Energy & Fuels 2009, 23, 2169–2175
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Quantitative Evaluation of Minerals in Lignites and Intraseam Sediments from the Achlada Basin, Northern Greece Nikolaos Koukouzas,*,† Colin R. Ward,‡ Dimitra Papanikolaou,† and Zhongsheng Li‡ Centre for Research and Technology Hellas, Institute for Solid Fuels Technology and Applications, Mesogeion AVe. 357-359, GR-15231 Halandri, Athens, Greece, and School of Biological, Earth and EnVironmental Sciences, UniVersity of New South Wales, Sydney, NSW 2052, Australia ReceiVed December 16, 2008. ReVised Manuscript ReceiVed January 27, 2009
Seven core samples (five lignite samples and two intraseam nonlignite rock samples) from the Achlada open-cut mine in northern Greece were characterized by X-ray diffraction (XRD) and X-ray fluorescence (XRF) techniques. Quantitative evaluation of the mineral phases in each sample was made from the powder X-ray diffractograms using Siroquant commercial interpretation software, which is based on Rietveld principles. The main minerals in the low-temperature ash (LTA) ash of the lignites are kaolinite and illite, with bassanite and quartz in minor proportions. The nonlignite rock samples mainly consist of illite, mica (2M1), and kaolinite (poorly ordered), along with quartz, chlorite (ferroan), feldspar (albite), rutile, and dolomite. Oriented-aggregate XRD study further shows the presence of smectite, and interstratified illite/smectite (I/S), in the clay fractions of the lignite and rock samples, with the mineral matter of the lignites being richer in kaolinite, smectite, and I/S than in mineral matter of the nonlignite materials. The differences in mineralogy between the lignite and the rock materials probably reflect selective concentration of minerals in the original peat during deposition, combined with authigenic precipitation of minerals such as kaolinte in the peat deposit. Inferred chemical analyses derived from the XRD data show reasonably good correlations with chemical data obtained by direct ash analysis, especially if the smectite and I/S are taken into account. This provides a link between mineralogical and chemical studies that may be valuable in evaluating the behavior of the lignite under different utilization conditions.
1. Introduction Lignite is the major source for the generation of electricity in Greece, where it is used to produce some of the leastexpensive and most cost-effective electric power within the European Community. There are 60 lignite basins in Greece,1 and the annual lignite production in 2006 was estimated at 62.50 Mt.2 According to data from the Public Power Corporation, this lignite was consumed to generate 31977 GWh of electricity. The exploitation of lignite in Greece has a very long history. Significant achievements and a large amount of experience, gained during many years of mining operations, have placed the Greek lignite-mining industry in the leading position in Europe.2,3 The Achlada open-cut mine is the most recently opened mine in Greece, having started operation in 2001. Its lignite output is ∼2.5 Mt per year, all of which is used by the Meliti (330 MW) power station.4 The lignite derived from the Achlada mine is higher in rank than the lignite from other mines in the region, giving it a higher heating value for equivalent ash percentages. The objective of the present paper is to examine the mineralogical composition of representative lignite samples from * Author to whom correspondence should be addressed. Tel.: +30 210 6501771. Fax: +30 210 6501598. E-mail:
[email protected]. † Centre for Research and Technology Hellas, Institute for Solid Fuels Technology and Applications. ‡ School of Biological, Earth and Environmental Sciences, University of New South Wales. (1) Koukouzas, N. Miner. Wealth 1998, 106, 53–68. (2) Kavouridis, K.; Koukouzas, N. Energy Policy 2008, 36, 693–703. (3) Koukouzas, N. Int. J. Coal Geol. 2007, 71, 276–286. (4) Koukouzas, N.; Vassilatos, C. J. Chem. Technol. Biotechnol. 2008, 83, 20–26.
Achlada, and also the associated intraseam nonlignite sediments, to correlate the data with the chemical composition of the lignite ash. The chemical composition of the combustion products from this coal characteristically shows high concentrations of calcium and sulfur, which are responsible for the unsuitability of the fly ash in concrete production. For example, the concentration of calcium for fly ash derived from the combustion of Achlada’s lignite ranges from 22% to 34%, while the concentration of sulfur fluctuates from 4% to 8%.5 2. Geological Setting The Achlada region is located in the eastern part of the Florina Basin in northwest Macedonia (Greece), extending in a NNW-SSE direction from Monastiri (Former Yugoslav Republic of Macedonia (FYROM)) up to the hills of Kozani through the cities of Florina, Amynteo, and Ptolemais (see Figure 1). The basin is almost 100 km wide.6 The largest lignite deposits of Greece, which were formed in the Monastiri-Florina-Ptolemais-Kozani graben, are classified into two types: Ptolemais-type “earthy” lignite and Komnina-type xylitic lignite.3 The Achlada lignite, which is of the xylitic type, is of Lower Neogene age, along with the other lignites of the Florina sedimentary basin (Vegora, Petres, Vevi, and Lofi). The Achlada lignite is older than the lignite of the Ptolemais type, which is (5) Koukouzas, N.; Tsikardani, E.; Papanikolaou, D. Fly Ash Utilisation Programme (FAUP), Technology Information, Forecasting & Assessment Council (TIFAC), Department of Science & Technology (DST), 2005. (6) Karakatsanis, S.; Koukouzas, N.; Pagonas, M.; Zelilidis, A. Bull. Geol. Soc. Greece 2007, Vol. XXXX, (Part 1), 76–84.
10.1021/ef8010993 CCC: $40.75 2009 American Chemical Society Published on Web 03/09/2009
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Figure 1. Geological map of the area studied, modified from the Institute of Geology and Mineral Exploration (IGME) Florina and Vevi geological map.
of Upper Neogene age and is found in Ptolemais, Proastio, Perdikas, and Amynteo. The strata beneath the basin include the Pelagonian zone of Palaeozoic and pre-Palaeozoic crystal schists, a Mesozoic carbonate cover, and ophiolites. The Neogene sediments that fill the basin contain lignite seams, on top of which fluviotorrential or terrestrial deposits are present.7 The lignite-bearing sediments are Late Miocene to Pliocene in age. The lowermost Neogene horizon, known as the Basal Conglomerate, begins the succession of sediments that fill the basin. The middle horizon (the Vevi-Achlada Formation), which is exposed in the Vevi and Achlada lignite mines, overlies the Basal Conglomerate and is represented by alternations of clayey, sandy, and marly sediments, as well as by lignite seams. Clayey diatomite and phosphatic nodules have also been identified in these sediments.8 The lignite seams and associated interseam sediments have a total thickness of ∼35 m. The uppermost horizon (Lofi Formation), overlying the Vevi-Achlada Formation, continues the Neogene succession, with alternations of clays, marly breccias, sandy conglomerates, and lime marl beds. On the top of this sandy-clay horizon is a marly limestone bed, which covers the entire basin. A thin cover of limnodeltaic, fluviotorrential, and terrestrial Quaternary sediments completes the lithological column in this part of the basin. 3. Materials and Methods Seven samples were collected from a mine exposure in the Achlada region, a column section of which is shown in Figure 2. These included five lignite samples and two rock samples. The total thickness of the section studied is 16.5 m, representing the upper part of the mining succession. The coding of the lignite and rock samples is presented in Table 1. The five lignite samples are C1, C2, C3, C4, and C5, and the two rock samples are R1 and R2. Proximate and ultimate analyses and heating value determinations were conducted, according to ASTM standards (see Table 2). The chemical composition of the lignite ashes, prepared at 815 °C, as well as that of the rocks, was determined by X-ray fluorescence spectrometry, using a Philips PW 2400 spectrometer and associated SuperQ software. (7) Kotis, Th.; Koukouzas, N.; Papanicolaou, C.; Foscolos, A.; Stamatakis, M. Ann. Geol. Pays Hell. 2004, 40, 143–158. (8) Koukouzas, N. Miner. Wealth 1992, 81, 39–52.
Figure 2. Column section represented by the seven samples collected for the study. Table 1. Thicknesses Represented by Lignite and Rock Samples sample code
sample description
interval thickness (m)
C1 R1 C2 C3 R2 C4 C5
xylite marl xylite marly lignite sand marly lignite lignite
3.5 0.3 5.0 1.5 0.2 2.0 4.0
Subsamples that were taken from the five lignite samples and the two rock samples were ground to fine powder. Subsequent analysis suggests that some of these were not fully equivalent to the samples detailed in Tables 1 and 2, because of inhomogeneities in the bulk material from which they were taken. However, they indicate a similar range of quality variation within the lignite seam. The powdered lignite samples were subjected to low-temperature oxygen-plasma ashing using an IPC four-chamber asher, as outlined in Australian Standard 1038, Part 22; the mass percentage of lowtemperature ash (LTA) was determined in each case. Each LTA was further powdered, and then analyzed via powder XRD, using a Philips Model PW1830 diffractometer with Cu KR radiation and a graphite monochromator. Diffractograms were run in a 2θ range of 2°-60°, with steps of 0.04° and a counting time of 2 s. The powdered rock samples were also analyzed in this way, without the low-temperature ashing procedure. Quantitative analyses of the mineral phases in each LTA or rock sample were made from the X-ray diffractograms using Siroquant, which is commercial interpretation software9 that is based on principles originally developed by Rietveld.10 Further details of the software interpretation process have been given by Ward et al.11
4. Results 4.1. Basic Lignite Properties. The Achlada lignite has a fixed carbon content in the range of 25.5% and 45.6%, daf (9) Taylor, J. C. Powder Diffr. 1991, 6, 2–9. (10) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65–71. (11) Ward, C. R.; Matulis, C E.; Taylor, J. C.; Dale, L. S. Int. J. Coal Geol. 2001, 46, 67–82.
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Table 2. Analysis Results and Energy Contents for the Five Lignite Samples C1-C5 (a) on a Dry Basis and (b) on a Dry, Ash-Free (daf) Basis Value parameter
C1
C2
C3
C4
C5
(a) Analysis Results and Energy Contents, Given on a Dry Basis proximate analysis (wt %, dry basis) ash 41.9 41.4 59.6 52.4 38.0 volatile matter 36.7 37.2 29.6 35.5 33.6 fixed carbon 21.4 21.4 10.8 12.1 28.4 ultimate analysis (wt %, dry basis) H 3.0 2.8 1.9 2.0 2.6 C 36.8 38.9 24.6 28.6 39.9 0.4 0.5 0.5 0.6 0.4 CO2 N 0.6 0.9 0.9 0.8 0.9 S 1.1 1.1 1.5 2.8 2.4 O 17.8 16.3 13.5 14.8 17.5 gross heating value 3658 3869 2358 2699 3872 (kcal/kg, dry basis) net heating value 3463 3667 2317 2533 3662 (kcal/kg, dry basis) (b) Analysis Results and Energy Contents, Given on a Dry, Ash-Free (daf) Basis proximate analysis (wt %, daf) volatile matter 63.1 63.6 68.7 74.6 54.2 fixed carbon 36.9 36.4 31.3 25.5 45.6 gross heating value 6292 6604 5835 5668 6247 (kcal/kg, daf) net heating value 5949 6256 5732 5320 5908 (kcal/kg, daf)
(Table 2b), which is similar to that of Ptolemais lignite (36%-48%, daf); the volatile matter content of Achlada lignite is in the range of 54.2%-74.6%, daf (see Table 2b), whereas that of Ptolemais lignite is in the range of 54%-61%, daf.12 The volatile matter percentage, in particular, is used as a basic parameter in the power station to select appropriate combustor operating conditions that will minimize the loss of unburned fuel and maintain good flame stability. Achlada lignite exhibits a lower moisture content (∼40%, on an as-received basis), compared to Ptolemais lignite (∼55%, on an as-received basis). The carbon content is in the range of 24.55%-39.94%, the hydrogen content is 1.86%-2.96%, the nitrogen content is 0.57%-0.93%, the oxygen content is 13.45%-17.54%, and the sulfur content is 1.06%-2.78%. The carbonate carbon (CO2) content is in the range of 0.39%-0.64%. The lignite samples in the present study have nitrogen contents of
