Fundamental trends in FT-IR spectra of coal in the thermoplastic range

Abstract

In the coke making process, the chemical and physical changes in the thermoplastic range are important factors to decide the coke strength. In this range, the main chemical changes such as macromolecular structural conversion, aromatic and aliphatic structural changes and the de-volatilization occur. Those chemical changes are affected by coal properties and conversely, that means it is good chances to find out some connections between coal properties and chemical changes. The change of coal spectra, especially aromatic C-H, aliphatic C-H from raw coals to heated coals at 500oC was found by Fourier transform infrared (FT-IR). Twenty three coking coals spectra were checked by FT-IR with KBR disk method. Some of them were analyzed from raw coals to heated coals at 500oC. The wave numbers of 3030, 2920~2850cm-1 are associated to aromatic C-H and aliphatic C-H, respectively. A_(aro_CH)⁄A_(ali_CH) is closely related to coal reflectance and aromaticity. On the other hands, it is roughly related to coal fluidity because aliphatic hydrogen affects coal fluidity more than aromatic hydrogen. High volatile coals have high amount of aliphatic and low A_(aro_CH)⁄A_(ali_CH) . This indicates that high volatile coals have high fluidity due to the abundance of aliphatic C-H which is easy to be broken in the thermoplastic range.For heated coals, FT-IR spectra showed that aliphatic C-H of each coal was almost broken because volatiles are mainly released from aliphatic C-H during the primary de-volatilization. However, aromatic C-H is still being broken until 500oC due to strong bond energy. So, A_(aro_CH)⁄A_(ali_CH) is sharply increased during thermoplastic range. The fluidity of coal blends improved due to abundant aliphatic C-H of high volatile coals when high volatile coal was mixed to coal blend. However, the reflectance decreased due to poor microstructure of high volatile coals. Additionally, it was found that the spectral ratio of coal blends could be calculated by the spectra of each coal without FT-IR measurement. The spectra of heated coal blends show similar trends to those for heated single coals. The more high volatile coals in coal blending are used, the more loss of aliphatic C-H will be resulted because high volatile coals have abundant aliphatic C-H and it mainly affects coal fluidity.