Co-pyrolysis of coal and papermaking black liquor: Synergistic effect, catalysis and migration of alkali metal sodium

Abstract

This study investigated the characteristics and mechanism of co-pyrolysis of Shenmu coal (SM) and papermaking black liquor (BL). Experiments were conducted in a fixed-bed reactor at temperatures ranging from 500℃ to 700℃, with varying mass ratios of SM and BL. The pyrolysis products were characterized using multiple analytical instruments. Alkali metals (sodium) in BL exhibited significant catalytic activity during co-pyrolysis, promoting volatile product formation and tar cracking. At 500℃, a notable synergistic effect was observed in the co-pyrolysis of SM and BL, with the highest gas yield and optimal tar quality achieved at the SM₇BL₃. As the temperature increased to 600℃, the distribution and properties of co-pyrolysis products were further optimized, resulting in the maximum tar yield and a significant increase in gas production. Analyses using XRF, SEM, XRD, and FTIR revealed that BL addition reduced the char’s crystallinity and graphitization degree, while increasing its porosity and reactivity. The study also elucidated the migration behavior of sodium during co-pyrolysis. Although most sodium remained in the solid phase, its presence facilitated migration to the gas phase, thereby enhancing gas product formation. This research provides important experimental data and theoretical support for the development of coal and biomass co-pyrolysis technology, contributing to the co-pyrolysis process optimization. It benefits energy efficiency and product quality improvements while offering new pathways for the resource utilization of BL.