The effect of pyrolysis temperature and feedstock on biochar agronomic properties

As biochar gains popularity within agricultural applications, proper selection of feedstocks and pyrolysis conditions are crucial to maximising its added value. In this study, biochars were characterised for elemental composition, potentially toxic elements, the sum of 16 US EPA polycyclic aromatic hydrocarbons and biochar agronomic properties (pH, acid-buffering capacity, biochar stability, electrical conductivity, cation exchange capacity and water holding capacity) to provide more insight concerning biochar properties and pyrolysis parameters that influence the biochar agronomic properties. Eighteen feedstocks for biochar production were screened based on four selection criteria: moisture content, ash content, fixed carbon content and the presence of potentially toxic elements. Eight feedstocks (spent peat, woody green waste, tree bark, applewood, flax shives, spent coffee grounds, insect frass and chicken manure) were selected and processed into 24 biochars (at three pyrolysis temperatures: 450, 600 and 750 °C) using slow pyrolysis in a pilot-scale rotary kiln reactor. The results showed that feedstock and pyrolysis temperature are crucial factors for biochar composition, toxicity and agronomic properties. The sum of 16 US EPA polycyclic aromatic hydrocarbons showed a significant increase for biochars produced at 750 °C compared to those produced in the 450–600 °C range. Furthermore, some potentially toxic elements (Zn, Cu, Cr) revealed a decrease in concentration above 600 °C. It is recommended not to exceed a pyrolysis temperature of 600 °C in the reactor configuration used. Lastly, feedstock-independent models were constructed for pH, electrical conductivity and acid-buffering capacity through the biochar properties measured. These models may provide a gateway to reverse engineer biochars based on suitable feedstock and pyrolysis temperature selection.