Towards enhanced understanding of the synergistic effects between potassium and calcium in biomass catalyzed pyrolysis

In this study, the influences of different parameters on the products of K-Ca synergistically catalyzed biomass pyrolysis and the possible chemical reactions during the process were investigated. Results indicated that, according to the standard Gibbs energy calculations, Ca(OH)₂ was more likely to react with carboxyl groups (precursors of CO₂) in biomass than KOH at low temperature (< 600 °C), effectively preventing the generation of CO₂. Increasing the amount of Ca(OH)₂ significantly enhanced this interaction, resulting in gradually decrease of CO₂ and CO, and remarkably promoted the generation of H₂. With a considerable decrease in the O-containing species, the hydrocarbons and phenols (phenol accounting for 49 %) became the main species in organic liquid products. For solid products, KOH can react and remove most of the oxygen-containing functional groups, while Ca(OH)₂ can fixed O-CO groups, resulting in a large amount of O-CO groups remaining. Additionally, bimetallic carbonate K₂Ca(CO₃)₂ was formed at Ca(OH)₂/KOH/soybean straw ratios below 0.25:1:2, whereas K₂Ca₂(CO₃)₃ was generated at higher ratios exceeding 0.25:1:2. At higher temperatures (> 600 °C), the O-CO groups transformed into more stable CO groups, and K₂Ca₂(CO₃)₃ decomposed into K₂Ca(CO₃)₂, which increased the content of CO via hydrogenation. As analyzed above, the mechanism of K-Ca synergistically catalyzed biomass pyrolysis was elucidated.