Evolution of Physical and Chemical Structures in Lignite during Dewatering Process and Their Effects on Combustion Reactivity

The Inner Mongolia lignite was selected as the feedstock and dewatered by hydrothermal, fixed-bed, and microwave upgrading techniques. The physicochemical structures of the raw coal and dewatered coal samples were determined by N2 adsorption, Fourier transform infrared, and chemical titration methods. The combustion behavior of coals was investigated by thermogravimetric analyzer, and the results demonstrated that the combustion reactivity of the dewatered coals was enhanced. The apparent activation energies of the coal samples were obtained from nonisothermal single heating rate method, and the results illuminated that the combustion process of these coals could be divided into three stages according to the variations of their apparent activation energies. Those variations were used as the medium to investigate the correlations between combustion reactivity parameters and physicochemical structures. It was found that at stage I of the combustion process, the ignition temperature of the coal was mainly affected by intrinsic chemical reactivity; at stage II, the maximum combustion reaction rate of the coal was affected by intrinsic chemical reactivity and specific area diffusion, and the temperature corresponding to the maximum combustion reaction rate was mainly affected by intrinsic chemical reactivity; and at stage III, the burnout temperature was affected by intrinsic chemical reactivity, and the overall reaction rate was controlled by gas diffusion rate.