Physicochemical properties and combustion kinetics of dried lignite

High water content in lignite leads to its low combustion efficiency, which hinders its large-scale utilization. Herein, lignite was dried under nitrogen atmosphere and changes in its pore structure and functional groups were analyzed. The combustion behavior of dried lignite was simulated using a thermogravimetric method. Moreover, the relationship between the lignite surface structure and combustion process of dried lignite was established based on grey correlation analysis. The results reveal that the pore structure of dried lignite was considerably damaged. At a drying temperature of 90 °C, the largest surface roughness and irregularity as well as the smallest the total pore volume and average pore diameter of lignite observed. As the drying temperature increased, the total pore volume first increased and then decreased; moreover, the average pore diameter increased from 13.55 to 15.75. With increasing drying temperature, the CH3/CH2 value of lignite initially increased up to 0.088 and then gradually decreased after 160 °C. At temperatures higher than 90 °C, the number of hydroxyl and carbonyl groups began to decrease. The worst and best combustion performances of lignite were observed when the drying process were conducted at 90 and 230 °C respectively. According to the results of grey correlation analysis, the average pore diameter exhibited the highest correlation coefficient (0.83) with the combustion performance of lignite. The drying process of lignite changes its average pore diameter; therefore, the combustion performance can be improved by adjusting the pore structure of lignite during the drying process. Furthermore, it is recommended to dry lignite at 230 °C to reduce energy loss during the drying and combustion processes and improve the utilization efficiency of lignite.