Macromolecular structure evolution and its significance for perhydrous coal under drying and pyrolysis conditions

ABSTRACT

No. 13-1 perhydrous coal (PHC) was dried by fluidized bed drying with N₂ used as the fluidizing medium in drying characteristics experiments. The PHC was characterized before and after the low-temperature drying process by Fourier transform infrared (FTIR). The drying temperatures tested were 100, 150, and 200°C. Changes in the major functional groups of PHC before and after the drying process were examined on the basis of the FTIR curve-fitting analysis. In addition, the PHC was pyrolyzed through a thermogravimetric analyzer. The pyrolysis temperature ranged from 300 to 650°C, and the temperature increment value was 50°C. The macromolecular structure of PHC before and after pyrolysis was characterized by Raman. The Raman parameters were obtained by spectral curve-fitting analysis. The FTIR results show that following the drying process, the aliphatic hydrocarbons in PHC progressively weakened with increasing drying temperature. Compared with the methyl groups, the oxidation activity of the methylene groups was higher in the low-temperature drying process. As the drying temperature increased, the main oxygen functional groups decreased. Overall, drying provides the chemical environment for hydrogen transfer in PHC, leading to a decline in aliphatic hydrocarbons and a rise in aromatic hydrocarbon content, and accelerates the polycondensation and ordering of large molecules in PHC. The Raman results show that with increasing pyrolysis temperature, the macromolecular structure of PHC becomes more and more similar to that of graphite. The Raman spectral parameters are closely correlated to reflectance. Considering the stability and reliability of the Raman parameters, the full width at half maximum (FWHM) of D2- and G-band was used to characterize the metamorphic grade of the PHC. The FWHM of the D2- and G-bands showed a linear relationship with PHCR₀ in the range of 2.0–7.6%, indicating that the FWHM parameters can be used to determine the PHC rank.

KEYWORDS:

• Drying
• macromolecular structure evolution
• perhydrous coal
• pyrolysis

Acknowledgment

We acknowledge joint support from the National Natural Science Foundation of China (No. 41502152, No. 51404260), the Visitor Foundation of State Key Laboratory of Coal Mine Disaster Dynamics and Control (Chongqing University) (No. 2011DA105287-FW201501), and the Basic Research Program of Jiangsu Province (No. BK20140206).