An innovative method for carbonization and activation of low-rank coal via a self-sustaining counter-current combustion process and its reinforcement strategy.

Gradation and quality utilization of low-rank coal is promising. In this study, a special counter-current combustion process, characterized by a combustion front traversing the bed against the air supply direction, was innovatively proposed to achieve the carbonization and activation of low-rank coal. This combustion process was first analysed experimentally in a fixed bed test rig. At an air equivalence ratio as low as 0.25, this combustion process was of a front temperature of 889 °C and self-sustainedly traversed the fuel bed at a propagating rate of 0.33 m/h. Accompanied by its propagation, about 82.5% of volatile matter inside the coal was removed. Due to different reaction phases inside the bed, the solids were in-situ activated by self-produced active components. Based on this, intensification experiments of carbonization and activation were carried out in a moving bed test rig. By moving the bed, the heat transfer near the combustion front tended to a quasi-equilibrium state and the residence time of the solids in the high temperature zone was greatly prolonged. The carbonization and activation effects on the solids were promoted. The carbonization effect almost approached the limit because the removal ratio of volatile matter inside the coal was as high as 98.4%. When external heating and active substances intervened, activation reactions of the solids (namely, C–H 2 O and C–CO 2) were intensified in the char reaction phase. The specific surface area and pore volume of the activated char increased up to 594 m2/g and 0.371 cm3/g. Correspondingly, its adsorption values of iodine and methylene blue were 732 and 76 mg/g. This work provides a technical solution for the in-situ high-quality utilization of low-rank coal. • A method to carbonize and activate coal by a combustion process is proposed. • Dynamically propagating combustion front removes 98.4% of volatile matter. • Activation of solids is related to bed structure with different conversion zones. • Intervention of auxiliary heating and active substances promotes the activation. [ABSTRACT FROM AUTHOR]