Evolution of the Char Structure of Lignite under Heat Treatment and Its Influences on Combustion Reactivity†

Abstract

A lignite and its demineralized sample were pyrolyzed in a tube furnace under various temperatures from 773 to 1673 K. The resulting chars were systematically characterized with Raman spectroscopy for carbon microstructure, Fourier transform infrared spectroscopy for functional groups, and N2 adsorption isotherm for pore structure. The reactivity of char combustion was measured with thermogravimetric analysis based on a non-isothermal approach to derive the reactivity index and kinetic data. Over the range of heat-treatment temperatures studied, char structure evolution had different behaviors before and after 1073 K. Increasing the treatment temperature from 773 to 1073 K led to the increases in the concentrations and the growth of aromatic ring sizes indicated by the evolutions of Raman parameters, the decreases in functional groups, and the increase of smaller pores. When the treatment temperature was increased from 1073 to 1673 K, the evolutions of Raman band area ratios and pore structure indicated that the main change was the microstructure ordering. Mineral matter was found to have little impact on the structure evolution of the lignite char. Combustion reactivity index and activation energy were observed to increase with the treatment temperature because of char structure evolution and ordering. Mineral matter was found to have a catalytic effect on the reactivity but no influence on the activation energy. Fairly good linear correlations were found between the reactivity index and band area ratios of ID1/IG and IG/IALL when considering the structure evolution behaviors at lower and higher temperature regions, respectively.