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Propagation characteristics of reverse combustion significantly impact the conversion of solid fuels in packed beds, especially under variable reaction atmosphere conditions. In this study, the propagation of reverse combustion was experimentally studied in an oxygen-limited and -enriched O 2 /N 2 flow with an oxygen concentration range of 21–100 % for a bed packed with rice husk. The temperature profile measured by thermocouples along the bed height was processed to appreciate the driving mechanism of combustion front, as well as the reaction structure inside the bed. Besides, the contribution of volatiles gas-phase oxidation and char surface oxidation to reverse combustion was also evaluated. Results show that compared to the air case, an oxygen-enriched condition broadens the operating range of oxygen flow rate for the oxygen-limited reverse combustion. Under high oxygen concentration conditions, the low-temperature oxidation of rice husk inside the reaction front becomes enhanced. Such chemical reaction effect widens the whole reaction front thickness and thereby reduces the oxidation intensity in local regions, resulting in a faster ignition rate and a lower temperature for combustion front. It is found that this process is dominated by the gas-phase oxidation of volatiles, of which oxygen consumed is above 60 % of the supplied. As oxygen concentration increases, the gas-phase oxidation of volatiles becomes further enhanced. Correspondingly, the oxidation ratio of char is gradually approaching a limit, about 12 %, especially at the oxygen concentration above 50 %. These findings will facilitate a better fundamental understanding of driving mechanism of this special combustion process and thereby provide an important theoretical basis for efficient thermal conversion of solid fuels. [ABSTRACT FROM AUTHOR]

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]