Formation and chemical speciation of arsenic-, chromium-, and nickel-bearing coal combustion PM2.5

Fuel Processing Technology 85 (2004) 701 – 726 www.elsevier.com/locate/fuproc

Formation and chemical speciation of arsenic-, chromium-, and nickel-bearing coal combustion PM2.5 Kevin C. Galbreath *, Christopher J. Zygarlicke Energy and Environmental Research Center, University of North Dakota, Box 9018, Grand Forks, ND 58202-9018, USA

Abstract Mode of occurrence analyses indicate that As, Cr, and Ni in the Illinois No. 6 (Herrin) coal are generally associated with relatively large discrete mineral grains, whereas these elements are much more strongly associated with macerals and fine-grained minerals in Absaloka subbituminous (McKay or Rosebud seam) coal. The coals were burned using conventional and low-NOx conditions in an c 7-kW combustion system to evaluate the importance of elemental modes of occurrence and combustion conditions on As, Cr, and Ni volatility and speciation. Chemical analyses of size-classified ( c 0.4 – 7.7 Am) Illinois No. 6 fly ash samples indicated that As, Cr, and Ni concentrations and relative enrichment/depletion (RED) factors generally increased with decreasing particle size which is consistent with an elemental vaporization – particle surface deposition process. Similar semivolatile partitioning systematics were noted for Ni in Absaloka fly ashes. Consistent with nonvolatility, As and Cr in Absaloka fly ashes were characterized by relatively uniform particle-size distributions (PSDs) and RED factors. Low-NOx combustion conditions promoted the elemental vaporization – particle surface deposition process, as evidenced by greater elemental concentrations on fineparticle surfaces. The As, Cr, and Ni speciation of PM2.5 samples were determined using X-ray absorption finestructure spectroscopy (XAFS). Differences in Illinois No. 6 and Absaloka coal combustion conditions did not significantly affect As, Cr, or Ni speciation. As5 +O4-containing phases occur in Illinois No. 6 and Absaloka PM2.5. Cr3 +/Cr6 + is much greater in Illinois No. 6 PM2.5 relative to Absaloka PM2.5. The predominance of maceral-bound Cr3 + and oxygen functional groups in

* Corresponding author. Tel.: +1-701-777-5127; fax: +1-701-777-5181. E-mail address: [email protected] (K.C. Galbreath). 0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2003.11.015

702

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

Absaloka coal may have promoted Cr6 + formation. Illinois No. 6 and Absaloka PM2.5 contain similar NiO-containing phases, possibly ferrite spinel. D 2004 Elsevier B.V. All rights reserved. Keywords: Illinois No. 6 coal; Absaloka coal; XAFS

1. Introduction Coal is the most commonly burned fuel for commercial power generation in the United States, accounting for 51.8% of the electricity generated in 2000 [1]. Coal-burning power plants are significant anthropogenic sources of trace element emissions to the atmosphere [2– 7]. Sixteen trace elements (As, Be, Cd, Cl, Co, Cr, F, Hg, Mn, Ni, P, Pb, Sb, Se, Th, and U) are among the 189 hazardous pollutants identified in the 1990 Clean Air Act Amendments [8]. Most trace elements are emitted primarily in association with fly ash. Several elements, however, including Cl, F, and Hg are largely gaseous emissions. Coal fly ashes possess bimodal size distributions with most of the particle mass occurring in a large supermicron-size mode and a smaller submicron-size mode [9,10]. Significant proportions of trace elements may be emitted in association with submicron fly ash particles because particles ranging in aerodynamic diameter from 0.1 to 1 Am preferentially penetrate existing industrial gas-cleaning devices and the smaller particles are generally enriched in semivolatile trace elements [11 – 13]. These fine trace-elementbearing particles are susceptible to human inhalation and, therefore, pose significant health concerns [14]. A recent assessment of the inhalation cancer and noncancer risks associated with trace element emissions from coal-burning utility boilers suggests that As, Cr, and Ni pose the greatest risk from inhalation exposure [7,15]. Arsenic, Cr, and Ni are primary contributors to the inhalation risk estimate mainly because of their known carcinogenic and toxicogenic potencies when in a trivalent oxidation state, hexavalent oxidation state, and subsulfide form (i.e., As3 +, Cr6 +, and Ni3S2), respectively [7,15 – 21]. Trace elements in coal combustion flue gases are generally classified based on volatility and partitioning among bottom ash, fly ash, and flue gas. Nonvolatile elements are distributed between bottom ash and fly ash; semivolatile elements are enriched in fly ash; and volatile elements compose flue gas. This classification scheme is imprecise, and many different classifications have been presented [12]. Semivolatile trace elements generally increase in concentration with decreasing fly ash particle size because of elemental vaporization –ash surface deposition processes [10,22 – 30]. Vapor deposition models have been proposed to account for the dependence of semivolatile trace element concentrations on particle size [22,31]. According to these models, trace element concentrations are inversely proportional to particle diameter (dp); 1/dp and 1/dp2 in the free molecular and continuum transport regimes, respectively. A dp of c 0.4 Am corresponds to the transition from the free molecular to the continuum transport regime in coal combustion flue gases [32,33]. An elemental concentration– particle-size distribution (PSD) trend approximated by 1/dp is indicative of a vapordeposition process, in the continuum regime, controlled by surface chemical reaction(s)

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

703

and/or pore diffusion, whereas a trend characterized by 1/dp2 is consistent with processes controlled by physical condensation and/or film diffusion reaction. Linak and Wendt [34] reviewed the application of these models and concluded that most trace element concentration –PSD trends are consistent with the 1/dp model, indicating that surface chemical reaction and/or pore diffusion processes generally control the deposition of trace elements on ash particles. Trace elements in coal combustion flue gas and fly ash occur in various gaseous and solid chemical forms. The gas –solid partitioning, PSDs, and chemical forms of trace elements are functions of coal composition and mineralogy, elemental modes of occurrence in coal, and combustion conditions (e.g., temperature, residence time, oxygen fugacity). Trace elements in coal are associated with various minerals and macerals [35 – 37]. Mineral-associated trace elements can occur as mineral-forming elements or as nonessential elements that substitute into the crystal structure of another mineral. Trace elements can also adsorb on minerals, such as clays. Maceral-associated trace elements are chemically bound to organic groups, complexes, and chelates. Combustion conditions in utility boilers vary widely because of differences in coal properties, furnace geometry and size, burner type, and boiler configuration and load. In addition, strategies to minimize NOx emissions (e.g., burner optimization, low-NOx burner, air staging, flue gas recirculation, fuel staging) significantly affect combustion conditions. Variations in the residence time –temperature histories of trace element-bearing minerals and macerals probably affect elemental vaporization and deposition processes. Pulverized bituminous Illinois No. 6 and subbituminous Absaloka coals were used to evaluate the importance of coal properties, elemental modes of occurrence, and combustion conditions on As, Cr, and Ni volatility and speciation. Density separation, computercontrolled scanning electron microscopy (CCSEM), and published analysis results were used to identify differences in elemental modes of occurrence for the two coals. The pulverized coals were burned using conventional and low-NOx conditions in an c 7-kW combustion system to assess the effects of these two common combustion strategies on elemental volatilities and PSDs. The As, Cr, and Ni concentrations of size-classified fly ash samples were determined to evaluate elemental PSDs and the mechanisms that control elemental partitioning during combustion and fly ash formation. In addition, the phase composition and As, Ni, and Cr speciation of fly ash samples containing particles c 2.5 Am in aerodynamic diameter (PM2.5) were determined using X-ray diffraction (XRD) and X-ray absorption fine-structure spectroscopy (XAFS).

2. Experimental 2.1. Fly ash sample production and collection Pulverized (i.e., 70– 80% of the coal particles < 75 Am) Illinois No. 6 and Absaloka coals were burned in a downfired cylindrical furnace, depicted in Fig. 1, at c 42 MJ/ h (40,000 Btu/h). Total residence time in the furnace is estimated to be c 2.5 s. The coals were burned using conventional and low-NOx combustion conditions as defined in Table 1. During the low-NOx conditions, lower temperatures in Furnace Sections 2 –5

704

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

Fig. 1. Schematic of the c 7-kW downfired combustion system.

relative to conventional combustion conditions and primary N2 rather than air were used to inhibit the formation of NOx by producing a relatively low temperature, oxygen deficient, coal-rich zone. A greater secondary airflow and higher temperatures in Furnace Sections 6 –8 were used to achieve coal burnout. Duplicate samples of aerodynamically sized fly ash and flue gas were collected from the baghouse inlet of the 42-MJ/h combustor using a five-stage cascade cyclone sampler [38] connected in series with an EPA Method 29 sampling train [39]. The cyclones were coated with polytetrafluoroethylene (PTFE) to minimize metal contamination. Nominal 50% cutoff diameters (d50) for the cyclone stages were 0.7, 1.6, 2.5, 4.3, and 7.7 Am, based on the actual flue gas conditions (temperature, pressure, gas composition, etc.). Scanning electron microscopy (SEM) was used to verify and document the size selectivity of the cyclones [40]. The filter catch d50 was c 0.4 Am. Cyclone measurement results were extrapolated using a cubic spline fit procedure described by McCain et al. [41] to compare fly ash-size distributions. 2.2. Trace element analysis Bulk coal samples, density-separated coal samples, and aerodynamically sized fly ash samples were prepared for trace element analysis using microwave-assisted digestion

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

705

Table 1 Conventional and low-NOx combustion parameters Parameter, unit

Conventional

Low NOx

Primary air, l/min Primary N2 Secondary air, l/min Purge air, l/min Total gas, l/min Fuel feed rate, lb/h Preheat air, jC Secondary air, jC Section 2, jC Section 3, jC Section 4, jC Section 5, jC Section 6, jC Section 7, jC Section 8, jC Heat-exchanger outlet, jC Baghouse inlet, jC

30 NA 137 3 170 3 778 666 1110 1300 1370 1300 1220 970 870 390 180

NAa 16 151 3 170 3 620 520 970 1280 1280 1280 1230 1040 910 400 170

a

Not applicable.

methods (U.S. Environmental Protection Agency [EPA] Method 3050 and American Society for Testing and Materials [ASTM] Method D3683). Trace element analyses were conducted using flame- or graphite furnace-atomic absorption spectroscopy or inductively coupled plasma-atomic emission spectroscopy as described in EPA Methods D3682, 249.2, and 6010, respectively. As a quality control measure, South African Bureau of Standards and National Institute of Standards and Technology standard reference materials 20 (coal) and 1633b (coal fly ash), respectively, were analyzed concurrently with the Illinois No. 6 and Absaloka coals and fly ashes. Results compared favorably with relative percent differences of generally < 20 [40]. 2.3. Density separations Specific gravity (SG) solutions were made by mixing reagent-grade tetrachloroethylene (SG c 1.6) with reagent-grade petroleum ether (SG c 0.8). 20 g of coal were placed into each of four centrifuge flasks. A 320 ml SG solution was added to each flask. Pulverized coal samples were stirred with a teflon stirring rod to optimize coal –solution contact. After mixing, the flasks were centrifuged at 2500 rpm for 20 min. Float fractions were separated by decanting onto a vacuum filter containing rapid-grade filter paper. Sink fractions were rinsed onto a separate filter paper. The float and sink samples were dried in a nitrogen-purged oven and then weighed. Float and sink fractions from each coal were inspected using SEM to evaluate mineral –maceral separation effectiveness. Float fractions were composed predominantly of organic (maceral-rich) coal particles with small proportions of minerals. Most minerals in the float fractions were present as tiny ( < 5 Am in longest dimension) inclusions

706

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

within coal particles. Sink fractions were composed predominantly of larger discrete mineral grains. 2.4. Computer-controlled scanning electron microscopy (CCSEM) Bulk coal samples were embedded in carnauba wax, cross-sectioned, polished (ASTM Standard Practice D2797), and sputter-coated with carbon. CCSEM analyses on the prepared coal samples were performed using a JEOL JSM-5800 SEM equipped with a Noran Instruments Voyager X-ray analysis and automation system. Mineral detection and sizing were performed using digital image analysis techniques. Mineral identification and quantification were performed using the procedures described in Galbreath et al. [42]. 2.5. X-ray diffraction (XRD) XRD analyses were performed on PM2.5 samples. Samples were hand-ground in an agate mortar and pestle and mounted on a ‘‘zero’’-background quartz plate for analysis. XRD patterns were collected over 5 to 70 j2h with a Phillips X’Pert diffractometer system (graphite monochromatized CuK~ radiation, 0.02 j2h steps, 1 s count time per step). 2.6. X-ray absorption fine-structure spectroscopy (XAFS) XAFS analyses of PM2.5 samples were conducted to determine the chemical speciation of Cr, Ni, and As. Cr and As K-edge XAFS spectra were obtained from Beam Line IV-3 at the Stanford Synchrotron Radiation Laboratory, Stanford University, California, and Ni K-edge spectra were obtained from Beam Line X-18B of the National Synchrotron Light Source at Brookhaven National Laboratory, New York. Cr0 and Ni0 foils as well as a thin-smear mount of As2O3 were used as primary standards for the Cr, Ni, and As K-edge XAFS measurements. Ash samples were suspended in the monochromatic X-ray beam in 6-Am-thick polypropylene bags. XAFS spectra were collected using a 13-Ge fluorescent X-ray detector. Spectra were collected at X-ray energies ranging from c 100 eV below to c 600 eV above the Cr, Ni, and As Kabsorption edges (5989, 8333, and 11,867 eV, respectively). K-edge XAFS spectra were divided into two distinct regions for analysis: the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) regions. The EXAFS spectral regions were processed, using relatively standardized procedures as described in Brown et al. [43], to create radial structure function (RSF) spectra that provide structural information, including average interatomic distances and the number and chemical ˚ radius of the absorbing Cr, Ni, and As atoms. identity of atoms within about a 5 A The position of each peak in an RSF spectrum represents the radial distance from each shell of neighboring atoms to the central analyte atom. However, the RSF spectra are not ˚ less than the true phase shift-corrected; thus RSF peak positions are c 0.3 –0.5 A distances. Unfortunately, analyses of the EXAFS regions of Cr and Ni K-edge XAFS spectra were uninformative because of interferences from the relatively abundant V and Fe in the samples. XANES and RSF spectra of reagent-grade compounds, acquired in

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

707

Table 2 Trace element composition of the Illinois No. 6 coal, ppm Element

Subsample 1

Subsample 2

Subsample 3

Mean

F 2ra

As Cr Ni

2.1 20 11

2.1 21 11

2 20 11

2.1 20 11

0.1 1 0

a

95% confidence limits ( F 2r) calculated from the mean of three analyses.

previous investigations [44 – 48], were used essentially as ‘‘fingerprints’’ for identifying Cr, Ni, and As species.

3. Results 3.1. Coal trace element compositions As, Cr, and Ni were analyzed in triplicate for the Illinois No. 6 and Absaloka coals. As indicated in Tables 2 and 3, the Illinois No. 6 coal contains higher concentrations of As, Cr, and Ni relative to the Absaloka coal. 3.2. Trace element modes of occurrence Mineral-rich and maceral-rich fractions were separated from the pulverized Illinois No. 6 and Absaloka coals using a 1.45 SG solution. The relative proportions and ash contents of float and sink fractions as well as the ash contents of the bulk coal samples were used to determine ash mass balance closures and the relative proportions of inorganic matter that are associated with the maceral-rich float fraction and mineral-rich sink fraction for each coal. Ash mass balance closures were acceptable with values of z 95%. Total ash contents of the two bulk coal samples are essentially identical, as shown in Fig. 2. Inorganic matter in the Illinois No. 6 coal is distributed approximately equally between the float and sink fractions. Inorganic matter in the Absaloka coal, however, is partitioned more strongly into the float fraction relative to the sink fraction, indicating that most of the mineral matter is finely dispersed in macerals. As, Cr, and Ni analysis results for the Illinois No. 6 and Absaloka coal samples and their corresponding float – sink fractions are presented in Tables 4 and 5, respectively. Elemental mass balance closures are also reported in Tables 4 and 5. Based on the imprecision in trace element analyses and ash recoveries, elemental mass balance closures Table 3 Trace element composition of the Absaloka coal, ppm Element

Subsample 1

Subsample 2

Subsample 3

Mean

F ra

As Cr Ni

1.1 8.8 4

2.2 11 4.2

1.6 12 4.1

1.6 11 4.1

0.9 3 0.2

a

95% confidence limits ( F 2r) calculated from the mean of three analyses.

708

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

Fig. 2. Bar diagram of ash density separation analysis results. The relative proportions of inorganic matter contained within the float and sink fractions of each coal are indicated within the bars.

of 70% to 130% were considered acceptable. The elemental concentrations from Tables 4 and 5 together with the float –sink data (Fig. 2) were used to determine elemental partitioning between the maceral-rich float fraction and mineral-rich sink fraction for each coal. Arsenic density separation results for the Illinois No. 6 coal were inconclusive because of a low mass balance closure (Table 4). Arsenic is a chalcophile element; thus it is probably associated with the abundant pyrite (Table 6) in Illinois No. 6 coal. Indeed, Galbreath et al. [49] reported As concentrations of V 457 ppm based on electron microprobe analyses of 600 pyrite grains in the sink fraction of an Illinois No. 6 coal. Kolker et al. [50] also identified pyrite as the dominant As host in Illinois No. 6 coal using XAFS, selective leaching, and electron microprobe analyses. In general, As mode of occurrence analyses of bituminous coals indicate that pyrite is a dominant As host [35 – 37,51]. Density separation results for the Absaloka coal suggest that c 32% of the As is associated with pyrite in the sink fraction and c 68% is associated with macerals in the float fraction. XAFS analyses of subbituminous coals from western U.S. (including Table 4 Illinois No. 6 coal trace element analysis results Element, ppm

Bulk coal

Float (SG V 1.45 g/cm3)

Sink (SG >1.45 g/cm3)

Elemental mass balance closure, %

As Cr Ni

3.3 17 13

< 0.8 18 14

5.2 42 63

31 124 156

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

709

Table 5 Absaloka coal trace element analysis results Element, ppm

Bulk coal

Float (SG V 1.45 g/cm3)

Sink (SG >1.45 g/cm3)

Elemental mass balance closure, %

As Cr Ni

1.1 8.8 4

0.86 9.2 3.4

2.6 26 5.2

99 130 91

Powder River Basin [PRB] coals) and Canada, similar to Absaloka coal, indicate that As is present primarily as As3 + in oxygen coordination [50,52]. The organic association indicated by the density separation procedure combined with XAFS evidence is consistent with the presence of As3 + dispersed through macerals and bound to them via carboxyl groups or other hydrated oxygen functional groups. During combustion, As-bearing pyrite and macerals will decompose rapidly, and gaseous As0 will be liberated. Arsenic is expected to oxidize readily in the flue gas environment forming gaseous As oxides (e.g., AsO[g], As2O3[g], and As2O5[g]) [32,53 –55]. Mass balance calculations indicate that 74% of the Cr in Illinois No. 6 coal is associated primarily with the maceral-rich float fraction. XAFS analyses by Huggins et al. [47] indicate that an amorphous CrO(OH) phase occurs within the macerals of Illinois No. 6 coal. Electron microprobe analyses by Galbreath et al. [49] also indicate that Cr in Illinois No. 6 coal is associated with very fine-grained illite (nominal formula—K[Al, Fe, Cr]2[Si, Al]4O10
[(OH)2, H2O]). Illite is a major mineral in Illinois No. 6 coal (Table 6). Srinivasachar et al. [56] showed that, during Illinois No. 6 coal combustion, K and Fe are not significantly volatilized from illite, but rather are incorporated into molten illite. Analogous to the behavior of K and Fe, Cr is probably incorporated into a chemically complex aluminosilicate melt phase during the formation of Illinois No. 6 coal fly ash. Similar to the Illinois No. 6 coal, 70% of the Cr in Absaloka coal is associated with the maceral-rich float fraction. XAFS evidence for Cr3 +UO bonds in subbituminous coals Table 6 Coal mineral compositions, wt.% Mineral

Illinois No. 6

Absaloka

Quartz Hematite Rutile Calcite Dolomite Kaolinite Mixed Clays Illite Pyrite Gypsum Barite Apatite

16 0.8 NDa 5.9 ND 9.8 6.8 13 47.7 ND ND ND

22.4 4.8 0.6 9.9 1.5 38.9 3.6 3.6 11.7 1.2 0.7 1

a

Not detected.

710

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

[47] suggests that Cr3 + is bound to macerals through carboxyl groups or other hydrated oxygen functional groups. During Absaloka coal combustion, the maceral-associated fraction of Cr is probably liberated as gaseous chromium oxides and oxyhydroxides. This hypothesis is consistent with thermodynamic modeling results which predict that Cr is liberated from coal during combustion as a mixture of CrO 2 (g), CrO(OH)(g), CrO2(OH)(g), CrO(OH)2(g), and CrO2(OH)2(g) [53,57]. Nickel float – sink separation results for the Illinois No. 6 coal were inconclusive for deciphering Ni modes of occurrence because of a high mass balance closure (Table 4). Electron microprobe analyses of pyrite in Illinois No. 6 coal by Galbreath et al. [49] indicate, however, that pyrite is an important host with Ni concentrations of V 1360 ppm. Approximately 80% of the Ni in Absaloka coal is associated with the maceral-rich float fraction. 3.3. Mineralogies Mineralogies of the Illinois No. 6 and Absaloka coals are compared in Table 6. The Illinois No. 6 coal is enriched in mixed clays, illite, and pyrite but depleted in quartz, hematite, rutile, calcite, dolomite, kaolinite, gypsum, barite, and apatite relative to the Absaloka coal. The pyrite contents reported in Table 6, however, are biased high. This well-known CCSEM bias is related to the image segmentation technique of thresholding and to the large gray-scale range in backscattered electron images of pyrite-bearing coals [42,58]. Nevertheless, the results in Table 6 are useful for comparing coal mineralogies on a semiquantitative basis. 3.4. Coal combustion flue gas compositions The coal combustion flue gas compositions produced from burning the pulverized Illinois No. 6 and Absaloka coals under conventional and low-NOx combustion conditions are compared in Table 7. The coals were burned using similar excess O2 concentrations under both combustion conditions. NOx concentrations were reduced by about 35% as a result of the low-NOx combustion strategy. Relative to the Absaloka flue gases, SO2 concentrations in the Illinois No. 6 coal combustion flue gases were significantly higher primarily because of the much greater pyrite content in the Illinois No. 6 coal (Table 6). Table 7 Coal combustion flue gas compositions Illinois No. 6

Excess O2, vol.% CO2, vol.% CO, ppmv SO2, ppmv NOx, ppmv NO, ppmv

Absaloka

Conventional

Low NOx

Conventional

Low NOx

4.8 15.1 51 2220 992 746

5.07 13.2 11 2040 640 441

5.35 15.8 26 500 1060 810

4.66 13.9 15 471 717 532

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

711

3.5. PSDs PSDs for the Illinois No. 6 and Absaloka coal fly ashes are presented in Figs. 3 and 4, respectively. The Illinois No. 6 coal fly ash is finer relative to the Absaloka coal fly ash produced under conventional combustion conditions. The low-NOx combustion conditions (Table 1) produce a coarser fly ash for a given coal. SEM analyses showed that the ash samples produced from the conventional combustion of coal consist primarily of discrete spherical particles, whereas particle agglomerates are characteristic of the low-NOx ash samples. Particle agglomeration is the result of sintering between individual ash particles. The longer residence times at relatively high temperatures (>900 jC) and slower mixing of coal and air associated with the low-NOx testing conditions enhance the sintering propensity of fine ash particles [59]. 3.6. As, Cr, and Ni volatility and PSD Samples of size-classified fly ash were collected in duplicate using five cascade cyclones and a backup filter. As, Cr, and Ni analyses were performed in duplicate on the coarser ash fractions (d50 z 4.3 Am). The finer ash fractions (d50 < 4.3 Am) from duplicate samples were combined to provide sufficient quantities of representative fine ash samples for reliably analyzing trace elements. Trace element concentrations for fly ashes collected on the cyclone backup filter upstream of the impinger train assembly actually

Fig. 3. Comparison of PSDs for the Illinois No. 6 coal fly ashes produced under conventional (solid lines) and low-NOx (dashed lines) combustion conditions. Solid symbols represent five-stage multicyclone measurement results; open symbols represent extrapolation (spline-fit) results.

712

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

Fig. 4. Comparison of PSDs for the Absaloka coal fly ashes produced under conventional (solid lines) and lowNOx (dashed lines) combustion conditions. Large symbols represent five-stage multicyclone measurement results; small symbols represent extrapolation (spline fit) results.

represent the composition of particles ranging in diameter from essentially zero to the d50 of the last cyclone ( c 0.7 Am). A d50 of 0.4 Am was assumed for the backup filter. As, Cr, and Ni were not detected in the impinger solutions of the EPA Method 29 sampling train. Trace element analysis results of the size-fractionated fly ash samples were used to evaluate As, Cr, and Ni volatility and PSDs. As, Cr, and Ni enrichments and depletions of the size-classified fly ash fractions relative to bulk Illinois No. 6 and Absaloka coal ashes were evaluated using the following relative enrichment/depletion (RED) factor devised by Meij [30]: RED factor ¼ elemental concentration in fly ash  coal ash fractionHelemental concentration in coal RED factors that increase with increasing particle size or are consistently less than c 2 over all particle-size fractions are indicative of nonvolatile elements. RED factors of b1 are indicative of elemental volatility. Semivolatile elements are characterized by trends of increasing RED factors with decreasing particle size. Regression analyses of elemental concentration and corresponding d50 results were conducted to evaluate the relative importance of deposition mechanisms, surface reaction, and/or pore diffusion (elemental concentration ~1/dp) versus film condensation (elemental concentration ~1/dp2), in controlling semivolatile element enrichments on fine ash particles. As, Cr, and Ni concentrations for the size-fractionated fly ash samples produced from burning the pulverized Illinois No. 6 coal under conventional and low-NOx combustion

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

713

Fig. 5. As, Cr, and Ni concentrations of size-classified fly ashes produced from the conventional and low-NOx combustion of Illinois No. 6 coal.

Fig. 6. RED factors for the size-classified fly ashes produced from the conventional and low-NOx combustion of Illinois No. 6 coal.

714

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

Fig. 7. As, Cr, and Ni concentrations versus 1/d50 for the Illinois No. 6 coal fly ashes; r2 = Pearson correlation coefficient.

Fig. 8. As, Cr, and Ni concentrations of size-classified fly ashes produced from the conventional and low-NOx combustion of Absaloka coal.

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

715

Fig. 9. RED factors for the size-classified fly ashes produced from the conventional and low-NOx combustion of Absaloka coal.

Fig. 10. Ni concentrations versus 1/d50 for the Absaloka coal fly ashes produced under conventional and low-NOx combustion conditions; r2 = Pearson correlation coefficient.

716

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

Table 8 PM2.5 inorganic phase assemblages Mineral, formula

Illinois No. 6 Conventional

Low NOx

Conventional

Low NOx

Glass Quartz, SiO2 Ferrite spinel,b AB2O4 Mullite, Al6Si2O13 Anhydrite, CaSO4 Lime, CaO Periclase, MgO

Xa X X X X

X X X X X

X X X

X X X X

X X

X X

a b

Absaloka

‘‘X’’ denotes that the corresponding crystalline phase was identified using XRD. E.g., where A2 + = Fe, Mg, Co, Cu, Ni and B3 + = Al, Fe, Cr.

conditions are presented in Fig. 5. RED factors are presented in Fig. 6 for the conventional and low-NOx combustion fly ashes. Cr was nonvolatile during the conventional combustion of Illinois No. 6 coal as indicated by consistent concentrations and RED factors. However, Cr was partially volatilized during low-NOx combustion as evidenced by trends of increasing concentration and RED factors with decreasing particle size. As and Ni concentrations and RED factors for both Illinois No. 6 fly ashes generally increase with decreasing particle size. These trends are indicative of semivolatile trace element behavior.

Fig. 11. As K-edge XANES spectra for Illinois No. 6 and Absaloka PM2.5 fractions produced under conventional and low-NOx combustion conditions. Value of zero on the abscissa corresponds to the actual AS K-edge absorption energy of 11,867 eV.

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

717

Linear regression analyses of elemental concentrations in Fig. 7 indicate that As, Cr, and Ni concentrations are consistent with the 1/dp dependence expected for a surface reaction- or pore diffusion-controlled deposition process. Low-NOx combustion conditions generally promoted the elemental vaporization– particle surface deposition process, as evidenced by greater elemental concentrations on fine-particle surfaces. As, Cr, and Ni concentrations for the size-fractionated fly ash samples produced from burning pulverized Absaloka coal under conventional and low-NOx combustion conditions are presented in Fig. 8. RED factors for the conventional and low-NOx Absaloka fly ashes are presented in Fig. 9. As and Cr were not volatilized and subsequently deposited on fly ash surfaces as evidenced by their relatively consistent concentrations and RED factors (Figs. 8 and 9). Ni was partially volatilized during conventional and low-NOx combustion as indicated by systematic increases in RED factors and concentrations with decreasing particle size (Figs. 8 and 9). Ni concentrations in the Absaloka coal fly ashes correlate to 1/dp, as indicated in Fig. 10, suggesting that Ni deposition on fine ash particles was a surface reaction- or pore diffusion-controlled process. 3.7. Inorganic phase compositions Inorganic phase compositions of PM2.5 were determined using XRD. As indicated in Table 8, glass, quartz, and ferrite spinel are ubiquitous in the Illinois No. 6 and Absaloka

Fig. 12. As RSF spectra for Illinois No. 6 and Absaloka PM2.5 fractions produced under conventional and lowNOx combustion conditions.

718

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

coal PM2.5 produced under conventional and low-NOx combustion conditions. Mullite is also an ubiquitous phase in Illinois No. 6 PM2.5, but was only detectable in Absaloka PM2.5 produced under low-NOx conditions. Lime and periclase are absent from Illinois No. 6 PM2.5, but are present in Absaloka PM2.5. Anhydrite is present in Illinois No. 6 PM2.5 because of the much higher S content of Illinois No. 6 coal. 3.8. As, Cr, and Ni speciation of PM2.5 Arsenic XANES and RSF spectra for the Illinois No. 6 and Absaloka PM2.5 are presented in Figs. 11 and 12. The As K-edge absorption in XANES spectra occurs over a 10-eV interval beginning at c 0 eV for As0 and at c 4 eV for As5 + compounds [44,45]. The absorption edge increases in height, decreases in full-width-half maximum, and shifts to greater energy with increasing As oxidation state. Similarities in the XANES spectra, Fig. 11, indicate that the As oxidation state for the four ash samples is the same. Locations of the most intense peak in both types of spectra, XANES peak at 3.6 F 0.1 eV and 4.2 F 0.1 eV for the Illinois No. 6 and Absaloka PM2.5, respectively, ˚ are indicative of the As5 + oxidation state; furthermore, these and RSF peak at c 1.3 A spectral features are consistent with As5 + in fourfold coordination with oxygen to form AsO4 units [44,45]. Subtle differences in the shape, width, and location of the major XANES peak, however, indicate that the As5 +O4 units occur in different phases in the Illinois No. 6 and Absaloka PM2.5.

Fig. 13. Cr K-edge XANES spectra for Illinois No. 6 and Absaloka PM2.5 fractions produced under conventional and low-NOx combustion conditions. Value of zero on the abscissa corresponds to the actual Cr K-edge absorption energy of 5989 eV.

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

719

Cr XANES spectra for the Illinois No. 6 and Absaloka PM2.5 are presented in Fig. 13. XANES spectra for PM2.5 produced from the conventional and low-NOx combustion of a given coal are essentially identical, indicating that combustion conditions did not affect Cr speciation. Spectra for the Illinois No. 6 and Absaloka PM2.5, however, are significantly different, indicating that differences in Cr modes of occurrence and/or coal composition and hence flue gas composition strongly affected Cr speciation. The presence of Cr6 + is distinguished from Cr3 + in XANES spectra by an intense preedge peak that accounts for about 95% of the edge step absorption, compared to only about 5% for Cr3 + [46,60]. Height differences in the preedge peak were used to quantify the proportions of Cr3 +and Cr6 +. Based on the calibration of Huggins et al. [46], it is estimated that 6% of the total Cr (170 ppm) in Illinois No. 6 PM2.5 is present as Cr6 +, whereas 43% of the total Cr (130 ppm) in Absaloka PM2.5 is present as Cr6 +. Unfortunately, Cr K-edge EXAFS spectra could not be used to more rigorously investigate the speciation of Cr because of spectral interferences. Presented in Fig. 14 are Ni XANES spectra for the Illinois No. 6 and Absaloka PM2.5. Spectral signal-to-noise ratios in Fig. 14 are relatively low because of the low Ni and relatively high Fe contents of PM2.5. Both sets of spectra for the PM2.5 samples are essentially identical, suggesting that differences in combustion conditions did not significantly affect Ni speciation. The absence of a preedge peak and the presence of an intense peak at c 16 eV is characteristic of Ni2 + in coordination with oxygen [48].

Fig. 14. Ni K-edge XANES spectra for Illinois No. 6 and Absaloka PM2.5 fractions produced under conventional and low-NOx combustion conditions. Value of zero on the abscissa corresponds to the actual Ni K-edge absorption energy of 8333 eV.

720

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

4. Discussion 4.1. Arsenic Arsenic concentrations in the Illinois No. 6 fly ashes generally increase with decreasing ash particle size (Fig. 5). These trends approximate 1/dp (Fig. 7) suggesting that As deposition on ash particles was surface reaction- and/or pore diffusion-controlled. XAFS analyses indicate that As is present in the Illinois No. 6 and Absaloka PM2.5 fractions as As+ 5O4 units. These units, however, occur in different phases in the Illinois No. 6 and Absaloka PM2.5. Arsenic phases in the coal fly ashes could not be identified from examining a limited XAFS spectral database of As compounds. However, the presence of carboxyl-bound As3 + and Ca in the Absaloka coal and CaO in the Absaloka coal fly ashes (Table 8) as well as thermodynamic modeling results [32,53] and results from an experimental investigation of As –CaO interaction [61] suggest strongly that Ca3(AsO4)2 is the dominant species in the Absaloka fly ashes. Huggins et al. [44] and Huffman et al. [45] have also postulated the presence of Ca3(AsO4)2 based on an XAFS analysis of lignitic coal fly ash. In addition, Irgolic et al. [62] identified Ca3(AsO4)2 in coal fly ash using a combination of laser microprobe mass analysis, selective leaching/chromatography, and electron microprobe measurements. 4.2. Chromium Although Cr was nonvolatile during the conventional combustion of Illinois No. 6 coal as indicated by consistent concentrations and RED factors, it was partially volatilized during low-NOx combustion, as evidenced by trends of increasing concentration and RED factors with decreasing particle size. Chromium RED factor and concentration – PSDs in the Absaloka fly ashes are characteristic of a nonvolatile element. XAFS analyses of PM2.5 samples indicate that the conventional and lowNOx combustion conditions did not significantly affect Cr speciation. However, different Cr modes of occurrence and/or coal compositions (i.e., flue gas compositions) strongly affected Cr speciation. Chromium in Illinois No. 6 PM2.5 is present predominantly as Cr3 + ( c 94%), whereas Cr in Absaloka PM2.5 is composed, on average, of 57% Cr3 + and 43% Cr6 +. The proportions of Cr6 + in Absaloka PM2.5 are much larger than any previous XAFS measurements of ashes produced under typical coal combustion conditions [46]. A significant proportion, 52 F 5%, of Cr6 + was found, however, in a high-temperature ash prepared by heating a Wyodak PRB subbituminous coal at 500 jC in air for 24 h [46]. The predominance of maceral-bound Cr3 + and oxygen functional groups in PRB subbituminous coals may promote Cr6 + formation during combustion and/or fly ash formation. 4.3. Nickel Nickel is distributed similarly in the fly ashes produced by burning Illinois No. 6 and Absaloka coal under conventional and low-NOx conditions; i.e., Ni concentrations and RED factors generally increase with decreasing particle size. The application of surface

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

721

deposition models (Fig. 7) to Ni concentrations suggests that surface reaction- and/or pore diffusion-controlled Ni deposition occurs on fly ash particles ranging from a d50 of c 0.7 to 7.7 Am. During the conventional combustion of Absaloka coal, Ni was nonvolatile. Nickel concentration- and RED factor-particle size systematics suggest, however, that Ni was semivolatile during the low-NOx combustion of Absaloka coal (Figs. 8 and 9). Presumably, the longer residence times at the relatively high temperatures (>900 jC) associated with low-NOx combustion conditions promoted Ni volatilization. Nickel XAFS analyses indicate the presence of Ni2 + oxide phases in the four Illinois No. 6 and Absaloka PM2.5 samples. Apparently, Ni2 + oxide phases are associated with the surfaces of Absaloka fly ash produced under low-NOx combustion conditions. Ferrite spinel (e.g., [Fe2 +, Mg, Ni] [Fe3 +, Al]2O4), identified in the PM2.5 samples (Table 8), is also a likely host for Ni. Ferrite spinel is a thermal decomposition product of pyrite in coal [63,64]. Chemical analyses of ferrite spinel in coal fly ashes indicate that it is an important host for transition metals such as Co, Cr, Cu, and Ni, but lacks As, Sb, Hg, Pb, and Se [65 – 67].

5. Summary and conclusions Density separation analysis, CCSEM, and published XAFS analysis results were used to determine As, Cr, and Ni modes of occurrence in bituminous Illinois No. 6 and subbituminous Absaloka coals. As indicated in Table 9, trace elements in the Illinois No. 6 coal are generally associated with relatively large discrete mineral grains, especially pyrite. Conversely, trace elements in the Absaloka coal are much more strongly associated with macerals and very fine-grained minerals. Pulverized Illinois No. 6 and Absaloka coals were burned using conventional and lowNOx conditions in an c 42-MJ/h combustion system. Size-classified fly ash and flue gas samples were collected using cascade cyclones in series with chilled impinger solutions and then chemically analyzed to evaluate the importance of coal composition and mineralogy, elemental modes of occurrence, and combustion conditions on trace element volatility. Trace element volatilities of the Illinois No. 6 and Absaloka coals are summarized in Table 10. Nonvolatile elements are characterized by relatively uniform PSDs or are more concentrated in coarse fly ash particles. Semivolatile elements increase in concentration with decreasing particle size and their concentration– PSD trends are consistent with the surface deposition models of Davison et al. [22] or Flagan and

Table 9 Summary of dominant trace element modes of occurrence Element

Illinois No. 6

Absaloka

As Cr Ni

Pyrite Maceral (CrO[OH]) + illite Pyrite

Pyrite + maceral Maceral Maceral

722

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

Table 10 Summary of Illinois No. 6 and Absaloka coal trace element volatility Coal

Nonvolatile a

Illinois No. 6 Absaloka a b

Cr As and Cr

Semivolatile As, Cr,b and Ni Ni

Conventional combustion conditions. Low-NOx combustion conditions.

Friedlander [31]. Low-NOx combustion conditions generally promoted the elemental vaporization – particle surface deposition process, as evidenced by greater elemental concentrations on fine particle surfaces. The inorganic phase composition and elemental (As, Cr, and Ni) speciation of PM2.5 fractions were determined using XRD and XAFS. The Illinois No. 6 and Absaloka PM2.5 fractions produced during conventional and low-NOx combustion contain aluminosilicate glass, quartz (SiO2), ferrite spinel (AB2O4; e.g., where A2 + = Fe, Mg, Ni, Co, Cu and B3 + = Al, Fe, Cr), and mullite (Al6Si2O13). These phases may be useful tracers for investigating the influence of coal-burning power plant emissions on ambient PM2.5 composition. Absaloka PM2.5 fractions are distinguished from Illinois No. 6 PM2.5 fractions by the presence of lime (CaO) and periclase (MgO) and lack of anhydrite (CaSO4). The higher sulfur content of Illinois No. 6 coal promoted anhydrite formation. XAFS speciation results for As, Cr, and Ni in Illinois No. 6 and Absaloka PM2.5 fractions are summarized in Table 11. Differences in Illinois No. 6 and Absaloka coal combustion conditions did not significantly affect As, Cr, or Ni speciation. Arsenic is present as different As+ 5O4-containing phases in the Illinois No. 6 and Absaloka PM2.5 fractions. Theoretical and empirical evidence strongly suggest that Ca3(AsO4)2 is the dominant species in the Absaloka fly ashes [44,45,53,61,62]. Presumably, the presence of maceral-bound As and Ca in Absaloka coal, which is lacking in the Illinois No. 6 coal, promotes the formation of Ca3(AsO4)2. The reactive capture of As by CaO during Absaloka ash formation renders As nonvolatile. The Cr3 + – Cr6 + ratio is much greater in the Illinois No. 6 PM2.5 fractions relative to Absaloka PM2.5 fractions, indicating that differences in coal compositions and/or Cr modes of occurrence affected Cr oxidation. The predominance of maceral-bound Cr3 + and oxygen functional groups in Absaloka coal may have promoted the formation of Cr6 + during combustion and/or fly ash formation. Ni2 + oxide phases occur in the Illinois No. 6 and Absaloka PM2.5 fractions. The XRD identification of ferrite spinel in the PM2.5 fractions combined with published chemical analyses [66,67] indicate that ferrite spinel is a likely oxide mineral host for Ni and other transition metals such as Cr, Co, and Cu. Table 11 Summary of As, Cr, and Ni speciation results Coal

As

Cr

Ni

Illinois No. 6 Absaloka

As5 +O4 As5 +O4

c 94% Cr3 +, c 6% Cr6 + c 57% Cr3 +, c 43% Cr6 +

Ni2 +Ox Ni2 +Ox

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

723

Acknowledgements The authors thank Frank Huggins and Gerald Huffman from the University of Kentucky for performing the XAFS analyses on selected fly ash samples. Although the research described in this article has been funded in part by the U.S. Environmental Protection Agency through Grant No. R 824854-01-0 to the University of North Dakota, it has not been subjected to the Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. References [1] U.S. Department of Energy, Coal Industry Annual 2000, Energy Information Administration, Washington, DC, 2000DOE/EIA-0584. [2] J.M. Pacyna, Atmospheric trace elements from natural and anthropogenic sources, in: J.O. Nriagu, C.I. Davidson (Eds.), Toxic Metals in the Atmosphere, Wiley, New York, 1986. [3] D.C. Chilvers, P.J. Peterson, in: T.C. Hutchinson, K.M. Meema (Eds.), Lead, Mercury, Cadmium and Arsenic in the Environment, Wiley, New York, 1987, p. 279. [4] J.O. Nriagu, J.M. Pacyna, Quantitative assessment of worldwide contamination of air, water, and soils by trace metals, Nature 333 (1988) 134 – 139. [5] J.O. Nriagu, Global metal pollution: poisoning the biosphere? Environment 32 (7) (1990) 7 – 33. [6] Electric Power Research Institute. Electric Utility Trace Substances Synthesis Report; Report No. EPRI TR104614, Project 3081, 1994. [7] U.S. Environmental Protection Agency, Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units—Final Report to Congress, vol. 1, Office of Air Quality, Planning and Standards, Research Triangle Park, NC, 1998, EPA-453/R-98-004a. [8] Provisions for attainment and maintenance of national ambient air quality standards, U.S. Public Law, 1990, pp. 101 – 549 http://www.epa.gov/oar/caa/contents. [9] M.W. McElroy, R.C. Carr, D.S. Ensor, G.R. Markowski, Size distribution of fine particles from coal combustion, Science (1982 Jan.) 13 – 19. [10] E.I. Kauppinen, T.A. Pakkanen, Coal combustion aerosols: a field study, Environ. Sci. Technol. 24 (1990) 1811 – 1818. [11] A.D. Shendrikar, D.S. Ensor, S.J. Cowen, G.J. Woffinden, M.W. McElroy, Size-dependent penetration of trace elements through a utility baghouse, Atmos. Environ. 17 (1983) 1411 – 1421. [12] L.B. Clarke, L.L. Sloss, Trace Elements-Emissions from Coal Combustion and Gasification, IEA Coal Research, London, UK, 1992 (July) IEACR/49, 111 pp. [13] M. Mohr, S. Ylatalo, N. Klippel, E.I. Kauppinen, O. Riccius, H. Burtscher, Submicron fly ash penetration through electrostatic precipitators at two coal powerplants, Aerosol Sci. Tech. 24 (1996) 191 – 204. [14] C.A. Pope, Review: epidemiological basis for particulate air pollution health standards, Aerosol Sci. Tech. 32 (2000) 4 – 14. [15] C. French, W. Peters, B. Maxwell, G. Rice, A. Colli, R. Bullock, J. Cole, E. Heath, J. Turner, B. Hetes, D.C. Brown, D. Goldin, H. Behling, D. Loomis, C. Nelson, Assessment of health risks due to hazardous air pollutant emission from electric utilities, Drug Chem. Toxicol. 20 (4) (1997) 375 – 386. [16] N.C. Korte, Q. Fernando, A review of arsenic (III) in groundwater, Crit. Rev. Environ. Control 21 (1991) 1 – 39. [17] R.K. Bharma, M. Costa, Trace elements: aluminum, arsenic, cadmium, mercury, and nickel, Environmental Toxicants: Human Exposures and Their Health Effects, 1992, pp. 575 – 632. [18] Nickel Producers Environmental Research Association (NiPERA), Occupational exposure limits criteria document for nickel and nickel compounds, Summary, Conclusions, and Recommendations, vol. 1, Prepared for the European Commission, Directorate-General V, Public Health and Safety at Work Direcorate. Prepared by NiPERA, Durham, NC, in collaboration with Eurometaux.

724

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

[19] L. Benramdane, F. Bressolle, J.J. Vallon, Arsenic speciation in humans and food products: a review, J. Chromatogr. Sci. 37 (1999) 330 – 344. [20] D.E. Kimbrough, Y. Cohen, A.M. Winer, L. Creelman, C. Mabuni, A critical assessment of chromium in the environment, Crit. Rev. Environ. Sci. Tech. 29 (1) (1999) 1 – 46. [21] Toxicology Excellence for Risk Assessment (TERA), Toxicological Review of Soluble Nickel Salts, Prepared for the Metal Finishing Association of Southern California, U.S. Environmental Protection Agency, and Health Canada. Prepared by TERA, Cincinnati, OH, under subcontract in part with Science Applications International, EPA Contract No. 68-C7-0011 (1999). [22] R.L. Davison, D.F.S. Natusch, J.R. Wallace, C.A. Evans Jr., Dependence of concentration on particle size, Environ. Sci. Technol. 8 (1974) 1107 – 1113. [23] D.H. Klein, A.W. Andren, J.A. Carter, J.F. Enery, C. Feldman, W. Fulkerson, W.S. Lyon, J.C. Ogle, Y. Talmi, Van Hook, R.I. Van Hook, N. Bolton, Pathways of thirty-seven trace elements through coal-fired power plant, Environ. Sci. Technol. 9 (1975) 973 – 979. [24] E.S. Gladney, J.A. Small, G.E. Gordon, W.H. Gordon, W.H. Zoller, Composition and size distribution of instack particulate matter at a coal-fired power plant, Atmos. Environ. 10 (1976) 1071 – 1077. [25] R.D. Smith, J.A. Campbell, K.K. Nielson, Concentration dependence upon particle size of volatilized elements in fly ash, Environ. Sci. Technol. 13 (1979) 553 – 558. [26] A.H. Biermann, J.M. Ondov, Application of surface-deposition models to size-fractionated coal fly ash, Atmos. Environ. 14 (1980) 289 – 295. [27] R.J. Quann, M. Neville, M. Janghorbani, C.A. Mims, A.F. Sarofim, Mineral matter and trace-element vaporization in a laboratory-pulverized coal combustion system, Environ. Sci. Technol. 16 (1982) 776 – 781. [28] G.R. Markowski, R. Filby, Trace element concentration as a function of particle size in fly ash from a pulverized coal utility boiler, Environ. Sci. Technol. 19 (1985) 796 – 804. [29] J.M. Ondov, C.E. Choquette, W.H. Zoller, G.E. Gordon, A.H. Biermann, R.E. Heft, Atmospheric behavior of trace elements on particles emitted from a coal-fired power plant, Atmos. Environ. 23 (1989) 2193 – 2204. [30] R. Meij, Trace element behavior in coal-fired power plants, Fuel Process. Technol. 39 (1994) 199 – 217. [31] R.C. Flagan, S.K. Friedlander, Particle formation in pulverized coal combustion: a review, in: D.T. Shaw (Ed.), Recent Developments in Aerosol Science, Wiley, New York, 1978, pp. 25 – 29. [32] L.E. Bool, J.J. Helble, A laboratory study of the partitioning of trace elements during pulverized coal combustion, Energy Fuels 9 (1995) 880 – 887. [33] M. Neville, A.F. Sarofim, The fate of sodium during pulverized coal combustion, Fuel 64 (1985) 384 – 390. [34] W.P. Linak, J.O.L. Wendt, Trace metal transformation mechanisms during coal combustion, Fuel Process. Technol. 39 (1994) 173 – 198. [35] D.J. Swaine, Trace Elements in Coal, Butterworth, London, UK, 1990, 276 pp. [36] R.B. Finkelman, Modes of occurrence of potentially hazardous elements in coal: levels of confidence, Fuel Process. Technol. 39 (1994) 21 – 34. [37] F.E. Huggins, G.P. Huffman, Application of XAFS spectroscopy to coal geochemistry, in: M.D. Dyar, C. McCammon, M.W. Schaefer (Eds.), Mineral Spectroscopy: A Tribute to Roger G. Burns, Special Publication, vol. 5, The Geochemical Society, Washington University, St. Louis, MD, 1996, pp. 133 – 151. [38] W.B. Smith, R.R. Wilson Jr., Development and Laboratory Evaluation of a Five Stage Cyclone System, EPA 600/7-78-008 (NTIS PB 279084), U.S. Environmental Protection Agency, Research Triangle Park, NC, 1978, 66 pp. [39] U.S. Environmental Protection Agency, Method 29—Determination of Metals Emissions from Stationary Sources, U.S. EPA, Emission Measurement Technical Information Center, TM-029 (http://www.epa.gov/ ttm/emc/promgate.html). [40] Center for Air Toxic Metals Final Technical Report, vol. III, Energy & Environmental Research Center Publication, University of North Dakota, Grand Forks, ND, 2000 Sept., October 1, 1996 – September 30, 2000. [41] J.D. McCain, S.S. Dawes, W.E. Farthing, Procedures Manual for the Recommended ARB Sized Chemical Sample Method (Cascade Cyclones). Prepared by Southern Research Institute as an Account of Work for The California Air Resources Board, Report No. SoRI-EAS-86-467, May 1986, 120 pp.

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

725

[42] K. Galbreath, C. Zygarlicke, G. Casuccio, T. Moore, P. Gottlieb, N. Agron-Olshina, G. Huffman, A. Shah, N. Yang, J. Vleeskens, G. Hamburg, Collaborative study of quantitative coal mineral analysis using computer-controlled scanning electron microscopy, Fuel 75 (1996) 424 – 430. [43] G.E. Brown Jr., G. Calas, G.A. Waychunas, J. Petiau, X-ray absorption spectroscopy and its applications in mineralogy and geochemistry, in: P.H. Ribbe (Ed.), Spectroscopic Methods in Mineralogy and Geology, Mineral. Soc. Am. Rev. Mineral., vol. 18 (11), Book Crafters Inc., Chelsea, Michigan, 1988, pp. 431 – 512. [44] F.E. Huggins, J.J. Helble, N. Shah, J. Zhao, S. Srinivasachar, J.R. Morency, F. Lu, G.P. Huffman, Forms of occurrence of arsenic in coal and their behavior during coal combustion, Prepr. Pap.-Am. Chem. Soc., Div. Environ. Chem. 38 (1993) 265 – 271. [45] G.P. Huffman, F.E. Huggins, N. Shah, J. Zhao, Speciation of arsenic and chromium in coal and combustion ash by XAFS spectroscopy, Fuel Process. Technol. 39 1994, pp. 47 – 62. [46] F.E. Huggins, M. Najih, G.P. Huffman, Direct speciation of chromium in coal combustion by-products by Xray absorption fine-structure spectroscopy, Fuel 78 (1999) 233 – 242. [47] F.E. Huggins, N. Shah, G.P. Huffman, A. Kolker, S. Crowley, C.A. Palmer, R.B. Finkelman, Mode of occurrence of chromium in four US coals, Fuel Process. Technol. 63 2000, pp. 79 – 92. [48] K.C. Galbreath, C.J. Zygarlicke, D.L. Toman, F.E. Huggins, G.P. Huffman, Nickel and chromium speciation of residual oil combustion ash, Combust. Sci. Technol. 134 (1 – 6) (1998) 243 – 262. [49] K.C. Galbreath, R.A. DeWall, C.J. Zygarlicke, IEA Coal Research Collaborative Programme on Modes of Occurrence of Trace Elements in Coal—Results from the Energy and Environmental Research Center; Final Report prepared for Dr. L. Dale, Division of Coal and Energy Technology, Lucas Heights Science and Technology Centre, Australia, Report No. 99-EERC-11-03, Nov, 1999, 20 pp. [50] A. Kolker, F.E. Huggins, C.A. Palmer, N. Shah, S.S. Crowley, G.P. Huffman, R.B. Finkelman, Mode of occurrence of arsenic in four U.S. coals, Fuel Process. Technol. 63 (2 – 3) 2000, pp. 167 – 178. [51] D.A. Spears, L.I. Manzanares-Papayanopoulos, C.A. Booth, The distribution and origin of trace elements in a UK coal: the importance of Pyrite, Fuel 78 (1999) 1671 – 1677. [52] F.E. Huggins, F. Goodarzi, C.J. Lafferty, Mode of occurrence of arsenic in subbituminous coals, Energy Fuels 10 (1996) 1001 – 1004. [53] F. Frandsen, K. Dam-Johansen, P. Rasmussen, Trace elements from combustion and gasification of coal—an equilibrium approach, Pror. Energy Combust. Sci. 20 (1994) 115 – 138. [54] S. Srinivasachar, J.R. Morency, B.E. Wyslouzil, Presented at the 85th Annual Meeting of the Air and Waste Management Association, June 21 – 26, 1992, Kansas City, MO. Paper 92-41.07, Air and Waste Managament Association, Pittsburgh, PA (1992). [55] R.M. Winter, R.R. Mallepalli, K.P. Hellem, S.W. Szydlo, Combust. Sci. Technol. 101 (1994) 45 – 58. [56] S. Srinivasachar, J.J. Helble, A.A. Boni, N. Shah, G.P. Huffman, F.E. Huggins, Mineral behavior during coal combustion: 2. Illite transformations, Pror. Energy Combust. Sci. 16 (1990) 293 – 302. [57] W.P. Linak, J.V. Ryan, J.O.L. Wendt, Formation and destruction of hexavalent chromium in a laboratory swirl flame incinerator, Combust. Sci. Technol. 116 – 117 (1996) 479 – 498. [58] N.K. Tovey, D.H. Krinsley, Mineralogical mapping of scanning electron micrographs, Sediment. Geol. 75 (1991) 109 – 123. [59] K.C. Galbreath, C.J. Zygarlicke, D.P. McCollor, D.L. Toman, Development of a stack-plume opacity index for subbituminous coal-fired utility boilers, in: C. Shiao-Hung (Ed.), Proceedings of the 12th Annual International Pittsburgh Coal Conference Coal-Energy and the Environment, Center for Energy and Research, University of Pittsburgh, Pittsburgh, PA, 1995 (Sept. 11 – 15), pp. 19 – 26. [60] M. Najih, F.E. Huggins, G.P. Huffman, Determination of chromium oxidation states in coal combustion products by XAFS spectroscopy, Prepr. Pap.-Present. Am. Chem. Soc., Div. Fuel Chem. 40 (1995) 808 – 812. [61] S. Mahuli, R. Agnihotri, S. Chauk, A. Ghosh-Dastidar, L.-S. Fan, Mechanism of arsenic sorption by hydrated lime, Environ. Sci. Technol. 31 (1997) 3226 – 3231. [62] K.J. Irgolic, G. Haas, W. Goessler, Arsenic compounds in fly ash from coal-fired power plants, Proceedings of the 4th International Conference on Managing Hazardous Air Pollutants, EPRI, Palo Alto, CA, 1999, pp. 2-3 – 2-16, TR-111024. [63] S. Srinivasachar, J.J. Helble, A.A. Boni, Mineral behavior during coal combustion: 1. Pyrite transformations, Pror. Energy Combust. Sci. 16 (1990) 281 – 292.

726

K.C. Galbreath, C.J. Zygarlicke / Fuel Processing Technology 85 (2004) 701–726

[64] L.D. Hulett, A.J. Weinberger, K.J. Northcutt, M. Ferguson, Chemical Species in Fly Ash from Coal-Burning Power Plants Science, 210 (1980) 1356 – 1358. [65] K.C. Galbreath, C.J. Zygarlicke, Mercury Transformations in Coal Combustion Flue Gas. Fuel Processing Technology, 65 – 66 (2000) 289 – 310. [66] R.J. Lauf, Nucl. Chem. Waste Manag. 5 (1985) 231 – 236. [67] J.C. Hower, R.F. Rathbone, J.D. Robertson, G. Peterson, A.S. Trimble, Petrology, mineralogy, and chemistry of magnetically separated sized fly ash, Fuel 78 (1999) 197 – 203.