Modes of Occurrence of Trace Elements in Coal
Short Course

By Allan Kolker, Leslie Ruppert, and Curtis A. Palmer

Eastern Energy Resources Team, U.S. Geological Survey, Mail Stop 956, Reston, VA 20192.

ABSTRACT

This short course summarizes basic principles and recent developments in understanding the modes of occurrence of trace elements in coal, with particular emphasis on elements that pose potential environmental problems, such as mercury, arsenic, selenium and chromium. Recent and ongoing research is presented with emphasis on approaches used at the U.S. Geological Survey (USGS). This short course provides an up-to-date overview of the distribution of trace metals in coal, information needed to understand sources and sinks for potential contaminants in a wide range of environmental and coal use issues. Cases in which a knowledge of trace-metal distribution in coal is needed include developing models for coal-fired powerplant emissions and predicting trace-element behavior during coal cleaning. Other possible applications include controlling the release of metals to ground water from coal and coal-combustion materials and minimizing health consequences of coal use in domestic settings. Because trace-metal behavior is a consequence of geologic setting, understanding its distribution can also tell us about the geologic processes that took place during and after coal formation.

[Note: The structure and overall content of this outline is as presented at Coal Quality: Global Priorities in September, 2001. In some cases, the content has been modified to reflect developments in the intervening period.]

INTRODUCTION

This short course focuses on modes of occurrence of trace elements in coal and the importance of trace elements in the overall context of coal quality. Emphasis is placed on those elements of technological, economic, and environmental significance. We will review the current state of knowledge and describe state-of-the-art methods for quantifying element modes of occurrence in coal.

COURSE OUTLINE

- Section 1: Introduction
- Section 2: Coal Formation and Diagenesis
- Section 3: Coal Mineralogy and Mineral Chemistry
- Section 4: Overview of Bulk Analytical Methods and USGS Selective Leaching Procedure
- Section 5: Overview of Microanalysis and Spectroscopic Methods
- Section 6: Coal-use Issues and Case Studies

COURSE INSTRUCTORS

Allan Kolker: (703) 648-6418 - akolker@usgs.gov

> Allan Kolker conducts research in the areas of coal geochemistry, trace elements, environmental geochemistry, application of microbeam analytical instruments, and geochemical health issues. Recent work includes ion microprobe and XAFS analysis of metals in U.S. coals, studies of arsenic in groundwater in southeastern Michigan, and work on the geochemistry of metal-enriched coals in the Donbas region of Ukraine. Allan served on the geology faculty at the University of Nebraska-Lincoln before joining the U.S. Geological Survey in 1996. He holds a Ph.D. in geochemistry from the State University of New York at Stony Brook, which was followed by postdoctoral work on trace metals in coal at Brookhaven National Laboratory.

Leslie Ruppert: (703) 648-6431 - lruppert@usgs.gov

> Leslie (Jingle) Ruppert has been involved in coal-related studies at the U.S. Geological Survey for more than 20 years. She has published extensively in the areas of coal quality and coal resource assessment. Most recently, she served as Project Chief for the Appalachian portion of the National Coal Resource Assessment. She has served in leadership roles in the Geological Society of America Coal Geology Division, the Executive Committee of the Pittsburgh Coal Conference, and the Editorial Board of the International Journal of Coal Geology. She holds an M.S. in geology from The George Washington University.

Curtis Palmer: (703) 648-6185 - cpalmer@usgs.gov

> Curtis Palmer is a research chemist at the U.S. Geological Survey, where he has more than 20 years of experience. He has authored over 100 publications in coal geochemistry and analytical geochemistry. Curtis has special expertise in the area of Instrumental Neutron Activation Analysis (INAA) and its application to coal samples. He helped develop the selective chemical leaching procedures for coal used at the U.S. Geological Survey. Curtis holds a Ph.D. in analytical chemistry from Washington State University.

Section 1: Introduction—Impact of Trace Metals

Understanding the distribution of trace metals in coal is needed to predict the environmental impact of coal use in a variety of settings. Information on minor constituents in coal is needed to develop models for powerplant emissions, and to predict coal behavior upon cleaning. Other applications include controlling release of metals from coal and coal combustion materials to ground water, and minimizing health consequences of coal use in domestic settings.

Some Examples of the Effect of Inorganic Constituents on Coal Utilization

Element |  | Effect(s)
Sodium |  | Contributes to boiler fouling
Iron |  | Contributes to boiler slagging
Chlorine |  | Causes boiler corrosion; impacts mercury capture
Si (Quartz) |  | Results in erosion of combustors

Table 1. Average values for trace elements of environmental interest in U.S. coals

Table 1 results from USGS COALQUAL database (Bragg et al., 1998; after Finkelman, 1993; Kolker and Finkelman, 1998).

Mode of Occurrence Concept

Definition — Understanding the chemical form of an element present in coal.

Understanding the mode of occurrence of an element is important because it helps determine element behavior during coal combustion and the potential for removal during coal cleaning. It can also determine potential environmental impact, technological behavior and byproduct potential, in addition to providing information on geologic history.

Examples of Element Modes of Occurrence

Organic (Maceral) Association

1. Ionic bonding to coal macerals
2. Covalent bonding to coal macerals
3. Elements present in moisture

Inorganic (Mineral) Association

1. Elements present in solid solution in a mineral phase (for example,. arsenic substituting for sulfur in pyrite; chromium in illite/smectite; cadmium in sphalerite)
2. Elements that fill a site in a mineral phase (Essential Structural Constituents; for example, lead in galena)

Table 2. Mode of occurrence results for elements of environmental/human health interest, and confidence in that association [Results after Finkelman (1994); Kolker and Finkelman (1998)]

About Moisture

Generally, calorific value increases and moisture decreases with increasing rank of coal. It is necessary to know the moisture content, and correct for it (if analyzed on a dry basis) to accurately express elemental concentrations in coal. Moisture contents range from about 2 percent in bituminous coals to as much as 30 percent in low rank coals. Analyses expressed on a dry basis can differ from whole coal values by as much as 30 percent if moisture is not taken into account.

Geologic Factors that Influence Coal Chemistry (discussed further in Section 2)

The chemical composition of coal can vary as a result of a variety of geologic factors, some of which include:

- Burial and diagenetic changes.
- Stratigraphic and lateral variation in coal-bearing horizons.
- Interaction of coal with hydrothermal fluids (for example, northern Alabama, Guizhou Province China, western Washington).
- Movement of fluids along fractures.
- Cleat (fracture-filling) mineralization during coal formation.

Section 2: Geologic Controls on Coal Quality

Coal Formation and Diagenesis

I. Geologic Controls on Coal Chemistry

> A. Conclusions:

> The concentration, variation (both laterally and vertically), and modes of occurrence of trace elements are controlled by geologic and geochemical processes that begin in the peat stage of coalification.

> If we understand the processes that control elements we have a good chance of predicting:

> 1. Where elements are
> 2. How they are bound
> 3. What will happen to elements during coal utilization

II. What controls coal quality?

> A. Allogenic controls:

> > 1. Climate
> > 2. Tectonism
> > 3. Eustasy

> B. Autogenic controls:

> > 1. Depositional environment
> > 2. Hydrology
> > 3. Sediment influx
> > 4. Peat preservation and burial
> > 5. Diagenetic reactions
> > 6. Rank
> > 7. Weathering

II. A. 1. Climate

1. Climatic factors control the type of peat, the type and rate of vegetation, and sediment input into the mire or peat swamp.
2. Domed (convex upward) peat:
   1. Characteristic of ever wet, tropical climates domed, rain fed (ombrogenous) peat bodies.
   2. Examples include the Sumatra foreland basin, and Lower Mississippian to mid Middle Pennsylvanian Central Appalachian Basin coals.
   3. Characterized by:
      Doming, radial drainage, flushing.
      Low in nutrients.
      Eolian input, dissolved solids minimal.
      Limited buffering capacity.
      Rapid peat development.
      Minimal degradation of organic matter.
      Upland soils anchored.
3. Planar (flat lying) peats:
   1. Characteristic of seasonally, wet temperate climates planar, ground water and rainwater fed (rheotropic) peat bodies.
   2. Examples include U.S. coastal swamps; late Middle to Upper Pennsylvanian Appalachian basin coal.
   3. Characterized by:
      High sediment/nutrient input from eolian, fluvial, and groundwater sources.
      Leaching can be high.
      Relatively high pH (>4) degrades organics.

II. A.2. Tectonism

1. Controls the rate of base level change climatic fluctuations may raise or lower the water table, but base level rise is critical to build up thick peat.
2. Fluctuations in base level are dependent on tectonic subsidence, eustasy, and compaction.
3. For the preservation of economic coals, a continuous rise in groundwater (subsidence) and relatively low relief of hinterlands are needed to restrict sediment influx.
4. Above (a–c) conditions occur in foreland basins.

II. A.3. Eustasy

1. Increased water depths with marine flooding elevates water tables in nonmarine and continental settings.
2. Elevated water tables allow for plant growth, peat development, and coal preservation in the accommodation space.

II. B. 1. Depositional environment

1. Affects the geometry, extent, and boundaries of peat bodies.
2. Affects bed and dissolved load of streams and peat swamp.
3. Controls the level of the underlying substrate.
4. Exerts considerable control on ground water chemistry.

II. B. 2. Hydrology

1. The geometry of peat bodies is controlled to a large extent by their hydrologic setting.
2. Characteristics of peats in different hydrologic settings:

> 1. Ombrotrophic peats (rainwater fed)
>    Low in ash few dissolved solids.
>    Often domed limits detrital influx.
>    Sediment input eolian volcanics and dust.
>    Highly acidic.
>    Low diversity of plants, stunted in middle.

> 2. Rheotropic peats (groundwater fed)
>    High in dissolved solids.
>    Often planar.
>    Sediment rich.

> 3. Mesotrophic peats (ground water and rainwater fed)
>    Intermediate ash contents.

> 4. Brackish or saline water will not form economic coal deposits.

II. B. 3. Sediment influx

1. In order to form economic peat accumulations, the rate and amount of sediment input must be low.
b
2. Eolian Sources (domed and planar peats)
   Dust
   Volcanic ash fall material amphibole, pyroxene, quartz, feldspar, glass, etc.
   Cosmic dust
   Sea spray
3. Water born sediment sources (predominantly planar peats and/or edges of domes)

> 1. Sediment composition is dependent on the geologic setting

II. B. 4. Peat preservation and burial

1. Anoxic conditions control the rate and degree of humification (decay within peat profile)
2. Very acidic conditions result in generation of humic acid from organics.
3. Burial and subsidence must be rapid.

II. B. 5 Diagenetic reactions

1. What gets into the peat doesn't necessarily stay there.
2. Multiple organic acids are produced that may:

> 1. Dissolve and alter mineral matter.
> 2. Break loosely bonded organic and inorganic complexes.
> 3. Create bonds between inorganic elements and organic complexes or form syngenetic mineral phases.
> 4. Ions available from ground and surface waters can be incorporated into minerals.

|  |  | Figure 1. Scanning electron microscope (SEM) image of an eolian, volcanic quartz grain showing v-shaped etch pits. The quartz grain was isolated from a domed peat body in Indonesia.

3. Organically associated elements are the most likely to be mobilized during diagenesis. These elements can be leached from the system or re precipitated into epigenetic mineral phases.
4. After burial:

> 1. As rank increases, organic bonds weaken, releasing elements.
> 2. Elements continue to be added, subtracted, and moved within the system.
> 3. Epigenetic mineral phases form.
> 4. Leaching continues.

|  |  | Figure 2. SEM image of a siliceous plant phytolith that is pitted, or etched, by organic acids. The silica that is released by organic acid dissolution can be incorporated in other mineral phases.

II. B. 6. Cleating

1. Cleats form in two ways:

> 1. Moisture/volatile loss occurs once moisture falls below about 20 percent (subbituminous range).
> 2. Tectonic forces/differential compaction.

2. Cleats can be coated with minerals, then further infilled.
3. Cleat infilling includes (but is not limited to):

> 1. pyrite
> 2. kaolinite
> 3. sphalerite
> 4. calcite
> 5. gypsum

4. Multi generational cleat infilling not uncommon.

|  |  | Figure 3. Moisture and volatile loss create a system of joints, or cleats, that are perpendicular to one another. The face cleat is the major cleat system and the minor butt cleat commonly forms at right angles to the face cleat.

II. B. 7 Faults

1. Faults can act as conduits for fluid flow in coals and coal basins.

> 1. Tends to increase mineral matter and trace element content of coal beds
> 2. Example- Warrior Basin, Alabama

|  |  | Figure 4. Photograph of a large fault in the Warrior Basin, Alabama. The fault served as a conduit for sulfide-rich waters that contained high concentrations of potentially hazardous trace elements. Photo credit: M. Goldhaber, USGS.

II. B. 8 Weathering

1. Effect of weathering on coal

> 1. Atmospheric weathering and action of ground waters effect the elemental and mineral composition of coal beds.
> 2. Development of new suites of minerals by:
>    oxidation (pyrite to sulfate and iron oxyhydroxide phases); inclusion of water in clay lattice (allophane)
> 3. Removal of some remaining organically bound minerals.
> 4. Removal of some organics concentrating inorganic elements.

III. Conclusions

1. Coal quality results from a continuum of processes that start in the peat stage and continue through coalification.
2. Geometry of a peat body (planar vs. domed) factors heavily in the quality of the resulting coal.
3. The concentration, variation (both laterally and vertically), and mode of occurrence of elements are controlled by geologic and chemical processes that start during peat development and continue through coalification. Understanding those processes will allow us to predict coal quality trends before mining.

Section 3: Coal Mineralogy and Mineral Chemistry

Table 3. Summary of major mineral phases found in coal and their minor element associations

The major mineral phases present in coal are remarkably consistent, as summarized above.

A | B
C | D
Figure 5. Examples of different forms of pyrite in Donets Basin (Ukraine) coal samples (SEM/BSE). Source: Kolker et al., 2002. A, Framboid cluster; B, microcleat; C, cell filling; D, deformed cell filling.

For minerals such as pyrite, there can be multiple forms or generations in the same coal sample or series of coal samples. The examples shown above are all from the Donets Basin of Ukraine.

Table 4. Common minor and trace phases in coal and their major and minor elemental constituents

Minor and trace elements can occur as essential structural constituents in discrete mineral phases (for example, Zr in zircon and Pb in galena) or in solid solution (for example, Cd in sphalerite).

Determination of Coal Mineralogy

Method involves a combination of mineral concentration by low-temperature ashing (LTA) and X-ray diffraction analysis (XRD) of the concentrate.

Key elements of LTA/XRD:

- Mineral matter is concentrated by slowly consuming organic matter at 175γC in an oxygen atmosphere.
- Mineral identification by X-Ray diffraction of LTA.
- Results for a mixture of minerals are solved by a computer program.
- Computer-controlled SEM is an alternate approach. Both techniques are semi-quantitative.

Figure 6. X-ray diffraction spectrum showing peaks for selected minerals in a low temperature ash fraction of coal (courtesy Frank Dulong, USGS).

Mode of Occurrence for Some Elements of Environmental Interest in Coal

Arsenic, mercury, selenium, and chromium, are of particular interest because of their potential environmental/human health impacts.

Arsenic mode of occurrence in coal:

- Pyrite is the primary host of arsenic in bituminous coals; Arsenopyrite is very rare, except in proximity to hydrothermal ore deposits.
- Arsenic contents vary widely within and between pyrite grains.
- In low rank coals, pyrite is a less important host of arsenic.
- Coal cleaning reduces pyrite content, but framboids may remain in organic fraction.
- Pyrite oxidation releases arsenic to the environment and changes arsenic oxidation state.

Documented health effects from arsenic are rare. Some examples include:

1. Guizhou, Province, China: Arsenic toxicity due to consumption of foods dried through domestic use of ultra-high-arsenic coals (up to 30,000 ppm).
2. Central Slovakia, 1970's: Arsenic toxicity from use of local coals in a powerplant.

Arsenic in Coal

|  |  | Figure 7. Electron microprobe elemental map of arsenic-rich pyrite (bright bands) occurring as overgrowths on pyrite framboids (dark voids) in an Alabama bituminous coal. Figure shows multiple pyrite generations with differing arsenic contents. Source: Goldhaber et al., 2003. See section on microanalysis for a description of this method.

|  | Figure 8. Selective leaching results for arsenic in 13 coal samples, but two of which (Wyodak-1 and Rosebud-McKay) are bituminous. The results show a significant pyrite association (yellow) for the eleven bituminous samples, and a lesser pyrite association for the two low-rank coals. Source: Palmer et al., 1998. See section on USGS selective leaching procedure for a description of this method.

|  | Figure 9. Comparison of average concentrations of arsenic in pyrite from five different bituminous coal samples. Data are averages of electron microprobe point analyses, ranging from 60 to 96 analyses per sample. Samples include a Springfield coal sample from Indiana (Spfld), two Pittsburgh coal samples from West Virginia (Pitts-1 and Pitts-2), and samples from the Black Creek (Al TP-2) and Mary Lee (AL LM-1) coal groups from Alabama. Data are from Kolker et al., 2003.

Examples of arsenic variation in pyrite in coal

A | B
Figure 10. Wavelength-dispersive electron microprobe elemental maps of pyrite in Alabama samples LM-1 (A) and TP-1 (B). LM-1 map shows pyrite distribution (iron map shown in left-hand side of A) and arsenic distribution (right-hand side of A). Maps show arsenic-enriched domains (bright regions) and arsenic-poor regions (darker areas). From results presented by Kolker et al. (2003).

Table 5. Analyses of coals used domestically in areas of pervasive arsenic poisoning, southwest Guizhou Province, China (Belkin et al., 1997).

Table shows results for reconnaissance sampling of metal-enriched coals used domestically in areas of pervasive arsenic poisoning, southwest Guizhou Province, China. Note extraordinary enrichment of As, Sb, and Hg, compared to averages for these elements in U.S. coals (24 ppm, 1.2 ppm, and 0.17 ppm, respectively; Bragg et al., 1998).

Arsenic in Coal

Arsenic in Coal

Figure 12. Arsenic field test kit developed in China to identify arsenic-rich coals in the field. A commercial version of this kit (above) is being introduced by a U.S. manufacturer. Field testing of Chinese coals has resulted in closure of several workings with coal having the highest arsenic contents. Inset at lower left shows colorimetric scale for arsenic content in test samples.

Mercury in Coal

Summary of the mode of occurrence of mercury:

o Pyrite (FeS2) is the most common mercury association in bituminous coals.
o The mercury content of pyrite is variable and can be correlated with arsenic and other air toxic metals.
o In low rank coals, an organic association is common.
o In very mercury-rich coals, discrete mercury minerals such as HgSe, HgS (cinnabar), and/or native mercury may be present.

Figure 13. Example of mercury variation in coal samples controlled by pyrite, indicated by the correlation between mercury and pyritic sulfur. Samples are from the Donets Basin, Ukraine. Figure is modified from Kolker et al., 2002, with one outlier (Hg = 4.5 ppm) removed.

Selenium in coal

An important organic association is indicated by most studies.

Other forms of selenium in coal:

o Measurable Se in some pyrite; Pyrite oxidation may contribute Se to ground water.

o Selenides (for example, PbSe clausthalite) is relatively common in coal, unlike other sediments (Hower and Robertson, 2003).

o Selenium is sensitive to in-situ oxidation, but less so than As.

Chromium in coal

Modes of occurrence of chromium:

o Silicate (illite) and organic-hosted forms are dominant.
o Chromium may also occur in Fe-Ti-Cr oxide minerals, if present.
o Chromium is not prone to in-situ oxidation in coal, unlike Fe, As, Se.

Figure XX

Figure 14. Plot of Cr vs. Mg in illite-smectite in several bituminous coal samples. Results were obtained by SHRIMP-RG ion microprobe and confirm selective leaching studies commonly indicating a silicate association for Cr in coal due to its substitution in illite-smectite. See sections on microanalysis and selective leaching for descriptions of these respective analytical methods. Figure is from Kolker et al., 2000.
Chlorine in Coal

Chlorine content is an important parameter because of corrosive effects of HCl and Hg-Cl complexing in coal-fired powerplants. Chlorine contents are strongly influenced by salinity or paleo-salinity. Salinity increases with depth or paleo-depth.

Table 6. Variation in average chlorine contents in the Appalachian Basin (from Bragg et al., 1991)

Basin
Average Cl in Coal (ppm)*
Northern Appalachian 850
Central Appalachian 950
Southern Appalachian 310

Table 7. Showing stratigraphic variation of chlorine content in Appalachian Basin coal samples (From Bragg et al., 1991)

Age Formation Number of Samples Mean Cl (ppm)
Lower Permian (?) Dunkard Group 44 162
Upper Pennsylvanian Monongahela Formation 73 477
Upper Pennsylvanian Conemaugh Formation 41 828
Middle Pennsylvanian Allegheny Formation 709 1097
Middle Pennsylvanian Kanawha Formation 36 1408
Lower Pennsylvanian New River Formation 56 1503

Trace metals in coal macerals

o There is a fairly limited number of determinations, either in-situ microanalysis or bulk analysis of maceral separates.
o Most information is for the vitrinite, liptinite and inertinite groups.
o There is a large variation in metal contents within and between coals.
Summary of concentration ranges for various metals in coal macerals (from compilation by Kolker and Finkelman, 1998):

sub-ppm: Hg
sub-ppm to few ppm: Sb, Th, U
ppm to 10's ppm: Cr, Ni, As, V
ppm to 100's ppm: Fe, Mn

Summary: Trace elements in coal

" Arsenic:
o Arsenic-bearing pyrite is the dominant arsenic form in unweathered bituminous coals.
o Presence of an oxidized form (arsenate) is a function of the degree of pyrite oxidation.
o A greater organic fraction of arsenic is present in low-rank coals.

" Chromium:
o Illite and organic-hosted forms are dominant.
o The oxidation state of Cr in coal is all Cr(III); In rare cases Cr(VI) has been determined in coal ash.

" Mercury:
o Pyrite is the most significant host of Hg.
o A greater organic fraction of mercury is present in low-rank coals.

" Selenium:
o Selenium exhibits multiple forms; it is common to have a significant organic fraction, even in bituminous coals.
o The oxidation state of Se changes with pyrite decomposition, but less so than Fe, As.

" Chlorine:
o Chlorine content variable; controlled by salinity or paleo-salinity.
o Affects Hg emissions by Hg-Cl complexing.

" Macerals:
o Largest fraction of trace elements in low-rank coals.
o May be a significant host of transition metals.

Section 4: Overview of Bulk Analytical Methods and USGS Selective Leaching Procedure

Bulk Analytical Methods: Coal Quality Characterization

" American Society of Testing and Materials (ASTM) Procedures

o USGS uses contract labs such as Geochemical Testing and Wyoming Analytical Laboratories

" Major and Trace Elements

o USGS uses internally developed procedures, some of these are included in ASTM protocols, and are preformed by USGS personnel

Bulk Analysis and Selective Leaching Procedure

ASTM Procedures

" Ultimate analysis:
o ASTM Methods D3176-D3179
o Moisture, C, H, O, N, total S

" Proximate analysis:
o ASTM Methods D3172-D3175
o Moisture, volatile matter, fixed carbon and ash

" Other techniques:
o Sulfur forms, calorific values,
o Hargrove grindability free swelling index, ash fusability,
o Specific gravity, equilibrium moisture

Analytical Methods Overview and Relative Merits

Elemental Analysis

Routine Methods:

Multi-Element Techniques:
o Inductively Coupled Plasma- Atomic Emission Spectroscopy (ICP-AES)
o Inductively Coupled Plasma- Mass Spectroscopy (ICP-MS)

Single Element Techniques:
o Cold Vapor Atomic Absorption (CVAA; Hg)
o Hydride Generation Atomic Absorption (HGAA; Se)

Non-Routine Methods:

Instrumental Neutron Activation analysis (INAA)

Bulk Analysis and Selective Leaching Procedure

Low Temperature Ashing (LTA)

USGS Ashing Procedure is similar to ASTM procedure:
" Samples are heated from 25 oC to 200 oC in about 1 hour.
" Samples are heated at 200 oC for 1.5 hrs.
" Temperature is increased to 350 oC and held constant for 2 hrs.
" Temperature is increased to 525 oC and held constants for 36 hrs.
" Samples are slowly cooled (1-2 hr).
" Samples are examined and re-ignited at 525 oC if combustion is incomplete.
" Samples are homogenized.

Advantages of Ashing

o Increases concentration and apparent detection limits
o Makes it easier to place many elements into solution
o Ash is more stable for long term storage (archiving)
o Can improve sample homogeneity

Disadvantages of Ashing

o Some elements may volatilize
o Volatility may be matrix dependent
" Amounts volatilized may be different for each sample and element
" Occasionally elements not normally considered volatile are volatile for a given sample
" Potential of cross-contamination of volatile components
o Larger sample needed
o Additional steps and time
o Elements may need to be recalculated to a whole coal basis
o Care must be taken to ensure ashing is complete
o Mechanical losses can effect results
Bulk Analysis and Selective Leaching Procedure

Methods requiring LTA

ICP-AES (Inductively-coupled plasma atomic emission spectroscopy)

Advantages
o Rapid
o Low Cost
o Multi-element

Disadvantages
o Requires dissolution of ash
o Moderate sensitivity

ICP-AES

Two dissolution procedures (sinter and acid digestion)

Sinter Method (Ash is fused at 445 oC with Na2O2)
" Advantages:
Dissolves species difficult to digest by acid dissolution
Conserves volatile elements during acid dissolution
" Disadvantages:
High dissolution ratio reduces sensitivity
High salt content can cause instrument problems
" Elements Determined:
Major elements in ash except Na
Trace elements: B, Ba, Zr

Acid Digestion Method
" Advantages
Low dissolution ratio--Higher sensitivity
Low salt content no Na contamination
" Disadvantages
Some elements are volatile, eg. B, Se, Cl
Some elements are associated with insoluble minerals, eg. Zr, B
" Elements Determined
Major element: Na2O
Trace elements: Be, Co Cr, Cu, Li, Mn, Ni, Sc, Sr, Th, V, Y, Zn

ICP-MS
(Inductively-coupled-plasma mass spectrometry)

" Much higher sensitivity than ICP-AES (10 to 1000 times)
" Higher cost instrument
" Some elements have interferences- giving poorer results than ICP-AES; Others similar results to ICP-AES
" Same dissolutions as ICP-AES but sinter dissolution is not routinely analyzed because the use of the highly ionic solution requires special setup and requires additional maintenance
" Acid digestion- can determine: Ag, As, Au, Bi, Cd, Cs, Ga, Ge, Mo, Nb, Pb, Rb, Sb, Sn, Te, Tl, U
" Sinter digestion- can determine: 13 Rare earth elements, Hf, Ta and W

Whole Coal Techniques

Cold Vapor Atomic Absorption (CVAA):

" Single element method (Hg)
" Requires sample dissolution
" 5 to 10 percent of coals have Hg contents below detection limit of 0.02 ppm
" Reliable and accurate (ASTM method)

Ion Chromotography:

" Single element method (Cl).
" Requires sample dissolution
" Less than 5 percent of coals have Cl contents below detection limit of 150 ppm
" Reliable and accurate

Whole-coal techniques

Hydride generation atomic absorption (HGAA):

" Used for a single element (Se)
" Requires sample dissolution
" Several elements (especially heavy and transition metals) in high concentrations can interfere with the results

Instrumental neutron activation analysis (INAA):

" Time-consuming multi-element technique.
" Highly linear response with few interferences.
" Can be used on a small sample size.
" No ashing or sample dissolution required.
" High sensitivity.
" High cost- requires sample irradiation in a nuclear reactor.
" Elements determined include: Na, K, Fe, Sc, Cr, Co, Ni, Zn, As, Se, Br, Rb, Sr, Mo, Sb, Cs, Ba, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, Hf, Ta, W, Th, U.
" Determination of other elements is possible: Al, Ca, Mg, Ti, S, V, Cl, I, Mn, Dy, Hg.

Overview of Bulk Analytical Methods

Quality Control and Assurance

" Accuracy - degree of agreement between the measured value and the "true" or proposed value

" Standard Reference Materials
o CLB-1 - coal
o NIST 1632b - coal (bituminous)
o NIST 1633a - coal fly ash

" Certified Calibration Standards

" Precision - degree of agreement between measured values under repetitive testing of a sample; reproducibility of results.

" Duplicate samples.

Overview of Bulk Analytical Methods

Summary- Analytical Approaches:

" Multi-element techniques provide methods to obtain a large and varied amount of data in a relatively short time.

" Cost of instruments for multi-element techniques can be very high.

" Some elements in coal cannot be determined using multi-element techniques due to volatility and matrix interferences.
Overview of Selective Leaching Procedure

" A Multi-Element Semi-Quantitative Approach

o Semi-Quantitative Leaching Results
o Microprobe Analysis- Concentration of elements in minerals
o Qualitative SEM- Mineral identification in whole samples and leached residues
o Semi-quantitative XRD- Concentration of minerals in coal

Figure 18. Overview of steps in the USGS Selective Leaching Procedure
Overview of Selective Leaching Procedure

Sequential Leaching:

" Duplicate, 5 gram 60-mesh samples are shaken in a centrifuge tube in 35 ml of ammonium acetate for 18 hrs.
o The resulting solution is saved for analysis by ICP-AES and ICP-MS.
o A 300 mg split of the resulting solid is analyzed by INAA and a 200 mg split is analyzed by CVAA for Hg. Additional splits may be taken for specialized experiments (XAFS, SEM, etc).

" The remaining solid from each sample is leached in the same manner with hydrochloric acid (HCl) and subsequently with hydrofluoric acid (HF), with analysis splits taken in each step.

" The remaining solid after leaching with HF is leached in a flask with nitric acid (HNO3).
o The procedure is similar to ASTM method for determining pyritic sulfur
o Solutions and solid samples are analyzed as in other steps.

Overview of Selective Leaching Procedure

Selective Leaching Results:

Several elements are mainly associated with pyrite as demonstrated by a large percentage of the element leached by HNO3 (nitric acid)

Figure 19. Stacked bar plots showing selective leaching results for 16 coal samples. Red bar segments indicate fractions leached by ammonium acetate; green segments indicate fractions leached by hydrochloric acid; blue segments indicate fractions leached by hydrofluoric acid; yellow segments represent fractions leached by nitric acid (see Fig. 18).
Overview of Selective Leaching Procedure

Selective Leaching Results:

o Other elements are mainly associated with silicates as demonstrated by a large percentage of the element leached by hydrofluoric Acid.

Figure 20. Selective leaching results showing leaching by HF, indicating as silicate association for Be and Cr (Palmer et al., 1998).

Overview of Selective Leaching Procedure

Selective Leaching Results:

" When the majority of an element is soluble with HCl then the element is likely associated with carbonates or monosulfides.
" Elements associated with calcite are partially soluble in ammonium acetate.

Figure 21. Leaching plots showing elements having an HCl leaching association.
Overview of Selective Leaching Procedure

Selective Leaching Results:

" For some elements a significant fraction is unleached indicating that the element is organically associated

Figure 22. Selective leaching results for antimony in 16 coal samples, each showing significant unleached (inferred organic) fractions.

Overview of Selective Leaching Procedure

In some cases mineral species are encapsulated or insoluble.

" Examination by SEM can usually be used to determine encapsulated or insoluble species.

Figure 23. SEM images of raw coal (left), and de-mineralized coal (right), the solid residue remaining after the leaching procedure.
Overview of Selective Leaching Procedure

" Phosphates are also soluble in HCl.
" Some elements can be in mineral forms such as phosphates that are encapsulated by silicates.
o Produces significant amounts of element leached by HNO3.
o Can be tested by leaching with HCl after HF leach.

Figure 24. Example of element associated with phosphates that is leached by HNO3

In some cases, leaching of an element is not dominated by a single solvent, indicating multiple modes of occurrence:

Figure 25. Example of element with multiple associations

Conclusions- Selective Leaching Approach

" The modes of occurrence of elements in coal can be semi-quantitatively determined by selective leaching

" Since each solvent dissolves more than one species, supporting techniques (SEM, microprobe etc.) are needed to determine the exact mode of occurrence.

" There is a great deal of consistency for leaching results within a given rank of coal.

Section 5: Overview of Microanalysis and Spectroscopic Methods

Leaching vs. Microanalysis

Microanalysis and Spectroscopic Methods

Methods of Determining Element Associations in Coal:

" XRD Mineralogy
" Bulk Chemical Testing (INAA, ICP-AES, ICP-MS, XRF, etc.)
" SEM with BSE
" Selective Leaching
" Electron Microprobe
" Ion Microprobe
" X-ray absorption fine structure (XAFS)
" Laser Ablation ICP-MS

Methods listed in boldface are examples of element determinations by microanalysis.
Electron-beam Instruments
(SEM and Electron Microprobe)

Signals produced by interaction of samples and an electron beam:

" Secondary Electrons
(used for imaging)

" Back Scattered Electrons
(imaging + composition)

" Characteristic X-rays
(elemental composition)

Microanalysis and Spectroscopic Methods

Scanning Electron Microscope (SEM):

Used mainly for imaging and obtaining compositional data using energy-dispersive analysis. Energy dispersive analyzer has excellent spatial resolution, but detection is limited to the tenths of weight percent level.

Fig 27. USGS Jeol 840* Scanning Electron Microscope in Reston, VA

Microanalysis and Spectroscopic Methods

Electron Microprobe:

Microanalysis and Spectroscopic Methods

Electron Microprobe Results:

Point Mode: Quantitative analysis of a 1-2 micrometer spot for major and minor elements. Detection limit depends on operating conditions and element of interest. Best results detection limit is about 100 ppm.

Elemental Maps: Semi-quantitative maps of element distribution over a specified area.

Figure 31. Electron microprobe elemental maps of Fe (left) and Cd (right) showing Cd-bearing sphalerite (Sph) enclosing pyrite framboids (py) that lack cadmium. Host is sandstone above coal-bearing interval in Allegheny Formation, West Virginia. Scale bar is 50 micrometers. Source: Kolker et al., 2001.

Microanalysis and Spectroscopic Methods

Ion Microprobe:

Utilizes an ion beam rather than an electron beam.

Capable of analyzing at the ppm level with good spatial resolution.

Stanford/USGS SHRIMP-RG [Sensitive High-Resolution Ion Microprobe Reverse Geometry]:

" Uses a primary beam of O2- or Cs+ ions
" Capable of element detection in the ppm range
" Area analyzed has a 10-15 micron spot size
" Capable of determining isotope ratios at high spatial resolution. Useful in age determinations on single zircons and other applications.

Figure 32. Stanford/USGS SHRIMP-RG Ion microprobe at Stanford University.

Microanalysis and Spectroscopic Methods

Ion Microprobe (continued):

Figure 33. Reflected light image of Illinois Basin coal sample showing illite-smectite band (I/S), pyrite framboids (Py), and two15-micrometer-wide SHRIMP-RG analysis points (A1, A2) for illite-smectite.

Microanalysis and Spectroscopic Methods

Laser Ablation ICP-MS:

" Pulsed Nd (YAG) laser coupled to dedicated ICP-MS. Excavation rate is about 3 m/sec.
" Results for Cr, Mn, Co, Ni, Cu, Zn, As, Se, Mo, Cd, Sb, Tl, Pb, and Hg, in pyrite.
" Sample/standard matrix match not critical as both are introduced into ICP-MS in an argon plasma.
" Best results for Hg with 50 m beam (about 3 times that of SHRIMP-RG).

Figure 34. Example of laser-ablation ICP-MS results for single pyrite grains in 3 bituminous coal samples. Results confirm mercury association with pyrite in these samples (Kolker et al., 2002).

Element Speciation

Different forms of an element can have very different behaviors.
Examples:
" Trivalent chromium is an essential nutrient; hexavalent chromium is a carcinogen.

" Trivalent arsenic (arsenite) is more toxic than pentavalent arsenic (arsenate).

Approaches to speciation determination:

" Classical approach using ion-exchange column chemistry.

" Spectroscopic methods such as X-ray absorption fine structure (XAFS) and 57Fe Mössbauer analysis (primarily solids)

" Selective leaching, where leached form of an element corresponds to a particular species.

" Coupled ion-chromatography-ICP/MS.

Microanalysis and Spectroscopic Methods

X-Ray Absorption Fine Structure (XAFS):

" Can determine elemental species in powdered coal samples.
" Requires high-energy synchrotron radiation source.
" Need samples with several ppm of an element; limited atomic number range, best for transition metals, As, Se.
" To be quantitative, need to do least-squares fitting of spectra for unknowns to spectra of calibration standards.

Microanalysis and Spectroscopic Methods

Summary of XAFS Results:

Arsenic:
" Pyrite and arsenate (equivalent to HCl + HF-leached As) are main forms in bituminous coals.
" Arsenate and As (III) are main forms in low rank coals.
" Fraction of arsenate is primarily a function of pyrite oxidation.

Chromium:
" Two major forms identified:
o Cr3+/illite
o Org. associated Cr (Amorph. CrOOH)
" Chromite- Common only in coals unusually rich in Cr.
" Oxidation State- Always Cr3+ in coal (rare Cr 6+ in some fly ash)

Figure 37. XAFS spectra of Alabama coal sample showing pyritic arsenic and arsenate forms.
Comparison of XAFS and Selective Leaching Results for Arsenic

Figure 38. Comparison of XAFS and leaching results for Ohio (bituminous) and North Dakota (lignite) coal samples. Determinations show formation of arsenate from As/pyrite over time. Leaching results are sum of arsenic leached by HCl and HF. Data from Huggins et al., 2002.

Arsenic XAFS of leached residues:

Section 6: Coal-use Issues and Case Studies

Trace elements and coal use:

" Legislative limits on mercury emissions from coal-fired powerplants are sought by the EPA. This issue balances potential health effects vs. multi-billion-dollar cost of controls.

" Fine particulate matter (PM2.5) concentrates trace metals relative to coarser ash fractions, and are more readily inhaled.

" Water quality issues: Acid mine drainage (AMD). Disposal of coal preparation wastes. Use of coal combustion products- considered non-hazardous under the Resource Conservation Recovery Act (RCRA).

Each of these subjects has prompted considerable research and an entire short course could be devoted to any of them. We will briefly discuss pyrite oxidation and the remainder of the session will be devoted to mercury.

Pyrite Oxidation

Some basic points:

" Oxidation of pyrite results in acid mine drainage and releases metals such as arsenic into the environment.

" Oxidation of pyrite in coal occurs spontaneously over time. Arsenic oxidation proceeds more rapidly than iron oxidation.

" Pyrite oxidation has important implications for coal transport and handling, potentially resulting in leaching of metals from coal piles.

Pyrite Oxidation

Oxidation Experiments- pyrite in coal:

Oxidation experiments (Kolker et al., 2003) test the hypothesis that arsenic-rich pyrite is more susceptible to oxidation than pyrite with little or no arsenic. Samples were selected to represent low, medium and high contents of arsenic in pyrite (Fig. 8). The experiment uses As XAFS and Fe-Mössbauer spectroscopic methods to monitor pyrite oxidation in coal stored under controlled conditions. Results to date show a nearly complete range of arsenic oxidation is produced by experimental conditions. Arsenic and iron show parallel oxidation. Humidity and/or presence of water, and oxygen availability are most important factors controlling oxidation state.

Table 8. Comparison of arsenic XAFS and iron
Mössbauer results for two powdered bituminous coal
samples, comparing dry samples stored in argon vs.
wetted samples stored in air, for a period of 3 weeks.

Springfield Coal
Arsenic ( percent as pyrite vs. arsenate) Iron ( percent as pyrite vs. jarosite)
Pyrite Arsenate Pyrite Jarosite
Argon 92 8 91 8
Air Wet 70 30 82 16
Pittsburgh #1 Coal
Argon 93 7 98 2
Air Wet 81 19 86 14

Pyrite Oxidation Experiments:

Figure 40. Coal oxidation experiment showing gas-ported desiccators (argon, oxygen, and air atmospheres; above left), and wetted samples stored in air (above right). Air-wet samples show rapid arsenic and iron oxidation relative to equivalent samples kept under dry gas atmospheres.

Mercury Emissions from Coal-fired powerplants.

" December, 2000, EPA determines to limit mercury emissions from coal-fired powerplants; timetable superceded by Clear Skies.

" Clear Skies*- Multi-pollutant plan links Hg, SO2 and NOx. Reduces 1999 Hg emissions (48 tons) to 26 tons by 2010 and 15 tons by 2018.

" Multi-pollutant plan for industrial/commercial boilers. Limits new boilers to 3 lbs Hg/trillion BTU and existing plants to 7 lbs/trillion BTU. Legislation initially sought by early 2004 and compliance in 3 years.

*http://www.epa.gov/clearskies/

Figure 42. Distribution of U.S. mercury emissions by source. Combustion sources account for nearly 90 percent of all emissions. This includes sources that are currently regulated, such as medical waste incinerators, sources for which specific regulations have been proposed, and sources that are not currently regulated.

Health Risks from Mercury

" Exposure due to consumption of methyl-mercury in fish.

" Nervous system effects and developmental disorders. Documented effects of chronic exposure at low levels. Risk to fetuses and infants is greatest.

" Strong association with kidney damage and disease.

" Likely association with increases in lung cancer, and possible cardiovascular effects.

Mercury in U.S. Coals

USGS COALQUAL Database.

" USGS database includes data for about 40 elements and many coal-use parameters.

Figure 43. Histogram of mercury concentrations (remnant moisture, whole coal basis) for conterminous US from the COALQUAL database. One outlier removed. Mean is 0.17 ppm, median = 0.11 and standard deviation of 0.17

EPA- ICR Data base

EPA database reflects mercury content of commercial coals delivered in 1999 to U.S. powerplants 25 MW.

Comparison of USGS COALQUAL and EPA ICR databases

" Subsets1 give averages of 0.10 ppm for ICR and 0.17 ppm for COALQUAL. Difference reflects cleaned vs. in-ground values, and increased use of low-S western coals. Source: Quick et al. 2003, Environmental Geology.

Mercury loading to U.S. powerplants

Variables are calorific value, mercury concentration, and moisture. Can compare mercury output by expressing mercury on an equivalent energy basis:

Example:

High-volatile B Bituminous Coal
Calorific value = 13,500 Btu/lb
Hg = 0.1 ppm Hg (equivalent basis)

1 lb Hg 1 lb coal 7.4 lbs Hg
--------------- X ------------------- = -------------
107 lb coal 1.35 x 105 Btu 1012 Btu

Figure 44. Means for mercury input loadings (in-ground coal) for selected coal regions in the US. Hg and Btu/lb calculated to as-received basis.

Comparing mercury loadings of coals:

Table 9. Average Mercury Loadings Appalachian Coal Regions. USGS Results from Tewalt et al., 2001 (as-received basis).

Coal Region Mean Hg (ppm) Mean Calorific Value (Btu/lb) Mean Hg Loading*
(lbs Hg/1012 Btu)
Northern Appalachian 0.24 12,440 18.8
Central Appalachian 0.15 13,210 11.3
Southern Appalachian 0.21 12,760 17.0

Benefit of Coal Cleaning:

Table 10. Table shows calorific value and mercury contents for raw and cleaned coal averages for 24 eastern bituminous coal samples (dry, equal-energy basis). Results from B. Toole-O'Neil et al., Fuel, v. 78, p. 47-54, 1999.

Calorific value raw coal (Btu/lb)

Hg content raw coal (ppm) Calorific value cleaned coal (Btu/lb)
Hg content cleaned coal
(ppm)
Percent Hg reduction
(equal energy basis)
10,704 0.23 13,730 0.16 37

Most Eastern coal producers practice coal cleaning and selective mining; Delivered Hg contents are lower than USGS in-ground averages.

Estimating yearly emissions from U.S. powerplants:

o Need to accurately know tonnages and moisture contents of coals having a particular mercury content.

o Estimates:
1) 63 1Mg, based on ICR state averages, EIA tonnage data, for powerplants 50 MW only (Quick et al., 2003).

2) 68 Mg, based entirely on ICR data2 (Kilgroe et al., 2002).

3) 111 Mg, based on COALQUAL state averages (Quick et al.). Reduced to about 70 Mg with estimated 37 percent reduction by cleaning (equal energy basis).

1Mg is one metric ton; 1 metric ton = 1.1023 U.S. tons
2EPA-600/R-01-109, April 2002

Atmospheric Mercury Deposition:

Highest projected rates of mercury wet deposition are in the eastern third of U.S., due to climatic factors, the distribution of coal burning powerplants, and other factors.

Figure XX.. Projected Hg deposition

Figure 45. Map showing projected total mercury wet deposition from sources in the U.S., Canada, and global background levels of elemental mercury. Map is based on Regional Lagragian Model of Air Pollution (RELMAP) simulation of Bullock (2000). An improved model incorporating more recent information about mercury emissions and mercury chemistry is currently being developed (Russ Bullock, personal communication, 2004; map courtesy of Russ Bullock, EPA/NOAA).

Mercury Deposition Network

" Subprogram of National Atmospheric Deposition Program initiated in 1995 to monitor mercury levels in precipitation.
" Current network consists of about 80 standardized sites in U.S. and Canada.
" Weekly wet deposition samples are determined by cold vapor atomic fluorescence at Frontier Geosciences, Inc.
" Data distribution and program management by Illinois State Water Survey.

Data available at: http//:nadp.sws.uiuc.edu

Figure 46. Distribution of stations in the National Atmospheric Deposition Program, Mercury Deposition Network, as of 2003. Network is primarily for total mercury in wet deposition. Map is from NADP-MDN website at: http://nadp.sws.uiuc.edu/mdn/sites.asp
MDN in mid-Atlantic region:

" VA-08 Culpeper (USGS/George Mason University) and VA-28 Shenandoah National Park Big Meadows (National Park Service) started in Oct./Nov. 2002 to help fill a gap in the network in the mid-Atlantic region.
" VA-28: Reference for ecological and water quality studies in Shenandoah National Park.
" VA-08 and VA-28: Regional background for mercury emissions prior to mandated changes.

Figure 47. View of MDN Station VA-08 near Culpeper, VA. Facilities include recording rain guage (left foreground) and collector assembly (right rear).

References

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