Further study on the correlations between biochar combustion reactivity and physiochemical structure at microscale by combining petrography and Raman spectroscopy

This study integrates petrographic methods and Raman spectroscopy to analyzed the co-evolutionary relationship between the characteristic morphologies and chemical structures of coconut shell and corncob biochar. An innovative classification method for biochar morphology based on porosity, pore shape and arrangement were introduced, categorizing into four types: random pore structure, cellular pore structure, solid structure, and strip pore structure. The increase in pyrolysis temperature induces a dynamic equilibrium in the reconstruction of biochar morphology. During 350–1000 °C, the proportion of the random pore structure increases from 22 % to 43 %, while the solid structure decreases from 44 % to 17 %. The random pore structure exhibits higher degree of aromatization and more stable carbon skeleton structure, resulting in lower reactivity. In contrast, the solid structure with lower degree of aromatization, undergoes more rapid transformation during pyrolysis, transitioning into other morphological types. By the correlation between morphology ratios, chemical structure, and combustion reactivity, and considering the differences in thermal reaction rates and extents between these morphologies, an innovative structural index (AD/AG)Ⅲ is introduced to develop a predictive model for biochar combustion reactivity. Compared to conventional methods only considering overall chemical structure, this model improves prediction accuracy, increasing the average fitting R2 from 0.693 to 0.889.