municipal solid waste (MSW) has considerably expanded, with an annual generation of MSW (in 2012) of over 170 million tons.[1] The conventional hierarchy of disposal system methodologies includes (1) prevention, (2) reuse, (3) recycling, (4) combustion with heat recovery and (5) landfill. Combustion with heat recovery has significant advantages, since it offers a large reduction of both volume ( > 90%) and mass ( > 70%), and it allows heat recovery [2] as well. In China, already 24.7% of the total MSW flow was incinerated in 2012 and this route will continue to be applied more widely.[1] However, waste incineration is still a controversial issue among social and scientific communities due to the occurrence of secondary pollution, in particular by the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs).[3] Especially, the 17 congeners of PCDD/Fs with their 2,3,7,8 positions substituted by chlorine are quite toxic and induce a variety of adverse health problems, such as sarcoma, lymphoma and stomach cancer.[4] These toxic pollutants are formed both via de novo synthesis and from precursors [5] and mainly emitted through the flue gas and the fly ash it still contains. According to the data of UNEP (Chemical Branch), the amount of dioxins in fly ash accounts for 58–88% of the total PCDD/Fs emitted from waste incin-
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
eration [6]; yet, other factors of influence may change this range, for example, the quality and efficiency of fly ash separation, its content of carbon and the gas cleaning and stack temperature. Concentrations of PCDD/Fs in fly ash were cited in the range of 0.34–7.53 ng International Toxic Equivalence Quantity (I-TEQ)/g;[7–11] yet, higher values may occur in case of poor combustion, since the presence of carbon enhances the adsorption capabilities of fly ash. According to Ibanez et al.,[12] fly ash generation in mechanical grate furnaces amounts to 3–5% of the original MSW mass. Some 35.8 million tons of MSW were burnt in China in 2012 and this amount will reach about 73.0 million tons in 2015,[13] so some 2.2–3.65 million tons of fly ash will annually be produced in China. Fly ash of waste incineration is reported in the national catalogue of hazardous waste in China,[14] and ash with more than 3 ng I-TEQ/g cannot be sent for landfill directly.[15] Therefore, it is essential to devise safe and effective treatment methods to dispose of the dioxins in fly ash. A leading country in this respect is Japan, since thermal treatment of fly ash is imposed by law and the total output of dioxins from MSW incineration is restricted to 4 μg ITEQ/ton treated; the total output of dioxins encompasses flue gas, fly ash, neutralization residues and bottom ash. A survey of the techniques used can be retrieved from a dedicated website [16]: these comprehend both consecutive
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treatment of fly ash and integrated (incineration, gasification or pyrolysis/melting) processes. The latter feature mainly three types of technology:
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• Vertical or blast furnace gasification and melting (Nippon Steel and NKK, today JFE-Holdings). • Fluidized bed gasification and melting (Ebara Co., Hitachi Zosen Co., Kawasaki Heavy Industries Ltd., Sanki Engineering Co., Ltd. and Sumitomo Heavy Industries, Ltd.) • Rotary kiln pyrolysis and consecutive melting (Hitachi Ltd.). Tsukishima Kikai Co., Ltd. proposed a rotary kiln incineration/melting system. Thermal desorption is one of the most effective methods to treat fly ash safely, with two kinds of technologies available: one proceeds at high temperature (fly ash melting furnaces, heated by plasma, coke, etc.) and another treatment is conducted at low temperature in an oxygenfree (N2 ) atmosphere. Stieglitz et al. found that when fly ash is heated in air at 600°C for 2 h, 95% of the de novo dioxins formed are decomposed, but they will likely regenerate in the exhaust cooling section.[17] Vogg et al. showed that 90% of dioxins are degraded when fly ash was heated at 300°C under inert atmosphere for 2 h.[18] Hagenmaier et al.[19] was the first to recognize the possibilities of annealing fly ash in an oxygen-deficient atmosphere; they indicate four key points to ensure the degradation of dioxins during the process of low-temperature thermal destruction of PCDD/Fs: absence of oxygen; reaction temperature between 250 and 400°C; (at least) 1 h residence time and exhaust gas temperature below 60°C. Technical methods for post-treatment of fly ash were developed by Deutsche Babcock (the Hagenmaier drum, applied mainly by licensees in Japan) as well as by a large number of mainly Japanese enterprises.[16] In this study, fly ash from MSW incineration was subjected to thermal treatment in a tubular reactor under either inert (nitrogen) or reducing (nitrogen and hydrogen) atmosphere in a wide range of temperatures (200°C, 300°C and 400°C). The PCDD/Fs concentration in ash was analysed before and after treatment to establish the effect of temperature and atmosphere. In addition, the PCDD/Fs desorbed from fly ash and reporting to the carrier gas was also trapped and analysed. Materials and methods The fly ash used in these experiments was sampled from an MSW mechanical grate incinerator in the northern part of Zhejiang province. The daily capacity of the incinerator is 800 tons and the air pollution control system consists of a semi-dry scrubber with injection of lime slurry, activated carbon injection and a bag-house filter collecting the fly ash together with the spent activated carbon. The ash sample was stored in glass bottles, under ambient conditions and in the dark.
Figure 1.
Sketch of the experimental system.
Major elements of fly ash were determined by energy dispersive spectroscopy (EDS) and the mineralogy was determined by X-ray diffraction (XRD). XRD patterns were recorded in the range of 2θ = 5–60°. The experiments were carried out at atmospheric pressure in a tubular reactor surrounded by a cylindrical furnace (Figure 1). A metered gas flow passes through the bed of fly ash fitted in the tubular reactor. The furnace has selfgoverned heater and temperature controller hardware. The amount of fly ash treated was 2 g and the gas flow was 300 ml/min. Each test at 200°C, 300°C and 400°C lasted 60 min. Two different carrier gas compositions were tested: an inert gas (N2 ) and a reducing one (N2 + 10 vol. % H2 ). The PCDD/Fs in the flue gas were absorbed first on XAD2 resin and then in toluene kept in an ice bath. The solid fly ash residue and the gas phase samples (XAD trap and toluene absorber) were separately analysed. After the clean-up procedure, according to the EPA 1613 method, high-resolution gas chromatography with high-resolution mass spectrometry (JMS-800D, JEOL, Japan) with a DB-5MS column (60 m × 0.25 mm × 0.25 μm) was applied to analyse the PCDD/Fs. All isotope standards were purchased from the Cambridge Isotope Laboratories. The target compounds were tetra- to octa-CDD/Fs (in principle, 136 isomers). The main purpose of the experiments is to check the influence of temperature and of hydrogen addition to the nitrogen carrier gas on the resulting removal efficiency (RE) and the destruction efficiency (DE). These are defined for the PCDD/Fs as Concentration of PCDD/Fs in the solid phase RE(%) = 1 − Concentration of PCDD/Fs in the origin ash × 100. ⎛
⎞ Concentration of PCDD/Fs in the solid phase ⎜ +Concentration of PCDD/Fs in the gas phase ⎟ ⎟ DE(%) = ⎜ ⎝1 − Concentration of PCDD/Fs in the origin ash ⎠ × 100. The RE is larger than the DE; the difference is due to the transfer of non-converted PCDD/Fs to the carrier gas.
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Figure 2. XRD pattern of the fly ash tested. Table 1. Elemental composition of the fly ash tested. Non-metal elements, wt. %
Metals, wt. %
C O Si S Cl P
Na Mg Al K Ca Ti Fe
10.5 33.45 15.4 1.79 4.38 0.95
2.37 2.11 5.79 2.77 16.6 0.57 3.38
Results and discussion Fly ash characteristics. Table 1 gives the elemental composition of the fly ash sample (as derived from EDS) and Figure 2 shows its XRD pattern. The major crystalline phases are identified as silica (SiO2 ), gypsum (CaSO4 .2H2 O), halite (NaCl), calcite (CaCO3 ), aragonite (CaCO3 ), aluminium metal (Al) and (more tentatively) bytownite, a kind of Ca-enriched feldspar. The hydrated lime sprayed in the semi-wet spray scrubber and the activated carbon injection result in a high concentration of Ca and carbon in the original fly ash sample. Both elements may have influenced the results discussed further: Ca-compounds are often regarded as strong suppressants of PCDD/Fs’ formation and promoters of PCDD/Fs’ destruction, whereas the presence of activated carbon enhances the retention of PCDD/Fs in the residue. Dioxins load. The total concentration of PCDD/Fs in the residue and the distribution over the various isomer groups of PCDD/Fs are given in Table 2; its I-TEQ value was 4.36 ng I-TEQ/g (or 211 ng/g), clearly beyond the standard of authorized landfill ( < 3 ng I-TEQ/g). Dioxins fingerprint. The ratio of polychlorinated dibenzo-pdioxin (PCDD) to polychlorinated dibenzofuran (PCDF) in the original fly ash sample is 0.66 (Table 3), a value typical for
combustion sources.[20] The weight average chlorination level is also recorded in Table 3. Figure 3 compares several fingerprints of fly ash and desorbed PCDD/Fs. Thermal treatment of fly ash. The total removal and destruction efficiencies of the various isomer groups of the PCDD/Fs and the I-TEQ concentration are presented in Table 4 for the three different temperatures tested. An RE of 32.6%, 88.9% and 95.9% was attained using N2 at 200°C, 300°C and 400°C, respectively, whereas under reducing atmosphere, these values reached 41.25%, 89.65% and 92.85%, respectively. Hydrogen addition thus improved the result at 200°C, yet deteriorated the 400°C result. Thermal treatment of fly ash – residual PCDD/Fs (Table 4). After thermal treatment in a N2 atmosphere, the residual PCDD/Fs in the fly ash residue was dramatically reduced, with the total initial concentration of 211 ng/g decreasing to 142, 23.4 and 8.72 ng/g at temperatures of 200°C, 300°C and 400°C, respectively. This corresponds to an RE of 32.6%, 88.9% and 95.9%, respectively. The DE attains 32.1%, 87.7% and 88.3%, respectively. Similarly, the I-TEQ value decreased from 4.36 ng/g to 2.77, 0.55 and 0.22 ng I-TEQ/g, respectively. These values correspond to an I-TEQ DE of 35.6%, 86.5% and 87.2%, respectively. These values are only slightly different from the DE values for the PCDD/Fs. Both the RE and the DE are higher for PCDFs than for PCDDs at 300°C and 200°C; yet, this tendency reverses at 400°C. Thus, the removal and destruction of PCDDs are more strongly temperature-dependent than those of PCDFs. When applying a reducing atmosphere (N2 + 10 vol. % H2 ), the concentration of PCDD/Fs decreased from 211 to 124, 21.8 and 15.1 ng/g, corresponding to a DE of 40.7%, 88.5% and 72.5%. The I-TEQ value declines from an initial value of 4.36 to 2.44, 0.47 and 0.31 ng/g, corresponding to a DE of 43.6%, 88.25% and 79.4%, respectively. The residual values are thus lower than those in nitrogen at 200°C and (marginally) at 300°C; yet, this improved DE is not confirmed at 400°C. The RE and the DE values are higher for
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Concentration of PCDD/Fs in the original fly ash, the treated fly ash and the amount desorbed (in ng/g of the original fly ash).
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Origin ash T4CDD P5CDD H6CDD H7CDD O8CDD Total PCDD T4CDF P5CDF H6CDF H7CDF O8CDF Total PCDF Total PCDD/Fs I-TEQ
15.19 22.85 30.54 9.83 5.34 83.75 46.27 41.07 25.08 12.32 2.31 127.05 210.79 4.36
N2 Solid 11.14 14.68 20.68 7.06 3.78 57.33 30.03 27.66 17.06 8.11 1.85 84.72 142.05 2.77
200°C N2 + 10%H2 Gas Solid Gas
300°C N2 N2 + 10%H2 Solid Gas Solid Gas
Solid
Gas
0.24 0.20 0.09 0.02 0.02 0.57 0.22 0.13 0.08 0.03 0.02 0.47 1.04 0.04
1.77 2.28 4.41 2.14 1.19 11.80 3.00 3.42 3.13 1.72 0.30 11.58 23.38 0.55
0.68 0.49 0.34 0.22 0.15 1.86 4.09 1.49 0.88 0.33 0.07 6.86 8.72 0.22
2.52 1.20 1.10 0.36 0.14 5.32 6.13 2.37 1.63 0.55 0.03 10.70 16.02 0.34
9.25 10.30 20.35 6.47 3.41 49.78 26.79 22.66 15.08 7.88 1.65 74.06 123.84 2.44
0.26 0.16 0.07 0.03 0.03 0.54 0.27 0.18 0.10 0.02 0.04 0.61 1.15 0.02
0.55 0.31 0.14 0.02 0.03 1.04 0.89 0.33 0.17 0.04 0.06 1.48 2.53 0.04
2.16 1.85 4.24 1.83 1.07 11.15 3.29 2.68 2.93 1.57 0.20 10.67 21.82 0.47
0.62 0.28 0.14 0.02 0.01 1.06 0.78 0.36 0.14 0.03 0.02 1.33 2.39 0.04
400°C N2
N2 + 10%H2 Solid Gas 0.71 0.50 0.64 0.24 0.10 2.19 6.83 2.89 2.10 0.87 0.17 12.87 15.07 0.31
8.05 2.80 2.17 1.16 0.55 14.73 11.77 8.12 5.08 2.77 0.48 28.21 42.95 0.58
Table 3. Weight average degree of chlorination and PCDD to PCDF ratio.
N2 N2 + 10%H2
Original ash
PCDD 5.61
200°C 300°C 400°C 200°C 300°C 400°C
5.61 5.89 5.29 5.69 5.80 5.32
Solid PCDF PCDD/PCDF 5.08 0.66 5.10 5.39 4.66 5.12 5.32 4.81
PCDFs than for PCDDs at 300°C and 200°C; yet, this tendency reverses at 400°C. Thus, the data suggest that the RE and the DE of PCDD, PCDF, PCDD/F and I-TEQ vary in a complex manner with temperature and addition of nitrogen and hydrogen. A rising temperature enhances both destruction and desorption, as well as their sum, that is, removal. The addition of hydrogen seems clearly positive at 200°C, marginally positive at 300°C; yet, it becomes clearly counterproductive at 400°C. The RE and the DE of PCDD seem to be slightly lower than for PCDF at 200°C. The difference even becomes marked
0.68 1.02 0.27 0.67 1.05 0.17
PCDD
4.88 4.72 4.95 4.90 4.62 4.87
Gas PCDF PCDD/PCDF
4.95 4.69 4.69 4.98 4.60 5.01
1.21 0.70 0.50 0.89 0.79 0.52
at 300°C; yet, it reverses at 400°C. The most important parameter is I-TEQ. Its RE and DE are higher than those for PCDF, PCDD/F and PCDD at 200°C; yet, this advantage disappears at 300°C and 400°C. It can be concluded that there is no marked advantage in adding hydrogen to the nitrogen carrier gas, even when the expense of such a measure is disregarded. All of these values, however, whether in nitrogen or in nitrogen + hydrogen, are lower than the dioxins limit value (3 ng I-TEQ/g) imposed for direct landfilling of fly ash, showing the benefit of both forms of thermal treatment.
Figure 3. Homologue profile of PCDD/Fs in the original and the treated ash.
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Table 4. RE and DE of PCDD/Fs isomer groups (%). 200°C
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N2 T4CDD P5CDD H6CDD H7CDD O8CDD Total PCDD T4CDF P5CDF H6CDF H7CDF O8CDF Total PCDF Total PCDD/Fs TEQ
RE
DE
26.66 35.73 32.29 28.24 29.30 31.54 35.09 32.65 31.97 34.19 19.76 33.32 32.61 36.38
25.06 34.85 31.99 28.06 28.99 30.86 34.62 32.33 31.66 33.98 18.86 32.95 32.12 35.57
300°C N2 + 10%H2 RE DE
39.08 54.91 33.37 34.17 36.19 40.56 42.09 44.81 39.89 36.06 28.71 41.71 41.25 44.09
37.37 54.21 33.16 33.86 35.70 39.91 41.51 44.38 39.48 35.88 27.11 41.23 40.70 43.57
N2 RE 88.35 90.01 85.56 78.22 77.66 85.91 93.52 91.67 87.51 86.00 86.88 90.88 88.91 87.45
DE
400°C N2 + 10%H2 RE DE
N2 RE
DE
84.76 85.79 81.73 95.55 78.97 88.65 91.89 90.68 97.87 92.63 85.09 86.10 85.66 98.90 95.28 78.04 81.40 81.19 97.79 94.15 77.16 80.02 79.81 97.25 94.64 84.67 86.68 85.42 97.77 91.42 91.60 92.89 91.20 91.16 77.91 90.87 93.47 92.59 96.38 90.61 86.82 88.32 87.75 96.49 90.01 85.69 87.28 87.04 97.30 92.88 84.40 91.34 90.62 97.12 95.89 89.72 91.60 90.56 94.60 86.18 87.71 89.65 88.51 95.86 88.26 86.52 89.16 88.25 94.98 87.21
Concentration of PCDD/Fs in the solid phase × 100, Notes: RE(%) = 1 − Concentration of PCDD/Fs in the origin ash
Concentration of PCDD/Fs in the solid phase + Concentration of PCDD/Fs the gas phase × 100. DE(%) = 1 − Concentration of PCDD/Fs in the origin ash
N2 + 10%H2 RE DE 95.30 97.82 97.89 97.61 98.08 97.38 85.24 92.95 91.62 92.91 92.62 89.87 92.85 92.81
42.30 85.54 90.78 85.81 87.83 79.79 59.80 73.18 71.37 70.46 71.91 67.66 72.48 79.43
chlorinated PCDD/Fs. Both dechlorination and decomposition of the PCDD/Fs in fly ash contributed to the PCDD/Fs’ removal recorded in their study. Hung et al. [22] also found the same phenomenon. The higher vapour pressure of the lower chlorinated tetra- and penta-chlorinated PCDD/Fs would make easier their desorption [23] According to Rordorf,[24,25] the chemical stability and melting point of PCDDs are higher than PCDFs at the same chlorination degree; higher chlorinated PCDD/Fs have higher chemical stability and melting point. Clear decreases were found in the PCDD/Fs content of the fly ash as the temperature increased beyond 200°C in this study, similar to the previously reported data of Cunliffe et al.[26] and Vanberkel et al.[27]. PCDD is prone to dechlorination and decomposition reactions at 400°C.[28] According to Wang et al.,[29] dechlorination can be depicted as follows: Figure 4. The Arrhenius relationship between the amounts of the PCDD/Fs (in ng/g) desorbed and temperature.
C12 H4 Cl4 O2 + 4H2 → C1 2H8 O2 + 4HCl.
Thermal treatment of fly ash – desorption of PCDD/Fs. Finally, the amount of PCDD/Fs desorbed from fly ash, and then absorbed by XAD-2 and toluene solution, was measured. As given in Table 2, the amount of PCDD/Fs reporting to the gas phase at 200°C was 1.04 and 1.15 ng/g under inert and reducing atmospheres, respectively. It increased to 2.53 and 2.39 ng/g at 300°C, yet to 16.0 and 42.95 ng/g at 400°C. Similarly, the desorbed I-TEQ-value of PCDD/Fs also increased with rising treatment temperature, from merely 0.02–0.04 ng I-TEQ/g at 200°C and 300°C to more substantial values of 0.34 and 0.58 ng I-TEQ/g at 400°C. The amounts desorbed in a hydrogen/nitrogen atmosphere very well fit the Arrhenius-type temperature dependence as shown in Figure 4. Cunliffe and Williams [21] found that the desorbed PCDD/Fs congener profile resulted from dechlorination of higher
According to Cieplik et al.,[30] the reaction apparently started with the scission of a C−O bond, induced by H atom attack. The degradation was interpreted as thermolysis, through C−O bond homolysis, isomerisation and fragmentation, primarily to naphthalene, and C2 O as the intermediate to CO. Lower chlorinated PCDD/Fs were more easily destroyed than high chlorinated PCDD/Fs at 200°C and 300°C, but when the temperature turned to 400°C, the DE of high chlorinated PCDD/Fs was higher than that of the low chlorinated PCDD/Fs. Table 3 gives the weight average chlorination degree (as an average number of substituted chlorine) of the PCDD/Fs, as well as the ratio of PCDD to PCDF in the test residue (the solid phase) and in the gas phase, at different heating temperatures. Weight average chlorination degree. There were no appreciable changes in the mean degree of chlorination of the PCDD or PCDF in the treated fly ash at 200°C, compared with the original sample. The average chlorination level slightly rises at 300°C,
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yet then decreases at 400°C. In the gas phase, the evolution with temperature is exactly opposite. It is often postulated that the mean degree of chlorination decreases with rising temperature. According to an extensive study of MINIDIP data, this tendency is not really proven, however (Buekens A, personal communication on the MINIDIP data bank). The ratio of PCDD/PCDF. The ratio of PCDD to PCDF is often interpreted as an indication of the relative importance of the formation of PCDDs from precursors (mainly chlorophenol) to that of PCDFs by the de novo route.[20] In this particular case, with thermal treatment under inert or reducing gas conditions, the de novo route is strongly suppressed and any deviation of the original ratio (PCDD/PCDF = 0.66) is more likely to signify differences in the rate of decomposition, dechlorination and desorption. The data in Table 3 once more suggest a complex dependence on both temperature and gas flow composition.
Conclusions Since the first tests by Hagenmaier et al.,[19] the thermal treatment of MSW incineration fly ash has mainly been applied in Japan, following the introduction of stringent codes. The thermal treatment under either inert or reducing atmosphere and using the experimental setup described in this paper seem a promising technique in case landfill is strictly controlled (as prescribed in Japan and as will increasingly be the case in China). The PCDD/Fs in fly ash can largely be removed by heating at a temperature of 300– 400°C, the RE of PCDD/Fs can reach ca. 89–96% and the TEQ DE can attain ca. 87%; this will fully meet the requirements of the concentration of dioxin in fly ash to be sent for landfill. During thermal treatment, both dechlorination and degradation reactions play an important role. There is a little difference between inert and reducing atmosphere; they can both meet the requirement of reducing concentration of PCDD/Fs and nearly have the same efficiency at 200°C and 300°C, but when the temperature turns to 400°C, the efficiency in inert atmosphere is higher than the value of the reducing atmosphere. Generally, the inert atmosphere is more suitable for engineering applications based on this study.
Acknowledgements This research was supported by the National Basic Research Program of China (973 program, 2011CB201500). The authors would like to thank Prof. Dr. Mario Grosso from Politecnico di Milano, for his helpful suggestion and English writing assistance.
References [1] National Bureau of Statistics of China (NBSC). Chinese Annual Statistic 2013. 2013. Available from: http://www. stats.gov.cn/tjsj/ndsj/2013/indexch.htm. Chinese. [2] Stegemann JA, Schneider J, Baetz BW, Murphy KL. Lysimeter washing of MSW incinerator bottom ash. Waste Manage Res. 1995;13:149–165. [3] Olie K, Vermeulen PL, Hutzinger O. Chlorodibenzo-pdioxins and chlorodibenzofurans are trace components of fly ash and flue gas of some municipal incinerators in The Netherlands. Chemosphere. 1977;6:455–459.
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[4] Mitrou PI, Dimitriadis G, Raptis SA. Toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds. Eur J Intern Med. 2001;12:406–411. [5] McKay G. Dioxin characterisation, formation and minimisation during municipal solid waste (MSW) incineration: review. Chem Eng J. 2002;86:343–368. [6] UNEP Chemicalo. Standardized toolkit for identification and quantification of dioxin and furan releases. 2nd ed. Geneva: United Nations Environment Programme; 2005. [7] Buekens A, Huang H. Comparative evaluation of techniques for controlling the formation and emission of chlorinated dioxins/furans in municipal waste incineration. J Hazard Mater. 1998;62:1–33. [8] Chang MB, Chung YT. Dioxin contents in fly ashes of MSW incineration in Taiwan. Chemosphere. 1998;36:1959 –1968. [9] Yiying J, Yongfeng N, Huiming Y, Chen Zuosheng HY. Dioxins contents in fly ash of MSW incinerator in three city. Environ Sci. 2003;24:22–25. [10] Feng JH, Zhang H, Shao LM. Distribution of dioxins-like compounds in air pollution control residues from municipal solid waste incinerator. China Environ Sci. 2005;25:737– 741. [11] Gao XB, Wang W. PCDD/Fs distribution in MSWI fly ash and identification of the indicated isomer. Environ Sci. 2007;28:445–448. [12] Ibanez R, Andres A, Viguri JR, Ortiz I, Irabien JA. Characterisation and management of incinerator wastes. J Hazard Mater. 2000;79:215–227. [13] General Office of the State Councilo. The twelfth five-year plan for national municipal solid waste harmless treatment facilities construction plan (2011–2015). 2012. Available from: http://www.gov.cn/zwgk/2012–05/04/content _2129302.htm. Chinese. [14] The People’s Republic of China Ministry of Environmental Protection, National Development and Reform Commissiono. National catalogue of hazardous wastes. 2008. Available from: http://www.gov.cn/flfg/2008-06/17/content_101 9136.htm. Chinese. [15] The People’s Republic of China Ministry of Environmental Protectiono. Standard for pollution control on the landfill site of municipal solid waste (GB16889-2008). 2008. Available from: http://www.es.org.cn/download/1595-1.pdf. Chinese. [16] Japan Environmental Sanitation Center. Introduction of waste treatment and recycling technologies and companies in Japan. 2012. Available from: http://www.env.go.jp/ recycle/circul/venous_industry/en/applicable_fields/index. html [17] Stieglitz L, Zwick G, Beck J, Bautz H, Roth W. The role of particulate carbon in the denovo synthesis of polychlorinated dibenzodioxins and dibenzofurans in fly-ash. Chemosphere. 1990;20:1953–1958. [18] Vogg H, Stieglitz L. Thermal-behavior of pcdd/pcdf in fly-ash from municipal incinerators. Chemosphere. 1986;15:1373–1378. [19] Hagenmaier H, Kraft M, Brunner H, Haag R. Catalytic effects of fly-ash from waste incineration facilities on the formation and decomposition of polychlorinated dibenzopara-dioxins and polychlorinated dibenzofurans. Environ Sci Technol. 1987;21:1080–1084. [20] Huang H, Buekens A. On the mechanisms of dioxin formation in combustion processes. Chemosphere. 1995;31: 4099–4117. [21] Cunliffe AM, Williams PT. PCDD/PCDF Isomer patterns in waste incineration flyash and desorbed into the gas phase in relation to temperature. Chemosphere. 2007;66: 1929–1938.
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[22] Hung PC, Chen QH, Chang MB. Pyrolysis of MWI fly ash: effect on dioxin-like congeners. Chemosphere. 2013;92:857–863. [23] Chang MB, Cheng YC, Chi KH. Reducing PCDD/F formation by adding sulfur as inhibitor in waste incineration processes. Sci Total Environ. 2006;366:456–465. [24] Rordorf BF. Prediction of vapor-pressures, boiling points and enthalpies of fusion for 29 halogenated dibenzo-pdioxins. Thermochim Acta. 1987;112:117–122. [25] Rordorf BF. Prediction of vapor-pressures, boiling points and enthalpies of fusion for 29 halogenated dibenzo-paradioxins and 55 dibenzofurans by a vapor-pressure correlation method. Chemosphere. 1989;18:783–788. [26] Cunliffe AM, Williams PT. Influence of temperature on PCDD/PCDF desorption from waste incineration flyash under nitrogen. Chemosphere. 2007;66:1146–1152.
[27] Vanberkel OM, Olie K, Vandenberg M. Thermaldegradation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans on fly-ash from a municipal incinerator. Int J Environ Anal Chem. 1988;34: 51–67. [28] Addink R, Govers HAJ, Olie K. Desorption behavior of polychlorinated dibenzo-p-dioxins dibenzofurans on a packed fly-ash bed. Chemosphere. 1995;31:3945– 3950. [29] Wang W, Gao X, Zheng L, Lan Y. Reductive dechlorination of polychlorinated dibenzo-p-dioxins and dibenzofurans in MSWI fly ash by sodium hypophosphite. Sep Purif Technol. 2006;52:186–190. [30] Cieplik MK, Epema OJ, Louw R. Thermal hydrogenolysis of dibenzo-p-dioxin and dibenzofuran. Eur J Org Chem. 2002;50:2792–2799.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Influence of temperature and atmosphere on polychlorinated dibenzo-p-dioxins and dibenzofurans desorption from waste incineration fly ash a
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Jie Yang , Mi Yan , Xiaodong Li , Tong Chen , Shengyong Lu , Jianhua Yan & Alfons Buekens a
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, People's Republic of China b
College of Mechanical Engineering, Zhejiang University of Technology, Chaowang Road 18, Hangzhou 310014, People's Republic of China Accepted author version posted online: 22 Sep 2014.Published online: 03 Oct 2014.
Click for updates To cite this article: Jie Yang, Mi Yan, Xiaodong Li, Tong Chen, Shengyong Lu, Jianhua Yan & Alfons Buekens (2015) Influence of temperature and atmosphere on polychlorinated dibenzo-p-dioxins and dibenzofurans desorption from waste incineration fly ash, Environmental Technology, 36:6, 760-766, DOI: 10.1080/09593330.2014.960480 To link to this article: http://dx.doi.org/10.1080/09593330.2014.960480
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Environmental Technology, 2015 Vol. 36, No. 6, 760–766, http://dx.doi.org/10.1080/09593330.2014.960480
Influence of temperature and atmosphere on polychlorinated dibenzo-p-dioxins and dibenzofurans desorption from waste incineration fly ash Jie Yanga , Mi Yanb , Xiaodong Lia∗ , Tong Chena , Shengyong Lua , Jianhua Yana and Alfons Buekensa a State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, People’s Republic of China; b College of Mechanical Engineering, Zhejiang University of Technology, Chaowang Road 18, Hangzhou 310014, People’s Republic of China
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(Received 17 April 2014; final version received 27 August 2014 ) A fly ash sample was heated for 1 h to 200°C, 300°C and 400°C, in order to study the influence of temperature and gas phase composition on the removal of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from fly ash derived from municipal solid waste incineration. The tests were conducted by treating a fixed bed of fly ash both in an inert (nitrogen) and in a reducing (nitrogen + hydrogen) gas flow in a horizontal bench-scale quartz tubular reactor, heated by a surrounding tubular furnace. The results indicate that most of the PCDD/Fs in fly ash were removed by thermal treatment, especially when the temperature was higher than 300°C: the PCDD/Fs’ removal efficiency attained up to 96%. PCDD/Fs dechlorination and destruction were much more important than PCDD/Fs desorption, under either inert or reducing atmosphere. At 200°C and 300°C, the experiments with reducing atmosphere yielded slightly better results than those in nitrogen; yet, this tendency was reversed at 400°C. In general, both treatment modes can fully meet the requirements regarding the concentration of dioxins in fly ash to be sent for landfill in China. Keywords: fly ash; PCDD/Fs; temperature; atmosphere; desorption
Introduction With the rapid development of Chinese economy, also the production of municipal solid waste (MSW) has considerably expanded, with an annual generation of MSW (in 2012) of over 170 million tons.[1] The conventional hierarchy of disposal system methodologies includes (1) prevention, (2) reuse, (3) recycling, (4) combustion with heat recovery and (5) landfill. Combustion with heat recovery has significant advantages, since it offers a large reduction of both volume ( > 90%) and mass ( > 70%), and it allows heat recovery [2] as well. In China, already 24.7% of the total MSW flow was incinerated in 2012 and this route will continue to be applied more widely.[1] However, waste incineration is still a controversial issue among social and scientific communities due to the occurrence of secondary pollution, in particular by the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs).[3] Especially, the 17 congeners of PCDD/Fs with their 2,3,7,8 positions substituted by chlorine are quite toxic and induce a variety of adverse health problems, such as sarcoma, lymphoma and stomach cancer.[4] These toxic pollutants are formed both via de novo synthesis and from precursors [5] and mainly emitted through the flue gas and the fly ash it still contains. According to the data of UNEP (Chemical Branch), the amount of dioxins in fly ash accounts for 58–88% of the total PCDD/Fs emitted from waste incin-
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
eration [6]; yet, other factors of influence may change this range, for example, the quality and efficiency of fly ash separation, its content of carbon and the gas cleaning and stack temperature. Concentrations of PCDD/Fs in fly ash were cited in the range of 0.34–7.53 ng International Toxic Equivalence Quantity (I-TEQ)/g;[7–11] yet, higher values may occur in case of poor combustion, since the presence of carbon enhances the adsorption capabilities of fly ash. According to Ibanez et al.,[12] fly ash generation in mechanical grate furnaces amounts to 3–5% of the original MSW mass. Some 35.8 million tons of MSW were burnt in China in 2012 and this amount will reach about 73.0 million tons in 2015,[13] so some 2.2–3.65 million tons of fly ash will annually be produced in China. Fly ash of waste incineration is reported in the national catalogue of hazardous waste in China,[14] and ash with more than 3 ng I-TEQ/g cannot be sent for landfill directly.[15] Therefore, it is essential to devise safe and effective treatment methods to dispose of the dioxins in fly ash. A leading country in this respect is Japan, since thermal treatment of fly ash is imposed by law and the total output of dioxins from MSW incineration is restricted to 4 μg ITEQ/ton treated; the total output of dioxins encompasses flue gas, fly ash, neutralization residues and bottom ash. A survey of the techniques used can be retrieved from a dedicated website [16]: these comprehend both consecutive
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treatment of fly ash and integrated (incineration, gasification or pyrolysis/melting) processes. The latter feature mainly three types of technology:
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• Vertical or blast furnace gasification and melting (Nippon Steel and NKK, today JFE-Holdings). • Fluidized bed gasification and melting (Ebara Co., Hitachi Zosen Co., Kawasaki Heavy Industries Ltd., Sanki Engineering Co., Ltd. and Sumitomo Heavy Industries, Ltd.) • Rotary kiln pyrolysis and consecutive melting (Hitachi Ltd.). Tsukishima Kikai Co., Ltd. proposed a rotary kiln incineration/melting system. Thermal desorption is one of the most effective methods to treat fly ash safely, with two kinds of technologies available: one proceeds at high temperature (fly ash melting furnaces, heated by plasma, coke, etc.) and another treatment is conducted at low temperature in an oxygenfree (N2 ) atmosphere. Stieglitz et al. found that when fly ash is heated in air at 600°C for 2 h, 95% of the de novo dioxins formed are decomposed, but they will likely regenerate in the exhaust cooling section.[17] Vogg et al. showed that 90% of dioxins are degraded when fly ash was heated at 300°C under inert atmosphere for 2 h.[18] Hagenmaier et al.[19] was the first to recognize the possibilities of annealing fly ash in an oxygen-deficient atmosphere; they indicate four key points to ensure the degradation of dioxins during the process of low-temperature thermal destruction of PCDD/Fs: absence of oxygen; reaction temperature between 250 and 400°C; (at least) 1 h residence time and exhaust gas temperature below 60°C. Technical methods for post-treatment of fly ash were developed by Deutsche Babcock (the Hagenmaier drum, applied mainly by licensees in Japan) as well as by a large number of mainly Japanese enterprises.[16] In this study, fly ash from MSW incineration was subjected to thermal treatment in a tubular reactor under either inert (nitrogen) or reducing (nitrogen and hydrogen) atmosphere in a wide range of temperatures (200°C, 300°C and 400°C). The PCDD/Fs concentration in ash was analysed before and after treatment to establish the effect of temperature and atmosphere. In addition, the PCDD/Fs desorbed from fly ash and reporting to the carrier gas was also trapped and analysed. Materials and methods The fly ash used in these experiments was sampled from an MSW mechanical grate incinerator in the northern part of Zhejiang province. The daily capacity of the incinerator is 800 tons and the air pollution control system consists of a semi-dry scrubber with injection of lime slurry, activated carbon injection and a bag-house filter collecting the fly ash together with the spent activated carbon. The ash sample was stored in glass bottles, under ambient conditions and in the dark.
Figure 1.
Sketch of the experimental system.
Major elements of fly ash were determined by energy dispersive spectroscopy (EDS) and the mineralogy was determined by X-ray diffraction (XRD). XRD patterns were recorded in the range of 2θ = 5–60°. The experiments were carried out at atmospheric pressure in a tubular reactor surrounded by a cylindrical furnace (Figure 1). A metered gas flow passes through the bed of fly ash fitted in the tubular reactor. The furnace has selfgoverned heater and temperature controller hardware. The amount of fly ash treated was 2 g and the gas flow was 300 ml/min. Each test at 200°C, 300°C and 400°C lasted 60 min. Two different carrier gas compositions were tested: an inert gas (N2 ) and a reducing one (N2 + 10 vol. % H2 ). The PCDD/Fs in the flue gas were absorbed first on XAD2 resin and then in toluene kept in an ice bath. The solid fly ash residue and the gas phase samples (XAD trap and toluene absorber) were separately analysed. After the clean-up procedure, according to the EPA 1613 method, high-resolution gas chromatography with high-resolution mass spectrometry (JMS-800D, JEOL, Japan) with a DB-5MS column (60 m × 0.25 mm × 0.25 μm) was applied to analyse the PCDD/Fs. All isotope standards were purchased from the Cambridge Isotope Laboratories. The target compounds were tetra- to octa-CDD/Fs (in principle, 136 isomers). The main purpose of the experiments is to check the influence of temperature and of hydrogen addition to the nitrogen carrier gas on the resulting removal efficiency (RE) and the destruction efficiency (DE). These are defined for the PCDD/Fs as Concentration of PCDD/Fs in the solid phase RE(%) = 1 − Concentration of PCDD/Fs in the origin ash × 100. ⎛
⎞ Concentration of PCDD/Fs in the solid phase ⎜ +Concentration of PCDD/Fs in the gas phase ⎟ ⎟ DE(%) = ⎜ ⎝1 − Concentration of PCDD/Fs in the origin ash ⎠ × 100. The RE is larger than the DE; the difference is due to the transfer of non-converted PCDD/Fs to the carrier gas.
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Figure 2. XRD pattern of the fly ash tested. Table 1. Elemental composition of the fly ash tested. Non-metal elements, wt. %
Metals, wt. %
C O Si S Cl P
Na Mg Al K Ca Ti Fe
10.5 33.45 15.4 1.79 4.38 0.95
2.37 2.11 5.79 2.77 16.6 0.57 3.38
Results and discussion Fly ash characteristics. Table 1 gives the elemental composition of the fly ash sample (as derived from EDS) and Figure 2 shows its XRD pattern. The major crystalline phases are identified as silica (SiO2 ), gypsum (CaSO4 .2H2 O), halite (NaCl), calcite (CaCO3 ), aragonite (CaCO3 ), aluminium metal (Al) and (more tentatively) bytownite, a kind of Ca-enriched feldspar. The hydrated lime sprayed in the semi-wet spray scrubber and the activated carbon injection result in a high concentration of Ca and carbon in the original fly ash sample. Both elements may have influenced the results discussed further: Ca-compounds are often regarded as strong suppressants of PCDD/Fs’ formation and promoters of PCDD/Fs’ destruction, whereas the presence of activated carbon enhances the retention of PCDD/Fs in the residue. Dioxins load. The total concentration of PCDD/Fs in the residue and the distribution over the various isomer groups of PCDD/Fs are given in Table 2; its I-TEQ value was 4.36 ng I-TEQ/g (or 211 ng/g), clearly beyond the standard of authorized landfill ( < 3 ng I-TEQ/g). Dioxins fingerprint. The ratio of polychlorinated dibenzo-pdioxin (PCDD) to polychlorinated dibenzofuran (PCDF) in the original fly ash sample is 0.66 (Table 3), a value typical for
combustion sources.[20] The weight average chlorination level is also recorded in Table 3. Figure 3 compares several fingerprints of fly ash and desorbed PCDD/Fs. Thermal treatment of fly ash. The total removal and destruction efficiencies of the various isomer groups of the PCDD/Fs and the I-TEQ concentration are presented in Table 4 for the three different temperatures tested. An RE of 32.6%, 88.9% and 95.9% was attained using N2 at 200°C, 300°C and 400°C, respectively, whereas under reducing atmosphere, these values reached 41.25%, 89.65% and 92.85%, respectively. Hydrogen addition thus improved the result at 200°C, yet deteriorated the 400°C result. Thermal treatment of fly ash – residual PCDD/Fs (Table 4). After thermal treatment in a N2 atmosphere, the residual PCDD/Fs in the fly ash residue was dramatically reduced, with the total initial concentration of 211 ng/g decreasing to 142, 23.4 and 8.72 ng/g at temperatures of 200°C, 300°C and 400°C, respectively. This corresponds to an RE of 32.6%, 88.9% and 95.9%, respectively. The DE attains 32.1%, 87.7% and 88.3%, respectively. Similarly, the I-TEQ value decreased from 4.36 ng/g to 2.77, 0.55 and 0.22 ng I-TEQ/g, respectively. These values correspond to an I-TEQ DE of 35.6%, 86.5% and 87.2%, respectively. These values are only slightly different from the DE values for the PCDD/Fs. Both the RE and the DE are higher for PCDFs than for PCDDs at 300°C and 200°C; yet, this tendency reverses at 400°C. Thus, the removal and destruction of PCDDs are more strongly temperature-dependent than those of PCDFs. When applying a reducing atmosphere (N2 + 10 vol. % H2 ), the concentration of PCDD/Fs decreased from 211 to 124, 21.8 and 15.1 ng/g, corresponding to a DE of 40.7%, 88.5% and 72.5%. The I-TEQ value declines from an initial value of 4.36 to 2.44, 0.47 and 0.31 ng/g, corresponding to a DE of 43.6%, 88.25% and 79.4%, respectively. The residual values are thus lower than those in nitrogen at 200°C and (marginally) at 300°C; yet, this improved DE is not confirmed at 400°C. The RE and the DE values are higher for
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Concentration of PCDD/Fs in the original fly ash, the treated fly ash and the amount desorbed (in ng/g of the original fly ash).
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Origin ash T4CDD P5CDD H6CDD H7CDD O8CDD Total PCDD T4CDF P5CDF H6CDF H7CDF O8CDF Total PCDF Total PCDD/Fs I-TEQ
15.19 22.85 30.54 9.83 5.34 83.75 46.27 41.07 25.08 12.32 2.31 127.05 210.79 4.36
N2 Solid 11.14 14.68 20.68 7.06 3.78 57.33 30.03 27.66 17.06 8.11 1.85 84.72 142.05 2.77
200°C N2 + 10%H2 Gas Solid Gas
300°C N2 N2 + 10%H2 Solid Gas Solid Gas
Solid
Gas
0.24 0.20 0.09 0.02 0.02 0.57 0.22 0.13 0.08 0.03 0.02 0.47 1.04 0.04
1.77 2.28 4.41 2.14 1.19 11.80 3.00 3.42 3.13 1.72 0.30 11.58 23.38 0.55
0.68 0.49 0.34 0.22 0.15 1.86 4.09 1.49 0.88 0.33 0.07 6.86 8.72 0.22
2.52 1.20 1.10 0.36 0.14 5.32 6.13 2.37 1.63 0.55 0.03 10.70 16.02 0.34
9.25 10.30 20.35 6.47 3.41 49.78 26.79 22.66 15.08 7.88 1.65 74.06 123.84 2.44
0.26 0.16 0.07 0.03 0.03 0.54 0.27 0.18 0.10 0.02 0.04 0.61 1.15 0.02
0.55 0.31 0.14 0.02 0.03 1.04 0.89 0.33 0.17 0.04 0.06 1.48 2.53 0.04
2.16 1.85 4.24 1.83 1.07 11.15 3.29 2.68 2.93 1.57 0.20 10.67 21.82 0.47
0.62 0.28 0.14 0.02 0.01 1.06 0.78 0.36 0.14 0.03 0.02 1.33 2.39 0.04
400°C N2
N2 + 10%H2 Solid Gas 0.71 0.50 0.64 0.24 0.10 2.19 6.83 2.89 2.10 0.87 0.17 12.87 15.07 0.31
8.05 2.80 2.17 1.16 0.55 14.73 11.77 8.12 5.08 2.77 0.48 28.21 42.95 0.58
Table 3. Weight average degree of chlorination and PCDD to PCDF ratio.
N2 N2 + 10%H2
Original ash
PCDD 5.61
200°C 300°C 400°C 200°C 300°C 400°C
5.61 5.89 5.29 5.69 5.80 5.32
Solid PCDF PCDD/PCDF 5.08 0.66 5.10 5.39 4.66 5.12 5.32 4.81
PCDFs than for PCDDs at 300°C and 200°C; yet, this tendency reverses at 400°C. Thus, the data suggest that the RE and the DE of PCDD, PCDF, PCDD/F and I-TEQ vary in a complex manner with temperature and addition of nitrogen and hydrogen. A rising temperature enhances both destruction and desorption, as well as their sum, that is, removal. The addition of hydrogen seems clearly positive at 200°C, marginally positive at 300°C; yet, it becomes clearly counterproductive at 400°C. The RE and the DE of PCDD seem to be slightly lower than for PCDF at 200°C. The difference even becomes marked
0.68 1.02 0.27 0.67 1.05 0.17
PCDD
4.88 4.72 4.95 4.90 4.62 4.87
Gas PCDF PCDD/PCDF
4.95 4.69 4.69 4.98 4.60 5.01
1.21 0.70 0.50 0.89 0.79 0.52
at 300°C; yet, it reverses at 400°C. The most important parameter is I-TEQ. Its RE and DE are higher than those for PCDF, PCDD/F and PCDD at 200°C; yet, this advantage disappears at 300°C and 400°C. It can be concluded that there is no marked advantage in adding hydrogen to the nitrogen carrier gas, even when the expense of such a measure is disregarded. All of these values, however, whether in nitrogen or in nitrogen + hydrogen, are lower than the dioxins limit value (3 ng I-TEQ/g) imposed for direct landfilling of fly ash, showing the benefit of both forms of thermal treatment.
Figure 3. Homologue profile of PCDD/Fs in the original and the treated ash.
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Table 4. RE and DE of PCDD/Fs isomer groups (%). 200°C
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N2 T4CDD P5CDD H6CDD H7CDD O8CDD Total PCDD T4CDF P5CDF H6CDF H7CDF O8CDF Total PCDF Total PCDD/Fs TEQ
RE
DE
26.66 35.73 32.29 28.24 29.30 31.54 35.09 32.65 31.97 34.19 19.76 33.32 32.61 36.38
25.06 34.85 31.99 28.06 28.99 30.86 34.62 32.33 31.66 33.98 18.86 32.95 32.12 35.57
300°C N2 + 10%H2 RE DE
39.08 54.91 33.37 34.17 36.19 40.56 42.09 44.81 39.89 36.06 28.71 41.71 41.25 44.09
37.37 54.21 33.16 33.86 35.70 39.91 41.51 44.38 39.48 35.88 27.11 41.23 40.70 43.57
N2 RE 88.35 90.01 85.56 78.22 77.66 85.91 93.52 91.67 87.51 86.00 86.88 90.88 88.91 87.45
DE
400°C N2 + 10%H2 RE DE
N2 RE
DE
84.76 85.79 81.73 95.55 78.97 88.65 91.89 90.68 97.87 92.63 85.09 86.10 85.66 98.90 95.28 78.04 81.40 81.19 97.79 94.15 77.16 80.02 79.81 97.25 94.64 84.67 86.68 85.42 97.77 91.42 91.60 92.89 91.20 91.16 77.91 90.87 93.47 92.59 96.38 90.61 86.82 88.32 87.75 96.49 90.01 85.69 87.28 87.04 97.30 92.88 84.40 91.34 90.62 97.12 95.89 89.72 91.60 90.56 94.60 86.18 87.71 89.65 88.51 95.86 88.26 86.52 89.16 88.25 94.98 87.21
Concentration of PCDD/Fs in the solid phase × 100, Notes: RE(%) = 1 − Concentration of PCDD/Fs in the origin ash
Concentration of PCDD/Fs in the solid phase + Concentration of PCDD/Fs the gas phase × 100. DE(%) = 1 − Concentration of PCDD/Fs in the origin ash
N2 + 10%H2 RE DE 95.30 97.82 97.89 97.61 98.08 97.38 85.24 92.95 91.62 92.91 92.62 89.87 92.85 92.81
42.30 85.54 90.78 85.81 87.83 79.79 59.80 73.18 71.37 70.46 71.91 67.66 72.48 79.43
chlorinated PCDD/Fs. Both dechlorination and decomposition of the PCDD/Fs in fly ash contributed to the PCDD/Fs’ removal recorded in their study. Hung et al. [22] also found the same phenomenon. The higher vapour pressure of the lower chlorinated tetra- and penta-chlorinated PCDD/Fs would make easier their desorption [23] According to Rordorf,[24,25] the chemical stability and melting point of PCDDs are higher than PCDFs at the same chlorination degree; higher chlorinated PCDD/Fs have higher chemical stability and melting point. Clear decreases were found in the PCDD/Fs content of the fly ash as the temperature increased beyond 200°C in this study, similar to the previously reported data of Cunliffe et al.[26] and Vanberkel et al.[27]. PCDD is prone to dechlorination and decomposition reactions at 400°C.[28] According to Wang et al.,[29] dechlorination can be depicted as follows: Figure 4. The Arrhenius relationship between the amounts of the PCDD/Fs (in ng/g) desorbed and temperature.
C12 H4 Cl4 O2 + 4H2 → C1 2H8 O2 + 4HCl.
Thermal treatment of fly ash – desorption of PCDD/Fs. Finally, the amount of PCDD/Fs desorbed from fly ash, and then absorbed by XAD-2 and toluene solution, was measured. As given in Table 2, the amount of PCDD/Fs reporting to the gas phase at 200°C was 1.04 and 1.15 ng/g under inert and reducing atmospheres, respectively. It increased to 2.53 and 2.39 ng/g at 300°C, yet to 16.0 and 42.95 ng/g at 400°C. Similarly, the desorbed I-TEQ-value of PCDD/Fs also increased with rising treatment temperature, from merely 0.02–0.04 ng I-TEQ/g at 200°C and 300°C to more substantial values of 0.34 and 0.58 ng I-TEQ/g at 400°C. The amounts desorbed in a hydrogen/nitrogen atmosphere very well fit the Arrhenius-type temperature dependence as shown in Figure 4. Cunliffe and Williams [21] found that the desorbed PCDD/Fs congener profile resulted from dechlorination of higher
According to Cieplik et al.,[30] the reaction apparently started with the scission of a C−O bond, induced by H atom attack. The degradation was interpreted as thermolysis, through C−O bond homolysis, isomerisation and fragmentation, primarily to naphthalene, and C2 O as the intermediate to CO. Lower chlorinated PCDD/Fs were more easily destroyed than high chlorinated PCDD/Fs at 200°C and 300°C, but when the temperature turned to 400°C, the DE of high chlorinated PCDD/Fs was higher than that of the low chlorinated PCDD/Fs. Table 3 gives the weight average chlorination degree (as an average number of substituted chlorine) of the PCDD/Fs, as well as the ratio of PCDD to PCDF in the test residue (the solid phase) and in the gas phase, at different heating temperatures. Weight average chlorination degree. There were no appreciable changes in the mean degree of chlorination of the PCDD or PCDF in the treated fly ash at 200°C, compared with the original sample. The average chlorination level slightly rises at 300°C,
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yet then decreases at 400°C. In the gas phase, the evolution with temperature is exactly opposite. It is often postulated that the mean degree of chlorination decreases with rising temperature. According to an extensive study of MINIDIP data, this tendency is not really proven, however (Buekens A, personal communication on the MINIDIP data bank). The ratio of PCDD/PCDF. The ratio of PCDD to PCDF is often interpreted as an indication of the relative importance of the formation of PCDDs from precursors (mainly chlorophenol) to that of PCDFs by the de novo route.[20] In this particular case, with thermal treatment under inert or reducing gas conditions, the de novo route is strongly suppressed and any deviation of the original ratio (PCDD/PCDF = 0.66) is more likely to signify differences in the rate of decomposition, dechlorination and desorption. The data in Table 3 once more suggest a complex dependence on both temperature and gas flow composition.
Conclusions Since the first tests by Hagenmaier et al.,[19] the thermal treatment of MSW incineration fly ash has mainly been applied in Japan, following the introduction of stringent codes. The thermal treatment under either inert or reducing atmosphere and using the experimental setup described in this paper seem a promising technique in case landfill is strictly controlled (as prescribed in Japan and as will increasingly be the case in China). The PCDD/Fs in fly ash can largely be removed by heating at a temperature of 300– 400°C, the RE of PCDD/Fs can reach ca. 89–96% and the TEQ DE can attain ca. 87%; this will fully meet the requirements of the concentration of dioxin in fly ash to be sent for landfill. During thermal treatment, both dechlorination and degradation reactions play an important role. There is a little difference between inert and reducing atmosphere; they can both meet the requirement of reducing concentration of PCDD/Fs and nearly have the same efficiency at 200°C and 300°C, but when the temperature turns to 400°C, the efficiency in inert atmosphere is higher than the value of the reducing atmosphere. Generally, the inert atmosphere is more suitable for engineering applications based on this study.
Acknowledgements This research was supported by the National Basic Research Program of China (973 program, 2011CB201500). The authors would like to thank Prof. Dr. Mario Grosso from Politecnico di Milano, for his helpful suggestion and English writing assistance.
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