Revealing the anion-dependent effects on potassium-assisted biomass pyrolysis

Abstract

Metal additives could significantly improve the product quality of biomass pyrolysis. However, the effect of anions added with metal cations has not been fully understood. To shed light on the anion effect, biomass pyrolysis assisted with different potassium agents was launched at parallel conditions, and the product and element distributions were investigated. Compared to the weak influence of neutral agents (i.e., KCl), alkali agents (i.e., KOH) strongly promote the formation of char (20.0–31.3 wt%), H2 (15–92 cm3/g), and phenols (50–65 area %), but inhibit CO, carbohydrates, and furans. Oxidizing agents (i.e., KMnO4) perform similar effects with alkali agents and raise the yields of CO2 and H2O. Alkali agents facilitate carbon and hydrogen migration into biochar and gas, respectively, where higher basicity is related to enhancing the above tendency. Furthermore, proton affinities were calculated, and the correlation between dehydrogenation and additive properties including proton affinity and anion charge was first established. Based on the correlation results, the shielding effects of anions were disclosed, where the dehydrogenation effects of K+ were shielded by anions endowing low proton affinity. The competition effect of borate and phosphates anions hinders the K+ effect by introducing intermediate cross-linking structures. Besides, the self-decomposition of acetate, dehydrogenation catalyzed by iron and manganese, and dehydration catalyzed by chloride were recognized as parallel effects that do not severely influence the potassium effect. This study provides novel insight into the effect of additives in pyrolysis and will help reinvent the in-situ product-upgrading approach of biomass pyrolysis.