ASSESSMENT OF THE COAL RESOURCES OF THE KYRGYZ REPUBLIC:
COAL CHARACTER AND DISTRIBUTION, GEOLOGY, MINING, AND IMPORTANCE TO THE NATION'S FUTURE
USGS Open File Report 97-137A (English)
D. PHYSICAL AND CHEMICAL CHARACTERISTICS
1. Chapter summary and the concept of coal quality
The "quality" of coal depends on many things that influence how easily and profitably the coal can be produced and used. The location, quantity, position in the earth and ownership are considered elsewhere in this report. Physical and chemical properties of the coal are described here. These properties must be addressed systematically in their entirety; they can not simply be evaluated as "good" or "bad", for that depends on the particular time and situation and on the particular use.
The general nature of Kyrgyz coals is summarized here, and the systematic details are presented later.
a. Rank
Most Kyrgyz coals are subbituminous or high-volatile C bituminous rank in U.S. terms. In Kyrgyz terms they are brown-coal 3 or long-flame. In the international classification established after WW-II, and used still in Europe, their rank (class number) is "8". In the East Fergana (Uzgen Basin) region are coals with much higher rank, ranging up to anthracite.
Parameters described later in this chapter that may be used for various rank classifications are vitrinite reflectance, volatile matter (mineral-free or ash-free), heating value (various bases), and moisture content (inherent bed moisture). Some data on tar extracts and humic materials are available in the literature on Kyrgyz coals; these data have been used for ranking brown coals (Mironov, 1982; Taits and Andreyeva, 1983).
b. Calorific yield
On an as-received basis most Kyrgyz coals can be produced with heat yields of 20 to 25 MJ/kg (4700-6000 kcal/kg, 8500-11000 Btu/lb). Some production is known to fall below 15 MJ/kg because of dilution with rock. Several mines yield coal with calorific value slightly higher than the above ranges, and coals occur in the East Fergana (Uzgen) region with 30 MJ/kg and probably higher. This chapter lists calorific values in MJ/kg, kcal/kg, and Btu/lb, on as-received and dry bases, and calculated to mineral- and ash-free bases.
c. Mineral and rock content, and ash fusion properties
The mineral and rock contents of coal are usually reported somewhat indirectly, as weight percent ash after high-temperature ashing, dry-coal basis. Data over many years show that the Kyrgyz mines produced coal with ash values as low as 3% and as high as 40%, but most mines were able to produce coals with 10-20% average ash through much of their history (table 7; Solpuyev, 1994). We report ash, both dry and as-received basis and some values of calculated mineral content for samples we collected (tables 4, 7, 10). We report 4-point softening and fusion data for ash from our samples, under both reducing and oxidizing conditions (table 14), but we do not yet have information on boiler practice in heat or electricity stations in Kyrgyzstan with which to evaluate these ash data with respect to fouling of combustion chambers.
d. Organic type of coal
Two very different ways of reporting organic type are common: 1) Visible identification of macerals under the petrographic microscope, 2) Indirect indication from chemical analysis, usually just by ratios of hydrogen, carbon and oxygen. This chapter gives both maceral group petrographic analyses and chemical analyses.
Petrographically, Kyrgyz coals have a greater range of composition than most U.S. and European coals -- mainly a greater content of inertinite. We did not find coals with large amounts of liptinite in our samples nor in the literature. The Kyrgyz coals with high inertinite content appear to be similar to some other high-inertinite coals in the former Soviet Union; they may be similar to many high-inertinite coals of India, Australia or South Africa.
Chemically, the Kyrgyz coals have lower H/C ratios for a given rank than many North American and European coals, and some Kyrgyz coals have element compositions that place them clearly in the "fusinization development line" which characterizes development of coals under oxidizing conditions.
e. Sulfur and other inorganic elements
The sulfur content of Kyrgyz coals is relatively low, with major coals reported to have not over 2% on average, but the range is very great, with individual values from several important mines as high as 7%(tables 7, 9; fig. 18). Three of our samples slightly exceeded 2%. We report data on three forms of sulfur in the coal (pyritic, organic, sulfate) (table 4) , but comparable data from the literature on Kyrgyz coals are sparse. We report the major and minor elements in coal ash (tables 11, 12, 13), which range greatly, but that is not unusual. Similar older data from Kyrgyzstan are reported. We report trace element content of our samples and explain their environmental significance, but have not yet found comparable older data on Kyrgyz coals.
2. Data and samples of this study
a. Kinds of raw data included
This chapter lists basic properties that characterize coals in Kyrgyzstan based on fifteen mine samples collected by the USGS at twelve separate deposits representative of existing mines and several potentially significant new mines. The data include the following which are commonly requested internationally to evaluate coals for utility combustion (Unsworth and others, 1991).
In addition, we provide data used to evaluate processing and industrial use and environmental consequences of coal use.
On numerous graphs we show the relationships between various parameters and also their general validity. Additionally, we compare these data with some published and unpublished older data on Kyrgyz coals, mostly from the Soviet era, so that the large body of older information on coal quality can be judged by our data.
b. Information not included
There is a significant amount of information on Kyrgyz coals which we do not present or discuss here because it is beyond the scope of this preliminary survey. For example, tests of pyrolysis behavior and products, mineral transformations at high temperature, semicoke and coke formation, sorptive properties, tar production and type, briquetting and pelleting tests, extraction with organic solvents, humic acid fractionation, etc. The potential value of such information is difficult to judge, for in the most accessible sources the data are usually presented isolated from the basic information about the character of each sample. However, the systematic field and laboratory records from the Soviet era appear to be preserved in large measure -- though the loss of access because of retirements, switching jobs, and emigration and even the physical loss or disorganization of records may be happening at an accelerating rate.
c. Source and nature of samples
The USGS team sampled coals from fresh mine faces at most sites (Table 3). Where possible, channels of consistent depth across the exposed coal were cut and bagged at the face. At several sites only grab samples were obtainable. Rarely was the entire seam sampled, however, because it was not exposed entirely or because hazardous and precipitous walls overhung it in surface pits. In several underground mines the samples represent nearly the entire mined seam. Samples were put in heavy plastic bags which were folded and sealed with tape; then each bag was put in a second bag and taped again. The intention was to make possible "bed moisture" determination. In this we succeeded well as judged by comparison of "as received" and "equilibrium" moisture, figure 7.
3. General coal chemistry (USGS samples)
Data from the basic analyses of coal are listed in table 4. They are repeated in figures with other data farther in this chapter.
Various coal users in different countries are accustomed to reporting data on different basis, for example "as-received" and "as-analyzed"; "with inherent moisture", "bed-moist" and "in-mine"; "working","as-shipped" and "as-delivered"; "air-dry" (room temperature) and "air-dry" (107C); "ash-free" and "mineral matter-free" (calculated from ash or weight of low-temperature ash). It is beyond the scope of this report to report data on all bases used, and in many cases it is difficult to determine accurately the true basis of an analysis. We attempt to present data in several forms 1) Common U.S. practice, 2) Common practice in the former Soviet Union (the basis of most Kyrgyz data), 3) Common international trade practice. Most of the basic analyses of the USGS samples were done under ASTM standards, as listed on the sheets of raw data in the appendix.
4. Coal rank / Organic maturation
a. Standard ASTM rank: Calorific value and volatile matter (or fixed carbon)
In US practice, "rank", the degree of metamorphism or progressive alteration in the natural series from lignite to anthracite, is used in a restricted way for multiple channel samples that represent a seam (or designated part) in a particular region. None of our samples, and probably none of the samples listed in the literature, fit that restriction. However, the ASTM term Apparent Rank is valid for our samples since we describe the conditions of the sampling. The apparent ASTM rank of our samples is on table 5. We use "rank" also as a geological term equivalent to "organic maturation", but only the rank on table 5 is ASTM apparent rank.
b. USSR rank of coals sampled by the USGS
With the exception of Kyzyl-Bulak (sample K-13), rank of coals at all the sites sampled by the USGS has been reported in the literature, and summarized recently by Solpuyev (1994). His USSR rank designation is in the right column on table 5. Most coals mined in Kyrgyzstan are the highest brown coal rank (B3) or the lowest stone coal rank (D, Long Flame). Our samples are representative, and five of the mines we sampled are cited as " B3" coal and seven are cited as " D" coal.
Because most coals of Kyrgyzstan are low rank (subbituminous and high volatile C bituminous [B3 and D]), the apparent rank is measured by heating value (calculated mineral-free). Only one of our samples, K-6, has relatively high rank and hence in the ASTM system is classified by volatile matter. Other parameters, such as elemental carbon, bed moisture, and vitrinite reflectance, are good measures of geological rank and are discussed in the following paragraphs on character of Kyrgyz coals.
c. Rank by elemental carbon, %Cdaf
Elemental carbon is a good chemical measure of rank, with perhaps the widest useful range of the chemical parameters. It is useful through the entire range of bituminous coals and anthracites, though volatile matter is better in the medium- and low-volatile bituminous range. The relationship between elemental carbon and calorific value, two different measures of rank, forms a fairly narrow band (fig. 8), which shows the consistency of our sampling and our analysis.
d. Rank by vitrinite reflectance, %Ro
Vitrinite reflectance indicates rank over a wide range like elemental carbon does; reflectance has the additional advantage that it is more independent of coal type, and it is not influenced by minerals so can be used to rank coaly shales (but not reliably oil shales). The vitrinite reflectance of our Kyrgyz coals is on table 6 and is plotted against other rank measures: elemental carbon, figure 9; calorific value, figure 10; and fixed carbon, figure 11.
5. General proximate and ultimate data
Basic analytical data of the USGS samples are in table 4 and are discussed earlier in this chapter. We gathered published and unpublished older data on Kyrgyz coals, some of which is on table 7, in order to relate the US data to data produced under sampling and analytical practice in the former USSR. We include here mainly information for the mines and seams we sampled.
The mineral matter in coal is not usually analyzed, but ash left after burning coal at high temperature is used as an inexpensive means to estimate the mineral content of coal, and the ash in a laboratory test is also an approximation of the ash left from industrial combustion. The ash reported in table 4 results from ASTM analysis at 750oC. ISO standard ash determination is at 815oC, and recent practice in the USSR agrees with ISO (800-830oC range). Note, however, that USSR practice uses a different slow heating sequence in analysis of brown coals and in analysis of stone coals; since produced Kyrgyz coals are about half brown coal and half stone coal, there may be some discontinuity in the Soviet data on ash. USGS analyses for this project included ashing also at 900oC in preparation for analysis of the major / minor / trace elements. All but three samples yielded equal or slightly lower ash at the higher temperature, but the difference is small (table 10).
6. Organic matter composition and type
a. Maceral composition (group analysis)
We petrographically analyzed the maceral group proportions in the USGS samples by visual point counting on polished sections of crushed coal. The resulting data are, therefore, volume estimates based on area -- not weight -- of each component. Table 8 lists the results on three bases: 1) Total point count of four maceral groups plus minerals, 2) Mineral percent plus percentage of four maceral groups on a mineral-free basis, 3) Three maceral group percentages (vitrinite, inertinite, liptinite) on a mineral- and mixtinite-free basis. Mixtinite is very fine organic matter, which can not be identified reliably by optical microscopy, mixed with some fine mineral matter. Using blue-excitation fluorescence it is seen to include much liptinite, but point counting this liptinite is unreliable because fluorescent objects below the polished surface could be counted by mistake. We estimate that the mixtinite contains roughly 1/3 vitrinite, 1/3 minerals, 1/3 liptinite, and only a small amount of inertinite. If the mixtinite-free data on table 8 had an added share of liptinite to account for the liptinite in the mixtinite, the indicated liptinite content would be increased significantly, by several percent, in the high-mixtinite samples.
Figure 12 is a ternary diagram of the maceral group percentages which shows the great range of composition and allows comparison with other data. The great abundance and range of inertinite places many Kyrgyz coals far outside the range of most U.S. and European coals. This should be kept firmly in mind by engineers and other coal specialists. Technology of some important Jurassic coals in the former USSR and technology of "Gondwana" coals from India and Pakistan, Australia, South Africa, and Brazil may be more applicable to Kyrgyz coals than would be technology familiar in North America and Europe.
Petrographic analyses from older USSR literature and unpublished analyses given in Solpuyev (1994) are compared with our analyses of samples from the same mines (fig.13). In about half the cases we have information that the samples reported are from the same seam we sampled. The agreement is notable -- much better than agreement of our analyses of major and minor elements, for instance, even though maceral analyses can be very subjective. One explanation for the good agreement comes to mind: Maceral analyses are rarely done on chance grab or run-of-mine samples, but usually on high quality channel samples or core.
b. Elemental H, C, N, O composition
The type of organic matter in coal is indicated chemically by the content of hydrogen, carbon, and oxygen. Normally the nitrogen and sulfur content is reported at the same time as other organic constituents, though their content is not easily connected with type of organic matter. Unlike petrographic determination of the organic type of coal, the chemical determination is complicated by large chemical changes as the coal changes from lignite to higher rank. Hence, normally only chemical ratios and cross relations between parameters can be used to determine organic type. Figure 14, a classical van Krevelen diagram, shows the low H/C ratio at a given rank (measured by O/C) of many Kyrgyz coals resulting from their high inertinite content. Figure 15 shows the same low hydrogen content, relative to rank as measured by vitrinite reflectance. Our data on hydrogen does not agree very well with data from the literature on Kyrgyz coals. Figure 16 indicates the linear regression line of the LITERATURE / USGS plot for hydrogen, compared with a 1:1 perfect agreement line. Figure 17 shows the same comparison for elemental carbon, with much better agreement. Note, however, the great spread of values from the literature in both cases. We believe that this spread results mainly from differences in sampling and the natural variation in the coals from a given site, not from analytical discrepancies.
7. Sulfur
Sulfur content of the 15 USGS coal samples ranges from 0.32 percent to 2.53 percent (table 9). There is some agreement between the sulfur in the USGS samples and sulfur reported in the literature (fig.18), but also a striking deviation in coals determined by the USGS to contain relatively high sulfur. It is unlikely that this deviation results from analytical discrepancies. More likely, samples reported in the literature may have been "premium" material from a given mine or seam. Sulfate sulfur ranges from 0.04 percent to 0.15 percent and probably represents the oxidation of pyritic sulfur to ferrous sulfate that took place during the interval of time between sampling of the coal and the analyses of the coal. Alternatively, in some instances there may have been some oxidation of the coal prior to our taking the sample. The sum of the sulfate sulfur and the pyritic sulfur is approximately the same as the amount of organic sulfur in most samples. This agrees with a general rule that in much coals half the sulfur is pyritic and half is organic.
There is no reason to expect the presence of any significant amount of any sulfide mineral in these coals other than pyrite or marcasite (both FeS2). There is excess iron available in each sample to stoichiometrically combine with the sulfur to make the FeS2.
Coal utilization within the United States is greatly influenced by federal regulations based on the Clean Air Act of 1970, that set standards for emissions of sulfur dioxide (SO2) from new steam electricity generating plants. The limit was set at 1.2 pounds of SO2 per million Btu of heat input. Coals that meet the requirements are referred to as "compliance " coals. Four of the 15 samples collected and analyzed meet the standards for compliance (less than 1.2 pounds SO2 per million Btu): three are close to being compliance and would produce between 1.2 and 1.3 pounds per million Btu; and the remaining eight samples are non-compliance as we sampled them.
In the United States, there are several ways in which a non-compliance coal could be utilized in a new electricity generating facility. These include blending the higher sulfur coal with a coal of lower sulfur content, cleaning the coal prior to burning to reduce the pyritic sulfur, and removal of the SO2 from the stack gasses by scrubbing the combustion gasses. Presently there is no means of reducing SO2 emissions in power and heat stations in Kyrgyzstan to a level comparable to U.S. standards, although the SO2 emissions are less than in other CIS countries (Mezgin and others, 1993). In 1996 refurbishment of boilers in the main Bishkek thermoelectric plant is scheduled; possibly emissions will be reduced as a result.
8. Major, minor and trace elements (mainly in minerals)
a. Major and minor elements
Mineral matter in coal consists primarily of the following major and minor elements: Si, Al, Fe, Mg, Ca, Na, K, Ti, P, and Mn. We have reported the concentrations of these as oxides of the elements in percentage of the ash of the coal in tables 10 and 11. They are also shown as elements (not oxides) in percent of the total coal sample in tables 12 and 13, later in this chapter.
Coals in which the mineral matter consists principally of alumino-silicate minerals, generally representing the detrital material which had washed into the peat forming environment (swamp), have relatively high SiO2 and Al2O3 contents. Coals that are high in Fe2O3 and CaO represent either the deposition of iron and calcium carbonate minerals early in the coal formation process, or the precipitation of these minerals along fractures and bedding following coal formation.
Combustion Engineers have developed several indices with which to classify the tendency of a coal to form bonded deposits on the boiler. These are referred to as fouling indices and generally take the form:
Fouling index= (base/acid) x Na2O, where base is the sum of CaO, Fe2O3, MgO, Na2O, and K2O. Acid is the sum of SiO2, Al2O3, and TiO2.
Using this formula, several of the coals analyzed have fouling indices in the range that might cause concern if the coals were being considered for use in a large pulverized-coal fired steam boiler. However, these measures of fouling are based on experiences with, primarily, bituminous coals of North America, are not precise, and would only indicate the need for further testing of the
coals with the higher fouling index values; something that would be done in case of any further major development of coals in the Kyrgyz Republic. In lower rank coals (lignite) the Na2O content is more heavily weighted when fouling is predicted and only coals with less than 2% Na2O are considered to be "low fouling".
The limited number of samples reported upon here makes it difficult to generalize concerning the major elements in coals of the Kyrgyz Republic. We do recognize significant variability within a very small area. For example, samples taken only a few tens of meters apart, from two different coal beds at Dzhergalan (samples K4 and K5) exhibit very different ash chemistry. In sample K4 the "base elements" of Fe2O3 and CaO are dominant, whereas in K5 the "acid elements" of SiO2 and Al2O3 are dominant.
b. Trace elements
Trace elements determined on the 15 coal samples are shown in tables 12 and 13 and include nearly all elements of environmental and industrial concern. The values were compared with average values for coals of the world, or of the United States for those elements for which world-wide data were not available, and anomalous values were noted. For this purpose, "anomalous" was defined as one tenth or less than the mean or ten times greater than the world-wide mean of the element.
The most significant observation concerning the trace element data is that there is very little that is worth noting about it. There were very few values that were high relative to the world-wide means. They are limited to a single manganese value in a sample from Kok Yangak (K10) and three elevated zinc values (K3, K13, and K14). The mode of occurrence of the Mn is not known and the single high value requires validation. The Mn may be associated with carbonate mineral, but this is only conjecture at this time.
Zinc values are significantly higher than the world coal average in three of the samples. Elevated zinc values generally indicate the presence of sphalerite (ZnS) in coals, and the sphalerite generally occurs along fractures in the coal. Zinc is not considered a problem element in coal inasmuch as it is refractory and remains in the ash when the coal is burned.
Zinc is the only one of the heavy metals to be found in elevated concentrations in the coals sampled for this report. Other elements, which are of interest because of their potential environmental impacts, such as As, Be, Cd, Co, Cr, Ni, P, Pb, and U are all present in amounts similar to or less than the world-wide averages. The relatively low concentrations of Se, Hg, And Cl, which are also potentially deleterious elements, were mentioned previously. The number of samples collected was certainly limited and, although they came from many different coal regions, they can not be considered to represent all of the coals of the Kyrgyz Republic. However the uniformly low trace element concentrations and the very small number of elevated values suggests that trace and minor element concentrations in Kyrgyz coals should not, generally, pose a problem for the continued mining and utilization of the coals.
c. Environmentally significant or volatile elements analyzed in whole coal
In addition to determining the organic elements C, H., O, and N, 45 additional major, minor and trace elements were also determined (tables 12 , 13 ). Several of those were selected because of their potential deleterious effects on coal utilization or on the environment. These include Cl, Se, and Hg.
The amount of chlorine reported was generally below the lower limit of detection of 0.015% and the highest value reported was 0.089%. Boiler operators are generally not concerned about chlorine in coal unless the value exceeds 0.20%. All of the coals sampled were well below the level at which one would be concerned.
Mercury and selenium are of potential concern in coal combustion because the elements are volatile and, when in significant concentration, are deleterious to animals, including humans. Concentrations of these two elements in the 15 coal samples from the Kyrgyz Republic that we have analyzed are approximately an order of magnitude less than (one-tenth the value) the average concentration of these elements in other coals of the world
9. Technical character of coals
a. Heating value
The heating value of coals is determined in the laboratory in a bomb calorimeter, but there are many different ways to report the results -- most only roughly indicate the natural coal with moisture and mineral matter as it is actually used. In some countries, such as the US, the results are commonly reported on a moist basis, but with mineral matter calculated to zero. This requires that the samples actually are of such good quality that bed moisture analysis is valid. Figure 19 shows the moist, mineral-free calorific value of the bed-moist USGS samples from Kyrgyzstan, in MJ/kg. Most of the data available in the literature on Kyrgyz coals are dry basis, ash free; such values for the USGS samples are shown in figures 20 and 21. Heating value reported previously for Kyrgyz coals ranges widely among samples from a given mine (fig.22), probably because the character of samples was not well defined when collected or when the results were reported. The results from the USGS samples agree roughly in most cases with the mean of values reported from earlier analyses (fig.22).
b. Ash fusion temperature (reducing / oxidizing)
Ash fusion temperatures are used to predict the slagging and fouling character of coals during combustion. Table 14 shows our analyses for coals from Kyrgyzstan, both in reducing and oxidizing conditions of analysis. The four temperatures reported are initial deformation, softening, hemisphere and fluid (flow) temperature. Our data show temperatures in reducing atmosphere slightly lower than in oxidizing atmosphere, which is characteristic of iron-bearing coals. Ash fusion temperature may be useful mainly where coals are stoker fired, not fed pulverized, but we consider they should be available for our suite of samples because our data are probably the only information of familiar type that combustion engineers outside the former USSR would have on Kyrgyz coals.
c. Hardgrove Grindability index
The Hardgrove Grindability index (HGI) is widely used in engineering of coal combustion. The HGI for the USGS samples of Kyrgyz coals are in table 14. A higher HGI generally predicts coals that are more easily pulverized with lower energy cost, of importance in designing coal-fired generators. Perhaps more important, however, in the present Kyrgyz coal industry is the problem of excess fines produced in mining and handling coal. The HGI may be a good predictor of which coals should be mined to maximize chunk products and which to maximize fine products for particular markets. However, to realize that use of the HGI test, one would need maceral analysis, for Unsworth and others (1991) indicate strong influence of maceral composition on coal breakage and on the HGI.
d. Free swelling index
The free swelling index (FSI) is an inexpensive test to predict the swelling and caking properties of coal. In addition, where coals are known to swell or cake normally, low FSI values can indicate weathering in situ or in storage, which lowers the value of coal. The use in this present study is very limited, for the coals presently produced in Kyrgyzstan are mostly not caking. Two of the coals sampled could have been expected to cake or agglomerate -- the low-volatile bituminous coal from Kara-Tyube (K-6) and the high-volatile B bituminous coal from Kum-Bel (K-11) (table 5). The latter does have FSI of 4.0 as expected, but the former shows FSI of 0 (tables 4, 14), probably because the rank is too high to maintain caking properties, but possibly because the sample was weathered.
10. Sources of additional similar data on Kyrgyz coals
At the present time the data resulting from this project may be the only information on Kyrgyz coals in forms familiar to engineers and geologists who work in the US and elsewhere outside the former USSR. However, there is much scattered information on Kyrgyz coals in the Russian-language technical literature, though more on geology than technology. Two recent publications on properties and technology of Kyrgyz coals are Barsanayev, S. B., editor, 1991, Improving methods of exploiting coal deposits of Central Asia (mainly physical properties of coal and rock in mining) and Dzhamanbayev, A. S., 1983, Coals of Kyrgyzstan and rational ways of using them (mainly chemical and industrial and briquetting properties).
Additionally there is a very large body of "fond" information and raw data on file at the Ministry of Geology and Mineral Resources in Bishkek. Probably much of that same information is preserved in Tashkent, Uzbekistan (from which much coal work in all of Central Asia was organized) and in Russia -- but access to information on particular coals or sites is likely to be difficult. Both "fond" and published information on properties, geology and resources of Kyrgyz coals have been compiled recently by T.S. Solpuyev, head of the Coal Division of the Ministry of Geology and Mineral Resources of the Republic of Kyrgyzstan (Solpuyev, T.S., 1994). Solpuyev's monumental 420 page compilation is particularly valuable because of his great practical field experience in Kyrgyz coal regions and because it serves to identify many unpublished works on coals of Kyrgyzstan.
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