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Environmental impacts of coal combustion: A risk approach to assessment of emissions
Fuel 89 (2010) 810–816
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Environmental impacts of coal combustion: A risk approach to assessment of emissions Peter F. Nelson a,b,*, Pushan Shah a,b, Vlad Strezov a,b, Brendan Halliburton a,c, John N. Carras a,c a
CRC for Coal in Sustainable Development, Brisbane, Australia Graduate School of the Environment, Macquarie University, NSW 2109, Australia c CSIRO Energy Technology, Lucas Heights Laboratory, Private Mail Bag 7, Bangor 2234, Australia b
a r t i c l e
i n f o
Article history: Received 11 September 2008 Received in revised form 3 March 2009 Accepted 3 March 2009 Available online 22 March 2009 Keywords: Coal combustion Trace elements Speciation
a b s t r a c t This paper summarises some of the work performed in the Cooperative Research Centre for Coal in Sustainable Development (CCSD) on emissions from current power generation. A comprehensive approach was taken in the CCSD program to assessing environmental issues of concern for the power, and by implication the coal, industries. Here results of sampling on full scale operating plants are described, and detailed data on emission fluxes, particle size distributions, trace element concentrations as a function of particle size, and speciation of the trace elements are illustrated. The results show that particle capture in electrostatic precipitators (ESPs) is significantly less efficient than in fabric filters (FFs), particularly for submicron material, and that significant enrichment is observed in the finer particle sizes emitted from both ESPs and FFs. Results for the speciation of chromium, arsenic and selenium in coals, bottom ash and fly ash are also presented. The majority of chromium in fly ash is present in the less toxic Cr3+ form. Speciation of arsenic in feed coals is variable but the dominant form of As in fly ash is the less toxic As5+. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Environmental regulations and agreements, enacted at both the national and international level, present a significant challenge to the future viability and operations of the coal and utility industries. Technological and political initiatives to mitigate or adapt to greenhouse gas emissions are likely to have a significant impact. It is likely that pressures to reduce emissions of carbon dioxide and pollutant gases and particles will intensify. Electric power producers will also seek to significantly reduce emissions from conventional pulverised coal combustion and to develop new technologies such as integrated gasification combined cycle (IGCC), oxy-firing and pressurized fluidized bed combustion (PFBC) with inherently lower emissions. Environmental concerns are already presenting, and will continue to present, challenges to the coal industry in reducing emissions. Environmental concerns presently associated with the use of coal include: gaseous and particulate emissions produced in the combustion process, notably NOx, SO2 and toxic trace elements; emissions of CO2 and the implications of such emissions for global warming; * Corresponding author. Address: Graduate School of the Environment, Macquarie University, NSW 2109, Australia. Tel.: +61 2 98506958; fax: +61 2 98507972. E-mail address: [email protected] (P.F. Nelson). 0016-2361/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2009.03.002
emissions to air, water and land from operations associated with the mining of coal and the subsequent disposal of ash and spoil. The industry needs to be well positioned to respond to these challenges, particularly in a more competitive environment where other fuels, such as gas and biomass, are being seriously considered as alternatives to coal. There are also air quality issues that have the potential to impact on existing and future markets for thermal coals. These include emissions of fine particles and toxic compounds, and impacts of industrial NOx on regional air quality. During the past seven years the Australian Cooperative Research Centre for Coal in Sustainable Development (CCSD) developed a detailed program of research on emissions from current power generation. A comprehensive approach was taken to assessing environmental issues of concern for the power, and by implication the coal, industries. The objectives included: Development of a database of pollutant emissions (gas, particulate and trace toxic compounds) from Australian power stations, utilising validated sampling and analytical procedures. Measurement of trace element release and transformations from selected Australian coals. Development of techniques (correlations, predictors and models) which relate emissions to coal properties, furnace design and operating conditions.
P.F. Nelson et al. / Fuel 89 (2010) 810–816
Use of data and techniques generated above to develop reduction strategies for the environmental impacts of pollutant emissions to land, air and water from coal utilisation in current power plants where this is assessed as being required. Assess the policy settings which may be necessary to achieve improvements in the sustainability of coal combustion. In this paper results for trace element emissions from full scale power plants and for speciation of trace element emissions are summarised. Further details are available in the CCSD reports and publications from which these results are drawn.
2. Observations of trace element enrichment at full scale and development of size dependent emission factors Accurate and representative reporting of emissions of trace toxic species from all sources is a high priority in order for their environmental impact to be assessed. There are at least three options which could be considered for the reporting of emissions: Direct measurements of emissions at the location required to report. Modelling of emissions based on coal properties and composition, high temperature chemistry and the performance of particle collection devices. Development of a database of emissions based on measurements at a large number of plants, and the calculation of emission factors for emissions from this database. The last approach is the one that has been adopted by many national pollutant inventories, drawing heavily on available US data and presents the best opportunity, currently, to develop procedures for estimating emissions of trace species from coal combustion plant. However there are a number of problems which emerge when the available data are considered. These include:
data data data data
quality; accessibility; variability; quantity.
In part, these problems are related to the complexity of the processes by which emissions of trace elements occur. For instance, the more volatile trace elements may be emitted in the gas phase or enriched on the fine (sub-micron) particulate fraction, and hence escape capture by electrostatic precipitators or bag filters. Alternatively, trace elements may reside in the fly ash collected by gas cleaning devices or in the bottom ashes or slags. Their ultimate fate, in the latter case, will depend on the utilisation and/or disposal options chosen for the ash or slag, and in many cases will be determined by the leachability of the trace elements. Further complexity arises due to post-combustion reactions and transformations of trace elements which can play an important role in determining their deportment in combustion [1,2]. For example, Seames and Wendt [3,4] have established a relationship between the concentration of solid phase arsenic, selenium and cadmium to calcium in supermicron particles, suggesting formation of trace element/Ca complexes. Interactions with iron have also been reported, and possible control strategies using sorbents have been investigated [5,6]. Current knowledge of these processes is incomplete, and modelling or estimation techniques, which account for all these effects are still in the process of development. The US data for trace elements has been critically reviewed by Helble [2]. He shows that the US databases provide information on coal rank, ash content, sulphur content, trace element concen-
811
trations, coal higher heating value, trace element emissions rate, and particle emissions rate. Emissions of individual trace elements are reported based on the following relationship:
Ei ¼ Ai;in ð1 gi Þ ¼ C i ð1 gi Þ=H
ð1Þ
where Ei is the emission on a mass per fuel energy content basis; Ai,in is the concentration of the trace element i at the inlet to the particle collection device (mass per unit fuel energy content); Ci is the concentration (mass fraction) of trace element i in the coal on an asreceived basis; gi is the capture efficiency of trace element i in the particle collection device; and H is the higher heating value of the coal on an energy content per unit mass basis. The particle collection efficiency, g, of a particle collection device is defined [2] as:
g ¼ 1 PMout =PMin
ð2Þ
where PMout,in is the particulate matter concentration (mass per unit heat input) at the outlet or inlet to the particle collection device. PMout can be expressed [2] in terms of coal parameters as:
PMout ¼ PMin ð1 gÞ ¼ fa ð1 gÞ=H
ð3Þ
where fa is the mass fraction of ash in the coal on an as-received basis. Combination of Eqs. (1) and (3) gives an expression for trace element emissions as a function of measurable parameters:
Ei ¼
C i PM out ð1 gi Þ fa ð1 gÞ
ð4Þ
However the broad range of trace element emissions observed at different plant has led to the development of a modified version of Eq. (4):
Ei ¼ ai
b ðC i PM out Þ i fa
ð5Þ
where ai and bi are empirical factors. Eq. (5) is the form recommended by EPRI for interpretation of the DOE and PISCES data (see [2] and references quoted there). It is also the basis for the equations used in the Australian National Pollutant Inventory (NPI) workbook,1 but it should be recognised that this is an empirical approach, and one which does not allow for the enrichment of many trace elements in the fine particle sizes. As these particles are more difficult to capture in electrostatic precipitators, this simplification may be significant. Helble [2] has developed a model, which includes trace element concentrations as a function of particle size, and size dependent particulate capture efficiencies. Using this model he is able to show that the predictions of emitted concentrations of volatile trace elements such as arsenic and selenium can be improved. At present, data for Australian coals and facilities is not extensive enough for the refinements incorporated in Helble’s model, and the approach used in the NPI, and based largely on US data, should be the preferred method for reporting emissions. It had been known since the work of Davison et al. [7] that the fine particle fraction of fly ash could be enriched in trace elements compared with the fraction of trace elements in the parent coal. This is due to the volatilisation of some elements in the boiler and their subsequent condensation in the cooler sections of the flue gas stream. There has been considerable work investigating these observations for a variety of electrostatic precipitators (ESP) stations burning different coals [2]. These studies have found different behaviour for different elements and their transport
1 See http://www.npi.gov.au/handbooks/approved_handbooks/ffossilfuel.html, accessed 11th September 2008.
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through the ESP. While there are exceptions, most studies find the same elements enriched in the fine particle fraction. For instance most studies examined by Helble [2] found enrichment for the elements As, Cd, Pb, Sb and Se while, most found depletion in Mn. There are also significant impacts of flue gas desulphurization (FGD) scrubbing solutions on metal retention which are important in many locations worldwide where FGD is used to reduce SO2 emissions [8]. As noted above, the deposition mechanism of the vapourised trace elements onto ash particle surfaces can result in a mechanism-specific correlation between trace element concentration and particle size. For example, deposition rate limited by ash particle surface reaction kinetics is governed by the reaction rate at the surface area, and leads to a 1/dp dependence, where dp is the particle diameter. Such relationships, together with ESP and fabric filter (FF) particle penetration information, may be useful in developing practical models of trace element emission. As an example of the required data, Fig. 1 shows enrichment factors for arsenic and cadmium, calculated according to the definition of Meij [9], as a function of particle diameter. The enrichment factor (EF) is a useful way of examining trace element behaviour under combustion conditions. The EF is defined as the ratio of an elemental concentration in the fly ash sample relative to elemental concentration in the coal. To provide normalisation relative to the total mineral content of the coal, EFs are often calculated from the ratios of specific elemental concentrations in the fly ash and coal to those of matrix elements in the fly ash and coal samples. Thus, the EF may be calculated from:
EF ¼ ð½Xs =½Ms Þ=ð½Xc =½Mc Þ
ð6Þ
where [X]s and [X]c represent the mass of element X in the sample and coal, respectively, and [M]s and [M]c represent the content of the matrix element in the sample and coal, respectively. 3. Methods Measurements are reported here for two power stations located in Queensland: Tarong South which utilises an Electrostatic Precipitator, and Tarong North which utilises a fabric filter. Measurements at these full scale power plants were made of particle size and chemistry before and after particle collection devices [10,11], enabling particle and elemental penetration rates as a function of particle size to be determined. At these plants, dimensions of the ducts carrying the particle laden flue gas are typically 4 m 4 m. Representative measurements
100
Enrichment Factor
Arsenic Cadmium
on such large ducts are difficult and time consuming. Issues to be addressed include: Representative sampling across the large duct cross-sections. Measurements of gas flows and gas temperature. Particle sizing methods and the associated elemental concentrations as a function of particle size. The large differences in particle concentrations before and after the gas cleaning technology for both ESP and FF stations, but particularly for the FF stations. Temperature distributions were measured using custom fabricated thermocouple probes and commercially available electronic thermocouple meters. The particle sampling methodologies utilised in this project were based on the Australian Standard (AS) 4323.1-1995, AS 4323.2 (1995) as well as the United States Environmental Protection Agency (USEPA) standard procedures for measuring particle emissions from stationary sources, methods 1, 2, 5 and 17 [12]. Particle mass and concentration measurements were completed following USEPA method 5 – Determination of Particulate Emissions from Stationary Sources, with ‘in-stack’ sampling methodology. ‘In-stack’ sampling, as opposed to external ‘out-of-stack” sampling, was chosen as the preferred technique due the length of sample probe required to fully examine the exhaust ducts at some power stations ie 5 m and due to the deposition and possible re-entrainment, of particles in such long sample lines. Mass measurements in the highly concentrated particle exhaust, that is, upstream of the power station gas cleaning device, utilised a combination of an initial high capacity cyclone followed by an absolute filter where the cyclone was used to intercept the bulk of particulate burden prior to the filter membrane. This combination reduced the problem of overloading the filter paper and enabled realistic sampling times to be used. Mass measurements undertaken downstream of the power station gas cleaning device where the concentration of the particle burden was significantly reduced, utilised an absolute filter unit only. The measurements involving the size selective collection of the particulate burden for elemental analyses, were the main focus of this study. These measurements were made with a cascade impaction particle sampler based on the ‘microslot’ design [13] but specially developed for this project to enable sampling of the very low particle loadings experienced at the outlet of the FF. The fine particle samples were removed from the impactor plates by washing with a chlorinated hydrocarbon solvent. These were then digested in an acid solution and sub-samples taken for analysis. The major elements were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The majority of the trace elements were determined by inductively coupled plasma mass spectrometry (ICP-MS), while selenium was determined by hydride generation/AFS and mercury by cold vapour/AFS.
10
4. Results and discussion 4.1. Capture and distribution of trace elements in fine particles 1
0.1 0.1
1
10
100
Particle Diameter (µm) Fig. 1. Elemental enrichment factors for emitted fly ash as a function of particle diameter; Australian power station equipped with ESP; data from [11].
Results obtained at two nearby power stations are particularly useful in assessing fine particle and trace element emissions from Australian power stations. In this case the coal supply is common but one plant employs an ESP, and the other a FF. Although the coal used is sourced from the same mine, there are temporal variations in the composition of the coals burnt in the power stations. As an indication Fig. 2 shows the variation in the trace species As, Be, Cd, Mo, Sb and Se for the coal used at Tarong South power station during an intensive sampling period for the current study. The coal
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P.F. Nelson et al. / Fuel 89 (2010) 810–816
31/10/2003 - 10:00 1/11/2003 -12:00 2/11/2003 - 10:40 3/11/2003 - 9:20 4/11/2003 - 11:00
3
Concentration (mg/kg)
2.5
31/10/2003 -13:10 1/11/2003 -18:00 2/11/2003 -14:20 3/11/2003 - 14:00 4/11/2003 - 16:20
2 1.5 1 0.5 0 As
Be
Cd
Mo
Sb
Se
Element Fig. 2. Concentrations of arsenic, beryllium, cadmium, molybdenum, antimony and selenium in the PF coal feed over the duration of the sampling campaign for Tarong South power station.
14
Penetration (%)
12
ESP equipped FF equipped
10 8
fraction. The data in Fig. 5, however, exhibit significant scatter and these trends may be masked by the scatter. The scatter is a result of sampling variations, analytical uncertainties and other issues. There are also clear differences between the trace elemental distributions and concentrations at the two power stations which are related to variations in the feed coal composition (see Fig. 2).
10
Ca
5
0
Fe 100 3
Concentration (mg/m )
samples were taken twice daily over a five day period. The data displayed in Fig. 2 reveal that the concentration of the elements Cd, Sb and Be remained relatively constant (within the scatter of the data) over the course of the sampling campaign. The elements arsenic, molybdenum and selenium, while also scattered, suggested an overall increase in concentration. These variations demonstrate the importance of collecting samples of coal feed in conjunction with the emissions sampling, and also potentially result in variations in amounts of trace elements emitted. Fig. 3 shows particle penetration as a function of particle size at these two plants. The much higher capture efficiency of the FF station is evident, even for submicron sized particles. Figs. 4 and 5 show the concentrations of selected elements in the pre-cleaned gas as a function of particle size. Results for the major ash species, Al and Si, are quite consistent at the two power stations as are the data for Fe, apart from one point. The bulk elements (see Fig. 4) also show an increasing concentration in the flue gas as a function of particle size. This would be expected on the basis of the overall particle size distribution and for elements that do not show any enrichment in elemental composition as a function of particle size. For the more volatile elements, some of which are shown in Fig. 5 the trend of increasing concentration with particle diameter may be moderated to some extent by enrichment in the fine particle
50
0
Al
2000
1000
0
6
Si
4000 4
2000
2 0
0 0.1
1
10
100
Aerodynamic diameter, d50 (µm) Fig. 3. Penetration as a function of particle size for an ESP and a FF equipped Australian power station; data from [11].
0.1
1
10
Aerodynamic diameter (µm) Fig. 4. Concentrations of selected elements in the pre-cleaned gas from ESP (d) and FF (s) power stations; data from [11].
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P.F. Nelson et al. / Fuel 89 (2010) 810–816
gested in the finer particle sizes. This is particularly clear for both power stations for the trace elements As and Cd. As finer particles are captured less efficiently by ESPs, higher than expected emissions of the more volatile components may result. The general behaviour of the volatile trace elements is broadly consistent with previous observations [2,9,14].
Se
0.008
0.004
0.000
4.2. Speciation of metals in power station emissions and waste products
Cr
It is widely recognised that distribution, mobility and bioavailability of any element not only depends on their total concentrations but, critically on their chemical forms and oxidation states (speciation) [15]. As3+ is about 50 times more toxic than As5+ and several times more toxic than organic forms such as monomethilarsonate (MMA) and dimethylarsinate (DMA) [16]. Arsenic affects the gastrointestinal tract, circulatory system, liver, kidney and skin [17]. Apart from the environmental and health impacts, it has been also observed that arsenic present in coal, particularly volatilised As2O3 generated during combustion, may deactivate selective catalytic reduction (SCR) catalysts used for NOx control [18]. 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 highly toxic. Selenium toxicity depends on its oxidation form and quantity. There is a small margin between selenium sufficiency and toxicity and hence concentrations of selenium species in the environment need to be carefully evaluated [19]. Chromium is a naturally occurring metallic element that has two oxidation states which are stable in the environment: trivalent (Cr3+) and hexavalent chromium (Cr6+). Cr3+ is an essential nutrient for humans and animals and has a low level of toxicity. Cr6+ originates mostly from anthropogenic sources and is a known carcinogen [20]. Thus, it is important to analyse these trace element species in coal and combustion residues for the purpose of comprehensive
3
Concentration (mg/m )
1.0 0.5 0.0
Cd 0.01
0.00
As
0.03 0.02 0.01 0.00 0.1
1
10
Aerodynamic diameter (µm) Fig. 5. Concentrations of selected elements in the pre-cleaned gas from ESP (d) and FF (s) power stations; data from [11].
Fig. 6 shows the compositional data as a function of particle size for the emissions from the two power stations. It is evident that, for many of the most toxic elements, significant enrichment is sug-
Concentration mg/kg
1000000
ESP
12 µm 0.9 µm
100000
7.8 µm 0.6 µm
3.7 µm 0.4 µm
1.5 µm 0.2 µm
10000 1000 100 10 1 0.1 0.01 Al
As
Ba
Be
Ca
Cd
Co
Cr
Cu
Fe
Hg
Mg
Element 1000000
7.8 µm 0.9 µm
FF
Concentration mg/kg
100000
3.7 µm 0.6 µm
1.5 µm 0.4 µm
10000 1000 100 10 1 0.1 0.01 0.001 0.0001 Al
As
Ba
Be
Ca
Cd
Co
Cr
Cu
Fe
Hg
Mg
Element Fig. 6. Trace element composition of particles of indicated particle size (aerodynamic diameter) sampled from the outlet of the ESP and FF; data from [11].
P.F. Nelson et al. / Fuel 89 (2010) 810–816
815
Table 1 Speciation of chromium in coal and ash samples. Power station
A B C D E
Cr3+ (mg/kg)
Cr6+ (mg/kg)
Coal
Bottom ash
Fly ash
Coal
Bottom ash
Fly ash
10 11 12 9.4 10
41 75 36 39 26
144 130 114 52 30
0.14 0.17 0.18 0.08 0.07
ND ND ND ND ND
0.17 ND 1.3 0.72 0.86
ND denotes not detected.
environmental impact of coal fired power stations. Here results are reported for arsenic, chromium and selenium in coal and coal combustion products from different Australian power stations. Five different power stations across Australia, two in New South Wales, two in Western Australia and one in Queensland, utilising bituminous ranked coals were chosen for collection of samples of coal, bottom ash, fly ash and flue gas. Representative samples of coal, bottom ash and fly ash were obtained from each power station. A range of techniques have been used to determine speciation. The speciation of chromium in coal and ash samples was determined according to the procedure reported by Shah et al. [21] and is given in Table 1. Enrichment of chromium in fly ash and bottom ash indicates the non-volatile nature of chromium under power station combustion conditions. The maximum concentration of Cr6+ in fly ash was 1.3 mg/kg while Cr6+ was not detected in any of the bottom ash samples. Thus the majority of chromium in fly ash is present in the less toxic Cr3+ form. Semi-quantitative analysis of arsenic was performed using the procedure reported by Shah et al. [22] and semi-quantitative results for arsenic speciation are given in Table 2. It was observed that arsenic is present in a range of chemical forms in coal such as arsenic/pyrite, As3+ and As5+. Arsenic was less than the detection limit of the instrument for the bottom ash samples. In the case of fly ash samples, arsenic is mainly present in As5+ and a small but variable proportion, i.e., 5–15% of toxic As3+. Selenium XANES spectra for one power station coal and fly ash sample is shown in Fig. 7. Selenium speciation was observed qualitatively using the procedure discussed by Shah et al. [22]. It was observed that organic or elemental selenium is the dominant form in feed coal compared to other forms, i.e., Se4+ and Se6+ while Se4+ is the dominant form in fly ash samples. Small quantities of Se6+ were also detected. Organic or reduced forms of selenium were not detected in power station fly ash samples. One of the main objectives of this work was to analyse arsenic, chromium and selenium species in coal and coal combustion products. After measuring total concentrations of these elements, it was
Table 2 Semi-quantitative analysis of arsenic species. Power station
As/pyrite (%)
As3+ (%)
As5+ (%)
A
Coal Fly ash
ND ND
90 <5
10 >95
B
Coal Fly ash
12 ND
26 15
62 85
C
Coal Fly ash
ND ND
30 <10
70 >90
D
Coal Fly ash
45 ND
45 10
10 90
E
Coal Fly ash
10 ND
25 10
65 90
ND denotes not detected.
Fig. 7. Speciation of selenium in coal and fly ash sample by XANES.
found that arsenic and selenium exhibited volatile nature while chromium was found to be relatively non-volatile under the combustion conditions of power stations. Arsenic in feed coal was present in various chemical forms such as arsenic/pyrite, trivalent (As3+) and pentavalent form (As5+). Selenium was mainly found in organic or elemental form and chromium was mainly present in Cr3+. In bottom ash, arsenic and selenium species were not detected and hexavalent chromium (Cr6+) was also not detected. In fly ash, arsenic was mainly present in As5+ with only minor amounts of As3+. Selenium was present mainly in Se4+ and chromium mainly as Cr3+ with minor amounts of Cr6+. 5. Conclusions Reliable assessment of the potential environmental impacts of trace gaseous and particulate emissions from coal fired power stations requires detailed data on emission fluxes, particle size distributions, trace element concentrations as a function of particle size, and speciation of the trace elements. The results summarised in this paper show that: particle capture efficiency in ESPs is significantly less efficient than in FFs, particularly for submicron material where penetrations of 10% or so were observed for ESP equipped power stations; for many of the most toxic elements, significant enrichment is observed in the finer particle sizes emitted from both ESPs and FFs; the majority of chromium in fly ash is present in the less toxic Cr3+ form; Cr6+ is not detected in bottom ash; speciation of arsenic in feed coals is variable but the dominant form of As in fly ash is the less toxic As5+. Acknowledgements The authors acknowledge the support of the CRC for Coal in Sustainable Development (CCSD) which is funded in part by the Coop-
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erative Research Centres Program of the Commonwealth Government of Australia. References [1] Sterling RO, Helble JJ. Reaction of arsenic vapor species with fly ash compounds: kinetics and speciation of the reaction with calcium silicates. Chemosphere 2003;51:1111–9. [2] Helble JJ. A model for the air emissions of trace metallic elements from coal combustors equipped with electrostatic precipitators. Fuel Process Technol 2000;63:125–47. [3] Seames WS, Wendt JOL. Partitioning of arsenic, selenium, and cadmium during the combustion of Pittsburgh and Illinois #6 coals in a self-sustained combustor. Fuel Process Technol 2000;63:179–96. [4] Seames WS, Wendt JOL. The partitioning of arsenic during pulverized coal combustion. Proc Combust Inst 2000;28:2305–12. [5] Gale TK, Wendt JOL. In-furnace capture of cadmium and other semi-volatile metals by sorbents. Proc Combust Inst 2005;30:2999–3007. [6] Fernandez A, Wendt JOL, Witten ML. Health effects engineering of coal and biomass combustion particulates: influence of zinc, sulfur and process changes on potential lung injury from inhaled ash. Fuel 2005;84:1320–7. [7] Davison RL, Natusch DFS, Wallace JR, Evans CAJ. Trace elements in fly ash – dependence of concentration on particle size. Environ Sci Technol 1974;8:1107–13. [8] Meij R, Winkel HT. The emissions and environmental impact of PM10 and trace elements from a modern coal-fired power plant equipped with ESP and wet FGD. Fuel Process Technol 2004;85:641–56. [9] Meij R. Trace element behavior in coal-fired power plants. Fuel Process Technol 1994;39:199–217.
[10] Halliburton B, Chase DR, Carras JN, Nelson PF. Fine particle emissions from power stations. CCSD project 2.1, task 5. Interim report on emissions from a fabric filter power station. CRC for coal in sustainable development; 2004. p. 20. [11] Halliburton BW, Carras JN, Nelson PF. Fine particle emissions from power stations. CRC for coal in sustainable development; 2006. p. 97. [12] USEPA. USEPA methods 1, 2, 4, and 17 [internet site]. Available from:; 2005 [21st February 2005]. [13] Hering S, Gundel L, Daisey JM. A microslot impactor for organic aerosol sampling. J Aerosol Sci 1997;28:1283–90. [14] Senior CL, Helble JJ, Sarofim AF. Emissions of mercury, trace elements, and fine particles from stationary combustion sources. Fuel Process Technol 2000;65– 66:263–88. [15] Ebdon L. Trace element speciation for environment, food, and health. Cambridge: Royal Society of Chemistry; 2002. [16] Hodgson E, Mailman RB, Chambers JE. Dictionary of toxicology. London: Macmillan; 1988. [17] Mandal BK, Suzuki KT. Arsenic round the world: a review. Talanta 2002;58:201–35. [18] Staudt JE, Engelmeyer T, Weston WH, Sigling R. The impact of arsenic on coal fired power plants equipped with SCR. In ICAC Forum. Houstan; 2002. [19] Greenwood NN, Earnshaw A. Chemistry of the elements. Boston: ButterworthHeinemann; 1997. [20] US Department of Health and Human Services. Agency for toxic substances and disease registry, toxicological profile for chromium. US Department of Health and Human Services, Public Health Service: Atlanta; 2000. [21] Shah P, Strezov V, Prince K, Nelson PF. Speciation of As, Cr, Se and Hg under coal fired power station conditions. Fuel 2008;87:1859. [22] Shah P, Strezov V, Stevanov C, Nelson PF. Speciation of arsenic and selenium in coal combustion products. Energy Fuels 2007;21:506–12.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Environmental impacts of coal combustion: A risk approach to assessment of emissions Peter F. Nelson a,b,*, Pushan Shah a,b, Vlad Strezov a,b, Brendan Halliburton a,c, John N. Carras a,c a
CRC for Coal in Sustainable Development, Brisbane, Australia Graduate School of the Environment, Macquarie University, NSW 2109, Australia c CSIRO Energy Technology, Lucas Heights Laboratory, Private Mail Bag 7, Bangor 2234, Australia b
a r t i c l e
i n f o
Article history: Received 11 September 2008 Received in revised form 3 March 2009 Accepted 3 March 2009 Available online 22 March 2009 Keywords: Coal combustion Trace elements Speciation
a b s t r a c t This paper summarises some of the work performed in the Cooperative Research Centre for Coal in Sustainable Development (CCSD) on emissions from current power generation. A comprehensive approach was taken in the CCSD program to assessing environmental issues of concern for the power, and by implication the coal, industries. Here results of sampling on full scale operating plants are described, and detailed data on emission fluxes, particle size distributions, trace element concentrations as a function of particle size, and speciation of the trace elements are illustrated. The results show that particle capture in electrostatic precipitators (ESPs) is significantly less efficient than in fabric filters (FFs), particularly for submicron material, and that significant enrichment is observed in the finer particle sizes emitted from both ESPs and FFs. Results for the speciation of chromium, arsenic and selenium in coals, bottom ash and fly ash are also presented. The majority of chromium in fly ash is present in the less toxic Cr3+ form. Speciation of arsenic in feed coals is variable but the dominant form of As in fly ash is the less toxic As5+. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Environmental regulations and agreements, enacted at both the national and international level, present a significant challenge to the future viability and operations of the coal and utility industries. Technological and political initiatives to mitigate or adapt to greenhouse gas emissions are likely to have a significant impact. It is likely that pressures to reduce emissions of carbon dioxide and pollutant gases and particles will intensify. Electric power producers will also seek to significantly reduce emissions from conventional pulverised coal combustion and to develop new technologies such as integrated gasification combined cycle (IGCC), oxy-firing and pressurized fluidized bed combustion (PFBC) with inherently lower emissions. Environmental concerns are already presenting, and will continue to present, challenges to the coal industry in reducing emissions. Environmental concerns presently associated with the use of coal include: gaseous and particulate emissions produced in the combustion process, notably NOx, SO2 and toxic trace elements; emissions of CO2 and the implications of such emissions for global warming; * Corresponding author. Address: Graduate School of the Environment, Macquarie University, NSW 2109, Australia. Tel.: +61 2 98506958; fax: +61 2 98507972. E-mail address: [email protected] (P.F. Nelson). 0016-2361/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2009.03.002
emissions to air, water and land from operations associated with the mining of coal and the subsequent disposal of ash and spoil. The industry needs to be well positioned to respond to these challenges, particularly in a more competitive environment where other fuels, such as gas and biomass, are being seriously considered as alternatives to coal. There are also air quality issues that have the potential to impact on existing and future markets for thermal coals. These include emissions of fine particles and toxic compounds, and impacts of industrial NOx on regional air quality. During the past seven years the Australian Cooperative Research Centre for Coal in Sustainable Development (CCSD) developed a detailed program of research on emissions from current power generation. A comprehensive approach was taken to assessing environmental issues of concern for the power, and by implication the coal, industries. The objectives included: Development of a database of pollutant emissions (gas, particulate and trace toxic compounds) from Australian power stations, utilising validated sampling and analytical procedures. Measurement of trace element release and transformations from selected Australian coals. Development of techniques (correlations, predictors and models) which relate emissions to coal properties, furnace design and operating conditions.
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Use of data and techniques generated above to develop reduction strategies for the environmental impacts of pollutant emissions to land, air and water from coal utilisation in current power plants where this is assessed as being required. Assess the policy settings which may be necessary to achieve improvements in the sustainability of coal combustion. In this paper results for trace element emissions from full scale power plants and for speciation of trace element emissions are summarised. Further details are available in the CCSD reports and publications from which these results are drawn.
2. Observations of trace element enrichment at full scale and development of size dependent emission factors Accurate and representative reporting of emissions of trace toxic species from all sources is a high priority in order for their environmental impact to be assessed. There are at least three options which could be considered for the reporting of emissions: Direct measurements of emissions at the location required to report. Modelling of emissions based on coal properties and composition, high temperature chemistry and the performance of particle collection devices. Development of a database of emissions based on measurements at a large number of plants, and the calculation of emission factors for emissions from this database. The last approach is the one that has been adopted by many national pollutant inventories, drawing heavily on available US data and presents the best opportunity, currently, to develop procedures for estimating emissions of trace species from coal combustion plant. However there are a number of problems which emerge when the available data are considered. These include:
data data data data
quality; accessibility; variability; quantity.
In part, these problems are related to the complexity of the processes by which emissions of trace elements occur. For instance, the more volatile trace elements may be emitted in the gas phase or enriched on the fine (sub-micron) particulate fraction, and hence escape capture by electrostatic precipitators or bag filters. Alternatively, trace elements may reside in the fly ash collected by gas cleaning devices or in the bottom ashes or slags. Their ultimate fate, in the latter case, will depend on the utilisation and/or disposal options chosen for the ash or slag, and in many cases will be determined by the leachability of the trace elements. Further complexity arises due to post-combustion reactions and transformations of trace elements which can play an important role in determining their deportment in combustion [1,2]. For example, Seames and Wendt [3,4] have established a relationship between the concentration of solid phase arsenic, selenium and cadmium to calcium in supermicron particles, suggesting formation of trace element/Ca complexes. Interactions with iron have also been reported, and possible control strategies using sorbents have been investigated [5,6]. Current knowledge of these processes is incomplete, and modelling or estimation techniques, which account for all these effects are still in the process of development. The US data for trace elements has been critically reviewed by Helble [2]. He shows that the US databases provide information on coal rank, ash content, sulphur content, trace element concen-
811
trations, coal higher heating value, trace element emissions rate, and particle emissions rate. Emissions of individual trace elements are reported based on the following relationship:
Ei ¼ Ai;in ð1 gi Þ ¼ C i ð1 gi Þ=H
ð1Þ
where Ei is the emission on a mass per fuel energy content basis; Ai,in is the concentration of the trace element i at the inlet to the particle collection device (mass per unit fuel energy content); Ci is the concentration (mass fraction) of trace element i in the coal on an asreceived basis; gi is the capture efficiency of trace element i in the particle collection device; and H is the higher heating value of the coal on an energy content per unit mass basis. The particle collection efficiency, g, of a particle collection device is defined [2] as:
g ¼ 1 PMout =PMin
ð2Þ
where PMout,in is the particulate matter concentration (mass per unit heat input) at the outlet or inlet to the particle collection device. PMout can be expressed [2] in terms of coal parameters as:
PMout ¼ PMin ð1 gÞ ¼ fa ð1 gÞ=H
ð3Þ
where fa is the mass fraction of ash in the coal on an as-received basis. Combination of Eqs. (1) and (3) gives an expression for trace element emissions as a function of measurable parameters:
Ei ¼
C i PM out ð1 gi Þ fa ð1 gÞ
ð4Þ
However the broad range of trace element emissions observed at different plant has led to the development of a modified version of Eq. (4):
Ei ¼ ai
b ðC i PM out Þ i fa
ð5Þ
where ai and bi are empirical factors. Eq. (5) is the form recommended by EPRI for interpretation of the DOE and PISCES data (see [2] and references quoted there). It is also the basis for the equations used in the Australian National Pollutant Inventory (NPI) workbook,1 but it should be recognised that this is an empirical approach, and one which does not allow for the enrichment of many trace elements in the fine particle sizes. As these particles are more difficult to capture in electrostatic precipitators, this simplification may be significant. Helble [2] has developed a model, which includes trace element concentrations as a function of particle size, and size dependent particulate capture efficiencies. Using this model he is able to show that the predictions of emitted concentrations of volatile trace elements such as arsenic and selenium can be improved. At present, data for Australian coals and facilities is not extensive enough for the refinements incorporated in Helble’s model, and the approach used in the NPI, and based largely on US data, should be the preferred method for reporting emissions. It had been known since the work of Davison et al. [7] that the fine particle fraction of fly ash could be enriched in trace elements compared with the fraction of trace elements in the parent coal. This is due to the volatilisation of some elements in the boiler and their subsequent condensation in the cooler sections of the flue gas stream. There has been considerable work investigating these observations for a variety of electrostatic precipitators (ESP) stations burning different coals [2]. These studies have found different behaviour for different elements and their transport
1 See http://www.npi.gov.au/handbooks/approved_handbooks/ffossilfuel.html, accessed 11th September 2008.
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through the ESP. While there are exceptions, most studies find the same elements enriched in the fine particle fraction. For instance most studies examined by Helble [2] found enrichment for the elements As, Cd, Pb, Sb and Se while, most found depletion in Mn. There are also significant impacts of flue gas desulphurization (FGD) scrubbing solutions on metal retention which are important in many locations worldwide where FGD is used to reduce SO2 emissions [8]. As noted above, the deposition mechanism of the vapourised trace elements onto ash particle surfaces can result in a mechanism-specific correlation between trace element concentration and particle size. For example, deposition rate limited by ash particle surface reaction kinetics is governed by the reaction rate at the surface area, and leads to a 1/dp dependence, where dp is the particle diameter. Such relationships, together with ESP and fabric filter (FF) particle penetration information, may be useful in developing practical models of trace element emission. As an example of the required data, Fig. 1 shows enrichment factors for arsenic and cadmium, calculated according to the definition of Meij [9], as a function of particle diameter. The enrichment factor (EF) is a useful way of examining trace element behaviour under combustion conditions. The EF is defined as the ratio of an elemental concentration in the fly ash sample relative to elemental concentration in the coal. To provide normalisation relative to the total mineral content of the coal, EFs are often calculated from the ratios of specific elemental concentrations in the fly ash and coal to those of matrix elements in the fly ash and coal samples. Thus, the EF may be calculated from:
EF ¼ ð½Xs =½Ms Þ=ð½Xc =½Mc Þ
ð6Þ
where [X]s and [X]c represent the mass of element X in the sample and coal, respectively, and [M]s and [M]c represent the content of the matrix element in the sample and coal, respectively. 3. Methods Measurements are reported here for two power stations located in Queensland: Tarong South which utilises an Electrostatic Precipitator, and Tarong North which utilises a fabric filter. Measurements at these full scale power plants were made of particle size and chemistry before and after particle collection devices [10,11], enabling particle and elemental penetration rates as a function of particle size to be determined. At these plants, dimensions of the ducts carrying the particle laden flue gas are typically 4 m 4 m. Representative measurements
100
Enrichment Factor
Arsenic Cadmium
on such large ducts are difficult and time consuming. Issues to be addressed include: Representative sampling across the large duct cross-sections. Measurements of gas flows and gas temperature. Particle sizing methods and the associated elemental concentrations as a function of particle size. The large differences in particle concentrations before and after the gas cleaning technology for both ESP and FF stations, but particularly for the FF stations. Temperature distributions were measured using custom fabricated thermocouple probes and commercially available electronic thermocouple meters. The particle sampling methodologies utilised in this project were based on the Australian Standard (AS) 4323.1-1995, AS 4323.2 (1995) as well as the United States Environmental Protection Agency (USEPA) standard procedures for measuring particle emissions from stationary sources, methods 1, 2, 5 and 17 [12]. Particle mass and concentration measurements were completed following USEPA method 5 – Determination of Particulate Emissions from Stationary Sources, with ‘in-stack’ sampling methodology. ‘In-stack’ sampling, as opposed to external ‘out-of-stack” sampling, was chosen as the preferred technique due the length of sample probe required to fully examine the exhaust ducts at some power stations ie 5 m and due to the deposition and possible re-entrainment, of particles in such long sample lines. Mass measurements in the highly concentrated particle exhaust, that is, upstream of the power station gas cleaning device, utilised a combination of an initial high capacity cyclone followed by an absolute filter where the cyclone was used to intercept the bulk of particulate burden prior to the filter membrane. This combination reduced the problem of overloading the filter paper and enabled realistic sampling times to be used. Mass measurements undertaken downstream of the power station gas cleaning device where the concentration of the particle burden was significantly reduced, utilised an absolute filter unit only. The measurements involving the size selective collection of the particulate burden for elemental analyses, were the main focus of this study. These measurements were made with a cascade impaction particle sampler based on the ‘microslot’ design [13] but specially developed for this project to enable sampling of the very low particle loadings experienced at the outlet of the FF. The fine particle samples were removed from the impactor plates by washing with a chlorinated hydrocarbon solvent. These were then digested in an acid solution and sub-samples taken for analysis. The major elements were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The majority of the trace elements were determined by inductively coupled plasma mass spectrometry (ICP-MS), while selenium was determined by hydride generation/AFS and mercury by cold vapour/AFS.
10
4. Results and discussion 4.1. Capture and distribution of trace elements in fine particles 1
0.1 0.1
1
10
100
Particle Diameter (µm) Fig. 1. Elemental enrichment factors for emitted fly ash as a function of particle diameter; Australian power station equipped with ESP; data from [11].
Results obtained at two nearby power stations are particularly useful in assessing fine particle and trace element emissions from Australian power stations. In this case the coal supply is common but one plant employs an ESP, and the other a FF. Although the coal used is sourced from the same mine, there are temporal variations in the composition of the coals burnt in the power stations. As an indication Fig. 2 shows the variation in the trace species As, Be, Cd, Mo, Sb and Se for the coal used at Tarong South power station during an intensive sampling period for the current study. The coal
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31/10/2003 - 10:00 1/11/2003 -12:00 2/11/2003 - 10:40 3/11/2003 - 9:20 4/11/2003 - 11:00
3
Concentration (mg/kg)
2.5
31/10/2003 -13:10 1/11/2003 -18:00 2/11/2003 -14:20 3/11/2003 - 14:00 4/11/2003 - 16:20
2 1.5 1 0.5 0 As
Be
Cd
Mo
Sb
Se
Element Fig. 2. Concentrations of arsenic, beryllium, cadmium, molybdenum, antimony and selenium in the PF coal feed over the duration of the sampling campaign for Tarong South power station.
14
Penetration (%)
12
ESP equipped FF equipped
10 8
fraction. The data in Fig. 5, however, exhibit significant scatter and these trends may be masked by the scatter. The scatter is a result of sampling variations, analytical uncertainties and other issues. There are also clear differences between the trace elemental distributions and concentrations at the two power stations which are related to variations in the feed coal composition (see Fig. 2).
10
Ca
5
0
Fe 100 3
Concentration (mg/m )
samples were taken twice daily over a five day period. The data displayed in Fig. 2 reveal that the concentration of the elements Cd, Sb and Be remained relatively constant (within the scatter of the data) over the course of the sampling campaign. The elements arsenic, molybdenum and selenium, while also scattered, suggested an overall increase in concentration. These variations demonstrate the importance of collecting samples of coal feed in conjunction with the emissions sampling, and also potentially result in variations in amounts of trace elements emitted. Fig. 3 shows particle penetration as a function of particle size at these two plants. The much higher capture efficiency of the FF station is evident, even for submicron sized particles. Figs. 4 and 5 show the concentrations of selected elements in the pre-cleaned gas as a function of particle size. Results for the major ash species, Al and Si, are quite consistent at the two power stations as are the data for Fe, apart from one point. The bulk elements (see Fig. 4) also show an increasing concentration in the flue gas as a function of particle size. This would be expected on the basis of the overall particle size distribution and for elements that do not show any enrichment in elemental composition as a function of particle size. For the more volatile elements, some of which are shown in Fig. 5 the trend of increasing concentration with particle diameter may be moderated to some extent by enrichment in the fine particle
50
0
Al
2000
1000
0
6
Si
4000 4
2000
2 0
0 0.1
1
10
100
Aerodynamic diameter, d50 (µm) Fig. 3. Penetration as a function of particle size for an ESP and a FF equipped Australian power station; data from [11].
0.1
1
10
Aerodynamic diameter (µm) Fig. 4. Concentrations of selected elements in the pre-cleaned gas from ESP (d) and FF (s) power stations; data from [11].
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P.F. Nelson et al. / Fuel 89 (2010) 810–816
gested in the finer particle sizes. This is particularly clear for both power stations for the trace elements As and Cd. As finer particles are captured less efficiently by ESPs, higher than expected emissions of the more volatile components may result. The general behaviour of the volatile trace elements is broadly consistent with previous observations [2,9,14].
Se
0.008
0.004
0.000
4.2. Speciation of metals in power station emissions and waste products
Cr
It is widely recognised that distribution, mobility and bioavailability of any element not only depends on their total concentrations but, critically on their chemical forms and oxidation states (speciation) [15]. As3+ is about 50 times more toxic than As5+ and several times more toxic than organic forms such as monomethilarsonate (MMA) and dimethylarsinate (DMA) [16]. Arsenic affects the gastrointestinal tract, circulatory system, liver, kidney and skin [17]. Apart from the environmental and health impacts, it has been also observed that arsenic present in coal, particularly volatilised As2O3 generated during combustion, may deactivate selective catalytic reduction (SCR) catalysts used for NOx control [18]. 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 highly toxic. Selenium toxicity depends on its oxidation form and quantity. There is a small margin between selenium sufficiency and toxicity and hence concentrations of selenium species in the environment need to be carefully evaluated [19]. Chromium is a naturally occurring metallic element that has two oxidation states which are stable in the environment: trivalent (Cr3+) and hexavalent chromium (Cr6+). Cr3+ is an essential nutrient for humans and animals and has a low level of toxicity. Cr6+ originates mostly from anthropogenic sources and is a known carcinogen [20]. Thus, it is important to analyse these trace element species in coal and combustion residues for the purpose of comprehensive
3
Concentration (mg/m )
1.0 0.5 0.0
Cd 0.01
0.00
As
0.03 0.02 0.01 0.00 0.1
1
10
Aerodynamic diameter (µm) Fig. 5. Concentrations of selected elements in the pre-cleaned gas from ESP (d) and FF (s) power stations; data from [11].
Fig. 6 shows the compositional data as a function of particle size for the emissions from the two power stations. It is evident that, for many of the most toxic elements, significant enrichment is sug-
Concentration mg/kg
1000000
ESP
12 µm 0.9 µm
100000
7.8 µm 0.6 µm
3.7 µm 0.4 µm
1.5 µm 0.2 µm
10000 1000 100 10 1 0.1 0.01 Al
As
Ba
Be
Ca
Cd
Co
Cr
Cu
Fe
Hg
Mg
Element 1000000
7.8 µm 0.9 µm
FF
Concentration mg/kg
100000
3.7 µm 0.6 µm
1.5 µm 0.4 µm
10000 1000 100 10 1 0.1 0.01 0.001 0.0001 Al
As
Ba
Be
Ca
Cd
Co
Cr
Cu
Fe
Hg
Mg
Element Fig. 6. Trace element composition of particles of indicated particle size (aerodynamic diameter) sampled from the outlet of the ESP and FF; data from [11].
P.F. Nelson et al. / Fuel 89 (2010) 810–816
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Table 1 Speciation of chromium in coal and ash samples. Power station
A B C D E
Cr3+ (mg/kg)
Cr6+ (mg/kg)
Coal
Bottom ash
Fly ash
Coal
Bottom ash
Fly ash
10 11 12 9.4 10
41 75 36 39 26
144 130 114 52 30
0.14 0.17 0.18 0.08 0.07
ND ND ND ND ND
0.17 ND 1.3 0.72 0.86
ND denotes not detected.
environmental impact of coal fired power stations. Here results are reported for arsenic, chromium and selenium in coal and coal combustion products from different Australian power stations. Five different power stations across Australia, two in New South Wales, two in Western Australia and one in Queensland, utilising bituminous ranked coals were chosen for collection of samples of coal, bottom ash, fly ash and flue gas. Representative samples of coal, bottom ash and fly ash were obtained from each power station. A range of techniques have been used to determine speciation. The speciation of chromium in coal and ash samples was determined according to the procedure reported by Shah et al. [21] and is given in Table 1. Enrichment of chromium in fly ash and bottom ash indicates the non-volatile nature of chromium under power station combustion conditions. The maximum concentration of Cr6+ in fly ash was 1.3 mg/kg while Cr6+ was not detected in any of the bottom ash samples. Thus the majority of chromium in fly ash is present in the less toxic Cr3+ form. Semi-quantitative analysis of arsenic was performed using the procedure reported by Shah et al. [22] and semi-quantitative results for arsenic speciation are given in Table 2. It was observed that arsenic is present in a range of chemical forms in coal such as arsenic/pyrite, As3+ and As5+. Arsenic was less than the detection limit of the instrument for the bottom ash samples. In the case of fly ash samples, arsenic is mainly present in As5+ and a small but variable proportion, i.e., 5–15% of toxic As3+. Selenium XANES spectra for one power station coal and fly ash sample is shown in Fig. 7. Selenium speciation was observed qualitatively using the procedure discussed by Shah et al. [22]. It was observed that organic or elemental selenium is the dominant form in feed coal compared to other forms, i.e., Se4+ and Se6+ while Se4+ is the dominant form in fly ash samples. Small quantities of Se6+ were also detected. Organic or reduced forms of selenium were not detected in power station fly ash samples. One of the main objectives of this work was to analyse arsenic, chromium and selenium species in coal and coal combustion products. After measuring total concentrations of these elements, it was
Table 2 Semi-quantitative analysis of arsenic species. Power station
As/pyrite (%)
As3+ (%)
As5+ (%)
A
Coal Fly ash
ND ND
90 <5
10 >95
B
Coal Fly ash
12 ND
26 15
62 85
C
Coal Fly ash
ND ND
30 <10
70 >90
D
Coal Fly ash
45 ND
45 10
10 90
E
Coal Fly ash
10 ND
25 10
65 90
ND denotes not detected.
Fig. 7. Speciation of selenium in coal and fly ash sample by XANES.
found that arsenic and selenium exhibited volatile nature while chromium was found to be relatively non-volatile under the combustion conditions of power stations. Arsenic in feed coal was present in various chemical forms such as arsenic/pyrite, trivalent (As3+) and pentavalent form (As5+). Selenium was mainly found in organic or elemental form and chromium was mainly present in Cr3+. In bottom ash, arsenic and selenium species were not detected and hexavalent chromium (Cr6+) was also not detected. In fly ash, arsenic was mainly present in As5+ with only minor amounts of As3+. Selenium was present mainly in Se4+ and chromium mainly as Cr3+ with minor amounts of Cr6+. 5. Conclusions Reliable assessment of the potential environmental impacts of trace gaseous and particulate emissions from coal fired power stations requires detailed data on emission fluxes, particle size distributions, trace element concentrations as a function of particle size, and speciation of the trace elements. The results summarised in this paper show that: particle capture efficiency in ESPs is significantly less efficient than in FFs, particularly for submicron material where penetrations of 10% or so were observed for ESP equipped power stations; for many of the most toxic elements, significant enrichment is observed in the finer particle sizes emitted from both ESPs and FFs; the majority of chromium in fly ash is present in the less toxic Cr3+ form; Cr6+ is not detected in bottom ash; speciation of arsenic in feed coals is variable but the dominant form of As in fly ash is the less toxic As5+. Acknowledgements The authors acknowledge the support of the CRC for Coal in Sustainable Development (CCSD) which is funded in part by the Coop-
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erative Research Centres Program of the Commonwealth Government of Australia. References [1] Sterling RO, Helble JJ. Reaction of arsenic vapor species with fly ash compounds: kinetics and speciation of the reaction with calcium silicates. Chemosphere 2003;51:1111–9. [2] Helble JJ. A model for the air emissions of trace metallic elements from coal combustors equipped with electrostatic precipitators. Fuel Process Technol 2000;63:125–47. [3] Seames WS, Wendt JOL. Partitioning of arsenic, selenium, and cadmium during the combustion of Pittsburgh and Illinois #6 coals in a self-sustained combustor. Fuel Process Technol 2000;63:179–96. [4] Seames WS, Wendt JOL. The partitioning of arsenic during pulverized coal combustion. Proc Combust Inst 2000;28:2305–12. [5] Gale TK, Wendt JOL. In-furnace capture of cadmium and other semi-volatile metals by sorbents. Proc Combust Inst 2005;30:2999–3007. [6] Fernandez A, Wendt JOL, Witten ML. Health effects engineering of coal and biomass combustion particulates: influence of zinc, sulfur and process changes on potential lung injury from inhaled ash. Fuel 2005;84:1320–7. [7] Davison RL, Natusch DFS, Wallace JR, Evans CAJ. Trace elements in fly ash – dependence of concentration on particle size. Environ Sci Technol 1974;8:1107–13. [8] Meij R, Winkel HT. The emissions and environmental impact of PM10 and trace elements from a modern coal-fired power plant equipped with ESP and wet FGD. Fuel Process Technol 2004;85:641–56. [9] Meij R. Trace element behavior in coal-fired power plants. Fuel Process Technol 1994;39:199–217.
[10] Halliburton B, Chase DR, Carras JN, Nelson PF. Fine particle emissions from power stations. CCSD project 2.1, task 5. Interim report on emissions from a fabric filter power station. CRC for coal in sustainable development; 2004. p. 20. [11] Halliburton BW, Carras JN, Nelson PF. Fine particle emissions from power stations. CRC for coal in sustainable development; 2006. p. 97. [12] USEPA. USEPA methods 1, 2, 4, and 17 [internet site]. Available from: