Assessing CO2 storage in Danish biochars using inertinite benchmarking

This study evaluates the carbon fractions and CO₂ removal (CDR) potential of a suite of Danish biochar samples produced from typical agricultural feedstocks (wheat straw, wood), digestate, and organic waste across a range of pyrolysis temperatures. The CO₂ storage potential per unit mass of biochar is expressed using the directly measured inertinite carbon concentration (C_inert, wt.%) determined through the inertinite benchmarking methodology. All biochar samples display a pronounced increase in mean random reflectance (R_o) at carbonization temperatures above ∼550 °C. The inertinite fraction (F_inert) increases systematically with carbonization temperature and reaches values > 80 % above ∼575 °C (corresponding to mean R_o values of ∼3-4 %, depending on feedstock). In wood-derived, lignocellulosic biochar produced at carbonization temperatures >575 °C (mean R_o ∼3 %) the F_inert exceeds 98 %. This biochar has the largest CO₂ storage potential, yielding C_inert values of 85.6-91.8 wt% and a CDR of 3.05 tons CO_2e per ton biochar (without considering the potential priming effect in the soil). Wheat straw and digestate biochar also attain a maximum inertinite carbon content at carbonization temperatures >575 °C, with peak C_inert values of 60.6-67.5 wt% and 39.9-50.4 wt%, respectively. The CDR is 2.21 and 1.41 tons CO_2e per ton biochar, respectively. Sewage sludge biochar shows the lowest CDR potential per unit mass, due to its low carbon concentration. Total carbon (C_tot) was found to be, within analytical uncertainty, essentially identical to measured organic carbon (C_org), indicating that separate determination of C_org is unnecessary for the studied biochar types.