Assessing CO2 storage in Danish biochars using inertinite benchmarking

This study evaluates the carbon fractions and CO 2 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 CO 2 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 CO 2 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. • The inertinite fraction increases with carbonization temperature. • Biochar carbonized above 575 °C consists of more than 80 % inertinite. • Wood-biochar is richest in inert carbon and has the highest CDR potential. • CDR potential of biochar decreases in the order wood > straw > digestate > sewage sludge.