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 degrees 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 H-2. 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-C=O groups, resulting in a large amount of O-C=O 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 degrees 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.