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Element associations in ash from waste combustion in fluidized bed
Waste Management 30 (2010) 1273–1279
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Element associations in ash from waste combustion in fluidized bed K. Karlfeldt Fedje a,*, S. Rauch b, P. Cho a, B.-M. Steenari a a b
Department of Chemical and Biological Engineering, Division of Environmental Inorganic Chemistry, Chalmers University of Technology, Kemivägen 10, 412 96 Göteborg, Sweden Department of Civil and Environmental Engineering, Division of Water Environment Technology, Chalmers University of Technology, Sven Hultins Gata 8, 412 96 Göteborg, Sweden
a r t i c l e
i n f o
Article history: Received 10 October 2008 Accepted 7 September 2009 Available online 21 October 2009
a b s t r a c t The incineration of MSW in fluidized beds is a commonly applied waste management practice. The composition of the ashes produced in a fluidized bed boiler has important environmental implications as potentially toxic trace elements may be associated with ash particles and it is therefore essential to determine the mechanisms controlling the association of trace elements to ash particles, including the role of major element composition. The research presented here uses micro-analytical techniques to study the distribution of major and trace elements and determine the importance of affinity-based binding mechanisms in separate cyclone ash particles from MSW combustion. Particle size and the occurrence of Ca and Fe were found to be important factors for the binding of trace elements to ash particles, but the binding largely depends on random associations based on the presence of a particle when trace elements condensate in the flue gas. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Combustion of municipal solid waste (MSW) is a commonly used management method for several reasons, including an efficient reduction of the waste volume and a substantial heating value of MSW (Reimann, 2006). Fluidized bed combustion is increasingly applied since this process is flexible towards variations in fuel properties and reasonably robust. Volatilisation of metal species may occur during combustion, and the extent not only depends on the temperature, but also on the gas composition. Thermodynamic equilibrium calculations and measurements during incineration show that a high concentration of chlorine as well as a reducing atmosphere increases the volatility of metals like Cd, Pb and Zn, whereas the presence of sulphur can reduce the volatility (Verhulst et al., 1995; Sun et al., 2004; Zhang et al., 2008). Condensation of volatile metal compounds as kernels of fly ash particles, or on already available particle surfaces, will occur when the temperature of the gas decreases. Thus, a number of reaction paths are possible, most of them leading to an enrichment of the volatised species in the fly ash (Haynes et al., 1982; Flagan and Sarofim, 1984). As the surface area available on a particle increases rapidly as the particle diameter decreases, it could be expected that the amount of volatile metal compounds found in different ash particle size fractions depends on the particle diameter. However, published results indicate that the distribution patterns of metals in ash particles also depend on the ash matrix. Cereda and coworkers found that coal ash particles could be grouped based on their major content and the presence of trace elements was not * Corresponding author. Tel.: +46 31 772 2864; fax: +46 31 772 2853. E-mail address: [email protected] (K. Karlfeldt Fedje). 0956-053X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2009.09.012
equally distributed between the groups (Cereda et al., 1995). Particles rich in Al and Si also had high content of for instance Cu and Ni. In another study Cd, Cu and Pb showed affinity for particles rich in quarts (Lind et al., 1999). Such processes would include a site specific step, i.e. a reaction step where the affinity of the metal to a specific ash matrix mineral plays an important role. Knowledge of this would open for an active choice of bed material or mineral additive in the fluidized bed to bind metals in stable, insoluble forms. One example is the use of kaolin to bind potassium in biomass combustion and thus decrease the problems with ash sintering and corrosion caused by KCl (Davidsson et al., 2007). However, the occurrence of affinity driven processes and association between elements on ash particles in MSW ashes have not been definitely verified so far and the present work was carried out to investigate this further. During the last decades, development of microprobes technique has opened up the possibility for analyses using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A laser beam is used as sampling device for online analysis by ICPMS, thereby providing sensitive multi-element analysis at micrometric resolution (Durrant, 1999; Gunther and Hattendorf, 2005). The LA-ICP-MS is applied on solids and is widely used for geochemical samples, but also for such various fields as material sciences, biology, and ornithology, the technique is becoming more common (Koch et al., 2002; Reinhardt et al., 2003; Ek et al., 2004; Jackson et al., 2004). However, analysis of ash, using this method, is still not common and the few studies found focus on coal fly ash (Spears, 2004; Spears and Martinez-Tarrazona, 2004). Therefore we found it interesting to apply it to our ash samples with the aim to study the distribution of major and minor elements on single ash particles of different appearance and matrix composition.
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Electron microscopy and energy dispersive X-ray fluorescence spectrometry was also applied. Both these methods are designed for surface analysis which gives the opportunity to compare the composition of the particle surfaces with bulk composition data. The choice of cyclone ash from combustion of MSW in a fluidized bed boiler was based on the fact that fluidized bed combustion is frequently used for MSW and the cyclone ash represents a large part of the total amount of ash produced at such a plant. In addition, the potential utilization of cyclone ash for construction purposes is promising as the content of metal species is rather low compared to that of the filter ash.
2. Experimental Ash samples were collected from the cyclones of 20 MW bubbling fluidized bed (BFB) boilers using a sand consisting of quartz and feldspars, as bed material. The power plant is located in south western Sweden and is exclusively used for the incineration of MSW. At the time of the sampling the waste burned consisted of 50% household waste, 25% burnable fraction from household and light industry waste, 15% paper and packaging, 10% RDF (well sorted household waste). The combustion temperature was about 850 °C. When the ashes 1 and 2 were produced ammonia was added for reduction of NOx emissions. In the later case (ash 2) sodium bicarbonate was also added with the aim to bind HCl and SO2. In this boiler the cyclone is placed after a section containing heat transfer surfaces which means that the temperature has been decreased to about 150 °C. One sample (denoted ash 3) was dry sieved into seven particle size fractions (>180, 125–180, 90–125, 63–90, 45–63, 32–45 and <32 lm) for investigation of the dependence of ash properties on particle size.
2.2. Single particle surface chemical composition The shape and surface structure of ash particles were studied by environmental scanning electron microscopy (ESEM) using an Electro-Scan 2020 operated at 20 kV for secondary electron imaging. Further characterisation was performed by electron microscopy with EDX detection for the determination of major elements (Al, Ca, Cl, Fe, K, Na, Si and Ti) using a FEI Quanta 200 FEG ESEM operated at 15 kV and coupled to an Oxford Inca 300 EDX system. The detection limit of the EEX-system is about 1 wt% and the penetration depth is about 1 lm. The composition of single ash particles was determined by LAICP-MS. Analysis was performed on a Cetac LSX-200 laser system coupled to a PE Sciex Elan 6000 ICP-MS. Due to beam size limitations and the need for multi-element data acquisition, only particles with a diameter larger than 100 lm could be analysed. Selected major (Al, Ca, Fe, K, Mg, Na, S, Si, Ti) and trace (As, Ba, Cd, Co, Cr, Cu, Pb, Mn, Ni, Sr, V, Zn) elements were analysed in approximately 100 particles of different shapes and sizes from samples 1 and 2 respectively. Ablation was applied in the form of a scan line across the particles at a speed of 10 lm s1 andwith a beam diameter of 25 lm while the energy was optimised to provide a sufficient signal. For the ICP-MS a short dwell time, 5 ms, was used for multi-element acquisition together with a short quadrupole settling time. Calibration was done using an ash reference material (NIST 1633b) pressed in the form of a pellet. Results for each element and particles were averaged. It is important to note that LA-ICP-MS is considered to be a surface analysis technique with the settings used. Visual observation of the particles after ablation supports that only material at the surface of the particles was ablated, although the penetration depth could not be determined. 3. Results 3.1. Particle size distribution
2.1. Bulk ash characterisation Total elemental composition of bulk samples was determined after total acid digestion using inductively coupled plasma with either optical emission spectroscopic or mass spectrometric detection (ICP-OES and ICP-MS) in an earlier work concerning samples 1 and 2 (Karlfeldt and Steenari, 2007) and for sample 3, in this work, using atomic absorption spectroscopy (AAS) with the flame AAS set up for major elements and the graphite furnace set up for trace elements. Crystalline compounds were identified with a detection limit of about 2 wt% by qualitative X-ray powder diffractometry (XRD) using a Siemens D5000 X-ray powder diffractometer with Cu characteristic radiation and a scintillation detector. Identification of compounds was carried out using the Joint Committee of Powder Diffraction Standards database (2006). The particle size distribution for particles smaller than 118 lm was determined using a Malvern 2600c laser diffractometer. The distribution is determined from a diffraction pattern resulting from a laser irradiated suspension of ash and water. Sample 3 contained 5 wt% of magnetic material, which was removed to hinder ash particles to stick on the magnetic stirrer used in the measurement cell. The content of magnetic material was lower in ashes 1 and 2. Specific surface area was determined according to the BET (Brunauer, Emmet, Teller) five point method (Webb and Orr, 1997) using a Micromeritics Accelerated Surface Area and Porosimetry System 2010 instrument for ashes 1 and 2 in an earlier work (Karlfeldt and Steenari, 2007) and using a Micromeritics TriStar 3000 for ash 3 in this work.
The particle size distributions and specific surface area data for the ash samples are presented in Fig. 1 and Table 1. Probably there had been a slight difference in the separation efficiency of the cyclone between the sampling dates 1, 2 and 3 since sample 3 contained more material in the larger particle size fractions than the other samples did. Specific surface areas were similar with a slightly higher value for sample 3 (Table 1a). The specific surface area values of size fractionated ash 3 samples (Table 1b) did not follow the expected trend with the surface area per weight unit increasing to the double when the particle diameter is decreased by a factor 2 (Appendix 1). Instead, the largest specific surface area was observed for the largest particle size fraction. The surface area showed a dependence on particle diameter similar to the theoretical one only for the smallest size fractions. This indicates that the particles have other forms than spherical and thus the theoretical model does not describe the particles properly. When studying the particles in larger magnification further, the size fraction >180 lm included a lot of particle aggregates and hollow particles with cracks and holes (Fig. 2). Since the inner surface in such particles is accessible for gas adsorption in the BET measurement, they will get a large specific surface area value. 3.2. Composition and mineralogy of bulk samples and particle size fractions The bulk ash sample element concentrations are summarised in Table 2. The high level of sodium in sample 2 is a result of the addi-
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12 11
% within each size fraction
10
Ash 1
9 8
Ash 2
7 6
Ash 3
5 4 3 2 1 0
Size fraction, m Fig. 1. Particle size distribution of original ashes 1, 2 and 3. (Data for ashes 1 and 2 from Karlfeldt and Steenari (2007)).
Table 1 Specific surface area of: (a) original ashes 1, 2 and 3 and (b) the size fractioned ash 3. (Data for ashes 1 and 2 from Karlfeldt and Steenari (2007)). (a) Ash Specific surface area, m2/g (b) Ash size fraction Specific surface area, m2/g
1 1.2 dp > 180 lm 5.2
dp < 180 lm 1.4
2 0.8 dp < 125 lm 1.1
dp < 90 lm 1.3
3 3.2
dp < 63 lm 1.8
dp < 45 lm 1.43
dp < 32 lm 2.6
Table 2 Total content of major and trace elements for the cyclone ashes studied. (Data for ashes 1 and 2 from Karlfeldt and Steenari (2007)).
Fig. 2. ESEM-image, magnification 500, of the size fraction with particles >180 lm in ash 3. The bar corresponds to 100 lm.
tion of NaHCO3 to the combustion zone. Otherwise the concentrations of major elements are similar in all samples. Earlier work on ash from this boiler has shown that the concentrations of most elements remain relatively constant over the years (Abbas et al., 1999; Karlfeldt and Steenari, 2007; Wilewska-Bien et al., 2007). Zinc and copper are the most abundant trace elements, with concentrations of 3000–7000 mg/kg ash and 5000–10000 mg/kg respectively. Sample 3 in this work has the highest concentrations of Cu and Zn observed, whereas the other two samples have compositions that are very typical for this boiler. Representative contents of chlorine and sulphur as sulphate are 40 g/kg ash and 25 g/kg ash respectively and total dissolved solids (TDS) is about 5% (Abbas et al., 2003). The contents of some major and trace elements in the sieved fractions of ash 3 are given in Table 3. Trace elements are typically
Major elements g/kg ash
Ash 1
Ash 2
Ash 3
Trace elements mg/kg ash
Ash 1
Ash 2
Ash 3
Al Ca Fe K Mg Na P S Si Ti
105 130 30 20 15 30 10 10 180 10
105 120 30 20 15 40 10 5 195 10
125 135 40 20 20 25 10 10 180 15
LOI (% TS)
<1
1
2
As Ba Cd Co Cr Cu Hg Mn Mo Ni Pb Sn Sr V Zn
30 2700 10 30 550 5900 <1 2100 10 220 1500 120 400 50 9000
50 3000 10 20 630 3800 <1 1900 10 160 1100 70 370 60 5900
40 4400 10 30 410 7300 <1 1800 30 210 1500 190 380 60 9600
enriched in smaller particles, whereas the distribution of major elements in different particle size fractions was less predictable. Calcium, Mg and Fe were enriched in smaller particles, whereas K was more abundant in larger particles and Na rather evenly distributed. Na and K often occur as volatile chlorides and therefore were expected to be enriched on small particles. However, the presence of Na and K in feldspar minerals, such as those present in the bed sand, probably covers the effect of chloride enrichment (Table 3). The XRD analysis of original ashes showed the same basic mineralogy for all ash samples with minerals including quartz (SiO2), feldspars ((Ca,Na)AlSi3O8 and KAlSi3O8), Al metal, and alkaline earth compounds (e.g. CaCO3) (Table 4). An estimation of com-
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Table 3 Elemental composition of different particle size fractions of ash 3. The amounts are given in g/kg dry ash and mg/kg dry ash for major and trace elements, respectively. dp > 180 lm
dp < 180 lm
dp < 125 lm
dp < 90 lm
dp < 63 lm
dp < 45 lm
dp < 32 lm
Major elements Ca 89 Fe 15 K 20 Mg 11 Na 22
102 16 19 12 23
135 17 17 14 21
154 20 13 15 21
175 21 15 17 23
183 29 11 19 21
180 34 13 20 24
Trace elements Cd 16 Cr 260 Cu 5790 Mn 1370 Pb 1300 Zn 6790
18 300 5800 1590 1220 5000
20 320 6870 1750 1360 7470
20 320 8890 2170 1830 10300
27 420 10500 2460 2460 13600
32 410 10900 2240 2700 15100
40 460 14600 2410 4030 20200
Table 4 Crystalline compounds identified in the ash samples 1, 2 and 3. (The data for 1 and 2 have been published elsewhere (Karlfeldt and Steenari, 2007)). Ash Mineral NaCl KCl CaO CaCO3 CaSO4 Ca3Al2O6 SiO2 Ca2Al2SiO7 (Na,Ca)AlSi3O8 KAlSi3O8 Al Fe2O3 Fe3O4 MgFe2O4
1
2
3
Minor Trace Minor Minor Minor Minor Major Major Minor Minor Major Trace
Major Trace Trace Major Minor Minor Major Major Minor Minor Major Minor Minor
Trace Trace Major Minor Trace Major Major Major Major Major Minor Minor Minor
pound concentrations has been made and Major means >10 wt%, Minor 5–10 wt% and Trace <5 wt%. All of these content estimations are, however, only approximate. Similar results were found in other studies on comparable ashes (Abbas et al., 2001; Wilewska-Bien et al., 2007; Mahmoudkhani et al., 2008). Quartz and feldspars are remnants of elutriated bed sand. The sand used contains <2% of other minerals than quarts and feldspars. The iron content was similar in all samples, but crystalline iron oxides could only be identified in sample 3. This may be due to the presence of iron species in very small crystals and non-crystalline phases in samples 1 and 2. The different particle size fractions in ash 3 were also examined by XRD and comparison of the peak heights showed that the amount of NaCl and KCl increased with decreasing particle size.
This confirms the expected enrichment of volatile alkali metal chlorides on small particles (Tables 3 and 4). Based on XRD results the amounts of CaCO3, CaSO4, Ca2Al2SiO7 and Fe2O3 also increased as the particle size decreased, whereas the amounts of SiO2 and Almetal decreased. The amounts of CaSO4, Ca2Al2SiO7 and NaCl in the fraction containing particles <32 lm were about twice as high as in the fraction passing the 125 lm sieve. 3.3. Major element composition of single ash particles The ESEM images (Fig. 3) show particles of different sizes and shapes in both ashes 1 and 2, as has also been reported earlier (Fujimori et al., 2002; Karlfeldt and Steenari, 2007). Spherical particles, which are probably formed by local melting, are common in these ashes. However, ESEM images did not indicate that spherical particles would be more abundant in ash 2 than in other ashes despite the increased sodium content and the possible formation of sodium silicate systems with low melting point (about 800 °C) or NaCl–silicate eutectics at even lower temperature (Hong et al., 1993). Two sub-groups were considered in the EDX investigation based on the particle shape, i.e. spherical and non-spherical particles. The element composition of spherical particles seemed to be more homogenous than that of non-spherical ones. The associations of major elements was studied by correlation between elements (correlation coefficient |>0.6|; p < 0.05) and the results are given in Appendix 2. The most significant correlation found was that between Al and Si in both groups. This was expected due to the presence of aluminium silicates. A negative correlation was found between Ca and Al in both sub-groups in ash 1 and in non-spherical particles in ash 2, and between Ca and Si in nonspherical particles in both ashes. Spherical particles typically con-
Fig. 3. ESEM-images, magnification 300, of the original ash samples 1 and 2 (Karlfeldt and Steenari, 2007). The bar corresponds to 150 lm. (a) Ash 1 and (b) ash 2.
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tained less Ca and more Si than non-spherical particles. To a minor extent a similar trend was observed for Al. Chlorine and Na are correlated in spherical particles in both ashes, suggesting that NaCl is adsorbed on particles present in the flue gas and that local eutectic melts cause the particles to become spherical. However, there was no correlation between Na or Cl and other major elements. Thus, the results suggest that there are at least three types of spherical particles: Ca-rich, Al–Si rich and NaCl rich.
tude for both major and trace elements. This indicates that studies, based on bulk element content, may not be adequate to explain trace element association in single ash particles. The distribution of major and trace elements in individual ash particles was investigated by studying correlations (correlation coefficient > 0.6; p < 0.05) and associations (co-occurrence of major and trace elements in trace element rich particles). Correlations and associations are a more common feature in ash 1 than in ash 2. We attribute this difference to the addition of Na during combustion, as a result of which Na is found in all particles in ash 2 and this probably covers possible correlations and association between other elements. For ash 1 significant correlations were found for Cd–Mg, Fe–Ni and Sr–Ca. In addition, particles rich in As, Cd, Co and Cu were also found to contain specific major elements, i.e. Ca and Fe. The absence of correlations for the latter elements is attributed to the fact that while trace element rich particles contain major elements, not all the particles containing major elements also contain these trace elements. We therefore suggest that the binding of these trace elements includes an affinity effect, i.e. that As, Cd, Co and Cu bind to available particles with a higher affinity for specific particles types. Calcium-rich particles appear to play a major carrier role for these elements, as particles with high content of Ca were also observed to contain several trace elements. In contrast, neither correlation nor association was found for Cr, Pb, Ti and Zn and major elements. We suggest that the absence of association for these elements is due to a random process, i.e. these elements bind to available
3.4. Correlations between contents of major and trace elements in single ash particles The investigation of possible correlations between the presence of trace elements in individual ash particles (diameter > 100 lm) and the major matrix elements of those particles was carried out using LA-ICP-MS. The fairly good agreement between the concentrations obtained in bulk analysis and single particle analysis (Fig. 4) shows that the number of particles selected for closer analysis is sufficiently large and that the selected particle size is representative of the bulk ash samples. Considering that LA-ICP-MS only measures the content of the surface, the similarity between bulk analysis and the average result of LA-ICP-MS analysis indicates that the surface composition of ash particles is similar to the composition in the inner parts of the particles. However, large differences were observed between single particles with concentrations spanning over several orders of magni-
(a)
100 LA-ICP-MS 10
Concentration (%)
Composition 1 0.1 0.01 0.001 0.0001
Pb
Ba
Cd
Sr
As
Zn
Cu
Ni
Co
Mn
V
Cr
Fe
Ti
K
Ca
S
Si
Al
Na
(b)
Mg
0.00001
100 LA-ICP-MS
Concentration (%)
10
Composition
1 0.1 0.01 0.001 0.0001
Pb
Ba
Cd
Sr
As
Zn
Cu
Co
Ni
Mn
Cr
V
Fe
Ti
Ca
K
S
Si
Al
Na
Mg
0.00001
Fig. 4. Comparison of total concentrations determined after dissolution of the bulk sample and results from LA-ICP-MS analysis on 100 single particles. (a) Ash 1 and (b) ash 2.
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ash particles regardless of their composition. However, the reasons for affinity or lack of affinity are presently not clear. 4. Discussion In this study the particle size was found to be important for the occurrence of trace elements on ash particles. This enrichment has been explained as partly a result of condensation and accumulation of volatile species in the flue gas cleaning systems (Verhulst et al., 1995; Zhang et al., 2008). In addition, a variety of processes affecting this enrichment have been suggested including the type of incineration technique used, the composition of the fuel and the addition of absorbents (Abbas et al., 2001; Sun et al., 2004; Selcuk et al., 2005; Fan et al., 2006; Zhang et al., 2008). Anyhow, published data on the distribution of individual elements includes noticeable differences. Lead is enriched in small particles in all the studied references, regardless of fuel and incineration technique used, while Cd shows opposite trends (Bogdanovi et al., 1995; Mattigod et al., 1999; Thipse et al., 2002; Arditsoglou et al., 2004; Liao et al., 2005). Variations in distribution for the same elements on and inside ash particles have also been reported (Fujimori et al., 2002; Camerani-Pinzani et al., 2004). There is also data available that indicates that trace elements are not homogenously distributed even within a single particle, pointing at the importance of the ash particle matrix (Camerani-Pinzani et al., 2002).This indicates that although trace elements are generally enriched in, or on, small particles, properties of the individual elements as well as parameters specific in each incineration situation are important for the binding of trace elements to ash particles. The results obtained in this work suggest that As, Cd, Co and Cu bind to available particles with a higher affinity for calcium and iron containing particles and that species of Cr, Pb, Ti and Zn mainly adsorb onto fly ash particles through a random process. The importance of Ca as a trace element carrier has been discussed earlier. Giere et al. (2003) found that Ca and S were important hosts for V and Zn and Seames and Wendt (2007) established that As and Se were associated to Ca and Fe in coal fly ash. Results showing that calcium compounds in MSW fly ash can carry Cd was also obtained in the work of Camerani-Pinzani et al. (2007). Based on the results obtained in this work it is not possible to determine if the vaporous trace metal species adsorb directly onto certain matrix mineral surfaces or if the trace elements specie reacts with this component after a random condensation on the surface. To do so, the trace metal speciation, i.e. the chemical compounds that the trace metals occur in, has to be determined. This is not easily done since the metal concentrations are very low. The most promising method for trace metal speciation is synchrotron based X-ray absorption spectroscopy (XAS) (CameraniPinzani et al., 2002). However, knowledge of the trace metal speciation can be valuable for elucidating of for example leaching properties and efforts to gather XAS data for trace metals are being done by our group and others. Appendix 1. Relation between particle diameter and surface area With the assumptions that all particles are spherical and have the same material density the volumes and surfaces areas of two particles with diameters d1 and d2 = ½d1 can be calculated. The respective masses and surface areas are called m1, m2, A1 and A2. From the relation d2 = ½d1 it then follows that: d2 ¼1=2d1
!
Asf a€r;2 =m2 ¼2 Asf a€r;1 =m1
Appendix 2 Correlation (correlation coefficient |>0.6|; p < 0.05) between major elements in spherical and non-spherical particles. (a) Ash 1 and (b) ash 2. (a) Ash 1 Spherical particles Si/Ca Si/Al 0.51 0.71 Ca/Al 0.68
Si/Fe 0.50 Ca/Fe 0.07 Al/Fe 0.07
Non-spherical particles Si/Ca Si/Al Si/Fe 0.79 0.70 0.03 Ca/Al Ca/Fe 0.68 0.24 Al/Fe 0.16
(b) Ash 2 Spherical particles Si/Ca Si/Al 0.18 0.82 Ca/Al 0.11
Si/Fe 0.23 Ca/Fe 0.03 Al/Fe 0.04
Non-spherical particles Si/Ca Si/Al Si/Fe 0.60 0.82 0.21 Ca/Al Ca/Fe 0.61 0.23 Al/Fe 0.23
Si/Cl 0.59 Ca/Cl 0.03 Al/Cl 0.52 Fe/Cl 0.23
Si/Na 0.36 Ca/Na 0.38 Al/Na 0.11 Fe/Na 0.26 Cl/Na 0.69
Si/K 0.03 Ca/K 0.25 Al/K 0.12 Fe/K 0.18 Cl/K 0.69 Na/K 0.26
Si/Ti 0.28 Ca/Ti 0.58 Al/Ti 0.26 Fe/Ti 0.08 Cl/Ti 0.20 Na/Ti 0.37 K/Ti 0.17
Si/Cl 0.39 Ca/Cl 0.32 Al/Cl 0.17 Fe/Cl 0.29
Si/Na 0.70 Ca/Na 0.71 Al/Na 0.89 Fe/Na 0.17 Cl/Na 0.11
Si/K 0.06 Ca/K 0.02 Al/K 0.17 Fe/K 0.24 Cl/K 0.60 Na/K 0.16
Si/Ti 0.21 Ca/Ti 0.20 Al/Ti 0.25 Fe/Ti 0.05 Cl/Ti 0.83 Na/Ti 0.08 K/Ti 0.68
Si/Cl 0.67 Ca/Cl 0.28 Al/Cl 0.54 Fe/Cl 0.21
Si/Na 0.65 Ca/Na 0.31 Al/Na 0.50 Fe/Na 0.22 Cl/Na 0.99
Si/K 0.36 Ca/K 0.23 Al/K 0.06 Fe/K 0.69 Cl/K 0.12 Na/K 0.17
Si/Ti 0.32 Ca/Ti 0.29 Al/Ti 0.18 Fe/Ti 0.86 Cl/Ti 0.16 Na/Ti 0.15 K/Ti 0.56
Si/Cl 0.56 Ca/Cl 0.25 Al/Cl 0.41 Fe/Cl 0.01
Si/Na 0.08 Ca/Na 0.52 Al/Na 0.19 Fe/Na 0.08 Cl/Na 0.38
Si/K 0.38 Ca/K 0.07 Al/K 0.31 Fe/K 0.82 Cl/K 0.48 Na/K 0.09
Si/Ti 0.17 Ca/Ti 0.10 Al/Ti 0.32 Fe/Ti 0.01 Cl/Ti 0.33 Na/Ti 0.37 K/Ti 0.28
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Element associations in ash from waste combustion in fluidized bed K. Karlfeldt Fedje a,*, S. Rauch b, P. Cho a, B.-M. Steenari a a b
Department of Chemical and Biological Engineering, Division of Environmental Inorganic Chemistry, Chalmers University of Technology, Kemivägen 10, 412 96 Göteborg, Sweden Department of Civil and Environmental Engineering, Division of Water Environment Technology, Chalmers University of Technology, Sven Hultins Gata 8, 412 96 Göteborg, Sweden
a r t i c l e
i n f o
Article history: Received 10 October 2008 Accepted 7 September 2009 Available online 21 October 2009
a b s t r a c t The incineration of MSW in fluidized beds is a commonly applied waste management practice. The composition of the ashes produced in a fluidized bed boiler has important environmental implications as potentially toxic trace elements may be associated with ash particles and it is therefore essential to determine the mechanisms controlling the association of trace elements to ash particles, including the role of major element composition. The research presented here uses micro-analytical techniques to study the distribution of major and trace elements and determine the importance of affinity-based binding mechanisms in separate cyclone ash particles from MSW combustion. Particle size and the occurrence of Ca and Fe were found to be important factors for the binding of trace elements to ash particles, but the binding largely depends on random associations based on the presence of a particle when trace elements condensate in the flue gas. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Combustion of municipal solid waste (MSW) is a commonly used management method for several reasons, including an efficient reduction of the waste volume and a substantial heating value of MSW (Reimann, 2006). Fluidized bed combustion is increasingly applied since this process is flexible towards variations in fuel properties and reasonably robust. Volatilisation of metal species may occur during combustion, and the extent not only depends on the temperature, but also on the gas composition. Thermodynamic equilibrium calculations and measurements during incineration show that a high concentration of chlorine as well as a reducing atmosphere increases the volatility of metals like Cd, Pb and Zn, whereas the presence of sulphur can reduce the volatility (Verhulst et al., 1995; Sun et al., 2004; Zhang et al., 2008). Condensation of volatile metal compounds as kernels of fly ash particles, or on already available particle surfaces, will occur when the temperature of the gas decreases. Thus, a number of reaction paths are possible, most of them leading to an enrichment of the volatised species in the fly ash (Haynes et al., 1982; Flagan and Sarofim, 1984). As the surface area available on a particle increases rapidly as the particle diameter decreases, it could be expected that the amount of volatile metal compounds found in different ash particle size fractions depends on the particle diameter. However, published results indicate that the distribution patterns of metals in ash particles also depend on the ash matrix. Cereda and coworkers found that coal ash particles could be grouped based on their major content and the presence of trace elements was not * Corresponding author. Tel.: +46 31 772 2864; fax: +46 31 772 2853. E-mail address: [email protected] (K. Karlfeldt Fedje). 0956-053X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2009.09.012
equally distributed between the groups (Cereda et al., 1995). Particles rich in Al and Si also had high content of for instance Cu and Ni. In another study Cd, Cu and Pb showed affinity for particles rich in quarts (Lind et al., 1999). Such processes would include a site specific step, i.e. a reaction step where the affinity of the metal to a specific ash matrix mineral plays an important role. Knowledge of this would open for an active choice of bed material or mineral additive in the fluidized bed to bind metals in stable, insoluble forms. One example is the use of kaolin to bind potassium in biomass combustion and thus decrease the problems with ash sintering and corrosion caused by KCl (Davidsson et al., 2007). However, the occurrence of affinity driven processes and association between elements on ash particles in MSW ashes have not been definitely verified so far and the present work was carried out to investigate this further. During the last decades, development of microprobes technique has opened up the possibility for analyses using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A laser beam is used as sampling device for online analysis by ICPMS, thereby providing sensitive multi-element analysis at micrometric resolution (Durrant, 1999; Gunther and Hattendorf, 2005). The LA-ICP-MS is applied on solids and is widely used for geochemical samples, but also for such various fields as material sciences, biology, and ornithology, the technique is becoming more common (Koch et al., 2002; Reinhardt et al., 2003; Ek et al., 2004; Jackson et al., 2004). However, analysis of ash, using this method, is still not common and the few studies found focus on coal fly ash (Spears, 2004; Spears and Martinez-Tarrazona, 2004). Therefore we found it interesting to apply it to our ash samples with the aim to study the distribution of major and minor elements on single ash particles of different appearance and matrix composition.
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Electron microscopy and energy dispersive X-ray fluorescence spectrometry was also applied. Both these methods are designed for surface analysis which gives the opportunity to compare the composition of the particle surfaces with bulk composition data. The choice of cyclone ash from combustion of MSW in a fluidized bed boiler was based on the fact that fluidized bed combustion is frequently used for MSW and the cyclone ash represents a large part of the total amount of ash produced at such a plant. In addition, the potential utilization of cyclone ash for construction purposes is promising as the content of metal species is rather low compared to that of the filter ash.
2. Experimental Ash samples were collected from the cyclones of 20 MW bubbling fluidized bed (BFB) boilers using a sand consisting of quartz and feldspars, as bed material. The power plant is located in south western Sweden and is exclusively used for the incineration of MSW. At the time of the sampling the waste burned consisted of 50% household waste, 25% burnable fraction from household and light industry waste, 15% paper and packaging, 10% RDF (well sorted household waste). The combustion temperature was about 850 °C. When the ashes 1 and 2 were produced ammonia was added for reduction of NOx emissions. In the later case (ash 2) sodium bicarbonate was also added with the aim to bind HCl and SO2. In this boiler the cyclone is placed after a section containing heat transfer surfaces which means that the temperature has been decreased to about 150 °C. One sample (denoted ash 3) was dry sieved into seven particle size fractions (>180, 125–180, 90–125, 63–90, 45–63, 32–45 and <32 lm) for investigation of the dependence of ash properties on particle size.
2.2. Single particle surface chemical composition The shape and surface structure of ash particles were studied by environmental scanning electron microscopy (ESEM) using an Electro-Scan 2020 operated at 20 kV for secondary electron imaging. Further characterisation was performed by electron microscopy with EDX detection for the determination of major elements (Al, Ca, Cl, Fe, K, Na, Si and Ti) using a FEI Quanta 200 FEG ESEM operated at 15 kV and coupled to an Oxford Inca 300 EDX system. The detection limit of the EEX-system is about 1 wt% and the penetration depth is about 1 lm. The composition of single ash particles was determined by LAICP-MS. Analysis was performed on a Cetac LSX-200 laser system coupled to a PE Sciex Elan 6000 ICP-MS. Due to beam size limitations and the need for multi-element data acquisition, only particles with a diameter larger than 100 lm could be analysed. Selected major (Al, Ca, Fe, K, Mg, Na, S, Si, Ti) and trace (As, Ba, Cd, Co, Cr, Cu, Pb, Mn, Ni, Sr, V, Zn) elements were analysed in approximately 100 particles of different shapes and sizes from samples 1 and 2 respectively. Ablation was applied in the form of a scan line across the particles at a speed of 10 lm s1 andwith a beam diameter of 25 lm while the energy was optimised to provide a sufficient signal. For the ICP-MS a short dwell time, 5 ms, was used for multi-element acquisition together with a short quadrupole settling time. Calibration was done using an ash reference material (NIST 1633b) pressed in the form of a pellet. Results for each element and particles were averaged. It is important to note that LA-ICP-MS is considered to be a surface analysis technique with the settings used. Visual observation of the particles after ablation supports that only material at the surface of the particles was ablated, although the penetration depth could not be determined. 3. Results 3.1. Particle size distribution
2.1. Bulk ash characterisation Total elemental composition of bulk samples was determined after total acid digestion using inductively coupled plasma with either optical emission spectroscopic or mass spectrometric detection (ICP-OES and ICP-MS) in an earlier work concerning samples 1 and 2 (Karlfeldt and Steenari, 2007) and for sample 3, in this work, using atomic absorption spectroscopy (AAS) with the flame AAS set up for major elements and the graphite furnace set up for trace elements. Crystalline compounds were identified with a detection limit of about 2 wt% by qualitative X-ray powder diffractometry (XRD) using a Siemens D5000 X-ray powder diffractometer with Cu characteristic radiation and a scintillation detector. Identification of compounds was carried out using the Joint Committee of Powder Diffraction Standards database (2006). The particle size distribution for particles smaller than 118 lm was determined using a Malvern 2600c laser diffractometer. The distribution is determined from a diffraction pattern resulting from a laser irradiated suspension of ash and water. Sample 3 contained 5 wt% of magnetic material, which was removed to hinder ash particles to stick on the magnetic stirrer used in the measurement cell. The content of magnetic material was lower in ashes 1 and 2. Specific surface area was determined according to the BET (Brunauer, Emmet, Teller) five point method (Webb and Orr, 1997) using a Micromeritics Accelerated Surface Area and Porosimetry System 2010 instrument for ashes 1 and 2 in an earlier work (Karlfeldt and Steenari, 2007) and using a Micromeritics TriStar 3000 for ash 3 in this work.
The particle size distributions and specific surface area data for the ash samples are presented in Fig. 1 and Table 1. Probably there had been a slight difference in the separation efficiency of the cyclone between the sampling dates 1, 2 and 3 since sample 3 contained more material in the larger particle size fractions than the other samples did. Specific surface areas were similar with a slightly higher value for sample 3 (Table 1a). The specific surface area values of size fractionated ash 3 samples (Table 1b) did not follow the expected trend with the surface area per weight unit increasing to the double when the particle diameter is decreased by a factor 2 (Appendix 1). Instead, the largest specific surface area was observed for the largest particle size fraction. The surface area showed a dependence on particle diameter similar to the theoretical one only for the smallest size fractions. This indicates that the particles have other forms than spherical and thus the theoretical model does not describe the particles properly. When studying the particles in larger magnification further, the size fraction >180 lm included a lot of particle aggregates and hollow particles with cracks and holes (Fig. 2). Since the inner surface in such particles is accessible for gas adsorption in the BET measurement, they will get a large specific surface area value. 3.2. Composition and mineralogy of bulk samples and particle size fractions The bulk ash sample element concentrations are summarised in Table 2. The high level of sodium in sample 2 is a result of the addi-
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12 11
% within each size fraction
10
Ash 1
9 8
Ash 2
7 6
Ash 3
5 4 3 2 1 0
Size fraction, m Fig. 1. Particle size distribution of original ashes 1, 2 and 3. (Data for ashes 1 and 2 from Karlfeldt and Steenari (2007)).
Table 1 Specific surface area of: (a) original ashes 1, 2 and 3 and (b) the size fractioned ash 3. (Data for ashes 1 and 2 from Karlfeldt and Steenari (2007)). (a) Ash Specific surface area, m2/g (b) Ash size fraction Specific surface area, m2/g
1 1.2 dp > 180 lm 5.2
dp < 180 lm 1.4
2 0.8 dp < 125 lm 1.1
dp < 90 lm 1.3
3 3.2
dp < 63 lm 1.8
dp < 45 lm 1.43
dp < 32 lm 2.6
Table 2 Total content of major and trace elements for the cyclone ashes studied. (Data for ashes 1 and 2 from Karlfeldt and Steenari (2007)).
Fig. 2. ESEM-image, magnification 500, of the size fraction with particles >180 lm in ash 3. The bar corresponds to 100 lm.
tion of NaHCO3 to the combustion zone. Otherwise the concentrations of major elements are similar in all samples. Earlier work on ash from this boiler has shown that the concentrations of most elements remain relatively constant over the years (Abbas et al., 1999; Karlfeldt and Steenari, 2007; Wilewska-Bien et al., 2007). Zinc and copper are the most abundant trace elements, with concentrations of 3000–7000 mg/kg ash and 5000–10000 mg/kg respectively. Sample 3 in this work has the highest concentrations of Cu and Zn observed, whereas the other two samples have compositions that are very typical for this boiler. Representative contents of chlorine and sulphur as sulphate are 40 g/kg ash and 25 g/kg ash respectively and total dissolved solids (TDS) is about 5% (Abbas et al., 2003). The contents of some major and trace elements in the sieved fractions of ash 3 are given in Table 3. Trace elements are typically
Major elements g/kg ash
Ash 1
Ash 2
Ash 3
Trace elements mg/kg ash
Ash 1
Ash 2
Ash 3
Al Ca Fe K Mg Na P S Si Ti
105 130 30 20 15 30 10 10 180 10
105 120 30 20 15 40 10 5 195 10
125 135 40 20 20 25 10 10 180 15
LOI (% TS)
<1
1
2
As Ba Cd Co Cr Cu Hg Mn Mo Ni Pb Sn Sr V Zn
30 2700 10 30 550 5900 <1 2100 10 220 1500 120 400 50 9000
50 3000 10 20 630 3800 <1 1900 10 160 1100 70 370 60 5900
40 4400 10 30 410 7300 <1 1800 30 210 1500 190 380 60 9600
enriched in smaller particles, whereas the distribution of major elements in different particle size fractions was less predictable. Calcium, Mg and Fe were enriched in smaller particles, whereas K was more abundant in larger particles and Na rather evenly distributed. Na and K often occur as volatile chlorides and therefore were expected to be enriched on small particles. However, the presence of Na and K in feldspar minerals, such as those present in the bed sand, probably covers the effect of chloride enrichment (Table 3). The XRD analysis of original ashes showed the same basic mineralogy for all ash samples with minerals including quartz (SiO2), feldspars ((Ca,Na)AlSi3O8 and KAlSi3O8), Al metal, and alkaline earth compounds (e.g. CaCO3) (Table 4). An estimation of com-
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Table 3 Elemental composition of different particle size fractions of ash 3. The amounts are given in g/kg dry ash and mg/kg dry ash for major and trace elements, respectively. dp > 180 lm
dp < 180 lm
dp < 125 lm
dp < 90 lm
dp < 63 lm
dp < 45 lm
dp < 32 lm
Major elements Ca 89 Fe 15 K 20 Mg 11 Na 22
102 16 19 12 23
135 17 17 14 21
154 20 13 15 21
175 21 15 17 23
183 29 11 19 21
180 34 13 20 24
Trace elements Cd 16 Cr 260 Cu 5790 Mn 1370 Pb 1300 Zn 6790
18 300 5800 1590 1220 5000
20 320 6870 1750 1360 7470
20 320 8890 2170 1830 10300
27 420 10500 2460 2460 13600
32 410 10900 2240 2700 15100
40 460 14600 2410 4030 20200
Table 4 Crystalline compounds identified in the ash samples 1, 2 and 3. (The data for 1 and 2 have been published elsewhere (Karlfeldt and Steenari, 2007)). Ash Mineral NaCl KCl CaO CaCO3 CaSO4 Ca3Al2O6 SiO2 Ca2Al2SiO7 (Na,Ca)AlSi3O8 KAlSi3O8 Al Fe2O3 Fe3O4 MgFe2O4
1
2
3
Minor Trace Minor Minor Minor Minor Major Major Minor Minor Major Trace
Major Trace Trace Major Minor Minor Major Major Minor Minor Major Minor Minor
Trace Trace Major Minor Trace Major Major Major Major Major Minor Minor Minor
pound concentrations has been made and Major means >10 wt%, Minor 5–10 wt% and Trace <5 wt%. All of these content estimations are, however, only approximate. Similar results were found in other studies on comparable ashes (Abbas et al., 2001; Wilewska-Bien et al., 2007; Mahmoudkhani et al., 2008). Quartz and feldspars are remnants of elutriated bed sand. The sand used contains <2% of other minerals than quarts and feldspars. The iron content was similar in all samples, but crystalline iron oxides could only be identified in sample 3. This may be due to the presence of iron species in very small crystals and non-crystalline phases in samples 1 and 2. The different particle size fractions in ash 3 were also examined by XRD and comparison of the peak heights showed that the amount of NaCl and KCl increased with decreasing particle size.
This confirms the expected enrichment of volatile alkali metal chlorides on small particles (Tables 3 and 4). Based on XRD results the amounts of CaCO3, CaSO4, Ca2Al2SiO7 and Fe2O3 also increased as the particle size decreased, whereas the amounts of SiO2 and Almetal decreased. The amounts of CaSO4, Ca2Al2SiO7 and NaCl in the fraction containing particles <32 lm were about twice as high as in the fraction passing the 125 lm sieve. 3.3. Major element composition of single ash particles The ESEM images (Fig. 3) show particles of different sizes and shapes in both ashes 1 and 2, as has also been reported earlier (Fujimori et al., 2002; Karlfeldt and Steenari, 2007). Spherical particles, which are probably formed by local melting, are common in these ashes. However, ESEM images did not indicate that spherical particles would be more abundant in ash 2 than in other ashes despite the increased sodium content and the possible formation of sodium silicate systems with low melting point (about 800 °C) or NaCl–silicate eutectics at even lower temperature (Hong et al., 1993). Two sub-groups were considered in the EDX investigation based on the particle shape, i.e. spherical and non-spherical particles. The element composition of spherical particles seemed to be more homogenous than that of non-spherical ones. The associations of major elements was studied by correlation between elements (correlation coefficient |>0.6|; p < 0.05) and the results are given in Appendix 2. The most significant correlation found was that between Al and Si in both groups. This was expected due to the presence of aluminium silicates. A negative correlation was found between Ca and Al in both sub-groups in ash 1 and in non-spherical particles in ash 2, and between Ca and Si in nonspherical particles in both ashes. Spherical particles typically con-
Fig. 3. ESEM-images, magnification 300, of the original ash samples 1 and 2 (Karlfeldt and Steenari, 2007). The bar corresponds to 150 lm. (a) Ash 1 and (b) ash 2.
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tained less Ca and more Si than non-spherical particles. To a minor extent a similar trend was observed for Al. Chlorine and Na are correlated in spherical particles in both ashes, suggesting that NaCl is adsorbed on particles present in the flue gas and that local eutectic melts cause the particles to become spherical. However, there was no correlation between Na or Cl and other major elements. Thus, the results suggest that there are at least three types of spherical particles: Ca-rich, Al–Si rich and NaCl rich.
tude for both major and trace elements. This indicates that studies, based on bulk element content, may not be adequate to explain trace element association in single ash particles. The distribution of major and trace elements in individual ash particles was investigated by studying correlations (correlation coefficient > 0.6; p < 0.05) and associations (co-occurrence of major and trace elements in trace element rich particles). Correlations and associations are a more common feature in ash 1 than in ash 2. We attribute this difference to the addition of Na during combustion, as a result of which Na is found in all particles in ash 2 and this probably covers possible correlations and association between other elements. For ash 1 significant correlations were found for Cd–Mg, Fe–Ni and Sr–Ca. In addition, particles rich in As, Cd, Co and Cu were also found to contain specific major elements, i.e. Ca and Fe. The absence of correlations for the latter elements is attributed to the fact that while trace element rich particles contain major elements, not all the particles containing major elements also contain these trace elements. We therefore suggest that the binding of these trace elements includes an affinity effect, i.e. that As, Cd, Co and Cu bind to available particles with a higher affinity for specific particles types. Calcium-rich particles appear to play a major carrier role for these elements, as particles with high content of Ca were also observed to contain several trace elements. In contrast, neither correlation nor association was found for Cr, Pb, Ti and Zn and major elements. We suggest that the absence of association for these elements is due to a random process, i.e. these elements bind to available
3.4. Correlations between contents of major and trace elements in single ash particles The investigation of possible correlations between the presence of trace elements in individual ash particles (diameter > 100 lm) and the major matrix elements of those particles was carried out using LA-ICP-MS. The fairly good agreement between the concentrations obtained in bulk analysis and single particle analysis (Fig. 4) shows that the number of particles selected for closer analysis is sufficiently large and that the selected particle size is representative of the bulk ash samples. Considering that LA-ICP-MS only measures the content of the surface, the similarity between bulk analysis and the average result of LA-ICP-MS analysis indicates that the surface composition of ash particles is similar to the composition in the inner parts of the particles. However, large differences were observed between single particles with concentrations spanning over several orders of magni-
(a)
100 LA-ICP-MS 10
Concentration (%)
Composition 1 0.1 0.01 0.001 0.0001
Pb
Ba
Cd
Sr
As
Zn
Cu
Ni
Co
Mn
V
Cr
Fe
Ti
K
Ca
S
Si
Al
Na
(b)
Mg
0.00001
100 LA-ICP-MS
Concentration (%)
10
Composition
1 0.1 0.01 0.001 0.0001
Pb
Ba
Cd
Sr
As
Zn
Cu
Co
Ni
Mn
Cr
V
Fe
Ti
Ca
K
S
Si
Al
Na
Mg
0.00001
Fig. 4. Comparison of total concentrations determined after dissolution of the bulk sample and results from LA-ICP-MS analysis on 100 single particles. (a) Ash 1 and (b) ash 2.
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ash particles regardless of their composition. However, the reasons for affinity or lack of affinity are presently not clear. 4. Discussion In this study the particle size was found to be important for the occurrence of trace elements on ash particles. This enrichment has been explained as partly a result of condensation and accumulation of volatile species in the flue gas cleaning systems (Verhulst et al., 1995; Zhang et al., 2008). In addition, a variety of processes affecting this enrichment have been suggested including the type of incineration technique used, the composition of the fuel and the addition of absorbents (Abbas et al., 2001; Sun et al., 2004; Selcuk et al., 2005; Fan et al., 2006; Zhang et al., 2008). Anyhow, published data on the distribution of individual elements includes noticeable differences. Lead is enriched in small particles in all the studied references, regardless of fuel and incineration technique used, while Cd shows opposite trends (Bogdanovi et al., 1995; Mattigod et al., 1999; Thipse et al., 2002; Arditsoglou et al., 2004; Liao et al., 2005). Variations in distribution for the same elements on and inside ash particles have also been reported (Fujimori et al., 2002; Camerani-Pinzani et al., 2004). There is also data available that indicates that trace elements are not homogenously distributed even within a single particle, pointing at the importance of the ash particle matrix (Camerani-Pinzani et al., 2002).This indicates that although trace elements are generally enriched in, or on, small particles, properties of the individual elements as well as parameters specific in each incineration situation are important for the binding of trace elements to ash particles. The results obtained in this work suggest that As, Cd, Co and Cu bind to available particles with a higher affinity for calcium and iron containing particles and that species of Cr, Pb, Ti and Zn mainly adsorb onto fly ash particles through a random process. The importance of Ca as a trace element carrier has been discussed earlier. Giere et al. (2003) found that Ca and S were important hosts for V and Zn and Seames and Wendt (2007) established that As and Se were associated to Ca and Fe in coal fly ash. Results showing that calcium compounds in MSW fly ash can carry Cd was also obtained in the work of Camerani-Pinzani et al. (2007). Based on the results obtained in this work it is not possible to determine if the vaporous trace metal species adsorb directly onto certain matrix mineral surfaces or if the trace elements specie reacts with this component after a random condensation on the surface. To do so, the trace metal speciation, i.e. the chemical compounds that the trace metals occur in, has to be determined. This is not easily done since the metal concentrations are very low. The most promising method for trace metal speciation is synchrotron based X-ray absorption spectroscopy (XAS) (CameraniPinzani et al., 2002). However, knowledge of the trace metal speciation can be valuable for elucidating of for example leaching properties and efforts to gather XAS data for trace metals are being done by our group and others. Appendix 1. Relation between particle diameter and surface area With the assumptions that all particles are spherical and have the same material density the volumes and surfaces areas of two particles with diameters d1 and d2 = ½d1 can be calculated. The respective masses and surface areas are called m1, m2, A1 and A2. From the relation d2 = ½d1 it then follows that: d2 ¼1=2d1
!
Asf a€r;2 =m2 ¼2 Asf a€r;1 =m1
Appendix 2 Correlation (correlation coefficient |>0.6|; p < 0.05) between major elements in spherical and non-spherical particles. (a) Ash 1 and (b) ash 2. (a) Ash 1 Spherical particles Si/Ca Si/Al 0.51 0.71 Ca/Al 0.68
Si/Fe 0.50 Ca/Fe 0.07 Al/Fe 0.07
Non-spherical particles Si/Ca Si/Al Si/Fe 0.79 0.70 0.03 Ca/Al Ca/Fe 0.68 0.24 Al/Fe 0.16
(b) Ash 2 Spherical particles Si/Ca Si/Al 0.18 0.82 Ca/Al 0.11
Si/Fe 0.23 Ca/Fe 0.03 Al/Fe 0.04
Non-spherical particles Si/Ca Si/Al Si/Fe 0.60 0.82 0.21 Ca/Al Ca/Fe 0.61 0.23 Al/Fe 0.23
Si/Cl 0.59 Ca/Cl 0.03 Al/Cl 0.52 Fe/Cl 0.23
Si/Na 0.36 Ca/Na 0.38 Al/Na 0.11 Fe/Na 0.26 Cl/Na 0.69
Si/K 0.03 Ca/K 0.25 Al/K 0.12 Fe/K 0.18 Cl/K 0.69 Na/K 0.26
Si/Ti 0.28 Ca/Ti 0.58 Al/Ti 0.26 Fe/Ti 0.08 Cl/Ti 0.20 Na/Ti 0.37 K/Ti 0.17
Si/Cl 0.39 Ca/Cl 0.32 Al/Cl 0.17 Fe/Cl 0.29
Si/Na 0.70 Ca/Na 0.71 Al/Na 0.89 Fe/Na 0.17 Cl/Na 0.11
Si/K 0.06 Ca/K 0.02 Al/K 0.17 Fe/K 0.24 Cl/K 0.60 Na/K 0.16
Si/Ti 0.21 Ca/Ti 0.20 Al/Ti 0.25 Fe/Ti 0.05 Cl/Ti 0.83 Na/Ti 0.08 K/Ti 0.68
Si/Cl 0.67 Ca/Cl 0.28 Al/Cl 0.54 Fe/Cl 0.21
Si/Na 0.65 Ca/Na 0.31 Al/Na 0.50 Fe/Na 0.22 Cl/Na 0.99
Si/K 0.36 Ca/K 0.23 Al/K 0.06 Fe/K 0.69 Cl/K 0.12 Na/K 0.17
Si/Ti 0.32 Ca/Ti 0.29 Al/Ti 0.18 Fe/Ti 0.86 Cl/Ti 0.16 Na/Ti 0.15 K/Ti 0.56
Si/Cl 0.56 Ca/Cl 0.25 Al/Cl 0.41 Fe/Cl 0.01
Si/Na 0.08 Ca/Na 0.52 Al/Na 0.19 Fe/Na 0.08 Cl/Na 0.38
Si/K 0.38 Ca/K 0.07 Al/K 0.31 Fe/K 0.82 Cl/K 0.48 Na/K 0.09
Si/Ti 0.17 Ca/Ti 0.10 Al/Ti 0.32 Fe/Ti 0.01 Cl/Ti 0.33 Na/Ti 0.37 K/Ti 0.28
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