FTIR and Raman spectroscopy characterization of functional groups in various rank coals

Coals varying in rank from lignite to anthracite were comprehensively characterized with regard to chemical structure using FTIR and Raman spectroscopy. Structural parameters were determined from spectrum deconvolution, and correlations were established for these parameters. The evolution of coal structure is complex with three stage characteristics at Ro = 0.4–1.3%, 1.3–2.0% and 2.0–4.0%, respectively. At Ro = 0.4–1.3%, the rapid decline in oxygen-containing functional groups accompanied with the enrichment of aliphatic compounds is the main characteristic. When Ro = 1.3–2.0%, the sharp changes of the band position difference (G-D1), AD1/AG and apparent aromaticity (fa) are caused by the strongest aromatization at this period. The aromatic rings and degree of graphitization are significantly enhanced, with the length of aliphatic chains (A(CH2)/A(CH3)) decreasing to the lowest level. When Ro = 2.0–4.0%, the slow variation of parameters (fa, ‘C’, G-D1 and AD1/AG) confirms that most of heterocycle compounds and amorphous carbon structures have been lost. The size of aromatic clusters and graphite crystallite continuously increase, and the microcrystalline structure in coal is gradually perfected, closer to graphite structure. These findings can provide a comprehensive understanding of chemical and physical properties of coals.