Global potential of decentralized waste biomass to solid carbon for scalable carbon removal

Livestock manure and crop residues are abundant agricultural by-products whose mismanagement releases ~1.56 Gt CO2-eq yr-1, about 22% of agricultural greenhouse gas emissions. Here, we propose a circular bioeconomy system to recover their energy, nutrients, and carbon. The proposed solar-powered, farm-scale system integrates anaerobic digestion, biomethanation, and methane pyrolysis to convert these residues into stable solid carbon, recycling hydrogen internally and returning nutrients to the soil as digestate. We apply a coupled mass, energy, and cost model globally using spatially explicit biomass and solar datasets. We estimate a sequestration potential of ~596 Mt C yr-1 (~2.2 Gt CO2-eq yr-1, ~6% of global fossil CO2 emissions), with a median levelized cost of 926 USD t-1 C for crop residue (5th–95th percentile: 904–980 USD t-1 C), and 1,534 USD t-1 C for manure (5th–95th percentile: 1,510–1,589 USD t-1 C), assuming no revenue from the carbon product. Capital expenditure dominates (~89% of cost, led by anaerobic digestion) while solar electricity contributes only ~11%. Under tailored process conditions, the product can be recovered as carbon black, graphite, carbon nanofibers, graphene, or nanotubes; both its market prices and carbon-credit prices could exceed the sequestration cost, making distributed carbon removal a potential source of farm income.