PAHs
Chapter on polycyclic aromatic hydrocarbons in biochar: formation, process controls, toxicity framing, and EBC/IBI/S668 testing. Report context: 2026-09-16.
Related notes: Contaminants and Quality · Biochar Quality from Process · Pyrolysis Systems and Temperatures · Testing EBC IBI S668 · Heavy Metals and Ash · Product Standards EBC IBI · Key Sources
1. Formation and process dependence
Pyrolysis of organic materials generates PAHs (EBC 10.5E §7.11). Content depends primarily on:
- Peak temperature and vapor residence
- Separation of biochar from pyrolysis gases
- Post-pyrolysis condensation of PAH-laden vapors onto cool porous char
EBC states feedstock type has a negligible influence relative to process conditions (§7.11). Clean combustion of pyrolysis gases usually destroys PAHs in the gas phase; contamination rises when gases adsorb or condense on the solid. Controlled vapor quenching and high-T separation help.
Even simple flame-curtain kilns can achieve low PAH with correct operation; some industrial designs have historically produced elevated PAH as a process-quality signal (EBC §7.11; Kon-Tiki emissions/char study Cornelissen et al. 2016).
2. Which PAH lists matter
| List | Role |
|---|---|
| 16 EPA PAH | Classic monitoring suite; EU fertiliser regulation uses Σ16 = 6 mg kg⁻¹ DM for soil amendments (EBC citing EU 2019/1009) |
| 8 EFSA PAH | Carcinogenic subset used for tighter EBC controls |
| REACH extras | Benzo[e]pyrene & benzo[j]fluoranthene ≤ 1 mg kg⁻¹ each for all EBC classes |
EBC analysis of 936 certified samples found non-carcinogenic PAHs usually >80% of Σ16; 99% of samples meeting Σ16 ≤ 6 mg kg⁻¹ also had Σ8 EFSA < 1 mg kg⁻¹—motivating the dual limit to stop post-treatments that strip light PAHs while leaving heavy carcinogens (EBC §7.11).
3. EBC limit summary (v10.5E)
| Class | Σ16 EPA PAH | Σ8 EFSA PAH |
|---|---|---|
| FeedPlus, Agro, AgroOrganic | 6.0 ± 2.4 mg kg⁻¹ DM | 1.0 mg kg⁻¹ DM |
| Urban, Materials | Declaration of Σ16 to CSI | 1.0 mg kg⁻¹ DM |
| Basic | Declaration | 4 mg kg⁻¹ DM |
(EBC 10.5E Tab.1 / §7.11). Units in EBC tables also appear as g t⁻¹ (= mg kg⁻¹).
4. Bioavailability and risk posture
Hilber and co-workers (cited extensively in EBC) show soil-applied low-PAH biochar often acts as a net sink for environmental PAHs rather than a source; bioavailability of biochar-bound PAHs is typically very low—but rumen studies showed substantial desorption of PAHs from high-PAH chars, motivating strict FeedPlus limits (EBC §7.11). Absent biochar-specific risk assessments, regulators applied compost/digestate-like limits (precautionary).
PCB / PCDD/F: EBC requires quantification when Σ8 EFSA PAH > 1 mg kg⁻¹; limits 0.2 mg kg⁻¹ PCB and 20 ng kg⁻¹ I-TEQ PCDD/F (§7.10). FeedPlus has stricter dioxin/PCB packages (§10.7).
5. Testing notes
- Standard soil PAH methods often under-extract biochar—use EBC-endorsed methods/labs (EBC §7.11).
- North American method pointers: standardizes biochar lab testing §12.1.1 · PDF (EPA extraction/analysis pathways).
- IBI standards similarly require PAH declaration/limits by grade—see IBI standards portal · Product Standards EBC IBI.
6. Practice takeaway
- Engineer no cool-down in PAH-rich vapor.
- Dual-track Σ16 and Σ8 (plus REACH pair) for soil/feed grades.
- Treat elevated PAH as a process failure, not a feedstock inevitability.
Primary links
- EBC 10.5E PDF
- Cornelissen et al. 2016
- ANSI S668 PDF · standardizes biochar lab testing
- IBI standards
- EU fertiliser regulation context via EBC citations: EU 2019/1009