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Speciation of Arsenic and Selenium in Coal Combustion Products† Pushan Shah,*,‡ Vladimir Strezov,‡ Chris Stevanov,§ and Peter F. Nelson‡ CRC for Coal in Sustainable DeVelopment, Graduate School of the EnVironment, Macquarie UniVersity, Sydney, NSW 2109 Australia, and CRC for Coal in Sustainable DeVelopment, Department of Chemical Engineering, UniVersity of Newcastle, Callaghan, NSW 2308 Australia ReceiVed August 20, 2006. ReVised Manuscript ReceiVed October 6, 2006
Coal combustion is one of the main anthropogenic sources of toxic trace element emissions to the environment. Various species and oxidation states of the trace elements released from power stations may determine their ultimate environmental fate and health impacts. This study focuses on speciation of arsenic and selenium present in the coal combustion products. Speciation analysis in representative coal, bottom ash, and fly ash obtained from four different Australian power stations was carried out in this work. Laboratory ash and char were also produced by carrying out combustion and pyrolysis experiments in a laboratory based drop tube furnace. The synchrotron based nondestructive speciation analysis method X-ray absorption fine structure spectrometry (XAFS) was applied for arsenic and selenium speciation analysis of the selected coal, ash, and char samples. The semiquantitative analysis of arsenic revealed variations in arsenic species in the coal samples indicating the presence of As/pyrite, arsenite (As3+), and arsenate (As5+) with the latter as a dominant form. Arsenic in power station fly ash samples was found to be mainly in an arsenate form with little presence of arsenite (As3+). Selenium speciation in coal samples indicated organic/reduced or elemental forms as dominant selenium species along with presence of selenite (Se4+)/selenate (Se6+). Selenium in fly ash was mainly found to be selenite with a minor presence of selenate. Char produced by pyrolysis indicated different speciation behavior of arsenic and selenium compared to coal and ash samples, which might be due to their further reactions with other volatilized species produced during pyrolysis and/or retained mineral matter.
Introduction Coal fired power stations are one of the largest anthropogenic contributors of trace elements to the environment. There is a growing environmental concern over the emissions, deposition, and management of the toxic trace elements released during combustion of coal. Trace elements in coal are present in very small concentrations of several parts per million, which can accumulate over time, if not managed in a sustainable way. Their occurrence and concentration vary between coals, primarily because of the geological variety of individual coal basins. Understanding their mobility, accumulation, and speciation is of vast importance as the specific compounds of the trace metals may have significant environmental impacts on human health and ecosystems.1 The oxidation state of the trace elements can determine its level of toxicity and carcinogenic potency.2 Therefore, speciation of the trace metals in coals and their combustion products is important for conducting comprehensive risk assessment of the trace metal emissions from coal based power plants and to further project their sustainable management options. Arsenic and selenium are two of the trace metals commonly enriched in the fly ash and depleted in the bottom ash.3 Under † Presented at the 2006 Sino-Australia Symposium on Advanced Coal Utilization Technology, July 12-14, 2006, Wuhan, China. * Corresponding author. E-mail:
[email protected]. ‡ Macquarie University. § University of Newcastle. (1) Swaine, D. J. Trace elements in coal; Butterworth: London, 1990. (2) Ebdon, L.; Royal Society of Chemistry (Great Britain). Trace element speciation for enVironment, food, and health; Royal Society of Chemistry: Cambridge, UK, 2002. (3) Clarke, L. Fuel 1993, 72 (6), 731.
combustion conditions, selenium is more volatile than arsenic and may be released out of the stack with the flue gas.4 Arsenic is found mainly in three valency states -3, +3, and +5. As3+ is about 50 times more toxic than As5+ and several times more toxic than the organic forms monomethilarsonate (MMA) and dimethylarsinate (DMA).5 The toxicity of arsenic is mainly due to its reactions with sulfydryl groups of proteins and enzymes and subsequent inhibition of enzyme functions. The gastrointestinal tract, circulatory system, liver, kidney, and skin are the organs most affected by arsenic. Apart from the environmental and health impacts, it has been observed that arsenic present in coal, particularly volatilized As2O3 generated during coal combustion, may deactivate the catalysts used in selective catalytic reduction (SCR) units. This may have a large adverse effect on the success of NOx control with SCR and lead to increased costs.6 Duker et al.7 have identified that under oxidizing conditions As5+ compounds are a more stable arsenic form while in reducing conditions As3+ compounds predominate. Selenium is found in the environment in multiple states such as -2, 0, +4, and +6. Selenium is one of the essential trace elements which acts as an antioxidant and protects cell membranes. Overexposure to Se, on the other hand, is known to be harmful, and both Se deficiency and toxicity have occurred in Australia. Selenium toxicity depends on its oxidation form (4) Meij, R. Fuel 1993, 72 (5), 718. (5) Hodgson, E.; Mailman, R. B.; Chambers, J. E. Dictionary of Toxicology; Macmillan: London, 1988. (6) Staudt, J. E.; Engelmeyer, T.; Weston, W. H.; Sigling, R. The impact of arsenic on coal fired power plants equipped with SCR. Presented at the ICAC Forum, Houston, TX, 2002. (7) Duker, A. A.; Carranza, E. J. M.; Hale, M. EnViron. Int. 2005, 31 (5), 631.
10.1021/ef0604083 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/18/2006
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Table 1. Proximate and Ultimate Analysis of Coals (Air Dried Basis) proximate analysis (%)
ultimate analysis (%)
coal
air dried moisture
ash
volatile matter
fixed carbon
C
H
N
O
S
A B C D CRC 240 CRC 283
13.3 11.1 4.0 2.8 2.1 2.2
5.4 6.8 32.5 20.9 13.2 14.8
33.4 33.0 26.6 28.2 29.9 29.7
47.9 49.1 36.9 48.1 54.8 53.3
60.6 60.1 48.93 62.62 84 83.7
3.20 3.45 3.48 3.92 5.74 5.45
1.19 1.17 0.97 1.47 2.12 1.81
34.9 34.68 46.5 31.89 7.9 8.98
0.11 0.6 0.12 0.1 0.24 0.06
and quantity; however, the gap between selenium sufficiency and toxicity is very narrow.8 There have been several registered outbreaks of selenium toxicity in domestic animals due to consumption of forage containing selenium with concentrations greater than 1 mg/kg.9 Human fatalities directly attributed to selenium poisoning have not been registered; however, the potential hazards exist and are always kept in perspective. The mode of occurrence of selenium in coal appears to be related to the organic material (70-80%), while a portion of Se between 5 and 10% is associate with the pyrite or other sulfide minerals, such as galena and clausthalite.10 The very low concentration of trace elements present in the coal and coal combustion products makes speciation of trace elements a great challenge. Several authors11-16 have successfully applied X-ray absorption fine structure spectrometry (XAFS) for speciation of selected trace elements in coal fly ash and incinerator ash materials. X-ray absorption fine structure spectrometry (XAFS) is a nondestructive and direct technique for trace element speciation analysis. A synchrotron radiation source is used for XAFS spectrometry. This method provides detailed information about local binding and structure for the elements of interest in a given sample. XAFS spectra can be divided in two regions: X-ray absorption near edge structure spectrometry (XANES) and extended X-ray absorption fine structure spectrometry (EXAFS). Particularly, an X-ray absorption near edge structure spectrum of the XAFS gives information about coordination chemistry, molecular orbits, band structure, and multiple scattering. Although XANES spectra are difficult to interpret, they give a “fingerprint” for particular chemical species. This method was successfully applied for semiquantitative analysis of arsenic species in coal and ash samples.11 In the case of selenium speciation, XANES analysis was applied for shale, soil, and sediment samples,17,18 while, in case of coal and coal combustion products, XANES technique has not been well explored. In this work, XANES was used to understand speciation of arsenic and selenium in coal combustion products generated (8) Greenwood, N. N.; Earnshaw, A. Chemistry of the elements; Butterworth-Heinemann: Boston, 1997. (9) Finkelman, R. B. Fuel Process. Technol. 1994, 39 (1-3), 21. (10) Wu, L. L. Selenium Accumulation and uptake by crop and grassland plant species. In EnVironmental Chemistry of Selenium; Frankenberg, W. T., Engberg, R. A., Eds.; Marcel Dekker: New York, 1998; pp 657-686. (11) Huffman, G. P.; Huggins, F. E.; Shah, N.; Zhao, J. Fuel Process. Technol. 1994, 39 (1-3), 47. (12) Huggins, F. E.; Huffman, G. P.; Kolker, A. Energy Fuels 2002, 16 (5), 1167. (13) Huggins, F. E.; Huffman, G. P.; Linak, W. P.; Miller, C. A. EnViron. Sci. Technol. 2004, 38 (6), 1836. (14) Huggins, F. E.; Najih, M.; Huffman, G. P. Fuel 1999, 78 (2), 233. (15) Huggins, F. E.; Shah, N.; Huffman, G. P.; Kolker, A.; Crowley, S.; Palmer, C. A.; Finkelman, R. B. Fuel Process. Technol. 2000, 63 (2-3), 79. (16) Huggins, F. E.; Shah, N.; Huffman, G. P.; David, R. J. Fuel Process. Technol. 2000, 65-66, 203. (17) Ryser, A. L.; Strawn, D. G.; Marcus, M. A.; Johnson-Maynard, J. L.; Gunter, M.; Moller, G. Geochem. Trans. 2005, 6 (1), 1. (18) Zawislanski, P. T.; Benson, S. M.; TerBerg, R.; Borglin, S. E. EnViron. Sci. Technol. 2003, 37 (11), 2415.
from four different Australian power stations. This work investigated the importance of arsenic and selenium due to the unique character of the Australian black coals and under operating conditions pertinent to Australian power stations. There is limited understanding of the effect of heating conditions on formation of the individual arsenic and selenium species. In the current work, the effect of combustion and pyrolyzing conditions on arsenic and selenium speciation is also studied using XANES analytical technique. Experimental Details Four Australian power stations were selected for this study. One power station was located in New South Wales, one in Queensland, and two were from Western Australia. The ultimate and proximate analysis of the representative coals used in each of the selected power stations, namely power stations A, B, C, or D, as well as two coals used for laboratory analysis are shown in Table 1. Coals were first pulverized to -50 µm before proceeding with the analysis. Bottom ash from each power station was collected manually from the hopper at the bottom of the boiler, and a representative sample was obtained. Fly ash collection activity was outsourced, and representative fly ash samples were obtained from each power station. Sulfate sulfur in coal samples was determined according to the Australian standard, AS 1038.11. Approximately 1 g of coal was digested with 50 mL of 50% HCl. The sulfate passed into solution and the pyrites, which are insoluble in dilute HCl, were retained in the residue. The solution was filtered and the sulfur content of the filtrate was determined by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). The residue from the sulfate extraction was digested with HNO3 to decompose the pyritic sulfur. The solution was filtered and the filtrate was measured for the pyritic iron concentration by ICP-AES spectrometry. The pyritic sulfur content was then calculated from this result. The elemental concentrations of arsenic and selenium in the coal and ash samples were determined with ICP-AES according to the Australian standard, AS 1038.10.2. Coal and ash samples weighing 0.25 g were mixed with 1 g of Eschka fusion mixture and heated in a muffle furnace at 800 °C for 2 h. The mixture was then extracted with 50% HCl and the analytes were determined by ICP-AES after generating their volatile hydrides. Drop Tube Furnace Experiments. Experiments were conducted on two coals obtained from the CCSD coal bank, referred to as CRC240 and CRC283. Samples were first crushed to the size range of 45-90 µm, which is consistent to the size range used in pulverized fuel (PF) boilers. The removal of the finer and coarser coal particles eliminates feeder problems and assists in maintaining consistent feed rates during the experiments. The coal samples were air-dried for 24 h prior to utilization. Pyrolysis and combustion experiments were conducted in a drop tube furnace (DTF) located at the University of Newcastle chemical engineering laboratories. The experimental setup used for the experiments is shown in Figure 1. The Tetlow built DTF
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Shah et al. Table 3. Total Arsenic and Selenium Analysis of Power Station Coal and Ash Samples (Dry Basis) arsenic (As) (mg/kg)
selenium (Se) (mg/kg)
power station
coal
bottom ash
fly ash
coal
bottom ash
fly ash
A B C D
0.4 1.9 1.9 1.6
2.6 0.4 0.5 11
8.4 10 19 7.7 (P)a 11(S)b
0.7 0.9 0.2 0.7
0.2 ND ND 0.4
4.4 3.4 1.0 1.3 (P) 2.1 (S)
a
(P) ) primary fly ash. b (S) ) secondary fly ash.
Table 4. Total Arsenic and Selenium in Laboratory Coal, Ash, and Char Samples (Dry Basis) arsenic (As) (mg/kg)
selenium (Se) (mg/kg)
type
coal
ash
char
coal
ash
char
CRC 240 CRC 283
0.9 0.4
1.6 0.6
1.2 0.5
1.0 0.5
0.8 0.2
1.1 0.3
Table 5. Relative Enrichment Factors arsenic bottom ash
fly ash
bottom ash
A B C D
0.351 0.014 0.086
1.134 0.35 3.25 1.00 (P)a, 1.43 (S)b
0.015
a
Figure 1. Schematic of experimental setup. Table 2. Summary of Experimental Conditions
coal feed rate temperature oxygen concentration primary gas secondary gas quench gas
(g/h) (°C) (%) (L/min) (L/min) (L/min)
pyrolysis
combustion
3-5 1400 0.5-1 1.2 4 8.8
3-5 1400 21 1.2 4 8.8
has an internal diameter of 86 mm, with a length of 1080 mm (measured between probe tips). Pyrolysis and combustion experiments were conducted with a furnace wall temperature of 1400 °C and residence times were estimated to be approximately 6 s. Combustion experiments were conducted with air as the carrier gas, and pyrolysis experiments were conducted in nitrogen with 0.5-1% oxygen, depending on the volatile content and the feed rate of the coal. The minute amount of oxygen was added to the pyrolysis experiments to combust the volatiles released during devolatilisation and to prevent soot formation. The oxygen concentration in the pyrolysis experiments was controlled by two mass flow controllers. A Brooks 5850E series was used to control the nitrogen flow rate, and a Tylan model FC-280 was used for air flow control. Medical grade air was used as the carrier gas in the combustion experiments and as the oxygen source in the pyrolysis experiments to ensure that char and ash samples were not contaminated with trace elements from the local environment. The feed rate to the DTF was 3-5 g/h, pneumatically supplied via a syringe feeder into the furnace. The air source was divided into primary and secondary supplies. The primary air supply was connected to the syringe feeder, and the secondary air supply was dispersed through the feeder probe’s heat shield. Gas flow rates were kept constant throughout the experiments as shown in Table 2. The char/ash was rapidly cooled in the collection probe and separated from the gas via a cyclone. A glass fiber filter situated after the cyclone captured the finer particles/aerosols. Char and ash samples were collected every 30 min and stored at a temperature below 0 °C to prevent further reactions.
selenium
power station
0.119
fly ash 0.33 0.256 1.625 0.388 (P), 0.63 (S)
(P) ) primary fly ash. b (S) ) secondary fly ash.
Speciation Analysis Using XAFS. The synchrotron radiation facility at the Australian National Beamline Facility (ANBF), located at BL20, Photon Factory, Tsukuba, Japan, was used for XAFS analysis. The beamline BL20 delivers monochromatic (4-25 KeV) or “white beam” synchrotron X-rays to the experimental station in a single hutch. The experiment used a water cooled channel cut Si(111) monochromator. The intensity of the incident beam was monitored with an ion chamber, and the resulting fluorescence energy which was detected using 36 element Ge energy dispersive detectors located at 90° to the incident X-ray beam. The energy scale of the spectra for each element was calibrated with respect to the primary standard. For arsenic speciation analysis in the samples, As2O3 was used as the primary standard, while for selenium speciation analysis it was the elemental selenium powder (-300 mesh). “SPEC” software from Certified Scientific Software with the operating system Red Hat Linux v6.2 was used for beamline control and data acquisition. The K-edge spectra were obtained for arsenic and selenium in all samples. Results and Discussion Arsenic and selenium concentrations in power stations coal, bottom ash, and fly ash as well as laboratory generated char and ash samples are given in Tables 3 and 4. The relative enrichment (RE) factor was calculated for the power station bottom ash and fly ash and is shown in Table 5. A relative enrichment factor, defined elsewhere,4 was calculated by the following formula:
RE ) (element concentration in ash)/ (element concentration in coal) × (% ash content in coal)/100 RE factors are used to classify trace elements in terms of their volatility observed through elemental concentration in bottom ash and fly ash. As per the classification of trace elements based on RE factor,4 the factors calculated are for power station bottom
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Table 6. K-Edge Energies of Standard Reference Compounds reference compounds
K-edge energies (eV) Arsenic
arsenic trioxide arsenopyrite arsenic sulfide disodium methylarsenate sodium arsenite sodium arsenate
11867.69 11864.26 11865.65 11870.11 11868.06 11871.12 Selenium
elemental selenium sodium selenite sodium selenate
12655.79 12661.24 12663.05
ash and fly ash indicate the volatile nature of arsenic and selenium under combustion conditions with selenium being more volatile than arsenic. A higher value of RE factors in fly ash indicates higher concentration of condensing volatile arsenic or selenium on fly ash particles and depletion of the concentration in the bottom ash. Similar to the literature findings,19 both arsenic and selenium were found in this work to be enriched in fly ash and depleted in bottom ash. Selected reference standard compounds for arsenic and selenium were used to obtain K-edge energies of their various oxidation states (Table 6). All reference compounds were diluted to 1% (wt) by mixing with alumina before XANES analysis. XANES spectra of arsenic and selenium reference compounds are shown in Figures 2 and 3. K-edge energies of various arsenic and selenium reference compounds are listed in Table 6. Least-Squares Fitting and Linear Combination Analysis. The initial sharp peaks observed in XANES spectra were due to the transition of photoelectrons from the 1s level to the 4p level, and these transition energies increase with the increasing oxidation state of the compounds.11 The step heights in all spectra were normalized to unity, and background removal was performed by the XAFS graphical utility software ATHENA.20 As discussed elsewhere,11 XANES spectra were fit to a series of peaks representing s f p level transition, scattering resonance, and step function which represents photoelectron transition to continuum. ATHENA allows least-squares fitting of XANES spectra with a combination of mathematical functions such as the arctangent step function and Gaussian and Lorentzian functions. To quantify the peak area under each oxidation state for arsenic, several standard compound mixtures were prepared by dilution to 1 wt % of the primary compound in alumina. Figure 4 gives an example of fitting arsenic XANES spectrum of fly ash from power station A with arctangent and Gaussian functions representing peaks contributed by arsenite and arsenate species. ATHENA also allows fitting linear combinations of standard spectra to an unknown spectrum, with an example shown in Figure 5.20 Successful quantification with this method is possible if proper reference standard spectra are obtained.21 Speciation of Arsenic in Coals. Arsenic in all analyzed coals, as shown in Figure 6, was found to be associated with pyrite (As/pyrite), arsenite (As3+), and arsenate (As5+). The peak location for different forms of arsenic was identified using the first derivative of their corresponding XANES spectra and the K-edge energies of standard reference compounds listed in Table 6. Semiquantitative speciation of the arsenic species was performed using two methods, linear combination and leastsquares fitting analysis, which were applied to verify the (19) Swaine, D. J.; Goodarzi, F. EnVironmental aspects of trace elements in coal; E Kluwer Academic Publishers: Boston, 1995. (20) Ravel, B.; Newville, M.; J. Synchrotron Radiat. 2005, 12, 537. (21) Ravel, B. Private communication.
Figure 2. XANES spectra of arsenic standard compounds.
Figure 3. XANES spectra of selenium standard compounds.
accuracy of the semiquantitative analysis. Both methods gave the same findings which are displayed in Table 7. Results show that coal utilized in power station D had higher arsenic associated with pyrite, while, for other coals such as A and C, it was difficult to quantify the presence of arsenic
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Figure 6. Arsenic XANES spectra of power station coal and ash samples. Figure 4. Example of least-squares fitting of fly ash arsenic XANES spectra using ATHENA.
Table 7. Semiquantitative Determination of Arsenic Species in Coal Samples coal A B C D CRC 240 CRC 283
As/pyrite (%) 12 45 4.2
As3+ (%)
As5+ (%)
90 26.4 30 44 28.8 45
10 61.6 70 11 67 55
Table 8. Forms of Sulfur Analysis for Various Coals (Dry Basis) forms of sulfur analysis
Figure 5. Example of linear combination of fly ash arsenic XANES spectra using ATHENA.
associated with pyrites because of low As/pyrite content. Coal utilized in power station A was found to have the highest arsenite content compared to the other coals. Different variations in arsenite and arsenate concentrations can be seen in the rest of the coals with the arsenate compounds being the more dominant form of arsenic. Previous work discussed that, in exposure to air, the pyrite associated arsenic may be oxidized and converted to other arsenic forms,11 which may be one of the reasons for large variations in the percentage of As/pyrite content in the coals analyzed in the current work.
coal
sulfate sulfur (%)
pyritic sulfur (%)
A B C D CRC 240 CRC 283
0.02 0.11 0.02 ND 0.19 0.02
0.09 0.49 0.10 0.10 0.05 0.04
The forms of sulfur analysis are given in Table 8. It was observed that coals have relatively low pyritic sulfur, and coal utilized in power station B was found to have the highest pyritic sulfur content of 0.49%. In the case of the sulfate form of sulfur, CRC 240 coal had the highest concentration of 0.19%, while sulfate sulfur concentration for coal utilized in power station D was below the detection limit. Arsenic may exhibit different modes of occurrence in coal; one of which is the pyrite by substituting (solid solution) for sulfur in the pyrite structure.11 However, according to the results presented in Tables 7 and 8, it was observed that As/pyrite content in coal had no direct correlation with pyritic sulfur content of coal; hence, there was no direct influence of pyritic sulfur content on As/pyrite concentration in the analyzed coal samples. Speciation of Selenium in Coals. In the case of selenium, as discussed elsewhere,17 XANES spectra are not always consistent with formal oxidation states due to variation of actual
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Figure 7. Example of qualitative speciation analysis of selenium using the first derivative of the coal (power station C) Se XANES spectra.
electronic density corresponding to varying degrees of covalent bonding. Hence, many selenium reference compounds, particularly organic, elemental (Se0), and reduced selenium (Se2-) compounds, are found to give K-edge positions close to each other despite variation in oxidation state. This makes identification of the oxidation state difficult if based solely on identification of the peaks. Instead, the entire XANES spectrum may be useful to differentiate between these selenium species by comparing the entire spectrum. Selenite (Se4+) and selenate (Se6+) are found to be giving XANES spectra consistent with the oxidation state. In the current work, reference compounds representing selenite and selenate were used, while those representing organic or reduced selenium were not applied. Unlike arsenic speciation, in the case of selenium speciation, the two methods for semiquantitative analysis, the linear combination and least-squares fitting analysis, showed inconsistency in the semiquantitative assessment for the individual species. For this reason, selenium speciation in the current work was determined qualitatively by taking the first derivative of each unknown XANES spectra which facilitated the visual examination of the presence and location of each oxidation state of selenium. One example for qualitative analysis of coal utilized in power station C is shown in Figure 7. The figure gave a clear indication of the points associated with the organic/ elemental or reduced form of selenium and selenate. Overall, in all coal samples, it was observed that selenium is present in all three forms: elemental/organic or reduced, selenite (Se4+), and selenate (Se6+). Variation in speciation can be observed, but in general, the organic/elemental or reduced form of selenium was found to be the dominant form in all of the analyzed coal samples, as shown in Figure 8. Arsenic and Selenium in Power Station Ash Samples. The XANES spectra of the power stations’ coal and ash samples are summarized in Figures 6 and 8. For bottom ash samples, the spectra revealed lower elemental concentrations of the arsenic and selenium compounds which were below the instrumental detection limits. In the case of the fly ash samples, the arsenic species were clearly detected, and the semiquantitative speciation analysis of all power station fly ash is given
Figure 8. Selenium XANES spectra of power station coal and ash samples. Table 9. Semiquantitative Determination of Arsenic Species in Fly Ash Samples power station
As 3+ (%)
As 5+ (%)
A B C Dsprimary fly ash Dssecondary fly ash
90 90 85
in Table 9. It can be observed that majority of arsenic in the fly ash was in the less harmful arsenate (As5+) form (Figure 6). There was a variation in the arsenic species present in the power station fly ash samples which was due to the utilization of different coals and operating conditions of the specific power stations. Selenium speciation in fly ash, determined through the first derivative of the XANES spectra, identified primarily selenite forms of selenium. However, a small presence of selenate may also be possible. It was found that under combustion conditions the majority of selenium in the organic/elemental or reduced form was either converted to higher oxidation states such as selenite/selenate or partly escaped from the stack. Laboratory Ash and Char Samples. Arsenic and selenium XANES spectra of coals and laboratory generated ash and char samples are shown in Figures 9 and 10. Ash samples produced under the prescribed laboratory conditions contained similar concentrations of arsenic and selenium as the power station bottom ash samples. The arsenic XANES spectra for both laboratory generated ash samples could not be detected. Selenium in laboratory ash was found mainly in the form of selenite.
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selenium exhibited different volatile natures during pyrolysis compared to combustion. Although both arsenic and selenium compounds could be volatilized at this operating temperature, further reactions with other volatilized species or mineral matter might have occurred forming more stable compounds in the final char samples. Arsenic XANES spectra of char generated from CRC 283 coal could not be detected, while the char obtained from CRC 240 coal indicated the presence of all three species of arsenic, namely, As/pyrite, arsenite, and arsenate. As discussed elsewhere, volatile metallic arsenic released due to the decomposition of arsenic compounds during pyrolysis might have interacted with mineral components of coal forming thermally stable arsenic compounds.22 Selenium XANES spectra showed the presence of organic, reduced, and elemental forms of selenium, selenite, and selenate which was found to be similar to the species observed in the coal samples. Conclusion
Figure 9. Arsenic XANES spectra of laboratory coal, ash, and char sample.
Figure 10. Selenium XANES spectra of laboratory coal, ash, and char samples.
Pyrolysis experiments were conducted at 1400 °C, and it was observed that, in terms of elemental concentrations, arsenic and
The XANES spectrum of XAFS was used to provide speciation of arsenic and selenium compounds in a nondestructive nature ensuring no interconversion of species during analysis. Both arsenic and selenium exhibited volatility under combustion conditions and were enriched in fly ash and depleted in bottom ash. Similar to the observations in the literature, in the current work, selenium was found to be more volatile compared to arsenic and in the power stations some amounts may escape from the stack with the flue gas. Semiquantitative arsenic speciation analysis of power station fly ash indicated that the majority of arsenic is present in the less toxic arsenate form. However, variations in speciation can be observed compared to feed coal for each power station. Variation in the speciation of arsenic in coal was associated with the total sulfur content and its different sulfur. Qualitative analysis of selenium speciation in fly ash indicated that majority of selenium is present in a selenite form. Selenium speciation in coal showed the presence of the organic/elemental or reduced form along with the presence of selenite and selenate. The organic/elemental or reduced form of selenium in coal was found to be the dominant form. Arsenic speciation analysis of ash produced by combustion in the laboratory drop tube furnace showed amounts below the detection limit of the XANES method, which was similar behavior to the power station bottom ash. Selenite was dominant selenium form found in laboratory ash. Different speciation was observed for char samples produced by pyrolysis experiments compared to ash samples and original coal samples. Char samples showed the presence of all species for arsenic and selenium, which might be due to further reactions of volatilized arsenic and selenium compounds with other volatilized species produced during pyrolysis and with the retained mineral matter forming stable inorganic forms of arsenic and selenium. Acknowledgment. The authors acknowledge the support of the Australian Synchrotron Research Program (ASRP) for access to the Australian National Beamline Facility (ANBF), Tsukuba, Japan, and that of the CRC for Coal in Sustainable Development (CCSD) which is funded in part by the Cooperative Research Centres Program of the Commonwealth Government of Australia. EF0604083 (22) Wang, J.; Tomita, A. Energy Fuels 2003, 17, 954.
