Environmental hotspots in the life cycle of a biochar-soil system

A life cycle assessment was conducted to study the environmental effects of a biochar-soil system and to identify the main environmental hotspots. Six scenarios were evaluated, which included the production of biochar from agricultural and forestry residual biomass pyrolyzed at 300, 400, and 500 °C, using a functional unit of 1 t of produced biochar. Modeling of the system and evaluation of impacts were performed using SimaPro selecting impact categories of climate change, human toxicity, freshwater eutrophication, and fossil depletion. According to the results, the climate change impact category presented the greatest relative importance in the life cycle of biochar, with greenhouse gas emission reductions of up to 2.74 t CO₂ eq t⁻¹ biochar when the biochar applied to soil is produced from forestry residual biomass at 500 °C. In relation to hotspots in the life cycle of biochar, transportation was the only stage identified that contributes environmental loads to the system, in contrast, carbon storage, natural gas avoided and urea avoided generate environmental benefits. Carbon storage in biochar is the main hotspot in the system associated to climate change, while the avoided use of natural gas and urea have great influence on fossil depletion, freshwater eutrophication, and human toxicity categories. These categories are highly sensible to allocation methodology options and the assumptions associated to the system boundaries expansion. This finding requires a comprehensive justification and to guarantee the data quality when the system expansion is considered in a LCA study of a biochar-soil system, including energy balance and syngas use, as well as, avoided urea estimation. This study considered one agricultural season, and future works should consider biochar amounts used as soil amendment in each agricultural season for evaluating residual effects of biochar use regarding fertilizers savings.