The critical role of anions in the porous biochar structure and potassium release during the potassium-assisted pyrolysis process

Abstract

Potassium-assisted pyrolysis is an efficient approach for preparing versatile porous biochar. However, among potassium additives, it is hard to decide which is more applicable due to the lack of understanding of the difference in their effects on the pyrolysis process. In this study, the critical impact of different anions in pyrolytic interaction was disclosed on the biochar structure and process sustainability via introducing a key property, proton affinity related to ionization of anions. The proton affinity of anions was found to have a crucial effect on the biochar structure. Anions with high proton affinity (pK_a1 > 4.76) lead to morphology rearrangement like cavity formation in particles, increase in the ratio and size of intact graphene domains, and development of porosity (1282–1582 m² g⁻¹). More importantly, the potassium release rate was measured as a metric for process sustainability. Potassium release at a relatively low or high temperature was differentiated into two stages viz. pyrolysis and activation potassium release. Anions with multiple charges and high proton affinity would strengthen pyrolytic potassium release (0.4–12%), while these selected anions with K₂O or K₂CO₃ as their final form had no obvious effect on the activation potassium release (0.88 ± 0.52%) below 800 °C. This study may provide a better understanding of the interaction between the char intermediate and potassium additives and assist in building a sustainable and efficient porous biochar preparation approach.