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Petrology, mineralogy and geochemistry of mined coals, western Venezuela
International Journal of Coal Geology 63 (2005) 68 – 97 www.elsevier.com/locate/ijcoalgeo
Petrology, mineralogy and geochemistry of mined coals, western Venezuela Paul C. Hackleya,T, Peter D. Warwicka, Eligio Gonza´lezb a U.S. Geological Survey, 956 National Center, Reston, VA 20192, USA INGEOMIN, Torre Oeste Parque, Central Piso 8, Caracas 1010, Venezuela
b
Received 1 January 2004; received in revised form 1 January 2005; accepted 6 February 2005 Available online 20 April 2005
Abstract Upper Paleocene to middle Miocene coal samples collected from active mines in the western Venezuelan States of Ta´chira, Me´rida and Zulia have been characterized through an integrated geochemical, mineralogical and petrographic investigation. Proximate, ultimate, calorific and forms of sulfur values, major and trace element, vitrinite reflectance, maceral concentrations and mineral matter content have been determined for 16 channel samples from 14 mines. Ash yield generally is low, ranging from b 1 to 17 wt.% (mean = 5 wt.%) on a dry basis (db). Total sulfur content is low to moderate, ranging from 1 to 6 wt.%, db (average =1.7 wt.%). Calorific value ranges from 25.21 to 37.21 MJ/kg (10,840–16,000 Btu/lb) on a moist, mineral-matter-free basis (average = 33.25 MJ/kg, 14,300 Btu/lb), placing most of the coal samples in the apparent rank classification of highvolatile bituminous. Most of the coal samples exhibit favorable characteristics on the various indices developed to predict combustion and coking behavior and concentrations of possible environmentally sensitive elements (As, Be, Cd, Cr, Co, Hg, Mn, Ni, Pb, Sb, Se, Th and U) generally are similar to the concentrations of these elements in most coals of the world, with one or two exceptions. Concentrations of the liptinite maceral group range from b 1% to 70 vol.%. Five samples contain N 20 vol.% liptinite, dominated by the macerals bituminite and sporinite. Collotelinite dominates the vitrinite group; telinite was observed in quantities of V1 vol.% despite efforts to better quantify this maceral by etching the sample pellets in potassium permanganate and also by exposure in an oxygen plasma chamber. Inertinite group macerals typically represent b 10 vol.% of the coal samples and the highest concentrations of inertinite macerals are found in distantly spaced (N 400 km) upper Paleocene coal samples from opposite sides of Lago de Maracaibo, possibly indicating tectonic controls on subsidence related to construction of the Andean orogen. Values of maximum reflectance of vitrinite in oil (R o max) range between 0.42% and 0.85% and generally are consistent with the high-volatile bituminous rank classification obtained through ASTM methods. X-ray diffraction analyses of low-temperature ash residues indicate that kaolinite, quartz, illite and pyrite dominate the inorganic fraction of most samples; plagioclase, potassium feldspar, calcite, siderite, ankerite, marcasite, rutile, anatase and apatite are present in minor or trace concentrations. Semiquantitative values of volume percent pyrite content show a strong correlation with pyritic sulfur and some sulfide-hosted trace element concentrations (As and Hg). This work provides a modern quality dataset for the western
T Corresponding author. Tel.: +1 703 648 6458; fax: +1 703 648 6419. E-mail address: [email protected] (P.C. Hackley). 0166-5162/$ - see front matter. Published by Elsevier B.V. doi:10.1016/j.coal.2005.02.006
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
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Venezuela coal deposits currently being exploited and will serve as the foundation for an ongoing coal quality research program in Venezuela. Published by Elsevier B.V. Keywords: Coal; Venezuela; Petrography; Geochemistry; Mineralogy; Macerals
1. Introduction Rapidly expanding coal production in Venezuela illustrates the need for widely accessible information on coal quality characteristics. Production of coal (8 Mt/year) has increased almost four-fold over the last decade, primarily due to the development of large deposits in the Guasare basin of northern Zulia (Fig. 1), and current production is expected to double over the next 5 years (Energy Information Administration, 2003). Total in-ground coal resources in Venezuela are estimated to be on the order of 10 Gt (Ministerio de Energı´a y Minas, 1996), placing Venezuela second in South America behind only Columbia in terms of resources (Fossil Energy International, 2002). Current coal production is concentrated in western Venezuela in the States of Zulia, Ta´chira and Me´rida (Fig. 1), where surface and underground mines produce high quality (low-ash, moderate-sulfur), high-volatile bituminous coal. Over 90% of the coal produced in Venezuela comes from the Guasare basin and all Guasare coal is exported to the United States, Italy, France, Spain, the Netherlands, the United Kingdom and other countries in Central and South America (Petro´leos de Venezuela, 2001a). Despite the recent dramatic increase in coal production and the significant in-ground coal resources of Venezuela, detailed characterization of the quality of western Venezuelan coals has been relatively underrepresented in English-language literature and the primary references are in Spanish (e.g., Escobar and Martı´nez, 1993; Ministerio de Energı´a y Minas, 1996; Escobar et al., 1997). In addition, information on the concentrations of environmentally sensitive trace elements, such as As, and Hg in western Venezuelan coals is not available, nor is an analysis of technological properties with respect to combustion as a thermal fuel resource. This study presents results of a quality characterization of in-ground coal and includes the majorand trace-element geochemistry, mineralogy and
petrography of representative samples of coals currently mined from western Venezuela. This work has been completed under the auspices of the U.S. Geological Survey’s (USGS) World Coal Quality Inventory project, a program designed to provide a modern library of coal quality data from commercially produced coals of most of the world’s major coal-producing countries (Finkelman and Lovern, 2001). The coal quality dataset presented herein provides a broad characterization of the western Venezuela coal deposits now being exploited, and will serve as the platform from which to study the Guasare coals currently targeted for rapidly expanding development.
2. Study area 2.1. Western Venezuela Bituminous coals of western Venezuela are hosted in upper Paleocene–middle Miocene strata located on the margins of the Maracaibo basin, a major structural depression flooded by Lago de Maracaibo (Fig. 1). Northwest of Lago de Maracaibo, in the northern Sierra de Perija of Zulia (Fig. 2A), extensive coal deposits are found in the Paleocene Marcelina Formation of the Guasare basin (Fig. 3). Southeast of Lago de Maracaibo, in the Cordillera de los Andes of the states of Me´rida and Ta´chira (Fig. 2B and C, respectively), coals primarily occur in the Paleocene Los Cuervos Formation and Eocene–Oligocene Carbonera Formation, and, less frequently, in the Miocene Palmar Formation (Fig. 3). The upper Paleocene deltaic Marcelina and Los Cuervos Formations in Zulia and Ta´chira, respectively, were deposited during regional regression, prograding out over a thick package of Cretaceous calcareous and deep water anoxic sediments into the Maracaibo basin (Parnaud et al., 1995). This sedimentation occurred in a foreland basin environment
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P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97 73ºW
72ºW
71ºW
12ºN
0
50
100
Area of Fig. 2A
150 km
SAMPLED COAL MINES 1. Minas Norte 2. Paso Diablo 3. Palmichosa 4. El Palmital 5. Las Dantas-La Vega 6. Río Escalante 7. Las Mesas-Río Pajitas 8. Las Mesas-Finca Familia Arellano 9. Caliche 10. Lobatera 11. Hato De La Virgen 12. Las Adjuntas 13. Cerro Capote & La Pajarita 14. Santo Domingo
1 2 Guasare
11ºN
aib Ma
rac
de Sie
rra
10ºN
ob
Per ij
asi n
a
basin
Lago de Maracaibo
Trujillo Zulia
3
9ºN Area of Fig. 2B
Mérida Area of Fig. 2C
9 8ºN
COLOMBIA
7, 8
4, 5 6
10 11 12 Táchira 13 14
s
de
n sA
e ad
lo
ler
dil or
C
State boundary 7ºN
Fig. 1. Shaded relief image of western Venezuela showing locations of sampled coal mines and State boundaries. Insets denote locations of Fig. 2A–C. Shaded relief elevation data from U.S. Geological Survey (2003) shuttle radar topography mission 90 m elevation data set (http:// srtm.usgs.gov/).
developed as a result of incipient oblique collision between the South American and Caribbean plates (Lugo and Mann, 1995; Milani and Filho, 2000). The paralic Eocene–Oligocene Carbonera Formation was sourced from uplifted areas to the west and south during continued oblique convergence. Continental sediments of the Miocene Palmar Formation were deposited as molasse shed from the margins of the emergent Me´rida Andes during rapid uplift of the orogen.
2.2. Zulia The State of Zulia (Fig. 2A) contains over 80% of the coal resources of Venezuela in two main districts: Guasare basin in the north and the Santa RosaCatatumbo district in the southern part of the State (Rodrı´guez, 1995). The Guasare basin is host to the two largest producing mines of Venezuela, Minas Norte and Paso Diablo, and potential resources of the basin are estimated at over 8.5 Gt (Ministerio de Energı´a y
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Minas, 1996). The coals are hosted within the Paleocene Marcelina Formation (Fig. 3), an intercalated sequence of dense gray sandstones, dark-gray to black shales and sandy shales. The coal beds mainly occur in the lower part of the Marcelina where 25–30 coal beds range in thickness from 1 to 13 m (Ruı´z, 1983). Estimates of the total thickness of the Marcelina range from 137 to 610 m (Sutton, 1946; Quijada and Kettle, 1985) and the unit is approximately 550 m thick in the area of the Paso Diablo mine. The open-pit Paso Diablo mine, located about 85 km northwest of Maracaibo, contains 22 coal beds which occur over an interval of approximately 400 m (Ministerio de Energı´a y Minas, 1996). The coals are consistent in both thickness and lateral continuity across the mine area, where the beds dip 5–158 to the southeast. The total thickness of coal averages 60 m. The mine currently is operated by Carbones del Guasare, a subsidiary of Petro´leos de Venezuela (Petro´leos de Venezuela, 2001b). Sixteen coal beds in the Paso Diablo mine, ranging between 0.5 and 13 m in thickness, are forecast to produce about 200 Mt of high volatile bituminous coal with low sulfur content and ash yield. The Norte mine hosts 33 coal beds of bituminous rank, of which 21 are thicker than one m (Ministerio de Energı´a y Minas, 1996). At this locality, a 533 m section through the Marcelina contains 62.5 m total thickness of coal. Potential resources are almost 1 Gt, of which 59 Mt will be extracted by open-pit methods. Carbones de la Guajira, a subsidiary of Petroleos de Venezuela, has been producing coal from the Norte deposit since 1996 (Petro´leos de Venezuela, 2001c). 2.3. Me´rida The state of Me´rida (Fig. 2B) is host to important coal deposits in the Franja Nororiental and Rı´o Muyapa areas (Ministerio de Energı´a y Minas, 1996). In the Rı´o Muyapa region of northern Me´rida, coals occur in the Miocene Palmar Formation (Fig. 3), composed of yellow-white to gray, fine-grained, massive sandstones interbedded with dark-gray to black carbonaceous shales, coals and conglomerates (Sutton, 1946). The unit varies from 570 to 1300 m in thickness (Heybroek, 1953; Ministerio de Energı´a y Minas, 1996). Coal beds vary in thickness from 0.3 to 1.65 m (Ministerio de Energı´a y Minas, 1996) and
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currently are produced from the open-pit Palmichosa mine. The Franja Nororiental area encompasses an area of approximately 520 km2 in southwestern Me´rida, where coals are hosted in the Paleocene Los Cuervos Formation and the Eocene–Oligocene Carbonera Formation (Fig. 3). The Carbonera Formation is a transgressive sequence of claystones, mudstones, grayish carbonaceous shales and coals (Sutton, 1946). The sediments were deposited in a delta plain with little marine influence (Azpiritxaga and Casas, 1989; Ma´rquez and Mederos, 1989; Martı´nez et al., 2001). The Carbonera Formation averages around 470 m in thickness (Ramı´rez and Fields, 1969). The sediments are irregularly stratified, and immature sandstone beds of 5–10 m thickness are found dispersed throughout the sequence as are coals of 1– 3 m thickness (Gonza´lez de Juana et al., 1980). 2.4. Ta´chira The State of Ta´chira, in western Venezuela, is one of the major coal-producing regions of Venezuela and is host to six coal fields (Fig. 2C), including the Franja Nororiental (a southern continuation of the Franja Nororiental field in Me´rida), San Fe´lix-Rı´o Guaramito, Franja Centro-Occidental, Rubio, Franja de Las Delicias and Santa Domingo (Ministerio de Energı´a y Minas, 1996). Coal deposits of Ta´ chira are hosted in the Paleocene Los Cuervos Formation (Fig. 3) and the Eocene–Oligocene Carbonera Formation (Martı´nez et al., 2001). The Carbonera was described in the preceding section on the coal deposits of Me´rida. The Paleocene Los Cuervos Formation primarily consists of shales, mudstones and minor intercalated sandstones, and coal beds (Notestein et al., 1944), in an association interpreted to represent a deltaic environment (Ma´rquez and Mederos, 1989). Coal beds of 0.5–2.5 m thickness are found in the lowermost 75 m of the section (Notestein et al., 1944). Estimates of the thickness of the Los Cuervos range from 245 to 840 m (Notestein et al., 1944; Heybroek, 1953). The Los Cuervos is a time-equivalent correlative of the coal-bearing Marcelina Formation found to the northwest in Zulia on the western side of the Maracaibo basin (Miller and San Juan, 1963), interpreted to have been deposited in fluvio-deltaic
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Fig. 2. Simplified geologic maps of western Zulia (A), Me´rida (B) and Ta´chira (C), western Venezuela, showing locations of mines/prospects and coal fields. Locations of mines/prospects and coal fields compiled from Biewick and Weaver (1995) and Ministerio de Energı´a y Minas (1996). Geology from Bellizzia et al. (1976).
plains with little or no marine influence during regional regression (Martı´nez et al., 2001). 2.5. Previous coal characterization investigations The most complete general reference on western Venezuelan coal is a comprehensive treatise on the coal industry in Venezuela by the Ministerio de Energı´a y Minas (1996). Resource estimates and proximate and ultimate quality parameters are presented for many of the areas examined in the current study.
The only available data on trace element concentrations in Venezuelan coals were presented by Martı´nez et al. (2001), which contained a detailed geochemical study aimed at determining a correlation between trace element concentrations in Paleocene Ta´chira coal beds and in provenance lithologies at the time of peat deposition. Martı´nez et al. (2001) were able to demonstrate through factor analysis that the concentrations of some trace elements in the Los Cuervos Formation coals were a result of sedimentary input derived from outcrops of the dominantly volcaniclastic Jurassic La Quinta Formation.
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TÁCHIRA and MÉRIDA P A L E O G E N E / N E O G E N E
PLIOCENE
Necesidad Betijoque
MIOCENE
OLIGOCENE EOCENE PALEOCENE
GUAYABO GROUP
Isnotú Palmar
León Carbonera Mirador OROCUE GROUP
Los Cuervos Barco Catatumbo
ZULIA Necesidad La Villa Los Ranchos Cuiba Macoa EL FAUSTO GROUP Peroc Misoa Marcelina Guasare
Fig. 3. Simplified Paleogene to Neogene stratigraphy of Zulia, Me´rida and Ta´chira, western Venezuela. Coal-bearing formations labeled in bold type. Modified from Bellizzia et al. (1976) and Martı´nez et al. (2001).
Petrographic studies of western Venezuelan coals include a contribution describing the relative proportions of the vitrinite, liptinite and inertinite (subdivided into fusinite and semifusinite) maceral groups in 33 bench samples from four localities in the Marcelina Formation of the Guasare basin (Mazeaud and Monpart, 1977). These authors found relatively homogeneous maceral proportions with all of the samples dominated by vitrinite, and noted some correlation between increasing mineral matter and liptinite content. A general review of Guasare basin coal quality characteristics was presented by Heintz et al. (1976), including some values of average vitrinite reflectance. Many other studies have investigated various aspects of the Guasare deposits, including resource assessment (Rodrı´guez, 1986), coal systems (Corzo and Cruz, 1987), coal rank (Ardina, 1987), and mine engineering and economics (Herna´ndez and Monsalve, 1979; Etchart, 1987). A study of the Santa Domingo coal deposit of the Los Cuervos Formation in Ta´chira indicated that the maximum reflectance of vitrinite ranged from 0.42% to 0.54%, suggesting a subbituminous rank classification (Bar and Pen˜a, 1985). Coal samples from Ta´chira and Zulia were analyzed for use in a study of the effects of weathering on geochemical parameters in coal (Martı´nez and Escobar, 1995), including determinations of humic and carboxylic acids in fresh and weathered samples. A study of the organic geochemistry of coal samples from Ta´chira, Me´rida and Trujillo was
conducted by Tocco et al. (1995) to determine the source of oil accumulations in the southern part of the Maracaibo basin. These workers found that terpane and sterane biological markers from the Carbonera and Los Cuervos Formation coals were similar to the signatures of local oil seeps and concluded that the coals of these units were likely terrestrial source rocks. In a similar study, Cano´nico et al. (2004) used total organic carbon analyses, Rock-Eval pyrolsis, vitrinite reflectance and maceral composition to demonstrate that some Paleogene–Neogene coals of western Venezuela contain high oil-source rock potential. General overviews of the major-element chemistry, petrology and mineralogy of Venezuela’s coal deposits were presented in Escobar and Martı´nez (1993), and Escobar et al. (1997). They presented proximate, ultimate, calorific and forms of sulfur values, major element concentrations, semiquantitative estimates of mineral matter content and the relative proportions of the three coal maceral groups for 33 coal samples from all of Venezuela’s important coal deposits; these data are most relevant to the new information contained in the present study. To build upon the work of Escobar and Martı´nez (1993) and Escobar et al. (1997), many of the samples examined in the present study are from the same localities described in these two contributions. The major advancements of our new work on western Venezuela coals are to provide information on the concentrations of environmentally sensitive trace
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
elements and detailed petrologic data on coal maceral types and abundances.
3. Methodology 3.1. Samples For the present study, 16 channel coal samples were collected from 14 active coal mines located in the States of Zulia, Me´rida and Ta´chira. Samples were collected by personnel of INGEOMIN and sent to the USGS for analysis. Coal bed name, thickness and relative stratigraphic position in the host formation were not reported by INGEOMIN. The collected samples represent the coal currently being produced at each mine location. Available information pertaining to sample collection and mine location is summarized in Table 1.
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Materials (ASTM) methods and procedures (ASTM, 2002). Tests for ash fusion temperature, air dry loss, residual moisture and the free swelling index of each sample also were determined in the same commercial laboratory. Determinations of major and trace element concentrations were performed at the USGS according to the methods outlined in Bullock et al. (2002). Concentrations of 26 major element oxides and trace elements were determined on coal ash by inductively coupled plasma atomic emission spectroscopy (ICPAES), and concentrations of an additional 17 trace elements in coal ash were determined by inductively coupled plasma mass spectroscopy (ICP-MS). Mercury concentrations were determined in whole-coal samples following ASTM D6414 (ASTM, 2002), and Se concentrations were determined in whole-coal samples by hydride generation atomic absorption (Bullock et al., 2002). Chlorine concentrations in whole-coal samples were determined by ion chromatograph.
3.2. Chemical analysis 3.3. Maceral analysis Analyses for proximate, ultimate, calorific and sulfur forms were performed in a commercial laboratory according to American Society for Testing and
Representative splits of all coal samples were ground, cast in epoxy and polished for petrographic
Table 1 Sample location information for western Venezuelan coal samples Mine
Sample ID
North latitudea
Zulia Minas Norte Paso Diablo
Guarija Guasare
11.04216 11.05000
Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa
M1-QP M2-CDLV M3-RE M4-QPA
Ta´chira Las Mesas-Rı´o Pajitas Las Mesas-Finca Familia Arellano Caliche Las Adjuntas Las Adjuntas Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote a
West longitudea
Origin
Formation
Age
72.27273 72.17000
Surface Surface
Marcelina Marcelina
Upper Paleocene Upper Paleocene
8.44127 8.44818 8.43339 9.06982
71.69081 71.69062 71.79267 71.09390
Surface Surface Surface Surface
Carbonera Carbonera Carbonera Palmar
Eocene–Oligocene Eocene–Oligocene Eocene–Oligocene Middle Miocene
M5-RP M6-FA
8.39232 8.40981
71.93938 71.90385
Surface Surface
Carbonera Carbonera
Eocene–Oligocene Eocene–Oligocene
M7-LC M8-M1LA M8-M2LA M10-LC M11-HV M12-CA M13-LP M14-SC
8.08578 7.76185 7.76185 7.40134 7.84870 7.92288 7.73768 7.73768
72.23795 72.41608 72.41608 72.02841 72.36808 72.29072 72.39907 72.39907
Underground Surface Surface Surface Surface Underground Underground Underground
Carbonera Los Cuervos Los Cuervos Los Cuervos Carbonera Carbonera Carbonera Carbonera
Eocene–Oligocene Upper Paleocene Upper Paleocene Upper Paleocene Eocene–Oligocene Eocene–Oligocene Eocene–Oligocene Eocene–Oligocene
Latitude and longitude coordinates obtained by global positioning satellite (GPS) at time of sample collection.
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analyses following procedures outlined in Pontolillo and Stanton (1994). Two sample mounts were made from each sample. Measurements of maximum vitrinite reflectance in immersion oil (R o max) measurements were performed according to ASTM D2798 methods and procedures (ASTM, 2002). Following vitrinite reflectance measurements, the samples were examined in blue-light fluorescence using a modification of ASTM D 2799 (ASTM, 2002). A total of 500 identifications of liptinite or non-fluorescing organic/mineral matter were made on each sample mount. Vitrinite and inertinite macerals were identified under oil immersion with a standard white light source, using the maceral nomenclature proposed by the International Committee for Coal and Organic Petrology (ICCP) (ICCP, 1998, 2001). An additional 500 counts were performed on each sample mount in white light, for a total of 2000 counts of coal macerals and mineral matter per sample (Pontolillo and Stanton, 1994). All sample mounts were etched in potassium permanganate solution prior to white light petrographic analysis to facilitate measurement of telinite abundance. However, this procedure yielded a negligible improvement, illustrating the highly gelified nature of the samples. An experimental program aimed at etching the sample mounts in a low temperature oxygen plasma chamber also had poor results despite the considerable duration of some etching runs. 3.4. Mineral analysis Representative splits of coal samples were ground to 200 mesh (75 Am) and oven-dried overnight before ashing in a low-temperature asher. Low temperature ash (LTA) residues were cast into pressed pellets and analyzed on an automated powder diffractometer. X-ray diffraction (XRD) patterns were analyzed with a commercial reference pattern library (International Center for Diffraction Data, 1997) and also using a USGS program with a common coal mineral reference pattern library (Hosterman and Dulong, 1989). Detection of phases constituting less than 5 wt.% of the sample LTA residue is suspect on the diffractometer, and values of the program output of 5 wt.% or less are provided for informative purposes only.
4. Results and discussion 4.1. Coal quality Western Venezuelan coal samples generally are comparable in quality with the extensively developed coal deposits of the Powder River and Appalachian basins in the United States that also are used for thermal power generation. Proximate, ultimate, calorific and forms of sulfur values are summarized in Table 2. Total moisture content on an as-received basis (as-received) of the Venezuelan samples ranges from 0.69 wt.% in the sample from Lobatera to 16.66 wt.% in the Santo Domingo sample, with an average moisture of 4.39 wt.%. Moisture content is, in general, comparable to Appalachian basin coals of the eastern United States, which average 3.4 wt.% moisture (n = 3206; Bragg et al., 1998). Volatile matter content ranges from 36.03 wt.% [dry basis, (db)] in one of the Las Adjuntas samples (M8-M1LA) to 60.91 wt.% in the La Pajarita sample. Average volatile matter content of the coal samples is 45.76 wt.% (db), slightly higher than the average coal from the Appalachian basin (34 wt.% volatile matter; Bragg et al., 1998). Fixed carbon content ranges from 35.35 wt.% (db) in the sample from La Pajarita to 62.45 wt.% (db) in the M8-M1LA sample from the Las Adjuntas mine. Proximate results indicate that the western Venezuelan coal samples have a low average ash yield of 5.30 wt.% (db), ranging from 0.42 wt.% in the Paso Diablo sample to 17.20 wt.% ash in the sample from Las Dantas-La Vega in Me´rida (Fig. 4). The majority of the coal samples have low ash yields, ranging between 1 and 8 wt.% ash, with only the samples from Lobatera (12.83%), Rı´o Pajitas (15.65%) and Las Dantas La Vega (17.2%) containing more than 10 wt.% ash. Ash yields are relatively low in comparison with Paleocene subbituminous Powder River basin coals of Wyoming and Montana in the western United States, which average 12.30 wt.% ash (db) (Bragg et al., 1998), higher than all but the Lobatera, Rı´o Pajitas and Las Dantas La Vega samples. When compared to the Carboniferous bituminous coals of the central and northern Appalachians, the western Venezuelan coal samples are significantly lower in ash. Average ash yield of 3206 Appalachian basin coals is 11 wt.% (db), again higher than all but the Lobatera, Rı´o
Table 2 Proximate, ultimate, calorific and forms of sulfur values [dry basis except moisture (as-received) and calorific value (moist, mineral-matter-free)] for western Venezuela coal samples Mine
Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa Ta´chira Las Mesas-Rı´o Pajitas Las Mesas-Finca Familia Arellano Caliche Las Adjuntas (M8-M1LA) Las Adjuntas (M8-M2LA) Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote
Ash (%, 750 8C)
VM (%)
FC (%)
H (%)
C (%)
N (%)
S (%)
O (%)
Forms of sulfur Sulfate (%)
Pyritic (%)
Organic (%)
CV m,mmf (Btu/lb)
CV m,mmf (MJ/kg)
R o max (%)
2.78 3.04
1.88 0.42
38.29 39.90
59.83 59.68
5.60 5.66
82.31 83.43
1.51 1.60
0.41 0.37
8.29 8.52
0.02 0.03
0.01 0.01
0.38 0.33
14,652 14,575
34.08 33.90
0.72 0.71
3.49 2.03 6.75 11.25
1.94 17.20 3.71 5.48
45.33 45.15 48.30 44.45
52.73 37.65 47.99 50.07
6.29 5.52 6.05 5.20
79.59 63.58 74.65 70.92
1.78 1.34 1.50 1.62
0.87 6.34 0.97 3.85
9.53 6.02 13.12 12.93
0.02 0.15 0.02 0.04
0.07 3.62 0.05 2.67
0.78 2.57 0.90 1.14
14,375 14,951 13,110 11,973
33.44 34.78 30.49 27.85
0.51 0.56 0.57 0.51
1.46
15.65
39.68
44.37
5.08
66.92
1.24
6.06
5.05
0.06
5.23
0.77
15,090
35.10
0.62
4.47
8.03
55.20
36.77
6.90
73.25
1.30
0.45
10.07
0.02
0.09
0.34
14,537
33.81
n.d.
0.93 6.48
4.59 1.52
43.08 36.03
52.33 62.45
6.14 4.92
80.07 78.89
1.49 1.79
2.63 0.66
5.08 12.22
0.03 0.04
1.78 0.03
0.82 0.59
15,493 13,035
36.04 30.32
0.61 0.85
3.82
1.87
36.31
61.82
5.36
81.46
1.65
0.61
9.05
0.02
0.02
0.57
14,121
32.85
0.75
16.66 1.10 0.69 0.72 4.61
1.09 1.95 12.83 3.64 2.97
47.51 52.23 49.77 60.91 50.05
51.40 45.82 37.40 35.45 46.98
5.21 7.17 6.60 8.05 6.20
74.87 82.90 72.97 80.25 78.50
1.56 1.63 1.06 1.54 1.19
0.32 0.81 0.47 0.57 1.21
16.95 5.54 6.07 5.95 9.93
0.03 0.04 0.01 0.01 0.10
0.01 0.11 0.08 0.03 0.15
0.28 0.66 0.38 0.53 0.96
10,839 15,860 15,999 15,964 14,207
25.21 36.89 37.21 37.13 33.04
0.42 0.60 0.48 n.d. 0.61
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Zulia Minas Norte Paso Diablo
Moist (%, as-received)
Abbreviations: Moist=moisture, %=weight percent, Ash =ash yield, VM=volatile matter, FC=fixed carbon, H =hydrogen, C=carbon, N =nitrogen, S =sulfur, O=oxygen, CV=calorific value, m,mmf=moist, mineral-matter-free, Btu/lb=British thermal units per pound, MJ/kg=Megajoules per kilogram, R o max =maximum reflectance of vitrinite in immersion oil, n.d. =not determined.
77
78
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97 20 18
wt.% ash (db)
16 14 12 10 8 6 4 2 La s A S Pas La dj ant o D s A unt o D iab dj as ( om lo un M ta 8- ing s( M o M 1L 8- A M M2 ) in L A a H E sN ) at l P or o de a l m te l i Ce a Vi tal rro rge C n La apo Pa te Es jari ca ta la Fi n nc Ca te aF P lic a am lm he ili ich aA o re sa l Lo lan La o s D R bat an ío P era ta aj s- ita Po La s w Ve A der ga pp R al ive ac r hi (n an =5 (n 52 =3 ) ,2 06 )
0
MINE Fig. 4. Proximate analyses values for weight percent ash (dry basis) of western Venezuelan coals. Average ash yield for Powder River basin and Appalachian basin coal samples (Bragg et al., 1998) provided for comparison.
Pajitas and Las Dantas-La Vega samples. Coals from Paso Diablo and Minas Norte in the Guasare basin of Zulia, which constitute 90% of Venezuela’s currently produced coal (Petro´leos de Venezuela, 2001d), have exceptionally low ash yields of 0.42 and 1.88 wt.%, respectively. Note that ashing of the coal samples for major and trace element analyses was performed at a maximum temperature of 525 8C, resulting in slightly higher ash yields than the proximate analyses (compare ash values in Table 2 to ash values in following tables). Higher ash yield at lower temperatures is a result of the retention of some mineral matter in the ash, particularly carbonates or other phases with a volatile molecular component, and is even more pronounced for the results of low temperature ashing. Moisture values compiled with major and trace element concentrations were measured on whole-coal samples prior to ashing and represent moisture on an asdetermined basis (after processing and conditioning of the coal samples to 60 mesh or 250 Am). Western Venezuelan coals, in general, have low total sulfur values, ranging from 0.32 wt.% sulfur (db) in the Santo Domingo sample to 6.34 wt.% in the Las Dantas-La Vega sample (Fig. 5). The average sulfur content of the 16 samples is 1.66 wt.%. However, this value is skewed by the relatively high total sulfur
content of the samples from only four mines: Las Dantas La Vega (6.34 wt.%), Palmichosa (3.85 wt.%), Rı´o Pajitas (6.06 wt.%) and Caliche (2.63 wt.%) (Table 2). Eleven of the samples contain less than 1 wt.% total sulfur (median = 0.74 wt.%). For comparison, on a dry basis, Powder River coals contain on average 1.3 wt.% sulfur (median = 0.97 wt.%) and Appalachian coals contain 2.2 wt.% sulfur (median = 1.7 wt.%; Bragg et al., 1998). As with ash yield, the Guasare basin coal samples are exceptionally low in sulfur content: 0.37 wt.% sulfur for Paso Diablo and 0.41 wt.% for Minas Norte. Sulfur mostly occurs in the organic form, except in the four samples with the highest sulfur concentrations, in which total sulfur is dominated by the pyritic form (Fig. 6). As discussed below, X-ray diffraction and petrographic analyses confirm that the mineralogy of these samples mostly is composed of pyrite. Overall, total sulfur content in western Venezuelan coals is low to moderate. 4.2. Coal rank The western Venezuelan coal samples have an apparent rank of subbituminous A (one sample— Santo Domingo) to high volatile A bituminous following the ASTM D 388 classification (ASTM, 2002), similar to the average coal rank of the
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
79
7 6
wt.% S (db)
5 4 3 2 1
La s A S Pas La dj ant o D s A unt o D iab dj as ( om lo un M ta 8- ing s( M o M 1L 8- A M M2 ) in L A a H E sN ) at l P or o de alm te l i Ce a Vi tal rro rge C n La apo Pa te Es jari ca ta la Fi n nc C te a F P ali a c am lm he ili ich aA o re sa l Lo lan La o s D R bat an ío P era ta aj s- ita Po La s w de Ve A rR ga pp i al ve ac r ( hi n= an 5 (n 52) =3 ,2 06 )
0
MINE
Fig. 5. Ultimate analyses values for weight percent sulfur (dry basis) of western Venezuelan coals. Average sulfur content of Powder River basin and Appalachian basin coal samples (Bragg et al., 1998) provided for comparison.
Pyritic Sulfur 0
20
100
Río Pajitas 80
Palmichosa Caliche 40
60
Las Dantas-La Vega %
%
60
40
80
20
100
Organic 0 Sulfur
20
40
60 %
80
0 100 Sulfate
Sufur
Fig. 6. Ternary diagram depicting forms of sulfur values (dry basis) for western Venezuelan coals. Samples containing high pyritic sulfur are labeled.
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P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Pennsylvanian coals of the Appalachian basin. On a moist, mineral-matter-free basis (m,mmf), calorific value ranges from 25.21 MJ/kg (10,839 Btu/lb) for the Santo Domingo sample to 37.21 MJ/kg (15,999 Btu/ lb) for the Lobatera sample. The majority of the samples (12) fall in the high volatile A bituminous classification with calorific values greater than 32.56 MJ/kg (14,000 Btu/lb) and contain sufficient volatile matter to exclude classification on dry bases following ASTM guidelines. Of these samples, measured R o max values are highly variable. R o max values range from 0.48% in the Lobatera sample to 0.75% in one sample from Las Adjuntas (M8-M2LA). Maximum reflectance values for two of the samples with calorific values greater than 32.56 MJ/kg (14,000 Btu/lb), Finca Familia Arellano and La Pajarita, were not determined due to high liptinite content. Of the remaining 10 samples with N32.56 MJ/kg (14,000 Btu/lb) m,mmf, 7 have R o max values N 0.60%, consistent with the high volatile A bituminous classification obtained by ASTM D 388. Rank classification for the three other samples with N32.56 MJ/kg (14,000 Btu/lb) m,mmf is somewhat more problematic. The R o max values for Lobatera (0.48%), El Palmital (0.51%) and Las Dantas-La Vega (0.56%) fall within the subbituminous range. Suppression of R o max by high volume percent liptinite content in the Lobatera and Las Dantas-La Vega samples is a possibility. However, this artifact cannot account for the relatively low R o max value for El Palmital, which contains negligible liptinite. Three samples, Escalante, Palmichosa and Las Adjuntas (M8-M1LA) are classified as high volatile B bituminous by ASTM D 388 (30.23–32.56 MJ/kg, 13,000– 14,000 Btu/lb), and a fourth, Santo Domingo is classified as subbituminous A (24.42–26.75 MJ/kg, 10,500–11,500 Btu/lb). These four coal samples have the highest moisture content of the western Venezuelan coals studied here (Table 2). The R o max value obtained for the Las Adjuntas (M8-M1LA) sample is 0.85%, well within the bituminous range, and generally consistent with the ASTM rank classification of high volatile B bituminous. The slight differences in ASTM rank between the two Las Adjuntas samples may be explained by differing inertinite content; sample M8-M2LA has 19 vol.% inertinite, almost three times as much as the other Las Adjuntas sample and the relatively higher carbon content of the
inertinite fraction may explain the slight rank elevation. The Escalante and Palmichosa samples have R o max values of 0.57% and 0.51%, in the subbituminous A range and slightly lower than the ASTM rank classification. The Santo Domingo sample has a R o max value of 0.42%, consistent with the subbituminous rank classification obtained by ASTM D 388. 4.3. Maceral analysis Results of petrographic analyses (Table 3) illustrate the diverse character of coals currently produced in Venezuela. Concentrations of the liptinite maceral group range from b 1 vol.% in coal from Las Adjuntas (M8-M2LA) to almost 70 vol.% in the coal sample from La Pajarita (Fig. 7). Five of the 16 samples contained N 20 vol.% liptinite, which typically consisted of the macerals bituminite and sporinite. More than half of the western Venezuela coal samples consisted of 80 vol.% or more vitrinite. Inertinite macerals typically make up less than 10 vol.% of the samples and range to a maximum of 22 vol.% in the sample from Minas Norte. The vitrinite component in all samples mostly consisted of either collotelinite (Fig. 8A) or collodetrinite (Fig. 8B), or approximately equal concentrations of both macerals. Small percentages of corpogelinite were observed in most samples. Likewise, several samples contained V 1 vol.% telinite (Fig. 8C). The relative lack of telinite in coal samples collected from localities widely dispersed over western Venezuela may suggest that climate predominantly controlled the peat flora, and that the original plant materials were depleted in woody substance. The relative scarcity of suberinite (Fig. 8D), corkified cell walls of bark, which was found in only trace quantities in about half the samples, supports this interpretation. Alternatively, waterlogged conditions in the peat environment may have resulted in the bacterial alteration and recombination of humic substances, precipitating gels which filled in cell lumina and removed the apparent telinite structure from woody precursor materials (Cohen et al., 1987). Cutinite, occurring with abundant gelified vitrinite macerals in most of the samples, suggests the presence of leafy shrubs or trees in the peat-forming environment of many of the western Venezuela coals (e.g., Wu¨st et al., 2001).
Table 3 Petrographic data for Venezuelan coal samples Minas Norte
Paso Diablo
El Palmital
LDLV
Escalante
Palmichosa
Rı´o Pajitas
FFA
Caliche
Las Adjuntas
Santo Domingo
HDLV
Lobatera
La Pajarita
Cerro Capote
Sample ID
GUAJIRA
GUASARE
M1-QP
M2-CDLV
M3-RE
M4-QPA
M5-RP
M6-FA
M7-LC
M8-M1LA
Vitrinites Telinite Collotelinite Vitrodetrinite Collodetrinite Corpogelinite Gelinite Total Vitrinite
M8-M2LA
M10-LC
M11-HV
M12-CA
M13-LP
M14-SC
tr 24 X 42 3 X 69
X 19 X 64 1 X 84
1 86 X 1 5 X 93
X 59 X X 8 X 67
X 86 X X 2 X 88
X 90 X X 5 X 95
X 64 X 23 4 tr 91
X 3 2 44 3 X 52
1 98 X tr tr X 99
X 63 X 29 tr tr 92
X 13 X 66 1 1 81
X 19 X 44 16 tr 79
X 13 tr 52 4 tr 69
1 23 X 28 14 X 66
1 3 2 13 11 X 30
tr 14 X 50 26 X 90
Inertinites Fusinite Semifusinite Secretinite Macrinite Micrinite Funginite Inertodetrinite Total Inertinite
6 5 tr 7 tr tr 4 22
3 2 tr 5 X tr 2 12
tr X X 1 X tr tr 1
X X X tr X tr X tr
X X X tr X 1 X 1
X tr X tr tr tr X tr
1 1 X 1 X tr 1 4
X X X X X X X tr
X X X X X tr X tr
2 1 X 2 X tr 2 7
7 4 tr 5 tr tr 3 19
4 4 X 4 tr tr 3 15
tr tr X tr tr 2 1 3
X tr X 1 tr 1 tr 2
tr tr X tr X 1 X 1
X tr X X tr 1 1 2
Liptinites Sporinite Cutinite Resinite Fluorinite Bituminite Exsudatinite Alginite Suberinite Liptodetrinite Total Liptinite
tr 3 tr X 1 tr 2 X 3 9
tr tr tr X tr 1 1 X 2 4
tr 1 tr X 2 tr tr tr 3 6
3 1 tr X 18 1 1 tr 9 33
1 1 X 1 1 2 1 tr 4 11
1 1 1 X tr X tr tr 2 5
1 1 tr X 2 tr tr tr 1 5
4 X 2 X 32 X X X 10 48
tr tr tr X 1 tr X tr tr 1
tr X tr X 1 tr tr X tr 1
tr X X X tr tr X X tr tr
1 1 tr tr 1 tr tr tr 3 6
4 X tr X 18 tr tr X 6 28
2 tr 1 X 21 tr tr tr 8 32
3 tr 1 2 51 1 3 X 8 69
1 tr tr X 3 tr X tr 4 8
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Mine
Data are reported on a mineral-matter-free basis in volume percent. LDLV= Las Dantas-La Vega, FFA=Las Mesas-Finca Familia Arellano, HDLV=Hato de la Virgen, X =not present, tr =present in trace amounts (b1 vol.%).
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P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97 Inertinite 0 100
20
80
40
60
%
%
60
40
Minas Norte
80
100 Liptinite 0
La Pajarita 20 40 Finca Familia Arellano
60
80
20
0 100 Vitrinite
%
Fig. 7. Ternary diagram showing relative percentages of vitrinite, inertinite and liptinite maceral groups in western Venezuela coal samples. Samples containing high liptinite or inertinite concentrations are labeled.
Fig. 8. Photomicrographs of western Venezuela coals. (A) Palmichosa, (B) Rı´o Pajitas, (C) Palmichosa sample showing rare telinite, (D) Paso Diablo sample showing rare suberinite. All photomicrographs taken in white light under oil immersion.
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
The organic component of several samples was dominated or substantially represented by detrital organic material; particularly samples from Las Adjuntas (M8-M2LA) and Paso Diablo (Fig. 8D), in which total detritus (including collodetrinite) of the three maceral groups sum to 69 vol.% and 68 vol.%, respectively. The Finca Familia Arellano, Hato de la Virgen and Cerro Capote samples from the Carbonera Formation all contain 50 vol.% or greater detrital coal macerals (i.e., collodetrinite, vitodetrinite, inertodetrinite and liptodetrinite). The high content of detrital organic material probably indicates pervasive degradation of autochthonous plant materials. Previous workers interpreted the characteristics of Carbonera Formation coal deposits in Ta´chira to suggest a paralic origin with relatively little transport of the original plant materials prior to coalification (Bar and Pen˜a, 1985). There does not appear to be any correlation between ash content and detrital organic component, possibly indicating sustained decomposition of a domed peat surface protected from flooding. Collodetrinite represents the most abundant maceral in many
83
of the detritus-rich coal samples, including: Minas Norte, Lobatera, Santo Domingo and the aforementioned Cerro Capote, Paso Diablo, Finca Familia Arellano, Hato de la Virgen and Las Adjuntas (M8M2LA) samples. The presence of this maceral does not necessarily imply transportation prior to coalification, as the precursor material may form in-situ during fragmentation of the original plant material (e.g., Wu¨st et al., 2001). However, the dominance of this maceral in many of the western Venezuela coal samples indicates pervasive decomposition at the onset of the peat stage of coalification, occurring in neutral to weakly alkaline waters in an oxygen-rich environment (ICCP, 1998). The absence of telinite can be interpreted as further evidence for the neutral to weakly alkaline nature of the peat environment as acidic conditions would have prohibited or limited bacterial degradation and preserved plant structures (Shao et al., 2003). Macerals of the inertinite group typically represented less than 10 vol.% of the coal samples and were found to be highest in upper Paleocene samples
Fig. 9. Photomicrographs of western Venezuela coals. (A) Santo Domingo, (B) Minas Norte, (C) Finca Familia Arellano, (D) La Pajarita. Photomicrographs A and B taken in white light under oil immersion; C and D taken in blue light.
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from the Las Adjuntas and Santo Domingo (Fig. 9A) mines and from the two mines in the Guasare basin. The Minas Norte coal sample contained the highest inertinite concentration (22 vol.%), which consisted of nearly equal proportions of fusinite, semifusinite, macrinite and inertodetrinite (Fig. 9B). Funginite was observed in quantities of 2 vol.% or less in all of the coal samples (e.g., Fig. 9A), except for the Finca Familia Arellano sample in which funginite is absent. The presence of fungi indicates aerobic conditions occurred at least intermittently in the peat environment of most of the western Venezuela samples (Brady and Weil, 1996; Wu¨st et al., 2001), and is consistent with the preservation of collodetrinite and the relative absence of telinite. Inertinite concentrations are highest in the Paleocene coals of the Marcelina and Los Cuervos Formations, whereas in the younger coals of the Carbonera and Palmar Formations, liptinite group macerals consistently are present in greater concentrations than inertinite. This dominance of inertinite group macerals over liptinite in the Paleocene coals of western Venezuela previously was noted by Escobar and Martı´nez (1993) and Escobar et al. (1997), who proposed four possible mechanisms for the restriction of higher inertinite concentrations to the Paleocene: (1) different biota types, (2) different tectonic regimes, (3) fluctuations in the relative constancy of the water table and (4) important changes in paleoclimate. They preferred variations in phreatic level as a result of tectonic controls (e.g., McCabe, 1991) as the dominant mechanism in determining coal type (Escobar and Martı´nez, 1993; Escobar et al., 1997). We concur with this hypothesis and present the following additional observations. Coal of all ages in western Venezuela was deposited in a foreland basin environment; however, a more rapid subsidence rate during accumulation of the younger Carbonera and Palmar coals may have been related to the advanced emplacement and thrust stacking of the Lara nappes to the northeast. Slower subsidence rates prior to the main phase of nappe emplacement would have dominated coal accumulation in the Paleocene. In this model, higher inertinite concentrations in the Paleocene coal samples could be explained by relatively greater desiccation as a result of fluctuating water tables, whereas in the younger coals, more rapid subsidence would have prevented oxidation.
Several of the coals are distinctly sapropelic in character, particularly the samples from Finca Familia Arellano (Fig. 9C) and La Pajarita (Fig. 9D), both occurring in the Carbonera Formation and containing 48 and 69 vol.% total liptinite, respectively. The liptinite component of these samples is dominated by a bituminite groundmass enclosing uniformly sized particles of liptodetrinite and lesser amounts of sporinite. Isolated resinite blebs in the bituminite groundmass were present in both samples and significant quantities of alginite, flourinite, exudatinite and trace cutinite were present in the sample from La Pajarita. Collodetrinite hosted in bituminite groundmass dominates the vitrinite component of both of these samples. Relatively low ash yields (La Pajarita = 3.64 wt.%, Finca Familia Arellano =v8.03 wt.%, db) indicate relatively little sediment input and suggest the coal-forming materials were deposited in stagnated, standing water, protected from flooding and sediment influx. The Lobatera sample, also from the Carbonera, contained high liptinite content as well, at 32 vol.%. Tocco et al. (1995) noted that Carbonera coals exhibited good oil-generating potential, based on the results of total organic carbon, Rock-Eval, vitrinite reflectance and maceral analyses, and concluded that terrestrial oil seeps in the southern Maracaibo basin possibly were sourced in Carbonera coals. Our R o max data show that the Carbonera coals are immature to marginally mature with respect to oil generation, ranging in value from 0.48 to 0.62, similar to the results obtained by Tocco et al. (1995). The occurrence of rare exudatinite in these samples (Table 3) points to the generation of at least early asphaltic bitumens. However, the presence of trace flourinite and the ubiquity of bituminite, particularly in the La Pajarita sample, indicates that the Carbonera coals are relatively immature with respect to oil generation. Regardless of their maturity with respect to petroleum generation, the potentially high combustion efficiency of these liptinite-rich coal samples renders them an excellent thermal fuel source. 4.4. Mineral analysis The mineralogy of most of the western Venezuela coal samples is dominated by kaolinite and quartz, and less frequently, by pyrite. The results of XRD analyses of LTA residues are compiled in Table 4.
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
85
Table 4 Semiquantitative mineral matter content of Venezuelan coal samples in weight percent of low-temperature ash residue Mine
LTA
Phyllosilicates
Sulfides
Tectosilicates
Kln Ill Rct Chl Prl Py Mrc Sp Qtz Zulia Minas Norte Paso Diablo Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa
Carbonates
Oxides
Others
Kfs
Pl
Cal Ank Sd Hem Ant Rt Dsp Bhm Ap Bas Anh Anl
2.26 85 0.58 45
X X X X
X X
X X
tr X tr X
b 1 10 X 35
X tr
X tr
X X
tr 5
tr X X X
X X
X X
X X b1 X
X X 10 X
X X
X X
2.74 50 13.95 55
15 X 7 X
X X
X X
5 X 25 tr
X tr
10 tr
tr tr
tr tr
X X
tr X
tr X X tr
X X
2 X
X X
X X
X X
5 X
X X
X b1
X X X X X 10 14 1
2 X 55 5
b 1 10 tr 1
X X
X X
X X
X X
X X X 5
X X
X X
X X
X X
X X
X tr
X 5
X 3
4.48 80 9.51 X
Ta´chira LM-Rı´o Pajitas 23.66 55 LM-Finca Familia 8.77 40 Arellano Caliche 7.64 40 Las Adjuntas 2.34 90 (M8-M1LA) Las Adjuntas 2.35 70 (M8-M2LA) Santo Domingo 2.04 55 Hato de la Virgen 2.51 65 Lobatera 14.67 85 La Pajarita 4.46 90 Cerro Capote 4.26 90
X X X X
X X
X 40 X b 1 tr X
tr X
tr 50
X tr
tr X
X tr
X X
X 1 X X
X 5
X X
X 1
X X
X X
X tr
X X
1 X
X X X X
X X
X X
55 5 10 tr
tr X
X tr
X X
X tr
X X
X X
X X tr X
X X
X X
X X
X X
X X
X X
X X
b1 b1
10 X
X
X
tr X
X
15
X
tr
tr
X
tr
tr
X
X
X
X
X
tr
X
X
5 X X X X
X X X X X
b1 X X X X
tr 10 tr X 5
X tr b1 X b1
15 tr 10 5 X
5 X tr tr X
tr X X X X
tr 10 X tr X
X X X X X
tr tr tr tr X
X X tr 1 1
5 X X X X
X X b1 tr X
X tr X X X
4 1 X X X
X X X X X
tr X X X X
X X X X X
X 2 X X X
X X X X X
X 5 X X tr
Semiquantitative mineral matter content derived by least squares solution of overdetermined system of linear equations representing XRD pattern by USGS Fortran language program (Hosterman and Dulong, 1989). Values are rounded to nearest 5. Note that detection of phases constituting less than 5 wt.% of sample powder is suspect and therefore concentrations b5 wt.% following X-ray data reduction by the Fortran program are here noted as trace concentrations (tr). Actual values listed in italics are possible solutions of XRD pattern through least squares technique and are not as reliable as non-italicized values. Abbreviations: LTA=low-temperature ash yield of the coal sample in wt.%, X=not present. Mineral abbreviations after Kretz (1983): Kln=kaolinite, Ill=illite, Rct=rectorite (smectite), Chl=chlorite, Prl =pyrophyllite, Py =pyrite, Mrc=marcasite, Sp =sphalerite, Qtz=quartz, Kfs=K-feldspar, Pl=plagioclase, Cal=calcite, Ank=ankerite, Sd =siderite, Hem=hematite, Ant=anatase, Rt=rutile, Dsp=diaspore, Bhm=boehmite, Ap=apatite, Bas=bassanite, Anh=anhydrite, Anl=analcime.
Detection of minerals constituting less than 5 wt.% of the LTA sample is considered suspect and therefore concentrations b 5 wt.% following reduction of the XRD data are noted as trace concentrations (tr) in Table 4. Values are rounded to the nearest 5. Kaolinite is present in all of the samples except the coal from Palmichosa, in which the dominant mineral is pyrite. Illite occurs in significant quantities in the samples from El Palmital, Las Dantas La Vega, Las Adjuntas (M8-M2LA) and Santo Domingo. Quartz dominates the mineralogy of the sample from Finca Familia Arellano and also constitutes a significant fraction of the Paso Diablo coal. Pyrite constitutes the majority of the inorganic fraction of the sample from Palmichosa, Caliche, Las Dantas La Vega and Rı´o Pajitas. Note
that these four samples containing abundant pyrite in the LTA residue also have the highest pyritic sulfur content (Fig. 6). Syngenetic pyrite dominantly occurs as discrete framboidal aggregates (Fig. 10A), commonly aligned parallel to bedding planes (Fig. 10B), or less commonly as pore fillings in fusinite. Plagioclase and K-feldspar occur as minor or trace phases in about half of the samples: the carbonates calcite, siderite and ankerite also occur as minor or trace constituents in many of the samples. Bassanite, a hydrated calcium sulfate that is an artifact of the ashing process, was identified in six samples. Calcite constitutes 10 wt.% of the LTA residue for the Hato de la Virgen sample, significantly higher than the calcite content found by Escobar et al. (1997). Rutile, anatase and apatite were
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with high weight percent pyrite content; Las Dantas La Vega, Palmichosa, Rı´o Pajitas and Caliche. Recent experimental work has demonstrated that pyrite is equally abrasive with quartz on a volume percent basis when these phases are excluded from coal macerals (Wells et al., 2004). However, most pyrite in the western Venezuelan coals occurs as framboids within macerals (e.g., Fig. 10A) and is not likely to contribute to abrasion during coal pulverization. 4.5. Major element chemistry and technological properties
Fig. 10. Photomicrographs showing syngenetic pyrite framboids in western Venezuela coals. (A) Palmichosa, (B) Las Dantas-La Vega.
found in trace quantities in several samples. Apatite concentration of ~ 8 wt.% in the sample from Paso Diablo is somewhat suspect as one would expect to see a significant amount of P in the ash and the results of the ICP-AES analysis do not indicate a significant P2O5 component when compared to the other coal samples. Quartz constitutes ~ 50 wt.% of the LTA residue of the Finca Familia Arellano sample. High quartz content might be of concern due to erosion in the boiler tube during combustion, or wear in grinding mills for power stations utilizing pulverized coal. However, this coal sample has a high temperature ash yield of only 8.03 wt.%, perhaps obviating concerns of wear or erosion failures due to high quartz content. Pyrite has a similar hardness to quartz and is also of concern in erosion and wear, particularly for the western Venezuelan samples
The major element chemistry of the western Venezuelan coal samples is considered in the context of utilization. Concentrations of major element oxides are summarized in Table 5 on a dry, ash basis, and calculated ash indices are compiled in Table 6. Note that these indices are based on empirical formulas, which have been developed to predict the combustion behavior of coals in power stations (summarized in Vaninetti and Busch, 1982; Skorupska, 1993). These ash descriptor indices apply only to certain coal types or to boiler design (Vaninetti and Busch, 1982); here, we have used indices developed for Eastern US coals, similar in rank and quality parameters to western Venezuela coals. The base–acid ratio (B/A) can be used to indicate potential slagging properties and ranges from 0.03 in the Minas Norte coal to 3.5 in the Palmichosa sample. Most of the coal samples have a low slagging tendency on this index with B/A ratios in the range of 0.1–0.7. Only the Caliche (B/A=1.5) and Palmichosa (B/A= 3.5) samples have a B/A ratio exceeding a value of 1. High total sulfur content in these samples indicates the potential for severe slagging tendency with calculated slagging factors of 13.4 for Palmichosa and 3.9 for Caliche. High total sulfur in Las Dantas La Vega and Rı´o Pajitas also result in severe slagging factors of 2.9 and 4.5, respectively, despite the low to medium slagging tendencies indicated by the B/A ratios in these samples. Values for the silica ratio in the Palmichosa and Caliche samples are 0.11 and 0.27, respectively, indicating low slagging tendency, and the values for Las Dantas La Vega (0.58) and Rı´o Pajitas (0.44) are in the medium to high range. These a priori estimates of slagging properties are largely descriptive in nature and not necessarily indicative of actual slagging
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
87
Table 5 Major element analytical data (dry, ash basis) for western Venezuelan coal samples Mine Zulia Minas Norte Paso Diablo Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa Ta´chira LM-Rı´o Pajitas LM-Finca Familia Arellano Caliche Las Adjuntas (M8-M1LA) Las Adjuntas (M8-M2LA) Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote
Ash (525 8C)
Moist
SiO2
Al2O3
1.89 0.38
1.00 0.98
52.40 50.50
38.60 30.90
2.21 17.68 3.95 5.72
1.71 1.04 3.14 5.30
23.80 35.90 39.20 8.80
16.01 8.14
0.70 1.55
4.69 1.66
CaO
MgO
Na2O
K2O
Fe2O3
TiO2
P2O5
SO3
Total
0.71 1.72
0.30 1.11
0.49 3.13
0.05 0.05
1.69 3.74
1.47 1.51
0.37 0.29
0.52 2.02
96.60 94.97
21.30 22.90 29.40 11.50
8.55 0.62 6.30 4.54
2.75 0.55 1.34 1.16
3.97 0.20 1.55 0.03
0.96 1.21 0.36 0.36
13.80 24.60 8.47 66.60
0.86 0.85 0.64 0.23
0.14 0.07 0.04 0.08
17.50 1.21 10.10 8.98
93.62 88.10 97.40 102.29
29.70 58.00
22.70 18.20
0.16 2.44
0.03 0.32
0.06 0.16
0.31 0.20
38.20 4.16
0.65 3.35
0.22 0.11
0.13 2.54
92.16 89.48
0.46 2.91
18.30 41.50
14.40 35.60
0.39 1.96
1.61 1.85
0.05 0.85
0.05 0.13
47.40 8.34
0.29 0.91
0.04 0.03
1.11 5.05
83.64 96.22
1.93
1.76
43.20
26.30
3.66
1.02
0.65
0.49
10.10
1.02
0.02
5.50
91.96
1.19 1.99 13.03 3.63 3.03
6.72 0.18 0.20 0.51 1.40
20.60 27.20 53.40 39.80 37.10
26.90 27.10 30.60 33.50 34.90
11.10 7.31 0.69 4.52 0.51
1.04 0.94 0.99 0.75 0.28
0.14 1.30 0.12 0.26 0.06
0.35 0.16 0.81 0.27 0.08
13.20 22.30 5.21 6.94 19.40
3.11 0.67 1.60 1.21 1.43
0.05 0.06 0.03 1.61 0.10
17.80 8.92 0.49 3.12 0.96
94.30 95.96 93.95 91.99 94.82
Abbreviations: Ash=ash yield in wt.% (dry), Moist=as-determined moisture in wt.%. Total is sum of oxides on an ash basis. All values in weight percent. Note that the reported ash and moisture values differ between Tables 3 and 2; values reported in Table 3 (this table) were derived in USGS laboratories following methods described in Bullock et al. (2002) [ashing at 525 8C, rather than 750 8C; total moisture determined on coal after conditioning and processing to 60 mesh (250 Am) (as-determined moisture)].
potential. However, the four aforementioned samples with a high slagging factor value collectively have, on average, an ash softening temperature (reducing atmosphere) 190 8C lower than the average of the other 12 western Venezuela coal samples (Table 6), consistent with the ash descriptor indices for slagging propensity. These four samples contain the greatest Fe2O3, total S, pyritic S and pyrite of the western Venezuela coal samples. A strong negative correlation between ash fusion temperatures, Fe2O3 and pyrite content was noted by Rimmer and Davis (1990), who suggested that low ash fusion temperatures could be interpreted to indicate peat deposition in a brackish or a marine-influenced environment. Other western Venezuela samples with relatively low ash fusion temperatures are El Palmital, Hato de la Virgen and Paso Diablo. El Palmital contains 13.8 wt.% Fe2O3 in the high temperature ash and detectable quantities of pyrite and marcasite in the LTA residue. The Hato de la Virgen sample contains ~ 10 wt.% pyrite, and the
Paso Diablo sample contains ~ 7 wt.% ankerite and a small amount of pyrite in the LTA, possibly explaining the relatively low ash fusion temperatures for these samples. Ash in the samples with high fusion temperatures and low slagging propensity is dominated by Al2O3 and SiO2, and the minerals kaolinite and to a lesser extent quartz, similar to the results obtained by Vassilev et al. (1995) and Alastuey et al. (2001). The calculated fouling factors [Rf =(B/A)/ Na2O] for the western Venezuela coal samples range from 0.01 in the Lobatera and Finca Familia Arellano samples to 2.6 in the sample from El Palmital. Values for Rf are below 0.2 in three quarters of the coal samples, indicating, in general, a low fouling tendency for the western Venezuela coals, and only the samples from Hato de la Virgen (Rf = 0.75) and El Palmital indicate a high or severe fouling tendency. Concentrations of Na2O typically are V 1 wt.% in the high temperature ash and highest in the El Palmital sample (3.97 wt.% Na2O). The Paso Diablo sample
88
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Table 6 Ash descriptor indices and technological properties of western Venezuelan coals Mine
S
Zulia Minas Norte Paso Diablo
B:A
SF
SR
0.44 0.03 0.37 0.12
Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa
0.83 6.36 0.93 3.76
0.65 0.54 0.46 2.90 0.26 0.24 3.55 13.34
0.49 0.58 0.71 0.11
Ta´chira Rı´o Pajitas Finca Familia Arellano Caliche Las Adjuntas (M8-M1LA) Las Adjuntas (M8-M2LA) Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote
6.11 0.43 2.61 0.63 0.59 0.32 0.79 0.47 0.56 1.20
0.73 0.09 1.50 0.17 0.23 0.51 0.58 0.09 0.17 0.28
0.44 0.89 0.27 0.77 0.75 0.45 0.47 0.89 0.77 0.65
AFT-ID (8C) AFT-S (8C) AFT-H (8C) AFT-F (8C) Rf
0.02 0.95 1540+ 0.04 0.88 1070
4.46 0.04 3.92 0.11 0.13 0.16 0.46 0.04 0.10 0.33
AFI
Cl
FSI
1540+ 1110
1540+ 1140
1540+ 1220
0.02 0.01 0.37 0.01
b0.038 6.0 0.032 6.5
1090 1110 1300 1255
1110 1150 1370 1270
1130 1180 1375 1330
1140 1190 1380 1370
2.59 0.09 0.40 0.11
0.10 0.18 0.07 0.02
0.035 0.020 0.024 0.016
4.0 2.0 1.0 1.0
1140 1390 1140 1540+ 1320 1310 1250 1540+ 1510 1450
1165 1455 1150 1540+ 1365 1340 1280 1540+ 1540+ 1490
1175 1500 1160 1540+ 1390 1350 1290 1540+ 1540+ 1500
1210 1520 1165 1540+ 1420 1355 1330 1540+ 1540+ 1530
0.04 0.01 0.08 0.14 0.15 0.07 0.76 0.01 0.04 0.02
0.04 0.02 0.004 0.02 0.02 0.004 0.03 0.09 0.02 0.003
0.023 0.016 0.020 0.044 0.025 0.030 0.018 0.016 0.020 0.036
6.0 0.5 4.5 0.5 1.0 0.5 1.5 1.0 0.5 0.5
Abbreviations: S =total sulfur (ASTM D5016), B/A=base–acid ratio [(Fe2O3) +(CaO+MgO) +(Na2O+K2O)]/[(SiO2)+(Al2O3) +(TiO2)], SF=slagging factor [(B/A)(S)], SR=silica ratio (SiO2)/[(SiO2) +(Fe2O3)+(CaO)+(MgO)], AFT=ash fusion temperatures in 8C in a reducing atmosphere (ASTM D1857), AFT-ID =initial deformation temperature, AFT-S =softening temperature, AFT-H=hemispherical temperature, AFT-F =fluid temperature, Rf=fouling index [(B/A)/(Na2O)], AFI =alkali fouling index [(ash)(Na2O+(0.659)(K2O))]/100, Cl=chlorine (wt.%), FSI=free swelling index (ASTM D720). Note that S content listed in this table was measured in a USGS laboratory by ASTM D5016 and S in Table 2 was measured in a commercial laboratory by ASTM D4239.
also contains high Na2O (3.13 wt.%), indicating severe fouling propensity for these two samples. However, on the alkali fouling index, the El Palmital sample has a value of only 0.1 and the Paso Diablo sample a much lower value of 0.01, indicating low fouling tendency and suggesting caution in acceptance of the calculated high Rf value and Na2O concentration in the El Palmital sample as a predictor of fouling potential. Concentrations of Cl are below 0.04 wt.% in all of the samples, consistent with the prediction on the Rf index for low fouling potential overall for the western Venezuela coals. Most of the western Venezuela coal samples are agglomerating and the samples from Rı´o Pajitas and the two mines in the Guasare basin exhibit good coking properties on the free swelling index. Free swelling indices (ASTM D720) are reported in Table 6. Coals from Finca Familia Arellano, Las Adjuntas (M8M1LA), Santo Domingo, La Pajarita and Cerro Capote are non-agglomerating. Previous workers have argued that a correlation exists between high ash content (van
Krevelen, 1993; Alastuey et al., 2001), or high inertinite content (Neavel et al., 1986; Alastuey et al., 2001) and the non-agglomerating character of coals. However, in the western Venezuelan coal samples there does not appear to be a significant relationship between high ash yield or inertinite content and plasticity as measured by the free swelling index. The only possible exception to this statement is the Santo Domingo sample, which contains 15 vol.% inertinite. 4.6. Trace element chemistry One particular concern pertaining to coal utilization impacts are the concentrations in coal of the possible hazardous air pollutants (HAPs) that are identified in the United States Clean Air Act Amendments of 1990 (Public Law 101-549) and in the equivalent legislation of other developed nations. The HAPs include Sb, As, Be, Cd, Cr, Co, Pb, Mn, Hg, Ni, Se and the radionuclides Th and U. Concentrations of other elements such as B, Cu, Mo, Sn, Tl, Vand Zn may
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
be of environmental concern if the particular element is present in enriched concentrations in the utilized coal deposit (Swaine, 1990; Senior et al., 2000). At least two of the elements listed above–As and Hg–are thought to have particularly adverse human health effects associated with coal utilization (Finkelman et al., 2002). Other considerations of trace element concentrations in coal are important in the utility industry; for example, concentrations of the halogen chlorine in coals consumed for power generation are important due to the potential for Cl-induced corrosion in combustors (Davidson, 1996). Trace element chemistry of the western Venezuelan coals samples is compiled in Table 7 on a dry, whole-coal basis. Concentrations of the HAP elements in the Venezuelan coal samples, with one or two possible exceptions, generally are within the range of concentrations of these elements found in most coals of the world (compiled in Taylor et al., 1998), or to coals of the Appalachian basin (Bragg et al., 1998). The typical HAP concentrations in western Venezuelan coals indicate that there may be little exceptional environmental or health impact from emissions as a consequence of coal combustion. However, slight to significant elevations of HAPs and other trace element concentrations are noted in the four coal samples which contain significant pyrite in the LTA residues: Rı´o Pajitas, Las Dantas La Vega, Caliche and Palmichosa. The Caliche and Palmichosa samples have ash yields about one third that of the Rı´o Pajitas and Las Dantas La Vega samples, which have the highest ash yields of the sample suite (Table 2). Concentrations of As and Hg are high in the Rı´o Pajitas sample when compared to the other Venezuelan coal samples; however, in general, they are within or close to the higher end of the range of these elements found in most coals of the world (Fig. 11). The high concentrations of As and Hg in this particular sample may be the result of the leaching of these elements from overlying strata into the sampled coal bed (Manuel Martı´nez, University of Central Venezuela, Caracas, written communication, 2004). The sample from Las Dantas-La Vega contains significant concentrations of HAP elements and concentrations of Mo, Ni and V slightly higher than the maximum concentration reported for world coals. The samples from Caliche and Palmichosa contain slightly elevated concentrations of HAP elements compared to the majority of the
89
western Venezuela coal samples, but are well within the ranges of concentrations reported for world coals. In addition to these four pyrite-rich samples, the sample from Finca Familia Arellano contains relatively high ash content and slightly elevated HAP element concentrations compared to the other western Venezuelan coals, but contains little pyrite. The Lobatera sample also contains relatively high ash and slight elevations in some trace metal concentrations, but contains no sulfides. Investigations into the modes of occurrence of HAPs elements in coals have shown that some of the elements (typically As, Cd, Co, Hg, Pb and Sb) are preferentially sequestered in the inorganic fraction of the coal, and primarily occur in the sulfide minerals pyrite, marcasite, galena, stibnite and sphalerite, and in selenides such as clausthalite (Kolker and Finkelman, 1998; Senior et al., 2000; Palmer et al., 2002; Hower and Robertson, 2003). These same studies have shown that other HAP elements (Be, Cr, Mn, Ni, Se and U) occur both as integral components of the organic matter and within mineral matter including sulfides, silicates and carbonates. In the western Venezuela coal samples, concentrations of the dominantly sulfide-associated HAP elements appear to be correlated with weight percent pyrite and marcasite as determined by XRD analyses, suggesting that these HAPs are indeed sequestered in sulfides. Arsenic and Hg concentrations are highest in the Rı´o Pajitas sample (71.3 ppm As, 2.1 ppm Hg), which contains 40 wt.% pyrite. The concentration of Hg in this sample is twice the maximum concentration reported for typical world coals (Taylor et al., 1998), and 10 times the average concentration of Hg in Appalachian basin coals (Bragg et al., 1998). In addition to As and Hg, the Rı´o Pajitas sample also contains relatively high concentrations of the sulfide-associated HAP elements Cd, Co, Pb and Sb, compared to the rest of the western Venezuelan coals. Concentrations of Mo, Tl and Ni, which also are thought to be hosted in sulfide minerals (Kolker and Finkelman, 1998; Alastuey et al., 2001; Zhuang et al., 2003), exceed the maximum of typical world coals in this sample and also indicate a sulfide mode-of-occurrence. Concentrations of V (269 ppm) are high compared to typical world coals and concentrations of Cr (28 ppm) are relatively high compared to the other western Venezuela coals. The V may be hosted in kaolinite (e.g., Karayigit et al., 2000), which con-
90
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Table 7 Trace element analytical data for western Venezuelan coal samples (whole-coal, dry basis) Mine Zulia Minas Norte Paso Diablo Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa Ta´chira LM-Rı´o Pajitas LM-Finca Familia Arellano Caliche Las Adjuntas (M8-M1LA) Las Adjuntas (M8-M2LA) Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote Reporting limit (ash basis) Appalachian average (nN3000) World coals minimum World coals maximum
Ash (525 8C)
Moist
Ag
As
Au
1.89 0.38
1.00 0.98
b0.038 b0.008
b0.004 0.021
b0.19 b0.04
21.3 17.9
2.21 17.68
1.71 1.04
b0.045 b0.360
2.78 10.1
b0.23 b1.8
53.6 48.8
3.95 5.72
3.14 5.30
b0.080 b0.120
0.815 18.9
b0.4 b0.58
16.01 8.14
0.70 1.55
b0.330 b0.170
71.3 1.89
b1.7 b0.82
4.69 1.66
0.46 2.91
b0.094 b0.034
3.39 0.255
b0.47 b0.17
1.93
1.76
b0.039
0.395
1.19 1.99 13.03 3.63 3.03
6.72 0.18 0.20 0.51 1.40
0.024 b0.040 b0.270 b0.073 b0.061 2
0.832 0.415 0.065 0.388 0.176 0.20
0.06
25
0.02 2
0.5 80
B
Ba
Be 3.51 1.21
Bi
Cd
Cl
Co
0.094 0.011
0.019 0.005
0.010 0.006
384 323
0.51 0.11
83.2 302
0.236 2.67
b0.002 b0.018
0.070 1.5
346 202
1.43 19.6
35.5 69.8
166 95.6
0.961 1.81
b0.004 0.056
0.186 0.086
237 b160
1.93 4.79
15.7 12.0
30.7 256
2.53 1.69
0.240 0.064
1.99 2.42
232 b160
8.54 7.35
4.0 28.7
0.117 1.18
0.016 0.016
0.061 0.453
201 443
1.36 2.3
b0.2
7.18
45.3
1.74
0.016
0.106
254
2.82
b0.12 b0.2 b1.4 b0.37 b0.31 10
42.7 18.4 3.6 13.2 7.85 20
81.4 58.2 71.1 114 7.52 2
0.339 0.480 3.24 0.682 0.227 1
0.018 0.018 0.093 0.021 0.020 0.1
0.045 0.403 0.313 0.385 0.594 0.1
300 180 b160 201 355 150*
5 3.59 4.08 2.9 3.46 2
944
7.2
50 2000
0.5 30
0.77
32
78
2.7
1.1
0.098
b0.01 b0.01
5 400
20 1000
0.1 15
0.1 0.5
0.1 3
14.1 7.7
All values in Ag/g (ppm), except ash yield and moisture which are in wt.%. Ash=ash yield (dry), Moist=moisture (as-determined), n.r.=not reported, *=reporting limit on whole-coal basis. Values were derived following methods described in Bullock et al. (2002). Average values for Appalachian coals are from Bragg et al. (1998), and maximum and minimum values for most world coals are from Taylor et al. (1998) (citing Swaine, 1990; Bowen, 1979).
stitutes ~ 55 wt.% of the ash; the Cr concentration is not atypical of world coals and may also be associated with kaolinite (e.g., Querol et al., 1992), or with organic matter (e.g., Huggins et al., 2000). The sample from Las Dantas-La Vega contains the greatest high-temperature ash yield (17.20 wt.%, db) and the highest concentrations of Ba, Co, Cr, Cs, Cu, Li, Rb, Sc, U, Y and Zn. Analyses of the LTA residue (13.95 wt.%) indicate that it consists of ~ 25 wt.% pyrite and trace marcasite and sphalerite, as well as ~ 55 wt.% kaolinite and ~ 5 wt.% illite. The high Co and Cu concentrations probably are explained by the pyrite content; Zn may be occurring in the trace mineral sphalerite, as there are no carbonate minerals
present in this coal (e.g., Palmer and Lyons, 1996). Chromium probably is occurring in the clay minerals illite and kaolinite or in an organic association (e.g., Huggins et al., 2000). Uranium presumably is in an organic association (Finkelman, 1995; Shao et al., 2003), as no trace minerals such as monazite which may host U in coals (Karayigit et al., 2000) were detected. The large ion lithophile elements (LILE) Ba, Cs, Li and Rb probably are sequestered in the clay minerals (Shao et al., 2003). The presence of high Ba concentrations, relatively high S and the sapropelic nature of the coal suggest a coastal setting with occasional marine influence for this horizon of the Carbonera Formation.
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Cr
Cs
1.19 0.179
0.004 0.002
4.64 49.7
0.095 1.36
1.93 6.01
10.9 0.126
28 22.3
0.109 0.052
2.27 1.46
0.011 0.015
2.36
Cu 2.83 0.838
Ga
Ge
1.09 0.074
0.274 0.057
3.31 58
1.19 6.33
0.821 3.2
0.06 5.15
10.8 0.853
4.39 0.217
6.5 4.38
5.12 9.84
2.35 2.32
0.877 4.46
0.039
4.16
1.3 10.5 17.2 12.5 4.58 2
0.029 0.007 0.547 0.044 0.011 0.1
17 0.5 60
Hg b0.03 b0.03
4.91 0.419
Mn 0.672 0.373
Mo
Nb
0.068 0.464
0.383 0.04
Ni 0.765 0.224
Pb 0.873 0.267
Rb 0.036 0.012
Sb 0.053 0.01
1.76 19.1
5.23 18
0.949 10.1
0.225 1.64
3.53 63.1
0.614 8.01
1.17 14.9
0.163 1.41
b0.03 2.64
3.31 15.9
24.8 5.29
0.324 0.155
0.336 35
4.43 1.41
1.27 1.36
0.648 0.315
2.1 0.061
16.5 5.39
4.04 36
21.5 1.02
1.18 2.94
66.3 24.2
16 4.86
2.19 0.765
0.296 4.73
0.28 0.1
2.43 1.3
25.9 2.67
0.835 3.35
0.211 0.222
5.58 11.5
1.5 1.15
0.131 0.126
0.136 1.82
3.93
6.15
0.11
1.03
2.24
1.41
0.269
8.94
1.1
0.52
1.78
4.68 10.5 11.7 20.9 4.85 2
0.897 0.514 7.16 1.43 1.96 0.1
0.169 0.124 2.77 0.334 0.704 0.1
b0.03 0.03 0.03 0.03 b0.03 0.02*
0.457 1.38 12.4 3.74 1.57 4
3.88 3.27 42.5 5.01 26.8 2
2.48 1.7 0.86 1.59 0.267 0.2
0.559 0.207 3.44 0.464 0.552 0.1
23.3 7.28 4.99 13.1 7.52 4
0.896 0.762 5.22 2.01 1.32 0.5
0.302 0.092 7.2 0.613 0.103 0.1
0.255 0.277 0.612 0.323 0.243 0.1
0.98
17.8
6.3
4.5
0.2
18.1
25
3.1
2.4
17
8.8
0.3 5
0.5 50
0.5 50
0.02 1
1 80
5 300
0.1 10
16 33.1
1 20
0.051 0.2
Li
91
20.2 0.17
1 20
0.5 50
2 80
33.8 3.16
23
1.2
2 50
0.05 10
(continued on next page)
The Caliche and Palmichosa samples both contain abundant pyrite at ~ 55 wt.% of the LTA, yet contain much lower concentrations of HAP elements than the Las Dantas-La Vega and Rı´o Pajitas samples. The mode of occurrence of the HAPs and other elements in these samples is more problematic, yet there does seem to be a relationship between the As and Hg concentrations and pyrite content. Concentrations of these elements are highest in the four pyrite-rich samples compared to the rest of the western Venezuela coals. However, despite the high pyrite content, As and Hg concentrations in the Caliche and Palmichosa samples are well within the range of typical world coals. Selenium, which typically occurs associated with the
organic matter in coals (Coleman et al., 1993; Finkelman, 1995), occurs at a concentration of 7.3 ppm in the Caliche sample, relatively high compared to most of the western Venezuela coal samples. However, Se in this sample probably is associated with the organic matter, and the sample from Santo Domingo, which contains only trace pyrite, has a higher Se concentration of 7.5 ppm. The Palmichosa sample contains the highest concentration of B (70 ppm) of the western Venezuela coals and relatively high B/Be (39), indicating a marine influence (Hower et al., 2002), which is consistent with the relatively high sulfur and the inferred coastalmarine depositional environment of the Palmar Formation (Gonza´lez de Juana et al., 1980).
92
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Table 7 (continued ) Mine Zulia Minas Norte Paso Diablo Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa Ta´chira LM-Rı´o Pajitas LM-Finca Familia Arellano Caliche Las Adjuntas (M8-M1LA) Las Adjuntas (M8-M2LA) Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote Reporting limit (ash basis) Appalachian average (nN3000) World coals minimum World coals maximum
Sc 0.44 0.056
Se
Sn
4.4 3.4
0.13 0.2
0.18 0.25
44.8 1.01
3.31 0.939
1.9 2.7
0.41 1.26
6.13 9.19
5.5 0.47
4.39 3.84
0.783 0.642
7.3 1.6
b0.15 0.07
1.19
2.4
0.453 3.35 4.55 5.22 1.63 4
Sr
Th
Tl
0.018 0.006
0.378 0.038
b0.002 b0.0004
0.711 0.065
0.071 3.54
0.3 0.902
34.8 9.44
0.018 0.057
1.48 1.14
0.023 2.11
14.5 42.1
0.11 0.037
3.2 4.35
0.812 11.1
0.021 0.012
1
27.1
7.5 2.3 4.5 2.9 2.2 0.1*
0.34 0.08 3.6 1.98 2.18 3
32.6 27.3 18.2 99.8 3.97 1
4
3.8
0.99
1 10
0.2 10
1.6 13.4
1 10
40.8 5.44
Te
0.007 54.5
U 0.176 0.018
Y 3.59 0.388
0.455 0.091
Zn 0.393 2.14
Zr 5.02 1.11
1.91 154
2.56 31.8
2.54 139
0.36 0.595
18.7 12.5
10 5.28
4.74 13.5
13.7 0.228
2.93 1.11
269 49.8
18.7 16.3
15.9 33.2
b0.47 0.529
0.676 0.03
0.117 0.259
6.38 5.8
2.07 5.87
4.39 9.22
1.79 3.33
0.009
0.596
0.023
0.228
6.19
8.43
3.09
3.44
0.016 0.048 0.064 0.044 0.016 0.1
0.443 1.89 5.39 4.39 0.91 8
0.211 0.013 b0.014 0.004 0.058 1
0.14 0.6 0.573 0.755 0.246 0.1
9.03 28.9 44.8 47.9 12.9 2
1.61 7.58 23.2 11.5 2.34 1
13.1 6.18 13.9 2.13 19.4 4
8.96 3.53 23.7 7.84 5.85 5
106
44.7
2.9
1.6
22
20
22
15 500
n.r. n.r.
0.5 10
0.5 10
2 100
5 300
5 200
The sapropelic coal sample from Finca Familia Arellano contains moderate ash content (8.03 wt.%, db) and the highest Cd concentration of the western Venezuela coals. Significant concentrations of some other trace elements including Co, Cr, Cu, Nb, Ni, Pb, Sb, Sc, Zn and Zr also occur in this coal. The major element chemistry of the Finca Familia Arellano sample shows that it contains the highest SiO2 and TiO2, and the lowest Fe2O3 of the western Venezuela coals. About 50 wt.% of the LTA residue is quartz, the source of the high SiO2. TiO2 is associated with anatase, a stable titanium oxide typically found in detrital sediments. Moderate ash content containing high concentrations of quartz and detectable anatase may
0.77 b0.2 1
13.8 3.18
V
8.4 2 50
14.4 4.43 2.64
16.7 37.8
be interpreted to indicate a clastic influx of relatively mature sediments into the depositional environment of this coal. The presence of zircon is inferred by the high Zr concentrations in the LTA residue, although zircon was not detected by XRD. Cadmium in coals typically occurs associated with Fe or Zn sulfides (Kolker and Finkelman, 1998); however, no sphalerite was detected in the LTA residue of the Finca Familia Arellano sample and only trace pyrite was noted. Another possible mode of occurrence of Cd and Zn in this coal is in the trace calcite, the only carbonate detected in the LTA residue (e.g., Spears and Zheng, 1999). The sapropelic coal from Lobatera has a relatively high ash yield (12.83 wt.%, db) compared to most of
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
93
100
A 10
As (ppm)
1
0.1
0.01
M in as N or te Pa so D ia b Lo lo ba La te sA ra Ce rro dj un Ca ta s( po M te 8La M sA 1L A dj La un Pa ) ta ja s( rit M a 8M H a 2 to LA W or d ) ld e l co a V al s m irge in n im um Es c al Sa an Fi nt te nc o D aF o m am in ili a A go re lla no El Pa lm ita La sD Ca l lic an he ta A spp La al Ve ac ga hi Pa an l m av ic er ho ag sa e( n> 30 W 0 or Rí ld o 0) Pa co jit al sm as ax im um
0.001
10
B
Hg (ppm)
1
0.1
la
de
H
at
o
Pa so
D ia bl o Sa Vi nt rg o D en om in M go in as N Ce o rro rte Ca po te La Pa ja r Lo ita ba te ra Es ca l a Fi nt El nc e Pa aF lm La am i ta sA ili l aA dj un r e ta lla La s no sA (M dj 8un M ta 1L s( A M ) A 8pp M al 2L ac A hi ) P an av alm ic er ho La age sa s D (n > 30 an 00 ta sLa ) W V eg or a ld Ca co l al i c sm he ax im um Rí o Pa jit as
W or
ld
co
al
sm
in
im
um
0.01
Fig. 11. Logarithmic bar graphs of selected trace element concentrations in western Venezuelan coal samples. (A) As, (B) Hg. Average of Appalachian coal from Bragg et al. (1998), and maximum and minimum concentration of typical world coals from Taylor et al. (1998).
the western Venezuela coal samples and slightly elevated concentrations of some trace metals, including Be, Ga, Mn, Nb, Th and Y, compared to the rest of the western Venezuela coals. The LTA residue is dominantly composed of kaolinite (~ 85 wt.%), minor quartz (~ 10 wt.%) and traces of pyrite, Kfeldspar, siderite and hematite, similar to the results found by Escobar et al. (1997). Despite the relatively
high ash content and the presence of trace pyrite, concentrations of sulfide-associated HAP elements such as As and Hg are low (Fig. 11). The remainder of the studied coal samples yielded less than 5 wt.% high temperature ash and contain trace element concentrations typical of most coals of the world. Considered collectively, with the possible exception of the high-pyrite samples, the western
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Venezuelan coal samples contain typical concentrations of trace elements when compared to most world coals or to the coals of the Appalachian basin.
5. Conclusions Western Venezuela coals constitute an important natural resource for thermal power generation and currently are being exploited at a rapidly expanding rate. Analysis of 16 representative coal samples from 14 active mines in Ta´ chira, Me´ rida and Zulia illustrates that these coals are bituminous in rank, low in ash yield and low to moderate in sulfur content, indicating excellent potential for thermal use. Calculated values of various ash indices indicate that most of the coal samples will perform favorably in combustion or coking processes. For most of the coal samples, concentrations of the HAPs are similar to the concentrations of these elements in most coals of the world, indicating little potential for exceptional environmental impact as a consequence of utilization. Semiquantitative estimates of volume percent pyrite content show a strong correlation with pyritic sulfur and appear to be correlated with some sulfide-hosted trace element concentrations (As and Hg). Given the occurrence of these HAPs in the inorganic fraction of the coal, it is possible that beneficiation through the physical separation of pyrite could reduce these trace element concentrations prior to combustion in the coals that contain the highest concentrations. Petrographic determination of coal maceral composition indicates that diverse peat-forming environments existed in western Venezuela during the Paleocene through the Miocene. Sapropelic depositional centers were common, as suggested by high liptinite contents (N 20 vol.%) for 5 of the 16 samples examined. The maceral telinite constitutes less than 1 vol.% of all of the samples indicating either that the original plant materials were depleted in woody substance or that waterlogged conditions in the peat environment resulted in the bacterial alteration and recombination of humic substances, precipitating gels which filled cell lumina and removed the apparent telinite structure from woody precursor materials. Inertinite group macerals are present in the highest concentrations in distantly spaced upper Paleocene coals from opposite sides of the Maracaibo basin, possibly indicating
tectonic controls on subsidence related to stacking of the Lara nappes in the northeastern Me´rida Andes. The data presented herein provide a modern outline concerning the quality of coal currently produced in western Venezuela and will serve as the foundation for further studies of the deposits targeted for expanding development in the near future. Acknowledgments The authors would like to thank the following people for their help with this project: Mike Trippi and John Bullock for sample preparation, processing and geochemical analysis; Fiorella Simoni, Noelia Rodrı´guez, Steven Olmore, Jean Weaver and Ivette Torres for their help in coordinating communications between the USGS and INGEOMIN; and Frank Dulong and Nadine Piatak for assistance with sample ashing and XRD analysis. This manuscript benefited from helpful reviews by Leslie Ruppert, Jon Kolak and Jim Coleman of the U.S. Geological Survey, and Franco Urbani and Manuel Martı´nez of the University of Central Venezuela. References Alastuey, A., Jime´nez, A., Plana, F., Querol, X., Sua´rez-Ruı´z, I., 2001. Geochemistry, mineralogy, and technological properties of the main Stephanian (Carboniferous) coal seams from the Puertollano basin, Spain. International Journal of Coal Geology 45, 247 – 265. Ardina, E., 1987. Variaciones en el rango del Manto IV (principal) de la cuenca carbonifera del Zulia. Boletı´n Sociedad Venezolana de Geo´logos 31, 67. ASTM, 2002. Annual book of ASTM standards: petroleum products, lubricants, and fossil fuels. Gaseous Fuels; Coal and Coke, Sec. 5, vol. 5.06. ASTM International, West Conshohocken, PA. 650 pp. Azpiritxaga, I., Casas, B.J., 1989. Estudio sedimentolo´gico de las Mirador y Carbonera Formaciones en el Rı´o de Lobaterita, estado Ta´chira, Venezuela. Geos 29, 1 – 17. Bar, T.P., Pen˜a, R.P., 1985. Yacimiento del carbon, Santo Domingo, Estado Ta´chira. Congreso Geolo´gico Venezolano Memoria, no. 6. Tomo II, 772 – 793. Bellizzia, A.G., Pimentel, N.M., Bajo, R.O., 1976. Mapa geolo´gico estructural de Venezuela: Repu´blica de Venezuela. Ministerio de Minas e Hidrocarburos Direccio´n de Geologı´a, 30 sheets, map scale 1:500,000. Biewick, L.R.H., Weaver, J.N., 1995. The digital coal map of South America in ARC/INFO format. U.S. Geological Survey OpenFile Report 95-235, map scale 1:7,500,000.
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Petrology, mineralogy and geochemistry of mined coals, western Venezuela Paul C. Hackleya,T, Peter D. Warwicka, Eligio Gonza´lezb a U.S. Geological Survey, 956 National Center, Reston, VA 20192, USA INGEOMIN, Torre Oeste Parque, Central Piso 8, Caracas 1010, Venezuela
b
Received 1 January 2004; received in revised form 1 January 2005; accepted 6 February 2005 Available online 20 April 2005
Abstract Upper Paleocene to middle Miocene coal samples collected from active mines in the western Venezuelan States of Ta´chira, Me´rida and Zulia have been characterized through an integrated geochemical, mineralogical and petrographic investigation. Proximate, ultimate, calorific and forms of sulfur values, major and trace element, vitrinite reflectance, maceral concentrations and mineral matter content have been determined for 16 channel samples from 14 mines. Ash yield generally is low, ranging from b 1 to 17 wt.% (mean = 5 wt.%) on a dry basis (db). Total sulfur content is low to moderate, ranging from 1 to 6 wt.%, db (average =1.7 wt.%). Calorific value ranges from 25.21 to 37.21 MJ/kg (10,840–16,000 Btu/lb) on a moist, mineral-matter-free basis (average = 33.25 MJ/kg, 14,300 Btu/lb), placing most of the coal samples in the apparent rank classification of highvolatile bituminous. Most of the coal samples exhibit favorable characteristics on the various indices developed to predict combustion and coking behavior and concentrations of possible environmentally sensitive elements (As, Be, Cd, Cr, Co, Hg, Mn, Ni, Pb, Sb, Se, Th and U) generally are similar to the concentrations of these elements in most coals of the world, with one or two exceptions. Concentrations of the liptinite maceral group range from b 1% to 70 vol.%. Five samples contain N 20 vol.% liptinite, dominated by the macerals bituminite and sporinite. Collotelinite dominates the vitrinite group; telinite was observed in quantities of V1 vol.% despite efforts to better quantify this maceral by etching the sample pellets in potassium permanganate and also by exposure in an oxygen plasma chamber. Inertinite group macerals typically represent b 10 vol.% of the coal samples and the highest concentrations of inertinite macerals are found in distantly spaced (N 400 km) upper Paleocene coal samples from opposite sides of Lago de Maracaibo, possibly indicating tectonic controls on subsidence related to construction of the Andean orogen. Values of maximum reflectance of vitrinite in oil (R o max) range between 0.42% and 0.85% and generally are consistent with the high-volatile bituminous rank classification obtained through ASTM methods. X-ray diffraction analyses of low-temperature ash residues indicate that kaolinite, quartz, illite and pyrite dominate the inorganic fraction of most samples; plagioclase, potassium feldspar, calcite, siderite, ankerite, marcasite, rutile, anatase and apatite are present in minor or trace concentrations. Semiquantitative values of volume percent pyrite content show a strong correlation with pyritic sulfur and some sulfide-hosted trace element concentrations (As and Hg). This work provides a modern quality dataset for the western
T Corresponding author. Tel.: +1 703 648 6458; fax: +1 703 648 6419. E-mail address: [email protected] (P.C. Hackley). 0166-5162/$ - see front matter. Published by Elsevier B.V. doi:10.1016/j.coal.2005.02.006
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69
Venezuela coal deposits currently being exploited and will serve as the foundation for an ongoing coal quality research program in Venezuela. Published by Elsevier B.V. Keywords: Coal; Venezuela; Petrography; Geochemistry; Mineralogy; Macerals
1. Introduction Rapidly expanding coal production in Venezuela illustrates the need for widely accessible information on coal quality characteristics. Production of coal (8 Mt/year) has increased almost four-fold over the last decade, primarily due to the development of large deposits in the Guasare basin of northern Zulia (Fig. 1), and current production is expected to double over the next 5 years (Energy Information Administration, 2003). Total in-ground coal resources in Venezuela are estimated to be on the order of 10 Gt (Ministerio de Energı´a y Minas, 1996), placing Venezuela second in South America behind only Columbia in terms of resources (Fossil Energy International, 2002). Current coal production is concentrated in western Venezuela in the States of Zulia, Ta´chira and Me´rida (Fig. 1), where surface and underground mines produce high quality (low-ash, moderate-sulfur), high-volatile bituminous coal. Over 90% of the coal produced in Venezuela comes from the Guasare basin and all Guasare coal is exported to the United States, Italy, France, Spain, the Netherlands, the United Kingdom and other countries in Central and South America (Petro´leos de Venezuela, 2001a). Despite the recent dramatic increase in coal production and the significant in-ground coal resources of Venezuela, detailed characterization of the quality of western Venezuelan coals has been relatively underrepresented in English-language literature and the primary references are in Spanish (e.g., Escobar and Martı´nez, 1993; Ministerio de Energı´a y Minas, 1996; Escobar et al., 1997). In addition, information on the concentrations of environmentally sensitive trace elements, such as As, and Hg in western Venezuelan coals is not available, nor is an analysis of technological properties with respect to combustion as a thermal fuel resource. This study presents results of a quality characterization of in-ground coal and includes the majorand trace-element geochemistry, mineralogy and
petrography of representative samples of coals currently mined from western Venezuela. This work has been completed under the auspices of the U.S. Geological Survey’s (USGS) World Coal Quality Inventory project, a program designed to provide a modern library of coal quality data from commercially produced coals of most of the world’s major coal-producing countries (Finkelman and Lovern, 2001). The coal quality dataset presented herein provides a broad characterization of the western Venezuela coal deposits now being exploited, and will serve as the platform from which to study the Guasare coals currently targeted for rapidly expanding development.
2. Study area 2.1. Western Venezuela Bituminous coals of western Venezuela are hosted in upper Paleocene–middle Miocene strata located on the margins of the Maracaibo basin, a major structural depression flooded by Lago de Maracaibo (Fig. 1). Northwest of Lago de Maracaibo, in the northern Sierra de Perija of Zulia (Fig. 2A), extensive coal deposits are found in the Paleocene Marcelina Formation of the Guasare basin (Fig. 3). Southeast of Lago de Maracaibo, in the Cordillera de los Andes of the states of Me´rida and Ta´chira (Fig. 2B and C, respectively), coals primarily occur in the Paleocene Los Cuervos Formation and Eocene–Oligocene Carbonera Formation, and, less frequently, in the Miocene Palmar Formation (Fig. 3). The upper Paleocene deltaic Marcelina and Los Cuervos Formations in Zulia and Ta´chira, respectively, were deposited during regional regression, prograding out over a thick package of Cretaceous calcareous and deep water anoxic sediments into the Maracaibo basin (Parnaud et al., 1995). This sedimentation occurred in a foreland basin environment
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P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97 73ºW
72ºW
71ºW
12ºN
0
50
100
Area of Fig. 2A
150 km
SAMPLED COAL MINES 1. Minas Norte 2. Paso Diablo 3. Palmichosa 4. El Palmital 5. Las Dantas-La Vega 6. Río Escalante 7. Las Mesas-Río Pajitas 8. Las Mesas-Finca Familia Arellano 9. Caliche 10. Lobatera 11. Hato De La Virgen 12. Las Adjuntas 13. Cerro Capote & La Pajarita 14. Santo Domingo
1 2 Guasare
11ºN
aib Ma
rac
de Sie
rra
10ºN
ob
Per ij
asi n
a
basin
Lago de Maracaibo
Trujillo Zulia
3
9ºN Area of Fig. 2B
Mérida Area of Fig. 2C
9 8ºN
COLOMBIA
7, 8
4, 5 6
10 11 12 Táchira 13 14
s
de
n sA
e ad
lo
ler
dil or
C
State boundary 7ºN
Fig. 1. Shaded relief image of western Venezuela showing locations of sampled coal mines and State boundaries. Insets denote locations of Fig. 2A–C. Shaded relief elevation data from U.S. Geological Survey (2003) shuttle radar topography mission 90 m elevation data set (http:// srtm.usgs.gov/).
developed as a result of incipient oblique collision between the South American and Caribbean plates (Lugo and Mann, 1995; Milani and Filho, 2000). The paralic Eocene–Oligocene Carbonera Formation was sourced from uplifted areas to the west and south during continued oblique convergence. Continental sediments of the Miocene Palmar Formation were deposited as molasse shed from the margins of the emergent Me´rida Andes during rapid uplift of the orogen.
2.2. Zulia The State of Zulia (Fig. 2A) contains over 80% of the coal resources of Venezuela in two main districts: Guasare basin in the north and the Santa RosaCatatumbo district in the southern part of the State (Rodrı´guez, 1995). The Guasare basin is host to the two largest producing mines of Venezuela, Minas Norte and Paso Diablo, and potential resources of the basin are estimated at over 8.5 Gt (Ministerio de Energı´a y
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Minas, 1996). The coals are hosted within the Paleocene Marcelina Formation (Fig. 3), an intercalated sequence of dense gray sandstones, dark-gray to black shales and sandy shales. The coal beds mainly occur in the lower part of the Marcelina where 25–30 coal beds range in thickness from 1 to 13 m (Ruı´z, 1983). Estimates of the total thickness of the Marcelina range from 137 to 610 m (Sutton, 1946; Quijada and Kettle, 1985) and the unit is approximately 550 m thick in the area of the Paso Diablo mine. The open-pit Paso Diablo mine, located about 85 km northwest of Maracaibo, contains 22 coal beds which occur over an interval of approximately 400 m (Ministerio de Energı´a y Minas, 1996). The coals are consistent in both thickness and lateral continuity across the mine area, where the beds dip 5–158 to the southeast. The total thickness of coal averages 60 m. The mine currently is operated by Carbones del Guasare, a subsidiary of Petro´leos de Venezuela (Petro´leos de Venezuela, 2001b). Sixteen coal beds in the Paso Diablo mine, ranging between 0.5 and 13 m in thickness, are forecast to produce about 200 Mt of high volatile bituminous coal with low sulfur content and ash yield. The Norte mine hosts 33 coal beds of bituminous rank, of which 21 are thicker than one m (Ministerio de Energı´a y Minas, 1996). At this locality, a 533 m section through the Marcelina contains 62.5 m total thickness of coal. Potential resources are almost 1 Gt, of which 59 Mt will be extracted by open-pit methods. Carbones de la Guajira, a subsidiary of Petroleos de Venezuela, has been producing coal from the Norte deposit since 1996 (Petro´leos de Venezuela, 2001c). 2.3. Me´rida The state of Me´rida (Fig. 2B) is host to important coal deposits in the Franja Nororiental and Rı´o Muyapa areas (Ministerio de Energı´a y Minas, 1996). In the Rı´o Muyapa region of northern Me´rida, coals occur in the Miocene Palmar Formation (Fig. 3), composed of yellow-white to gray, fine-grained, massive sandstones interbedded with dark-gray to black carbonaceous shales, coals and conglomerates (Sutton, 1946). The unit varies from 570 to 1300 m in thickness (Heybroek, 1953; Ministerio de Energı´a y Minas, 1996). Coal beds vary in thickness from 0.3 to 1.65 m (Ministerio de Energı´a y Minas, 1996) and
71
currently are produced from the open-pit Palmichosa mine. The Franja Nororiental area encompasses an area of approximately 520 km2 in southwestern Me´rida, where coals are hosted in the Paleocene Los Cuervos Formation and the Eocene–Oligocene Carbonera Formation (Fig. 3). The Carbonera Formation is a transgressive sequence of claystones, mudstones, grayish carbonaceous shales and coals (Sutton, 1946). The sediments were deposited in a delta plain with little marine influence (Azpiritxaga and Casas, 1989; Ma´rquez and Mederos, 1989; Martı´nez et al., 2001). The Carbonera Formation averages around 470 m in thickness (Ramı´rez and Fields, 1969). The sediments are irregularly stratified, and immature sandstone beds of 5–10 m thickness are found dispersed throughout the sequence as are coals of 1– 3 m thickness (Gonza´lez de Juana et al., 1980). 2.4. Ta´chira The State of Ta´chira, in western Venezuela, is one of the major coal-producing regions of Venezuela and is host to six coal fields (Fig. 2C), including the Franja Nororiental (a southern continuation of the Franja Nororiental field in Me´rida), San Fe´lix-Rı´o Guaramito, Franja Centro-Occidental, Rubio, Franja de Las Delicias and Santa Domingo (Ministerio de Energı´a y Minas, 1996). Coal deposits of Ta´ chira are hosted in the Paleocene Los Cuervos Formation (Fig. 3) and the Eocene–Oligocene Carbonera Formation (Martı´nez et al., 2001). The Carbonera was described in the preceding section on the coal deposits of Me´rida. The Paleocene Los Cuervos Formation primarily consists of shales, mudstones and minor intercalated sandstones, and coal beds (Notestein et al., 1944), in an association interpreted to represent a deltaic environment (Ma´rquez and Mederos, 1989). Coal beds of 0.5–2.5 m thickness are found in the lowermost 75 m of the section (Notestein et al., 1944). Estimates of the thickness of the Los Cuervos range from 245 to 840 m (Notestein et al., 1944; Heybroek, 1953). The Los Cuervos is a time-equivalent correlative of the coal-bearing Marcelina Formation found to the northwest in Zulia on the western side of the Maracaibo basin (Miller and San Juan, 1963), interpreted to have been deposited in fluvio-deltaic
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Fig. 2. Simplified geologic maps of western Zulia (A), Me´rida (B) and Ta´chira (C), western Venezuela, showing locations of mines/prospects and coal fields. Locations of mines/prospects and coal fields compiled from Biewick and Weaver (1995) and Ministerio de Energı´a y Minas (1996). Geology from Bellizzia et al. (1976).
plains with little or no marine influence during regional regression (Martı´nez et al., 2001). 2.5. Previous coal characterization investigations The most complete general reference on western Venezuelan coal is a comprehensive treatise on the coal industry in Venezuela by the Ministerio de Energı´a y Minas (1996). Resource estimates and proximate and ultimate quality parameters are presented for many of the areas examined in the current study.
The only available data on trace element concentrations in Venezuelan coals were presented by Martı´nez et al. (2001), which contained a detailed geochemical study aimed at determining a correlation between trace element concentrations in Paleocene Ta´chira coal beds and in provenance lithologies at the time of peat deposition. Martı´nez et al. (2001) were able to demonstrate through factor analysis that the concentrations of some trace elements in the Los Cuervos Formation coals were a result of sedimentary input derived from outcrops of the dominantly volcaniclastic Jurassic La Quinta Formation.
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TÁCHIRA and MÉRIDA P A L E O G E N E / N E O G E N E
PLIOCENE
Necesidad Betijoque
MIOCENE
OLIGOCENE EOCENE PALEOCENE
GUAYABO GROUP
Isnotú Palmar
León Carbonera Mirador OROCUE GROUP
Los Cuervos Barco Catatumbo
ZULIA Necesidad La Villa Los Ranchos Cuiba Macoa EL FAUSTO GROUP Peroc Misoa Marcelina Guasare
Fig. 3. Simplified Paleogene to Neogene stratigraphy of Zulia, Me´rida and Ta´chira, western Venezuela. Coal-bearing formations labeled in bold type. Modified from Bellizzia et al. (1976) and Martı´nez et al. (2001).
Petrographic studies of western Venezuelan coals include a contribution describing the relative proportions of the vitrinite, liptinite and inertinite (subdivided into fusinite and semifusinite) maceral groups in 33 bench samples from four localities in the Marcelina Formation of the Guasare basin (Mazeaud and Monpart, 1977). These authors found relatively homogeneous maceral proportions with all of the samples dominated by vitrinite, and noted some correlation between increasing mineral matter and liptinite content. A general review of Guasare basin coal quality characteristics was presented by Heintz et al. (1976), including some values of average vitrinite reflectance. Many other studies have investigated various aspects of the Guasare deposits, including resource assessment (Rodrı´guez, 1986), coal systems (Corzo and Cruz, 1987), coal rank (Ardina, 1987), and mine engineering and economics (Herna´ndez and Monsalve, 1979; Etchart, 1987). A study of the Santa Domingo coal deposit of the Los Cuervos Formation in Ta´chira indicated that the maximum reflectance of vitrinite ranged from 0.42% to 0.54%, suggesting a subbituminous rank classification (Bar and Pen˜a, 1985). Coal samples from Ta´chira and Zulia were analyzed for use in a study of the effects of weathering on geochemical parameters in coal (Martı´nez and Escobar, 1995), including determinations of humic and carboxylic acids in fresh and weathered samples. A study of the organic geochemistry of coal samples from Ta´chira, Me´rida and Trujillo was
conducted by Tocco et al. (1995) to determine the source of oil accumulations in the southern part of the Maracaibo basin. These workers found that terpane and sterane biological markers from the Carbonera and Los Cuervos Formation coals were similar to the signatures of local oil seeps and concluded that the coals of these units were likely terrestrial source rocks. In a similar study, Cano´nico et al. (2004) used total organic carbon analyses, Rock-Eval pyrolsis, vitrinite reflectance and maceral composition to demonstrate that some Paleogene–Neogene coals of western Venezuela contain high oil-source rock potential. General overviews of the major-element chemistry, petrology and mineralogy of Venezuela’s coal deposits were presented in Escobar and Martı´nez (1993), and Escobar et al. (1997). They presented proximate, ultimate, calorific and forms of sulfur values, major element concentrations, semiquantitative estimates of mineral matter content and the relative proportions of the three coal maceral groups for 33 coal samples from all of Venezuela’s important coal deposits; these data are most relevant to the new information contained in the present study. To build upon the work of Escobar and Martı´nez (1993) and Escobar et al. (1997), many of the samples examined in the present study are from the same localities described in these two contributions. The major advancements of our new work on western Venezuela coals are to provide information on the concentrations of environmentally sensitive trace
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
elements and detailed petrologic data on coal maceral types and abundances.
3. Methodology 3.1. Samples For the present study, 16 channel coal samples were collected from 14 active coal mines located in the States of Zulia, Me´rida and Ta´chira. Samples were collected by personnel of INGEOMIN and sent to the USGS for analysis. Coal bed name, thickness and relative stratigraphic position in the host formation were not reported by INGEOMIN. The collected samples represent the coal currently being produced at each mine location. Available information pertaining to sample collection and mine location is summarized in Table 1.
75
Materials (ASTM) methods and procedures (ASTM, 2002). Tests for ash fusion temperature, air dry loss, residual moisture and the free swelling index of each sample also were determined in the same commercial laboratory. Determinations of major and trace element concentrations were performed at the USGS according to the methods outlined in Bullock et al. (2002). Concentrations of 26 major element oxides and trace elements were determined on coal ash by inductively coupled plasma atomic emission spectroscopy (ICPAES), and concentrations of an additional 17 trace elements in coal ash were determined by inductively coupled plasma mass spectroscopy (ICP-MS). Mercury concentrations were determined in whole-coal samples following ASTM D6414 (ASTM, 2002), and Se concentrations were determined in whole-coal samples by hydride generation atomic absorption (Bullock et al., 2002). Chlorine concentrations in whole-coal samples were determined by ion chromatograph.
3.2. Chemical analysis 3.3. Maceral analysis Analyses for proximate, ultimate, calorific and sulfur forms were performed in a commercial laboratory according to American Society for Testing and
Representative splits of all coal samples were ground, cast in epoxy and polished for petrographic
Table 1 Sample location information for western Venezuelan coal samples Mine
Sample ID
North latitudea
Zulia Minas Norte Paso Diablo
Guarija Guasare
11.04216 11.05000
Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa
M1-QP M2-CDLV M3-RE M4-QPA
Ta´chira Las Mesas-Rı´o Pajitas Las Mesas-Finca Familia Arellano Caliche Las Adjuntas Las Adjuntas Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote a
West longitudea
Origin
Formation
Age
72.27273 72.17000
Surface Surface
Marcelina Marcelina
Upper Paleocene Upper Paleocene
8.44127 8.44818 8.43339 9.06982
71.69081 71.69062 71.79267 71.09390
Surface Surface Surface Surface
Carbonera Carbonera Carbonera Palmar
Eocene–Oligocene Eocene–Oligocene Eocene–Oligocene Middle Miocene
M5-RP M6-FA
8.39232 8.40981
71.93938 71.90385
Surface Surface
Carbonera Carbonera
Eocene–Oligocene Eocene–Oligocene
M7-LC M8-M1LA M8-M2LA M10-LC M11-HV M12-CA M13-LP M14-SC
8.08578 7.76185 7.76185 7.40134 7.84870 7.92288 7.73768 7.73768
72.23795 72.41608 72.41608 72.02841 72.36808 72.29072 72.39907 72.39907
Underground Surface Surface Surface Surface Underground Underground Underground
Carbonera Los Cuervos Los Cuervos Los Cuervos Carbonera Carbonera Carbonera Carbonera
Eocene–Oligocene Upper Paleocene Upper Paleocene Upper Paleocene Eocene–Oligocene Eocene–Oligocene Eocene–Oligocene Eocene–Oligocene
Latitude and longitude coordinates obtained by global positioning satellite (GPS) at time of sample collection.
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analyses following procedures outlined in Pontolillo and Stanton (1994). Two sample mounts were made from each sample. Measurements of maximum vitrinite reflectance in immersion oil (R o max) measurements were performed according to ASTM D2798 methods and procedures (ASTM, 2002). Following vitrinite reflectance measurements, the samples were examined in blue-light fluorescence using a modification of ASTM D 2799 (ASTM, 2002). A total of 500 identifications of liptinite or non-fluorescing organic/mineral matter were made on each sample mount. Vitrinite and inertinite macerals were identified under oil immersion with a standard white light source, using the maceral nomenclature proposed by the International Committee for Coal and Organic Petrology (ICCP) (ICCP, 1998, 2001). An additional 500 counts were performed on each sample mount in white light, for a total of 2000 counts of coal macerals and mineral matter per sample (Pontolillo and Stanton, 1994). All sample mounts were etched in potassium permanganate solution prior to white light petrographic analysis to facilitate measurement of telinite abundance. However, this procedure yielded a negligible improvement, illustrating the highly gelified nature of the samples. An experimental program aimed at etching the sample mounts in a low temperature oxygen plasma chamber also had poor results despite the considerable duration of some etching runs. 3.4. Mineral analysis Representative splits of coal samples were ground to 200 mesh (75 Am) and oven-dried overnight before ashing in a low-temperature asher. Low temperature ash (LTA) residues were cast into pressed pellets and analyzed on an automated powder diffractometer. X-ray diffraction (XRD) patterns were analyzed with a commercial reference pattern library (International Center for Diffraction Data, 1997) and also using a USGS program with a common coal mineral reference pattern library (Hosterman and Dulong, 1989). Detection of phases constituting less than 5 wt.% of the sample LTA residue is suspect on the diffractometer, and values of the program output of 5 wt.% or less are provided for informative purposes only.
4. Results and discussion 4.1. Coal quality Western Venezuelan coal samples generally are comparable in quality with the extensively developed coal deposits of the Powder River and Appalachian basins in the United States that also are used for thermal power generation. Proximate, ultimate, calorific and forms of sulfur values are summarized in Table 2. Total moisture content on an as-received basis (as-received) of the Venezuelan samples ranges from 0.69 wt.% in the sample from Lobatera to 16.66 wt.% in the Santo Domingo sample, with an average moisture of 4.39 wt.%. Moisture content is, in general, comparable to Appalachian basin coals of the eastern United States, which average 3.4 wt.% moisture (n = 3206; Bragg et al., 1998). Volatile matter content ranges from 36.03 wt.% [dry basis, (db)] in one of the Las Adjuntas samples (M8-M1LA) to 60.91 wt.% in the La Pajarita sample. Average volatile matter content of the coal samples is 45.76 wt.% (db), slightly higher than the average coal from the Appalachian basin (34 wt.% volatile matter; Bragg et al., 1998). Fixed carbon content ranges from 35.35 wt.% (db) in the sample from La Pajarita to 62.45 wt.% (db) in the M8-M1LA sample from the Las Adjuntas mine. Proximate results indicate that the western Venezuelan coal samples have a low average ash yield of 5.30 wt.% (db), ranging from 0.42 wt.% in the Paso Diablo sample to 17.20 wt.% ash in the sample from Las Dantas-La Vega in Me´rida (Fig. 4). The majority of the coal samples have low ash yields, ranging between 1 and 8 wt.% ash, with only the samples from Lobatera (12.83%), Rı´o Pajitas (15.65%) and Las Dantas La Vega (17.2%) containing more than 10 wt.% ash. Ash yields are relatively low in comparison with Paleocene subbituminous Powder River basin coals of Wyoming and Montana in the western United States, which average 12.30 wt.% ash (db) (Bragg et al., 1998), higher than all but the Lobatera, Rı´o Pajitas and Las Dantas La Vega samples. When compared to the Carboniferous bituminous coals of the central and northern Appalachians, the western Venezuelan coal samples are significantly lower in ash. Average ash yield of 3206 Appalachian basin coals is 11 wt.% (db), again higher than all but the Lobatera, Rı´o
Table 2 Proximate, ultimate, calorific and forms of sulfur values [dry basis except moisture (as-received) and calorific value (moist, mineral-matter-free)] for western Venezuela coal samples Mine
Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa Ta´chira Las Mesas-Rı´o Pajitas Las Mesas-Finca Familia Arellano Caliche Las Adjuntas (M8-M1LA) Las Adjuntas (M8-M2LA) Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote
Ash (%, 750 8C)
VM (%)
FC (%)
H (%)
C (%)
N (%)
S (%)
O (%)
Forms of sulfur Sulfate (%)
Pyritic (%)
Organic (%)
CV m,mmf (Btu/lb)
CV m,mmf (MJ/kg)
R o max (%)
2.78 3.04
1.88 0.42
38.29 39.90
59.83 59.68
5.60 5.66
82.31 83.43
1.51 1.60
0.41 0.37
8.29 8.52
0.02 0.03
0.01 0.01
0.38 0.33
14,652 14,575
34.08 33.90
0.72 0.71
3.49 2.03 6.75 11.25
1.94 17.20 3.71 5.48
45.33 45.15 48.30 44.45
52.73 37.65 47.99 50.07
6.29 5.52 6.05 5.20
79.59 63.58 74.65 70.92
1.78 1.34 1.50 1.62
0.87 6.34 0.97 3.85
9.53 6.02 13.12 12.93
0.02 0.15 0.02 0.04
0.07 3.62 0.05 2.67
0.78 2.57 0.90 1.14
14,375 14,951 13,110 11,973
33.44 34.78 30.49 27.85
0.51 0.56 0.57 0.51
1.46
15.65
39.68
44.37
5.08
66.92
1.24
6.06
5.05
0.06
5.23
0.77
15,090
35.10
0.62
4.47
8.03
55.20
36.77
6.90
73.25
1.30
0.45
10.07
0.02
0.09
0.34
14,537
33.81
n.d.
0.93 6.48
4.59 1.52
43.08 36.03
52.33 62.45
6.14 4.92
80.07 78.89
1.49 1.79
2.63 0.66
5.08 12.22
0.03 0.04
1.78 0.03
0.82 0.59
15,493 13,035
36.04 30.32
0.61 0.85
3.82
1.87
36.31
61.82
5.36
81.46
1.65
0.61
9.05
0.02
0.02
0.57
14,121
32.85
0.75
16.66 1.10 0.69 0.72 4.61
1.09 1.95 12.83 3.64 2.97
47.51 52.23 49.77 60.91 50.05
51.40 45.82 37.40 35.45 46.98
5.21 7.17 6.60 8.05 6.20
74.87 82.90 72.97 80.25 78.50
1.56 1.63 1.06 1.54 1.19
0.32 0.81 0.47 0.57 1.21
16.95 5.54 6.07 5.95 9.93
0.03 0.04 0.01 0.01 0.10
0.01 0.11 0.08 0.03 0.15
0.28 0.66 0.38 0.53 0.96
10,839 15,860 15,999 15,964 14,207
25.21 36.89 37.21 37.13 33.04
0.42 0.60 0.48 n.d. 0.61
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Zulia Minas Norte Paso Diablo
Moist (%, as-received)
Abbreviations: Moist=moisture, %=weight percent, Ash =ash yield, VM=volatile matter, FC=fixed carbon, H =hydrogen, C=carbon, N =nitrogen, S =sulfur, O=oxygen, CV=calorific value, m,mmf=moist, mineral-matter-free, Btu/lb=British thermal units per pound, MJ/kg=Megajoules per kilogram, R o max =maximum reflectance of vitrinite in immersion oil, n.d. =not determined.
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P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97 20 18
wt.% ash (db)
16 14 12 10 8 6 4 2 La s A S Pas La dj ant o D s A unt o D iab dj as ( om lo un M ta 8- ing s( M o M 1L 8- A M M2 ) in L A a H E sN ) at l P or o de a l m te l i Ce a Vi tal rro rge C n La apo Pa te Es jari ca ta la Fi n nc Ca te aF P lic a am lm he ili ich aA o re sa l Lo lan La o s D R bat an ío P era ta aj s- ita Po La s w Ve A der ga pp R al ive ac r hi (n an =5 (n 52 =3 ) ,2 06 )
0
MINE Fig. 4. Proximate analyses values for weight percent ash (dry basis) of western Venezuelan coals. Average ash yield for Powder River basin and Appalachian basin coal samples (Bragg et al., 1998) provided for comparison.
Pajitas and Las Dantas-La Vega samples. Coals from Paso Diablo and Minas Norte in the Guasare basin of Zulia, which constitute 90% of Venezuela’s currently produced coal (Petro´leos de Venezuela, 2001d), have exceptionally low ash yields of 0.42 and 1.88 wt.%, respectively. Note that ashing of the coal samples for major and trace element analyses was performed at a maximum temperature of 525 8C, resulting in slightly higher ash yields than the proximate analyses (compare ash values in Table 2 to ash values in following tables). Higher ash yield at lower temperatures is a result of the retention of some mineral matter in the ash, particularly carbonates or other phases with a volatile molecular component, and is even more pronounced for the results of low temperature ashing. Moisture values compiled with major and trace element concentrations were measured on whole-coal samples prior to ashing and represent moisture on an asdetermined basis (after processing and conditioning of the coal samples to 60 mesh or 250 Am). Western Venezuelan coals, in general, have low total sulfur values, ranging from 0.32 wt.% sulfur (db) in the Santo Domingo sample to 6.34 wt.% in the Las Dantas-La Vega sample (Fig. 5). The average sulfur content of the 16 samples is 1.66 wt.%. However, this value is skewed by the relatively high total sulfur
content of the samples from only four mines: Las Dantas La Vega (6.34 wt.%), Palmichosa (3.85 wt.%), Rı´o Pajitas (6.06 wt.%) and Caliche (2.63 wt.%) (Table 2). Eleven of the samples contain less than 1 wt.% total sulfur (median = 0.74 wt.%). For comparison, on a dry basis, Powder River coals contain on average 1.3 wt.% sulfur (median = 0.97 wt.%) and Appalachian coals contain 2.2 wt.% sulfur (median = 1.7 wt.%; Bragg et al., 1998). As with ash yield, the Guasare basin coal samples are exceptionally low in sulfur content: 0.37 wt.% sulfur for Paso Diablo and 0.41 wt.% for Minas Norte. Sulfur mostly occurs in the organic form, except in the four samples with the highest sulfur concentrations, in which total sulfur is dominated by the pyritic form (Fig. 6). As discussed below, X-ray diffraction and petrographic analyses confirm that the mineralogy of these samples mostly is composed of pyrite. Overall, total sulfur content in western Venezuelan coals is low to moderate. 4.2. Coal rank The western Venezuelan coal samples have an apparent rank of subbituminous A (one sample— Santo Domingo) to high volatile A bituminous following the ASTM D 388 classification (ASTM, 2002), similar to the average coal rank of the
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
79
7 6
wt.% S (db)
5 4 3 2 1
La s A S Pas La dj ant o D s A unt o D iab dj as ( om lo un M ta 8- ing s( M o M 1L 8- A M M2 ) in L A a H E sN ) at l P or o de alm te l i Ce a Vi tal rro rge C n La apo Pa te Es jari ca ta la Fi n nc C te a F P ali a c am lm he ili ich aA o re sa l Lo lan La o s D R bat an ío P era ta aj s- ita Po La s w de Ve A rR ga pp i al ve ac r ( hi n= an 5 (n 52) =3 ,2 06 )
0
MINE
Fig. 5. Ultimate analyses values for weight percent sulfur (dry basis) of western Venezuelan coals. Average sulfur content of Powder River basin and Appalachian basin coal samples (Bragg et al., 1998) provided for comparison.
Pyritic Sulfur 0
20
100
Río Pajitas 80
Palmichosa Caliche 40
60
Las Dantas-La Vega %
%
60
40
80
20
100
Organic 0 Sulfur
20
40
60 %
80
0 100 Sulfate
Sufur
Fig. 6. Ternary diagram depicting forms of sulfur values (dry basis) for western Venezuelan coals. Samples containing high pyritic sulfur are labeled.
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P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Pennsylvanian coals of the Appalachian basin. On a moist, mineral-matter-free basis (m,mmf), calorific value ranges from 25.21 MJ/kg (10,839 Btu/lb) for the Santo Domingo sample to 37.21 MJ/kg (15,999 Btu/ lb) for the Lobatera sample. The majority of the samples (12) fall in the high volatile A bituminous classification with calorific values greater than 32.56 MJ/kg (14,000 Btu/lb) and contain sufficient volatile matter to exclude classification on dry bases following ASTM guidelines. Of these samples, measured R o max values are highly variable. R o max values range from 0.48% in the Lobatera sample to 0.75% in one sample from Las Adjuntas (M8-M2LA). Maximum reflectance values for two of the samples with calorific values greater than 32.56 MJ/kg (14,000 Btu/lb), Finca Familia Arellano and La Pajarita, were not determined due to high liptinite content. Of the remaining 10 samples with N32.56 MJ/kg (14,000 Btu/lb) m,mmf, 7 have R o max values N 0.60%, consistent with the high volatile A bituminous classification obtained by ASTM D 388. Rank classification for the three other samples with N32.56 MJ/kg (14,000 Btu/lb) m,mmf is somewhat more problematic. The R o max values for Lobatera (0.48%), El Palmital (0.51%) and Las Dantas-La Vega (0.56%) fall within the subbituminous range. Suppression of R o max by high volume percent liptinite content in the Lobatera and Las Dantas-La Vega samples is a possibility. However, this artifact cannot account for the relatively low R o max value for El Palmital, which contains negligible liptinite. Three samples, Escalante, Palmichosa and Las Adjuntas (M8-M1LA) are classified as high volatile B bituminous by ASTM D 388 (30.23–32.56 MJ/kg, 13,000– 14,000 Btu/lb), and a fourth, Santo Domingo is classified as subbituminous A (24.42–26.75 MJ/kg, 10,500–11,500 Btu/lb). These four coal samples have the highest moisture content of the western Venezuelan coals studied here (Table 2). The R o max value obtained for the Las Adjuntas (M8-M1LA) sample is 0.85%, well within the bituminous range, and generally consistent with the ASTM rank classification of high volatile B bituminous. The slight differences in ASTM rank between the two Las Adjuntas samples may be explained by differing inertinite content; sample M8-M2LA has 19 vol.% inertinite, almost three times as much as the other Las Adjuntas sample and the relatively higher carbon content of the
inertinite fraction may explain the slight rank elevation. The Escalante and Palmichosa samples have R o max values of 0.57% and 0.51%, in the subbituminous A range and slightly lower than the ASTM rank classification. The Santo Domingo sample has a R o max value of 0.42%, consistent with the subbituminous rank classification obtained by ASTM D 388. 4.3. Maceral analysis Results of petrographic analyses (Table 3) illustrate the diverse character of coals currently produced in Venezuela. Concentrations of the liptinite maceral group range from b 1 vol.% in coal from Las Adjuntas (M8-M2LA) to almost 70 vol.% in the coal sample from La Pajarita (Fig. 7). Five of the 16 samples contained N 20 vol.% liptinite, which typically consisted of the macerals bituminite and sporinite. More than half of the western Venezuela coal samples consisted of 80 vol.% or more vitrinite. Inertinite macerals typically make up less than 10 vol.% of the samples and range to a maximum of 22 vol.% in the sample from Minas Norte. The vitrinite component in all samples mostly consisted of either collotelinite (Fig. 8A) or collodetrinite (Fig. 8B), or approximately equal concentrations of both macerals. Small percentages of corpogelinite were observed in most samples. Likewise, several samples contained V 1 vol.% telinite (Fig. 8C). The relative lack of telinite in coal samples collected from localities widely dispersed over western Venezuela may suggest that climate predominantly controlled the peat flora, and that the original plant materials were depleted in woody substance. The relative scarcity of suberinite (Fig. 8D), corkified cell walls of bark, which was found in only trace quantities in about half the samples, supports this interpretation. Alternatively, waterlogged conditions in the peat environment may have resulted in the bacterial alteration and recombination of humic substances, precipitating gels which filled in cell lumina and removed the apparent telinite structure from woody precursor materials (Cohen et al., 1987). Cutinite, occurring with abundant gelified vitrinite macerals in most of the samples, suggests the presence of leafy shrubs or trees in the peat-forming environment of many of the western Venezuela coals (e.g., Wu¨st et al., 2001).
Table 3 Petrographic data for Venezuelan coal samples Minas Norte
Paso Diablo
El Palmital
LDLV
Escalante
Palmichosa
Rı´o Pajitas
FFA
Caliche
Las Adjuntas
Santo Domingo
HDLV
Lobatera
La Pajarita
Cerro Capote
Sample ID
GUAJIRA
GUASARE
M1-QP
M2-CDLV
M3-RE
M4-QPA
M5-RP
M6-FA
M7-LC
M8-M1LA
Vitrinites Telinite Collotelinite Vitrodetrinite Collodetrinite Corpogelinite Gelinite Total Vitrinite
M8-M2LA
M10-LC
M11-HV
M12-CA
M13-LP
M14-SC
tr 24 X 42 3 X 69
X 19 X 64 1 X 84
1 86 X 1 5 X 93
X 59 X X 8 X 67
X 86 X X 2 X 88
X 90 X X 5 X 95
X 64 X 23 4 tr 91
X 3 2 44 3 X 52
1 98 X tr tr X 99
X 63 X 29 tr tr 92
X 13 X 66 1 1 81
X 19 X 44 16 tr 79
X 13 tr 52 4 tr 69
1 23 X 28 14 X 66
1 3 2 13 11 X 30
tr 14 X 50 26 X 90
Inertinites Fusinite Semifusinite Secretinite Macrinite Micrinite Funginite Inertodetrinite Total Inertinite
6 5 tr 7 tr tr 4 22
3 2 tr 5 X tr 2 12
tr X X 1 X tr tr 1
X X X tr X tr X tr
X X X tr X 1 X 1
X tr X tr tr tr X tr
1 1 X 1 X tr 1 4
X X X X X X X tr
X X X X X tr X tr
2 1 X 2 X tr 2 7
7 4 tr 5 tr tr 3 19
4 4 X 4 tr tr 3 15
tr tr X tr tr 2 1 3
X tr X 1 tr 1 tr 2
tr tr X tr X 1 X 1
X tr X X tr 1 1 2
Liptinites Sporinite Cutinite Resinite Fluorinite Bituminite Exsudatinite Alginite Suberinite Liptodetrinite Total Liptinite
tr 3 tr X 1 tr 2 X 3 9
tr tr tr X tr 1 1 X 2 4
tr 1 tr X 2 tr tr tr 3 6
3 1 tr X 18 1 1 tr 9 33
1 1 X 1 1 2 1 tr 4 11
1 1 1 X tr X tr tr 2 5
1 1 tr X 2 tr tr tr 1 5
4 X 2 X 32 X X X 10 48
tr tr tr X 1 tr X tr tr 1
tr X tr X 1 tr tr X tr 1
tr X X X tr tr X X tr tr
1 1 tr tr 1 tr tr tr 3 6
4 X tr X 18 tr tr X 6 28
2 tr 1 X 21 tr tr tr 8 32
3 tr 1 2 51 1 3 X 8 69
1 tr tr X 3 tr X tr 4 8
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Mine
Data are reported on a mineral-matter-free basis in volume percent. LDLV= Las Dantas-La Vega, FFA=Las Mesas-Finca Familia Arellano, HDLV=Hato de la Virgen, X =not present, tr =present in trace amounts (b1 vol.%).
81
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P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97 Inertinite 0 100
20
80
40
60
%
%
60
40
Minas Norte
80
100 Liptinite 0
La Pajarita 20 40 Finca Familia Arellano
60
80
20
0 100 Vitrinite
%
Fig. 7. Ternary diagram showing relative percentages of vitrinite, inertinite and liptinite maceral groups in western Venezuela coal samples. Samples containing high liptinite or inertinite concentrations are labeled.
Fig. 8. Photomicrographs of western Venezuela coals. (A) Palmichosa, (B) Rı´o Pajitas, (C) Palmichosa sample showing rare telinite, (D) Paso Diablo sample showing rare suberinite. All photomicrographs taken in white light under oil immersion.
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
The organic component of several samples was dominated or substantially represented by detrital organic material; particularly samples from Las Adjuntas (M8-M2LA) and Paso Diablo (Fig. 8D), in which total detritus (including collodetrinite) of the three maceral groups sum to 69 vol.% and 68 vol.%, respectively. The Finca Familia Arellano, Hato de la Virgen and Cerro Capote samples from the Carbonera Formation all contain 50 vol.% or greater detrital coal macerals (i.e., collodetrinite, vitodetrinite, inertodetrinite and liptodetrinite). The high content of detrital organic material probably indicates pervasive degradation of autochthonous plant materials. Previous workers interpreted the characteristics of Carbonera Formation coal deposits in Ta´chira to suggest a paralic origin with relatively little transport of the original plant materials prior to coalification (Bar and Pen˜a, 1985). There does not appear to be any correlation between ash content and detrital organic component, possibly indicating sustained decomposition of a domed peat surface protected from flooding. Collodetrinite represents the most abundant maceral in many
83
of the detritus-rich coal samples, including: Minas Norte, Lobatera, Santo Domingo and the aforementioned Cerro Capote, Paso Diablo, Finca Familia Arellano, Hato de la Virgen and Las Adjuntas (M8M2LA) samples. The presence of this maceral does not necessarily imply transportation prior to coalification, as the precursor material may form in-situ during fragmentation of the original plant material (e.g., Wu¨st et al., 2001). However, the dominance of this maceral in many of the western Venezuela coal samples indicates pervasive decomposition at the onset of the peat stage of coalification, occurring in neutral to weakly alkaline waters in an oxygen-rich environment (ICCP, 1998). The absence of telinite can be interpreted as further evidence for the neutral to weakly alkaline nature of the peat environment as acidic conditions would have prohibited or limited bacterial degradation and preserved plant structures (Shao et al., 2003). Macerals of the inertinite group typically represented less than 10 vol.% of the coal samples and were found to be highest in upper Paleocene samples
Fig. 9. Photomicrographs of western Venezuela coals. (A) Santo Domingo, (B) Minas Norte, (C) Finca Familia Arellano, (D) La Pajarita. Photomicrographs A and B taken in white light under oil immersion; C and D taken in blue light.
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from the Las Adjuntas and Santo Domingo (Fig. 9A) mines and from the two mines in the Guasare basin. The Minas Norte coal sample contained the highest inertinite concentration (22 vol.%), which consisted of nearly equal proportions of fusinite, semifusinite, macrinite and inertodetrinite (Fig. 9B). Funginite was observed in quantities of 2 vol.% or less in all of the coal samples (e.g., Fig. 9A), except for the Finca Familia Arellano sample in which funginite is absent. The presence of fungi indicates aerobic conditions occurred at least intermittently in the peat environment of most of the western Venezuela samples (Brady and Weil, 1996; Wu¨st et al., 2001), and is consistent with the preservation of collodetrinite and the relative absence of telinite. Inertinite concentrations are highest in the Paleocene coals of the Marcelina and Los Cuervos Formations, whereas in the younger coals of the Carbonera and Palmar Formations, liptinite group macerals consistently are present in greater concentrations than inertinite. This dominance of inertinite group macerals over liptinite in the Paleocene coals of western Venezuela previously was noted by Escobar and Martı´nez (1993) and Escobar et al. (1997), who proposed four possible mechanisms for the restriction of higher inertinite concentrations to the Paleocene: (1) different biota types, (2) different tectonic regimes, (3) fluctuations in the relative constancy of the water table and (4) important changes in paleoclimate. They preferred variations in phreatic level as a result of tectonic controls (e.g., McCabe, 1991) as the dominant mechanism in determining coal type (Escobar and Martı´nez, 1993; Escobar et al., 1997). We concur with this hypothesis and present the following additional observations. Coal of all ages in western Venezuela was deposited in a foreland basin environment; however, a more rapid subsidence rate during accumulation of the younger Carbonera and Palmar coals may have been related to the advanced emplacement and thrust stacking of the Lara nappes to the northeast. Slower subsidence rates prior to the main phase of nappe emplacement would have dominated coal accumulation in the Paleocene. In this model, higher inertinite concentrations in the Paleocene coal samples could be explained by relatively greater desiccation as a result of fluctuating water tables, whereas in the younger coals, more rapid subsidence would have prevented oxidation.
Several of the coals are distinctly sapropelic in character, particularly the samples from Finca Familia Arellano (Fig. 9C) and La Pajarita (Fig. 9D), both occurring in the Carbonera Formation and containing 48 and 69 vol.% total liptinite, respectively. The liptinite component of these samples is dominated by a bituminite groundmass enclosing uniformly sized particles of liptodetrinite and lesser amounts of sporinite. Isolated resinite blebs in the bituminite groundmass were present in both samples and significant quantities of alginite, flourinite, exudatinite and trace cutinite were present in the sample from La Pajarita. Collodetrinite hosted in bituminite groundmass dominates the vitrinite component of both of these samples. Relatively low ash yields (La Pajarita = 3.64 wt.%, Finca Familia Arellano =v8.03 wt.%, db) indicate relatively little sediment input and suggest the coal-forming materials were deposited in stagnated, standing water, protected from flooding and sediment influx. The Lobatera sample, also from the Carbonera, contained high liptinite content as well, at 32 vol.%. Tocco et al. (1995) noted that Carbonera coals exhibited good oil-generating potential, based on the results of total organic carbon, Rock-Eval, vitrinite reflectance and maceral analyses, and concluded that terrestrial oil seeps in the southern Maracaibo basin possibly were sourced in Carbonera coals. Our R o max data show that the Carbonera coals are immature to marginally mature with respect to oil generation, ranging in value from 0.48 to 0.62, similar to the results obtained by Tocco et al. (1995). The occurrence of rare exudatinite in these samples (Table 3) points to the generation of at least early asphaltic bitumens. However, the presence of trace flourinite and the ubiquity of bituminite, particularly in the La Pajarita sample, indicates that the Carbonera coals are relatively immature with respect to oil generation. Regardless of their maturity with respect to petroleum generation, the potentially high combustion efficiency of these liptinite-rich coal samples renders them an excellent thermal fuel source. 4.4. Mineral analysis The mineralogy of most of the western Venezuela coal samples is dominated by kaolinite and quartz, and less frequently, by pyrite. The results of XRD analyses of LTA residues are compiled in Table 4.
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
85
Table 4 Semiquantitative mineral matter content of Venezuelan coal samples in weight percent of low-temperature ash residue Mine
LTA
Phyllosilicates
Sulfides
Tectosilicates
Kln Ill Rct Chl Prl Py Mrc Sp Qtz Zulia Minas Norte Paso Diablo Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa
Carbonates
Oxides
Others
Kfs
Pl
Cal Ank Sd Hem Ant Rt Dsp Bhm Ap Bas Anh Anl
2.26 85 0.58 45
X X X X
X X
X X
tr X tr X
b 1 10 X 35
X tr
X tr
X X
tr 5
tr X X X
X X
X X
X X b1 X
X X 10 X
X X
X X
2.74 50 13.95 55
15 X 7 X
X X
X X
5 X 25 tr
X tr
10 tr
tr tr
tr tr
X X
tr X
tr X X tr
X X
2 X
X X
X X
X X
5 X
X X
X b1
X X X X X 10 14 1
2 X 55 5
b 1 10 tr 1
X X
X X
X X
X X
X X X 5
X X
X X
X X
X X
X X
X tr
X 5
X 3
4.48 80 9.51 X
Ta´chira LM-Rı´o Pajitas 23.66 55 LM-Finca Familia 8.77 40 Arellano Caliche 7.64 40 Las Adjuntas 2.34 90 (M8-M1LA) Las Adjuntas 2.35 70 (M8-M2LA) Santo Domingo 2.04 55 Hato de la Virgen 2.51 65 Lobatera 14.67 85 La Pajarita 4.46 90 Cerro Capote 4.26 90
X X X X
X X
X 40 X b 1 tr X
tr X
tr 50
X tr
tr X
X tr
X X
X 1 X X
X 5
X X
X 1
X X
X X
X tr
X X
1 X
X X X X
X X
X X
55 5 10 tr
tr X
X tr
X X
X tr
X X
X X
X X tr X
X X
X X
X X
X X
X X
X X
X X
b1 b1
10 X
X
X
tr X
X
15
X
tr
tr
X
tr
tr
X
X
X
X
X
tr
X
X
5 X X X X
X X X X X
b1 X X X X
tr 10 tr X 5
X tr b1 X b1
15 tr 10 5 X
5 X tr tr X
tr X X X X
tr 10 X tr X
X X X X X
tr tr tr tr X
X X tr 1 1
5 X X X X
X X b1 tr X
X tr X X X
4 1 X X X
X X X X X
tr X X X X
X X X X X
X 2 X X X
X X X X X
X 5 X X tr
Semiquantitative mineral matter content derived by least squares solution of overdetermined system of linear equations representing XRD pattern by USGS Fortran language program (Hosterman and Dulong, 1989). Values are rounded to nearest 5. Note that detection of phases constituting less than 5 wt.% of sample powder is suspect and therefore concentrations b5 wt.% following X-ray data reduction by the Fortran program are here noted as trace concentrations (tr). Actual values listed in italics are possible solutions of XRD pattern through least squares technique and are not as reliable as non-italicized values. Abbreviations: LTA=low-temperature ash yield of the coal sample in wt.%, X=not present. Mineral abbreviations after Kretz (1983): Kln=kaolinite, Ill=illite, Rct=rectorite (smectite), Chl=chlorite, Prl =pyrophyllite, Py =pyrite, Mrc=marcasite, Sp =sphalerite, Qtz=quartz, Kfs=K-feldspar, Pl=plagioclase, Cal=calcite, Ank=ankerite, Sd =siderite, Hem=hematite, Ant=anatase, Rt=rutile, Dsp=diaspore, Bhm=boehmite, Ap=apatite, Bas=bassanite, Anh=anhydrite, Anl=analcime.
Detection of minerals constituting less than 5 wt.% of the LTA sample is considered suspect and therefore concentrations b 5 wt.% following reduction of the XRD data are noted as trace concentrations (tr) in Table 4. Values are rounded to the nearest 5. Kaolinite is present in all of the samples except the coal from Palmichosa, in which the dominant mineral is pyrite. Illite occurs in significant quantities in the samples from El Palmital, Las Dantas La Vega, Las Adjuntas (M8-M2LA) and Santo Domingo. Quartz dominates the mineralogy of the sample from Finca Familia Arellano and also constitutes a significant fraction of the Paso Diablo coal. Pyrite constitutes the majority of the inorganic fraction of the sample from Palmichosa, Caliche, Las Dantas La Vega and Rı´o Pajitas. Note
that these four samples containing abundant pyrite in the LTA residue also have the highest pyritic sulfur content (Fig. 6). Syngenetic pyrite dominantly occurs as discrete framboidal aggregates (Fig. 10A), commonly aligned parallel to bedding planes (Fig. 10B), or less commonly as pore fillings in fusinite. Plagioclase and K-feldspar occur as minor or trace phases in about half of the samples: the carbonates calcite, siderite and ankerite also occur as minor or trace constituents in many of the samples. Bassanite, a hydrated calcium sulfate that is an artifact of the ashing process, was identified in six samples. Calcite constitutes 10 wt.% of the LTA residue for the Hato de la Virgen sample, significantly higher than the calcite content found by Escobar et al. (1997). Rutile, anatase and apatite were
86
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
with high weight percent pyrite content; Las Dantas La Vega, Palmichosa, Rı´o Pajitas and Caliche. Recent experimental work has demonstrated that pyrite is equally abrasive with quartz on a volume percent basis when these phases are excluded from coal macerals (Wells et al., 2004). However, most pyrite in the western Venezuelan coals occurs as framboids within macerals (e.g., Fig. 10A) and is not likely to contribute to abrasion during coal pulverization. 4.5. Major element chemistry and technological properties
Fig. 10. Photomicrographs showing syngenetic pyrite framboids in western Venezuela coals. (A) Palmichosa, (B) Las Dantas-La Vega.
found in trace quantities in several samples. Apatite concentration of ~ 8 wt.% in the sample from Paso Diablo is somewhat suspect as one would expect to see a significant amount of P in the ash and the results of the ICP-AES analysis do not indicate a significant P2O5 component when compared to the other coal samples. Quartz constitutes ~ 50 wt.% of the LTA residue of the Finca Familia Arellano sample. High quartz content might be of concern due to erosion in the boiler tube during combustion, or wear in grinding mills for power stations utilizing pulverized coal. However, this coal sample has a high temperature ash yield of only 8.03 wt.%, perhaps obviating concerns of wear or erosion failures due to high quartz content. Pyrite has a similar hardness to quartz and is also of concern in erosion and wear, particularly for the western Venezuelan samples
The major element chemistry of the western Venezuelan coal samples is considered in the context of utilization. Concentrations of major element oxides are summarized in Table 5 on a dry, ash basis, and calculated ash indices are compiled in Table 6. Note that these indices are based on empirical formulas, which have been developed to predict the combustion behavior of coals in power stations (summarized in Vaninetti and Busch, 1982; Skorupska, 1993). These ash descriptor indices apply only to certain coal types or to boiler design (Vaninetti and Busch, 1982); here, we have used indices developed for Eastern US coals, similar in rank and quality parameters to western Venezuela coals. The base–acid ratio (B/A) can be used to indicate potential slagging properties and ranges from 0.03 in the Minas Norte coal to 3.5 in the Palmichosa sample. Most of the coal samples have a low slagging tendency on this index with B/A ratios in the range of 0.1–0.7. Only the Caliche (B/A=1.5) and Palmichosa (B/A= 3.5) samples have a B/A ratio exceeding a value of 1. High total sulfur content in these samples indicates the potential for severe slagging tendency with calculated slagging factors of 13.4 for Palmichosa and 3.9 for Caliche. High total sulfur in Las Dantas La Vega and Rı´o Pajitas also result in severe slagging factors of 2.9 and 4.5, respectively, despite the low to medium slagging tendencies indicated by the B/A ratios in these samples. Values for the silica ratio in the Palmichosa and Caliche samples are 0.11 and 0.27, respectively, indicating low slagging tendency, and the values for Las Dantas La Vega (0.58) and Rı´o Pajitas (0.44) are in the medium to high range. These a priori estimates of slagging properties are largely descriptive in nature and not necessarily indicative of actual slagging
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
87
Table 5 Major element analytical data (dry, ash basis) for western Venezuelan coal samples Mine Zulia Minas Norte Paso Diablo Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa Ta´chira LM-Rı´o Pajitas LM-Finca Familia Arellano Caliche Las Adjuntas (M8-M1LA) Las Adjuntas (M8-M2LA) Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote
Ash (525 8C)
Moist
SiO2
Al2O3
1.89 0.38
1.00 0.98
52.40 50.50
38.60 30.90
2.21 17.68 3.95 5.72
1.71 1.04 3.14 5.30
23.80 35.90 39.20 8.80
16.01 8.14
0.70 1.55
4.69 1.66
CaO
MgO
Na2O
K2O
Fe2O3
TiO2
P2O5
SO3
Total
0.71 1.72
0.30 1.11
0.49 3.13
0.05 0.05
1.69 3.74
1.47 1.51
0.37 0.29
0.52 2.02
96.60 94.97
21.30 22.90 29.40 11.50
8.55 0.62 6.30 4.54
2.75 0.55 1.34 1.16
3.97 0.20 1.55 0.03
0.96 1.21 0.36 0.36
13.80 24.60 8.47 66.60
0.86 0.85 0.64 0.23
0.14 0.07 0.04 0.08
17.50 1.21 10.10 8.98
93.62 88.10 97.40 102.29
29.70 58.00
22.70 18.20
0.16 2.44
0.03 0.32
0.06 0.16
0.31 0.20
38.20 4.16
0.65 3.35
0.22 0.11
0.13 2.54
92.16 89.48
0.46 2.91
18.30 41.50
14.40 35.60
0.39 1.96
1.61 1.85
0.05 0.85
0.05 0.13
47.40 8.34
0.29 0.91
0.04 0.03
1.11 5.05
83.64 96.22
1.93
1.76
43.20
26.30
3.66
1.02
0.65
0.49
10.10
1.02
0.02
5.50
91.96
1.19 1.99 13.03 3.63 3.03
6.72 0.18 0.20 0.51 1.40
20.60 27.20 53.40 39.80 37.10
26.90 27.10 30.60 33.50 34.90
11.10 7.31 0.69 4.52 0.51
1.04 0.94 0.99 0.75 0.28
0.14 1.30 0.12 0.26 0.06
0.35 0.16 0.81 0.27 0.08
13.20 22.30 5.21 6.94 19.40
3.11 0.67 1.60 1.21 1.43
0.05 0.06 0.03 1.61 0.10
17.80 8.92 0.49 3.12 0.96
94.30 95.96 93.95 91.99 94.82
Abbreviations: Ash=ash yield in wt.% (dry), Moist=as-determined moisture in wt.%. Total is sum of oxides on an ash basis. All values in weight percent. Note that the reported ash and moisture values differ between Tables 3 and 2; values reported in Table 3 (this table) were derived in USGS laboratories following methods described in Bullock et al. (2002) [ashing at 525 8C, rather than 750 8C; total moisture determined on coal after conditioning and processing to 60 mesh (250 Am) (as-determined moisture)].
potential. However, the four aforementioned samples with a high slagging factor value collectively have, on average, an ash softening temperature (reducing atmosphere) 190 8C lower than the average of the other 12 western Venezuela coal samples (Table 6), consistent with the ash descriptor indices for slagging propensity. These four samples contain the greatest Fe2O3, total S, pyritic S and pyrite of the western Venezuela coal samples. A strong negative correlation between ash fusion temperatures, Fe2O3 and pyrite content was noted by Rimmer and Davis (1990), who suggested that low ash fusion temperatures could be interpreted to indicate peat deposition in a brackish or a marine-influenced environment. Other western Venezuela samples with relatively low ash fusion temperatures are El Palmital, Hato de la Virgen and Paso Diablo. El Palmital contains 13.8 wt.% Fe2O3 in the high temperature ash and detectable quantities of pyrite and marcasite in the LTA residue. The Hato de la Virgen sample contains ~ 10 wt.% pyrite, and the
Paso Diablo sample contains ~ 7 wt.% ankerite and a small amount of pyrite in the LTA, possibly explaining the relatively low ash fusion temperatures for these samples. Ash in the samples with high fusion temperatures and low slagging propensity is dominated by Al2O3 and SiO2, and the minerals kaolinite and to a lesser extent quartz, similar to the results obtained by Vassilev et al. (1995) and Alastuey et al. (2001). The calculated fouling factors [Rf =(B/A)/ Na2O] for the western Venezuela coal samples range from 0.01 in the Lobatera and Finca Familia Arellano samples to 2.6 in the sample from El Palmital. Values for Rf are below 0.2 in three quarters of the coal samples, indicating, in general, a low fouling tendency for the western Venezuela coals, and only the samples from Hato de la Virgen (Rf = 0.75) and El Palmital indicate a high or severe fouling tendency. Concentrations of Na2O typically are V 1 wt.% in the high temperature ash and highest in the El Palmital sample (3.97 wt.% Na2O). The Paso Diablo sample
88
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Table 6 Ash descriptor indices and technological properties of western Venezuelan coals Mine
S
Zulia Minas Norte Paso Diablo
B:A
SF
SR
0.44 0.03 0.37 0.12
Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa
0.83 6.36 0.93 3.76
0.65 0.54 0.46 2.90 0.26 0.24 3.55 13.34
0.49 0.58 0.71 0.11
Ta´chira Rı´o Pajitas Finca Familia Arellano Caliche Las Adjuntas (M8-M1LA) Las Adjuntas (M8-M2LA) Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote
6.11 0.43 2.61 0.63 0.59 0.32 0.79 0.47 0.56 1.20
0.73 0.09 1.50 0.17 0.23 0.51 0.58 0.09 0.17 0.28
0.44 0.89 0.27 0.77 0.75 0.45 0.47 0.89 0.77 0.65
AFT-ID (8C) AFT-S (8C) AFT-H (8C) AFT-F (8C) Rf
0.02 0.95 1540+ 0.04 0.88 1070
4.46 0.04 3.92 0.11 0.13 0.16 0.46 0.04 0.10 0.33
AFI
Cl
FSI
1540+ 1110
1540+ 1140
1540+ 1220
0.02 0.01 0.37 0.01
b0.038 6.0 0.032 6.5
1090 1110 1300 1255
1110 1150 1370 1270
1130 1180 1375 1330
1140 1190 1380 1370
2.59 0.09 0.40 0.11
0.10 0.18 0.07 0.02
0.035 0.020 0.024 0.016
4.0 2.0 1.0 1.0
1140 1390 1140 1540+ 1320 1310 1250 1540+ 1510 1450
1165 1455 1150 1540+ 1365 1340 1280 1540+ 1540+ 1490
1175 1500 1160 1540+ 1390 1350 1290 1540+ 1540+ 1500
1210 1520 1165 1540+ 1420 1355 1330 1540+ 1540+ 1530
0.04 0.01 0.08 0.14 0.15 0.07 0.76 0.01 0.04 0.02
0.04 0.02 0.004 0.02 0.02 0.004 0.03 0.09 0.02 0.003
0.023 0.016 0.020 0.044 0.025 0.030 0.018 0.016 0.020 0.036
6.0 0.5 4.5 0.5 1.0 0.5 1.5 1.0 0.5 0.5
Abbreviations: S =total sulfur (ASTM D5016), B/A=base–acid ratio [(Fe2O3) +(CaO+MgO) +(Na2O+K2O)]/[(SiO2)+(Al2O3) +(TiO2)], SF=slagging factor [(B/A)(S)], SR=silica ratio (SiO2)/[(SiO2) +(Fe2O3)+(CaO)+(MgO)], AFT=ash fusion temperatures in 8C in a reducing atmosphere (ASTM D1857), AFT-ID =initial deformation temperature, AFT-S =softening temperature, AFT-H=hemispherical temperature, AFT-F =fluid temperature, Rf=fouling index [(B/A)/(Na2O)], AFI =alkali fouling index [(ash)(Na2O+(0.659)(K2O))]/100, Cl=chlorine (wt.%), FSI=free swelling index (ASTM D720). Note that S content listed in this table was measured in a USGS laboratory by ASTM D5016 and S in Table 2 was measured in a commercial laboratory by ASTM D4239.
also contains high Na2O (3.13 wt.%), indicating severe fouling propensity for these two samples. However, on the alkali fouling index, the El Palmital sample has a value of only 0.1 and the Paso Diablo sample a much lower value of 0.01, indicating low fouling tendency and suggesting caution in acceptance of the calculated high Rf value and Na2O concentration in the El Palmital sample as a predictor of fouling potential. Concentrations of Cl are below 0.04 wt.% in all of the samples, consistent with the prediction on the Rf index for low fouling potential overall for the western Venezuela coals. Most of the western Venezuela coal samples are agglomerating and the samples from Rı´o Pajitas and the two mines in the Guasare basin exhibit good coking properties on the free swelling index. Free swelling indices (ASTM D720) are reported in Table 6. Coals from Finca Familia Arellano, Las Adjuntas (M8M1LA), Santo Domingo, La Pajarita and Cerro Capote are non-agglomerating. Previous workers have argued that a correlation exists between high ash content (van
Krevelen, 1993; Alastuey et al., 2001), or high inertinite content (Neavel et al., 1986; Alastuey et al., 2001) and the non-agglomerating character of coals. However, in the western Venezuelan coal samples there does not appear to be a significant relationship between high ash yield or inertinite content and plasticity as measured by the free swelling index. The only possible exception to this statement is the Santo Domingo sample, which contains 15 vol.% inertinite. 4.6. Trace element chemistry One particular concern pertaining to coal utilization impacts are the concentrations in coal of the possible hazardous air pollutants (HAPs) that are identified in the United States Clean Air Act Amendments of 1990 (Public Law 101-549) and in the equivalent legislation of other developed nations. The HAPs include Sb, As, Be, Cd, Cr, Co, Pb, Mn, Hg, Ni, Se and the radionuclides Th and U. Concentrations of other elements such as B, Cu, Mo, Sn, Tl, Vand Zn may
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
be of environmental concern if the particular element is present in enriched concentrations in the utilized coal deposit (Swaine, 1990; Senior et al., 2000). At least two of the elements listed above–As and Hg–are thought to have particularly adverse human health effects associated with coal utilization (Finkelman et al., 2002). Other considerations of trace element concentrations in coal are important in the utility industry; for example, concentrations of the halogen chlorine in coals consumed for power generation are important due to the potential for Cl-induced corrosion in combustors (Davidson, 1996). Trace element chemistry of the western Venezuelan coals samples is compiled in Table 7 on a dry, whole-coal basis. Concentrations of the HAP elements in the Venezuelan coal samples, with one or two possible exceptions, generally are within the range of concentrations of these elements found in most coals of the world (compiled in Taylor et al., 1998), or to coals of the Appalachian basin (Bragg et al., 1998). The typical HAP concentrations in western Venezuelan coals indicate that there may be little exceptional environmental or health impact from emissions as a consequence of coal combustion. However, slight to significant elevations of HAPs and other trace element concentrations are noted in the four coal samples which contain significant pyrite in the LTA residues: Rı´o Pajitas, Las Dantas La Vega, Caliche and Palmichosa. The Caliche and Palmichosa samples have ash yields about one third that of the Rı´o Pajitas and Las Dantas La Vega samples, which have the highest ash yields of the sample suite (Table 2). Concentrations of As and Hg are high in the Rı´o Pajitas sample when compared to the other Venezuelan coal samples; however, in general, they are within or close to the higher end of the range of these elements found in most coals of the world (Fig. 11). The high concentrations of As and Hg in this particular sample may be the result of the leaching of these elements from overlying strata into the sampled coal bed (Manuel Martı´nez, University of Central Venezuela, Caracas, written communication, 2004). The sample from Las Dantas-La Vega contains significant concentrations of HAP elements and concentrations of Mo, Ni and V slightly higher than the maximum concentration reported for world coals. The samples from Caliche and Palmichosa contain slightly elevated concentrations of HAP elements compared to the majority of the
89
western Venezuela coal samples, but are well within the ranges of concentrations reported for world coals. In addition to these four pyrite-rich samples, the sample from Finca Familia Arellano contains relatively high ash content and slightly elevated HAP element concentrations compared to the other western Venezuelan coals, but contains little pyrite. The Lobatera sample also contains relatively high ash and slight elevations in some trace metal concentrations, but contains no sulfides. Investigations into the modes of occurrence of HAPs elements in coals have shown that some of the elements (typically As, Cd, Co, Hg, Pb and Sb) are preferentially sequestered in the inorganic fraction of the coal, and primarily occur in the sulfide minerals pyrite, marcasite, galena, stibnite and sphalerite, and in selenides such as clausthalite (Kolker and Finkelman, 1998; Senior et al., 2000; Palmer et al., 2002; Hower and Robertson, 2003). These same studies have shown that other HAP elements (Be, Cr, Mn, Ni, Se and U) occur both as integral components of the organic matter and within mineral matter including sulfides, silicates and carbonates. In the western Venezuela coal samples, concentrations of the dominantly sulfide-associated HAP elements appear to be correlated with weight percent pyrite and marcasite as determined by XRD analyses, suggesting that these HAPs are indeed sequestered in sulfides. Arsenic and Hg concentrations are highest in the Rı´o Pajitas sample (71.3 ppm As, 2.1 ppm Hg), which contains 40 wt.% pyrite. The concentration of Hg in this sample is twice the maximum concentration reported for typical world coals (Taylor et al., 1998), and 10 times the average concentration of Hg in Appalachian basin coals (Bragg et al., 1998). In addition to As and Hg, the Rı´o Pajitas sample also contains relatively high concentrations of the sulfide-associated HAP elements Cd, Co, Pb and Sb, compared to the rest of the western Venezuelan coals. Concentrations of Mo, Tl and Ni, which also are thought to be hosted in sulfide minerals (Kolker and Finkelman, 1998; Alastuey et al., 2001; Zhuang et al., 2003), exceed the maximum of typical world coals in this sample and also indicate a sulfide mode-of-occurrence. Concentrations of V (269 ppm) are high compared to typical world coals and concentrations of Cr (28 ppm) are relatively high compared to the other western Venezuela coals. The V may be hosted in kaolinite (e.g., Karayigit et al., 2000), which con-
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P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Table 7 Trace element analytical data for western Venezuelan coal samples (whole-coal, dry basis) Mine Zulia Minas Norte Paso Diablo Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa Ta´chira LM-Rı´o Pajitas LM-Finca Familia Arellano Caliche Las Adjuntas (M8-M1LA) Las Adjuntas (M8-M2LA) Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote Reporting limit (ash basis) Appalachian average (nN3000) World coals minimum World coals maximum
Ash (525 8C)
Moist
Ag
As
Au
1.89 0.38
1.00 0.98
b0.038 b0.008
b0.004 0.021
b0.19 b0.04
21.3 17.9
2.21 17.68
1.71 1.04
b0.045 b0.360
2.78 10.1
b0.23 b1.8
53.6 48.8
3.95 5.72
3.14 5.30
b0.080 b0.120
0.815 18.9
b0.4 b0.58
16.01 8.14
0.70 1.55
b0.330 b0.170
71.3 1.89
b1.7 b0.82
4.69 1.66
0.46 2.91
b0.094 b0.034
3.39 0.255
b0.47 b0.17
1.93
1.76
b0.039
0.395
1.19 1.99 13.03 3.63 3.03
6.72 0.18 0.20 0.51 1.40
0.024 b0.040 b0.270 b0.073 b0.061 2
0.832 0.415 0.065 0.388 0.176 0.20
0.06
25
0.02 2
0.5 80
B
Ba
Be 3.51 1.21
Bi
Cd
Cl
Co
0.094 0.011
0.019 0.005
0.010 0.006
384 323
0.51 0.11
83.2 302
0.236 2.67
b0.002 b0.018
0.070 1.5
346 202
1.43 19.6
35.5 69.8
166 95.6
0.961 1.81
b0.004 0.056
0.186 0.086
237 b160
1.93 4.79
15.7 12.0
30.7 256
2.53 1.69
0.240 0.064
1.99 2.42
232 b160
8.54 7.35
4.0 28.7
0.117 1.18
0.016 0.016
0.061 0.453
201 443
1.36 2.3
b0.2
7.18
45.3
1.74
0.016
0.106
254
2.82
b0.12 b0.2 b1.4 b0.37 b0.31 10
42.7 18.4 3.6 13.2 7.85 20
81.4 58.2 71.1 114 7.52 2
0.339 0.480 3.24 0.682 0.227 1
0.018 0.018 0.093 0.021 0.020 0.1
0.045 0.403 0.313 0.385 0.594 0.1
300 180 b160 201 355 150*
5 3.59 4.08 2.9 3.46 2
944
7.2
50 2000
0.5 30
0.77
32
78
2.7
1.1
0.098
b0.01 b0.01
5 400
20 1000
0.1 15
0.1 0.5
0.1 3
14.1 7.7
All values in Ag/g (ppm), except ash yield and moisture which are in wt.%. Ash=ash yield (dry), Moist=moisture (as-determined), n.r.=not reported, *=reporting limit on whole-coal basis. Values were derived following methods described in Bullock et al. (2002). Average values for Appalachian coals are from Bragg et al. (1998), and maximum and minimum values for most world coals are from Taylor et al. (1998) (citing Swaine, 1990; Bowen, 1979).
stitutes ~ 55 wt.% of the ash; the Cr concentration is not atypical of world coals and may also be associated with kaolinite (e.g., Querol et al., 1992), or with organic matter (e.g., Huggins et al., 2000). The sample from Las Dantas-La Vega contains the greatest high-temperature ash yield (17.20 wt.%, db) and the highest concentrations of Ba, Co, Cr, Cs, Cu, Li, Rb, Sc, U, Y and Zn. Analyses of the LTA residue (13.95 wt.%) indicate that it consists of ~ 25 wt.% pyrite and trace marcasite and sphalerite, as well as ~ 55 wt.% kaolinite and ~ 5 wt.% illite. The high Co and Cu concentrations probably are explained by the pyrite content; Zn may be occurring in the trace mineral sphalerite, as there are no carbonate minerals
present in this coal (e.g., Palmer and Lyons, 1996). Chromium probably is occurring in the clay minerals illite and kaolinite or in an organic association (e.g., Huggins et al., 2000). Uranium presumably is in an organic association (Finkelman, 1995; Shao et al., 2003), as no trace minerals such as monazite which may host U in coals (Karayigit et al., 2000) were detected. The large ion lithophile elements (LILE) Ba, Cs, Li and Rb probably are sequestered in the clay minerals (Shao et al., 2003). The presence of high Ba concentrations, relatively high S and the sapropelic nature of the coal suggest a coastal setting with occasional marine influence for this horizon of the Carbonera Formation.
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Cr
Cs
1.19 0.179
0.004 0.002
4.64 49.7
0.095 1.36
1.93 6.01
10.9 0.126
28 22.3
0.109 0.052
2.27 1.46
0.011 0.015
2.36
Cu 2.83 0.838
Ga
Ge
1.09 0.074
0.274 0.057
3.31 58
1.19 6.33
0.821 3.2
0.06 5.15
10.8 0.853
4.39 0.217
6.5 4.38
5.12 9.84
2.35 2.32
0.877 4.46
0.039
4.16
1.3 10.5 17.2 12.5 4.58 2
0.029 0.007 0.547 0.044 0.011 0.1
17 0.5 60
Hg b0.03 b0.03
4.91 0.419
Mn 0.672 0.373
Mo
Nb
0.068 0.464
0.383 0.04
Ni 0.765 0.224
Pb 0.873 0.267
Rb 0.036 0.012
Sb 0.053 0.01
1.76 19.1
5.23 18
0.949 10.1
0.225 1.64
3.53 63.1
0.614 8.01
1.17 14.9
0.163 1.41
b0.03 2.64
3.31 15.9
24.8 5.29
0.324 0.155
0.336 35
4.43 1.41
1.27 1.36
0.648 0.315
2.1 0.061
16.5 5.39
4.04 36
21.5 1.02
1.18 2.94
66.3 24.2
16 4.86
2.19 0.765
0.296 4.73
0.28 0.1
2.43 1.3
25.9 2.67
0.835 3.35
0.211 0.222
5.58 11.5
1.5 1.15
0.131 0.126
0.136 1.82
3.93
6.15
0.11
1.03
2.24
1.41
0.269
8.94
1.1
0.52
1.78
4.68 10.5 11.7 20.9 4.85 2
0.897 0.514 7.16 1.43 1.96 0.1
0.169 0.124 2.77 0.334 0.704 0.1
b0.03 0.03 0.03 0.03 b0.03 0.02*
0.457 1.38 12.4 3.74 1.57 4
3.88 3.27 42.5 5.01 26.8 2
2.48 1.7 0.86 1.59 0.267 0.2
0.559 0.207 3.44 0.464 0.552 0.1
23.3 7.28 4.99 13.1 7.52 4
0.896 0.762 5.22 2.01 1.32 0.5
0.302 0.092 7.2 0.613 0.103 0.1
0.255 0.277 0.612 0.323 0.243 0.1
0.98
17.8
6.3
4.5
0.2
18.1
25
3.1
2.4
17
8.8
0.3 5
0.5 50
0.5 50
0.02 1
1 80
5 300
0.1 10
16 33.1
1 20
0.051 0.2
Li
91
20.2 0.17
1 20
0.5 50
2 80
33.8 3.16
23
1.2
2 50
0.05 10
(continued on next page)
The Caliche and Palmichosa samples both contain abundant pyrite at ~ 55 wt.% of the LTA, yet contain much lower concentrations of HAP elements than the Las Dantas-La Vega and Rı´o Pajitas samples. The mode of occurrence of the HAPs and other elements in these samples is more problematic, yet there does seem to be a relationship between the As and Hg concentrations and pyrite content. Concentrations of these elements are highest in the four pyrite-rich samples compared to the rest of the western Venezuela coals. However, despite the high pyrite content, As and Hg concentrations in the Caliche and Palmichosa samples are well within the range of typical world coals. Selenium, which typically occurs associated with the
organic matter in coals (Coleman et al., 1993; Finkelman, 1995), occurs at a concentration of 7.3 ppm in the Caliche sample, relatively high compared to most of the western Venezuela coal samples. However, Se in this sample probably is associated with the organic matter, and the sample from Santo Domingo, which contains only trace pyrite, has a higher Se concentration of 7.5 ppm. The Palmichosa sample contains the highest concentration of B (70 ppm) of the western Venezuela coals and relatively high B/Be (39), indicating a marine influence (Hower et al., 2002), which is consistent with the relatively high sulfur and the inferred coastalmarine depositional environment of the Palmar Formation (Gonza´lez de Juana et al., 1980).
92
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
Table 7 (continued ) Mine Zulia Minas Norte Paso Diablo Me´rida El Palmital Las Dantas-La Vega Escalante Palmichosa Ta´chira LM-Rı´o Pajitas LM-Finca Familia Arellano Caliche Las Adjuntas (M8-M1LA) Las Adjuntas (M8-M2LA) Santo Domingo Hato de la Virgen Lobatera La Pajarita Cerro Capote Reporting limit (ash basis) Appalachian average (nN3000) World coals minimum World coals maximum
Sc 0.44 0.056
Se
Sn
4.4 3.4
0.13 0.2
0.18 0.25
44.8 1.01
3.31 0.939
1.9 2.7
0.41 1.26
6.13 9.19
5.5 0.47
4.39 3.84
0.783 0.642
7.3 1.6
b0.15 0.07
1.19
2.4
0.453 3.35 4.55 5.22 1.63 4
Sr
Th
Tl
0.018 0.006
0.378 0.038
b0.002 b0.0004
0.711 0.065
0.071 3.54
0.3 0.902
34.8 9.44
0.018 0.057
1.48 1.14
0.023 2.11
14.5 42.1
0.11 0.037
3.2 4.35
0.812 11.1
0.021 0.012
1
27.1
7.5 2.3 4.5 2.9 2.2 0.1*
0.34 0.08 3.6 1.98 2.18 3
32.6 27.3 18.2 99.8 3.97 1
4
3.8
0.99
1 10
0.2 10
1.6 13.4
1 10
40.8 5.44
Te
0.007 54.5
U 0.176 0.018
Y 3.59 0.388
0.455 0.091
Zn 0.393 2.14
Zr 5.02 1.11
1.91 154
2.56 31.8
2.54 139
0.36 0.595
18.7 12.5
10 5.28
4.74 13.5
13.7 0.228
2.93 1.11
269 49.8
18.7 16.3
15.9 33.2
b0.47 0.529
0.676 0.03
0.117 0.259
6.38 5.8
2.07 5.87
4.39 9.22
1.79 3.33
0.009
0.596
0.023
0.228
6.19
8.43
3.09
3.44
0.016 0.048 0.064 0.044 0.016 0.1
0.443 1.89 5.39 4.39 0.91 8
0.211 0.013 b0.014 0.004 0.058 1
0.14 0.6 0.573 0.755 0.246 0.1
9.03 28.9 44.8 47.9 12.9 2
1.61 7.58 23.2 11.5 2.34 1
13.1 6.18 13.9 2.13 19.4 4
8.96 3.53 23.7 7.84 5.85 5
106
44.7
2.9
1.6
22
20
22
15 500
n.r. n.r.
0.5 10
0.5 10
2 100
5 300
5 200
The sapropelic coal sample from Finca Familia Arellano contains moderate ash content (8.03 wt.%, db) and the highest Cd concentration of the western Venezuela coals. Significant concentrations of some other trace elements including Co, Cr, Cu, Nb, Ni, Pb, Sb, Sc, Zn and Zr also occur in this coal. The major element chemistry of the Finca Familia Arellano sample shows that it contains the highest SiO2 and TiO2, and the lowest Fe2O3 of the western Venezuela coals. About 50 wt.% of the LTA residue is quartz, the source of the high SiO2. TiO2 is associated with anatase, a stable titanium oxide typically found in detrital sediments. Moderate ash content containing high concentrations of quartz and detectable anatase may
0.77 b0.2 1
13.8 3.18
V
8.4 2 50
14.4 4.43 2.64
16.7 37.8
be interpreted to indicate a clastic influx of relatively mature sediments into the depositional environment of this coal. The presence of zircon is inferred by the high Zr concentrations in the LTA residue, although zircon was not detected by XRD. Cadmium in coals typically occurs associated with Fe or Zn sulfides (Kolker and Finkelman, 1998); however, no sphalerite was detected in the LTA residue of the Finca Familia Arellano sample and only trace pyrite was noted. Another possible mode of occurrence of Cd and Zn in this coal is in the trace calcite, the only carbonate detected in the LTA residue (e.g., Spears and Zheng, 1999). The sapropelic coal from Lobatera has a relatively high ash yield (12.83 wt.%, db) compared to most of
P.C. Hackley et al. / International Journal of Coal Geology 63 (2005) 68–97
93
100
A 10
As (ppm)
1
0.1
0.01
M in as N or te Pa so D ia b Lo lo ba La te sA ra Ce rro dj un Ca ta s( po M te 8La M sA 1L A dj La un Pa ) ta ja s( rit M a 8M H a 2 to LA W or d ) ld e l co a V al s m irge in n im um Es c al Sa an Fi nt te nc o D aF o m am in ili a A go re lla no El Pa lm ita La sD Ca l lic an he ta A spp La al Ve ac ga hi Pa an l m av ic er ho ag sa e( n> 30 W 0 or Rí ld o 0) Pa co jit al sm as ax im um
0.001
10
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Hg (ppm)
1
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la
de
H
at
o
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D ia bl o Sa Vi nt rg o D en om in M go in as N Ce o rro rte Ca po te La Pa ja r Lo ita ba te ra Es ca l a Fi nt El nc e Pa aF lm La am i ta sA ili l aA dj un r e ta lla La s no sA (M dj 8un M ta 1L s( A M ) A 8pp M al 2L ac A hi ) P an av alm ic er ho La age sa s D (n > 30 an 00 ta sLa ) W V eg or a ld Ca co l al i c sm he ax im um Rí o Pa jit as
W or
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um
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Fig. 11. Logarithmic bar graphs of selected trace element concentrations in western Venezuelan coal samples. (A) As, (B) Hg. Average of Appalachian coal from Bragg et al. (1998), and maximum and minimum concentration of typical world coals from Taylor et al. (1998).
the western Venezuela coal samples and slightly elevated concentrations of some trace metals, including Be, Ga, Mn, Nb, Th and Y, compared to the rest of the western Venezuela coals. The LTA residue is dominantly composed of kaolinite (~ 85 wt.%), minor quartz (~ 10 wt.%) and traces of pyrite, Kfeldspar, siderite and hematite, similar to the results found by Escobar et al. (1997). Despite the relatively
high ash content and the presence of trace pyrite, concentrations of sulfide-associated HAP elements such as As and Hg are low (Fig. 11). The remainder of the studied coal samples yielded less than 5 wt.% high temperature ash and contain trace element concentrations typical of most coals of the world. Considered collectively, with the possible exception of the high-pyrite samples, the western
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Venezuelan coal samples contain typical concentrations of trace elements when compared to most world coals or to the coals of the Appalachian basin.
5. Conclusions Western Venezuela coals constitute an important natural resource for thermal power generation and currently are being exploited at a rapidly expanding rate. Analysis of 16 representative coal samples from 14 active mines in Ta´ chira, Me´ rida and Zulia illustrates that these coals are bituminous in rank, low in ash yield and low to moderate in sulfur content, indicating excellent potential for thermal use. Calculated values of various ash indices indicate that most of the coal samples will perform favorably in combustion or coking processes. For most of the coal samples, concentrations of the HAPs are similar to the concentrations of these elements in most coals of the world, indicating little potential for exceptional environmental impact as a consequence of utilization. Semiquantitative estimates of volume percent pyrite content show a strong correlation with pyritic sulfur and appear to be correlated with some sulfide-hosted trace element concentrations (As and Hg). Given the occurrence of these HAPs in the inorganic fraction of the coal, it is possible that beneficiation through the physical separation of pyrite could reduce these trace element concentrations prior to combustion in the coals that contain the highest concentrations. Petrographic determination of coal maceral composition indicates that diverse peat-forming environments existed in western Venezuela during the Paleocene through the Miocene. Sapropelic depositional centers were common, as suggested by high liptinite contents (N 20 vol.%) for 5 of the 16 samples examined. The maceral telinite constitutes less than 1 vol.% of all of the samples indicating either that the original plant materials were depleted in woody substance or that waterlogged conditions in the peat environment resulted in the bacterial alteration and recombination of humic substances, precipitating gels which filled cell lumina and removed the apparent telinite structure from woody precursor materials. Inertinite group macerals are present in the highest concentrations in distantly spaced upper Paleocene coals from opposite sides of the Maracaibo basin, possibly indicating
tectonic controls on subsidence related to stacking of the Lara nappes in the northeastern Me´rida Andes. The data presented herein provide a modern outline concerning the quality of coal currently produced in western Venezuela and will serve as the foundation for further studies of the deposits targeted for expanding development in the near future. Acknowledgments The authors would like to thank the following people for their help with this project: Mike Trippi and John Bullock for sample preparation, processing and geochemical analysis; Fiorella Simoni, Noelia Rodrı´guez, Steven Olmore, Jean Weaver and Ivette Torres for their help in coordinating communications between the USGS and INGEOMIN; and Frank Dulong and Nadine Piatak for assistance with sample ashing and XRD analysis. This manuscript benefited from helpful reviews by Leslie Ruppert, Jon Kolak and Jim Coleman of the U.S. Geological Survey, and Franco Urbani and Manuel Martı´nez of the University of Central Venezuela. References Alastuey, A., Jime´nez, A., Plana, F., Querol, X., Sua´rez-Ruı´z, I., 2001. Geochemistry, mineralogy, and technological properties of the main Stephanian (Carboniferous) coal seams from the Puertollano basin, Spain. International Journal of Coal Geology 45, 247 – 265. Ardina, E., 1987. Variaciones en el rango del Manto IV (principal) de la cuenca carbonifera del Zulia. Boletı´n Sociedad Venezolana de Geo´logos 31, 67. ASTM, 2002. Annual book of ASTM standards: petroleum products, lubricants, and fossil fuels. Gaseous Fuels; Coal and Coke, Sec. 5, vol. 5.06. ASTM International, West Conshohocken, PA. 650 pp. Azpiritxaga, I., Casas, B.J., 1989. Estudio sedimentolo´gico de las Mirador y Carbonera Formaciones en el Rı´o de Lobaterita, estado Ta´chira, Venezuela. Geos 29, 1 – 17. Bar, T.P., Pen˜a, R.P., 1985. Yacimiento del carbon, Santo Domingo, Estado Ta´chira. Congreso Geolo´gico Venezolano Memoria, no. 6. Tomo II, 772 – 793. Bellizzia, A.G., Pimentel, N.M., Bajo, R.O., 1976. Mapa geolo´gico estructural de Venezuela: Repu´blica de Venezuela. Ministerio de Minas e Hidrocarburos Direccio´n de Geologı´a, 30 sheets, map scale 1:500,000. Biewick, L.R.H., Weaver, J.N., 1995. The digital coal map of South America in ARC/INFO format. U.S. Geological Survey OpenFile Report 95-235, map scale 1:7,500,000.
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