Role of alkali sodium on the catalytic performance of red mud during coal pyrolysis

Red mud (RM) as large quantity of industrial wastes could serve as a low-cost catalyst for catalytic upgrading of coal pyrolysis volatiles. However, the catalytic performance of raw RM for improving the quality of tar was limited because of its composition and structure. In this paper, the RM-based catalyst (RMC) for coal pyrolysis was prepared by using acid-alkali treatment. It was found that changing sodium content could modify the characteristics of RMC. The BET and NH3-TPD showed that the surface area and acid strength of RMC increased evidently with the reduction of sodium content. The pyrolysis characteristics of samples by mix shaping of coal and RMC with different sodium contents were evaluated in a fixed-bed reactor. The addition of RMC into coal could upgrade tar quality, thus increasing the yield of light tar (boiling point < 360 degrees C) and decreasing the yield of heavy tar. The content of sodium in the RMC had a great influence on product distribution of coal catalytic pyrolysis. The low-content sodium with high concentration acids contributed to the formation of coke. Whereas, with the increase of sodium content in RMC, coking was suppressed but more gaseous products were produced. After adding 4 wt% RMC with sodium content of 3.2 wt% into coal, the yield of light tar increased to 7.7 wt% and the light fraction in tar was as high as 74%, much higher than those from coal pyrolysis without catalyst.