Investigation into the evolution of char structure using Raman spectroscopy in conjunction with coal petrography; Part 1

Simple procedure for an estimation of the coal rank using micro-Raman spectroscopy

“…This increases the number of the parameters describing the line because it is necessary to obtain the percentage of contribution of the Gaussian and the Lorentzian functions. In many papers that report a fit with a pseudo-Voigt function, the percentage of each curve to the total is not explicitly stated (Beyssac et al, 2003;Guedes et al, 2010), or else indicated as percentage 100% L + G (Chabalala et al, 2011), implying de facto a Lorentzian fit. Ferrari and Robertson (2000) claim that the Lorentzian fit is normally 800 1000 1200 1400 1600 1800 1.…”

“…The history of different approaches to coal analysis with Raman spectroscopy has been reviewed by Potgieter-Vermaak et al (2011). Raman spectroscopy has been used to estimate coal rank Kelemen and Fang, 2001;Liu et al, 2013;Marques et al, 2009;Nestler et al, 2003;Quirico et al, 2005;Ulyanova et al, 2014;Valentim et al, 2010;Zerda et al, 1981) and to track maturity changes in coals or kerogens after some additional thermal treatment (Bernard et al, 2010;Chabalala et al, 2011;Green et al, 1983;Johnson et al, 1986;Li et al, 2006a,b;Liu et al, 2014;Sheng, 2007;Zaida et al, 2007;Zhang et al, 2011;Zhou et al, 2014), high pressure application (Bustin et al, 1995), anisotropic stress (Urban et al, 2003), or metamorphic transformation (Bonal et al, 2006;Lahfid et al, 2010;Rahl et al, 2005;Wopenka and Pasteris, 1993).…”

“…Cuesta et al (1994) used 4 functions "principally in order to improve fittings in the deconvolution of spectra", while Zaida et al (2007) describe the instability in the fit parameters of the minor bands (non-G or non-D). According to several authors (Bernard et al, 2010;Beyssac et al, 2003;Chabalala et al, 2011;Lahfid et al, 2010;Sadezky et al, 2005;Sheng et al, 2007;Ulyanova et al, 2014;Urban et al, 2003;Valentim et al, 2010;Zhou et al, 2014), Raman spectra of carbonaceous material contain up to five vibrational modes that can be adjusted with five Gaussian or Lorentzian functions. Next to the above-mentioned G-and D(D1)-bands, three additional vibrations due to disorder in the crystal lattice were included: the D2 vibration, centered~1620 cm −1 , related to disorder inside the graphite planes (E 2g ); the D3 vibration, centered around 1500 cm −1 , due to defects outside the plane of aromatic layers like tetrahedral carbons; and the D4 band, centered around 1200 cm −1 , attributed to disordered graphitic lattice (A 1g symmetry) or ionic impurities.…”

“…These additional bands can be clearly seen in some highly ordered carbonaceous materials like anthracite and graphite (Bernard et al, 2010;Rodrigues et al, 2013;Tan et al, 2004). Guedes et al (2010) used 6 lines to fit the Raman spectra of coals; however, their fit does not include the D2 vibration mentioned by Sadezky et al (2005) or Chabalala et al (2011). Several authors utilized as many as 10 different lines between 1200 and 1800 cm −1 to fit the spectra from chars of brown coal (Li et al, 2006a,b;Zhang et al, 2011) or biomass (Keown et al, 2007).…”

Changes in nascent char structure during the gasification of low-rank coals in CO2

“…The quantitative analysis of catalysts is relatively easy but the direct quantification of char structural features is a challenge. Raman spectroscopy has been used as a powerful technique to obtain information about the carbon skeleton in char during the pyrolysis/gasification of low-rank coals [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] and biomass 27]. The reaction pathways between gasification in O2, CO2 and H2O are different at 800 °C [9,18,19].…”