Effect of various potassium agents on product distributions and biochar carbon sequestration of biomass pyrolysis

Biomass to biochar through pyrolysis is promising for carbon-negative biomass energy. Potassium could enhance biochar formation whereas the difference among potassium agents has not been understood. Herein, 13 potassium agents were adopted, and the product compositions and carbon distributions of slow and intermediate pyrolysis were investigated. Results show that neutral agents have a weak influence on the products, while high-basicity agents enhance biochar formation, decrease organics yields, and promote H2 generation (9–144 cm3/g) and phenols selectivity (64–79 area%). Especially, due to the anion properties, borate and phosphates deviate from the basicity tendency; acetate contributes to increasing acetic acid selectivity (up to 43 area%) while oxidizing agents promote CO2 and H2O yields. Potassium performs obvious enhancement of biochar carbon sequestration, increasing by 2–49 % for intermediate pyrolysis and -8–51 % for slow pyrolysis with biochar carbon retention reaching 55 % and 77 %, respectively. Meanwhile, the thermally unstable phases of biochar were also generated. General correlation analysis indicates that CO yields and phenols selectivity highly relate to biochar carbon retention. This study sheds light on the key role of acid-base properties and anion properties, and provides insight into AAEM-catalytic biomass pyrolysis.