Guidelines for assessment of biochar’s stability through Organic Petrography and micro-Raman Spectroscopy

Biochar is a carbon-rich residue of biomass pyrolysis or pyro-gasification that mimics natural coal macerals, transforming biomass into inertinite-like carbon for long-term storage. To directly assess nine commercial biochar’s stability, this study applies optical microscopies (organic petrography and reflectance measurements) combined with micro-Raman spectroscopy. These techniques, commonly used to evaluate the thermal maturity of geological organic matter, are here adopted to identify differences in degree of carbonization. Bulk chemical indicators, especially the H/C molar ratio, are also included for comparison. While molar ratios remain a useful proxy of overall biochar stability, spectroscopic and petrographic results provided the necessary resolution for study complex heterogeneous biochar.

Biochar reflectance (BCR_o) emerged as key indicators of carbonization uniformity. Unimodal, narrow BCR_o distributions reflect homogeneous thermal degradation, whereas bimodal or skewed patterns identify incomplete or uneven carbonization linked to inefficient heat transfer, short residence times and/or heterogeneous biomass traits. Using the value of 2% as inertinite benchmark (IBR_o2%), reflectance data effectively discriminated incompletely carbonized domains from fully stabilized aromatic structures. Furthermore, Raman spectra showed systematic evolution of the D1 and G bands, with D1-G separation, intensity (iD1/iG), area (aD1/aG), and width (wD1/wG) ratios increasing with BCR_o. These parameters defined two carbonization stages across the dataset. Biochar with BCR_o > 3% show inertinite-like signatures consistent with high thermal maturity. In contrast, samples dominated by low-reflectance fractions (BCR_o < 2%) are characterized by Raman spectral features typical of poorly carbonized, labile material.

By integrating micro-Raman spectroscopy with reflectance measurements, this study introduces a set of rapid diagnostic parameters for evaluating the carbonization efficiency and long-term stability of commercial biochar. The approach enables rapid discrimination between poorly carbonized and fully inertinite-like materials, offering practical benchmarks for CDR applications and for optimizing conditions in real production scenarios. For geoscientists, this multi-proxy framework provides a comprehensive guideline to characterize biochar’s formation, properties, and long-term stability, aligning its evaluation with established concepts of inert organic carbon in the geological record.