Trace Metal Emissions in Fine Particles from Coal ... - ACS Publications

Nov 7, 2006 - New South Wales 2109, Australia ... the association between mortality and other health effects and particle mass show relatively consist...
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Energy & Fuels 2007, 21, 477-484

477

Trace Metal Emissions in Fine Particles from Coal Combustion† Peter F. Nelson* CRC for Coal in Sustainable DeVelopment, Graduate School of the EnVironment, Macquarie UniVersity, New South Wales 2109, Australia ReceiVed August 20, 2006. ReVised Manuscript ReceiVed September 20, 2006

Fine-particle concentrations in the atmosphere and related human health impacts have been the subject of significant scientific research and regulatory development over the past couple of decades. Finer particles have been shown in epidemiological studies to be more strongly associated with adverse health outcomes than coarser particles, although the causal mechanisms responsible have not been definitively established. While the association between mortality and other health effects and particle mass show relatively consistent magnitudes of effects, there is considerably less agreement and consistency in the results from studies that have examined associations between health impacts and the composition of the fine particles. It has been known for more than 20 years that trace metals are enriched in the fine particles formed during coal combustion. Some of these metals are toxic at high concentrations, and reliable estimates of their emission rates in coal-fired plants are required to assist assessment of the relationships between fine-particle exposure and health impacts. In this paper, the current understanding of the health effects of fine particles, including the influence of composition, and the factors that determine trace-element release and emission from full-scale plants, is reviewed.

Introduction Atmospheric fine particles (less than about 1-2 µm in diameter) are receiving increased attention as a consequence of their effects on human health, visibility, acid deposition, and global climate. Much of the recent attention has been directed at the fine-particle fraction because of the potential impacts on human health. Statistical analyses of urban air pollution in the United States, Europe, and elsewhere have revealed a strong correlation between fine-particle concentrations and short-term impacts on health, such as mortality.1,2 Recent results3 have extended these findings to long-term impacts and found that exposure to fine particles is associated with very substantial increases in the risk of lung cancer and cardiopulmonary mortality. The legislative response to these findings has been rapid. The U.S. Environmental Protection Agency (EPA), for example, has recently drafted a major revision of the Air Quality Standard for particulate matter (PM)2.5. Australia has recently announced a PM2.5 reporting guideline under the National Environment Protection Measure (NEPM) process. The World Health Organization (WHO)4 declined to set an air-quality guideline for PM, in part because the evidence suggests that there may be health impacts even at very low concentrations. In Europe, estimates of the health costs from the exposure of the public to PM have † Presented at the 2006 Sino-Australia Symposium on Advanced Coal Utilization Technology, July 12-14, 2006, Wuhan, China. * To whom correspondence should be addressed. Telephone: 61-2-98506958. Fax: 61-2-9850-7972. E-mail: [email protected]. (1) Dockery, D.; Pope, C. A.; Xu, X. An association between air pollution and mortality in six US cities. N. Engl. J. Med. 1993, 329, 1753-1759. (2) Wilson, R.; Spengler, J. Particles in Our Air: Concentrations and Health Effects; Harvard University Press: Cambridge, MA, 1996. (3) Pope, C. A.; Burnett, R. T.; Thun, M. J.; Calle, E. E.; Krewski, D.; Ito, K.; Thurston, G. D. Lung cancer, cardiopulmonary mortality, and longterm exposure to fine particulate air pollution. J. Amer. Med. Assoc. 2002, 287, 1132-1141. (4) WHO Guidelines for Air Quality. http://www.who.int/peh/air/ Airqualitygd.htm.

shown that these can exceed those due to traffic accidents.5 Hence, atmospheric fine particles will come under increased scrutiny. Fine particles arise from both natural and anthropogenic, and primary and secondary, sources. Here, primary refers to particles directly emitted by sources such as diesel vehicles, industrial processes, and bushfires, and secondary refers to atmospheric gas-to-particle conversion processes. Coal-fired power stations contribute both primary fine particles, from the inorganic material in the coal, and secondary aerosol, from atmospheric conversion of SO2 and NOx to sulfate and nitrate. In this paper, the focus is on direct emissions of fine particles formed in the combustion process and the toxic metals associated with these particles. Utilization of coal in combustion and gasification processes results in potential releases of trace elements in a number of ways. The more volatile may be emitted in the gas phase or enriched on the fine (submicron) particulate fraction and, hence, escape capture by electrostatic precipitators (ESPs), fabric filters (FFs), or other air-pollution control devices (APCDs). 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 upon the utilization and/or disposal options chosen for the ash or slag and, in many cases, will be determined by the leachability of the trace elements. The nature of the fine particles released from coal-fired power stations has been studied for many years. Early work is excellently summarized and reviewed by Damle and coworkers.6 More recent work at full scale7 and pilot scale8 has extended our understanding of the size and composition as a (5) London, S. J.; Romieu, I. Health costs due to outdoor air pollution by traffic. The Lancet 2000, 356, 782-783. (6) Damle, A. S.; Ensor, D. S.; Ranade, M. B. Coal combustion aerosol formation mechanisms: A review. Aerosol Sci. Technol. 1982, 1, 119133. (7) Kauppinen, E. I.; Pakkanen, T. A. Coal combustion aerosols: A field study. EnViron. Sci. Technol. 1990, 24, 1811-1818.

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function of size. The current understanding of combustion aerosols or fine particles has been reviewed by Lighty and coworkers.9 The release of trace elements10 to the environment is an area of increasing concern given the relatively high toxicity of some of these species. As a consequence, pollution control agencies in many countries are evaluating the environmental impacts of emissions of toxic trace elements and, in some cases, instituting regulations to limit these emissions. For example, the U.S. EPA announced in December 2000 that it would regulate emissions of mercury and other air toxics from coal- and oil-fired power plants. The decision was focused on mercury, as the air toxic of greatest concern. The announcement committed the EPA to proposing regulations by December 15, 2003 and to finalizing regulations by December 15, 2004. As is the case in many countries, the largest domestic source of mercury in the U.S.A. is the coal-fired power industry, whose 1300 plants emit 48 tons annually. Mercury was first listed as a hazardous air pollutant (HAP) in the Clean Air Act (CAA) in 1990.11 The CAA sought to control mercury emissions by establishing standards and recommending control technologies. More recently, the federal U.S. government has sought to remove the listing of mercury as a HAP under the CAA based on the fact that this approach did not constitute the most effective measure for its control. In 2003, the Clear Skies Act was developed to reduce emissions of NOx and SOx and to establish controls for mercury emissions.12 The Clear Skies Act elicited a very strong negative response from environmental and community groups, mainly because of its lack of regard for public health and safety issues. The method favored by industry for mercury control and reductions, a cap and trade scheme, was thought to have the potential to lead to toxic hotspots, hence, make water-quality goals difficult to achieve, and lead to increased exposure for some individuals.13 It has also been pointed out that the cap and trade scheme suggests an action which the EPA’s own water agency program expressly discourages - trading in persistent bio-accumulative toxics.13 The Clear Skies Act was not passed by the U.S. Senate, and ultimately, it was the Final Mercury Rule that was enacted in March 2005.14 This Rule establishes targets for mercury emissions, although a detailed framework for how these targets might be achieved through a cap and trade system has not been released as yet. The U.S. EPA now has entire discretion to regulate mercury emissions. However, controversy continues, and the agency is being sued over allegations that corruption may have occurred in the process of developing and enacting this new mercury legislation. A judicial review is currently in progress.15 Because all aspects of the mercury issue have been covered extensively elsewhere,16-20 the focus of this paper will be on other trace metals emitted during coal combustion. (8) Galbreath, K. C.; Toman, D. L.; Zygarlicke, C. J.; Pavlish, J. H. Trace element partitioning and transformations during combustion of bituminous and subbituminous U. S. coals in a 7-kW combustion system. Energy Fuels 2000, 14, 1265-1279. (9) Lighty, J. S.; Veranth, J. M.; Sarofim, A. F. Combustion aerosols: Factors governing their size and composition and implications to human health. J. Air Waste Manage. Assoc. 2000, 50, 1565-1618. (10) Trace elements of concern include those regulated under the 1990 U.S. Clean Air Act amendments (antimony, arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury, nickel, and selenium, as well as the radionuclides uranium and thorium). (11) See http://www.epa.gov/oar/oaq_caa.html/, accessed August 4, 2005. (12) See http://www.epa.gov/clearskies/, accessed August 5, 2005. (13) Christen, K. Mercury trading scheme raises concerns. EnViron. Sci. Technol. 2005, 126A-127A. (14) See http://www.epa.gov/air/mercuryrule/rule.htm, accessed August 5, 2005.

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Senior et al.21 argue that emissions of trace elements from coal combustion are determined by modes of occurrence of the elements in the coal, transformations of the elements into vapor and particles in the furnace, and the ability of the vapors and particles to penetrate the APCDs. It has also been shown that postcombustion reactions and transformations of trace elements can play an important role in determining their deportment in combustion.22 In this paper, current understanding of the health impacts of toxic metals associated with fine particles are reviewed and our understanding of trace-metal release, transformation, and emission is summarized. Health Impacts PM and Health Impacts. The relationship between exposure to air pollutants and potential health impacts has been recognized for many years, at least since increasing industrial development in Europe resulted in large increases in emissions of black smoke and acid gases. The quantitative relationship between extreme air-pollution events and excess mortality has also been established for around 50 years, since the famous “London Smog” of 1952. In that event, a strong rise in air-pollution levels, particularly particles and SO2, was followed by sharp increases in mortality and morbidity. A recent reanalysis23 of the London Smog estimates that about 12 000 excess deaths occurred from December 1952 to February 1953 because of acute and persisting effects of the event. Pollution levels during the London Smog were 5-19 times above the current U.K. and other international regulatory standards and guidelines23 and were similar to current levels in some rapidly developing regions. Figure 1 shows the relationship between mortality and SO2 for the London Smog.23 Effects of long-term exposure to much lower levels of pollutants have been more difficult to establish, in part because of the difficulties in separating the impacts of confounding factors on health outcomes. Recent epidemiological research, however, on the basis of long-term observations in cities in the developed world, has consistently revealed an association between air pollution, particularly fine particles, and human health impacts. Statistical analyses of urban air pollution worlwide have revealed a correlation between PM concentrations and shortterm impacts on health.1,2,24 Recent results3 have extended these findings to long-term impacts. For example, Pope et al.3 found (15) See http://www.oag.state.ny.us/press/2005/may/may18d_05.html, accessed August 5, 2005. (16) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Status review of mercury control options for coal-fired power plants. Fuel Process. Technol. 2003, 82, 89-165. (17) Senior, C. L.; Helble, J. J.; Sarofim, A. F., Emissions of mercury, trace elements, and fine particles from stationary combustion sources. Fuel Process. Technol. 2000, 65-66, 263-288. (18) Sondreal, E. A.; Benson, S. A.; Pavlish, J. H. Status of research on air quality: Mercury, trace elements, and particulate matter. Fuel Process. Technol. 2000, 65-66, 5-19. (19) Zhuang, Y.; Thompson, J. S.; Zygarlicke, C. J.; Pavlish, J. H. Development of a mercury transformation model in coal combustion flue gas. EnViron. Sci. Technol. 2004, 38, 5803-5808. (20) Sondreal, E. A.; Benson, S. A.; Pavlish, J. H.; Ralston, N. V. An overview of air quality III: Mercury, trace elements, and particulate matter. Fuel Process. Technol. 2004, 85, 425-440. (21) Senior, C. L.; Helble, J. J.; Sarofim, A. F., Emissions of mercury, trace elements, and fine particles from stationary combustion sources. Fuel Process. Technol. 2000, 65, 263-288. (22) Seames, W. S.; Wendt, J. O. L. The partitioning of arsenic during pulverized coal combustion. Proc. Combust. Inst. 2000, 28, 2305-2312. (23) Bell, M. L.; Davis, D. L. Reassessment of the lethal London fog of 1952: Novel indicators of acute and chronic consequences of acute exposure to air pollution. EnViron. Health Perspect. 2001, 109, 389-394.

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Figure 1. Relationship between mortality and air pollution during and following the London “Smog” Event of 1952 (from Bell and Davis).23 Reproduced with permission from Environmental Health Perspectives.

Figure 2. Percent excess mortality for a 10 µg m-3 increase in PM10 (summarized from refs 24-26).

that each 10 µg m-3 increase in the concentration of fine particles (PM2.5) was associated with an 8% increased risk of lung cancer mortality. A similar magnitude of impacts has been observed worldwide. Figure 2 presents a summary of the magnitude of the effects observed for PM10 at a range of locations.24-26 In summary, these studies suggest that atmospheric particles have substantial impacts on human health, with more recent data indicating that PM2.5 has more significant impacts than PM10. A feature of many of the studies that have now been conducted in many different countries is the convergence of the results obtained. Uniquely for a “criteria” air pollutant, the interpretation of these epidiemological studies has not indicated a threshold below which no effects occur. On this basis, the WHO decided not to recommend a health goal for PM, at this stage, on the grounds that4 “the available information does not allow a judgment to be made of concentrations below which no effects would be expected”. There have, however, been some recent developments in the analysis of PM and health effects data that has resulted in a revision of the magnitude of the effects. In addition, there has also been recent evidence for a threshold below which some effects are not observed. In 2002, researchers at John Hopkins University and Health Canada identified problems with the statistical model used in the majority of the studies of health effects.25 In response, much (24) HEI. Understanding the Health Effects of Components of the Particulate Matter Mix: Progress and Next Steps; Health Effects Institute: Boston, MA, April 2002; p 20. (25) HEI. ReVised Analyses of Time-Series Studies of Air Pollution and Health; Health Effects Institute: Boston, MA, May 2003; p 4.

of the statistical data in the U.S. were reanalysed using modified procedures that addressed the identified problems. The conclusions from this reanalysis25 included the following points: (i) In general, the estimates of effects decreased substantially, but the qualitative conclusions did not change. (ii) Across the 90 cities included in the Health Effects Institute (HEI) funded National Morbidity, Mortality, and Air Pollution Study (NMMAPS), the revised mean effect on mortality decreased from a 0.41% increase per 10 µg m-3 increase in the PM10 concentration to 0.21-0.27%, an overall decrease of nearly 50%. (iii) Smaller decreases in effects (8-10%) were found in estimates for hospitalizations for cardiovascular diseases and chronic obstructive pulmonary diseases. (iv) The effect on pneumonia hospitilizations was substantially reduced. The issue of thresholds has also been extensively examined recently. The HEI summary26 of recent work in this area provides some evidence for a threshold for some effects. Composition of Ambient PM and Health Effects. The fine (PM2.5) fraction of ambient PM largely consists of carbon (elemental and organic), metals, sulfate, and nitrate. The relative contributions of these components varies spatially and temporally and will be determined, inter alia, by proximity to sources, time of day and year, and other factors. Recent reviews9,27,28 have summarized the current understanding in the area of PM composition and health efffects and what is currently known about the size and composition of combustion aerosols and the organic fraction, and also of the spatial variability in composition. It is beyond the scope of this paper to review this information, since much relates to urban aerosols produced from the combustion of liquid fuels and wood and to secondary organic aerosols. Some comments on the composition and formation mechanisms for particles from coal combustion are, however, relevant. There remains intense activity in the area of PM and health effects and, particularly, in investigating causal relations between fine-particle composition and health effects. The introduction to Okeson et al.29 summarizes recent studies, with a particular emphasis on combustion-generated fine particles. Briefly, the key issues are considered to be the following: (i) The magnitude of the impact of PM on human health depends upon the PM (26) HEI. The National Morbidity, Mortality, and Air Pollution Study: Concentration-Response CurVes and Thresholds for the 20 Largest US Cities; Health Effects Institute: Boston, MA, 2004; p 2. (27) Jacobson, M. C.; Hansson, H. C.; Noone, K. J.; Charlson, R. J. Organic atmospheric aerosols: Review and state of the science. ReV. Geophys. 2000, 38, 267-294. (28) Monn, C. Exposure assessment of air pollutants: A review on spatial heterogeneity and indoor/outdoor/personal exposure to suspended particulate matter, nitrogen dioxide and ozone. Atmos. EnViron. 2001, 35, 1-32.

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mass, size distribution, composition (polyaromatic hydrocarbons and metals such as Fe, V, and Zn), the presence of biogenic components (endotoxins, pollens, bacteria, and viruses), and other factors. (ii) Particle-size distribution “appears to have a modulating effect on the degree of toxicity beyond that anticipated due merely to deposition issues”; as discussed earlier, there is evidence of increasing toxicty with a decreasing particle size. (iii) Relative roles for soluble and insoluble components of PM require further elucidation. A major study of atmospheric concentrations, exposure assessment, and health impacts is being conducted by the Electric Power Research Institute (EPRI) in the Aerosol Research and Inhalation Epidemiology Study (ARIES). An overview of ARIES is provided in a fact sheet available from the EPRI web site.30 The objectives of ARIES are the following: (i) Investigate via epidemiology and exposure studies associations between air quality and human health. (ii) Provide input for consideration of the National Ambient Air Quality Standard (NAAQS) and for the subsequent development of State Implementation Plans (SIPs) What distinguishes ARIES from predecessor studies is its focus on an unprecedented range of potential agents in the air, including volatile organic compounds (VOCs), aeroallegens, and specific PM components, in addition to PM mass (the basis for most previous investigations). The factsheet30 also summarizes results to date in ARIES. Of note are the following findings: (i) Daily mortality results show that the best model fits for all-cause mortality in those aged 65 and older are observed for CO, NO2, PM2.5, coarse PM, SO2, and ozone, followed by elemental and organic carbon. (ii) Hospital emergency department visits for cardiovascular disease are associated with NO2, CO, elemental carbon (EC), and organic carbon (OC). (iii) Hospital emergency department visits for respiratory visits are associated with PM10, PM2.5, PM2.5 - water-soluble metals, NO2, CO and SO2. Studies of the associations between, on the one hand, mortality and other health effects and, on the other, particle mass show relatively consistent magnitudes of effects. In some of these studies, these associations have also been explored for components of the fine particles, but in this case, there is significantly less agreement and consistency. In the original “Six Cities” study,1 it is reported that “mortality was most strongly associated with air pollution fine particulates, including sulfates”. The data from the “Six Cities” study was also used to examine associations between mortality and fine PM from different sources.31 In this work, chemical tracers were used to attribute source contributions to PM. Lead was used as a tracer for motor vehicles; selenium for coal combustion sources; and silicon as an indicator of crustal material. On this basis, crustal material was found to be not associated, motor vehicle sources were found to be responsible for 3.4% per 10 µg m-3 increase in PM2.5, and coal combustion sources were found to be responsible for 1.1% per 10 µg m-3 increase in PM2.5. By contrast, recent results of the ARIES study,32 discussed above, reveal a somewhat different picture. These results, derived from a very extensive field sampling campaign in (29) Okeson, C. D.; Riley, M. R.; Fernandez, A.; Wendt, J. O. L. Impact of the composition of combustion generated fine particles on epithelial cell toxicity: Influences of metals on metabolism. Chemosphere 2003, 51, 1121-1128. (30) EPRI. ARIES: Aerosol Research and Inhalation Epidemiology Study; EPRI: Palo Alto, CA, August 12, 2004. (31) Laden, F.; Neas, L. M.; Dockery, D. W.; Schwartz, J., Association of fine particulate matter from different sources with daily mortality in six US cities. EnViron. Health Perspect. 2000, 108, 941-947.

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Atlanta, GA, appear to show that the pollutants most strongly associated with the health effects appear to be CO and carboncontaining particulates. However, the details of the effects are complex and clearly require additional work in other locations to derive more general conclusions. The HEI is also sponsoring work examining causal relationships between health impacts and exposure to PM. Of particular relevance to emissions from coal-fired power stations is a study by Aust and co-workers33 on the effects of metals bound to PM on lung cells. The basis for this study was the hypothesis that inhaled coal fly ash could be a health hazard because metals solubilized from fly ash within lung cells may cause toxic reactions. It was found33 that (i) coal fly ash particles entered lung cells and stimulated synthesis of the protein ferritin (ferritin binds iron and is produced in response to increasing iron levels); (ii) the presence of ferritin indicates that iron was released intracellularly and was available to provoke an inflammatory response by forming reactive oxygen species; and (iii) there was indirect evidence for the formation of intracellular reactive oxygen species because lung epithelial cells exposed to coal fly ash synthesized the inflammatory mediator interleukin-8. The investigators concluded that there is a plausible connection between the intracellular release of a transition metal from particles, formation of reactive oxygen species, and lung inflammation. Table 1 provides an overview of the current understanding of the biological effects of the various components of the PM. It is clear from this brief summary that the inter-related effects of particle size, composition, and other characteristics and health effects are not yet completely understood, which in part may be attributable to the significant temporal and spatial variation observed in PM characteristics. Trace-Element Concentrations and Modes of Occurrence Trace toxic metals are present in coals in varying concentrations. The range and variation has been extensively reviewed in the past, and references only are provided here.34-36 A recent initiative from the U.S. Geological Survey, the World Coal Quality Inventory (WoCQI),37 will establish an electronic database with information on most coal properties, including trace-element contents, facilitating comparisons. Partners in some 40 countries are involved. Inorganic material, including the trace elements, can occur in coal in different mineralogical environments, and this can have a significant effect on their behavior or deportment during combustion. Three broad classes of inorganic material in coals have been described: (1) inorganic material which was present (32) Klemm, R. J.; Lipfert, F. W.; Wyzga, R. E.; Gust, C. Daily mortality and air pollution in Atlanta: Two years of data from ARIES. Inhalation Toxicol. 2004, 16, 131-141. (33) Aust, A. E.; Ball, J. C.; Hu, A. A.; Lighty, J. S.; Smith, K. R.; Straccia, A. M.; Veranth, J. M.; Young, W. C. Particle Characteristics Responsible for Effects on Human Lung Epithelial Cells. HEI Research Report Number 110; Health Effects Institute: Boston, MA, December 2002; p 86. (34) Swaine, D. J. Trace Elements in Coal; Butterworth: Woburn, MA, 1990. (35) Swaine, D. J. The contents and some related aspects of trace elements in coals. In EnVironmental Aspects of Trace Elements in Coal; Swaine, D. J., Goodarzi, F., Eds.; Kluwer Academic Publishers: Norwell, MA, 1995; pp 5-23. (36) Davidson, R. M.; Clarke, L. B. Trace Elements in Coal; IEA PER/ 21; IEA Coal Research: London, U.K., February 2000. (37) Finkelman, R. B. In The World Coal Quality InVentory (WoCQI): A Tool for Addressing Global Energy, Technology, Economic, EnVironmental, and Human Health Issues; The Pittsburgh Coal Conference, Newcastle, Australia, December 2001; p 4.

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Table 1. Chemical Components of PM10 and Their Biological Effects (from the Health Effects Institute)24 component

major subcomponents

described biologic effects

metals

iron, vanadium, nickel, copper, platinum, and others

organic compounds

many are adsorbed onto particles; some volatile or semi-volatile organic species from particles themselves viruses, bacteria and their endotoxins, animal and plant debris (such as pollen fragments), and fungal spores sulfate (usually as ammonium sulfate) nitrate (usually as ammonium/sodium nitrate acidity (H+)

biologic origin ions

reactive gases

ozone, peroxides, aldehydes

particle core

carbonaceous material

in the original sediment layers which formed the organic structure of the coal. This is usually organically bound and described as an inherently-bound metal. (2) Some inorganic material may not be incorporated into the organic structure. Instead, the structure forms around it; this is described as a mineral inclusion. (3) Some material enters the sediment layers after they have decomposed or mix into the coal during geological events after the coal seam has formed and during mining operations; these are described as mineral exclusions. Trace elements can reside in any of these structures. These residences or modes of occurrence have been studied with a range of analytical techniques.38-44 Recent results have been reviewed by Davidson45,46 who summarizes a collaborative research project in which modes of occurrence were determined in four coal samples from Australia (high-volatile bituminous), U.K. (bituminous from a power station feed), U.S. (Illinois number 6, a high-volatile bituminous), and an eastern Canadian coal (high-volatile bituminous). Agreement between the range of techniques was variable and quite poor for some crucial elements. Some techniques were also unable to be used for selected elements. This highlights the fact that completely reliable techniques have yet to be developed for the analysis of trace-element modes of occurrence. The importance of the trace-element mode of occurrence may be illustrated by considering the results of Teng (doctoral thesis (38) Wang, J.; Sharma, A.; Tomita, A. Determination of the modes of occurrence of trace elements in coal by leaching coal and coal ashes. Energy Fuels 2003, 17, 29-37. (39) Finkelman, R. B., Trace elements in coalsEnvironmental and health significance. Biol. Trace Elem. Res. 1999, 67, 197-204. (40) Vassilev, S. V.; Vassileva, C. G. Geochemistry of coals, coal ashes and combustion wastes from coal-fired power stations. Fuel Process. Technol. 1997, 51, 19-45. (41) Huggins, F. E.; Huffman, G. P., Modes of occurrence of trace elements in coal from XAFS spectroscopy. Int. J. Coal Geol. 1996, 32, 31-53. (42) Huggins, F. E.; Zhao, J. M.; Shah, N. H.; Huffman, G. P. Modes of occurrence of trace elements in coal from XAFS spectroscopy. Abstr. Pap. Am. Chem. Soc. 1994, 207, 98-FUEL. (43) Palmer, C. A.; Krasnow, M. R.; Finkelman, R. B.; Dangelo, W. M. Reliability and reproducibility of leaching procedures to estimate the modes of occurrence of trace elements in coal. Abstr. Pap. Am. Chem. Soc. 1994, 207, 107-FUEL. (44) Huggins, F. E. Overview of analytical methods for inorganic constituents in coal. Int. J. Coal Geol. 2002, 50, 169-214. (45) Davidson, R. M. Modes of Occurrence of Trace Elements in Coal: Results from an International CollaboratiVe Program; IEA Coal Research: London, U.K., 2000. (46) Davidson, R. M., Modes of occurrence of trace elements in coal: Results from an international collaborative program. Abstr. Pap. Am. Chem. Soc. 2000, 220, U388-U388.

can trigger inflammation, cause DNA damage, and alter cell permeability by inducing production of reactive oxygen species (particularly hydroxyl free radicals) in tissues some may cause mutations; some may cause cancer; and others can act as irritants and induce allergic reactions plant pollens can trigger allergic responses in the airways of sensitive individuals; viruses and bacteria can provoke immune defense responses in the airways sulfuric acid at relatively high concentrations can impair muccociliary clearance and increase airway resistance in people with asthma; acidity may change the solubility (and availability of metals and other compounds adsorbed onto particles may adsorb onto particles and be transported into lower airways, causing injury carbon induces lung irritation, epithelial cell proliferation, and fibrosis after long-term exposure

in Mechanical Engineering, MIT, as quoted by Senior et al.).17 Teng measured the fractional vaporization of As, Se, Cr, and Co under conditions approximating those found in full-scale boilers. It was found that the fractional vaporization was similar for all four elements at ∼0.2-0.4. This is surprising because (i) the oxides of As and Se have high vapor pressures (As2O3 boils at 465 °C, and SeO2 and SeO3 boil at 317 and 180 °C, respectively) and might be predicted to completely vaporize under these conditions and (ii) the oxides of Cr and Co are refractory with very high boiling points and, hence, might be predicted to not vaporize to any significant extent. These differences have been rationalized17 on the basis of modes of occurrence. Chromium and cobalt are often organically bound in northern hemisphere coals, although comparative studies of modes of occurrence have shown poor agreement for these elements. Elements associated with the organics would be expected to be released with the volatiles regardless of their vapor pressure and are often lost during American Society for Testing and Materials (ASTM) devolatilization tests. In contrast, As and Se are often associated with the pyrite in northern hemisphere coals. Their release has been postulated to be retarded because they must diffuse out of the pyrhoittite particles formed in the early stages of combustion. A refinement to the model of trace-element vaporization has been developed to account for this diffusional resistance.17 These results have clear implications for coals, such as Australian coals, where pyrite contents are generally low and trace-element associations may be rather different, leading to different fractional vaporizations. Vaporization, Condensation, and Fine-Particle Formation Particle-size distributions of ash produced during coal combustion have long been known to be multi-modal in character. The majority of the ash is greater than 1 µm in size and results from pulverized fuel burnout, leading to an ash residue and particle fragmentation processes. A much smaller proportion of fine material is also produced. The nature and composition of this material is of great importance, since (i) collection efficiency in electrostatic precipitators is lowest for particles in the 0.1-1 µm size range47; (ii) in the context of trace-element deportment, the nature of the fine ash is very important because trace elements have been observed to be enriched in the fine fraction; (iii) the major health effects observed recently for air pollutants are associated most strongly with fine PM; and (iv) light scattering is greatest for

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PM in the submicron range; hence visibility impacts are highest for this range. Submicron PM produced in combustion processes largely arises from vaporization and condensation processes, although recent results have revealed a contribution, probably from fragmentation processes, to particles in the 0.7-3.0 µm size range.48 The mechanism for the formation of submicron-size particles is widely understood to result from homogeneous condensation of flame-volatilized species, namely, the refractory oxides SiO2, CaO, MgO, and Fe2O3. Flagan and co-workers49,50 described the vaporization/condensation processes. The main features of this mechanism are as follows: (i) During combustion, highly reducing conditions can exist inside coal particles. (ii) Under these conditions, refractory oxides can be reduced to more volatile suboxides or elements; for example, in the case of Si:

SiO2 (s) + CO (g) f SiO (g) + CO2 (g) (iii) The volatile species is transported away from the particle into the bulk gas, where O2 concentrations are significantly higher and the suboxide or element is reoxidized

SiO (g) + O2 (g) f SiO2 (g) + O (g) (iv) Provided that the vapor pressure of the oxide exceeds the saturated vapor pressure spontaneous condensation will occur and nuclei of the submicron fume will be formed. This model was extended by Sarofim and co-workers51-53 to account for observed distributions of inorganic species in the fine ash products from coal combustion and further by Haynes,54 who developed a detailed kinetic model for silica vaporization and condensation. Submicron particle formation is important in trace-element deportment, because although trace toxic species such as As and Se can also vaporize as a result of the formation of volatile combustion products, these species are often not present in sufficient concentrations to homogeneously condense. Condensation of these species upon existing particles is, in that case, more likely. The fine particles contribute relatively more to the available surface area; therefore, enrichments in the fine-particle fraction are often observed as detailed above. Simple models to describe the mechanisms that give rise to trace-element enrichment have been discussed by a number of (47) Helble, J. J. A model for the air emissions of trace metallic elements from coal combustors equipped with electrostatic precipitators. Fuel Process. Technol. 2000, 63, 125-147. (48) Linak, W. P.; Miller, C. A.; Seames, W. S.; Wendt, J. O. L.; Ishinomori, T.; Endo, Y.; Miyamae, S. On trimodal particle size distributions in fly ash from pulverized-coal combustion. Proc. Combust. Inst. 2003, 29, 441-447. (49) Flagan, R. C.; Friedlander, S. K. Particle formation in pulverised coal combustionsA review. In Recent DeVelopments in Aerosol Science; Shaw, D. T., Ed.; Wiley: New York, 1978. (50) Taylor, D. D.; Flagan, R. C. Aerosol Sci. Technol. 1982, 1, 103117. (51) Quann, R. J.; Neville, M.; Janghorbani, M.; Mims, C. A.; Sarofim, A. F. Mineral matter and trace-element vaporization in a laboratorypulverized coal combustion system. EnViron. Sci. Technol. 1982, 16, 776781. (52) Quann, R. J.; Sarofim, A. Proc. Combust. Inst. 1982, 19, 14291440. (53) Haynes, B. S.; Neville, M.; Quann, R. J.; Sarofim, A. F. Factors governing the surface enrichment of fly ash in volatile trace species. J. Colloid Interface Sci. 1982, 87, 266-278. (54) Haynes, B. S. Chemical Kinetic Computations of Vaporisation of Mineral Constituents during Coal Combustion; Final report project 4.4; CRC for Black Coal Utilisation: Australia, 1999; p 47.

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authors.53,55-57 Friedlander55 discussed condensation upon existing particles for situations where the particles were much larger (the continuum regime) or much smaller (the free molecular regime) than the mean free path. Condensation in the continuum regime for diffusion-controlled condensation would lead to traceelement enrichment with a dp-2 dependence. In the free molecular regime for very small particles, the dependence would be expected to be dp-1. Haynes et al.,53 from a study of fine particles (0.4-12.2 µm) using a laboratory-based combustor, showed that the dp-1 relationship fitted all of the elements studied (As, Sb, K, Mn, Na, V, and W) well, apart from Na, which followed an inverse square law with particle size. The data were able to be explained by considering surface reaction control rather than diffusion control as the rate-limiting mechanism. Linak and Wendt57,58 have summarized the simple models for concentration dependence on particle size as follows: (i) a dp-1 relationship for either free molecular condensation when the particle size is much less than the gas mean free path or for external particle surface-controlled reactions for large and small particles and (ii) a dp-2 relationship for diffusion-controlled condensation on particles with diameters much greater than the gas mean free path. Observations of 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. There are at least three options, which could be considered for the reporting of emissions: (1) Direct measurements of emissions at the location required to report. While this would appear to be the ideal situation, representative measurements are difficult to obtain and very expensive for many trace species. The measurements require detailed probing of the large power station ducts and vary with load, type of coal, and operational condition of the particle-capture technology (e.g., ESP rapping or tears in fabric filter bags). Reliance on direct measurements alone could result in inconsistencies in reporting and large changes in the amounts reported year to year from individual facilities. (2) Modeling of emissions based on coal properties and composition, high-temperature chemistry, and the performance of airpollution control devices is attractive in the long term. However, the ability to undertake this modeling has not yet advanced to a stage where it can be used with confidence to report emissions. (3) 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. This approach is the one that has been adopted by many national pollutant inventories, drawing heavily on available U.S. data. The database and emission factor approach would appear to present the best opportunity currently to develop procedures for estimating emissions of trace species from coal combustion plant. However, there are a number of problems that emerge when the available data are considered. These include (i) data quality (significant problems of sampling and analysis have not always been adequately addressed in some studies, particularly (55) Friedlander, S. K. Smoke, Dust and Haze, 1977. (56) Davison, R. L.; Natusch, D. F. S.; Wallace, J. R.; Evans, C. A. J. Trace elements in fly ashsDependence of concentration on particle size. EnViron. Sci. Technol. 1974, 8, 1107-1113. (57) Linak, W. P.; Wendt, J. O. L. Trace metal transformation mechanisms during coal combustion. Fuel Process. Technol. 1994, 39, 173-198. (58) Linak, W. P.; Wendt, J. O. L. Toxic metal emissions from incineration: Mechanisms and control. Prog. Energy Combust. Sci. 1993, 19, 145-185.

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Energy & Fuels, Vol. 21, No. 2, 2007 483

the older ones); (ii) data accessibility (the data are not always available in the open literature; for example, EPRI has a large data set that is only completely available to its members); (iii) data variability (for reasons not yet completely understood, significant variability in the deportment of selected trace elements in full-scale plants is observed); and (iv) data quantity (in many cases, the amount of emission data of acceptable quality for a particular substance is limited). In part, these problems are related to the complexity of the processes by which emissions of trace elements occur. As noted above, the more volatile trace elements may be emitted in the gas phase or enriched on the fine (submicron) 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 upon the utilization 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 because of postcombustion reactions and transformations of trace elements, which can play an important role in determining their deportment in combustion.47,59 For example, Seames and Wendt22,60 have established a relationship between the concentration of solid-phase arsenic, selenium, and cadmium to calcium in supermicron particles, suggesting the formation of trace metal/Ca complexes. Interactions with iron have also been reported, and possible control strategies using sorbents have been investigated.61,62 Current knowledge of these processes is incomplete, and modeling or estimation techniques, which account for all of these effects, are still in the process of development. Given this position, it is likely that existing and new field data will provide the basis for reporting for some time to come, despite the limitations and problems noted above. The most extensive data sets have been collected in the U.S. and Europe. The U.S. data for trace metals has been critically reviewed by Helble.47 He shows that the U.S. databases provide information on coal rank, ash content, sulfur content, trace-element concentrations, coal higher heating value, trace element emissions rate, and particle emissions rate. Emissions of individual trace elements are reported on the basis of the following relationship:

η ) 1 - PMout/PMin

Ei ) Ai,in(1 - ηi) ) Ci(1 - ηi)/H

(1)

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 air-pollution control device (mass per unit fuel energy content), Ci is the concentration (mass fraction) of trace element i in the coal on an as-received basis, and ηi is the capture efficiency of trace element i in the air-pollution control device The particle collection efficiency, η, of an air-pollution control device is defined47 as (59) Sterling, R. O.; Helble, J. J. Reaction of arsenic vapor species with fly ash compounds: Kinetics and speciation of the reaction with calcium silicates. Chemosphere 2003, 51, 1111-1119. (60) Seames, W. S.; Wendt, J. O. L. 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, 179-196. (61) Gale, T. K.; Wendt, J. O. L. In-furnace capture of cadmium and other semi-volatile metals by sorbents. Proc. Combust. Inst. 2005, 30, 29993007. (62) Fernandez, A.; Wendt, J. O. L.; Witten, M. L. 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, 1320-1327.

(2)

where PMout,in is the PM concentration (mass per unit heat input) at the outlet or inlet to the air-pollution control device. PMout can be expressed47 in terms of coal parameters as

PMout ) PMin(1 - η) ) fa(1 - η)/H

(3)

where fa is the mass fraction of ash in the coal on an as-received basis and H is the higher heating value of the coal on an energy content per unit mass basis. The combination of eqs 1 and 3 gives an expression for traceelement emissions as a function of measurable parameters

Ei )

CiPMout(1 - ηi) fa(1 - η)

(4)

However, the broad range of trace-element emissions observed at different plants has led to the development of a modified version of eq 4

[

E i ) ai

]

(CiPMout) fa

bi

(5)

Equation 5 is the form recommended by EPRI for interpretation of the DOE and PISCES data (see ref 42 and references quoted there). It is also the basis for the equations used in the Australian National Pollutant Inventory (NPI) workbook,63 but it should be recognized that this is an empirical approach and one that does not allow for the enrichment of many trace elements in the fine-particle sizes. Because these particles are more difficult to capture in electrostatic precipitators, this simplification may be significant. Helble47 has developed a model that includes trace-element concentrations as a function of the 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 are not extensive enough for the refinements incorporated in Helble’s model and the approach used in the NPI and based largely on U.S. data should be the preferred method for reporting emissions. It had been known since the work of Davison et al.56 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 volatilization 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 ESP stations burning different coals.47 These studies have found different behavior for different elements and their transport 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 Helble47 found enrichment for the elements As, Cd, Pb, Sb, and Se, while most found depletion in Mn. This is in general accord with the findings of Meij64 who carried out studies on ESPs at coal-fired power stations in The Netherlands and proposed that the observed results could be explained in terms of the element volatility as shown in Table (63) See http://www.npi.gov.au/handbooks/approved_handbooks/ffossilfuel.html, accessed June 19, 2006. (64) Meij, R., Trace element behavior in coal-fired power plants. Fuel Process. Technol. 1994, 39, 199-217.

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Table 2. Collection Efficiencies (CE) for Total Particles and Each Element, Adapted from Helble47 element

total particle CE

element CE

Hg Se As Cd Pb Ni Cr Co Be Mn Sb

99.1 99.0 99.2 99.2 99.2 99.1 99.2 98.9 99.0 99.2 98.9

28.9 49.1 96.1 96.1 96.8 97.6 98.0 98.2 98.3 98.5 98.5

2. The overall capture efficiencies for individual trace elements for ESPs were estimated by Helble47 for a number of elements. Table 2 illustrates the variation in the values for a number of studies. Table 2 shows that, for most elements, except Hg and Se, the two most volatile elements present, the element capture efficiency is almost as high as the total particle collection efficiency. However, for the reasons stated earlier, small variations between the overall collection efficiency and the individual collection efficiency may have resulted from enrichment in the fine fraction. As noted above, the deposition mechanism of the vaporized trace elements onto ash particle surfaces can result in a mechanism-specific correlation between the trace-element concentration and particle size. For example, the 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 FF particle penetration information, may be useful in developing practical models of trace-element emission. As an example of the required data, Figure 3 shows enrichment factors for arsenic and cadmium, calculated according to the definition of Meij,64 as a function of the particle diameter.65 The enrichment factor (EF) is a useful way of examining trace-element behavior under combustion conditions. The EF is defined as the ratio of an elemental concentration in (65) Halliburton, B.; Carras, J. N.; Nelson, P. F. 2006, manuscript in preparation.

Figure 3. Elemental enrichment factors for emitted fly ash as a function of the particle diameter. The Australian power station was equipped with ESP (data from ref 65).

the fly ash sample relative to the elemental concentration in the coal. To provide normalization 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 ) ([X]s/[M]s)/([X]c/[M]c) 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. Further data similar to that presented in Figure 3, for a range of coal types and APCDs, should enable the development of size-dependent emission factors and improve estimates of emissions of trace metals from coal combustion. Acknowledgment. The author wishes to acknowledge 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. Hugh Malfroy provided assistance with preparation of the health effects section of this paper. EF060405Q