Spatial migration characteristics of coal powder medium in a gas-solid zonal fluidized bed and impact on low-rank lignite upgrading

Coal occupies a dominant position in China’s energy structure, with lignite representing a particularly abundant resource. Enhancing the quality and efficient utilization of low-rank lignite is therefore of substantial significance for alleviating energy shortages and promoting sustainable energy development. In this study, a gas – solid area autogenous-medium fluidized bed was employed to upgrade low-rank, high-sulfur lignite through density-based dry separation. The research primarily focused on elucidating the migration and spatial distribution characteristics of coal-powder medium particles with differentiated densities within the area-specific fluidized bed. The dominant density composition of the coal-powder medium and the optimal fluidization area were identified. Furthermore, the effects of operating parameters on the spatial uniformity and stability of bed density, as well as on the distribution patterns of sulfur content, were systematically analyzed, leading to the delineation of functional Areas within the bed. The study also examined the significance levels of various operating factors and their interactions on separation performance, from which the optimal parameter combination was determined. Additionally, the mass distribution of materials across different areas of the bed after separation was investigated. Experimental results indicate that the optimal single-density and mixed-density compositions of coal-powder media for forming a stable area fluidized bed are ρ = 1.45 g/cm3 and ρ = 1.45 g/cm3 +1.65 g/cm3, respectively. Under these conditions, the coal-powder medium particles exhibit uniform and stable distribution throughout the bed, with minimal fluctuations in the mean bed-density profile, yielding an optimal fluidized medium layer in Area I. Three functional spatial Areas within the bed were identified: Area I functions as a low-density separation medium layer enriched in low-sulfur materials; Area II serves as a medium- and high-density stratified area where medium-sulfur materials are interspersed; and Area III acts as a high-density extraction layer uniformly enriched with high-sulfur materials. Among the operational factors, the degree of significance in influencing separation performance follows the order frequency > amplitude > gas velocity. The optimal parameter combination was determined to be A = 3.0 mm, f = 30 Hz, and v = 0.65 m/s. Under these optimal conditions, the upgraded low-rank high-sulfur lignite achieved a clean-coal ash content of 8.67% with a yield of 66.56%, whereas the tailings exhibited an ash content of 43.20% with a yield of 33.44%. The probable error was E = 0.12 g/cm3, demonstrating that the process effectively accomplished clean dry upgrading of low-rank high-sulfur lignite in the 25–0 mm size range.