Applied Radiation and Isotopes 90 (2014) 53–57
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Elemental characterization of coal, fly ash, and bottom ash using an energy dispersive X-ray fluorescence technique M. Tiwari, S.K. Sahu, R.C. Bhangare, P.Y. Ajmal, G.G. Pandit n Environmental Monitoring and Assessment Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
H I G H L I G H T S
Concentrations of 18 elements were determined in coal and ash samples using EDXRF. Mineral quantification up to 95% was carried out for fly and bottom ash samples. Enrichment ratios of elements were calculated in combustion residue with respect to coal. Enrichment factor with respect to crustal average was estimated for ash samples.
art ic l e i nf o
a b s t r a c t
Article history: Received 3 December 2013 Received in revised form 4 February 2014 Accepted 1 March 2014 Available online 12 March 2014
A total of 18 elements viz. Si, Al, Fe, Ca, Mg, K, Na, Sr, V, Zn, Mn, Cr, Cu, Pb, Ni, Co, As and Cd were analyzed in coal, fly ash and bottom ash samples collected across India using an EDXRF technique. Various indices such as element enrichment ratio, enrichment factor (with respect to crustal average) and mineral composition were calculated. Around 95% of mass was reconstructed using the concentration of elements in this study for fly and bottom ash. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Fly ash Bottom ash Fusion point EDXRF Enrichment ratio
1. Introduction Coal has many important uses worldwide. The most significant uses of coal are in electricity generation, steel production and in cement manufacturing. According to the World Coal Association around 7.6 billion tons of coal was used worldwide which includes 1 billion tons of brown coal. Global coal consumption has grown faster than any other fuel since 2000. The five largest coal users China, USA, India, Russia and Japan account for 76% of total global coal usage. Electricity production from coal sources in India was 68.56% in 2009. As a fuel, coal refers to all coals and brown coal of both primary (including hard coal and lignite-brown coal) and derived (including patent fuel, coke oven coke, gas coke, coke oven gas, and blast furnace gas) origins. Poor-quality coal with high ash yield results in the generation of large amounts of fly and bottom ash of varying properties when used in coal-fired power plants.
n
Corresponding author. Tel.: þ 91 2225590233; fax: þ91 225505151. E-mail address: [email protected] (G.G. Pandit).
http://dx.doi.org/10.1016/j.apradiso.2014.03.002 0969-8043/& 2014 Elsevier Ltd. All rights reserved.
Indian coal used in power plants generally has high ash yield (35– 45%) and is of low quality (Rajamane, 2003; Mathur et al., 2003; Sarkar et al., 2005). The major composition of fly ash is qualitatively similar to that of natural earthly materials such as soils and shales. Oxidized compounds of Si, Al, Fe and Ca account for nearly 90% of the composition of fly ash. Other elements such as Mg, K, Na, Ti and S occur as minor constituents and account for a small percentage of the bulk composition. Generally, all other elements occur in the parts per million ranges and, collectively, seldom exceed 1% of the bulk composition. Fly ash is associated with various useful constituents such as Ca, Mg, Mn, Fe, Cu, Zn, B, S and P, along with appreciable amounts of toxic elements such as Cr, Pb, Hg, Ni, V, As and Ba. The concentration of trace elements in ash is extremely variable and depends on the type and composition of the parent fuel, conditions during combustion and efficiency of emission control devices (Dogana and Kobya, 2006). Fly ash also contains radioactive elements which may come in contact with the general public when they are dispersed in air and water or are included in commercial products that contain fly ash (Pandit et al., 2011).
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M. Tiwari et al. / Applied Radiation and Isotopes 90 (2014) 53–57
Fly ash has gained considerable attention from the construction industry as a useful and increasingly important raw material. Once considered as a nuisance waste product with a disposal problem, fly ash is now recognized as a valuable substance which confers certain desirable characteristics in its many applications viz. cement manufacture, ceramics making and wastewater treatment (Diamadopoulos et al., 1993; Panday et al., 1985). The quality of fly ash and bottom ash is determined by the chemical makeup of coal and combustion parameters. So it is important to quantify the elemental composition of coal along with bottom and fly ash, so partitioning of elements can be determined. There are various analytic techniques (ICP-MS/AES, AAS, NAA, DPASV, etc.) capable to characterize the coal, fly ash and bottom ash with their own advantages like lower detection limit, sensitivity, large dynamic ranges, etc. Among simultaneous multi-elements analysis techniques, X-ray fluorescence (XRF) instrumentation has lower capital cost and is cheaper to use than Neutron Activation Analysis (NAA), is faster than ICP-AES and is widely available. XRF techniques require less sample preparation time, cost and also avoid use of hazardous chemicals. In recent years X-ray fluorescence analysis has emerged as a very powerful technique for elemental analysis of environmental samples (Vijayan et al., 1997; Cakir et al., 2003; Budak et al., 2006; Gupta et al., 2013). In view of the potential of fly ash as a useful industrial raw material and a health hazard, as a source for recovery of radioactive and valuable elements, studies of its elemental composition are highly desirable. In the present study coal, fly ash and bottom ash samples were collected from various coal-fired power plants situated across India. Major (Na, K, Si, Al, Ca, and Mg), heavy and toxic (Zn, V, Cr, Mn, Fe, Co, Ni, Cu, As, Pb, Sr, and Cd) elements in coal, fly ash and bottom ash were analyzed using EDXRF techniques. Voltage and current of X-ray tube and the application of secondary targets were optimized for the analysis of elements of different atomic numbers.
2. Methodology 2.1. Sample collection and processing The feed coal, fly ash and bottom ash samples were collected from eight coal-fired thermal power plants of India. Most of the power stations under study used Indian coal as the feed material; detail of coal type and their grading is described elsewhere (Sahu et al., 2009). A list of thermal power plants, their capacity and geographical coordinates is presented in Table 1. The samples were collected weekly over a period of one month to obtain true representative samples. The coal samples were obtained in duplicates from samplers located at the coal feeder of the boilers. The samples were further homogenized to obtain a gross sample. Bottom ash samples were collected from the asher of the boiler and fly ash were collected from the hoppers of the electrostatic
precipitators. The gross samples were milled and split carefully in accordance with ISO recommendations to obtain a representative subsample of particle size 230 mesh size for further chemical analyses (Bhangare et al., 2011). Each fine sieved (o 63 mm) coal, fly ash and bottom ash samples were homogeneously mixed with cellulose powder (Sigma-Aldrich) in a ratio of 1:1 and pelletized with a hydraulic pressure of 25 tones/cm2 for three minutes. 2.2. EDXRF set-up All measurements were carried out under vacuum, using a Xenemetrix EX-6600 EDXRF spectrometer. The instrument consists of an X-ray tube with a Rh anode as the source of X-rays with HVPS 60 kV, 6.6 mA power supply, a LN2 cooled Si(Li) detector with a resolution of 131 eV at, Mn Kα (5.9 keV) X-ray and a 8-sample turret that enables mounting and analyzing 8 samples at a time. Different secondary targets were used to excite the elements in the sample (Singh et al., 2011).The concentrations of 18 elements, namely Na, K, Si, Al, Ca, Zn, V, Cr, Mn, Fe, Co, Ni, Cu, As, Pb, Sr, Mg and Cd were measured. The built-in software was used for the quantitative analysis. To optimize the EDXRF sensitivities for the wide range of elements of interest, three different combinations of EDXRF parameters (including voltage and current) shown in Table 2 are employed for different elements for coal and ash samples. The secondary targets were used for particular line energies to reduce the relevant background intensities. A screenshot of EDXRF spectra produce by bottom ash using Zr as a secondary target is shown in Fig. 1. 2.3. Quality control and quality assurance Individual and mixture of elements coated on nucleopore aerosol membrane with known deposition per unit area (mg/cm2) were used for calibration of EDXRF. IAEA-405 and IAEA-433 reference materials were used for to verify calibration. Concentration of elements of interest analyzed in both RMs has shown good agreement with published reference sheet values. Pb and As are Table 2 Combinations of the parameters and X-ray filters used in the EDXRF setup to find the different elements. Elements
Voltage (kV)
27 Na, Mg, Al, Si, K, and Ca V, Cr, Mn, Fe, 40 Co, Ni, Cu, and Zn As, Sr, Cd, 50 and Pb
Current (lA)
Secondary target
Preset time (s)
Range (keV)
Atmosphere
4800
Ti
800
0–10
Vacuum
4000
Ge
300
0–40
Vacuum
4800
Zr
500
0–40
Vacuum
Table 1 List of thermal power plants under study with their capacity and geographical locations. Name of the thermal power plant
Capacity (MW)
Geographical coordinates
Chandrapur Thermal Power Station (Maharashtra) Koradi Thermal Power Station (Maharashtra) Dahanu Thermal Power Station(Maharashtra) Talcher Super Thermal Power Station (Orissa) Jindal Mega Power Plant Tamnar (Chhattisgarh) Simhadri Super Thermal Power Plant (Andhra Pradesh) Manguru Thermal Power Station (Andhra Pradesh) Vindhyachal Thermal Power Station (Madhya Pradesh)
2340 620 500 3000 1000 2000 90 3760
201000 24ʺN 791170 21ʺE 211140 52ʺN 791050 53ʺE 191570 12ʺN 721440 54ʺE 211050 49ʺN 851040 30ʺE 221060 16ʺN 831270 04ʺE 171350 42ʺN 831050 18ʺE 171560 14ʺN 801490 07ʺE 241050 53ʺN 821400 18ʺE
M. Tiwari et al. / Applied Radiation and Isotopes 90 (2014) 53–57
55
Table 3 Elements concentration (mg/kg) in coal, fly ash and bottom ash (mean7 standard deviation).
Fig. 1. Typical EDXRF spectra of bottom ash using Zr as a secondary target.
two elements that can be correlated by the dispersion of X-Ray energy since both share one of the main lines of dispersion in the EDXRF spectra (Pb Lα and As Kα are at the same energy). The area under the peak for PbLα and As Kα (between 10.40 keV and 10.60 keV) and PbLβ energy (around 12.61 keV) was measured. Pb concentration is calculated directly from its intensity of Lβ energy. Using the energy area ratio for Pb Lα to As Kα line, the As concentration was calculated. Measured concentrations (mg/kg) of elements were found within 95% of confidence interval mentioned in reference sheet of IAEA-405 and 433 RMs (Tiwari et al., 2013).
3. Result and discussions 3.1. Characterization of feed coal, bottom ash and fly ash Average elemental concentrations in feed coal, fly ash and bottom ash samples are shown in Table 3. The relative elemental abundance in coal was found to be in the order of Si4Al4Fe4Ca 4Mg 4K4Na 4Sr4V4Zn4Mn4Cr4Cu4Pb4Ni4Co4As4Cd. The order of elemental concentration resembles the order in which these metals are found in crustal samples. As Indian coal is high in ash content (Bhatt, 2003), these elements accumulate in combustion residue in large quantity. The fly ash samples are found to have the relative elemental abundance in the order of Si4Al4Fe4Ca4 Mg4K4Na4V4Mn4Sr4Zn4Cu4Cr4Pb4Co4As4Ni4Cd. Most of the elements were found to be well enriched in fly ash when compared with respective concentrations in coal. Elements such as Zn, Cr, Ni and Pb were found to be depleted in fly ash. The order of relative elemental concentration in bottom ash was found to be Si4Al4Fe4Ca4Mg4K4Na4Mn4Sr4V4Zn4Ni4Cr4 Cu4Pb4Co4Cd4As. Silicon was found most abundant among measured elements with 17.8%, 29.2% and 26.8% respectively in coal, fly ash and bottom ash samples. Concentration of Cd was found to be the lowest in coal, fly ash and bottom ash with concentrations 0.37 mg/kg, 0.62 mg/kg, and 0.40 mg/kg respectively. 3.2. Mineral constituents of coal, fly ash and bottom ash The presence of mineral matter and different ash constituent has an effect on ash resistivity and conductance which ultimately reduce the performance of electrostatic precipitator (ESP) (Mandal and Mandal, 2009). The pulverized ash after passing through the boiler flame consists of silicon dioxide (SiO2), iron oxide (Fe2O3), alkaline earth and metal oxide (CaO, Al2O3, MgO, Na2O, and K2O). Elements Si, Al, Fe, Ca, Mg, Na, and K were probably converted into
Element
Coal
Fly ash
Bottom ash
Na K Si Al Ca Mg Zn V Cr Mn Fe Co Ni Cu As Pb Sr Cd
911.5 7 196.1 19377 403 178,0007 35,412 32,1037 23,812 9569 7 706 6141.7 7 116.0 166.07 58.4 182.7 7 31.5 54.5 7 24.6 123.4 7 56.4 15,088 7 4026 8.487 4.14 28.6 7 10.5 49.77 29.8 2.08 7 0.62 28.63 7 13.1 217.17 56.8 0.377 0.09
1258 7 216 68197 1062 292,0007 52,410 123,5117 42,357 12,526 7 218 81317 289 74.3 7 19.0 659.8 7 293.0 51.4 7 24.1 308.2 7 139.5 36,756 7 3406 10.36 7 6.13 2.277 1.31 63.9 7 15.2 4.96 7 3.60 23.83 7 17.06 243.17 169.0 0.62 7 0.37
10577 154 45787 412 268,0007 12,041 139,6487 21,090 12,3177 335 71457 19.8 57.3 7 28.3 225.4 7 46.5 51.8 7 26.5 278.3 7 61.7 49,4717 5826 10.8 7 3.5 52.3 7 37.1 47.7 7 20.0 0.147 0.10 17.36 7 6.66 273.7 7 167.1 0.40 7 0.27
their most stable oxide to quantify total makeup of fly ash and bottom ash. Percentage distributions of silicon dioxide (SiO2), iron oxide (Fe2O3), alkaline earth and metal oxide (CaO, Al2O3, MgO, Na2O, and K2O) for fly ash and bottom ash are depicted in Figs. 2 and 3. Around 95% of mass could be accounted for using the concentration of elements in this study for fly ash and bottom ash. Silica (SiO2) was found to be most abundant in both fly ash and bottom ash with mean value of 62.57% and 57.43% of total mass respectively. Alumina (Al2O3) was found to be the second most abundant constituent of both fly ash and bottom ash, followed by iron oxide (Fe2O3). Concentration of alumina is found to be higher in bottom ash compared to fly ash. The slight difference in silica and alumina concentration in fly ash and bottom ash is probably due to melting points i.e. 1685 1C and 1775 1C for silica and alumina respectively. The un-accounted mass (5%) of fly ash and bottom ash may include organic compounds, sulfur compounds, P2O5, TiO2 and other trace elements and their compounds. Fly ash containing low concentrations of alkali metals generally does have high ash fusion temperatures. The fusion point generally varies according to the ratio between the acidic (SiO2 þ Al2O3) and basic (Fe2O3 þCaO þMgO þAlkalies) components and this ratio ranges between 3.05 and 9.50 (Roy, 1935; Du et al., 2014; Liu et al., 2013). The acidic/basic ratio for the fly ash samples analyzed is found to be having a mean value of 9.23. 3.3. Enrichment ratio (ER) The ER was calculated as the ratio of concentration of the element in ash to its concentration in coal. Elements tend to condense on fly ash as the temperature drops which results in their enrichment in it. Fly ash is the finer fraction of slag compared to bottom ash, which offers more surface area for condensation. Thus the enrichment factors of the trace elements, which have high affinity for small particles, are larger in fly ash than for bottom ash. The organically-bonded elements could partly volatilize and condense on the fine particles of fly ash, leading to a higher concentration in fly ash than in bottom ash (Dai et al., 2010). Elements K, Na, Si, V, Al, Mn, Cd, and Fe were found enriched in both fly and bottom ash as compared to coal. As in fly ash and Ni in bottom ash were found enriched as their enrichment ratio is more than 1. The enrichment ratios of As in bottom ash, Ni in fly ash, and Zn and Pb in both were below unity indicating that these elements might have escaped with the flue gases, due to their higher volatility. Ca, Mg, Cr, Co, Cu, and Sr were
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M. Tiwari et al. / Applied Radiation and Isotopes 90 (2014) 53–57
Fig. 4. Enrichment ratio for analyzed elements in fly ash and bottom ashwith respect to coal. Fig. 2. Mineral composition of fly ash values is in percentage. Other elements are analyzed elements in this study while unidentified are not analyzed such as organics, P2O5, TiO2, etc. Table 4 Enrichment factor of elements in coal, fly ash and bottom ash with respect to crustal average.
Fig. 3. Mineral composition of bottom ash values is in percentage. Other elements are analyzed elements in this study while unidentified are not analyzed such as organics, P2O5, TiO2, etc.
found with enriched ratio of 1 which shows that there is not much difference in their partitioning. The enrichment ratio was found higher in bottom ash as compared to fly ash for Al, Fe, Ni, and Sr elements. Enrichment ratios for each element in fly ash and bottom ash with respect to coal are shown in Fig. 4. 3.4. Enrichment factor with respect to crustal average The value of the enrichment factor (EF) was calculated for coal, fly ash and bottom ash with respect to crustal average using the following formula EF ¼
Msample =Fesample Maverage =Feaverage
where Msample is the concentration of the examined metal in sample (coal, fly ash or bottom ash), Fesample is the concentration
Element
Coal
Fly ash
Bottom ash
Na K Si Al Ca Mg Zn V Cr Mn Fe Co Ni Cu As Pb Sr Cd
0.10 0.22 1.45 1.02 0.97 1.07 6.43 4.89 1.54 0.41 1.00 1.27 1.58 4.61 1.12 4.37 1.76 10.58
0.06 0.31 0.98 1.61 0.52 0.58 1.18 7.25 0.60 0.42 1.00 0.64 0.05 2.43 1.10 1.49 0.81 7.34
0.03 0.16 0.67 1.36 0.38 0.38 0.68 1.84 0.45 0.28 1.00 0.50 0.88 1.35 0.02 0.81 0.68 3.55
of the iron in the sample for which EF to be calculated (coal, fly ash, and bottom ash). M average shale concentration of the element in the average shale, Feaverage shale concentration of the iron in the average shale (Sutherland, 2000). Enrichment factors of each element in coal, fly ash and bottom ash with respect to average crustal shale are shown in Table 4. Five contamination categories are generally recognized on the basis of the enrichment factor: EFo2, depletion to slightly enrichment; 2 rEF o5, moderate enrichment; 5 rEFo 20, significant enrichment; 20 rEF o40, very high enrichment; and EF4 40, extremely high enrichment (Pekey, 2003). For coal alkali metals Na, K, Ca and Mn were found to be depleted as compared to the crustal average as their EF were found to be less than unity. Si, Al, Cr, Co, Ni, As and Sr were found slightly enriched as compare to the crustal average as their EF ranged from 1 to 2, while V, Cu and Pb were found to be moderately enriched. Significant enrichment has shown by Zn and Cd in the coal samples. Fly ash samples exhibited depletion of Na, K, Si, Ca, Mg, Cr, Mn, Co, Ni, As, Pb and Sr while slight enrichment of Al with respect to the crustal averages. Cu in fly ash has shown moderate enrichment whereas vanadium and
M. Tiwari et al. / Applied Radiation and Isotopes 90 (2014) 53–57
cadmium were significantly enriched. V and Cu were slightly enriched and Cd was moderately enriched in bottom ash. All other measured elements were found to be depleted in bottom ash samples. 4. Conclusion The investigation has clearly shown that the EDXRF method is a powerful tool for fast simultaneous multi-element analysis of coal, fly ash and bottom ash. Alkali content in Indian coal, fly ash and bottom ash was found to be low which increase the fusion point. There was no significant difference in the distribution (in the order of concentration) of elements in coal, fly ash and bottom ash. Compared to coal, elements such as K, Na, Si, V, Al, Mn, Cd and Fe were found enriched in both fly and bottom ash. Around 95% of fly ash and bottom ash mass could be accounted using the elemental data. Toxic metals Pb, Ni and Zn were found significantly enriched in coal compared to average crustal shale which should raise concerns. V and Cd were found in higher concentration compared to crustal average in coal, fly ash, and bottom ash samples. References Bhangare, R.C., Ajmal, P.Y., Sahu, S.K., Pandit, G.G., Puranik, V.D., 2011. Distribution of trace elements in coal and combustion residues from five thermal power plants in India. Int. J. Coal Geol. 86, 349–356. Bhatt, M.S., 2003. Effect of ash in coal on the performance of coal fired thermal Power plants, part II: capacity and secondary energy effects. Energy Sources, Part A: Recovery Util. Environ. Eff. 28, 43–58. Budak, G., Aslan, I., Karabulut, A., Tırasoglu, E., 2006. Analysis of some elements in three Chrysolina (Coleoptera, Chrysomelidae) species by EDXRF spectrometry. J. Quant. Spectrosc. Radiat. Transf. 101, 195–200. Cakir, C., Budak, G., Karabulut, A., Sahin, Y., 2003. Analysis of trace elements in different three region coals in Erzurum (Turkey): a study using EDXRF. J. Quant. Spectrosc. Radiat. Transf. 76, 101–106. Dai, S., Zhao, L., Peng, S., Chou, C.L., Wang, X., Zhang, Y., Li, D., Sun, Y., 2010. Abundances and distribution of minerals and elements in high-alumina coal fly ash from the Jungar Power Plant, Inner Mongolia. China. Int. J. Coal Geol. 81, 320–332.
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Diamadopoulos, E., Ioannidis, S., Sakellaropoulos, G.P., 1993. As (V) removal from aqueous solution by fly ash. Water Res. 27 (12), 1773–1777. Dogana, O., Kobya, M., 2006. Elemental analysis of trace elements in fly ash sample of Yatag an thermal power plants using EDXRF. J. Quant. Spectrosc. Radiat. Transf. 101, 146–150. Du, S., Yang, H., Qian, K., Wang, X., Chen, H., 2014. Fusion and transformation properties of the inorganic components in biomass ash. Fuel Part B 117, 1281–1287 http://dx.doi.org/10.1016/j.fuel.2013.07.085. Gupta, D., Roy, S., Ghosh, R., Mitra, A.K., 2013. Chemical composition of soil evolving from municipal solid waste using energy dispersive X-ray fluorescence. X-ray Spectrom. 42, 268–275. Liu, B., He, Q., Jiang, Z., Xu, R., Hu, B., 2013. Relationship between coal ash composition and ash fusion temperatures. Fuel 105, 293–300. Mandal, P.K., Mandal, T.K., 2009. Electrostatic precipitator performance in Indian pulverized coal based thermal power stations – problems and solutions. Water Energy Res. Dig. 19 (4), 31–40. Mathur, R., Chand, S., Tezuka, T., 2003. Optimal use of coal for the power generation in India. Energy Policy 31, 319–331. Panday, K.K., Prasad, G., Singh, V.N., 1985. Copper(II) removal from aqueous solution by fly ash. Water Res. 19 (7), 869–873. Pandit, G.G., Sahu, S.K., Puranik, V.D., 2011. Natural radionuclides from coal fired thermal power plants – estimation of atmospheric release and inhalation risk. Radioprotection 46, S173–S179. Pekey, H., 2003. Heavy metals pollution assessment in sediments of the Izmit Bay, Turkey. Environ. Monit. Assess. 123, 219–231. Rajamane, N.P., 2003. Making of concrete green through use of fly ash.Green Business Opportunities. Confed. Indian Ind., 22–29 Roy, R.K.D., 1935. A critical study of some Indian coal ashes. Ind. Eng. Chem. 27, 1100. Sahu, S.K., Bhangare, R.C., Ajmal, P.Y., Sharma, S., Pandit, G.G., Puranik, V.D., 2009. Characterization and quantification of persistent organic pollutants in fly ash from coal fueled thermal power stations in India. Microchem. J. 92, 92–96. Sarkar, A., Rano, R., Mishra, K.K., Sinha, I.N., 2005. Particle size distribution profile of some Indian fly ash—a comparative study to assess their possible uses. Fuel Process. Technol. 86, 1221–1238. Singh, V., Agrawal, H.M., Joshi, G.C., Sudershan, M., Sinha, A.K., 2011. Elemental profile of agricultural soil by the EDXRF technique and use of the principal component analysis (PCA) method to interpret the complex data. Appl. Radiat. Isot. 69, 969–974. Sutherland, R.A., 2000. Bed sediment-associated trace metals in an urban stream, Oahu, Hawaii. Environ. Geol. 39, 611–627. Tiwari, M., Sahu, S.K., Bhangare, R.C., Ajmal, P.Y., Pandit, G.G., 2013. Depth profile of major and trace elements in estuarine core sediment using the EDXRF technique. Appl. Radiat. Isot. 80, 78–83. Vijayan, V., Behera, S.N., Ramamurthy, V.S., Puri, S., Shahi, J.S., Singh, N., 1997. Elemental composition of fly ash from a coal-fired thermal power plant: a study using PIXE and EDXRF. X-Ray Spectrom. 26, 65–68.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Elemental characterization of coal, fly ash, and bottom ash using an energy dispersive X-ray fluorescence technique M. Tiwari, S.K. Sahu, R.C. Bhangare, P.Y. Ajmal, G.G. Pandit n Environmental Monitoring and Assessment Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
H I G H L I G H T S
Concentrations of 18 elements were determined in coal and ash samples using EDXRF. Mineral quantification up to 95% was carried out for fly and bottom ash samples. Enrichment ratios of elements were calculated in combustion residue with respect to coal. Enrichment factor with respect to crustal average was estimated for ash samples.
art ic l e i nf o
a b s t r a c t
Article history: Received 3 December 2013 Received in revised form 4 February 2014 Accepted 1 March 2014 Available online 12 March 2014
A total of 18 elements viz. Si, Al, Fe, Ca, Mg, K, Na, Sr, V, Zn, Mn, Cr, Cu, Pb, Ni, Co, As and Cd were analyzed in coal, fly ash and bottom ash samples collected across India using an EDXRF technique. Various indices such as element enrichment ratio, enrichment factor (with respect to crustal average) and mineral composition were calculated. Around 95% of mass was reconstructed using the concentration of elements in this study for fly and bottom ash. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Fly ash Bottom ash Fusion point EDXRF Enrichment ratio
1. Introduction Coal has many important uses worldwide. The most significant uses of coal are in electricity generation, steel production and in cement manufacturing. According to the World Coal Association around 7.6 billion tons of coal was used worldwide which includes 1 billion tons of brown coal. Global coal consumption has grown faster than any other fuel since 2000. The five largest coal users China, USA, India, Russia and Japan account for 76% of total global coal usage. Electricity production from coal sources in India was 68.56% in 2009. As a fuel, coal refers to all coals and brown coal of both primary (including hard coal and lignite-brown coal) and derived (including patent fuel, coke oven coke, gas coke, coke oven gas, and blast furnace gas) origins. Poor-quality coal with high ash yield results in the generation of large amounts of fly and bottom ash of varying properties when used in coal-fired power plants.
n
Corresponding author. Tel.: þ 91 2225590233; fax: þ91 225505151. E-mail address: [email protected] (G.G. Pandit).
http://dx.doi.org/10.1016/j.apradiso.2014.03.002 0969-8043/& 2014 Elsevier Ltd. All rights reserved.
Indian coal used in power plants generally has high ash yield (35– 45%) and is of low quality (Rajamane, 2003; Mathur et al., 2003; Sarkar et al., 2005). The major composition of fly ash is qualitatively similar to that of natural earthly materials such as soils and shales. Oxidized compounds of Si, Al, Fe and Ca account for nearly 90% of the composition of fly ash. Other elements such as Mg, K, Na, Ti and S occur as minor constituents and account for a small percentage of the bulk composition. Generally, all other elements occur in the parts per million ranges and, collectively, seldom exceed 1% of the bulk composition. Fly ash is associated with various useful constituents such as Ca, Mg, Mn, Fe, Cu, Zn, B, S and P, along with appreciable amounts of toxic elements such as Cr, Pb, Hg, Ni, V, As and Ba. The concentration of trace elements in ash is extremely variable and depends on the type and composition of the parent fuel, conditions during combustion and efficiency of emission control devices (Dogana and Kobya, 2006). Fly ash also contains radioactive elements which may come in contact with the general public when they are dispersed in air and water or are included in commercial products that contain fly ash (Pandit et al., 2011).
54
M. Tiwari et al. / Applied Radiation and Isotopes 90 (2014) 53–57
Fly ash has gained considerable attention from the construction industry as a useful and increasingly important raw material. Once considered as a nuisance waste product with a disposal problem, fly ash is now recognized as a valuable substance which confers certain desirable characteristics in its many applications viz. cement manufacture, ceramics making and wastewater treatment (Diamadopoulos et al., 1993; Panday et al., 1985). The quality of fly ash and bottom ash is determined by the chemical makeup of coal and combustion parameters. So it is important to quantify the elemental composition of coal along with bottom and fly ash, so partitioning of elements can be determined. There are various analytic techniques (ICP-MS/AES, AAS, NAA, DPASV, etc.) capable to characterize the coal, fly ash and bottom ash with their own advantages like lower detection limit, sensitivity, large dynamic ranges, etc. Among simultaneous multi-elements analysis techniques, X-ray fluorescence (XRF) instrumentation has lower capital cost and is cheaper to use than Neutron Activation Analysis (NAA), is faster than ICP-AES and is widely available. XRF techniques require less sample preparation time, cost and also avoid use of hazardous chemicals. In recent years X-ray fluorescence analysis has emerged as a very powerful technique for elemental analysis of environmental samples (Vijayan et al., 1997; Cakir et al., 2003; Budak et al., 2006; Gupta et al., 2013). In view of the potential of fly ash as a useful industrial raw material and a health hazard, as a source for recovery of radioactive and valuable elements, studies of its elemental composition are highly desirable. In the present study coal, fly ash and bottom ash samples were collected from various coal-fired power plants situated across India. Major (Na, K, Si, Al, Ca, and Mg), heavy and toxic (Zn, V, Cr, Mn, Fe, Co, Ni, Cu, As, Pb, Sr, and Cd) elements in coal, fly ash and bottom ash were analyzed using EDXRF techniques. Voltage and current of X-ray tube and the application of secondary targets were optimized for the analysis of elements of different atomic numbers.
2. Methodology 2.1. Sample collection and processing The feed coal, fly ash and bottom ash samples were collected from eight coal-fired thermal power plants of India. Most of the power stations under study used Indian coal as the feed material; detail of coal type and their grading is described elsewhere (Sahu et al., 2009). A list of thermal power plants, their capacity and geographical coordinates is presented in Table 1. The samples were collected weekly over a period of one month to obtain true representative samples. The coal samples were obtained in duplicates from samplers located at the coal feeder of the boilers. The samples were further homogenized to obtain a gross sample. Bottom ash samples were collected from the asher of the boiler and fly ash were collected from the hoppers of the electrostatic
precipitators. The gross samples were milled and split carefully in accordance with ISO recommendations to obtain a representative subsample of particle size 230 mesh size for further chemical analyses (Bhangare et al., 2011). Each fine sieved (o 63 mm) coal, fly ash and bottom ash samples were homogeneously mixed with cellulose powder (Sigma-Aldrich) in a ratio of 1:1 and pelletized with a hydraulic pressure of 25 tones/cm2 for three minutes. 2.2. EDXRF set-up All measurements were carried out under vacuum, using a Xenemetrix EX-6600 EDXRF spectrometer. The instrument consists of an X-ray tube with a Rh anode as the source of X-rays with HVPS 60 kV, 6.6 mA power supply, a LN2 cooled Si(Li) detector with a resolution of 131 eV at, Mn Kα (5.9 keV) X-ray and a 8-sample turret that enables mounting and analyzing 8 samples at a time. Different secondary targets were used to excite the elements in the sample (Singh et al., 2011).The concentrations of 18 elements, namely Na, K, Si, Al, Ca, Zn, V, Cr, Mn, Fe, Co, Ni, Cu, As, Pb, Sr, Mg and Cd were measured. The built-in software was used for the quantitative analysis. To optimize the EDXRF sensitivities for the wide range of elements of interest, three different combinations of EDXRF parameters (including voltage and current) shown in Table 2 are employed for different elements for coal and ash samples. The secondary targets were used for particular line energies to reduce the relevant background intensities. A screenshot of EDXRF spectra produce by bottom ash using Zr as a secondary target is shown in Fig. 1. 2.3. Quality control and quality assurance Individual and mixture of elements coated on nucleopore aerosol membrane with known deposition per unit area (mg/cm2) were used for calibration of EDXRF. IAEA-405 and IAEA-433 reference materials were used for to verify calibration. Concentration of elements of interest analyzed in both RMs has shown good agreement with published reference sheet values. Pb and As are Table 2 Combinations of the parameters and X-ray filters used in the EDXRF setup to find the different elements. Elements
Voltage (kV)
27 Na, Mg, Al, Si, K, and Ca V, Cr, Mn, Fe, 40 Co, Ni, Cu, and Zn As, Sr, Cd, 50 and Pb
Current (lA)
Secondary target
Preset time (s)
Range (keV)
Atmosphere
4800
Ti
800
0–10
Vacuum
4000
Ge
300
0–40
Vacuum
4800
Zr
500
0–40
Vacuum
Table 1 List of thermal power plants under study with their capacity and geographical locations. Name of the thermal power plant
Capacity (MW)
Geographical coordinates
Chandrapur Thermal Power Station (Maharashtra) Koradi Thermal Power Station (Maharashtra) Dahanu Thermal Power Station(Maharashtra) Talcher Super Thermal Power Station (Orissa) Jindal Mega Power Plant Tamnar (Chhattisgarh) Simhadri Super Thermal Power Plant (Andhra Pradesh) Manguru Thermal Power Station (Andhra Pradesh) Vindhyachal Thermal Power Station (Madhya Pradesh)
2340 620 500 3000 1000 2000 90 3760
201000 24ʺN 791170 21ʺE 211140 52ʺN 791050 53ʺE 191570 12ʺN 721440 54ʺE 211050 49ʺN 851040 30ʺE 221060 16ʺN 831270 04ʺE 171350 42ʺN 831050 18ʺE 171560 14ʺN 801490 07ʺE 241050 53ʺN 821400 18ʺE
M. Tiwari et al. / Applied Radiation and Isotopes 90 (2014) 53–57
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Table 3 Elements concentration (mg/kg) in coal, fly ash and bottom ash (mean7 standard deviation).
Fig. 1. Typical EDXRF spectra of bottom ash using Zr as a secondary target.
two elements that can be correlated by the dispersion of X-Ray energy since both share one of the main lines of dispersion in the EDXRF spectra (Pb Lα and As Kα are at the same energy). The area under the peak for PbLα and As Kα (between 10.40 keV and 10.60 keV) and PbLβ energy (around 12.61 keV) was measured. Pb concentration is calculated directly from its intensity of Lβ energy. Using the energy area ratio for Pb Lα to As Kα line, the As concentration was calculated. Measured concentrations (mg/kg) of elements were found within 95% of confidence interval mentioned in reference sheet of IAEA-405 and 433 RMs (Tiwari et al., 2013).
3. Result and discussions 3.1. Characterization of feed coal, bottom ash and fly ash Average elemental concentrations in feed coal, fly ash and bottom ash samples are shown in Table 3. The relative elemental abundance in coal was found to be in the order of Si4Al4Fe4Ca 4Mg 4K4Na 4Sr4V4Zn4Mn4Cr4Cu4Pb4Ni4Co4As4Cd. The order of elemental concentration resembles the order in which these metals are found in crustal samples. As Indian coal is high in ash content (Bhatt, 2003), these elements accumulate in combustion residue in large quantity. The fly ash samples are found to have the relative elemental abundance in the order of Si4Al4Fe4Ca4 Mg4K4Na4V4Mn4Sr4Zn4Cu4Cr4Pb4Co4As4Ni4Cd. Most of the elements were found to be well enriched in fly ash when compared with respective concentrations in coal. Elements such as Zn, Cr, Ni and Pb were found to be depleted in fly ash. The order of relative elemental concentration in bottom ash was found to be Si4Al4Fe4Ca4Mg4K4Na4Mn4Sr4V4Zn4Ni4Cr4 Cu4Pb4Co4Cd4As. Silicon was found most abundant among measured elements with 17.8%, 29.2% and 26.8% respectively in coal, fly ash and bottom ash samples. Concentration of Cd was found to be the lowest in coal, fly ash and bottom ash with concentrations 0.37 mg/kg, 0.62 mg/kg, and 0.40 mg/kg respectively. 3.2. Mineral constituents of coal, fly ash and bottom ash The presence of mineral matter and different ash constituent has an effect on ash resistivity and conductance which ultimately reduce the performance of electrostatic precipitator (ESP) (Mandal and Mandal, 2009). The pulverized ash after passing through the boiler flame consists of silicon dioxide (SiO2), iron oxide (Fe2O3), alkaline earth and metal oxide (CaO, Al2O3, MgO, Na2O, and K2O). Elements Si, Al, Fe, Ca, Mg, Na, and K were probably converted into
Element
Coal
Fly ash
Bottom ash
Na K Si Al Ca Mg Zn V Cr Mn Fe Co Ni Cu As Pb Sr Cd
911.5 7 196.1 19377 403 178,0007 35,412 32,1037 23,812 9569 7 706 6141.7 7 116.0 166.07 58.4 182.7 7 31.5 54.5 7 24.6 123.4 7 56.4 15,088 7 4026 8.487 4.14 28.6 7 10.5 49.77 29.8 2.08 7 0.62 28.63 7 13.1 217.17 56.8 0.377 0.09
1258 7 216 68197 1062 292,0007 52,410 123,5117 42,357 12,526 7 218 81317 289 74.3 7 19.0 659.8 7 293.0 51.4 7 24.1 308.2 7 139.5 36,756 7 3406 10.36 7 6.13 2.277 1.31 63.9 7 15.2 4.96 7 3.60 23.83 7 17.06 243.17 169.0 0.62 7 0.37
10577 154 45787 412 268,0007 12,041 139,6487 21,090 12,3177 335 71457 19.8 57.3 7 28.3 225.4 7 46.5 51.8 7 26.5 278.3 7 61.7 49,4717 5826 10.8 7 3.5 52.3 7 37.1 47.7 7 20.0 0.147 0.10 17.36 7 6.66 273.7 7 167.1 0.40 7 0.27
their most stable oxide to quantify total makeup of fly ash and bottom ash. Percentage distributions of silicon dioxide (SiO2), iron oxide (Fe2O3), alkaline earth and metal oxide (CaO, Al2O3, MgO, Na2O, and K2O) for fly ash and bottom ash are depicted in Figs. 2 and 3. Around 95% of mass could be accounted for using the concentration of elements in this study for fly ash and bottom ash. Silica (SiO2) was found to be most abundant in both fly ash and bottom ash with mean value of 62.57% and 57.43% of total mass respectively. Alumina (Al2O3) was found to be the second most abundant constituent of both fly ash and bottom ash, followed by iron oxide (Fe2O3). Concentration of alumina is found to be higher in bottom ash compared to fly ash. The slight difference in silica and alumina concentration in fly ash and bottom ash is probably due to melting points i.e. 1685 1C and 1775 1C for silica and alumina respectively. The un-accounted mass (5%) of fly ash and bottom ash may include organic compounds, sulfur compounds, P2O5, TiO2 and other trace elements and their compounds. Fly ash containing low concentrations of alkali metals generally does have high ash fusion temperatures. The fusion point generally varies according to the ratio between the acidic (SiO2 þ Al2O3) and basic (Fe2O3 þCaO þMgO þAlkalies) components and this ratio ranges between 3.05 and 9.50 (Roy, 1935; Du et al., 2014; Liu et al., 2013). The acidic/basic ratio for the fly ash samples analyzed is found to be having a mean value of 9.23. 3.3. Enrichment ratio (ER) The ER was calculated as the ratio of concentration of the element in ash to its concentration in coal. Elements tend to condense on fly ash as the temperature drops which results in their enrichment in it. Fly ash is the finer fraction of slag compared to bottom ash, which offers more surface area for condensation. Thus the enrichment factors of the trace elements, which have high affinity for small particles, are larger in fly ash than for bottom ash. The organically-bonded elements could partly volatilize and condense on the fine particles of fly ash, leading to a higher concentration in fly ash than in bottom ash (Dai et al., 2010). Elements K, Na, Si, V, Al, Mn, Cd, and Fe were found enriched in both fly and bottom ash as compared to coal. As in fly ash and Ni in bottom ash were found enriched as their enrichment ratio is more than 1. The enrichment ratios of As in bottom ash, Ni in fly ash, and Zn and Pb in both were below unity indicating that these elements might have escaped with the flue gases, due to their higher volatility. Ca, Mg, Cr, Co, Cu, and Sr were
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Fig. 4. Enrichment ratio for analyzed elements in fly ash and bottom ashwith respect to coal. Fig. 2. Mineral composition of fly ash values is in percentage. Other elements are analyzed elements in this study while unidentified are not analyzed such as organics, P2O5, TiO2, etc. Table 4 Enrichment factor of elements in coal, fly ash and bottom ash with respect to crustal average.
Fig. 3. Mineral composition of bottom ash values is in percentage. Other elements are analyzed elements in this study while unidentified are not analyzed such as organics, P2O5, TiO2, etc.
found with enriched ratio of 1 which shows that there is not much difference in their partitioning. The enrichment ratio was found higher in bottom ash as compared to fly ash for Al, Fe, Ni, and Sr elements. Enrichment ratios for each element in fly ash and bottom ash with respect to coal are shown in Fig. 4. 3.4. Enrichment factor with respect to crustal average The value of the enrichment factor (EF) was calculated for coal, fly ash and bottom ash with respect to crustal average using the following formula EF ¼
Msample =Fesample Maverage =Feaverage
where Msample is the concentration of the examined metal in sample (coal, fly ash or bottom ash), Fesample is the concentration
Element
Coal
Fly ash
Bottom ash
Na K Si Al Ca Mg Zn V Cr Mn Fe Co Ni Cu As Pb Sr Cd
0.10 0.22 1.45 1.02 0.97 1.07 6.43 4.89 1.54 0.41 1.00 1.27 1.58 4.61 1.12 4.37 1.76 10.58
0.06 0.31 0.98 1.61 0.52 0.58 1.18 7.25 0.60 0.42 1.00 0.64 0.05 2.43 1.10 1.49 0.81 7.34
0.03 0.16 0.67 1.36 0.38 0.38 0.68 1.84 0.45 0.28 1.00 0.50 0.88 1.35 0.02 0.81 0.68 3.55
of the iron in the sample for which EF to be calculated (coal, fly ash, and bottom ash). M average shale concentration of the element in the average shale, Feaverage shale concentration of the iron in the average shale (Sutherland, 2000). Enrichment factors of each element in coal, fly ash and bottom ash with respect to average crustal shale are shown in Table 4. Five contamination categories are generally recognized on the basis of the enrichment factor: EFo2, depletion to slightly enrichment; 2 rEF o5, moderate enrichment; 5 rEFo 20, significant enrichment; 20 rEF o40, very high enrichment; and EF4 40, extremely high enrichment (Pekey, 2003). For coal alkali metals Na, K, Ca and Mn were found to be depleted as compared to the crustal average as their EF were found to be less than unity. Si, Al, Cr, Co, Ni, As and Sr were found slightly enriched as compare to the crustal average as their EF ranged from 1 to 2, while V, Cu and Pb were found to be moderately enriched. Significant enrichment has shown by Zn and Cd in the coal samples. Fly ash samples exhibited depletion of Na, K, Si, Ca, Mg, Cr, Mn, Co, Ni, As, Pb and Sr while slight enrichment of Al with respect to the crustal averages. Cu in fly ash has shown moderate enrichment whereas vanadium and
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cadmium were significantly enriched. V and Cu were slightly enriched and Cd was moderately enriched in bottom ash. All other measured elements were found to be depleted in bottom ash samples. 4. Conclusion The investigation has clearly shown that the EDXRF method is a powerful tool for fast simultaneous multi-element analysis of coal, fly ash and bottom ash. Alkali content in Indian coal, fly ash and bottom ash was found to be low which increase the fusion point. There was no significant difference in the distribution (in the order of concentration) of elements in coal, fly ash and bottom ash. Compared to coal, elements such as K, Na, Si, V, Al, Mn, Cd and Fe were found enriched in both fly and bottom ash. Around 95% of fly ash and bottom ash mass could be accounted using the elemental data. Toxic metals Pb, Ni and Zn were found significantly enriched in coal compared to average crustal shale which should raise concerns. V and Cd were found in higher concentration compared to crustal average in coal, fly ash, and bottom ash samples. References Bhangare, R.C., Ajmal, P.Y., Sahu, S.K., Pandit, G.G., Puranik, V.D., 2011. Distribution of trace elements in coal and combustion residues from five thermal power plants in India. Int. J. Coal Geol. 86, 349–356. Bhatt, M.S., 2003. Effect of ash in coal on the performance of coal fired thermal Power plants, part II: capacity and secondary energy effects. Energy Sources, Part A: Recovery Util. Environ. Eff. 28, 43–58. Budak, G., Aslan, I., Karabulut, A., Tırasoglu, E., 2006. Analysis of some elements in three Chrysolina (Coleoptera, Chrysomelidae) species by EDXRF spectrometry. J. Quant. Spectrosc. Radiat. Transf. 101, 195–200. Cakir, C., Budak, G., Karabulut, A., Sahin, Y., 2003. Analysis of trace elements in different three region coals in Erzurum (Turkey): a study using EDXRF. J. Quant. Spectrosc. Radiat. Transf. 76, 101–106. Dai, S., Zhao, L., Peng, S., Chou, C.L., Wang, X., Zhang, Y., Li, D., Sun, Y., 2010. Abundances and distribution of minerals and elements in high-alumina coal fly ash from the Jungar Power Plant, Inner Mongolia. China. Int. J. Coal Geol. 81, 320–332.
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