Biochars with low polycyclic aromatic hydrocarbon concentrations achievable by pyrolysis under high carrier gas flows irrespective of oxygen content or feedstock

The release of polycyclic aromatic hydrocarbons (PAHs) from biochar is a concern when applied in agriculture or other sectors. Therefore, the objective of this study was to produce “clean” biochars under pyrolysis conditions presumed to minimize their PAH content. Willow, Miscanthus, wheat straw and Sida hermaphrodita feedstocks were pyrolysed under controlled conditions in a laboratory pyrolyzer for 4 h at temperatures of 500, 600 and 700 °C under a continuous stream of nitrogen. Additionally, the oxygen content was varied from 0 up to 2.0% during the pyrolysis of Miscanthus. Elemental analyses confirmed that the biochars met the European Biochar Certificate (EBC) and International Biochar Initiative (IBI) guidelines. Their resulting PAH concentrations, quantified with an optimized analytical method, were consistently very low (<1.5 mg kg−1dw), and qualified them as premium grade (EBC) or Class 1 (IBI). Neither the feedstock nor the oxygen content had a significant influence on the PAH content. Although at 600 °C the PAH concentrations were significant lower than at the other temperatures, the temperature range of 500-700 °C is considered suitable to obtain minimal PAH concentrations. This work confirms the hypothesis that complete removal of gas-phase pyrosynthesized PAHs leads to biochars with low PAH concentrations.