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Size fractionation of waste-to-energy boiler ash enables separation of a coarse fraction with low dioxin concentrations
Waste Management 49 (2016) 110–113
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Size fractionation of waste-to-energy boiler ash enables separation of a coarse fraction with low dioxin concentrations E. Weidemann a,⇑, E. Allegrini b, T. Fruergaard Astrup b, T. Hulgaard c, C. Riber c, S. Jansson a a
Umeå University, Department of Chemistry, SE-90187 Umeå, Sweden Technical University of Denmark, Department of Environmental Engineering, Building 115, Lyngby 2800, Denmark c Rambøll Danmark S/S, Hannemanns Allé 53, 2300 København S, Denmark b
a r t i c l e
i n f o
Article history: Received 18 September 2015 Revised 22 January 2016 Accepted 23 January 2016 Available online 25 January 2016 Keywords: MSW Boiler ash PCDD/F Size fractionation
a b s t r a c t Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) formed in modern Waste-to-Energy plants are primarily found in the generated ashes and air pollution control residues, which are usually disposed of as hazardous waste. The objective of this study was to explore the occurrence of PCDD/F in different grain size fractions in the boiler ash, i.e. ash originating from the convection pass of the boiler. If a correlation between particle size and dioxin concentrations could be found, size fractionation of the ashes could reduce the total amount of hazardous waste. Boiler ash samples from ten sections of a boiler’s convective part were collected over three sampling days, sieved into three different size fractions – <0.09 mm, 0.09–0.355 mm, and >0.355 mm – and analysed for PCDD/F. The coarse fraction (>0.355 mm) in the first sections of the horizontal convection pass appeared to be of low toxicity with respect to dioxin content. While the total mass of the coarse fraction in this boiler was relatively small, sieving could reduce the amount of ash containing toxic PCDD/F by around 0.5 kg per tonne input waste or around 15% of the collected boiler ash from the convection pass. The mid-size fraction in this study covered a wide size range (0.09–0.355 mm) and possibly a low toxicity fraction could be identified by splitting this fraction into more narrow size ranges. The ashes exhibited uniform PCDD/F homologue patterns which suggests a stable and continuous generation of PCDD/F. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Incineration is an efficient method to reduce the non-recyclable fractions of municipal solid waste (MSW) and similar waste types in weight and volume while recovering the energy, however some concerns stem from the formation of toxic organohalogen compounds. The most infamous of these are the polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF). They were first identified in MSW incineration residues in the late 1970’s (Olie et al., 1977), but since then legislative measures and advanced air pollution control systems have reduced their emissions to air greatly which has transferred most of the PCDD/F from the stack to the solid residues. There are in total 210 PCDD/F congeners which differ with regard to the number and position of the chlorine substituents in the molecule. Their toxicity is expressed relative to the most toxic congener, 2,3,7,8TCDD, by the toxic equivalency factor, and the combined toxicity of a sample is expressed by the toxic equivalent (TEQ) concentra⇑ Corresponding author. E-mail address: [email protected] (E. Weidemann). http://dx.doi.org/10.1016/j.wasman.2016.01.027 0956-053X/Ó 2016 Elsevier Ltd. All rights reserved.
tion. Of the 210 PCDD/F congeners, 17 are assigned toxic equivalency factors (Van den Berg et al., 2006). PCDD/Fs formed during waste incineration are primarily found in ash fractions collected in the air pollution control system (fly ash) and in the energy recovery system (boiler ash). The total ash content of the input waste is typically in the range 15–25%, which distributes into bottom ash (80–90%), boiler ash (2–12‰) and fly ash. (10–20%) (United States Environmental Protection Agency, 2014; Avfall Sverige, 2015). The boiler ash thus corresponds to between 2 kg/t and 12 kg/t of incinerated waste (Hjelmar, 1996), which is in conformance with the studied plant. Fly ash and boiler ash are sometimes mixed and might be considered a hazardous waste, which must be disposed of accordingly (ISWA, 2008). Detailed literature on characterization of boiler ashes is relatively scarce (Allegrini et al., 2014; De Boom and Degrez, 2015; De Boom et al., 2011) and even fewer have focused on formation of persistent organic pollutants such as PCDD/F in boiler ash. The concentrations from different points along the boiler have been investigated and PCDD/F formation have been found to occur in the temperature range of 370–490 °C (Düwel et al., 1990; Lundin and Marklund, 2008; Mariani et al., 1990). In Allegrini et al.
E. Weidemann et al. / Waste Management 49 (2016) 110–113
(2014) a detailed characterization of boiler ash from a Danish Waste-to-Energy plant was presented, with samples collected at 10 different points along the boiler’s convective part. These boiler ash samples were analysed for grain size distribution, content of inorganic elements, leaching of metals, and concentrations of PCDD/F, but the relationship between boiler ash particle size and concentrations of PCDD/F were not investigated. This is therefore the objective of the present study in which we explored the possibility of identifying a size fraction of the boiler ash with a lower concentration of toxic dioxins and furans. Determination of a size fraction that could be separated from the boiler ash would reduce the total amount of ash to be handled as hazardous waste, thus reducing the management costs. Previous studies on size fractionation of MSW incineration ash in relation to PCDD/F concentrations have been performed on bottom ash (Chen et al., 2006), and on the insoluble part of the ash (Moon et al., 2002). No previous studies have targeted boiler ash specifically. To obtain detailed knowledge about the content of PCDD/F in the different grain size fractions of the boiler ash, samples from the ten individual hoppers in the different sections of the boiler were sieved into three different size fractions and analysed for PCDD/F. The objective was to investigate a potential correlation between dioxin concentrations and grain size distribution in the ash samples.
2. Material and methods 2.1. Sampling and analysis of boiler ash The boiler ash samples were collected from one line at a 27 MW (thermal input) Waste-to-Energy plant that incinerated mixed municipal solid waste from households, local industries, recycling stations, and the commercial/institutional sector. The boiler/ energy recovery system where the boiler ash is generated could be divided in 10 sections (denoted 1–10), with different functions, temperature, and piping configuration. Each of them corresponded to a bottom hopper for ash discharge. A schematic representation of the configuration of the horizontal part of the boiler’s convective part is shown in Fig. 1. The flue gas temperature within the boiler dropped from approximately 700 °C (section 1) to 500 °C (section 4), 340 °C (section 7), and 160 °C (section 10). Detailed information about the plant, sample collection, and preparation can be found in Allegrini et al. (2014). The ash samples were collected during three consecutive days (days 1–3) at the bottom of the individual hoppers (1–10) of the ten horizontal sections. Boiler ash was sampled for approximately 5.5 h during day 1 and day 2, and 3 h during day 3 (Allegrini et al., 2014). One sample was collected each sampling day. All the col-
111
lected material, between 2.5 kg and 22 kg for each section was mass-reduced by means of a 34 chute riffle-splitter (1 cm chute width) to obtain laboratory samples. Since the material dry weight were above 99%, no drying was done. The results from day 1 – section 10 were excluded from the calculations due to clogging problems in the hopper during sampling. Samples from day 1 and 3 were pooled over all sections before mass reduction and PCDD/F analysis. To further investigate the PCDD/F distribution, the individual samples from each section collected during day 2 were also sieved into three grain size fractions, fine: <0.09 mm, mid-size: 0.09–0.355 mm, and coarse: >0.355 mm. The grain size fractions were chosen to yield three distinct fractions. The three fractions were then individually analysed for PCDD/F concentrations. As each sample was unique, no PCDD/F analysis replicates were performed. The ashes were extracted by Soxhlet for 48 h in toluene, clean-up was done using multi-silica columns, and analysed using HRGC-MS equipped with a 60 m DB5-ms column (i.d. 0.25 lm) (Liljelind et al., 2003). Laboratory blanks were processed alongside the samples for quality assurance and blank concentrations were found to be below 10 % of sample concentrations.
3. Results and discussions The mid-size fraction was the largest fraction and constituted 50–60% of the total sample weight, the fine fraction was 35–41%, and the coarse fraction was 5–10%. The mass distribution between the three fractions was calculated by multiplying the daily ash production with the sieving distribution percentages (Fig. 2, left part of the graph). This enabled a mass flow of ash which could be related to the PCDD/F concentrations. As only the toxic equivalent (TEQ) concentrations can be used from a legislative and practical standpoint, this evaluation will focus on the TEQ concentrations of PCDD/F in the three particle size fractions. On weight basis (lg/kg ash), the lowest TEQ concentrations were found in the 0.09–0.355 mm fraction, but when normalized to the daily production of ash (Fig. 2, right part of the graph) the coarse fraction had the lowest loadings. The highest loadings were found in the fine fraction. The actual concentrations of PCDD/F varied between sampling days, which can be expected in full scale MSW incineration due to fuel heterogeneity. Tetra-chlorinated furans dominated the PCDF profiles, and octachlorinated dioxins dominated the PCDD. This is not uncommon for MSW incineration homologue profiles (i.e., the relative abundance of congeners with different degrees of chlorination) e.g., Lundin and Marklund (2008). Moreover, the profiles displayed a high degree of similarity between sampling days, indicating a stable and continuous generation of PCDD/F (Fig. 3). In MSW
Fig. 1. Schematic representation of the boilers’ convective part. Circled numbers represent different sections in the post combustion zone. The dotted line denotes an evaporator screen, the dashed line separates the economizer from the rest of the boiler.
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E. Weidemann et al. / Waste Management 49 (2016) 110–113
Fig. 2. Total amount of boiler ash in each sieving fraction by kg/day (left) and TEQ (toxic equivalent) concentrations (based on the 2005 WHO-TEFs) in lg TEQ/day (i.e. normalized to the daily production of ash in the specific size range) (right). For day 2, the presented concentrations were summarized from the concentrations in the individual grain size fractions.
Fig. 3. Relative homologue profiles of tetra to octa chlorinated PCDD/F for the different sampling days. The similar shape of the profiles indicates that generation of PCDD/F was comparable between sampling days.
incineration, the actual concentrations of PCDD/F can vary between sampling days, but the homologue profiles remain steady also on the boiler side of the air pollution cleaning devices (Weidemann et al., 2014). The homologue profiles may in such cases facilitate a determination of PCDD/F formation reaction consistency, as the relative distributions vary less than the total concentrations. For samples collected during day 2, boiler ash samples from the sections were combined in pairs (sections 1–2, 3–4, 5–6, 7–8, and 9–10) to investigate if the distribution of PCDD/F was uniform over the sections. We found that both TEQ loadings and concentrations in the coarse fraction of sections 1–6 were low compared to sections 7–10. The highest TEQ loadings were found in the fine fraction, except in section 9–10. The same was observed for the TEQ concentrations with exceptions for sections 7–8 and 9–10 (Fig. 4). The influence in temperature on the PCDD/F formation could be identified in Fig. 4. The coarse fraction from the higher temperature sections appear to have markedly less toxic dioxins and furans compared to the economizer sections. The low loading/low concentration fraction was identified using the day 2 samples. The low concentration of PCDF and PCDD in the coarse particles of the convection part of the boiler are likely related to particle
weight. Heavier particles would descend quicker into and out of the hoppers. This means that the residence time within the optimal temperature range for PCDD/F formation would be shorter, which could explain the lower TEQ concentrations. The PCDD/F generation could also be influenced by the usually low surface area (m2/kg) of the coarse particles and the surface composition of the particles with respect to elemental composition, availability of catalytically active metals such as copper, or surface area. PCDD/Fs are generally found in higher concentrations on high surface area particles, which normally correlates with decreasing particle size (Lu et al., 2012). This is not fully congruent with the findings in this study, as the highest toxic PCDD/F concentrations in lg/kg ash were found in the coarse fractions of the economizer. At this point this incongruence remains unclear. One possible explanation could be that the composition of the particle might change as the temperature decreases and fine particles from the superheaters might have agglomerated into coarser particles as they cool down in the economizer. However, surface characterization of the size fractions was outside of the scope of this study. This study does not consider other types of toxicity, e.g. heavy metals, but an extensive investigation into the characterization, leachability and economical aspects of identifying clean fractions
E. Weidemann et al. / Waste Management 49 (2016) 110–113
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Fig. 4. Result of the day 2 separation into paired sections. The TEQ loadings are shown in lg/day from the different sections of the post combustion zone presented by sieved fractions. The circles show the actual TEQ concentration in lg/kg. The dotted line denotes an evaporator screen, the dashed line separates the economizer from the rest of the boiler.
have been done on the same boiler ash by Allegrini et al. (2014). They concluded that based on chemical and physical characterization, as well of leaching behavior, there were no significant differences between the residues produced at the different sections. Ashes from all ten sections exhibited the same hazardous properties and release of elements such as Cl, K and Na and in all it was concluded that the selection of separate bottom ash streams in the convective part of the boiler, using current techniques was not justified from an environmental and/or economic perspective. Although the evaluation in Allegrini et al. (2014) did not address the toxicity of different size fractions of the ash, the potential benefit of identifying less toxic size fractions is obvious and was identified as an important factor for the current study.
4. Conclusions The coarse particles in the first six sections of the boiler appears to have low toxicity with respect to dioxin content (indicated by TEQ concentrations), and even though the total mass of the coarse fraction (>0.355 mm) in this boiler was relatively small, this might allow for separation and alternative handling of a low TEQ concentration fraction. Using the current sieving cut-off and ash production, the corresponding amount of boiler ash would be around 15% of the collected boiler ash from the convection pass. In this plant, that would correspond to 32 tonnes per year. Furthermore, the mid-size fraction in this study covers a quite wide range (0.09–0.355 mm) and further sub-fractionation could be possible thereby potentially increasing the low TEQ portion of the boiler ashes.
Acknowledgments Energinet.dk is acknowledged for funding the project PSO10627. The authors would like to thank FORCE Technology, Babcock & Wilcox Vølund and AffaldPlus, who were partners in the project. All PCDD/F analyses were done by Miljökemiskt Laboratorium at the Trace Analysis Platform (TAP), Department of Chemistry, Umeå University.
References Allegrini, E., Boldrin, A., Jansson, S., Lundtorp, K., Fruergaard Astrup, T., 2014. Quality and generation rate of solid residues in the boiler of a waste-to-energy plant. J. Hazard. Mater. 270, 127–136. Avfall, Sverige (Swedish Waste Management and Recycling association), 2015. Svensk Avfallshantering 2014 (Summary of Waste handling in Sweden, 2014). Chen, C.-K., Lin, C., Wang, L.-C., Chang-Chien, G.-P., 2006. The size distribution of polychlorinated dibenzo-p-dioxins and dibenzofurans in the bottom ash of municipal solid waste incinerators. Chemosphere 65, 514–520. De Boom, A., Degrez, M., 2015. Combining sieving and washing, a way to treat MSWI boiler fly ash. Waste Manage. 39, 179–188. De Boom, A., Degrez, M., Hubaux, P., Lucion, C., 2011. MSWI boiler fly ashes: magnetic separation for material recovery. Waste Manage. 31, 1505–1513. Düwel, U., Nottrodt, A., Ballschmiter, K., 1990. Proceedings of the ninth international symposium chlorinated dioxins and related compounds 1989 simultaneous sampling of PCDD/PCDF inside the combustion chamber and on four boiler levels of a waste incineration plant. Chemosphere 20, 1839–1846. Hjelmar, O., 1996. Disposal strategies for municipal solid waste incineration residues. J. Hazard. Mater. 47, 345–368. ISWA, 2008. Management of APC residues from W-t-E Plants – An overview of management options and treatment methods. Liljelind, P., Soderstrom, G., Hedman, B., Karlsson, S., Lundin, L., Marklund, S., 2003. Method for multiresidue determination of halogenated aromatics and PAHs in combustion-related samples. Environ. Sci. Technol. 37, 3680–3686. Lu, S.-Y., Du, Y., Yan, J.-H., Li, X.-D., Ni, M.-J., Cen, K.-F., 2012. Dioxins and their fingerprint in size-classified fly ash fractions from municipal solid waste incinerators in China-Mechanical grate and fluidized bed units. J. Air Waste Manag. Assoc. 62, 717–724. Lundin, L., Marklund, S., 2008. Distribution of mono to octa-chlorinated PCDD/Fs in fly ash from a municipal solid-waste incinerator. Environ. Sci. Technol. 42, 1245–1250. Mariani, G., Benfenati, E., Fanelli, R., 1990. Concentrations of PCDD and PCDF in different points of a modern refuse incinerator. Chemosphere 21, 507–517. Moon, M.H., Kang, D.J., Lim, H.B., Oh, J.O.E., Chang, Y.S., 2002. Continuous fractionation of fly ash particles by SPLIT for the investigation of PCDD/Fs levels in different sizes of insoluble particles. Environ. Sci. Technol. 36, 4416–4423. Olie, K., Vermulen, P.L., Hutzinger, O., 1977. Chlorodibenzo-p-dioxins and chlorodibenzofurans are trace components of fly ash and flue gas of some municipal incinerators in the Netherlands. Chemosphere 6, 455–459. United States Environmental Protection Agency, US.EPA., 2014. Wastes – NonHazardous Waste – Municipal Solid Waste – Basic Information, USA.. Van den Berg, M., Birnbaum, L.S., Denison, M., De Vito, M., Farland, W., Feeley, M., Fiedler, H., Hakansson, H., Hanberg, A., Haws, L., Rose, M., Safe, S., Schrenk, D., Tohyama, C., Tritscher, A., Tuomisto, J., Tysklind, M., Walker, N., Peterson, R.E., 2006. The 2005 world health organization reevaluation of human and mammalian toxic equivalency factors for dioxins and dioxin-like compounds. Toxicol. Sci. 93, 224–241. Weidemann, E., Marklund, S., Bristav, H., Lundin, L., 2014. In-filter PCDF and PCDD formation at low temperature during MSWI combustion. Chemosphere 102, 12–17.
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Size fractionation of waste-to-energy boiler ash enables separation of a coarse fraction with low dioxin concentrations E. Weidemann a,⇑, E. Allegrini b, T. Fruergaard Astrup b, T. Hulgaard c, C. Riber c, S. Jansson a a
Umeå University, Department of Chemistry, SE-90187 Umeå, Sweden Technical University of Denmark, Department of Environmental Engineering, Building 115, Lyngby 2800, Denmark c Rambøll Danmark S/S, Hannemanns Allé 53, 2300 København S, Denmark b
a r t i c l e
i n f o
Article history: Received 18 September 2015 Revised 22 January 2016 Accepted 23 January 2016 Available online 25 January 2016 Keywords: MSW Boiler ash PCDD/F Size fractionation
a b s t r a c t Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) formed in modern Waste-to-Energy plants are primarily found in the generated ashes and air pollution control residues, which are usually disposed of as hazardous waste. The objective of this study was to explore the occurrence of PCDD/F in different grain size fractions in the boiler ash, i.e. ash originating from the convection pass of the boiler. If a correlation between particle size and dioxin concentrations could be found, size fractionation of the ashes could reduce the total amount of hazardous waste. Boiler ash samples from ten sections of a boiler’s convective part were collected over three sampling days, sieved into three different size fractions – <0.09 mm, 0.09–0.355 mm, and >0.355 mm – and analysed for PCDD/F. The coarse fraction (>0.355 mm) in the first sections of the horizontal convection pass appeared to be of low toxicity with respect to dioxin content. While the total mass of the coarse fraction in this boiler was relatively small, sieving could reduce the amount of ash containing toxic PCDD/F by around 0.5 kg per tonne input waste or around 15% of the collected boiler ash from the convection pass. The mid-size fraction in this study covered a wide size range (0.09–0.355 mm) and possibly a low toxicity fraction could be identified by splitting this fraction into more narrow size ranges. The ashes exhibited uniform PCDD/F homologue patterns which suggests a stable and continuous generation of PCDD/F. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Incineration is an efficient method to reduce the non-recyclable fractions of municipal solid waste (MSW) and similar waste types in weight and volume while recovering the energy, however some concerns stem from the formation of toxic organohalogen compounds. The most infamous of these are the polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF). They were first identified in MSW incineration residues in the late 1970’s (Olie et al., 1977), but since then legislative measures and advanced air pollution control systems have reduced their emissions to air greatly which has transferred most of the PCDD/F from the stack to the solid residues. There are in total 210 PCDD/F congeners which differ with regard to the number and position of the chlorine substituents in the molecule. Their toxicity is expressed relative to the most toxic congener, 2,3,7,8TCDD, by the toxic equivalency factor, and the combined toxicity of a sample is expressed by the toxic equivalent (TEQ) concentra⇑ Corresponding author. E-mail address: [email protected] (E. Weidemann). http://dx.doi.org/10.1016/j.wasman.2016.01.027 0956-053X/Ó 2016 Elsevier Ltd. All rights reserved.
tion. Of the 210 PCDD/F congeners, 17 are assigned toxic equivalency factors (Van den Berg et al., 2006). PCDD/Fs formed during waste incineration are primarily found in ash fractions collected in the air pollution control system (fly ash) and in the energy recovery system (boiler ash). The total ash content of the input waste is typically in the range 15–25%, which distributes into bottom ash (80–90%), boiler ash (2–12‰) and fly ash. (10–20%) (United States Environmental Protection Agency, 2014; Avfall Sverige, 2015). The boiler ash thus corresponds to between 2 kg/t and 12 kg/t of incinerated waste (Hjelmar, 1996), which is in conformance with the studied plant. Fly ash and boiler ash are sometimes mixed and might be considered a hazardous waste, which must be disposed of accordingly (ISWA, 2008). Detailed literature on characterization of boiler ashes is relatively scarce (Allegrini et al., 2014; De Boom and Degrez, 2015; De Boom et al., 2011) and even fewer have focused on formation of persistent organic pollutants such as PCDD/F in boiler ash. The concentrations from different points along the boiler have been investigated and PCDD/F formation have been found to occur in the temperature range of 370–490 °C (Düwel et al., 1990; Lundin and Marklund, 2008; Mariani et al., 1990). In Allegrini et al.
E. Weidemann et al. / Waste Management 49 (2016) 110–113
(2014) a detailed characterization of boiler ash from a Danish Waste-to-Energy plant was presented, with samples collected at 10 different points along the boiler’s convective part. These boiler ash samples were analysed for grain size distribution, content of inorganic elements, leaching of metals, and concentrations of PCDD/F, but the relationship between boiler ash particle size and concentrations of PCDD/F were not investigated. This is therefore the objective of the present study in which we explored the possibility of identifying a size fraction of the boiler ash with a lower concentration of toxic dioxins and furans. Determination of a size fraction that could be separated from the boiler ash would reduce the total amount of ash to be handled as hazardous waste, thus reducing the management costs. Previous studies on size fractionation of MSW incineration ash in relation to PCDD/F concentrations have been performed on bottom ash (Chen et al., 2006), and on the insoluble part of the ash (Moon et al., 2002). No previous studies have targeted boiler ash specifically. To obtain detailed knowledge about the content of PCDD/F in the different grain size fractions of the boiler ash, samples from the ten individual hoppers in the different sections of the boiler were sieved into three different size fractions and analysed for PCDD/F. The objective was to investigate a potential correlation between dioxin concentrations and grain size distribution in the ash samples.
2. Material and methods 2.1. Sampling and analysis of boiler ash The boiler ash samples were collected from one line at a 27 MW (thermal input) Waste-to-Energy plant that incinerated mixed municipal solid waste from households, local industries, recycling stations, and the commercial/institutional sector. The boiler/ energy recovery system where the boiler ash is generated could be divided in 10 sections (denoted 1–10), with different functions, temperature, and piping configuration. Each of them corresponded to a bottom hopper for ash discharge. A schematic representation of the configuration of the horizontal part of the boiler’s convective part is shown in Fig. 1. The flue gas temperature within the boiler dropped from approximately 700 °C (section 1) to 500 °C (section 4), 340 °C (section 7), and 160 °C (section 10). Detailed information about the plant, sample collection, and preparation can be found in Allegrini et al. (2014). The ash samples were collected during three consecutive days (days 1–3) at the bottom of the individual hoppers (1–10) of the ten horizontal sections. Boiler ash was sampled for approximately 5.5 h during day 1 and day 2, and 3 h during day 3 (Allegrini et al., 2014). One sample was collected each sampling day. All the col-
111
lected material, between 2.5 kg and 22 kg for each section was mass-reduced by means of a 34 chute riffle-splitter (1 cm chute width) to obtain laboratory samples. Since the material dry weight were above 99%, no drying was done. The results from day 1 – section 10 were excluded from the calculations due to clogging problems in the hopper during sampling. Samples from day 1 and 3 were pooled over all sections before mass reduction and PCDD/F analysis. To further investigate the PCDD/F distribution, the individual samples from each section collected during day 2 were also sieved into three grain size fractions, fine: <0.09 mm, mid-size: 0.09–0.355 mm, and coarse: >0.355 mm. The grain size fractions were chosen to yield three distinct fractions. The three fractions were then individually analysed for PCDD/F concentrations. As each sample was unique, no PCDD/F analysis replicates were performed. The ashes were extracted by Soxhlet for 48 h in toluene, clean-up was done using multi-silica columns, and analysed using HRGC-MS equipped with a 60 m DB5-ms column (i.d. 0.25 lm) (Liljelind et al., 2003). Laboratory blanks were processed alongside the samples for quality assurance and blank concentrations were found to be below 10 % of sample concentrations.
3. Results and discussions The mid-size fraction was the largest fraction and constituted 50–60% of the total sample weight, the fine fraction was 35–41%, and the coarse fraction was 5–10%. The mass distribution between the three fractions was calculated by multiplying the daily ash production with the sieving distribution percentages (Fig. 2, left part of the graph). This enabled a mass flow of ash which could be related to the PCDD/F concentrations. As only the toxic equivalent (TEQ) concentrations can be used from a legislative and practical standpoint, this evaluation will focus on the TEQ concentrations of PCDD/F in the three particle size fractions. On weight basis (lg/kg ash), the lowest TEQ concentrations were found in the 0.09–0.355 mm fraction, but when normalized to the daily production of ash (Fig. 2, right part of the graph) the coarse fraction had the lowest loadings. The highest loadings were found in the fine fraction. The actual concentrations of PCDD/F varied between sampling days, which can be expected in full scale MSW incineration due to fuel heterogeneity. Tetra-chlorinated furans dominated the PCDF profiles, and octachlorinated dioxins dominated the PCDD. This is not uncommon for MSW incineration homologue profiles (i.e., the relative abundance of congeners with different degrees of chlorination) e.g., Lundin and Marklund (2008). Moreover, the profiles displayed a high degree of similarity between sampling days, indicating a stable and continuous generation of PCDD/F (Fig. 3). In MSW
Fig. 1. Schematic representation of the boilers’ convective part. Circled numbers represent different sections in the post combustion zone. The dotted line denotes an evaporator screen, the dashed line separates the economizer from the rest of the boiler.
112
E. Weidemann et al. / Waste Management 49 (2016) 110–113
Fig. 2. Total amount of boiler ash in each sieving fraction by kg/day (left) and TEQ (toxic equivalent) concentrations (based on the 2005 WHO-TEFs) in lg TEQ/day (i.e. normalized to the daily production of ash in the specific size range) (right). For day 2, the presented concentrations were summarized from the concentrations in the individual grain size fractions.
Fig. 3. Relative homologue profiles of tetra to octa chlorinated PCDD/F for the different sampling days. The similar shape of the profiles indicates that generation of PCDD/F was comparable between sampling days.
incineration, the actual concentrations of PCDD/F can vary between sampling days, but the homologue profiles remain steady also on the boiler side of the air pollution cleaning devices (Weidemann et al., 2014). The homologue profiles may in such cases facilitate a determination of PCDD/F formation reaction consistency, as the relative distributions vary less than the total concentrations. For samples collected during day 2, boiler ash samples from the sections were combined in pairs (sections 1–2, 3–4, 5–6, 7–8, and 9–10) to investigate if the distribution of PCDD/F was uniform over the sections. We found that both TEQ loadings and concentrations in the coarse fraction of sections 1–6 were low compared to sections 7–10. The highest TEQ loadings were found in the fine fraction, except in section 9–10. The same was observed for the TEQ concentrations with exceptions for sections 7–8 and 9–10 (Fig. 4). The influence in temperature on the PCDD/F formation could be identified in Fig. 4. The coarse fraction from the higher temperature sections appear to have markedly less toxic dioxins and furans compared to the economizer sections. The low loading/low concentration fraction was identified using the day 2 samples. The low concentration of PCDF and PCDD in the coarse particles of the convection part of the boiler are likely related to particle
weight. Heavier particles would descend quicker into and out of the hoppers. This means that the residence time within the optimal temperature range for PCDD/F formation would be shorter, which could explain the lower TEQ concentrations. The PCDD/F generation could also be influenced by the usually low surface area (m2/kg) of the coarse particles and the surface composition of the particles with respect to elemental composition, availability of catalytically active metals such as copper, or surface area. PCDD/Fs are generally found in higher concentrations on high surface area particles, which normally correlates with decreasing particle size (Lu et al., 2012). This is not fully congruent with the findings in this study, as the highest toxic PCDD/F concentrations in lg/kg ash were found in the coarse fractions of the economizer. At this point this incongruence remains unclear. One possible explanation could be that the composition of the particle might change as the temperature decreases and fine particles from the superheaters might have agglomerated into coarser particles as they cool down in the economizer. However, surface characterization of the size fractions was outside of the scope of this study. This study does not consider other types of toxicity, e.g. heavy metals, but an extensive investigation into the characterization, leachability and economical aspects of identifying clean fractions
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Fig. 4. Result of the day 2 separation into paired sections. The TEQ loadings are shown in lg/day from the different sections of the post combustion zone presented by sieved fractions. The circles show the actual TEQ concentration in lg/kg. The dotted line denotes an evaporator screen, the dashed line separates the economizer from the rest of the boiler.
have been done on the same boiler ash by Allegrini et al. (2014). They concluded that based on chemical and physical characterization, as well of leaching behavior, there were no significant differences between the residues produced at the different sections. Ashes from all ten sections exhibited the same hazardous properties and release of elements such as Cl, K and Na and in all it was concluded that the selection of separate bottom ash streams in the convective part of the boiler, using current techniques was not justified from an environmental and/or economic perspective. Although the evaluation in Allegrini et al. (2014) did not address the toxicity of different size fractions of the ash, the potential benefit of identifying less toxic size fractions is obvious and was identified as an important factor for the current study.
4. Conclusions The coarse particles in the first six sections of the boiler appears to have low toxicity with respect to dioxin content (indicated by TEQ concentrations), and even though the total mass of the coarse fraction (>0.355 mm) in this boiler was relatively small, this might allow for separation and alternative handling of a low TEQ concentration fraction. Using the current sieving cut-off and ash production, the corresponding amount of boiler ash would be around 15% of the collected boiler ash from the convection pass. In this plant, that would correspond to 32 tonnes per year. Furthermore, the mid-size fraction in this study covers a quite wide range (0.09–0.355 mm) and further sub-fractionation could be possible thereby potentially increasing the low TEQ portion of the boiler ashes.
Acknowledgments Energinet.dk is acknowledged for funding the project PSO10627. The authors would like to thank FORCE Technology, Babcock & Wilcox Vølund and AffaldPlus, who were partners in the project. All PCDD/F analyses were done by Miljökemiskt Laboratorium at the Trace Analysis Platform (TAP), Department of Chemistry, Umeå University.
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