Fig. 4: Atmospheric CO₂ removal costs (US$ t⁻¹ CO₂) for mineral-enriched biomass pyrolysis and subsequent biochar soil application.

The calculations consider stable carbon content in biochar based on proximate analysis (stable for ~100 years) and greenhouse gas emissions associated with feedstock and mineral generation, biochar production and biochar application. Simulation of biomass enrichment with (a, d) a refined, soluble mineral (potassium acetate) to bioenergy crop biomass (miscanthus)¹⁶, b, e mineral residues (wood ash) to forestry residues (woody residues)²⁵ and c, f ground rocks (vermiculite) to agricultural residues (rice straw)⁸⁷. The baseline scenarios in a–f assume biomass pyrolysis without mineral addition. Scenarios (a–c) are based on fixed feedstock and mineral costs and scenarios (d–f) on variable feedstock and mineral costs. In e only the feedstock costs were varied as the wood ash costs were assumed to be US$ 0 t⁻¹. In scenarios (d, f) the lower, middle, and upper lines reflect potassium acetate and vermiculite prices of US$ 750 t⁻¹, US$ 1000 t⁻¹ and US$ 1250 t⁻¹ and US$ 105 t⁻¹, US$ 140 t⁻¹ and US$ 175 t⁻¹, respectively. The vertical black line in scenarios (d–f) show the feedstock costs used in scenarios (a–c), respectively. The scenario parameters are reported in Table 2 and more details on the assumptions can be found in the Supplementary Methods and Supplementary Table 1.