Quantifying inertinite carbon in biochar

The carbon dioxide removal (CDR) potential of biochar is determined by the long-term stability of its biogenic carbon, derived from atmospheric CO₂ fixed by photosynthesis and stabilized in solid form. This stability (carbon permanence) is commonly assessed using decay models to evaluate resistance to re-emission as greenhouse gases. However, these models are limited, as they focus primarily on short-term degradation of labile carbon fractions and are not suited to project the behavior of the highly recalcitrant component of biochar over extended timescales. Inertinite represents highly aromatized and condensed carbon structures that are geochemically stable over millennia. This paper builds upon the Inertinite Benchmarking (IBR_o2) methodology, directly quantifying the stable carbon fraction in biochar rather than relying on modeling. The method combines thermochemical analysis and incident-light microscopy to measure the reactive (labile) component and solid carbonized macerals, respectively. Random reflectance analysis (R_o) provides a representative distribution of carbonization states, with R_o values >2.0 % defining the inertinite fraction after discounting reactive organic carbon. The R_o distribution is processed using kernel density estimation (KDE) and numerical integration to classify inertinite carbon with precision and statistical robustness. As CDR crediting can be linked to measured inertinite content, statistical validity is essential. A Monte Carlo simulation model evaluates uncertainties from sampling frequency and production variability. Results show that increased sampling reduces uncertainty and lowers the conservative safety margin needed for potential errors. This framework supports a justified safety margin applied to reported inertinite carbon and corresponding CDR values, enabling conservative and robust crediting. By combining direct quantification of inertinite carbon with probabilistic modeling of uncertainty, the IBR_o2 method offers a transparent and rigorous framework for assessing biochar permanence, aligned with emerging international certification and national inventory methodologies.