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Determination of the ash content of coal without ashing: A simple technique using laser-induced breakdown spectroscopy A.F.M. YUSUF HAIDER, M. Ahtesham Rony, and K. M. Abedin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef400566u • Publication Date (Web): 03 Jun 2013 Downloaded from http://pubs.acs.org on June 4, 2013
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Determination of the ash content of coal without ashing: A simple technique using laser-induced breakdown spectroscopy A.F.M.Y. Haider*, M.A. Rony and K.M. Abedin
Department of Physics, University of Dhaka, Dhaka-1000, Bangladesh
Abstract: The bituminous coal samples were analyzed by using the technique of Laser-Induced Breakdown Spectroscopy (LIBS). The fraction of the ash content of the coal was spectroscopically determined by a technique, requiring only two measurements of the LIBS spectrum of the coal and the results were compared with ash fraction determined by actual ashing of the coal in a furnace. An excellent correlation (r=0.99) was obtained between the spectroscopically determined ash fraction and the fraction of ash content determined by actual ashing of the coal. This excellent coefficient of correlation provides sufficient confidence in the measurement of ash content by this simple and quick method using the LIBS technique.
Key words: Fraction of ash content, silicon, carbon, LIBS, Coefficient of correlation.
PACS: 89.30.ag, 42.62.Fi
* Corresponding author: A.F.M. Y. Haider Email:
[email protected] 1 ACS Paragon Plus Environment
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1. Introduction Coal is a well-known, cheap and common fuel used in power generation, iron and steel industry, brick-making, and in a host of other industrial applications1. Because of its wide availability and cheapness, it is mined and used in many countries of the world. The leading producers of coal in the world are China, the United States, India, Australia and Russia2. Chemically, coal contains mainly elemental carbon and various volatile and non-volatile organic compounds. In addition, it contains a number of silicates, such as those of sodium, calcium, aluminum etc. and some carbonates or sulfates of calcium or magnesium in smaller amounts including ferrous sulfide3. The presence of all of these nonvolatile minerals contributes particularly to the formation of bottom ash in the combustion process of coal. It comprises the non-combustible part of the coal. The coal combustion products are the fly ash, boiler slag, bottom ash and the FGD (flue gas desulphurization) sludge. This coal ash is considered a nuisance, since it accumulates in the combustion chamber as a hard, difficult-toremove glassy residue, compromising the thermal efficiency of boilers, furnaces, etc. For this reason, it is vitally important to know the ash content of the coal beforehand for its use in industrial processes. Therefore, the most important measure of coal quality is its ash content. The standard procedure of determining the ash content of coal is ashing in a high temperature ashing furnace3. However, this ashing procedure is quite involved and time consuming and can no way be used for in situ measurement of ash content of coal. Due to its unique advantages, laser-induced breakdown spectroscopy (LIBS) has been extensively used, for the characterization of the ash-forming components of raw coal, and for the determination of the elemental constituents of the ashes formed after combustion4-9. Chemical analyses of coal are also well established. For example, Fiona Low et al.10 used optimized microwave digestion and ICP-OES technique for determining the ash forming inorganic elements in coal. Shunchum Yao et al.11 applied invariant and multivariate calibration methods to LIBS data to determine the un-burnt carbon in the fly ash. A fairly involved and complex method using nonlinearized multivariate dominant factor based partial least squares (PLS) model was applied by Jie Feng et al.12 to determine the carbon concentration in coal. However, for the specific determination of the ash content in coal, Gaft et al.13 used an empirical best-fit method to characterize the detected ash-forming components for their role in the ash formation to find a correlation between spectroscopic data and actual ash content, but in a later publication, declined to give details of the technique citing proprietary reasons14. Therefore, determination of ash content in coal by LIBS still remains an open question particularly in search of a simple, quick and inexpensive method to determine the ash content of coal for its use for general purposes. In our experiments, various coal samples from Bangladesh were subjected to LIBS analysis. After characterizing the elements found15 in the coal samples, we devised a simple spectroscopic method to determine the ash content, using LIBS and without actually ashing the coal sample. The procedure basically requires only two measurements in the LIBS spectrum. The method is simple and quick but effective for most practical purposes. An excellent correlation between the spectroscopically determined ash content and the actual measured ash content was observed.
2. Experimental setup In the present LIBS experiment the second harmonic at 532 nm from a Q-switched Nd:YAG laser (Spectra-Physics LAB-170-10) with pulse duration of 8 ns, repetition rate of 10 Hz was 2 ACS Paragon Plus Environment
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focused on the sample to produce a laser-induced micro-plasma. The pulse energy used in the experiments was about 40 mJ 16. The plasma was generated at the repetition rate of the laser, i.e. at 10 times per second. The light emitted from the plasma was focused by a fused quartz lens (f=50 mm) and collected by a 3 m long multimode silica optical fiber. The light was then transmitted through the fiber to its other end, which was placed at the entrance slit of a 750 mm focal length computerized Czerny-Turner spectrograph (Acton Model SP-2758). The spectrograph was equipped with three ruled gratings, viz., 2400, 600 and 300 grooves/mm blazed at 240, 500 and 300 nm, respectively, which were interchangeable under computer control, providing high and low-resolution spectra in the wavelength range of 200-960 nm. If 600 grooves/mm grating is used, a spectrum of about 38 nm in width can be captured without moving the grating, and for the 2400 grooves/mm grating, it is about 9 nm15. The output end of the spectrograph was coupled with an intensified and gated CCD camera (Princeton PI-MAX with Unigen II coating and programmable delay generator). The ICCD camera used has 1024X1024 pixels and was cooled to -20 C by a Peltier cooler to reduce noise. The ICCD camera was electrically triggered by the synchronous Q-switch pulse from the Nd: YAG laser after a software-controlled, adjustable time delay. In this way, the intense background initially created by the high-temperature plasma was largely eliminated, and the atomic/ionic emission lines of the elements were more clearly observed. Usually, spectra from a number of laser shots (about 40–80) were acquired and averaged to increase the signal-tonoise ratio. Samples were manually moved between exposures to prevent crater formation and to avoid other deleterious effects. The spectrum, captured by the ICCD camera, was transferred to the personal computer by USB cable. All the functions of the ICCD camera and the Acton spectrograph was fully controlled by WinSpec/32 software provided by the manufacturer.15 In the present experiments, a delay time of 1000 ns and a gate width of 50 µs were selected for the optimum signal to noise ratio. Experiments were performed in open air.
Samples of bituminous coal were collected from Barapukuria coal mine in the Dinajpur district of Bangladesh. The coal samples with different ash contents were collected by trial and error method from different locations in the coal mine. The collected hard coal samples were first washed with distilled water and dried in sunshine which mostly removed the surface moisture. The sample was then ground into a fine powder in a hand mortar. The ground powder was then passed through a 75 micron sieving machine and thoroughly stirred which makes the sample most homogeneous to eliminate the possibility of matrix effects in the LIBS experiments. A part (of about 2 gm) of the ground coal from each batch of samples was used to prepare small pellets by using a hand press with sufficient pressure (about 80 bars) for the LIBS experiment. The remaining part of the same batch of powdered coal sample was kept for ashing. A number of pellets were prepared from each batch of coal samples and subjected to LIBS experiments as described above. Ashing of the coal samples were performed by the following method: About 2 gm of finely ground and sieved coal sample from each of the various batches wherefrom pellets were made for the LIBS measurements were taken in a crucible and heated in an electrical muffled furnace at such a rate that the temperature reached 500 C in 1 hr. The sample was heated at this temperature for two hours. The temperature of the furnace was then raised from 500 to 750 C in another hour, and the heating was further continued for 2 more hours at that temperature. This heating protocol under atmospheric conditions resulted in the complete expulsion of all water and volatile organic compounds (VOCs), and complete combustion of all the carbon in the coal, resulting in coal ash3, 13. At the end of the heating, the furnace was switched off and 3 ACS Paragon Plus Environment
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allowed to cool down and the crucible was taken out and weighed. To ensure total combustion of coal the heating protocol was repeated until the weight of the ash became constant. The fraction of ash content of the coal was ascertained by using the following simple relation: The fraction of ash content = Mass of ash from the furnace/Mass of initial coal sample. Sample pellets were prepared from the coal ash following the method described earlier. These ash pellets were also subjected to LIBS experiments particularly for trace element detection.
3. Results and Discussions LIBS spectra of coal sample were taken in the spectral range of 240 to 800 nm. Besides the major elements carbon and silicon, a number of minor and trace elements, such as iron, titanium, aluminum, calcium, sodium, copper, zirconium, neodymium, ytterbium, cerium, samarium, dysprosium and gadolinium were identified15. A strong line of carbon was identified at 247.856 nm, along with the lines of silicon (at 250.69, 251.61, 252.85 and 288.16nm) using the high resolution grating of 2400 lines/mm blazed at 240 nm of our spectrometer. From the LIBS experiments, it was found that the intensities of the carbon lines relative to the intensities of the lines of other elements (such as Si, Na, Ca and Fe) vary over wide limits for coal samples with different ash fractions. This variation of the intensities of the different emission lines is, of course, indicative of the differing ash contents of the coal samples. In an attempt to quantify the results, we measured the ratio of the intensities of the silicon lines (at 250.7 nm) to the principal neutral carbon emission line at 247.8 nm. The intensity ratios were calculated by measuring the areas under the two lines and by taking the quotient of these two areas. The two lines fell on the same window of the spectrometer even when the highresolution grating of 2400 lines/mm was used, therefore, eliminating the necessity of any grating scanning. Ash contents of the individual coal samples were determined by the ashing procedure described in section 2. Figure 1 shows the LIBS spectra of the carbon and silicon lines in the vicinity of 250 nm for different ash contents determined by ashing the different coal samples. As the ash content is increased, a clear dominance of the silicon line at 250.69 nm over the carbon line at 247.856 nm is observed. Fraction of ash content in coal was defined as the ratio of ash to total coal sample. This can be written as A / (A + C), where A = mass of ash (assumed proportional to the number of silicon atoms) and C = mass of carbon in the coal samples (assumed proportional to the number of carbon atoms). Now, if A/C (which is proportional to ISi / IC 17) is substituted by x, then Fraction of ash content = A / (A+C) = ISi / (ISi + IC) = x / (1+x).
(1)
Now if one calculates the ratio ISi / (IC + ISi), and plots this ratio as a function of the measured fraction of ash content A/(A+C), one can assume a linear relation between the two. This is shown in Figure 2 with a coefficient of correlation of 0.99. Such a high value of the coefficient of correlation provides enough confidence in our assumption of linear relation between these two. Here, the quantity (IC + ISi), is proportional to the total amount of coal mineral, including the ash and carbon, and the ratio ISi/(IC + ISi) represents better the fraction of the ash content rather than the ratio ISi/IC (=x) especially when the ash is a large fraction of the total amount of coal. This is why we obtain a good linear fit in Figure 2, since this emission line intensity ratio, 4 ACS Paragon Plus Environment
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ISi/(IC + ISi), correctly represents the ash fraction of any coal sample, be it of high ash content or of low ash content. So far, we have not addressed the question; why the intensity of the silicon line at 250.7 nm is indicative of the actual amount of ash in coal? If we recall that the ash in coal mostly consist of various silicates such as those of sodium, calcium, aluminium and iron, and forms an approximate stoichiometric mixture of these silicates in coal ash, then we can assume that the intensity of silicon line will be a good indicator of ash content. In order to test this hypothesis, we measured the integrated line intensities of the lines of major metals such as iron, aluminium, titanium and calcium, and calculated the ratio of the intensities of these lines to that of the silicon line. We plotted these ratios as a function of the actual ash content (as measured by ashing) for various samples of coal. For example figures 3 and 4 shows the ratios of the intensities of Ti I line (at 392.459 nm) and Ca I line (at 393.364) to the line intensity of Si I at 390.56 nm in the same spectral window as a function of the fraction of ash content of the coal which were fitted with straight lines. In all the cases, the fitted straight lines were roughly horizontal (slope of the line is almost zero), implying that the ratios of the above line intensities are approximately independent of the ash content for the different coal samples tested over a significant range of measured ash contents. Therefore, it follows that the ratios of the atomic concentrations of the metals to that of silicon are nearly independent of the ash content of the coal samples and can be taken to be constants. This means that the concentration of silicon is proportional to the concentration of all the metal atoms in coal. Hence it can be assumed that the silicon concentration in coal is proportional to all ash forming constituents of the coal samples. Therefore it is reasonable to assume that the intensity of the silicon line is a measure of the entire residual ash of the coal samples for most practical purposes. The higher the ash content, the greater is the intensity of the silicon line. The ratio of the intensities of the silicon line to carbon line will be highly correlated with the ash content of the coal, and a measurement of this ratio will enable one to estimate the ash content of any coal samples without performing the ashing process. Once the calibration curve such as the one shown in Figure 2 is obtained, it will be a simple matter to measure the ash content of an unknown coal sample by simply measuring the intensity ratio of the silicon line to the carbon line in a LIBS experiment. The fraction of ash content can be read off directly from the graph (Fig. 2). This calibration curve provides one with a simple and straightforward method for determining the ash content of any coal sample in a matter of few minutes, without actually performing the ashing process.
4. Conclusion We have devised a simple but effective method of estimating the fraction of ash content of coal by LIBS without performing the ashing of the coal. The method is simple and quick, since only two measurements of the intensities of the adjacent carbon and silicon lines in LIBS spectrum of the coal is all that is required. This method can be reliably employed for the determination of fraction of ash content of coals with ash content varying over a wide range. The procedure can even be incorporated in an automated industrial process, for example, in the online evaluation of ash content of coal samples moving in a conveyer belt in a large-scale coal-fired power plant. The excellent correlation (coefficient of correlation, r=0.99) between the intensity ratios of Si I and C I emission lines and the fraction of ash content determined by the actual ashing of the
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coal samples (figure 2) provides sufficient confidence in the measurement of ash content by this simple and quick method.
References : (1) International Energy Agency, Coal Information; OECD/IEA. 2007. (2) http://www.worldcoal.org/coal/coal-mining/ (3) Speight, J. G. Handbook of Coal Analysis; John Wiley and Sons, 2005. (4) Mateo, M. P.; Nicolas, G.; Yanez, A. Characterization of Inorganic Species in Coal by Laser-induced breakdown spectroscopy using UV and IR radiations; Appl. Surface Sci. 2007, 254, 868. (5) Ctvrtnickova, T.; Mateo, M. P.; Yanez, A.; Nicolas, G.; Characterization of coal fly ash Components by laser-induced breakdown spectroscopy; Spectrochimica Acta, Part B. 2009, 64, 1093. (6) Ctvrtnickova, T.; Mateo, M.P.; Yanez, A.; Nicolas, G. Laser-induced breakdown spectroscopy for ash characterization for a coal fired power plant; Spectrochimica Acta, Part B. 2010, 65, 734. (7) Yin, W.; Zhang, L.; Dong, L.; Ma, W.; Jia, S. Design of a Laser-induced breakdown Spectroscopy system for On-line Quality Analysis of Pulverized Coal in Power Plants; Appl. Spectrosc. 2009, 63, 865. (8) Romero, C. E.; De Saro, R.; Craparo, J.; Weisberg, A.; Moreno, R.; Yao, Z. Laserinduced breakdown spectroscopy for coal characterization and assessing slagging propensity; Energy and Fuels. 2010, 24, 510. (9) Ctvrtnickova, T.; Mateo, M. P.; Yanez, A.; Nicolas, G. Application of LIBS and TMA for the determination of combustion predictive indices of coals and coal blends; Appl. Surface Sci. 2011, 257, 5447. (10) Low, F.; Zhang, L. Microwave digestion for the quantification of inorganic elements in coal and coal ash using ICP-OES; Talanta, 2012, 101, 346. (11) Yao, S.; Lu, J.; Zheng, J.; Dong, M. Analyzing unburnt carbon in fly ash using laserinduced breakdown spectroscopy with multivariate calibration method; J Anal. At. Spectrom. 2012, 27, 473. (12) Feng, J.; Wang, Z.; Li, L.; Li, Z. ; Ni, W. A Nonlinearized Multivariate Dominant FactorBased Partial Least Squares (PLS) Model for Coal analysis by Using Laser-induced Breakdown Spectroscopy; Appl. Spectros. 2013, 67(3), 291. (13) Gaft, M.; Sapir-Sofar, I.; Modiano, H.; Stana, R. Laser Induced Breakdown Spectroscopy for bulk mineral online analyses; Spectrochimica Acta Part B. 2007, 62, 1496. 6 ACS Paragon Plus Environment
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(14) Gaft, M.; Dvir, E.; Modiano, H.; Schone, U. Laser Induced Breakdown Spectroscopy machine for online ash analyses in coal; Spectrochimica Acta Part B. 2008, 63, 1177.
(15) Haider, A. F. M. Y.; Rony, M. A.; Lubna, R. S.; Abedin, K.,M. Detection of multiple elements in coal samples from Bangladesh by Laser-induced Breakdown Spectroscopy; Optics and Laser Technology, 2011, 43, 1405. (16) Haider, A. F. M. Y.; Lubna, R.S. ; Abedin, K.M. Elemental Analyses and Determination of Lead Content in Kohl (Stone) by Laser-Induced Breakdown Spectroscopy; Applied Spectroscopy, 2012, 66(4), 420. (17) Cremers, D.A.; Radziemski, R.J. Handbook of Laser-Induced Breakdown Spectroscopy; John Wiley: New York, 2006.
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L in ea r R e g re ssio n for D ata 1 _B : Y = A + B * X P aram e ter V alue E rror -----------------------------------------------------------A -0.0 5 58 0 .02 4 13 B 1 .0 98 5 7 0 .0 62 0 4 -----------------------------------------------------------R SD N P -----------------------------------------------------------0 .98 9 02 0 .04 6 07 9 < 0 .00 0 1 ------------------------------------------------------------
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Figure 3. ITi to ISi Vs fraction of ash content. A straight line fit is shown by the solid line.
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