Composition of PAHs in Biochar and Implications for Biochar Production

Abstract

The content of polycyclic aromatic hydrocarbons (PAHs) in biochar has been studied extensively; however, the links between biomass feedstock, production process parameters, and the speciation of PAHs in biochar are understudied. Such an understanding is crucial, as the health effects of individual PAHs vary greatly. Naphthalene (NAP) is the least toxic of the 16 US EPA PAHs but comprises the highest proportion of PAHs in biochar. Therefore, we investigate which parameters favor high levels of non-NAP PAHs (∑16 US EPA PAHs without NAP) in a set of 73 biochars. On average, the content of non-NAP PAHs was 9 ± 29 mg kg –1 (median 0.9 mg kg –1 ). Importantly, during the production of the biochars with the highest non-NAP PAH contents, the conditions in the post-pyrolysis area, where pyrolysis vapors and biochar are separated, favored condensation and deposition of PAHs on biochar. Under these conditions, NAP condensed to a lower degree because of its high vapor pressure. In biochars not contaminated through this process, the average non-NAP content was only 2 ± 3 mg kg –1 (median 0.5 mg kg –1 ). Uneven heat distribution and vapor trapping during pyrolysis and cool zones in the post-pyrolysis area need to be avoided. This demonstrates that the most important factor yielding high contents of toxic PAHs in biochar was neither a specific pyrolysis parameter nor the feedstock but the pyrolysis unit design, which can be modified to produce clean and safe biochar.

Preliminary Assessment of Pyrolysis Biochar Derived from Milk/Dairy Processing Sludge as a Potential Component of Fertilizers

“…At pilot scale, biochar leaving the pyrolysis reactor was separated from the gas and collected in an uninsulated vessel (temperature of 200−300 °C), which allows for fast cooling as opposed to laboratory scale where the biochar was left in the hot reactor to cool down and the temperature slowly dropped to room temperature. 21 A recent study shows that the PHAs present in the pyrolysis gas and vapors can physically sorb/condense onto biochar and contaminate it in the cool, post-pyrolysis zone (discharge chamber) 44 as the one used at pilot scale.…”

Biochar from animal manure: A critical assessment on technical feasibility, economic viability, and ecological impact

“…During pyrolysis, polycyclic aromatic hydrocarbons (PAHs) are formed. However, when adequate process conditions are maintained, they do not condense onto the biochar but are burned as part of the pyrolysis gas (Bucheli et al, 2015; Buss, Hilber, et al, 2022). In practice, product certification, for example, the European Biochar Certificate (EBC, 2012), controls the PAH content of commercial biochar using adapted analytical protocols (Hilber et al, 2012).…”

Atomistic Modeling of Lignocellulosic and Carbonaceous Fuels and Their Pyrolysis Reactions: A Review

“…However, not all the PAHs constitute biochar; a circular concatenation (≥5-ring PAH) must be achieved in the carbonization process. 122 The examination of resonance structures of radicals 27 Copyright 2015, American Chemical Society; Zhou et al 76 Copyright 2017, Wiley-VCH Verlag GmbH & Co; Hosoya et al 81 Copyright 2008, Elsevier; Pastorova et al 109 Copyright 1994, Elsevier; Yang et al 133 Copyright 2020, Elsevier. Phenanthrene intermediated was selected based on McGrath et al 118 is instrumental in elucidating because the radical's position can influence the reaction's favorability.…”