Transformation of functional groups during lignite heat-treatment and its effects on moisture re-adsorption properties

The surface functional groups of lignite are thought to be the significant factor influencing the moisture re adsorption behavior of dried coal. Several lignite samples with different surface functional groups were prepared by thermal treatment from 160 degrees C to 240 degrees C in N-2 and air. In situ FTIR and chemical titration methods were carried out to measure the functional groups of the raw coal and dried samples, and the results from these two methods were compared. FTIR spectra were deconvoluted to reveal the transformation of different functional groups, e.g. CH3, CH2, CH, anhydrides, esters, aldehydes, carboxylic acid, quinones, and ketones. The moisture re-adsorption behaviors of the heat-treated coals under relative humidity of 75% at 25 degrees C were measured, and the effect of functional groups on moisture re-adsorption property was investigated. The results showed that CHx functional groups decreased dramatically, especially in air, when heat-treating temperature was higher than 200 degrees C. For the CH, CH2, and CH3 functional groups, their decomposition activation energies are 29.6, 44.2 and 19.3 kJ.mol(-1), and their oxidation activation energies are 65.9, 36.1 and 36.5 kJ.mol(-1), respectively. In air, when the heat-treating temperature raised to 240 degrees C, aromatic esters, aliphatic esters and anhydrides all sharply increased. The quinones were probably produced from the transformation of phenolic hydroxyl (Ar-OH) at 240 degrees C in air. Moreover, coal samples heat-treated in N-2 and air showed different equilibrium moisture capacity, which can be attributed to the different contents of hydrophilic groups. The contents of ketones, aldehydes, COOH and Ar-OH had a good linear relationship with equilibrium moisture capacity. The hydrophilic abilities of ketones, aldehydes, COOH and Ar-OH in lignite are higher than that of quinones, esters and anhydrides.