A Fit-for-Purpose Biochar Index for Classifying Hard and Soft Biochars in Circular Carbon Storage Applications

As a multifunctional material for the circular bioeconomic applications and carbon storage, biochar is getting rapid attention. Addressing the lack of industrial application-oriented, standardized biochar characterization framework, the current study presents a fit-for-purpose biochar index (FitPBC) that quantitatively assesses biochar classification as hard biochar and soft biochar. Biochar derived from sludge, rice husk, tea waste, and Gliricidia, produced at different pyrolysis temperatures (300, 500, and 700 ℃), was evaluated. Using a multi-factor rating weighted framework, FPI calculations were carried out with weighted parameters representing carbon structural maturity, elemental stability, thermal resistance, and reactivity potential, in alignment with long-term carbon storage and permanence.  Demonstrating clear differences among biochar and pyrolysis temperatures, FPI values ranged from 0.24 to 0.46. Biochar produced at 300 ℃ generally exhibits moderate FPI values (0.33-0.41), with limited thermal stability, elevated functional reactivity, and elemental stability, characterizing them as soft biochar. Within the FPI framework, hard biochar (FPI > 0.41) produced at elevated temperatures (500-700 ℃) exhibits high condensed aromatic structures and low H/C (0.06-0.02), O/C (0.03-0.25) ratio due to gasification at high temperatures, indicating multi-decadal carbon storage applications. Out of the selected biochar pyrolysis temperature combinations, Gliricidia biochar at 700 ℃ achieved the maximum FPI value (0.46), driven by enhanced carbon structural maturity, which is favorable for the carbon permanence applications. The FitPBC Index provides a novel and flexible approach for the classification of biochar based on its use, which will help optimize the selection of the appropriate materials and conditions for its production for various uses, and will contribute to the principles of the bio-circular economy. Current refinements consider more variables for better robustness and define sub-classifications (ultra-soft, soft, transitional, hard, and ultra-hard).Keywords: Biochar Properties, Carbon Permanence, Circular Bioeconomy, Fit for Purpose Biochar, Sustainable Development Goals