Optimizing pyrolysis of sawdust and spent sulfite liquor: synergy, activation energy reduction and gas selectivity

Abstract

This study investigates the synergistic effects and pyrolysis kinetics of blends of sawdust (SD) and spent sulfite liquor (SSL) from a magnesium-based acid sulfite pulping process using thermogravimetric analysis (TGA). Experiments were conducted at heating rates of 5, 10, 15, 20, and 25 °C min⁻¹ over a temperature range of 25–900 °C. The most favorable synergistic interaction was observed for the blend containing 80% SSL and 20% SD. Kinetic parameters were determined using model-free isoconversional methods (Friedman, Ozawa–Flynn–Wall, and Vyazovkin). The activation energies for SD were 180.10, 163.95, and 144.60 kJ mol⁻¹, while SSL exhibited higher values of 284.90, 241.33, and 222.28 kJ mol⁻¹ for the respective models. Incorporating SD into SSL reduced the activation energy by approximately 20%, with the 80 : 20 blend showing values of 206.56, 195.66, and 177.31 kJ mol⁻¹. Char yield was unaffected by heating rate. Evolved gas analysis revealed distinct selectivity: the 80 : 20 blend favored hydrogen evolution (>0.06 wt%) near 550 °C which is 6 times more than SD alone, SD favored ethane (∼0.18 wt%) at 400 °C, and SSL favored methane (∼0.4 wt%) at 500 °C. These findings highlight the potential of SSL–SD blends for optimized pyrolysis performance and targeted gas production.