Using Raman spectroscopy to evaluate coal maturity: The problem. (15th March 2022)

• Record Type: Journal Article
• Authors: Cheng, Nannan; Pan, Jienan; Shi, Mengyan; Hou, Quanlin; Han, Yuzhen
• Is Part Of: Fuel. Volume 312(2022)
• Publication Date: 2022-03-15
• Subjects: Coal -- Maturity -- Raman spectroscopy -- Chemical structure evolution -- Stress; Fuel -- Periodicals; Coal -- Periodicals; Coal; Fuel; Periodicals; 662.6
• DOI: 10.1016/j.fuel.2021.122811
• Languages: English
• ISSNs: 0016-2361

Abstract

Highlights: The discrepancy between Raman parameters and maturity was found. The chemical structure evolution does not seem to active the Raman D peak. Stress can cause an increase of D peak by inducing structure defects. Errors caused in data acquisition procedures may contribute to the discrepancy. All factors need to be considered to obtain more representative Raman parameters. Abstract: Raman spectroscopy has increasingly been used as an alternative to evaluation the maturity of coals and organic matters, with more than 84% related articles published during the year 2010–2020. The disordered peak (D peak) in the Raman spectra is generally considered to represent the structure disorder in coals, thus it is expected increasing maturity would produce a decrease in D peak and Raman intensity ratio ID /IG . However, the many correlations between well-established maturity parameters (such as vitrinite reflectance, H/C, and volatile content) reveal that the D peak is also found to increase in literature. This work tries to give some insights into the factors causing the discrepancy above. Firstly, the maturation process is characterized by the removal of aliphatic linkage and non-carbon atoms, concurrently with the growth of aromatic layers. Compared with the maturity parameters, the D peak reflects more evolution information of structure ordering. Heat treatment results also show that the chemical structure does not always change to structure ordering before graphitization, and the evolution paths of different substances vary greatly. Hence the chemical structure evolution seems not a necessary requirement to active the D peak. Furthermore, stress can influence both the maturation and structure ordering evolution, which also causes an increase of D peak by inducing structure defects. Curve fitting methods and structure heterogeneity may also contribute to the discrepancy. Therefore, both errors caused in data acquisition procedures and factors influencing the structure ordering evolution need to be considered when using Raman spectroscopy to evaluate coal maturity.