Link between Fly Ash Properties and Polychlorinated Organic

Article pubs.acs.org/EF

Link between Fly Ash Properties and Polychlorinated Organic Pollutants Formed during Simulated Municipal Solid Waste Incineration Duong Ngoc Chau Phan, Stina Jansson,* and Jean-François Boily Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden S Supporting Information *

ABSTRACT: The relationship between the properties of fly ash generated during waste incineration and the thermal formation of persistent organic pollutants (POPs), such as polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), biphenyls (PCBs), and naphthalenes (PCNs), was investigated on two artificial wastes using a laboratory incinerator. Fly ash particles were sampled in the post-combustion zone at approximately 300 °C and were characterized with the following complementary techniques: X-ray diffraction (XRD), scanning electron microscopy−energy-dispersive X-ray (SEM−EDX), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. Flue gas samples were collected at the same location and analyzed for Mo-OCDDs, Mo-OCDFs, Tri-DCBs, and Di-OCNs. A strong correlation between fly ash characteristics and waste composition exists for several of the elements considered in this work. For instance, the waste containing the highest levels of Al produced more abundant Al-bearing minerals and elemental Al in the resulting fly ashes. Copper, an especially important POP formation catalyst, was not detected in the top 10 nm surface of fly ash particles but rather occurred within the top 2 μm, indicating that surface copper of catalytic importance for POP formation reactions was not available. Important contributions of ferric iron present in the abundant fly ash-building hematite phase could have also played an important role, especially given its documented contributions in chlorination pathways. Orthogonal projections to latent structures (OPLS) modeling resolved the relationship between fly ash properties and the post-combustion POP formation. These efforts showed that low levels of ashforming elements (i.e., Na, Mg, Fe, Ti, etc.) were associated with an increase in flue gas S levels, which, in turn, poison the Cl2 production via the Deacon process. Wastes with depleted levels of fly-ash-building elements should therefore be favored for minimizing PCDD, PCDF, PCB, and PCN release caused by incineration.

1. INTRODUCTION Municipal solid waste incineration (MSWI) is commonly used to dispose of household waste because it recovers energy and reduces the mass and volume of the waste by 70−90%. The formation and emission of persistent organic pollutants (POPs), such as polychlorinated dibenzo-p-dioxins (PCDDs), furans (PCDFs), biphenyls (PCBs), and naphthalenes (PCNs), by MSWI however poses significant environmental pollution problems.1−4 Two primary mechanisms drive the formation of these compounds: (i) de novo synthesis from carbonaceous particulates and aromatic hydrocarbons5−9 and (ii) synthesis from similarly structured chlorinated precursors, such as chlorophenols and chlorobenzenes.10−13 While it is possible for PCDDs and PCDFs to form directly in the gas phase (homogeneous formation) at high temperatures (500−800 °C),14 the yield is typically low. Laboratory-scale incineration experiments have also shown that high temperatures (>640 °C) produce very low or undetectable levels of PCDDs and PCDFs, supporting the notion that formation mainly occurs at lower temperatures.15,16 The major residual fraction generated from MSW incineration is bottom ash, non-combustible residue consisting of agglomerated minerals, metal pieces, sand, and glassy slag lumps.17 Fly ash particles carried by the flue gas stream are however a more important source of POP formation. They are, moreover, classified as a hazardous waste in the European Union (EU) because they can carry high levels of dioxins and © 2014 American Chemical Society

dioxin-like compounds, heavy metals, alkali chlorides, and soluble metal salts, thus a potential source of adverse health effects in humans and ecosystems.17,18 Furthermore, fly ash particle surfaces contain reactive sites catalyzing the formation of, e.g., PCDD in MSWI and even at temperatures as low as 250−450 °C in the post-combustion zone.19−21 Fly ash particles from MSWI plants contain various compounds that can promote or enhance the thermal formation of POPs, e.g., PCDDs, PCDFs, PCBs, and PCNs. The catalytic capability of fly ash depends upon not only concentrations but also metal speciation, mineralogical composition, and relative abundance of accessible elements at particle surfaces compared to those deeper in the bulk. Although fly ash chemistry has received considerable attention in a variety of contexts that covering mineralogy, morphology, bulk chemical and speciation compositions, and leaching properties,22−24 little information is currently available on their contribution to the thermal formation of POPs in the 250−450 °C temperature range. The main objective of this study was therefore to alleviate this knowledge gap by collecting an exhaustive physicochemical characterization of fly ash particles formed contemporaneously with PCDDs, PCDFs, PCBs, and PCNs during thermal Received: August 7, 2013 Revised: March 18, 2014 Published: March 18, 2014 2761

dx.doi.org/10.1021/ef4023459 | Energy Fuels 2014, 28, 2761−2769

Energy & Fuels

Article

mm inner diameter × 10 mm length). Details on the material compositions of these wastes can be found in Table 1 and Table S1 of the Supporting Information. The pellets were dried at room temperature and sampled for elemental analysis and characterization. Waste composition analyses were performed by ALS Scandinavia (Luleå, Sweden) according to the modified United States Environmental Protection Agency (U.S. EPA) method 200.8 using inductively coupled plasma−sector field mass spectrometry (ICP−SFMS). Solutions were prepared from 0.125 g of dried sample melted with 0.375 g of LiBO2 and dissolved in HNO3. Combustion experiments initially involved 2 h of propane combustion, followed by waste combustion for 4 h, after which a steady state was achieved,15 prior to commencement of sampling. Inorganic combustion gases (CO2, H2O, CO, HCl, SO2, NH3, NO2, NO, and N2O) were continuously monitored by Fourier transform infrared (FTIR) spectroscopy, while oxygen was measured using a zirconium dioxide cell. The FTIR instrument was zero-calibrated using N2(g). The average fuel-feeding rate during the experimental series was 0.57 ± 0.02 kg/h. The wastes were combusted in the primary chamber at 800 °C, and the flue gases were recombusted in the secondary chamber at 850 °C. The primary and secondary airflow were set to 80 and 30 L/min, respectively. The experimental campaign included a pre-burn day, triplicate combustion runs using waste A, a pre-burn day, and finally, duplicate combustion runs using waste B (triplicates were not available). To minimize memory effects, the postcombustion zone of the reactor was thoroughly cleaned between each day by applying negative pressure and fluxes of pressurized air. The freeboard was swept, and the bed sand material was replaced before each experiment. 2.2. Flue Gas Sampling and Analysis. POP data containing PCDD, PCDF, PCB, and PCN concentrations were retrieved from a previous study that investigated the effects of the difference in waste composition on the thermal formation of POPs.25 Briefly, samples of flue gas were collected isokinetically using the cooled-probe method described in the EN 1948:1 standard.30 The sampling procedure has been described in detail previously.16 Flue-gas samples were collected at a fixed location in the post-combustion zone where the gas temperature was approximately 300 °C (see P4 in Figure S1 of the Supporting Information). Flue-gas sampling was performed on 5 successive combustion days (3 days for waste A and 2 days for waste B) with the same settings to intend for the days to be as identical as possible with regard to combustion and temperature conditions. The flue gas samples were analyzed to determine their contents of Mo- to OCDD, Mo- to OCDF, Tri- to DCB, and Di- to OCN. The sample workup procedure and the protocols for the analysis of PCDDs, PCDFs, PCBs, and PCNs have been described in detail elsewhere.31,32 The samples were analyzed for PCDDs, PCDFs, and PCBs by gas chromatography−high-resolution mass spectrometry (GC−HRMS) using an AutoSpec ULTIMA NT 2000D highresolution mass spectrometer (Waters Corporation, Milford, MA) equipped with a J&W Scientific DB5-MS column (60 m × 0.25 mm inner diameter and 0.25 μm film thickness). PCN analysis was performed using the same GC−HRMS system equipped with a J&W Scientific DB5 column (60 m × 0.25 mm inner diameter and 0.25 μm film thickness). All flue-gas volumes were normalized to 11% O2, 1 atm, and 0 °C. The isotopically labeled compounds 37Cl4 2,3,7,8-TeCDD, 13C12 1,2,3,4,6,7-HxCDD, and 13C12 2,2′,3,3′,5,5′,6-HpCB were used as sampling standards and pre-spiked into the water flask of the sampling train to verify the efficiency of the sampling process. All samples, including blanks, were spiked with 13C12-labeled PCDD/Fs and PCB internal standards. The recoveries of the sampling and internal standards were satisfactory for all samples and well within the ranges specified by the relevant criteria. 2.3. Fly Ash Particle Sampling and Characterization. Fly ash sampling was performed at the same flue gas sampling location in the post-combustion zone, prior to the air cleaning device, corresponding to approximately 300 °C (see P4 in Figure S1 of the Supporting Information). Fly ash was isokinetically sampled for 1 h prior to flue gas sampling, when stable combustion conditions were achieved. A

combustion in laboratory-scale MSWI. The study builds upon previous investigations from this group on of the nature of the POPs formed under the same conditions and, particularly, the distributions of highly and lightly chlorinated organic pollutants.25 In this study, fly ash samples were collected from the combustion of two synthetic wastes (Table 1) and Table 1. Elemental Compositions of the Artificial Wastes and Fly Ash Formed during the Combustion Experimentsa characteristic Al (%, dw) Fe (%, dw) Ca (%, dw) Na (%, dw) Mg (%, dw) K (%, dw) Si (%, dw) P (%, dw) Cl (%, dw) O (%, dw) C (%, dw) Cu (%, dw) S (%, dw) moisture at 105 °C (%) ash at 550 °C (%, dw) Al (%, dw) Fe (%, dw) Ca (%, dw) Na (%, dw) Mg (%, dw) K (%, dw) Si (%, dw) P (%, dw)

waste Ab Fuel 2.0 4.4 0.87 0.4 0.068 0.32 6.6 0.12 0.8 29 49 0.015 0.09 8.6 11.4 Fly Ash 8.0−8.5 5.4−6.2 6.9−10.6 1.3−1.4 0.57−0.63 1.21−1.22 20.9−21.8 0.42

waste Bc 8.4 8.6 0.88 1.0 0.066 0.19 7.3 0.12 1.2 26 47 0.015 0.09 8.2 16.5 16.8−17.3 5.4−6.1 6.0−6.2 2.5−3.3 0.46−0.47 0.91−1.27 12.7−13.2 0.30

a

dw = dry weight. Fuel was analyzed by inductively coupled plasma− optical emission spectroscopy (ICP−OES). Fly ash was analyzed by inductively coupled plasma−sector field mass spectrometry (ICP− SFMS). Note that not all elements could be analyzed in the latter because of small sample sizes. bData from three combustion experiments. cData from two combustion experiments.

then characterized for mineralogy, size, morphology, and bulk and surface chemical composition. The surface-mediated hypothesis of POP formation was evaluated by performing multivariate correlation analyses of fly ash chemistry and the PCDD, PCDF, PCB, and PCN contents of the flue gas.

2. MATERIALS AND METHODS 2.1. Combustion Experiments. A 5 kW laboratory-scale fluidized-bed reactor was used for all experiments (see Figure S1 of the Supporting Information). Details of this setup have been described previously.26,27 Briefly, the reactor consists of a primary combustion zone, secondary combustion zone, post-combustion zone, and an aircleaning device, including a cyclone, filter box, wet scrubber, and activated carbon filter. The reactor is equipped with electrical heaters to set desired temperatures efficiently. Combustion experiments were performed using two artificial wastes: “waste A” was designed to simulate typical Swedish MSW, while “waste B” had a material content and elemental composition representative of European MSW.28 Because of the more extensive discharge of materials containing Fe and Al in Europe as a whole than in Sweden, these elements were more abundant in waste B than waste A.29 The mixed ingredients of each waste were ground into pellets (6 2762

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glass fiber filter (type A/E 142 mm, fine porosity, fast flow rate, particle retention of ≥1.0 μm, Pall Corporation, Ann Arbor, MI) was used to sample fly ash particles. To prevent water condensation and coagulation, the sampling filter temperature was maintained at 250 °C by an external heater during the entire sampling procedure. Elemental analysis of the resulting fly ash was performed according to the modified U.S. EPA method 200.8 by ICP−SFMS. Solutions were prepared from ashes previously fused with 0.4 g of LiBO2 and subsequently dissolved in diluted nitric acid. The crystalline mineral content of the fly ash was characterized by X-ray diffraction (XRD), using a Bruker D8 Advance instrument operating in θ−θ mode and equipped with an optical configuration consisting of a primary Göebel mirror, Cu Kα radiation (1.5418 Å), and a Våntec-1 detector. The PDF-2 databank33 together with Bruker software was used for initial qualitative identifications. The data were further analyzed using the Rietveld technique and data from ICSD to obtain semi-quantitative information on the presence of crystalline phases.34 FTIR spectra were collected using a VERTEX 80/80v spectrometer (Bruker) equipped with a DLaTGS detector. Measurements were made in vacuo on fly ash powders pressed onto a diamond cell of an attenuated total reflectance (Golden Gate, single bounce) accessory. All spectra were collected in the 600−4500 cm−1 range at a resolution of 4.0 cm−1 and forward/reverse scanning rate of 10 Hz. Each spectrum was an average of 100 scans. The Blackman−Harris threeterm apodization function was used to correct phase resolution. Specific surface area measurements were conducted on 90-point N2(g) adsorption/desorption isotherms (Tristar, Micrometrics) and analyzed with the Brunauer−Emmett−Teller (BET) method. These measurements were performed on samples previously dried for 12 h at 110 °C in an atmosphere of dry N2(g). Particle size and morphology were characterized by scanning electron microscopy (SEM) using a Cambridge 360 iXP microscope equipped with a LaB6 electron emitter (Leica Cambridge, Ltd., Cambridge, U.K.). The equipment was operated at 10 kV, 100 pA probe current, and 0° tilt angle. Three images were randomly chosen (size of approximately 1000 μm2) and recorded from each sample at magnifications of 1000×, 5000×, and 15000×. Furthermore, the chemical compositions of the top ∼2 μm of the fly ash particle surfaces were qualitatively and quantitatively analyzed with energy-dispersive X-ray (EDX) analysis. The samples remained uncoated for these analyses in an effort to obtain unbiased chemical compositions, unfortunately, at the cost of image quality. The top ∼10 nm of the fly ash particle surfaces was subjected to elemental analysis by X-ray photoelectron spectroscopy (XPS), using a Kratos Axis Ultra electron spectrometer equipped with a monochromatic Al Kα source operated at 150 W. A pass energy of 160 eV (with a step size of 1 eV) was used for survey scans, and 10 eV (with a step size of 0.1 eV) was used for separate photoelectron lines. To compensate for the charging of the surface, charge-neutralizing equipment was used and the binding energy (BE) scale was referenced to the C 1s peak at 285.0 eV, corresponding to adventitious carbon. The presence of elements on the particle surfaces was confirmed by matching the experimentally determined binding energies with corresponding values taken from the National Institute of Standards and Technology (NIST) XPS database.35 2.4. Multivariate Data Analysis. Multivariate analysis was performed using the SIMCA-P+ version 13.0 software (Umetrics AB, Umeå, Sweden). Orthogonal projections to latent structures (OPLS) regression was used to evaluate the flue gas and particle characterization data. This technique has been used successfully in several related studies25,36 and provides an objective overview of the correlations between various experimental observations. The OPLS model separates the systematic variation in the X matrix (the fly ash chemistry data) into two parts. The first part (Ypredictive) is linearly related to the Y matrix (homologue and congener fractions), while the second part is unrelated (Yorthogonal). The greatest advantage of this technique is that it enables analysis of the non-related variation in X. The correlations between the systematic variation in the X data set and specific responses in Y were investigated.

3. RESULTS AND DISCUSSION 3.1. Combustion Conditions. Stable combustion conditions were achieved throughout the experiments involving wastes A and B (Table 2 and Table S2 of the Supporting Table 2. Flue Gas Composition during Combustiona waste A

waste B

parameter

averageb

RSD (%)c

averageb

RSD (%)c

H2O (%) O2 (%) CO (ppm) HCl (ppm) SO2 (ppm)

6 12 7 346 17

4 9 42 5 24

5 14 2 371 4

1 1 59 3 17

a

RSD = relative standard deviation. The complete set of values is reported in the work by Phan et al.25 bData from three combustion experiments. cData from two combustion experiments.

Information), as manifested by low mean levels of CO (