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Atmospheric emissions of mercury from Australian point sources
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Atmospheric Environment 41 (2007) 1717–1724 www.elsevier.com/locate/atmosenv
Atmospheric emissions of mercury from Australian point sources Peter F. Nelson Graduate School of the Environment, Co-operative Research Centre for Coal in Sustainable Development, Macquarie University, Sydney 2109, Australia Received 19 May 2006; received in revised form 11 October 2006; accepted 17 October 2006
Abstract The UN Global Mercury Assessment (GMA) estimates that atmospheric emissions of mercury from Australian stationary combustion sources were 97.0 tonnes for the year of 1995. This is more than 90% of the estimated emissions from stationary combustion for the whole of North America, and seems abnormally high for a country with a population of around 20 million, in spite of the fact that most of Australia’s stationary energy supply is provided by coal. It is also significantly larger than previous estimates of mercury emissions from Australian sources. New estimates of Australian mercury emissions from stationary energy sources, based on both a top down and bottom up approach, are presented. These estimates can be reconciled for black coal fired power stations, but suggest that the bottom up approach (the Australian National Pollutant Inventory) significantly under-estimates emissions from brown coal fired plant, if mercury capture efficiencies in these plants are low, as observed for lignite-fired plant. The major uncertainties in these estimates are the coal mercury content in coals burnt in Australian power stations, and the mercury capture efficiency in particulate control devices used at these stations. Based on these estimates, Australian emissions of mercury from stationary energy are currently 2–8 tonnes/year, significantly lower than the GMA estimate. r 2006 Elsevier Ltd. All rights reserved. Keywords: Mercury; Coal combustion; Emission Inventory
1. Introduction Mercury is a potentially toxic trace metal found throughout the terrestrial, aquatic and marine ecosystems, as well as in the atmosphere. Complex processes of emission, transport, chemical transformation and storage are responsible for this ubiquity (UNEP, 2002). The atmosphere is recognised to be the dominant medium for transport of mercury in the environment (Fitzgerald et al., 1991; Lindquist Tel.: +61 2 98506958; fax: +61 2 9850 7972.
E-mail address: [email protected]. 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.10.029
et al., 1991), and there is considerable evidence (Petersen et al., 1995; Jackson, 1997; Pai et al., 1997; Fitzgerald et al., 1998; Xu et al., 2000, 2000; Petersen et al., 2001; Wangberg et al., 2001; Pirrone and Mahaffey, 2005) for a connection between anthropogenic mercury emissions and observed concentrations in remote regions, such as the Artic, due to long-range transport of elemental mercury from industrial areas. Hence, mercury is a global problem which may not only have local environmental impacts near point sources of emission, but also in remote locations. Mercury concentrates in the environment
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by bio-accumulation particularly in fish, and human exposure is mainly through dietary intake. The UN global mercury assessment (GMA) (UNEP, 2002) concluded ‘‘there is sufficient evidence of significant global adverse impacts from mercury to warrant further international action to reduce the risks to humans and wildlife from the release of mercury to the environment’’. In the view of some researchers, release of any mercury from anthropogenic sources which will lead to increases in the global pool, should be avoided since there is already evidence for significant impacts (Meili et al., 2003). As a global pollutant, an accurate knowledge of the amounts of mercury released worldwide and from all sources is an important input into models of mercury distribution in the global environment, and in the formulation of effective control strategies. Greer et al. (2006) review the global mercury market, and analyse scenarios for reductions in global supply and demand. The GMA (UNEP, 2002) also provides an extensive overview of the current knowledge of mercury sources, and environmental impacts, and includes an estimate of total global emissions of mercury from anthropogenic sources for 1995. The estimate is based on data collected by Pacyna and Pacyna (2002), and is summarised in Table 1. Table 1 shows that the major anthropogenic sources of emissions of mercury to the atmosphere are stationary combustion, non-ferrous metal production, pig iron and steel production, cement production and waste disposal. Approximately 1900 tonnes of anthropo-
genic mercury were estimated to be emitted, an apparent decrease of 10% since 1990 (Pacyna and Pacyna, 2002). Table 1 shows that Australian emissions of mercury from stationary combustion were estimated to be 97.0 tonnes per year in 1995. This is more than 90% of the estimated emissions from stationary combustion for the whole of North America, and more than 50% of the estimated emissions from stationary combustion for Europe. In spite of the fact that most of Australia’s stationary energy supply is provided by coal, this seems an abnormally high number for a country with a population of around 20 million. Mercury emissions from combustion sources have been recognised as a significant contributor to anthropogenic atmospheric emissions for many years (Carpi, 1997). There have been both previous and more recent estimates (Pirrone et al., 1996; Pacyna et al., 2006) of global mercury emissions than those reported in the GMA, and maps have been developed to depict the spatial distribution of emissions (Pacyna et al., 2003; Wilson et al., 2006). It is instructive to examine the range of estimates from Australian sources. Pirrone et al. (1996) reports estimates of global mercury emissions from 1983 to 1992; these authors do not explicitly identify Australian emissions, but they are presumably included in, and make up the largest part of, the Oceania category (Pirrone et al., 1996). An average coal mercury content of 0.2 mg g 1 is assumed based on available data, and 40% control efficiency is
Table 1 Global emissions of mercury from major anthropogenic sources in 1995 (Mg yr 1)a Continent
Europe Africa Asia North America South America Australiaa Oceaniaa TOTAL 1995 TOTAL 1990c a
Stationary combustion
Non-ferrous metal production
Pig iron and steel production
Cement production
185.5 197.0 860.4 104.8
15.4 7.9 87.4 25.1
10.2 0.5 12.1 4.6
26.2 5.2 81.8 12.9
26.9
25.4
1.4
5.5
97.0 2.9 1474.5 1295.1
4.4 — 165.6 394.4
0.3 — 29.1 28.4
0.7 0.1 132.4 114.5
Waste disposal
12.4 32.6 66.1
249.7 210.6 1074.3 213.5 59.2
0.1 — 111.2 139.0
Table from (Pacyna and Pacyna, 2002; UNEP, 2002) and personal communications with J. Pacyna. 325 tonnes of mercury emissions from gold production is not included (450% assumed to occur in Africa). c Estimates of maximum values, which were regarded as close to the best valuea. b
Total
102.5 3.0 1912.8b 2143.1d
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assumed from coal-fired plants in the developed countries, which would include Australia. Given those assumptions, Pirrone et al. (1996) estimate Oceania emissions from coal combustion of 6.3 and 8.6 Mg yr 1 in 1983 and 1992, respectively. By contrast in the most recent global inventory (Pacyna et al., 2006) emissions from Australian stationary combustion sources are explicitly identified at 109.6 ton in 2000, and it is claimed that Australia is the fifth largest emitter country. In this paper, estimates for anthropogenic mercury emissions in Australia are examined, based on both a top-down and bottom-up approach. Australia is the largest exporter of coals, and this, together with its reliance on coal combustion as an electricity source makes an accurate estimate of emissions of more than local interest. 2. Australian anthropogenic mercury emissions 2.1. Estimates based on the National Pollutant Inventory (NPI) Electricity production in Australia is largely derived from black and brown coal combustion (84–86% of total electricity generated in 2003–2004 (ESAA, 2005; NEMMCO, 2005)). Australian black coals are predominately bituminous coals, with varying volatile matter and ash content. Most are of Permian age (about 250 million years old), but there are also important deposits of lower-rank, younger Triassic, Jurassic and Cretaceous age coals. Power production in the states of New South Wales and Queensland largely use black coal combustion for power production. Australian brown coals are Tertiary in age and range from about 15–50 million years old. Brown coals are lower rank coals similar to US lignites, but with very high water contents. The state of Victoria derives most of its power from combustion of brown coals. Mining and mineral processing are also significant activities in Australia. As a consequence, potential point sources of mercury emission in Australia are coal-fired power stations, smelting and mineral processing, and iron and steel production. Reporting of point source emissions in Australia has been mandated since 1998, under the National Pollutant Inventory (NPI). The NPI, which is administered by the Department of Environment and Heritage (DEH), includes data on 90 substances released to the environment. It includes estimation of some area sources, in addition to point source
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emissions reported by industry. Details of the process are given on the NPI website (DEH, 2006). Data for point source emissions of mercury collated from the 2004–2005 NPI reports are given in Table 2. This compilation includes all mercury sources with emissions of greater than 5 kg yr 1, which together account for some 98% of total point source emissions. It is derived from the facility or plant data reported by individual emitters, and use of this data allows distinguishing brown and black coal power station sources. Some small differences are noted between the values reported in Table 2 and the consolidated values reported on the NPI website if a substance search is performed for 2004–2005 and ‘‘mercury & compounds’’ are chosen as the substance (see http://www.npi.gov.au/cgi-bin/ npidbsearch.pl?proc=substance). These differences are related to round off and the omission in the current study of sources emitting less than 5 kg yr 1. The calculated total emission from stationary energy sources (1.1 Mg yr 1) is therefore significantly less than the 97 Mg yr 1 quoted in the Global Mercury Assessment (UNEP, 2002). By contrast, the GMA estimate of Australian emissions from non ferrous metal production of 4.4 Mg yr 1 is less than the value of 11.6 Mg yr 1 from smelting and mineral processing, which is included in the NPI.
Table 2 Emissions of mercury to atmosphere from Australian point sources (45 kg yr 1) as reported in the Australian National Pollutant Inventorya Source category
Emissions to atmosphere (Mg yr 1)
Percentage of total point source emissions
Stationary power—Total Stationary power—Black coal Stationary power—Brown coal Smelting/mineral processing Iron and steel production Petroleum refining Cement Waste Coal mining Other mining
1.1 0.98 0.10 11.6 0.8 0.3 0.3 0.2 0.02 0.3
7.3 6.5 0.7 77 5.3 2.0 2.0 1.3 0.1 2.0
Total (sources greater than 5 kg yr 1) Total (all point sources)
14.8
98
15.1
a Data from NPI reports for 2004–2005 (available at http:// www.npi.gov.au/); all point sources with emissions greater than 5 kg yr 1 included.
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Inspection of the details of the NPI shows that the majority of this emission comes from facilities operated by Kalgoorlie Consolidated Gold Mines (KCGM), which operates a gold mining and processing operation in Western Australia. An emission of 7.7 Mg yr 1 has been reported from the gold ore roaster at KCGM, and this is the largest point source of mercury included in the NPI. The most recent global inventory (Pacyna et al., 2006) includes a figure of 7.7 tonnes for gold production from Australia. Inevitably, the data from a centralised reporting system such as the NPI is subject to uncertainty at a number of levels. Reporting is performed on the basis of workbooks based on world’s best practice in emissions estimation, and negotiated between government and industry. In many cases stack gas or plant measurements of emissions are not performed, but emission factors are used to calculate emissions. In the case of coal combustion some of these emission estimation techniques can only be used if contemporaneous measurements of coal mercury content are made. There is, however, limited validation or auditing of the NPI reporting system, and there are concerns with the completeness of the data reported. A recent review of the system (available at: http://www.npi.gov.au/about/ review/pubs/npi-review290405.pdf) recognises some of these deficiencies, noting that the NPI is ‘‘over reliant on American emission factors data’’, and has ‘‘suspected low capture rate of potential industry reporters, possibly around 50% or higher of potential’’. It is also important to examine some of the assumptions included in the NPI concerning emissions of mercury from coal-fired plant. The NPI workbook for coal-fired plant, recommends emission factors for mercury and compounds which are either based on generic emission factors (kg Hg per tonne of coal burnt) or on input concentrations of mercury in the coal feed, and reductions based on capture efficiencies for mercury in particulate control devices, either fabric filters (FFs) or electrostatic precipitators (ESPs). Australian power plants are not equipped with additional air pollution control devices such as wet scrubbers, or selective catalytic reduction (SCR) systems which might reduce emissions further (Pavlish et al., 2003). The capture efficiencies for the particulate control devices are based on USEPA data, and the implications of these assumptions are discussed below.
2.2. Estimates based on coal consumption and coal mercury content Emissions can also be estimated by a top-down approach for some point sources of emission. An upper limit for mercury emissions from coal-fired power stations and other coal using sources such as iron and steel manufacturing may be determined using data for coal consumption and coal mercury content. A summary of the results of these estimates is given in Table 3. Sources of the data used are given in the footnotes to Table 3. In brief, data on black and brown coal consumption for electricity, iron and steel, and cement production is derived from ABARE (2005); mercury contents in black coal are derived from extensive measurements of the composition of 100 Australian black coals (Dale, 2003; Riley et al., 2005), and of brown coals (Brockway et al., 1991). Riley et al. (2005) report an average concentration in black coals of 0.04 mg kg 1 as received basis, with a range of 0.02–0.13 mg kg 1, while Dale (2003) reports an average of 0.045 mg kg 1, with a range of 0.01–0.11 mg kg 1. In the calculations presented in Table 3 a value of 0.05 mg kg 1 has been assumed, and the impact of uncertainty in this value is discussed below. A coal mercury content of 0.05 mg kg 1 is low by world standards (Swaine, 1990; Dale, 2003; Riley et al., 2005), but is supported by an extensive database of Australian and world coal analysed for comparison (Dale, 2003; Riley et al., 2005). It is probably a consequence of the low S content in Australian coals, particularly the pyritic S, since mercury is most likely associated with this form of S (Swaine, 1990). In the case of the brown coals the mercury contents quoted by (Brockway et al., 1991) are on a dry basis, and average 0.08 ppm (mg kg 1). They have been corrected for the present purposes using a typical value for water contents in Australian brown coals and residual water content after drying. Values of mean moisture content reported by Allardice (1991), range from 1–2 kg kg 1 dry coal for the brown coal fields used in Australia’s major brown coal-fired power stations. For the current purposes a representative value of 1.5 kg kg 1 dry coal is assumed, and a value of 0.032 ppm Hg (as received) calculated. An uncertainty of 50% is assumed for the mercury content of the coal, and will be discussed further below. It is assumed that all mercury in the coal is released to the atmosphere
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Table 3 Summary of mercury emission estimates from Australian coal-related point sources based on top-down estimate and comparisons with NPI and GMA estimates Black coal
Coal consumption (Mt)a Mercury content (g tonne 1)b Atmospheric emissions (Mg yr 1) Total electricity generation (Mg yr 1) NPI estimates (Mg yr 1) GMA estimate (Mg yr 1)
Brown coal
Iron and steel
Cement industry
Other consumers
Electricity generation
Electricity generation
5.28 0.05 0.26
0.90 0.05 0.045
4.55 0.05 0.23
58.53 0.05 2.9
66.33 0.032 2.1
0.8
0.4
NA
0.98
5.0 0.10 97c
a
Data from ABARE (2005). Data for black coals (Dale, 2003; Riley et al., 2005); for brown coals (Brockway et al., 1991), as described in the text. c Includes contribution from oil and gas combustion; see text for discussion. b
Table 4 Emission estimate techniques for mercury used in the Australian NPI Coal and particle capture type
Emission estimate technique (kg tonne 1) For cases where coal mercury content is C (mg/kg, as received basis), and is measured during reporting period
Emission estimate technique (kg tonne 1) If no mercury measurements are made
Black coal stations with electro-static precipitators Black coal stations with fabric filters Brown coal stations (only use ESPs)
C 5.4 E 04 C 1.7 E 04 C 9.8E 04
4.2E 05 4.2E 05 1.6E 06
from each of these industrial processes, an assumption which will be further examined below. In Table 3, the NPI data includes all sources reporting, and some small differences in the data presented in Table 2 (where only sources with emissions greater than 5 kg yr 1), and that in Table 3 are a consequence of this difference. Inspection of the data in Table 3 reveals some significant differences between the estimates of mercury emissions based on the NPI, and the top down approach. So, for example, the NPI estimate of 0.8 Mg yr 1 for iron and steel production is greater than that based on the top-down approach (0.26 Mg yr 1), but in reasonable agreement. By contrast, the NPI reported values for stationary energy combustion (largely coal) are significantly lower than the estimates based on the top-down approach, particularly in the case of brown coal. Potential reasons for the differences in estimates for the stationary energy emissions will now be explored. The top down approach assumes 100% release to atmosphere of input coal mercury. However, there is extensive evidence (Pavlish et al., 2003) that existing air pollution control
devices (APCDs) on coal-fired power stations remove significant amounts of mercury, and the estimation techniques in the NPI include allowance for this mercury capture in APCDs, provided that coal mercury content is measured. The equations used for calculating mercury emissions in the NPI are given in Table 4, and are of two types. The more accurate version is that where mercury contents of feed coals are measured during the reporting period, and used to estimate emissions. In that case, mercury capture efficiencies for various cases are assigned to different types of coals and particle capture devices. Hence it is assumed that black coal fired in stations with ESPs have mercury capture efficiencies of 46%; black coal with FFs, capture efficiencies of 83%; and brown coal stations which only use ESPs, capture efficiencies of only 1.5%. These capture efficiencies are based on extensive measurements at many US coal-fired plants (USEPA, 2001). There is some question whether they will be strictly applicable to Australian plants as combustion controls to reduce NOx in US power stations often produce an ash relatively high in unburnt carbon. Carbon may be an effective
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sorbent for mercury capture and will result in potentially lower emissions (Pavlish et al., 2003). Carbon in ash tends to be lower in Australian plants, and hence mercury capture efficiencies may also be lower. For the purposes of this exercise uncertainties in capture efficiencies of 50% are assumed, and will be discussed below. The second emission estimate technique is a generic emission factor, again based on US data, for situations where coal mercury content is not measured. In this case, emission factors of 4.2 10 5 kg tonne 1 may be used for black coals, and 1.6 10 6 kg tonne 1 for brown coals. Hence for black and brown coals of similar mercury content, reported emissions for brown coals would be a factor of approximately 26 times lower if this reporting technique were to be used. This leads to a significant anomaly in the NPI, since existing evidence (USEPA, 2001) suggests that particle capture devices are significantly more efficient in capturing mercury from bituminous (black) coal combustion than from lignites, as oxidation of elemental mercury to oxidised forms which are more easily captured is significantly more facile in bituminous coals. We now consider the numbers reported in the NPI, and those from the top-down estimate, in the light of these capture efficiencies. For black coal power stations, the NPI estimates 0.98 Mg yr 1, and the top-down estimate is 2.9 Mg yr 1. Installed capacity of major black coal fired power plants in Australia (ESAA, 2005) shows that approximately 57% of total installed capacity (MW) is equipped with FFs, and 43% with ESPs. If these numbers are combined with the mercury capture efficiencies used in the NPI, an overall capture efficiency of 67% for Australian black coal fired plants is estimated. Hence the NPI estimate of 0.98 Mg yr 1, and the top-down estimate of 2.9 Mg yr 1, can be reconciled. This result also suggests that the black coal fired plants are largely reporting their emissions based on the preferable method of measuring mercury concentrations in the feed coals, rather than on the basis of the generic emission factor (4.2 10 5 kg tonne 1), as this would give total emissions, based on the total black coal use in power production of 58.53 Mton yr 1, of 2.5 tonnes yr 1. The situation is rather different for the brown coal-fired plants. For these power stations, the NPI estimate is 0.10 Mg yr 1, and the top down estimate is 2.1 Mg yr 1. These numbers cannot be
reconciled by use of the mercury capture efficiencies since the existing data for lignites (similar to Australian brown coals), suggests very low capture for these coals in ESPs. Instead the very low estimate of 0.10 Mg yr 1 suggests that the brown coal fired plants are largely reporting their emissions based the NPI generic emission factor (1.6 10 6 kg tonne 1), as this would give total emissions, based on the total brown coal use in power production of 66.63 Mtonne yr 1, of 0.106 Mg yr 1. This estimate is in good agreement with the NPI estimate, but is difficult to justify, unless Australian brown coals behave quite differently from similar lignites for which extensive measurements have been made. In fact, the reported NPI estimate suggests capture of 96% of the mercury from Australian brown coal fired plant, which seems unlikely. Victorian brown coals from the Gippsland basin (the major power production area) have chlorine contents which range from 0.04% to 0.35% dry basis) (Brockway and Higgins, 1991). These are significantly higher than many US lignites: Pavlish et al. (2003) report average Cl concentrations of 0.0139% (dry basis) for Fort Union lignites, and 0.0221% for Gulf Coast lignites. The higher Cl content in the Australian brown coals may lead to more extensive oxidation of elemental to oxidised mercury, but significant emission from the ESP equipped brown coal fired power stations will still be expected. It is concluded that the NPI provides a good basis for the estimation of mercury emissions from black coal power plants in Australia, but is likely to significantly underestimate emissions from brown coal fired stations if the simple generic emission factor is used. 3. Discussion and conclusions Estimates such as those described above are probably best expressed as a range of values which includes some measure of uncertainty. Here we concentrate on the uncertainty in the coal mercury contents, and in the capture efficiencies in the particulate control devices. Uncertainties in the total amounts of coal used in power generation are likely to be significantly smaller than these uncertainties. As noted, coal mercury contents in black coals are based on data for 100 Australian coals reported by workers at CSIRO (Dale, 2003; CSIRO, 2005; Riley et al., 2005). The agreement between the top
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down and NPI estimates for black coal suggests that 0.05 mg kg 1 is close to the value being measured for coal fed to these stations. Additional evidence for this estimate is provided by a survey of mercury contents in the feed coals to eight Australian power stations (Attalla et al., 2004), which revealed a mean of 0.0470.015 (one standard deviation) mg kg 1. One station in this survey had significantly higher concentrations but this analysis was atypical of mercury concentrations at this plant and has been excluded from this calculation. These results for mercury content suggest an uncertainty of around 750% for the coal mercury contents. It is likely that uncertainties in brown coal mercury content will be similar. There are also uncertainties associated with the capture efficiencies, here assumed to also be of the order of 50%. On this basis, a resulting overall uncertainty in mercury emissions of 775% seems reasonable; it is significantly higher than the 725% quoted as an ‘‘accuracy value’’ for fuel combustion by Pacyna et al. (2006). An estimate of total mercury emissions from Australian coal fired plant of 2–8 tonnes yr 1 would then appear reasonable, and would include the uncertainties associated with coal mercury content and capture efficiencies. This is larger than the NPI estimate of 1.1 Mg yr 1, largely because of the issues with brown coal fired plant discussed above, but very significantly lower than the GMA estimate of 97 tonnes yr 1 for stationary combustion sources. It is however in relatively good agreement with the earlier estimate of 6.3–8.6 Mg yr 1 quoted by Pirrone et al. (1996) for emissions between 1983 and 1992. The GMA estimate is for 1995 and the current discussion is for 2004/2005. However, coal use has increased significantly in that time (black coal use in power production in 1994/95 was 42.82 Mtonne; and brown coal use 48.72 Mtonne (ABARE, 2005)), and, hence, this difference in time does not account for the discrepancy. The latest version of the global mercury inventory reports emissions of 109.6 Mg yr 1 from Australian stationary combustion sources are also much higher than the estimate in Pirrone et al. (1996), or in the current work. Finally, we discuss possible reasons for this difference. The GMA is for total emissions, and will presumably include a contribution from oil and gas combustion. ABARE (2006) report fuel oil production in Australia in 2004/2005 of 1595 ML. Pacyna and Pacyna (2002) report a Hg emission factor for oil combustion of 0.06 g tonne 1 oil, and
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Pacyna et al. (2006) one of 0.0006 g tonne 1 oil. The latter appears too low and may be an error in the 2006 paper. Assuming an emission factor of 0.06 g tonne 1 oil and a density of 0.9 g cm 3, a potential total emission of 86,100 g yr 1 (or 0.086 Mg yr 1) for oil combustion from stationary sources may also contribute to the stationary combustion source. Pacyna et al. (2006) also state that ‘‘it is believed that mercury emissions during natural gas combustion are insignificant’’. Hence differences in the source categories included in the GMA estimate and the current one for stationary sources from coal combustion are unlikely to account for the discrepancy. The differences between the emissions estimated in this work and those in the GMA may be due to the use of differing emission factors. However, they do highlight the need to use a range of emission estimation techniques, and to check these by both top down and bottom up calculations. Acknowledgements The author acknowledges the financial support provided by the Cooperative Research Centre for Coal in Sustainable Development, which is funded in part by the Cooperative Research Centres Program of the Commonwealth Government of Australia. References ABARE. Australian Commodity Statistics 2005. 2005 [cited 18th April 2006]; Available from: /http://www.abare.gov.au/ ACS_Mini_Site/pdf/acs_minerals.pdfS. ABARE. Australian Energy Statistics 2006. 2006 [cited 10th October 2006]; Available from: /http://www.abareconomics. com/interactive/energy/excel.htmS. Allardice, D.J., 1991. The water in brown coal. In: Durie, R.A. (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization. ButterworthHeinemann Ltd., Oxford pp. 103–150. Attalla, M.I., Malfroy, H.R., Morgan, S., Riley, K.R., Nelson, P.F., 2004. Hazardous pollutants in power station emissions, Cooperative research Centre for Coal in Sustainable Development. Research Report 51, p. 56. Brockway, D.J., Higgins, R.S., 1991. Brown coal sampling, analysis and composition. In: Durie, R.A. (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization. Butterworth-Heinemann Ltd., Oxford pp. 247–278. Brockway, D.J., Ottrey, A.L., Higgins, R.S., 1991. Inorganic constituents. In: Durie, R.A. (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization. Butterworth-Heinemann Ltd, Oxford pp. 598–650.
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Carpi, A., 1997. Mercury from combustion sources: a review of the chemical species emitted and their transport in the atmosphere. Water Air and Soil Pollution 98, 241–254. CSIRO. Trace Elements in Australian Export Thermal Coals. Fact sheets prepared as a joint initiative of the Australian Coal AssociationResearch Program, CSIRO Energy Technology and the CRC for Coal in Sustainable Development. 2005 [cited 1st May 2006]; Available from: /http://www.det.csiro. au/science/e_e/Clean%20Air/trace_elements.htmS. Dale, L.S., 2003. Review of Trace Elements in Coal, Australian Coal Association Research Program. Project Number C11020, p. 65. DEH. National Pollutant Inventory. Australia’s national database of pollutant emissions. 2006 [cited 2006 18th April 2006]; Available from: /www.npi.gov.auS. ESAA, 2005. Electricity Gas Australia 2005. Energy Supply Association of Australia. Fitzgerald, W.F., Mason, R.P., Vandal, G.M., 1991. Atmospheric cycle and air-water exchange of mercury over midcontinental lacustrine regions. Water, Air and Soil Pollution 56, 745–767. Fitzgerald, W.F., Engstrom, D.R., Mason, R.P., Nater, E.A., 1998. The case for atmospheric mercury contamination in remote areas. Environmental Science & Technology 32, 1–7. Greer, L., Bender, M., Maxson, P., Lennett, D., 2006. Curtailing Mercury’s Global Reach. In: Starke, L. (Ed.), State of the World 2006. A Worldwatch Institute Report on Progress Toward a Sustainable Society. W.W. Norton & Company, New York pp. 96–114. Jackson, T.A., 1997. Long-range atmospheric transport of mercury to ecosystems, and the importance of anthropogenic emissions—a critical review and evaluation of the published evidence. Environmental Reviews 5, 99–120. Lindquist, O., Johansson, K., Aastrup, M., Andersson, A., Bringmark, L., Hovsenius, G., Hasansson, L., Iverfeldt, A., Meili, M., Timm, B., 1991. Water Air and Soil Pollution 55, 1–261. Meili, M., Bishop, K., Bringmark, L., Johansson, K., Munthe, J., Sverdrup, H., de Vries, W., 2003. Critical levels of atmospheric pollution: criteria and concepts for operational modelling of mercury in forest and lake ecosystems. The Science of The Total Environment 304, 83–106. NEMMCO. An introduction to Australia’s national electricity market. 2005 [cited 2006 18th April 2006]; Available from: /http://www.nemmco.com.au/nemgeneral/000-0187.pdfS. Pacyna, E.G., Pacyna, J.M., 2002. Global emission of mercury from anthropogenic sources in 1995. Water Air and Soil Pollution 137, 149–165. Pacyna, J.M., Pacyna, E.G., Steenhuisen, F., Wilson, S., 2003. Mapping 1995 global anthropogenic emissions of mercury. Atmospheric Environment 37, S109–S117. Pacyna, E.G., Pacyna, J.M., Steenhuisen, F., Wilson, S., 2006. Global anthropogenic mercury emission inventory for 2000. Atmospheric Environment 40, 4048–4063. Pai, P., Karamchandani, P., Seigneur, C., 1997. Simulation of the regional atmospheric transport and fate of mercury using a comprehensive Eulerian model. Atmospheric Environment 31, 2717–2732.
Pavlish, J.H., Sondreal, E.A., Mann, M.D., Olson, E.S., Galbreath, K.C., Laudal, D.L., Benson, S.A., 2003. Status review of mercury control options for coal-fired power plants. Fuel Processing Technology 82, 89–165. Petersen, G., Iverfeldt, A., Munthe, J., 1995. Atmospheric mercury species over central and northern europe—modelcalculations and comparison with observations from the Nordic air and precipitation network for 1987 and 1988. Atmospheric Environment 29, 47–67. Petersen, G., Bloxam, R., Wong, S., Munthe, J., Kruger, O., Schmolke, S.R., Kumar, A.V., 2001. A comprehensive Eulerian modelling framework for airborne mercury species: model development and applications in Europe 35, 3063–3074. Pirrone, N., Keeler, G.J., Nriagu, J.O., 1996. Regional differences in worldwide emissions of mercury to the atmosphere. Atmospheric Environment 30, 2981–2987. Pirrone, N., Mahaffey, K.R. (Eds.), 2005. Dynamics of Mercury Pollution on Regional and Global Scales: Atmospheric Processes and Human Exposures Around the World. Springer, Berlin p. 744. Riley, K., Dale, L., Devir, G., Williams, A., Holcolmbe, D., 2005. Background information for web site on trace elements in coal, Australian Coal Association Research Program. Project number C12060, p. 55. Swaine, D.J., 1990. Trace elements in coal. Boston: Butterworth, London. UNEP. Global Mercury Assessment Report. 2002 [cited; Available from: /http://www.chem.unep.ch/mercury/Report/ Final%20report/final-assessment-report-25nov02.pdfS. USEPA. Emergency Planning and Community Right-to-know Act Section 313: Guidance for Reporting Toxic Chemicals: Mercury and Mercury Compounds Category. 2001 [cited 2006 1st May 2006]; Available from: /http://www.epa.gov/ tri/guide_docs/2001/hg2001.pdfS. Wangberg, I., Munthe, J., Pirrone, N., Iverfeldt, A., Bahlman, E., Costa, P., Ebinghaus, R., Feng, X., Ferrara, R., Gardfeldt, K., Kock, H., Lanzillotta, E., Mamane, Y., Mas, F., Melamed, E., Osnat, Y., Prestbo, E., Sommar, J., Schmolke, S., SpaiJJn, G., Sprovieri, F., Tuncel, G., 2001. Atmospheric mercury distribution in Northern Europe and in the Mediterranean region. Atmospheric Environment 35, 3019–3025. Wilson, S.J., Steenhuisen, F., Pacyna, J.M., Pacyna, E.G., 2006. Mapping the spatial distribution of global anthropogenic mercury atmospheric emission inventories. Atmospheric Environment 40, 4621–4632. Xu, X.H., Yang, X.S., Miller, D.R., Helble, J.J., Carley, R.J., 2000. A regional scale modeling study of atmospheric transport and transformation of mercury. II. Simulation results for the northeast United States. Atmospheric Environment 34, 4945–4955. Xu, X.H., Yang, X.S., Miller, D.R., Helble, J.J., Carley, R.J., 2000. A regional scale modeling study of atmospheric transport and transformation of mercury. I. Model development and evaluation. Atmospheric Environment 34, 4933–4944.
Atmospheric Environment 41 (2007) 1717–1724 www.elsevier.com/locate/atmosenv
Atmospheric emissions of mercury from Australian point sources Peter F. Nelson Graduate School of the Environment, Co-operative Research Centre for Coal in Sustainable Development, Macquarie University, Sydney 2109, Australia Received 19 May 2006; received in revised form 11 October 2006; accepted 17 October 2006
Abstract The UN Global Mercury Assessment (GMA) estimates that atmospheric emissions of mercury from Australian stationary combustion sources were 97.0 tonnes for the year of 1995. This is more than 90% of the estimated emissions from stationary combustion for the whole of North America, and seems abnormally high for a country with a population of around 20 million, in spite of the fact that most of Australia’s stationary energy supply is provided by coal. It is also significantly larger than previous estimates of mercury emissions from Australian sources. New estimates of Australian mercury emissions from stationary energy sources, based on both a top down and bottom up approach, are presented. These estimates can be reconciled for black coal fired power stations, but suggest that the bottom up approach (the Australian National Pollutant Inventory) significantly under-estimates emissions from brown coal fired plant, if mercury capture efficiencies in these plants are low, as observed for lignite-fired plant. The major uncertainties in these estimates are the coal mercury content in coals burnt in Australian power stations, and the mercury capture efficiency in particulate control devices used at these stations. Based on these estimates, Australian emissions of mercury from stationary energy are currently 2–8 tonnes/year, significantly lower than the GMA estimate. r 2006 Elsevier Ltd. All rights reserved. Keywords: Mercury; Coal combustion; Emission Inventory
1. Introduction Mercury is a potentially toxic trace metal found throughout the terrestrial, aquatic and marine ecosystems, as well as in the atmosphere. Complex processes of emission, transport, chemical transformation and storage are responsible for this ubiquity (UNEP, 2002). The atmosphere is recognised to be the dominant medium for transport of mercury in the environment (Fitzgerald et al., 1991; Lindquist Tel.: +61 2 98506958; fax: +61 2 9850 7972.
E-mail address: [email protected]. 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.10.029
et al., 1991), and there is considerable evidence (Petersen et al., 1995; Jackson, 1997; Pai et al., 1997; Fitzgerald et al., 1998; Xu et al., 2000, 2000; Petersen et al., 2001; Wangberg et al., 2001; Pirrone and Mahaffey, 2005) for a connection between anthropogenic mercury emissions and observed concentrations in remote regions, such as the Artic, due to long-range transport of elemental mercury from industrial areas. Hence, mercury is a global problem which may not only have local environmental impacts near point sources of emission, but also in remote locations. Mercury concentrates in the environment
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by bio-accumulation particularly in fish, and human exposure is mainly through dietary intake. The UN global mercury assessment (GMA) (UNEP, 2002) concluded ‘‘there is sufficient evidence of significant global adverse impacts from mercury to warrant further international action to reduce the risks to humans and wildlife from the release of mercury to the environment’’. In the view of some researchers, release of any mercury from anthropogenic sources which will lead to increases in the global pool, should be avoided since there is already evidence for significant impacts (Meili et al., 2003). As a global pollutant, an accurate knowledge of the amounts of mercury released worldwide and from all sources is an important input into models of mercury distribution in the global environment, and in the formulation of effective control strategies. Greer et al. (2006) review the global mercury market, and analyse scenarios for reductions in global supply and demand. The GMA (UNEP, 2002) also provides an extensive overview of the current knowledge of mercury sources, and environmental impacts, and includes an estimate of total global emissions of mercury from anthropogenic sources for 1995. The estimate is based on data collected by Pacyna and Pacyna (2002), and is summarised in Table 1. Table 1 shows that the major anthropogenic sources of emissions of mercury to the atmosphere are stationary combustion, non-ferrous metal production, pig iron and steel production, cement production and waste disposal. Approximately 1900 tonnes of anthropo-
genic mercury were estimated to be emitted, an apparent decrease of 10% since 1990 (Pacyna and Pacyna, 2002). Table 1 shows that Australian emissions of mercury from stationary combustion were estimated to be 97.0 tonnes per year in 1995. This is more than 90% of the estimated emissions from stationary combustion for the whole of North America, and more than 50% of the estimated emissions from stationary combustion for Europe. In spite of the fact that most of Australia’s stationary energy supply is provided by coal, this seems an abnormally high number for a country with a population of around 20 million. Mercury emissions from combustion sources have been recognised as a significant contributor to anthropogenic atmospheric emissions for many years (Carpi, 1997). There have been both previous and more recent estimates (Pirrone et al., 1996; Pacyna et al., 2006) of global mercury emissions than those reported in the GMA, and maps have been developed to depict the spatial distribution of emissions (Pacyna et al., 2003; Wilson et al., 2006). It is instructive to examine the range of estimates from Australian sources. Pirrone et al. (1996) reports estimates of global mercury emissions from 1983 to 1992; these authors do not explicitly identify Australian emissions, but they are presumably included in, and make up the largest part of, the Oceania category (Pirrone et al., 1996). An average coal mercury content of 0.2 mg g 1 is assumed based on available data, and 40% control efficiency is
Table 1 Global emissions of mercury from major anthropogenic sources in 1995 (Mg yr 1)a Continent
Europe Africa Asia North America South America Australiaa Oceaniaa TOTAL 1995 TOTAL 1990c a
Stationary combustion
Non-ferrous metal production
Pig iron and steel production
Cement production
185.5 197.0 860.4 104.8
15.4 7.9 87.4 25.1
10.2 0.5 12.1 4.6
26.2 5.2 81.8 12.9
26.9
25.4
1.4
5.5
97.0 2.9 1474.5 1295.1
4.4 — 165.6 394.4
0.3 — 29.1 28.4
0.7 0.1 132.4 114.5
Waste disposal
12.4 32.6 66.1
249.7 210.6 1074.3 213.5 59.2
0.1 — 111.2 139.0
Table from (Pacyna and Pacyna, 2002; UNEP, 2002) and personal communications with J. Pacyna. 325 tonnes of mercury emissions from gold production is not included (450% assumed to occur in Africa). c Estimates of maximum values, which were regarded as close to the best valuea. b
Total
102.5 3.0 1912.8b 2143.1d
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assumed from coal-fired plants in the developed countries, which would include Australia. Given those assumptions, Pirrone et al. (1996) estimate Oceania emissions from coal combustion of 6.3 and 8.6 Mg yr 1 in 1983 and 1992, respectively. By contrast in the most recent global inventory (Pacyna et al., 2006) emissions from Australian stationary combustion sources are explicitly identified at 109.6 ton in 2000, and it is claimed that Australia is the fifth largest emitter country. In this paper, estimates for anthropogenic mercury emissions in Australia are examined, based on both a top-down and bottom-up approach. Australia is the largest exporter of coals, and this, together with its reliance on coal combustion as an electricity source makes an accurate estimate of emissions of more than local interest. 2. Australian anthropogenic mercury emissions 2.1. Estimates based on the National Pollutant Inventory (NPI) Electricity production in Australia is largely derived from black and brown coal combustion (84–86% of total electricity generated in 2003–2004 (ESAA, 2005; NEMMCO, 2005)). Australian black coals are predominately bituminous coals, with varying volatile matter and ash content. Most are of Permian age (about 250 million years old), but there are also important deposits of lower-rank, younger Triassic, Jurassic and Cretaceous age coals. Power production in the states of New South Wales and Queensland largely use black coal combustion for power production. Australian brown coals are Tertiary in age and range from about 15–50 million years old. Brown coals are lower rank coals similar to US lignites, but with very high water contents. The state of Victoria derives most of its power from combustion of brown coals. Mining and mineral processing are also significant activities in Australia. As a consequence, potential point sources of mercury emission in Australia are coal-fired power stations, smelting and mineral processing, and iron and steel production. Reporting of point source emissions in Australia has been mandated since 1998, under the National Pollutant Inventory (NPI). The NPI, which is administered by the Department of Environment and Heritage (DEH), includes data on 90 substances released to the environment. It includes estimation of some area sources, in addition to point source
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emissions reported by industry. Details of the process are given on the NPI website (DEH, 2006). Data for point source emissions of mercury collated from the 2004–2005 NPI reports are given in Table 2. This compilation includes all mercury sources with emissions of greater than 5 kg yr 1, which together account for some 98% of total point source emissions. It is derived from the facility or plant data reported by individual emitters, and use of this data allows distinguishing brown and black coal power station sources. Some small differences are noted between the values reported in Table 2 and the consolidated values reported on the NPI website if a substance search is performed for 2004–2005 and ‘‘mercury & compounds’’ are chosen as the substance (see http://www.npi.gov.au/cgi-bin/ npidbsearch.pl?proc=substance). These differences are related to round off and the omission in the current study of sources emitting less than 5 kg yr 1. The calculated total emission from stationary energy sources (1.1 Mg yr 1) is therefore significantly less than the 97 Mg yr 1 quoted in the Global Mercury Assessment (UNEP, 2002). By contrast, the GMA estimate of Australian emissions from non ferrous metal production of 4.4 Mg yr 1 is less than the value of 11.6 Mg yr 1 from smelting and mineral processing, which is included in the NPI.
Table 2 Emissions of mercury to atmosphere from Australian point sources (45 kg yr 1) as reported in the Australian National Pollutant Inventorya Source category
Emissions to atmosphere (Mg yr 1)
Percentage of total point source emissions
Stationary power—Total Stationary power—Black coal Stationary power—Brown coal Smelting/mineral processing Iron and steel production Petroleum refining Cement Waste Coal mining Other mining
1.1 0.98 0.10 11.6 0.8 0.3 0.3 0.2 0.02 0.3
7.3 6.5 0.7 77 5.3 2.0 2.0 1.3 0.1 2.0
Total (sources greater than 5 kg yr 1) Total (all point sources)
14.8
98
15.1
a Data from NPI reports for 2004–2005 (available at http:// www.npi.gov.au/); all point sources with emissions greater than 5 kg yr 1 included.
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Inspection of the details of the NPI shows that the majority of this emission comes from facilities operated by Kalgoorlie Consolidated Gold Mines (KCGM), which operates a gold mining and processing operation in Western Australia. An emission of 7.7 Mg yr 1 has been reported from the gold ore roaster at KCGM, and this is the largest point source of mercury included in the NPI. The most recent global inventory (Pacyna et al., 2006) includes a figure of 7.7 tonnes for gold production from Australia. Inevitably, the data from a centralised reporting system such as the NPI is subject to uncertainty at a number of levels. Reporting is performed on the basis of workbooks based on world’s best practice in emissions estimation, and negotiated between government and industry. In many cases stack gas or plant measurements of emissions are not performed, but emission factors are used to calculate emissions. In the case of coal combustion some of these emission estimation techniques can only be used if contemporaneous measurements of coal mercury content are made. There is, however, limited validation or auditing of the NPI reporting system, and there are concerns with the completeness of the data reported. A recent review of the system (available at: http://www.npi.gov.au/about/ review/pubs/npi-review290405.pdf) recognises some of these deficiencies, noting that the NPI is ‘‘over reliant on American emission factors data’’, and has ‘‘suspected low capture rate of potential industry reporters, possibly around 50% or higher of potential’’. It is also important to examine some of the assumptions included in the NPI concerning emissions of mercury from coal-fired plant. The NPI workbook for coal-fired plant, recommends emission factors for mercury and compounds which are either based on generic emission factors (kg Hg per tonne of coal burnt) or on input concentrations of mercury in the coal feed, and reductions based on capture efficiencies for mercury in particulate control devices, either fabric filters (FFs) or electrostatic precipitators (ESPs). Australian power plants are not equipped with additional air pollution control devices such as wet scrubbers, or selective catalytic reduction (SCR) systems which might reduce emissions further (Pavlish et al., 2003). The capture efficiencies for the particulate control devices are based on USEPA data, and the implications of these assumptions are discussed below.
2.2. Estimates based on coal consumption and coal mercury content Emissions can also be estimated by a top-down approach for some point sources of emission. An upper limit for mercury emissions from coal-fired power stations and other coal using sources such as iron and steel manufacturing may be determined using data for coal consumption and coal mercury content. A summary of the results of these estimates is given in Table 3. Sources of the data used are given in the footnotes to Table 3. In brief, data on black and brown coal consumption for electricity, iron and steel, and cement production is derived from ABARE (2005); mercury contents in black coal are derived from extensive measurements of the composition of 100 Australian black coals (Dale, 2003; Riley et al., 2005), and of brown coals (Brockway et al., 1991). Riley et al. (2005) report an average concentration in black coals of 0.04 mg kg 1 as received basis, with a range of 0.02–0.13 mg kg 1, while Dale (2003) reports an average of 0.045 mg kg 1, with a range of 0.01–0.11 mg kg 1. In the calculations presented in Table 3 a value of 0.05 mg kg 1 has been assumed, and the impact of uncertainty in this value is discussed below. A coal mercury content of 0.05 mg kg 1 is low by world standards (Swaine, 1990; Dale, 2003; Riley et al., 2005), but is supported by an extensive database of Australian and world coal analysed for comparison (Dale, 2003; Riley et al., 2005). It is probably a consequence of the low S content in Australian coals, particularly the pyritic S, since mercury is most likely associated with this form of S (Swaine, 1990). In the case of the brown coals the mercury contents quoted by (Brockway et al., 1991) are on a dry basis, and average 0.08 ppm (mg kg 1). They have been corrected for the present purposes using a typical value for water contents in Australian brown coals and residual water content after drying. Values of mean moisture content reported by Allardice (1991), range from 1–2 kg kg 1 dry coal for the brown coal fields used in Australia’s major brown coal-fired power stations. For the current purposes a representative value of 1.5 kg kg 1 dry coal is assumed, and a value of 0.032 ppm Hg (as received) calculated. An uncertainty of 50% is assumed for the mercury content of the coal, and will be discussed further below. It is assumed that all mercury in the coal is released to the atmosphere
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Table 3 Summary of mercury emission estimates from Australian coal-related point sources based on top-down estimate and comparisons with NPI and GMA estimates Black coal
Coal consumption (Mt)a Mercury content (g tonne 1)b Atmospheric emissions (Mg yr 1) Total electricity generation (Mg yr 1) NPI estimates (Mg yr 1) GMA estimate (Mg yr 1)
Brown coal
Iron and steel
Cement industry
Other consumers
Electricity generation
Electricity generation
5.28 0.05 0.26
0.90 0.05 0.045
4.55 0.05 0.23
58.53 0.05 2.9
66.33 0.032 2.1
0.8
0.4
NA
0.98
5.0 0.10 97c
a
Data from ABARE (2005). Data for black coals (Dale, 2003; Riley et al., 2005); for brown coals (Brockway et al., 1991), as described in the text. c Includes contribution from oil and gas combustion; see text for discussion. b
Table 4 Emission estimate techniques for mercury used in the Australian NPI Coal and particle capture type
Emission estimate technique (kg tonne 1) For cases where coal mercury content is C (mg/kg, as received basis), and is measured during reporting period
Emission estimate technique (kg tonne 1) If no mercury measurements are made
Black coal stations with electro-static precipitators Black coal stations with fabric filters Brown coal stations (only use ESPs)
C 5.4 E 04 C 1.7 E 04 C 9.8E 04
4.2E 05 4.2E 05 1.6E 06
from each of these industrial processes, an assumption which will be further examined below. In Table 3, the NPI data includes all sources reporting, and some small differences in the data presented in Table 2 (where only sources with emissions greater than 5 kg yr 1), and that in Table 3 are a consequence of this difference. Inspection of the data in Table 3 reveals some significant differences between the estimates of mercury emissions based on the NPI, and the top down approach. So, for example, the NPI estimate of 0.8 Mg yr 1 for iron and steel production is greater than that based on the top-down approach (0.26 Mg yr 1), but in reasonable agreement. By contrast, the NPI reported values for stationary energy combustion (largely coal) are significantly lower than the estimates based on the top-down approach, particularly in the case of brown coal. Potential reasons for the differences in estimates for the stationary energy emissions will now be explored. The top down approach assumes 100% release to atmosphere of input coal mercury. However, there is extensive evidence (Pavlish et al., 2003) that existing air pollution control
devices (APCDs) on coal-fired power stations remove significant amounts of mercury, and the estimation techniques in the NPI include allowance for this mercury capture in APCDs, provided that coal mercury content is measured. The equations used for calculating mercury emissions in the NPI are given in Table 4, and are of two types. The more accurate version is that where mercury contents of feed coals are measured during the reporting period, and used to estimate emissions. In that case, mercury capture efficiencies for various cases are assigned to different types of coals and particle capture devices. Hence it is assumed that black coal fired in stations with ESPs have mercury capture efficiencies of 46%; black coal with FFs, capture efficiencies of 83%; and brown coal stations which only use ESPs, capture efficiencies of only 1.5%. These capture efficiencies are based on extensive measurements at many US coal-fired plants (USEPA, 2001). There is some question whether they will be strictly applicable to Australian plants as combustion controls to reduce NOx in US power stations often produce an ash relatively high in unburnt carbon. Carbon may be an effective
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sorbent for mercury capture and will result in potentially lower emissions (Pavlish et al., 2003). Carbon in ash tends to be lower in Australian plants, and hence mercury capture efficiencies may also be lower. For the purposes of this exercise uncertainties in capture efficiencies of 50% are assumed, and will be discussed below. The second emission estimate technique is a generic emission factor, again based on US data, for situations where coal mercury content is not measured. In this case, emission factors of 4.2 10 5 kg tonne 1 may be used for black coals, and 1.6 10 6 kg tonne 1 for brown coals. Hence for black and brown coals of similar mercury content, reported emissions for brown coals would be a factor of approximately 26 times lower if this reporting technique were to be used. This leads to a significant anomaly in the NPI, since existing evidence (USEPA, 2001) suggests that particle capture devices are significantly more efficient in capturing mercury from bituminous (black) coal combustion than from lignites, as oxidation of elemental mercury to oxidised forms which are more easily captured is significantly more facile in bituminous coals. We now consider the numbers reported in the NPI, and those from the top-down estimate, in the light of these capture efficiencies. For black coal power stations, the NPI estimates 0.98 Mg yr 1, and the top-down estimate is 2.9 Mg yr 1. Installed capacity of major black coal fired power plants in Australia (ESAA, 2005) shows that approximately 57% of total installed capacity (MW) is equipped with FFs, and 43% with ESPs. If these numbers are combined with the mercury capture efficiencies used in the NPI, an overall capture efficiency of 67% for Australian black coal fired plants is estimated. Hence the NPI estimate of 0.98 Mg yr 1, and the top-down estimate of 2.9 Mg yr 1, can be reconciled. This result also suggests that the black coal fired plants are largely reporting their emissions based on the preferable method of measuring mercury concentrations in the feed coals, rather than on the basis of the generic emission factor (4.2 10 5 kg tonne 1), as this would give total emissions, based on the total black coal use in power production of 58.53 Mton yr 1, of 2.5 tonnes yr 1. The situation is rather different for the brown coal-fired plants. For these power stations, the NPI estimate is 0.10 Mg yr 1, and the top down estimate is 2.1 Mg yr 1. These numbers cannot be
reconciled by use of the mercury capture efficiencies since the existing data for lignites (similar to Australian brown coals), suggests very low capture for these coals in ESPs. Instead the very low estimate of 0.10 Mg yr 1 suggests that the brown coal fired plants are largely reporting their emissions based the NPI generic emission factor (1.6 10 6 kg tonne 1), as this would give total emissions, based on the total brown coal use in power production of 66.63 Mtonne yr 1, of 0.106 Mg yr 1. This estimate is in good agreement with the NPI estimate, but is difficult to justify, unless Australian brown coals behave quite differently from similar lignites for which extensive measurements have been made. In fact, the reported NPI estimate suggests capture of 96% of the mercury from Australian brown coal fired plant, which seems unlikely. Victorian brown coals from the Gippsland basin (the major power production area) have chlorine contents which range from 0.04% to 0.35% dry basis) (Brockway and Higgins, 1991). These are significantly higher than many US lignites: Pavlish et al. (2003) report average Cl concentrations of 0.0139% (dry basis) for Fort Union lignites, and 0.0221% for Gulf Coast lignites. The higher Cl content in the Australian brown coals may lead to more extensive oxidation of elemental to oxidised mercury, but significant emission from the ESP equipped brown coal fired power stations will still be expected. It is concluded that the NPI provides a good basis for the estimation of mercury emissions from black coal power plants in Australia, but is likely to significantly underestimate emissions from brown coal fired stations if the simple generic emission factor is used. 3. Discussion and conclusions Estimates such as those described above are probably best expressed as a range of values which includes some measure of uncertainty. Here we concentrate on the uncertainty in the coal mercury contents, and in the capture efficiencies in the particulate control devices. Uncertainties in the total amounts of coal used in power generation are likely to be significantly smaller than these uncertainties. As noted, coal mercury contents in black coals are based on data for 100 Australian coals reported by workers at CSIRO (Dale, 2003; CSIRO, 2005; Riley et al., 2005). The agreement between the top
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down and NPI estimates for black coal suggests that 0.05 mg kg 1 is close to the value being measured for coal fed to these stations. Additional evidence for this estimate is provided by a survey of mercury contents in the feed coals to eight Australian power stations (Attalla et al., 2004), which revealed a mean of 0.0470.015 (one standard deviation) mg kg 1. One station in this survey had significantly higher concentrations but this analysis was atypical of mercury concentrations at this plant and has been excluded from this calculation. These results for mercury content suggest an uncertainty of around 750% for the coal mercury contents. It is likely that uncertainties in brown coal mercury content will be similar. There are also uncertainties associated with the capture efficiencies, here assumed to also be of the order of 50%. On this basis, a resulting overall uncertainty in mercury emissions of 775% seems reasonable; it is significantly higher than the 725% quoted as an ‘‘accuracy value’’ for fuel combustion by Pacyna et al. (2006). An estimate of total mercury emissions from Australian coal fired plant of 2–8 tonnes yr 1 would then appear reasonable, and would include the uncertainties associated with coal mercury content and capture efficiencies. This is larger than the NPI estimate of 1.1 Mg yr 1, largely because of the issues with brown coal fired plant discussed above, but very significantly lower than the GMA estimate of 97 tonnes yr 1 for stationary combustion sources. It is however in relatively good agreement with the earlier estimate of 6.3–8.6 Mg yr 1 quoted by Pirrone et al. (1996) for emissions between 1983 and 1992. The GMA estimate is for 1995 and the current discussion is for 2004/2005. However, coal use has increased significantly in that time (black coal use in power production in 1994/95 was 42.82 Mtonne; and brown coal use 48.72 Mtonne (ABARE, 2005)), and, hence, this difference in time does not account for the discrepancy. The latest version of the global mercury inventory reports emissions of 109.6 Mg yr 1 from Australian stationary combustion sources are also much higher than the estimate in Pirrone et al. (1996), or in the current work. Finally, we discuss possible reasons for this difference. The GMA is for total emissions, and will presumably include a contribution from oil and gas combustion. ABARE (2006) report fuel oil production in Australia in 2004/2005 of 1595 ML. Pacyna and Pacyna (2002) report a Hg emission factor for oil combustion of 0.06 g tonne 1 oil, and
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Pacyna et al. (2006) one of 0.0006 g tonne 1 oil. The latter appears too low and may be an error in the 2006 paper. Assuming an emission factor of 0.06 g tonne 1 oil and a density of 0.9 g cm 3, a potential total emission of 86,100 g yr 1 (or 0.086 Mg yr 1) for oil combustion from stationary sources may also contribute to the stationary combustion source. Pacyna et al. (2006) also state that ‘‘it is believed that mercury emissions during natural gas combustion are insignificant’’. Hence differences in the source categories included in the GMA estimate and the current one for stationary sources from coal combustion are unlikely to account for the discrepancy. The differences between the emissions estimated in this work and those in the GMA may be due to the use of differing emission factors. However, they do highlight the need to use a range of emission estimation techniques, and to check these by both top down and bottom up calculations. Acknowledgements The author acknowledges the financial support provided by the Cooperative Research Centre for Coal in Sustainable Development, which is funded in part by the Cooperative Research Centres Program of the Commonwealth Government of Australia. References ABARE. Australian Commodity Statistics 2005. 2005 [cited 18th April 2006]; Available from: /http://www.abare.gov.au/ ACS_Mini_Site/pdf/acs_minerals.pdfS. ABARE. Australian Energy Statistics 2006. 2006 [cited 10th October 2006]; Available from: /http://www.abareconomics. com/interactive/energy/excel.htmS. Allardice, D.J., 1991. The water in brown coal. In: Durie, R.A. (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization. ButterworthHeinemann Ltd., Oxford pp. 103–150. Attalla, M.I., Malfroy, H.R., Morgan, S., Riley, K.R., Nelson, P.F., 2004. Hazardous pollutants in power station emissions, Cooperative research Centre for Coal in Sustainable Development. Research Report 51, p. 56. Brockway, D.J., Higgins, R.S., 1991. Brown coal sampling, analysis and composition. In: Durie, R.A. (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization. Butterworth-Heinemann Ltd., Oxford pp. 247–278. Brockway, D.J., Ottrey, A.L., Higgins, R.S., 1991. Inorganic constituents. In: Durie, R.A. (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization. Butterworth-Heinemann Ltd, Oxford pp. 598–650.
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P.F. Nelson / Atmospheric Environment 41 (2007) 1717–1724
Carpi, A., 1997. Mercury from combustion sources: a review of the chemical species emitted and their transport in the atmosphere. Water Air and Soil Pollution 98, 241–254. CSIRO. Trace Elements in Australian Export Thermal Coals. Fact sheets prepared as a joint initiative of the Australian Coal AssociationResearch Program, CSIRO Energy Technology and the CRC for Coal in Sustainable Development. 2005 [cited 1st May 2006]; Available from: /http://www.det.csiro. au/science/e_e/Clean%20Air/trace_elements.htmS. Dale, L.S., 2003. Review of Trace Elements in Coal, Australian Coal Association Research Program. Project Number C11020, p. 65. DEH. National Pollutant Inventory. Australia’s national database of pollutant emissions. 2006 [cited 2006 18th April 2006]; Available from: /www.npi.gov.auS. ESAA, 2005. Electricity Gas Australia 2005. Energy Supply Association of Australia. Fitzgerald, W.F., Mason, R.P., Vandal, G.M., 1991. Atmospheric cycle and air-water exchange of mercury over midcontinental lacustrine regions. Water, Air and Soil Pollution 56, 745–767. Fitzgerald, W.F., Engstrom, D.R., Mason, R.P., Nater, E.A., 1998. The case for atmospheric mercury contamination in remote areas. Environmental Science & Technology 32, 1–7. Greer, L., Bender, M., Maxson, P., Lennett, D., 2006. Curtailing Mercury’s Global Reach. In: Starke, L. (Ed.), State of the World 2006. A Worldwatch Institute Report on Progress Toward a Sustainable Society. W.W. Norton & Company, New York pp. 96–114. Jackson, T.A., 1997. Long-range atmospheric transport of mercury to ecosystems, and the importance of anthropogenic emissions—a critical review and evaluation of the published evidence. Environmental Reviews 5, 99–120. Lindquist, O., Johansson, K., Aastrup, M., Andersson, A., Bringmark, L., Hovsenius, G., Hasansson, L., Iverfeldt, A., Meili, M., Timm, B., 1991. Water Air and Soil Pollution 55, 1–261. Meili, M., Bishop, K., Bringmark, L., Johansson, K., Munthe, J., Sverdrup, H., de Vries, W., 2003. Critical levels of atmospheric pollution: criteria and concepts for operational modelling of mercury in forest and lake ecosystems. The Science of The Total Environment 304, 83–106. NEMMCO. An introduction to Australia’s national electricity market. 2005 [cited 2006 18th April 2006]; Available from: /http://www.nemmco.com.au/nemgeneral/000-0187.pdfS. Pacyna, E.G., Pacyna, J.M., 2002. Global emission of mercury from anthropogenic sources in 1995. Water Air and Soil Pollution 137, 149–165. Pacyna, J.M., Pacyna, E.G., Steenhuisen, F., Wilson, S., 2003. Mapping 1995 global anthropogenic emissions of mercury. Atmospheric Environment 37, S109–S117. Pacyna, E.G., Pacyna, J.M., Steenhuisen, F., Wilson, S., 2006. Global anthropogenic mercury emission inventory for 2000. Atmospheric Environment 40, 4048–4063. Pai, P., Karamchandani, P., Seigneur, C., 1997. Simulation of the regional atmospheric transport and fate of mercury using a comprehensive Eulerian model. Atmospheric Environment 31, 2717–2732.
Pavlish, J.H., Sondreal, E.A., Mann, M.D., Olson, E.S., Galbreath, K.C., Laudal, D.L., Benson, S.A., 2003. Status review of mercury control options for coal-fired power plants. Fuel Processing Technology 82, 89–165. Petersen, G., Iverfeldt, A., Munthe, J., 1995. Atmospheric mercury species over central and northern europe—modelcalculations and comparison with observations from the Nordic air and precipitation network for 1987 and 1988. Atmospheric Environment 29, 47–67. Petersen, G., Bloxam, R., Wong, S., Munthe, J., Kruger, O., Schmolke, S.R., Kumar, A.V., 2001. A comprehensive Eulerian modelling framework for airborne mercury species: model development and applications in Europe 35, 3063–3074. Pirrone, N., Keeler, G.J., Nriagu, J.O., 1996. Regional differences in worldwide emissions of mercury to the atmosphere. Atmospheric Environment 30, 2981–2987. Pirrone, N., Mahaffey, K.R. (Eds.), 2005. Dynamics of Mercury Pollution on Regional and Global Scales: Atmospheric Processes and Human Exposures Around the World. Springer, Berlin p. 744. Riley, K., Dale, L., Devir, G., Williams, A., Holcolmbe, D., 2005. Background information for web site on trace elements in coal, Australian Coal Association Research Program. Project number C12060, p. 55. Swaine, D.J., 1990. Trace elements in coal. Boston: Butterworth, London. UNEP. Global Mercury Assessment Report. 2002 [cited; Available from: /http://www.chem.unep.ch/mercury/Report/ Final%20report/final-assessment-report-25nov02.pdfS. USEPA. Emergency Planning and Community Right-to-know Act Section 313: Guidance for Reporting Toxic Chemicals: Mercury and Mercury Compounds Category. 2001 [cited 2006 1st May 2006]; Available from: /http://www.epa.gov/ tri/guide_docs/2001/hg2001.pdfS. Wangberg, I., Munthe, J., Pirrone, N., Iverfeldt, A., Bahlman, E., Costa, P., Ebinghaus, R., Feng, X., Ferrara, R., Gardfeldt, K., Kock, H., Lanzillotta, E., Mamane, Y., Mas, F., Melamed, E., Osnat, Y., Prestbo, E., Sommar, J., Schmolke, S., SpaiJJn, G., Sprovieri, F., Tuncel, G., 2001. Atmospheric mercury distribution in Northern Europe and in the Mediterranean region. Atmospheric Environment 35, 3019–3025. Wilson, S.J., Steenhuisen, F., Pacyna, J.M., Pacyna, E.G., 2006. Mapping the spatial distribution of global anthropogenic mercury atmospheric emission inventories. Atmospheric Environment 40, 4621–4632. Xu, X.H., Yang, X.S., Miller, D.R., Helble, J.J., Carley, R.J., 2000. A regional scale modeling study of atmospheric transport and transformation of mercury. II. Simulation results for the northeast United States. Atmospheric Environment 34, 4945–4955. Xu, X.H., Yang, X.S., Miller, D.R., Helble, J.J., Carley, R.J., 2000. A regional scale modeling study of atmospheric transport and transformation of mercury. I. Model development and evaluation. Atmospheric Environment 34, 4933–4944.