Study of Chemical Structure Changes of Chinese Lignite upon Drying in Superheated Steam, Microwave, and Hot Air

摘要

Chemical changes of Chinese lignite upon drying in superheated steam, microwave, and hot air have been studied in this paper using the Fourier transform infrared (FTIR) spectroscopy technique. The infrared (IR) Spectra of raw and dried samples were curve-fitted to a series of bands in aliphatic hydrogen (3000—2800 cm~(-1)) and carbonyl absorption (1850— 1500 cm~(-1)) zones. It has been found that aliphatic hydrogen absorbance decreased slightly with an increasing temperature during superheated steam drying, while absorption of carboxyl (COOH) and carbonyl (C=O) groups decreased drastically, indicative of the loss of oxygen functionalities with an increasing drying temperature. During steam drying, aromatic carbon and aromatic ring stretch absorption remained relatively unchanged up to 250 °C and decreased significantly thereafter because of some pyrolysis reactions that took place at higher drying temperatures. Microwave heating of lignite resulted in a significant decrease in the concentration of oxygen-containing functional groups. Aromatic carbon remained relatively unchanged under microwave drying conditions, while aliphatic hydrogen decreased slightly. The aromaticity of coal calculated from curve-fitted spectra of deconvoluted peaks showed a progressive increase with an increasing drying intensity under both steam and microwave drying conditions. Under air drying conditions, aliphatic hydrogen absorbance decreased drastically at 250 °C, while aromatic carbon remained unchanged. It was observed that oxidation in air mainly took place on aliphatic hydrogen sites, especially on methylene groups. Changes of carboxyl and carbonyl groups during air-dried samples showed a different trend compared to those dried in steam and microwave, increasing gradually up to 150 °C and then a sharp increase at 200 °C. The absorption of these groups decreased significantly at an increased air temperature up to 250 °C.