Pyrolysis of Agricultural Residues for Biochar Production: A Narrative Review of Technologies, Applications, and Business Opportunities

Agricultural residues represent an abundant and largely underexploited biomass resource that can be valorized using thermochemical conversion technologies within a circular economy framework. The improper management of these residues, chiefly open-field burning and uncontrolled disposal, contributes to air pollution, greenhouse gas emissions, and the loss of a valuable carbon feedstock. This narrative review examines pyrolysis as a route for converting agricultural residues into biochar, together with the co-products bio-oil and syngas. It covers the characteristics of candidate feedstocks and the influence of their chemical composition on product distribution; the principles, main types (slow, fast, and intermediate), and reactor configurations of pyrolysis; the physical, chemical, and carbon stability properties of the resulting biochar; and its environmental and agricultural applications, including soil amendment, environmental remediation, and climate change mitigation. Finally, the economic and business perspectives of biochar are considered, spanning market potential, plant scale, and business models, alongside the principal challenges and research gaps that currently limit large-scale deployment. Overall, pyrolysis of agricultural residues shows strong potential to support sustainable waste management, soil improvement, renewable energy recovery, and long-term carbon storage. Biochar can retain a substantial fraction of the feedstock carbon in a stable, aromatic form (with reported permanent removals of approximately 2.7 t CO2-eq per tonne of biochar). It can also improve soil water-holding capacity by up to 51% (and considerably more in sandy soils), while reported biochar selling prices of EUR 436–979 per tonne indicate emerging commercial viability. However, wider implementation will depend on improved feedstock management, biochar quality standardization, techno-economic optimization, and the development of viable value chains adapted to local agricultural conditions.