Effect of red mud-based additives on the formation characteristics of tar and gas produced during coal pyrolysis

Red mud (RM), an industrial solid waste, was introduced into the coal pyrolysis reaction system, and its effect on the formation characteristics of tar and gas products during coal pyrolysis was explored based on the regulation and characterization of RM structure along with a series of fixed-bed pyrolysis experiments. The results show that Fe₂O₃ in RM could react with reducing agents, e.g., solid carbon, H₂ and CO, in coal pyrolysis environment, along with the gradual reduction process of Fe₂O₃→Fe₃O₄→FeO→Fe. As a result, coal conversion as well as release of light gas, such as CO and CO₂, are both promoted. Influence tendency of RM after acid treatment (ARM) is consistent with that of the initial RM but the magnitude becomes greater. Compared with RM, ARM has a larger specific surface area and more developed mesoporous structure, thus providing a stronger regulation performance on tar composition. In the presence of ARM, contents of phenols, indenes and aliphatics in tar decrease, accompanied by the increase of monocyclic aromatic hydrocarbons, and the content of BTX and its yield increase by 76.25% and 47.90%, respectively. Furthermore, both the light components (boiling point, BP ≤ 360 °C) and light oil (BP ≤ 170 °C) in tar reach higher levels with addition of ARM. Therefore, ARM is an effective multifunctional catalyst for in-situ upgrading of tar during coal pyrolysis process.