Application of Micro-Raman Spectroscopy for the Quantitative Analysis of Vitrinite Reflectance in Medium and High Rank Colombian Coals

The structural changes and the degree of maturity of three Colombian coals from the Santander and Antioquia provinces were studied using a simple and fast curve fitting method of the RBS parameters (the distance between band G and band D), FWHM G (total width at half the maximum of the G band) and AR (area of the Raman spectrum), obtained by deconvolution and band resolution of the Micro-Raman spectra. The results were compared by morphological and chemical analysis using a SEM scanning electron microscope coupled with a solid state X ray EDS detector. A percentage of vitrinite reflectance (% Ro) of 4.3 %, 3.8 % and 0.07 % was obtained, a percentage of carbon by EDS analysis of 91.47 %, 87.78 %, 73.4 %, a morphological description by SEM analysis presenting smooth, rough surfaces and rough-lumpy and pore size between 0.7 μm 0.8 μm, 1.3 μm 1.5 μm and 0.2 μm 1 M. Sc. (c) Universidad Nacional de Colombia (Medellín-Antioquia, Colombia). rdurbanon@unal.edu.co. ORCID: 0000-0001-5289-0621. 2 Ph. D. Universidad Nacional de Colombia (Medellín-Antioquia, Colombia). haestupinand@unal.edu.co. ORCID: 0000-0002-9607-3364. 3 Ph. D. Universidad Nacional de Colombia (Medellín-Antioquia, Colombia). gneira@unal.edu.co. ORCID: 00000003-1178-1917. Application of Micro-Raman Spectroscopy for the Quantitative Analysis of Vitrinite Reflectance in Medium and High Rank Colombian Coals Revista Facultad de Ingeniería (Rev. Fac. Ing.) Vol. 29 (54), e12241. 2020. Tunja-Boyacá, Colombia. L-ISSN: 0121-1129, e-ISSN: 2357-5328, DOI: https://doi.org/10.19053/01211129.v29.n54.2020.12241 0.7 μm for the anthracite, semi-anthracite and bituminous samples, respectively. The results showed that based on this method, it was possible to obtain a model that allows to rapidly analyze and characterize the coal. The results compare very well to those of Colombian coals normalized by other methods to improve the precision of the measurement. The modified method, however, did not offer an appropriate fit for the lower range coal and produced extremely low reflectance values. This effect is possibly related with the molecular structure of organic matter in coal or the presence of fluorescence interference.

0.7 µm for the anthracite, semi-anthracite and bituminous samples, respectively. The results showed that based on this method, it was possible to obtain a model that allows to rapidly analyze and characterize the coal. The results compare very well to those of Colombian coals normalized by other methods to improve the precision of the measurement. The modified method, however, did not offer an appropriate fit for the lower range coal and produced extremely low reflectance values. This effect is possibly related with the molecular structure of organic matter in coal or the presence of fluorescence interference.

I. INTRODUCTION
Colombia is the main producer of coal in Latin America and the fourth exporter in the world, mainly to the Netherlands and Turkey with 30% of the total production, which is close to 80 million tons per year [1][2][3][4]. The characteristics of Colombian coals vary widely with the region of origin, and those from the north of the country (Guajira and Cesar provinces) account for approximately 90 % of the total production, being mainly used for power generation. The remaining 10 % of coal is produced in the Santander, Córdoba, Cundinamarca, Boyacá, Antioquia, and Valle del Cauca provinces, and is suitable for power, metallurgical and other industrial applications The traditional use of coal as a power source has recently been under scrutiny. The global trend in reducing greenhouse emissions, mainly in European Union countries, has promoted a reduced use of coal-fired power plants and their transformation into cleaner alternatives, as coal combustion is one of the main sources of carbon dioxide emissions. Other economies, however, still depend strongly from the production and use of coal and finding alternative technologies is of prime importance in economic, social and environmental terms.
In the last decade, coal and its derivatives have also been identified as an unconventional source of rare earth elements (REE) [5][6], which are critical raw materials for high technology industries such as fuel cells, mobile phones, high capacity batteries, permanent magnets, phosphors and metal catalysts [7][8]. The presence of REE in the organic constituents of coal (called macerals) depends on the geological regions [9], and is associated with volcanic ash deposits, characteristics related to the Colombian geography. Alternative applications of coals include their potential to be used as an adsorbent material, particularly anthracite due to its high carbon content and its structure of micropores, mesopores and macropores.
The wide range of Colombian coals in its different reserves where there are qualities ranging from subbituminous for power generation [10][11], bituminous for metallurgical applications, synthesis of chemical products and liquid fuels through pyrolysis [12], and anthracites as adsorbent materials [13]. For this reason, it is necessary to identify and integrate conventional and advanced characterization techniques to provide a fast and reliable analysis in order to determine the applicability of a particular coal. Traditionally, proximal analysis is the preferred technique for the technological characterization of coal and includes the determination of moisture, ash, volatile matter, and fixed carbon. This system serves well for the metallurgical and power generation industries, for which caking capacity and calorific value are of great importance. However, due to the potential for property overlap, the exact determination of the range must be more precise. An alternative to improve the characterization of carbons is through Scanning Electron Microscopy Coal can also be characterized more precisely using petrographic techniques based on the reflectance of vitrinite (% Ro) [1,14], which is the most abundant and important maceral in coals, genetically formed by thermal alteration of the lignin and cellulose, where their properties vary with range. It is a component that provides information on the degree of metamorphism or coalification (range) of the carbon, measuring its reflectance.
%Ro can be estimated with light microscopy, however, because mineral matter is not always easy to identify under the microscope, the count may not be very accurate, in addition to the long analysis and preparation times of the sample.
Raman spectroscopy is also a technique that can be used to quantify% Ro, with the advantages of easy sample preparation, non-destructive micrometer-scale characterization, short sampling times, and low energy consumption. Vitrinite reflectance has been used as a reliable indication of the thermal maturity of coal [15].
Zhang and Li [1], have proposed an easy and fast method to determine the coal range by means of Raman Spectroscopy using samples with% Ro previously analyzed with traditional petrographic techniques, which were correlated with RBS, FWHM-G and AR obtained from of Raman spectra.
The objective of this work was to characterize three Colombian coals with different maturity, using samples of anthracite, semi-anthracite and bituminous coals with 96.6%, 78% and 72% carbon, respectively. Scanning electron microscopy (SEM) allowed to correlate the morphology of the stacked layers with the range of the carbon, X-ray energy dispersion spectrometry (EDS), allowed the determination and mapping of the atomic composition and Micro-Raman spectroscopy to estimate the % Ro.

II. MATERIALS AND METHODS
Three coal samples from three different locations in Colombia were selected to be studied, and their analysis were performed following the ASTM D 388-2012 standard. A first sample from the Páramo del Almorzadero region in Santander Province, north-east of the country, with 96.6% fixed carbon (FC) content, dense, exhibited high hardness, semi-metallic black colour, low thermal expansion, the same as low ash and moisture content, being classified as an anthracite-type coal.
A second and third samples, both from the Amagá District in Antioquia Province, central part of Colombia, had 78% and 72% FC respectively, fragile and grayish in colour. Initial classification of these two samples showed, in the first case, a correspondence to a semi-anthracite to bituminous low volatile -type coal and a bituminous medium volatile -type coal, in the second. In order to differentiate them, and for simplicity, the samples were therefore named in the results as anthracite, semi-anthracite and bituminous, respectively.
Morphological and chemical analyses were performed with a Carl Zeiss EVO MA10 SEM scanning electron microscope and an acceleration voltage of 25KV. Images were acquired in a magnification range of 500X to 5000X with a resolution of 1024x884 pixels. A series of high-resolution SEM images of the coal samples were captured at a working distance of 8.5mm. A 10 mm 2 solid state Oxford brand X-ray EDS detector was used to obtain a semi-quantitative assessment of the coal constituent elements, in selected areas of analysis. The characterization was carried out in both specific mode with 3 points of interest and in mapping mode to determine the distribution of the elements in the selected areas. Composition mapping was presented in pairs for each coal sample with three different elements on each map.
A Horiba Jobin-Yvon LabRam Raman Confocal Microscope, equipped with a solid state 532nm laser, was used in order to determine the characteristic spectra and the respective spectral bands of interest in each sample, with the following operating conditions: 50mW power, set at 1% sample surface, 12-second acquisition time, 10second accumulation time, and a 50X microscope objective focused on a laser area between 2μm and 20μm in diameter on the macerals selected for the analysis.
Spectra were acquired using a 600 gr/mm grid. The swept spectral range was 500-2500 cm -1 .
The reflectance of vitrinite (%Ro) was evaluated by fitting the curve of the Raman spectra using the deconvolution method with the second derivative, which allows to find precise information on the adsorption bands. The correlation between the carbon percentage and the Raman characteristic parameters for %Ro were analyzed following the methodology proposed by Zhang and Li [1].
Raman spectra were processed using a commercial statistics analysis program. The adjustment of the curves was made in the spectral range of 800 to 2000 cm -1 . A spectrum baseline procedure was carried out, selecting for this the spectral region where the characteristic D and G bands were located, to which a deconvolution procedure with the second derivative was then performed achieving a convergence to the Lorentzian function with the best R 2 , in order to obtain the RBS, FWHM-G and AR parameters. Figure 1 shows the SEM images where the main morphological features of the three coal samples used in this study can be seen. Figures 1(a), Figure 1(c) and Figure   1(e) correspond to images at relatively low magnifications (600X), and Figures 1(b), Figure 1(d) and Figure 1(f) correspond to those at higher magnifications (2,400X and 4600X). The sample of anthracite, Figures 1(a) and (b), exhibited a relatively smooth surface with a pattern of exfoliation or stacked layers, whose laminar texture can be associated with a most ordered carbon structure. This material presents a microporous type structure, with pores in the range between 0.7 µm and 0.8 µm. In general, this sample presents a structure which is denser than that of the remaining  Table 1 shows the results of the elemental composition of each type of coal obtained by SEM-EDS analysis. In this Table, it is noticeable the presence of organic sulfur, whose weight percentage increases with carbon content, in addition to the presence of inorganic elements such as aluminum, iron and silicon that are generally found in the macerals forming aluminocarbonate compounds, aluminosulfates and probably quartz, plagioclase, siderite, pyrite, goethite and hematite [16].  Figure 2 shows the element distribution maps of each type of coal, as determined by using the EDS detector and the INCA analysis software. Table 2 shows the details of the colour code used to present the information in Figure    shaped D band with a steep slope or background. This is attributed to interference from strong fluorescence in the lower rank (less mature) coals [1]. Eventually, for the highest rank (more mature) coals, D band will split from G band, and both become sharper, intense and better defined. The width of D and G bands also decrease with the increase in vitrinite reflectance. In this work, G band was predominant for all coal types, as it can be seen in Figures 3b through to 3d. On the other hand, the separation between the G band and the D band was also larger for the highest coal range, as it was the respective intensity of the peak. The D bands are associated with chemical and structural defects in the crystal lattice; as the temperature increases, the chemical defects are removed from the crystal lattice and the carbon reorganizes into a more ordered carbon structure, until it reaches the stage of metamorphism where the carbon residue is transformed into perfectly ordered graphite [15]. Following from the former discussion, the spectra from the anthracitic coal samples examined in this work exhibited the typical behaviour expected for a high rank, high level of maturity coal, with an excellent fit for a second derivative approach. On the other hand, the bituminous-type sample presented a less defined fit for both bands, D and G, which was related to a lower maturation level (range) and the increased difficulties for fitting the curve following the deconvolution, as it was mentioned above.

III. RESULTS AND DISCUSSION
The necessary parameters for the calculation of vitrinite reflectance (% Ro) were derived from the linearization procedure that relates the terms RBS, FWHM-G and AR with %Ro using the regression of equations (1), (2) and (3). Once the spectra for the anthracitic samples were processed, the following two equations (5) and (6) were derived in order to calculate %Ro for anthracite and semi-anthracite, respectively. In this case, equation (3) from the model by Zhang and Li [1] was omitted because the spectra did not show noticeable changes in slope at the beginning nor the end of the spectra. In other words, equation 6 takes into account the presence of fluorescence, which is more likely to occur in the analysis of low rank, bituminous carbon. Table 3 summarizes the results of the parametrization obtained from the Raman spectra, the calculated %Ro and the percentage of carbon determined by EDS analysis. The result of FC by ASTM analysis of each sample is also included with the purpose of comparison.   Gustavo Neira-Arenas: Formal Analysis, Writingreview & editing.