Genesis, evolution and economic significance of the northern and the eastern parts of Karapınar-Ayrancı coal deposit

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Date

2016-07-25

Authors

Oskay, Rıza Görkem

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Abstract

During the last two decades electric power demand in Turkey is steadily increasing. As total installed capacity is being insufficient for covering the increasing demand, several new power plants fed with imported natural gas are being installed that increased the dependence of the country on imported resources. In this sense domestic coal, particularly this of low rank, is becoming important for the country to decrease the dependence on imported resources. Total coal reserves of Turkey amount to ~13 Gt with low-rank coals (i.e. lignite and sub-bituminous) sharing the greatest part. Karapınar-Ayrancı coal deposit is recently explored and proved being very important due to both its large reserves (1.8 Gt) and the geographical location in south central Turkey. There are plans to exploit the coal for power generation; therefore, determination of its potential environmental impacts and technical problems to be caused by coal is essential. The aim of the present study is to identify the palaeoenvironmental conditions during peat accumulation in the eastern and northern parts of Karapınar-Ayrancı coal deposit; furthermore, the potential environmental impacts from future coal mining and utilization, as well as potential technical problems from coal burning are assessed and measures are proposed. Several (coal and intercalating inorganic sediment) samples were picked up from cores obtained from the eastern and the northern parts of the deposit during the final exploration stages. The samples were analysed for mainly their petrological, mineralogical and geochemical features. Coal seam displays total thickness varying from 12.05 to 40.20 m with cumulative thickness of coal layers ranging from 2.00 to 9.10 m. The inorganic intercalations consist mainly of claystone, mudstone and marl, whereas the coal layers contain several mollusc-bearing layers. Matrix lithotype is the most common along with the mineral-rich one; char-rich lithotype is observed in the eastern part of the deposit only. The coal displays high ash yield (37.5 wt.%, on dry basis), and high contents in volatile matter (avg. 53.1 wt.%, on dry, ash-free basis), total sulphur (avg. 8.6%, on dry, ash-free basis) and hydrogen (avg. 6.8%, on dry, ash-free basis); its calorific value is low (avg. 11.71 MJ/kg, on moist, ash-free basis); it is a low-grade, low-rank coal (lignite). Huminite is the dominant maceral group in both parts of the deposit, whereas liptinite and inertinite display variable concentrations. In the northern part inertinite has generally lower values than in the eastern part. Telohuminite and detrohuminite are the most common huminite subgroups. Inertodetrinite and fusinite are the most common inertinite macerals. Liptinite is mostly represented by liptodetrinite and resinite (fluorinite type). X-ray diffractometry was performed on coal, inorganic sediments and coal ash residues produced after combustion at 750°C. Silicate minerals proved being common in coal; quartz is contained in all coal samples, whereas clay minerals, mica and feldspar show variable concentrations. Sulphur-bearing minerals are presented by sulphates and sulphides in coal. Bassanite is common and gypsum rare, whereas pyrite is the only sulphide contained mostly as minor phase. Carbonate is rarely detected in mollusc-bearing samples only. Halite is contained in coal from eastern part of the deposit. Inorganic samples display similar mineral composition, with the exception of carbonate minerals being dominant phases. Aluminosilicates and Ca-Mg silicates are dominant phases in the coal ash residues; oxides, sulphates (mostly anhydrite) and carbonate minerals (mostly dolomite) are usually contained as minor phases. The predominance of silicates can be explained by reaction of oxides with silicate melt or solid-phase reactions during ashing. Elemental composition of coal from both parts of the deposit shows slight variations due to different geological background and position within the palaeomire. Nevertheless, Al and Fe proved to be major elements (1.18% and 0.48%, respectively). From the remaining the following elements have been determined: As, B, Ba, Be, Bi, Cd, Cs, Co, Cr, Cs, Cu, F, Ga, Ge, Li, Mn, Mo, Ni, Pb, Rb, Re, Sr, Ti, U, V, W, Zn, and Zr. Most of these show intermediate affiliation; however, their mode of occurrence is variable from site to site in the deposit. In the northern part most elements are mostly affiliated with inorganic matter (mostly clastic-aluminosilicate minerals), whereas intermediate affiliation is more common in eastern part. Coal facies indices and lithological features of coal seam suggest that peat was accumulating under pure telmatic to limnotelmatic conditions; however, peat accumulation was ceased several times by water-level rising. Therefore, clastic (e.g. quartz, clay minerals) and authigenic (e.g. pyrite) mineral concentrations are variable. Similar trends are also obvious from elemental composition. High total sulphur, evaporite mineral (e.g. halite) and boron contents point to brackish-saline conditions during peat accumulation; however, high sulphur contents can be related to alkaline conditions. The presence of gastropod fragments along with silica-gel cavity infillings indicates alkaline conditions too. These can be related with a sulphate-rich karstic aquifer, which contributed to palaeomire water supply. Boron and chloride ions can be easily mobilised by circulating fluids within coal, thus, enriching it. Nevertheless, gathered gastropod and ostracod fauna also points to freshwater conditions. All these point out that in the studied parts of the deposit peat accumulated under mesotrophic conditions and the palaeomire was supplied by sulphate-rich, neutral to alkaline groundwater. The total geological reserves of entire Karapınar-Ayrancı deposit and the low calorific values of coal make it suitable as a feeding coal in power plants. On the other hand, the mineralogical and elemental compositions of the coal layers in the studied parts of the deposit could cause environmental impacts (e.g. acid mine drainage, hazardous and toxic gas emissions) and technical problems (e.g. corrosion, slagging) in the boilers of future power plants. These problems can be reduced applying proper coal beneficiation and flue gas control. Simple coal washing tests show some coal quality improvement. Thus, several further studies are essential before final decision on coal utilization and installation of power plants. Beyond these handicaps, Karapınar-Ayrancı coal’s exploitation might have great socio-economic importance at local, regional and national levels.

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Karapinar-Ayranci basin, Coal petrology/petrography, Palaeoenvironmental reconstruction, Lignite, Turkey, Peat-forming environment, Environmental impact, Mineralogy

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