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Energy & Fuels 2009, 23, 1518–1525
X-Ray Absorption Near Edge Structure Spectrometry Study of Nickel and Lead Speciation in Coals and Coal Combustion Products Pushan Shah,* Vladimir Strezov, and Peter F. Nelson CRC for Coal in Sustainable DeVelopment, Graduate School of the EnVironment, Macquarie UniVersity, Sydney, NSW 2109, Australia ReceiVed September 29, 2008. ReVised Manuscript ReceiVed January 10, 2009
The fate and environmental impacts of trace elements from coal fired power stations are a significant concern because of the large quantities of coal used as an energy source. The ultimate environmental fate and health impact of some of these trace elements is dependent on their various forms and oxidation states. Nickel and lead are two of the trace elements classified as “priority pollutants” by the National Pollutant Inventory (NPI) in Australia. This study attempts to understand speciation of nickel and lead in coal and coal combustion products from five coal fired power stations in Australia where bituminous rank coals are utilized. Non-destructive X-ray Absorption Near Edge Structure Spectrometry (XANES) was used to determine speciation of these metals. Semiquantitative speciation of nickel and lead was calculated using a linear combination fit of XANES spectra obtained for selected pure reference compounds. In all fly ash samples, 28-80% of nickel was present as nickel in NiSO4 form, which is a more toxic and more bioavailable form of nickel. Less toxic NiO was detected in fly ash samples in the range of 0-15%. Speciation of lead revealed that 65-70% is present as PbS in the feed coals. In all fly ash samples analyzed, lead comprised different proportions of PbCl2, PbO, and PbSO4. PbCl2 and PbSO4 contents varied between 30-70% and 30-60%, respectively. Chemical reactions resulting in nickel and lead transformation that are likely to have occurred in the post-combustion environment are discussed.
1. Introduction The trace elements and their compounds are naturally occurring in the coal matrix at very low concentrations. A number of these elements, such as As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Se, Zn, are classified as toxic and, during coal combustion, distribute to the bottom ash, fly ash, or flue gas in either gaseous or particulate forms. According to an estimate made in a recent study 4930 Mt of coal is utilized annually worldwide for electricity generation which produces 180 Mt of bottom ash and 410 Mt of fly ash.1 Fly ash capture efficiency was assumed to be 95%, which results in an estimated 390 Mt of fly ash being captured with the pollution control devices, while 20 Mt fly ash escapes in the environment as particulate emissions.1 In Australia 130 Mt coal is utilized annually contributing 85% of national electricity generation which results in the annual production of 13.5 Mt of fly ash. About 30% of the collected fly ash is reused while the rest is disposed of in ash dams and dry ash disposal systems. When introduced to the environment, fine particles and volatile trace elements can travel long distances and eventually may deposit far away from the source potentially contaminating the soil and river systems. Collection and deposition of coal ash in ash impoundments may also create environmental concerns as the toxic trace elements may further react and reform chemically with rainwater and may become part of either underground water or nearby aquatic systems such as lakes and rivers. Apart from their total concentrations, the form and valency of trace elements determines the level of toxicity and carcinogenic potency. Properties * To whom correspondence should be addressed. E-mail: pshah@ gse.mq.edu.au. Phone: +61-2-9850 7950. Fax: +61-2-9850 7972. (1) Mukherjee, A. B.; Zevenhoven, R. Sci. Total EnViron. 2006, 368 (1), 384–392.
such as toxicity, mobility, bioavailability, physical and chemical behavior of any chemical elements depend on the chemical form, for example, oxidation state, known as speciation. To successfully provide environmental control measures and assessment of impacts of coal emissions and combustion wastes to the environment, it is of great importance to have an understanding of speciation of trace elements. There are several high priority trace elements, such as As, Cd, Cr(VI), Hg, Ni, and Pb which require strict emission control in Australia.2 The present study will focus on investigation of the chemical speciation of two of these high priority pollutants, nickel and lead, in coal combustion products. According to the Australian National Pollutant Inventory, coal fired power stations are among the highest pollution sources of lead and nickel in the environment, and assessment, not only of their total emissions, but also of the chemical speciation of metals in these emissions, is a high priority. In the environment, nickel predominantly exists in the Ni(II) oxidation state. Other valences (-1, +1, +3, and +4) are less frequently encountered in the environment. Exposure to nickel polluted environments is widely accepted as potentially hazardous to humans. Some pathological effects from nickel poisoning consist of skin allergies, lung fibrosis, and cancer of the respiratory tract.3 The level of carcinogenicity of nickel depends on the species and amounts of nickel compounds. In particular, some Ni(II) containing compounds are considered to have severe genotoxic and mutagenic activities to humans. Various forms (2) Air toxics and indoor air quality in Australia, State of Knowledge Report, Environment Australia 2001. http://www.environment.gov.au/ atmosphere/airquality/publications/sok/chapter5.html, accessed on 17th May 2008. (3) Kasprzak, K. S.; Sunderman, F. W., Jr; Salnikow, K. Mutat. Res. 2003, 533 (1-2), 67–97.
10.1021/ef800824d CCC: $40.75 2009 American Chemical Society Published on Web 02/23/2009
XANES Study of Nickel and Lead Speciation in Coals
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Table 1. Ultimate and Proximate Analysis of Coal and Ash Samples proximate analysis (%) (ad basis)
ultimate analysis (%) (daf basis)
coal
air-dried moisture
ash
volatile matter
fixed carbon
C
H
N
Oa
S
A B C D E
13.3 11.1 2.8 4.0 2.8
5.4 6.8 26.8 32.5 20.9
33.4 33.0 26.5 26.6 28.2
47.9 49.1 43.9 36.9 48.1
74.5 73.2 81.7 77.1 82.1
3.9 4.2 5.3 5.5 5.1
1.5 1.4 2.0 1.5 1.9
20.0 20.6 10.7 15.7 10.8
0.1 0.6 0.3 0.2 0.1
a
Calculated by difference.
Table 2. Ni and Pb Concentrations and Corresponding Relative Enrichment (RE) Factors power station and sample description A B C D E
coal bottom fly ash coal bottom fly ash coal bottom fly ash coal bottom fly ash coal bottom fly ash
ash ash ash ash ash
total Ni mg/kg
total Pb mg/kg
16 200 270 23 180 280 15 21 36 5 14 14 6 12 38
3 24 74 6.5 15 100 14 6 57 12 9 23 13 22 46
RE, Ni
RE, Pb
0.43 1.33
0.68 0.91
0.53 0.83
0.16 1.05
0.38 0.64
0.11 1.09
0.91 0.91
0.24 0.62
0.42 1.32
0.35 0.74
such as Ni(CO)4 and Ni3S2 are considered hazardous and carcinogenic. From a public health point of view, the concentration of nickel associated with small particles that can be inhaled in lungs is of the greatest concern and such small particles may be released to the environment from coal fired power stations if not captured through existing particle capture devices such as electrostatic precipitators (ESPs) or fabric filters (FFs). Lead rarely occurs in its elemental state. In the earth’s crust it mainly exists in the Pb(II) oxidation state. All inorganic lead compounds are classified by the International Agency for Research on Cancer (IARC) as human carcinogens; however,
Figure 1. XANES spectra of nickel reference compounds.
Table 3. List of Reference Compounds Nickel reference compounds
K-edge energies (E0)
NiS NiO NiSO4 · 6H2O Nisacetate Niscarbonate NiFe2O4 Nishydroxide
8333.9 8342.6 8342.4 8342.5 8343.5 8343.7 8343.7 Lead
reference compounds
L3 edge energies (E0)
PbO PbCl2 PbCO3 PbSO4 PbS PbO2 Pb Pb3O4
13052.5 13054.0 13059.1 13057.5 13053.9 13038.3 13036.4 13056.1
lead carbonate is known to be the most toxic form of lead if inhaled or ingested.4 Once lead deposits on soil, it binds strongly with soil particles and remains in the top layer of soil increasing the risk of lead poisoning through the upper layer of soil. Lead causes deficiency in children’s cognitive development and intelligence, it is also linked with low birth weights, hearing problems, aggression, high blood pressure, and fertility problems in adult males. Lead poisoning is an ongoing problem in Australia.5 For example, in 2007 the deaths of thousands of birds in the remote coastal town of Esperance in far south Western Australia was caused by lead poisoning. Recent testing has shown one tenth of local children in Mount Isa in Queensland, the site of major mining and smelting operations, have high levels of lead in their blood.6 Although episodes related to lead pollution from coal combustion has not occurred, it is important to assess risk of lead exposure through coal utilization given that lead and compounds are listed as “priority pollutants” by NPI in Australia. In coal, nickel is intrinsically present in the range between 2 and 21 ppm and is believed to be associated with the pyrite and organic matter in bituminous coals.7 Small portion of nickel may be present in the clays and carbonates, or in a sulfide form. Nickel is hardly volatile and found distributed equally in bottom ash and fly ash. It has been previously reported that the (4) Australian Broadcasting Corporation (ABC), TV Program Transcript, http://www.abc.net.au/7.30/content/2007/s1878218.htm, accessed on 18th Jan2008. (5) Lisel O’Dwyer, The use of GIS in identifying risk of lead poisoning in Australia. 10th Colloquium of the Spatial Information Research Centre, University of Otago, New Zealand, 16-19 November, 1998. (6) Xstrata faces legal action over lead poisoning, Australian Broadcasting Corporation (ABC). http://www.abc.net.au/worldtoday/content/2008/ s2214631.htm, accessed on 5th May2008. (7) Nickel in Australian export thermal coal. CSIRO 2007. http:// www.csiro.au/resources/ps2rk.html, accessed on 5th Dec2007. (8) Swaine, D. J. Trace elements in coal; Butterworth: London; Boston, 1990; p 278.
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Figure 2. (a) Nickel XANES spectra for power station coal samples. (b) Nickel XANES spectra for power station fly ash samples. Table 4. Semi-Quantitative Analysis of Coal Samples for Nickel Speciationa power station A B C D E
a
Figure 3. Ni XANES spectra for bottom ash samples.
concentration range of lead for most coals would be 2-80 ppm while for Australian coals it would be 1.5-60 ppm.8 In a more recent study, the average concentration of lead in most coals is reported as 1- 22 ppm while in the case of Australian coals it has been reported in the range of 2-14 ppm.9 Lead in the bituminous coal is present predominantly in pyrite and other sulfide minerals. Minor presence of lead selenide has also been observed.9 In a study conducted on Chinese coals it was reported that majority of lead might be associated with pyrite, and a correlation was observed between the concentration of lead, ash, (9) Lead in Australian export thermal coal. CSIRO 2007 http:// www.csiro.au/resources/ps2r7.html, accessed on 5th Dec2007.
coal bottom fly ash coal bottom fly ash coal bottom fly ash coal bottom fly ash coal bottom fly ash
ash ash ash ash ash
NiS (%)
NiSO4 (%)
NiO (%)
R factor
50 35 50 55 ND 5 70 50 70 90 20 60 70 25 33
50 55 48 45 50 80 28 50 28 10 80 40 30 75 55
ND 10 2 ND 50 15 2 ND 2 ND ND ND ND ND 12
0.009 0.04 0.009 0.002 0.004 0.009 0.003 0.01 0.051 0.003 0.02 0.002 0.003 0.02 0.007
ND: not detected.
and sulfur content of the coal samples.10 During combustion, lead is highly volatile, and, upon cooling, it is expected to condense on fly ash particles. Speciation of nickel in coal and coal combustion products has been previously reported for coals and power station ash samples collected from North America using X-ray Absorption Fine Structure Spectrometry (XAFS).11-13 For Australian coals and coal combustion products, speciation of nickel has still not been performed. In the case of lead, there is still no data available in the literature for speciation in coal and coal combustion products, apart from some modeling attempts.14,15 In this work, speciation of both lead and nickel in coals and (10) Kunli, L; Jidong, L; Lianwu, C. EnViron. Geochem. Health 2005, 27, 31–37. (11) Goodarzi, F.; Huggins, F. E. J. EnViron. Monit. 2001, 3, 1–6. (12) Goodarzi, F.; Huggins, F. E. J. EnViron. Monit. 2004, 6 (10), 787– 791. (13) Goodarzi, F.; Huggins, F. E.; Sanei, H. Int. J. Coal Geol. 2008, 74 (1), 1–12. (14) Wang, J.; Tomita, A. Energy Fuels 2003, 17, 954–960. (15) Lundholm, K.; Nordin, A.; Backman, R. Fuel Process. Technol. 2007, 88 (11-12), 1061–1070.
XANES Study of Nickel and Lead Speciation in Coals
Figure 4. Example of linear combination analysis for fly ash sample from power station A for semiquantitative analysis of nickel.
Figure 5. XANES spectra of lead reference compounds.
coal combustion products collected from five Australian power stations is carried out using synchrotron based XANES methods. 2. Methodology Sampling of Coal and Ash Samples. Coal and ash sampling was arranged at five different power stations across Australia, two in New South Wales, two in Western Australia, and one from Queensland. All power stations use bituminous rank coals for electricity generation, and they are all equipped with either ESPs or FFs as particle capture devices. Flue gas desulfurization (FGD) is not installed in any of the studied power stations. For
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the purpose of this study, representative coal samples were collected from each power station. Bottom ash samples were collected from the hopper located at the bottom of the boiler furnace, while fly ash samples were collected from designated sampling locations at either ESPs or FFs. Elemental Analysis of Lead and Nickel. For Pb analysis, coal and ash samples were digested in an acid mixture (aqua regia, HF) in a microwave oven, and the resultant solution was analyzed by graphite furnace atomic absorption spectrometry (GFAAS). For Ni analysis, the ashed sample was fused with sodium carbonate/sodium tetraborate flux. The melt produced was dissolved in hydrochloric acid. The solution was then measured by Inductively Coupled Plasma with Optical Emission Spectroscopy (ICPOES). Speciation Analysis Using XANES. The synchrotron radiation facility at the Australian National Beamline Facility (ANBF) located in Tsukuba, Japan was used for the speciation measurements. Samples were mounted on sample holders and monochromatic (4-25 KeV) synchrotron X-rays were used for sample irradiation. Resulting fluorescence energy was detected using 36 element Ge SSD energy dispersive detectors. The energy scale of spectra for nickel and lead were calibrated with respect to the corresponding primary standards. In case of nickel, nickel metal and in case of lead, lead metal were used as primary standards. No monochromator drifts were observed during the experiments. In the case of nickel, K-edge spectra between 8150 and 8500 eV were obtained while in case of lead the L3 edge spectra for all samples was between 12830 and 13150 eV. Sample irradiation was performed at ambient conditions, and a double-crystal Si(111) sagittally focusing monochromator with detune capability for harmonic rejection was used. Beamline control and data acquisition was performed using “SPEC” software from Certified Scientific Software on a Linux based operating system. Raw data collected from the beamline were first processed with the software “Average” for merging data from multiple channels of detectors and scans into a single, averaged data set ready for XAFS background subtraction, splining, and Fourier transforming. Once the XANES spectra were compiled, a semiquantitative analysis was performed using the XAFS graphic utility software ATHENA.16 In this work, linear combination analysis (LCA) was used as a method for semiquantitative analysis. In LCA, it is assumed that XANES spectrum of unknown sample can be fitted as a linear combination of pure reference compounds. ATHENA allows users to select a combination of reference compounds to fit an unknown XANES spectrum. However, the prerequisite for a successful LCA is that data should be aligned and be appropriately normalized. While performing LCA, ATHENA allows users to select an option to keep the edge energy (E0) floating which automatically improves any inconsistencies in data alignment. Ideally E0 should not vary if alignment and normalization of raw XANES data are performed appropriately. Apart from floating E0, the software also allows selection of additional constraints. For example, the value of the fractional contribution of each reference compound can be restricted between 0 and 1 with the sum of the weight of all reference compounds forced to equal 1. In this work, XANES spectra of the samples and reference compounds were aligned with respect to their edge energy position. E0 was selected as floating, and the sum of the weights of the standard reference compounds was selected to equal 1. Statistically, R-factor value was applied to determine the equation fit. (16) Ravel, B.; Newville, M. Phys. Scr. 2005, T115, 1007–1010.
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R)
Shah et al.
- XANES ∑ (XANES ∑ XANES
2 Calculated )
Measured
2 Measured
The R-factor gives an indication of residual values after the linear combination fit is obtained. Once the linear combination analysis is performed and depending on the selection of the number of standard reference compounds, ATHENA software proposes possible combinations of results. Each combination is presented with corresponding R value, and generally the combination with the lowest R value is selected. 3. Results and Discussion Ultimate and proximate analysis of coal samples collected from five Australian power stations are presented in Table 1. Samples were bituminous rank and varied significantly in ash and air-dried moisture contents. The sulfur content of all feed coals was very low. Concentration of Ni and Pb in Coal and Ash. Total concentrations of lead and nickel for the coals and representative fly ash and bottom ash samples collected from the examined Australian power stations are displayed in Table 2. Overall, concentrations of Pb and Ni in bottom ash and fly ash were higher than in their corresponding feed coals. On the basis of the elemental analysis, relative enrichment factors (RE)17 were calculated for both lead and nickel in the bottom ash and fly ash, and are presented in Table 2. Relative enrichment factors calculated by the formula given below are useful in understanding volatility of trace element under combustion conditions. TE content in ash % Ash content in coal × TE content in coal 100 Trace elements in fly ash having RE greater than 1 show high volatility and enrichment of those elements on fly ash particles. RE factors which are less than 1 suggest that the elements present in a volatile phase undergo only partial condensation on the fly ash particles. On the basis of the RE RE )
factors a classification system was proposed for trace elements where they are divided into three broad classes.17 Class I elements are non volatile under combustion conditions. Class II elements are volatile under combustion conditions with condensation on fly ash particles while Class III elements are highly volatile and mostly escape through stack in their volatile form. In this study, the RE factors for Ni and Pb, as shown in Table 2, are found to vary between greater and less than 1. The variation in volatility behavior of these two elements suggests that for the examined power station conditions, both Ni and Pb can be classified as class II elements. Nickel and Lead Calibration Reference Materials. To quantitatively understand speciation, pure reference standards of lead and nickel were first analyzed using XANES. Selected reference standards along with K-edge energies (in case of nickel) and L3-edge energies (in case of lead) are presented in Table 3. All pure reference compounds selected were of analytical reagent grade, and they were diluted to 0.5% (wt) by mixing with analytical reagent grade alumina. XANES spectra of nickel reference standards are shown in Figure 1. In the case of nickel, mixtures of NiS/NiSO4 · 6H2O and NiS/NiO were prepared to a total Ni concentration of 0.5% (wt) diluted in alumina powder. NiS is less soluble and less toxic form of nickel while NiSO4 is more toxic and more bioavailable. Other nickel compounds, such as NiO, NiFe2O4, Ni-acetate, Ni-hydroxide, and Ni-carbonate are Ni(II) with Ni-O co-ordination environment are insoluble and less toxic forms of nickel. In the current work, the shape of NiS, NiSO4, and NiO XANES spectra was found to match with the XANES spectra obtained for the investigated coal and ash samples. For the purpose of semiquantitative analysis NiS, NiSO4, and NiO were considered as reference compounds. Speciation of Nickel in Coal and Ash. XANES spectra of nickel for all coal and fly ash samples are shown in panels a and b of Figure 2, respectively. XANES spectra of nickel in bottom ash samples are shown in Figure 3. It can be observed that XANES spectra for bottom ash samples have higher noise levels than the fly ash samples. It has been reported previously
Figure 6. (a) Lead XANES spectra for power station coal samples. (b) Lead XANES spectra for power station fly ash samples.
XANES Study of Nickel and Lead Speciation in Coals
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Table 5. Speciation of Nickel in Coal and Coal Combustion Products Found in Literature key findings on speciation of nickel reference
method
study location
coal
10
XAFS and leaching
Canada
11
XAFS
Canada
12
XAFS
Canada
mainly Ni(II) in oxygen co-ordination; presence of NiSO4 was observed in one coal sample. 80 or >90% as Ni(II) in oxygen co-ordination almost entirely as Ni(II) in oxygen co-ordination.
>90% as Ni(II) in oxygen co-ordination almost entirely as Ni(II) in oxygen co-ordination.
Table 6. Semi-Quantitative Analysis of Coal Samples for Lead Speciation power station
PbS
PbSO4
R factor
A B C D E
70 65 70 65 65
30 35 30 35 35
0.0005 0.0001 0.003 0.002 0.003
Table 7. Semi-Quantitative Analysis of Fly Ash Samples for Lead Speciation power station
PbCl2
PbSO4
PbO
R factor
A B C D E
50 50 40 30 70
30 50 30 60 30
20 ND 30 10 ND
0.0001 0.0002 0.0001 0.0002 0.0004
that nickel XANES spectrum can exhibit poor signal-to-noise ratio when iron is present in the sample matrix.12 As shown in Figure 1, the intensity of the absorption edge of NiFe2O4, Nihydroxide, Ni-carbonate, Ni-acetate, NiO, and NiSO4 · 6H2O is significantly higher than the absorption edge of NiS. XANES spectrum of NiS exhibits a broad shoulder immediately after the absorption edge at ∼8345 eV, which can also be seen in the Ni XANES spectra of coal, bottom ash, and fly ash shown in Figures 2 and 3, indicating the presence of NiS in variable proportions in all samples. Furthermore, the XANES spectra of NiO, NiFe2O4, and Ni hydroxide reference compounds show a prominent shoulder at ∼8365 eV, and the previous work has reported the possibility of the presence of Ni in NiO or NiFe2O4 forms in coal and ash samples.12 To confirm the likely nickel forms in the analyzed coal and ash samples, the linear combination analysis using combinations of the reference compounds NiS/NiSO4/NiO and NiS/NiSO4/NiFe2O4 were applied in ATHENA for Ni XANES spectra match fitting with the coal and ash samples spectra. NiS/NiSO4/NiO combinations provided the best fit result and were further applied for the semiquantitative analysis in the current work. The semiquantitative analysis of nickel for the coal, bottom ash and fly ash samples in this work was performed by applying a linear combination fit to the normalized XANES spectra. An example of the linear combination fit for one sample is presented in Figure 4 along with XANES spectra of pure reference compounds. Results from the semiquantitative analysis are shown in Table 4. As shown in Table 4, coal contains a greater proportion of nickel present as NiS compared to NiSO4. For all cases 50%-90% of nickel in coal is present as the less toxic and insoluble NiS. Little presence of less toxic NiO at 2% was detected for a coal sample from power station C. R-factors reported here are very low confirming the quality of the applied linear combination fit. R factor values obtained for nickel species in the bottom ash were greater than the R values for nickel species in coal and fly ash because of a weaker signal-to-noise ratio obtained with XANES. Bottom ash samples exhibit varying concentrations
Figure 7. Example of linear combination analysis for fly ash of power station A for semiquantitative analysis of lead.
of NiSO4. Bottom ash from power station B exhibits the highest concentration of NiSO4 at 80% while bottom ash from power station C shows the lowest at 50%. NiO was detected in the bottom ash samples of power stations A and B at 10 and 50%, respectively. The semiquantitative analysis performed on fly ash samples revealed that the chemical forms of nickel in the fly ash are considerably different to those detected in the corresponding coals. This difference was observed in all power station samples except for power station C. In the case of power stations A, B, D, and E, the content of NiSO4 was found to be higher in fly ash comparing to the corresponding feed coal, and there is a depletion of concentration of NiS. NiO was detected in fly ash samples from power stations A, B, C, and E in the range of up to 15%, while the more toxic NiSO4 varied in the range of 28-80%. R-factors obtained here also demonstrate the good quality of the linear combination fit. Previous work carried out on nickel speciation in coal and ash samples from North American power stations is summarized in Table 5. In these studies, it was reported that nickel in coal, bottom ash, and fly ash is present mainly as Ni(II) in oxygen co-ordination environment, which is a non- toxic form of nickel. Ni(II) present in a sulfide co-ordination environment was also low in samples studied by Goodarzi and Huggins11 and Goodarzi et al.12 In this work, the more bioavailable and toxic NiSO4 was found to be higher in the analyzed ash samples from Australian power stations comparing to the proportion of the toxic and carcinogenic nickel forms reported as Ni(II) in Ni-S coordina-
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tion environment for North American conditions (Table 5). It should be noted that there is a possibility for additional complex nickel compounds to be present in the coals or fly ash samples in smaller concentrations, such as nickel in aluminosilicate glass, as well as Ni(II) in Ni-S environment such as Ni3S2 and NiS2, which have not been considered in this study. Under high temperature combustion conditions, nickel is likely to volatilize and react with other constituents of flue gas during combustion and in the post-combustion zone resulting in subsequent changes in speciation of nickel in fly ash. The results obtained in this study indicate that Ni present as NiS in feed coals is likely to have been converted partially into NiSO4 and NiO under combustion conditions. According to Wang et al.18 the following oxidation reactions of NiS or Ni3S2 can occur in the temperature range of 660-700 °C: NiS + 3/2 O2 f NiO + SO2 NiS + O2 f Ni3S2+SO2 Ni3S2+7/2 O2 f 3NiO + 2SO2 NiS + 4H2O a NiSO4+H2 NiS can oxidize to NiO or form an intermediate product Ni3S2 which further reacts with oxygen to form NiO. Nickel oxide in the presence of oxides of sulfur can react further forming nickel sulfate.19 In a previous study using a thermodynamic equilibrium approach it was predicted that volatility behavior of nickel is similar in either the presence or the absence of sulfur.20 Moreover, at high temperature Ni-sulfates are predicted to be unstable while at temperatures lower than 625 °C solid or condensed phase Ni sulfates are predicted to replace solid phase Ni-oxides.21 Speciation of Lead in Coal and Ash. XANES spectra of lead reference compounds are shown in Figure 5. On the basis of the available information for the modes of occurrences of lead in coal,9 PbS was selected as one of the candidates for lead reference standards. PbSO4 was also selected as the most likely species present in coal given that PbSO4 is a product of oxidation of the most dominant lead species in coal, PbS. The previous work carried out using thermodynamic equilibrium approach suggests that PbO and PbCl2 are the most stable species in the fly ash.14,15 It is also suggested that, in the flue gas post-combustion cooling zone, PbSO4 may additionally form on the surface layer of the fly ash particles by chemical reaction between PbO and H2SO4.17 Thus, with the support of thermodynamic equilibrium studies and after studying the shape of the XANES spectra, mixtures of PbO, PbSO4, and PbCl2 were prepared to the total Pb concentrations of 0.5% (wt) diluted in alumina. XANES spectra of lead for all coal and fly ash samples are shown in panels a and b of Figure 6, respectively. XANES spectra for bottom ash samples exhibited poor signal-to-noise ratio. The semiquantitative analysis for lead was performed on the samples by applying the linear combination fit, with one example shown in Figure 7. Results of the semiquantitative analysis are displayed in Tables 6 and 7. As shown in Table 6, the semiquantitative analysis of coal was performed using PbS and PbSO4 as potential Pb species. (17) Meij, R. Fuel Process. Technol. 1994, 39, 199–217. (18) Swaine, D. J.; Goodarzi, F. EnVironmental aspects of trace elements in coal; Kluwer Academic Publishers: Dordrecht, 1995; p 312. (19) Wang, H.; Pring, A.; Ngothai, Y.; Neill, B. O. Am. Mineral. 2006, 91, 537–543. (20) Mehandru, S. P.; Anderson, A. B. J. Electrochem. Soc. 2003, 133 (4), 828–832.
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For all power station coals 65-70% of Pb is present as PbS while the remaining was detected as PbSO4. It is known that in the earth’s crust the naturally occurring mineral anglesite PbSO4 occurs as an oxidation product of the primary lead sulfide ore galena, that is, PbS. The semiquantitative analysis of lead in fly ash was performed accounting for PbO, PbCl2, and PbSO4 as potential Pb(II) species present in the fly ash. Results revealed that all fly ash samples contained PbCl2 and PbSO4 as Pb(II) species. In case of power station fly ash B and E, PbO was not detected in the samples. For these cases, PbO generated might have been converted to PbSO4 and condensed on the surface of the fly ash particle.17 It should be highlighted here that one of the limitations of the XANES analysis technique is selection of a limited reference standards to perform LCA; hence, the results reported in this work show only the relative concentration of the most likely predominant forms of lead in coal and fly ash. Other forms of lead species, such as lead phosphates and lead in aluminosilicate which could also be present in smaller concentrations, have not been considered in the current work. According to Wang and Tomita,13 under combustion and pyrolysis conditions, lead in coal is converted to gaseous species with the following possible chemical reactions: PbO + C f Pb(g) + CO PbS + CaO + C f Pb(g) + CaS + CO PbS(s) f PbS(g) PbS + 0.5C f Pb(g) + 0.5CS2 PbS f Pb(g) + 0.5S2 PbO(s) f PbO(g) In the flue gas cooling post-combustion atmosphere and in the presence of Cl and SO2, lead compounds may further react in the following chemical reactions:22 Cl + H2O a HCl + OH Pb + Cl a PbCl PbCl + Cl a PbCl2 PbCl + OH a PbO + HCl PbO + H2SO4 a PbSO4+H2O The presence of PbCl2, PbSO4, and PbO in the fly ash samples studied here as shown in Table 7 indicates that the above reaction mechanism is likely to have taken place in the studied power stations. 4. Conclusions Speciation of lead and nickel in coal and coal combustion products collected in Australian coal fired power stations was performed in this study. Five different coal fired power stations were selected for sampling, and total Pb and Ni contents were measured in coal, bottom ash, and fly ash samples using GFAAS and ICPOES, respectively. Both lead and nickel were found to be partially volatile at combustion temperatures with a varying degree of condensation on fly ash particles. XANES was used for speciation analysis. Semiquantitative speciation was calculated using linear combination fit of XANES spectra of reference (21) Linak, W. P.; Wendt, J. O. L. Combust. Sci. Technol. 1998, 134, 291–314. (22) Durlak, S. K.; Biswas, P.; Shi, J. J. Hazard. Mater. 1997, 56, 1– 20.
XANES Study of Nickel and Lead Speciation in Coals
compounds and their mixtures. One of the limitations of this study is in the selection of a limited range of standard reference compounds. The work presented here is the first experimental measurement on speciation of nickel in coal and coal combustion products under Australian power station combustion conditions. Speciation of nickel in coal revealed its presence mainly in the mixture of NiS and NiSO4. A little presence of NiO was detected in one of the coal samples at 2%. In all coal samples NiS form was present in the range of 50-90%. The more toxic NiSO4 was detected in all fly ash samples at the range of 28-80% of the total nickel present. NiO in the fly ash samples ranged between 0 and 15%. The proportion of the more toxic NiSO4 in all analyzed bottom ash samples varied between 50 to 80%. One bottom ash sample from power station B exhibited significant presence of NiO at 50% of the total nickel content. The proportion of the more toxic nickel compounds detected for the ash samples under Australian conditions is greater than what was reported in the literature for the North American power station conditions. The mechanism of nickel transformation under combustion conditions is likely to be the oxidation of nickel sulfide or subsulfide compounds followed by reaction with oxides of sulfur in the post-combustion environment.
Energy & Fuels, Vol. 23, 2009 1525
The present work is the first attempt to perform experimental measurements for lead speciation in coal and coal combustion products. Speciation of lead in coal revealed that lead was present as PbS and PbSO4. Approximately 65-70% of lead in coal was present as PbS. It was observed that fly ash from all power stations contained PbCl2 and PbSO4 while PbO was not detected in two of the fly ash samples. PbCl2 varied between 30-70%, while PbSO4 varied between 30-60% in all fly ash samples. The mechanism for lead transformation under combustion conditions is likely to be the oxidation of PbS followed by the formation of PbCl2, PbO, and PbSO4 in post-combustion environment in the presence of oxygen, chlorine and oxides of sulfur. Acknowledgment. The authors acknowledge the support of CRC for Coal in Sustainable Development (CCSD) which is funded in part by the Cooperative Research Centres Program of the Commonwealth Government of Australia. XANES analysis was performed at the Australian National Beamline Facility with support from the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. EF800824D
