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Energy & Fuels 2005, 19, 882-891
Trace Elements in High-Sulfur Assam Coals from the Makum Coalfield in the Northeastern Region of India Samit Mukherjee*,† and S. K. Srivastava Fuel Science Division, Central Fuel Research Institute (CSIR), Dhanbad 828 108 (Jharkhand), India Received September 6, 2004. Revised Manuscript Received January 26, 2005
The trace-element contents in several high-sulfur Assam coal samples from the Baragolai colliery of the Makum coalfield were estimated using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The coals are relatively rich in gallium (8-122 ppm) and poor in arsenic (0.04-0.24 ppm), boron (0.02-0.11 ppm), selenium (0.04-0.24 ppm), germanium (0.19-1.19 ppm), and barium (0.38-2.39 ppm), relative to most of the world coal. The contents of copper (9.8630.35 ppm), manganese (15.27-63.81 ppm), antimony (0.22-1.19 ppm), tin (0.18-1.43 ppm), lead (5.06-24.13 ppm), and vanadium (5.92-64.29 ppm) are in the same range as those for world coal. The mode of occurrence of all the aforementioned trace elements present in these coals have been determined. Assam coals have the following trace elements: arsenic, boron, lead, and selenium of great concern; copper and vanadium of moderate concern; barium, germanium, and manganese of minor concern; and tin of concern but present in negligible concentration in coal. The trace elements in Assam coals are harmful because of several reasons: (i) the presence of quartz (respirable R-quartz during mining and sample preparation leading to the disease silicosis); (ii) leaching of arsenic, boron, lead, selenium, copper, vanadium, barium, germanium, and manganese from fly ash/bottom ash due to regular rains, contaminating the water table; (iii) pyrite oxidation, ultimately leading to hydrated ferric oxide, which is an undesirable addition to surface water; and (iv) SO2 emission from thermal power plants, brickmaking, the cement industry, etc. The problem in the near future will be alarming with the growth of heat- and power-generating industries in and around the northeastern region of India, based on these coals. In this study on Assam coals, some trace elements such as gallium, germanium, and tin, which are generally less common, were observed; however, some trace elements, such as beryllium, cadmium, chromium, cobalt, fluorine, and nickel, that are present in most of the world coals were not observed. There seems to be no relation between the region of rank of coal on traceelement concentration. The concentration of trace elements in Assam coals have been found to be less than that in the Earth’s crust.
Introduction Sulfur and several trace elements in coal undergo major changes during coal combustion and lead to atmospheric pollution. Finkelman1 reported the presence of 25 trace elements, including antimony, arsenic, barium, beryllium, boron, cadmium, chlorine, chromium, cobalt, copper, fluorine, lead, mercury, manganese, molybdenum, nickel, potassium, selenium, silver, thallium, thorium, tin, vanadium, uranium, and zinc, in the coal. The concentrations of these elements vary from coal to coal, depending on the depositional environment and the various processes by which these elements have entered the coal during different stages * Author to whom correspondence should be addressed. Telephone: +91-326-2381001-10, ext. 336. Fax: +91-326-2381113. † Present address: State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, PRC. Telephone: + 86-351-4053832. Fax: + 86-351-4041153. E-mail address:
[email protected],
[email protected]. (1) Finkelman, R. B. Modes of Occurrence of Environmentally Sensitive Trace Elements of Coal. In Environmental Aspects of Trace Elements of Coal; Goodarzi, D. J., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1995; pp 24-50.
of coalification. With the increasing use of coal, the growing impact on the environment from potentially hazardous trace elements becomes a great concern; thus, the information about the concentration, distribution, and occurrence of the trace elements in coal is of great importance. Coal contains most of the elements in the periodic table,2 and the data on the potentially hazardous trace elements are most urgently needed in coal utilization and environmental assessment. PECH3 has placed trace elements in coal and residues produced from coal resource development in five groups: (a) of greatest concern (arsenic, boron, cadmium, mercury, molybdenum, lead, selenium); (b) of moderate concern (chromium, copper, fluorine, nickel, vanadium, zinc); (c) of minor concern (barium, bromine, chlorine, cobalt, germanium, lithium, manganese, strontium); (d) radioactive elements (polonium, radium, ruthenium, thorium, uranium), and (e) of concern, but with negligible con(2) Swaine, D. J. Crit. Rev. Anal. Chem. 1985, 15, 315. (3) Panel on the Trace Element Geochemistry of Coal Resource Development Related to Health (PECH). Trace-Element Geochemistry of Coal Resource Development Related to Environmental Quality of Health; National Academy Press: Washington, DC, 1980; pp 1-8.
10.1021/ef049775+ CCC: $30.25 © 2005 American Chemical Society Published on Web 03/12/2005
High-Sulfur Assam Coals from Northeastern India
centrations in coal and coal residues (silver, beryllium, tin, thallium). Swaine and Goodarzi4 proposed that the trace elements were in four groups, based on their environmental relevance in coal: Group I elements (arsenic, cadmium, chromium, mercury, selenium), which are known to be hazardous in some circumstances; Group II A (boron, chlorine, fluorine, manganese, molybdenum, nickel, lead) and Group II B (beryllium, copper, phosphorus, thorium, uranium, vanadium, zinc) elementssparticularly boron, manganese and molybdenumswhich should be taken into consideration for leachates from wastes and for reclamation after mining; and Group III elements (barium, cobalt, antimony, tin, thallium), the concentrations of which in coals are not expected to give troublesome effects. The elements associated with the mineral matter mostly remain concentrated in the ash.5 Sulfur and some elements such as fluorine, chlorine, bromine, mercury, selenium, molybdenum, arsenic, copper, lead are discharged either in elementary or in compound form into the environment during coal combustion. For arsenic, beryllium, cadmium, mercury, manganese, nickel, lead, selenium, fluorine, vanadium, and uranium that are emitted by coal-fired power plants, domestic and industrial coal combustion have a toxic response in test systems.6 Arsenic, antimony, selenium, beryllium, cadmium, chromium, cobalt, nickel, lead, mercury, manganese, and radioactive nuclides (e.g., uranium) present in coal have been identified as hazardous air pollutants. Trace elements in Assam coals are also likely to effect the catalyst used for various coal conversion reactions such as gasification and liquefaction, because these coals are potential raw material for coal liquefaction. The modes of occurrence of potentially hazardous elements in coal are of significance in any attempt to reduce their mobilization from combustion. The mode of occurrence refers to how the element is chemically bound and physically distributed throughout the coal. Knowledge of an element’s mode of occurrence enables the prediction of its distribution in a coal deposit, as well as its behavior during coal preparation and combustion. The mode of occurrence of an element can be inferred from indirect evidences such as float and sink data, from statistical correlations with other elements, or with other coal characteristics such as ash yield or from the element’s geochemical characteristics or from its behavior during heating or leaching of the coal.7-9 Because Indian coals are of drift origin, the trace elements present in the original coal are predominantly associated with the mineral matter, although some of them may be organically associated, from their association with the original plant material from which the coal was formed. Thus, most of the elements are associated with the mineral matter of the Indian coal; however, certain elements have an organic affinity. Makum coalfield in Assam is situated in the Dibrugarh district, covering an area of 150 km2 (30 km long and 5 km wide). (4) Swaine, D. J.; Goodarzi, F. In Environmental Aspects of Trace Elements of Coal; Goodarzi, D. J., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1995; pp 1-4. (5) Gentzis, T.; Goodarzi, F. Can. Energy Sources 1997, 19, 259. (6) Finkelman, R. B. Fuel Process Technol. 1994, 39, 21. (7) Wang, J.; Takaya, A.; Tomita, A. Fuel 2004, 83, 651. (8) Wang, J.; Sharma, A.; Tomita, A. Energy Fuels 2003, 17, 29. (9) Wang, J.; Tomita, A. Energy Fuels 2003, 17, 954.
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It has a reserves of >250 million tons. The area is within the latitudes 27°16′ and 27°18′ North and longitudes 95° 43′ and 95° 55′ East.10 The coalfield is situated along the outermost flank of the Patkai range. The southern and southeastern sides of the coalfields are hills that rise abruptly to a height of 300-500 m from the alluvial plains of the Buridihing and Tirap rivers. These hilly ranges are traversed by the Namdung, Ledopani, and Tirap rivers, whose courses expose sections of the coalbearing Tikak Parbat formation.10 The coal available in Assam and other states of the northeastern region of India are sub-bituminous in rank and are characterized by low ash content, high sulfur content, double tar content, high fluidity, a perhydrous nature, high volatile matter content, and a high caking index (for the caking variety of coals). The coals generally contain 2-6% sulfur, in which 70-90% of the sulfur is in the organic form. The major uses of the coal are as fuel for boiler, cement, brickmaking, and blending (57%) for making hard coke for iron extraction in blast furnaces. The Assam state gets regular rains and, because of the hilly area, the chances of leaching of toxic trace elements present in coal kept on the surface and due to reclamation of mine, etc., as well as that which is due to presence of oxidized pyritic sulfur present in Assam coal, becomes extremely fair, leading to pollution of the water table. Little work on determination of the trace-element content in the northeastern India region coal has since been made.11-17 Most of these works were concentrated on determination of one or two trace elements in the coal. Mukherjee and Dutta,11 Choudhury et al.,12 and Banerjee et al.13 spectrophotometrically detected the presence of germanium in the range of 0.02-0.12% in Assam coal. Ghosh et al.14 detected gallium in very low concentration (50 ppm) in some Assam coal samples. The presence of several rare-earth elements (such as terbium, europium, erbium, neodymium, and samarium) have also been reported by some workers.15,16 Baruah et al.17 reported the presence of 22.8-32.0 ppm of gold in some coal samples from the Tirap, Tikak, and Baragolai collieries of the northeastern India coalfield. No investigator has performed any systematic study and tried to find the modes of occurrence of trace elements and their relations with ash and sulfur in coal. The present paper is an attempt to fill this gap in the literature. In the context of clean coal technology, this work has great value. Thus, the present communication reports the contents of several trace elements (such as arsenic, boron, barium, copper, gallium, germanium, manganese, lead, antimony, selenium, tin, and vanadium) in some of the Assam coal samples and their modes of occurrences and correlations with ash, its constituents, and sulfur in coal. (10) D. G. M. Assam, Misc. Publ; vol. 1, Directorate of Geology and Mining, Government of Assam, Gauhati, 1982. (11) Mukherjee, B.; Dutta, R. Sci. Cult. 1949, 14, 538. (12) Choudhury, J. K.; Dutta, P. B.; Ghose, S. J. Sci. Ind. Res., Sect. B 1952, 11B, 146. (13) Banerjee, N. K.; Rao, D. K.; Rao, H. S.; Lahiri, A. Chem. Age India 1967, 2, 63. (14) Ghosh, B.; Biswas, D.; Banerjee, N. N. Indian J. Technol. 1979, 17, 61. (15) Mukherjee, B. Nature 1949, 63, 402. (16) Mukherjee, B. Fuel 1950, 29, 264. (17) Baruah, M. K.; Kotoky, P.; Borah, G. C. Fuel 1998, 77, 1867.
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Table 1. Characterization of Baragolai Coal Samples Value characteristic proximate analysis (wt %, as-received basis) moisture ash volatile matter fixed carbon ultimate analysis (wt %, dmmf basis) carbon hydrogen nitrogen sulfur oxygen (diff) calorific value (kcal/kg) caking index sulfur distribution pyritic sulfate organic
sample I
sample II
sample III
sample IV
3.2 3.8 42.7 50.3
5.4 8.4 41.4 44.8
5.4 23.9 33.4 37.3
3.3 4.4 41.5 50.8
75.6 5.6 1.4 4.3 13.1 8015 23
68.8 5.1 1.5 3.2 21.4 7526 20
64.2 4.8 1.3 2.6 27.1 7508 18
74.9 5.1 1.2 2.4 16.4 7975 21
0.80 0.52 2.98
Experimental Section
0.64 0.41 2.15
0.60 0.20 1.80
0.45 0.10 1.85
Table 2. Characterization of Coal Ash Composition (wt %)
Four coal samples (denoted as I, II, III, and IV) used in the present investigation were collected from different working mines of the Baragolai colliery of the Makum coalfield in Assam, India. All the samples were crushed to -212 µm fineness. The proximate analysis of the samples was performed using standard methods (Indian Standard 1350 (part 1), 1984). The percentage of carbon, hydrogen, and nitrogen were estimated using a Perkin-Elmer model 2400 elemental analyzer, and the total sulfur was determined by the Eschka method (ASTM D 3177). The percentage of oxygen was calculated by difference. The forms of sulfur were determined following ASTM D 2492 methods. The calorific value was determined using a high-pressure oxygen bomb calorimeter (ASTM D 3286). The caking index was determined using a standard method.18 The trace elements in ash were analyzed by a fusion method that was described by Boar and Ingram19 and by Nadkarni.20 For this, an ∼0.15 g sample of the lowtemperature ash (LTA) ( III ≈ IV. The oxygen content in sample III is highest among the four samples, indicating that the sample has been partially oxidized, resulting in lower values of carbon, hydrogen, nitrogen, and sulfur. This also causes the caking index to be reduced. Coal Ash Characterization. The ash analyses of the coal samples are presented in Table 2, which clearly indicates that the major constituent of coal ash is silica, followed by alumina and iron oxide. In addition, oxides of magnesium, calcium, potassium, sodium, and titanium are also present. SO3 has also been observed to be present. The loss on ignition is very small. Source of Trace Elements. Trace elements are important because they are linked with environmental issues and the health of plants, animals, and human beings. Due weighting must be given to the essentiality, nonessentiality, and toxicity, which are dependent on the concentration, the forms of the elements (i.e., speciation, pH, and oxidation-reduction conditions), and other factors. There are two sources of trace elements: natural and anthropogenic. Natural sources include the weathering of rocks, volcanoes, sea spray, thermal springs, lake and river sediments, vegetation, and forest fires, whereas anthropogenic sources involve metal mining and smelting, coal combustion, oil and wood, industrial operations, waste disposal, agricultural
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activities, motor tyre wear, and cremation. Volcanic activity is a significant source of several trace elements such as bromine, selenium, and antimony.21 The main source of zinc may be a smelter, a power station, a volcano, fertilizer production, vegetation, or motor tyre wear, depending on the distance of the sampling location from the source. The concept of geochemical cycling (i.e., the movement of elements in the various parts of the Earth, including the atmosphere) is of prime relevance to the proper understanding of the overall role of trace elements and especially their environmental significance.22 It is important to mention here that the system is dynamic rather than static and the biogeochemical cycling is now well-recognized.23 Modes of Contamination of Trace Elements. The toxicity/harmful effects of trace elements are known and are beyond the scope of this work. However, it is important to know how the trace elements come into contact with human beings, plants, animals, etc. In Indian coals, mercury, which is being burned, has been reported to be present. Mercury is widely used in industry, agriculture, and medicine, but its short-term poisoning and long-term ill effects are of concern; hence, its widespread use is curtailed. At least half of the mercury emitted to the atmosphere comes from anthropogenic activities, including coal combustion, but natural sources including volcanoes, bedrock, soil water, and vegetation are also relevant.24 The volatility of mercury leads to enhancing its global atmospheric distribution. The trace elements come in contact with (or contaminates during) coal mining, coal crushing, coal storage, coal beneficiation, coal combustion, pyritic sulfur in coal, waste disposal, petroleum, radioactivity, reclamation after mining. Trace elements are not expected to create problems during underground mining except for (i) minor local effects due to soluble elements present in the mine water, and (ii) the fine and respirable R-quartz particles during drilling or even during the crushing of coal, which are inhaled by the miners, causing the disease silicosis, which may lead to death. However, after mining has ceased, there may be problems due to weathering of coal around the surface of the mine area, especially from the degradation of pyrite, producing acidic leachates. Such runoff contaminates nearby waterways with an unsightly brown slurry, adding unwanted trace elements. In open cast mining, there may be changes in (i) topography, (ii) water table and water quality, (iii) aeration of subsurface materials (such as sediments and carbonaceous sediments), and (iv) bacterial population of soil. In many coals, the oxidation of pyrite results in a reduction in pH. The sulfuric acid thus obtained during the oxidation may leach trace elements (such as arsenic and selenium) from the mineral matter of coal and nearby rocks. Under certain conditions, pyrite oxidation may produce a slurry of hydrated ferric oxide, as is observed in the northeastern region of India, which is an undesirable addition to nearby surface waters. Trace elements in the over-
burden and soil, which will be used in the reclamation of the open cut after mining, must be taken into consideration while deciding the nature of the revegetation. Some care should be taken with boron and manganese, which affect the growth of some plants under special conditions of pH and drainage. The selenium and molybdenum present in coal ash will not disturb the plant growth; however, too much of these elements may produce plants that could upset grazing animals. Long-term storage of coals after mining and before treatment is not common. Various beneficiation methods are used to produce clean coal. The main intention is to reduce the mineral matter with a consequent reduction in the transportation and other costs-enhancing combustion efficiency. There will be simultaneous reductions in various trace elements, especially those associated with sulfide and other minerals. Significant reductions have been reported for arsenic, chromium, manganese, nickel, lead, cadmium, cobalt, beryllium, and lesser reductions have been observed for mercury, antimony, and selenium. During beneficiation, there is a proportionate increase in the trace element contents in the reject material. Thus, care should be taken for disposal of washery rejects, because that will the contaminate local waters. The major use of coal in India is for the generation of power by combustion on a large scale, because 58% of commercial energy is from coal. This means that most of the trace elements will be released and redistributed into bottom ash, fly ash, fine fly ash, and the gaseous phase. The bottom ash remains in the combustion area in the furnace, whereas the fly ash is conveyed through the system where ∼90% can be removed by electrostatic precipitation or fabric filters. However, a small proportion, mostly fine particles
