Forest residue biochars: feedstock, pyrolysis, and properties

Abstract

The European Union (EU) is fostering the bioeconomy as a strategy to achieve a climate-neutral and circular economy. In this context, valorization of forest wastes as pine sawdust into biochar is a key opportunity in Galicia (NW Spain), one of the most forested areas of the EU. Moreover, two abundant Galician shrubs, Erica cinerea L. and Ulex europaeus L. have not yet been explored as biochar feedstock, although these shrubs are associated with high biomass accumulation and an increase of forest fires in the region. Therefore, their conversion into biochar offers a sustainable waste management pathway. This study aims to characterize biochar produced from different Galician feedstocks (pine sawdust, Erica cinerea L., and Ulex europaeus L.). The biochar was produced under three pyrolysis temperatures (300, 400 and 500°C) and three pyrolysis times (30, 60 and 120 minutes). The results showed that nitrogen, carbon concentration, and C/N ratio in the biochar were greatly affected by feedstock initial composition and operating conditions during pyrolysis. Carbon concentration varied significantly among feedstocks, with all materials showing increased carbon concentration at higher pyrolysis temperatures. Sawdust consistently had the highest nitrogen concentration: up to 8% at 300 °C, after 120 min due to the preprocessing with ammonium polyphosphate (APP). The C/N ratio demonstrated distinct patterns, with sawdust maintaining a lower C/N ratio at lower temperatures, while Erica cinerea and Ulex europaeus showed a higher C/N ratio that varied with temperature and pyrolysis time. The results indicate that biochar derived from Erica cinerea L. and Ulex europaeus L. are suitable for soil carbon sequestration due to its higher and more stable C/N ratios, whereas APP processed pine sawdust biochar shows particular potential as a long-term source of nitrogen due to its consistently higher nitrogen concentration and lower C/N ratio under lower pyrolysis temperatures. However, it should be note that the effect of these biochars on soil depends on the specific soil conditions.